Written Discussion

855

Written Discussion - Contributions

DISCUSSION

SESSION II ELASTOHYDRODYNAMICS - 1 Paper II (i) Invited Paper - "The Effect of Roughness in EHL Contacts" by Dr C J Hooke (The University of Birmingham, UK). Professor J B Medley (University of Waterloo, Canada) Can you explain the physics of how the complementary wave develops in the inlet zone during rolling-sliding contacts? Reply by the Author Although conditions in an EHL contact are somewhat more complex, it is simplest to think of the conjunction as consisting of an inlet zone where the fluid film is generated; a central zone where the pressures are high and where any clearance created in the inlet will pass through at the velocity of entrainment and, finally, an exit zone where a localised restriction forms and where the pressures drop back to atmospheric. As the roughness passes through the inlet it alters conditions there, producing a change in clearance at the inlet to the central zone. For example a roughness with a wavelength slightly shorter than the inlet will tend to increase the clearance entering the conjunction when the peak (maximum clearance) is close to the end of the inlet region. Conversely when a trough (minimum clearance) is near the junction between the inlet and the high pressure zone it will tend to reduce the clearance entering the central region. These clearance changes will pass, relatively unchanged (ignoring for the moment any nonNewtonian effects) and constitute the complementary wave. The frequency with which

the clearance at the start of the central region varies depends on the velocity of the rough surface, v, and the wavelength of the roughness, X, giving a frequency v/X. Since the clearances are carried through the central zone at the entrainment velocity, u, the wavelength of the complementary wave will be Xu/v. Thus, where the rough surface is moving more slowly than its counterface, the wavelength of the complementary wave will be greater than that of the original surface roughness. Where it moves faster the complementary wave will have a reduced wavelength. Professor A A Lubrecht (LMC, INSA, Lyon, France) How do you treat low slip cases that fall between pure rolling and rolling and sliding? Reply by the Author The aim of the paper was to provide a framework within which actual contacts could be placed. Thus I intentionally only discussed the cases of pure rolling and of rolling/sliding where the sliding velocity was sufficient to induce non-Newtonian behaviour. However, at low slip velocities Fig 2 suggests that the complementary way will not decay significantly even under non-Newtonian conditions. Equally, there will be little difference between the wavelengths of the complementary wave and that of the original roughness. I would suggest that the approach used for pure rolling would apply under these conditions. If the non-Newtonian effects have not significantly altered conditions in the inlet then the data obtained for pure rolling should apply. However, it would be prudent to use a perturbation analysis for the actual operating conditions to obtain more accurate roughness attenuation and pressure ripple values.

Paper II (ii) "Complete Numerical Analysis for Mixed Boundary and Thermal Elastohydrodynamic Lubrication in Steady1 1 State Point Contacts" by L Gao , M Jiang , P 2 1 2 Yang , Z M Jin , D Dowson ('Qingdao

856 Polytechnic University, China, 2The University of Leeds, Leeds, UK).

was based only on the results for perfectly smooth surfaces.

Professor H P Evans (Cardiff University, UK). What is the coefficient of friction imposed on the boundary contact area in comparison with that of the fluid film area?

The importance of the convection terms in EHL studies has long been an argument between researchers. Some people, based on their experiences, claimed that convection effects can be ignored completely, while others, also based on their experiences, said that convection effects must be considered. However, the importance of the convection terms strongly depends on the speed conditions, that is, when the speed is very high, we should consider the convection, but, when the speed is relatively low, we do not need to consider the convection. In this paper the entrainment speed is only 20 mm/s, so we are confident that the effect of convection can be ignored. A more detailed discussion on this point was given elsewhere [Al], where the results with and without convection terms were compared.

It is surprising that there seems to be so little effect of boundary lubrication on the temperature in the film. Reply by the Author The coefficient of friction imposed on the boundary contact area was /% = 0.1, which was larger than the coefficient of friction of the whole contact. In all cases considered in this paper, boundary lubrication occurred only in low pressure regions, therefore its effect on the film temperature was not so significant. Dr Hao Xu (Glacier Vandervell Bearings, UK). The presenter stated in his answer to the previous question that the value of the stress in the boundary lubrication region is not important. This leads to the conclusion that the limiting oil film thickness for the region is not significant. However, we know from our experience that the presence of asperity contacts in a lubricated conjunction can lead to a significant increase in the oil film temperature. How would the authors comment on the point? Also, did the authors verify that the convection term is insignificant in EHL Analysis? From our experience, the convection can have an important impact on the peak oil film/solid temperature, as well as the temperature field in the EHL conjunction, in particular in the inlet film region, which may affect the oil film generation in the conjunction.

[Al] P. Yang, S. Qu, Q. Chang and F. Guo, "On the theory of thermal elastohydrodynamic lubrication at high slide-roll ratios — line contact solution," ASME Journal of Tribology, 123(1), 2001, 36-41.

Paper II (iv) "Unsteady EHL: An Inverse Hydrodynamic Formulation" by J F Booker (Cornell University, USA). Professor H P Evans (Cardiff University, Wales, UK). Is the Reynolds equation formulated with film thickness as the active variable? If so how does the pressure enter into its formulation - is it linearised or is it an active variable? Background to the question is that Reynolds equation has an inlet singularity when entrainment is included if it is formulated with 'h' as the active variable. Reply by the Author

Reply by the Author In this paper the surfaces were assumed to be perfectly smooth. However, we have also solved some cases for rough surfaces, where the temperature rise associated with asperities was quite high. The conclusion we made in this paper

All three discussers have extensive experience with unsteady EHL of counterformal contacts, so their comments are very much appreciated. Professor Evans raises issues pertinent to the present "inverse hydrodynamic formulation". As noted in Apps. A and D, the fluid film physics resides in finite element matrices which are developed either from a

857 related functional or from the application of Galerkin's method to the Reynolds differential equation itself. Either way, the pressure is assumed to vary bilinearly over each rectangular finite element, so that distributions within elements are interpolated from nodal values. As the paper describes, this FEA fluid formulation is simply solved (subject to structural constraint) for time rate-of-change of interior film thickness. Unfortunately, the present formulation has not yet been fully tested on problems with non-zero entrainment velocity, so the pitfalls suggested are still largely untested. Adding a nominal entrainment velocity (1 mm/s) to the instantaneous situation of Figs. 7-9 results only in breaking the symmetry of Fig. 7, as shown in Fig. 10. (Film pressure and thickness remain as in Figs. 8,9.) Evolution from that instant onwards is yet to be investigated. It seems likely that in unsteady EHL of counterformal contacts (as in unsteady massconservative cavitation) the introduction of entrainment changes the very nature of the problem and calls for special solution procedures (such as upwind differencing).

Hooke, C. J. 'The calculation of film thicknesses in soft, highly deformed contacts under dynamic conditions', Proc. Instn. Mech. Engrs. 201(C), 1987, 171-179. Use of clearances as the primary variables in dynamic EHL contacts works well if the pressures are well defined. For example, Hirano used this idea to calculate the clearances under reciprocating seals. However, the method was found to fail where there was significant hydrodynamic pressure generation as in the inlet zones of EHL contacts. How does the author's approach overcome this problem? For instance how does the present calculation cope with the initial stages of film formation where the ball-cup separation is larger? Reply by the Author The author is grateful for the very relevant additional references provided by Dr. Hooke. The concerns re entrainment raised by Dr. Hooke seem very similar to those of Professor Evans. As noted above, the present formulation has not yet been fully tested for such problems, so the potential degree of difficulty is still unknown. Clearly, this is an area for early investigation.

Dr C J Hooke (The University of Birmingham, UK). The use of film thickness as a primary variable has been most developed in soft EHL contacts. See the following references:

In the present numerical example no specific difficulties were observed in the initial stages of film formation.

Hirano, F., 'Dynamic problems in inverse hydrodynamic lubrication', Proc. Third Conf on Fluid Sealing, BHRA 1967

It is worth noting that the formulation must fail numerically in any situation in which elastic deformation becomes vanishingly small.

Hirano F and Kaneta, M, 'Theoretical investigation of the friction and sealing characteristic for reciprocating motion', Proc. Fifth Conf on Fluid Sealing, BHRA 1971

Professor A A Lubrecht (LMC, INSA, Lyon, France). What is the influence of the boundary conditions on the pressure and film thickness results presented?

Hooke, C. J. 'The elasto-hydrodynamic lubrication of soft, highly deformed contacts under conditions of nonuniform motion', Trans. A.S.M.E. series F, 1986,545-550.

Reply by the Author

Hooke, C. J. 'The lubrication of soft contacts', Proc. 13th Leeds- Lyon Symp., Leeds, Sept 1986, 1-6.

Professor Lubrecht's question raises the possibility that non-zero pressure boundary conditions might pose unforeseen difficulties for the present formulation.

858 The Numerical Example presented in the paper has 'flooded' (zero) pressure boundary conditions. In discrete terms, this means that all exterior nodal pressures are set to zero. Figures 11-13 show the instantaneous results if exterior nodal pressures are given a uniform value of 10 kPa and interior nodal film thicknesses are (arbitrarily) chosen as in Fig. 9.

Film Flow @ I = 0 10591

(qmax * 4 368e-09)

(qsum • 6.272e-O8)

It should be noted that the test described is a bit artificial, corresponding as it does to a narrow inlet groove surrounding (but not extending beyond) the square 'contact zone'. Comparison of Figs. 11-13 and corresponding Figs. 7-9 reveals generally predictable differences in instantaneous results. Fig. 11 shows that the elevation of (specified) exterior pressure results in a reversal of exterior flow direction at corner nodes (and a reduction in magnitude elsewhere). The net outward flow drops from 195.0 ug/s to 62.7 ug/s. Fig. 12 shows the uniform elevation of (specified) exterior pressure, coupled with subtle changes in interior pressure distribution contours (but without any change in the necessary average pressure of 32.5 kPa). Maximum (central) pressure drops slightly, from 54.3 kPa to 53.5 kPa.

Figure 11. Nodal outward flow resultants (p2 = 10 kPa)

p (kPa)

Fig. 13 shows the specified interior film thickness distribution, coupled with a general increase of exterior values, particularly visible at the corners of the 'contact zone'. Minimum (midside) film thickness rises from 5.14 um to 6.92 um.

y (mm)

The crucial matter of how the system will evolve from this instant onwards is yet to be investigated.

Figure 12. Film pressure (p2 = 10 kPa)

-0.5 -1

-1

-0.5

i

0.5

x (mm)

In any case, it seems likely that a much larger 'contact zone' would be less sensitive to boundary condition effects. h (um)

15 .

0.5

y (mm)

^j

-°-

5

x (mm)

Figure 10. Nodal outward flow resultants (ux = 1 mm/s)

y (mm)

Figure 13. Film thickness (P2 = 10 kPa)

859

SESSION III NANOTRIBQLOGY - 1 Paper HI (i) Invited Paper - "Some Aspects of Tribological Behaviour at the Microscale - With Particular Reference to MEMS and MMAs" by J A Williams and H R Le. (The University of Cambridge, Cambridge, UK). Professor G Poll (University of Hannover, Germany). The principal kinematics and designs used for MEMS were tried on a larger scale already 200 years ago, at least it appears to me from the presentation. Are we now doing things better, especially regarding tribology, due to the experience gained since then? Or is history "repeating itself and everything invented again? Reply by the Author Kinematically there are indeed some very close parallels between the potential design of MEMS mechanical arrangements and 18th or 19th century mechanical devices. With modern machine tools, tolerances on macroscopic components can be of the order of 1 part in 10,000 or better: within a MEMS, this sort of accuracy would mean dimensional control down to atoms - which is not possible - indeed the etch stage of component release really limits backlash to something like 1 part in 100. So there may very well be some ideas from, perhaps, the early instrument makers about retaining functionality while coping with inaccuracies in manufacture. Tribologically, I suspect that we are in uncharted territory - the material set is rather limited, full film lubrication is not an option because of the potentially disastrous effects of meniscus forces. Professor Koii Kato (Tohoka University, Japan). Is there any special or unique type of wear and wear problem of MEMS which is not observed in Macro systems? Reply by the Author In so far that there is an established set of wear mechanisms at the macro-scale, most of our models assume that each wear event occurs at a very small scale and that the results of a large number of these

interactions (which occur within the nominal or geometric contact area) must then be combined in some appropriate statistical manner to arrive at a macroscopic wear rate. Although 'small', each individual asperity-scale interaction is, at least in most analyses, supposed sufficiently large to be modelled by some method which treats the wearing material as a (usually homogeneous, isotropic) continuum. Both stages of this philosophy could be thought open to be challenged at the MEMS scale. The mechanical behaviour of Si (or the ceramics derived from it or deposited on it) may well be very different from that of a homogeneous, isotropic plastically deformable material. Within a MEMS the length scale of the contact patch and that of the surface topography (which is often governed by the fabrication process) may scale at different rates so that the nominal contact area may not be 'large' compared to the asperity size - although at this scale it is even more challenging than normal to define exactly what one means by an asperity! Professor F M Borodich (Cardiff University, Wales, UK). What do you think about the importance of studying the influence of environmental gases on nano-level friction of DLC:H coatings? Reply by the Author There is no doubt that the tribological performance, i.e. both friction levels and wear rates, of many ceramic materials, including DLC, can be influenced by the local environment. I can recommend the review on the physical behaviour of DLC by my colleague Professor Robertson in Material Science and Engineering R37 (4-6)(2002) pp 129-281. From a practical point of view, if it becomes necessary, in order to maintain adequate function, to hermetically seal a MEMS (whether in a vacuum or some specified atmosphere) to ameliorate the effects of local atmospheric changes (in humidity for example) there would be a severe cost penalty. Packaging of these devices is a very real issue. Professor A A Lubrecht (LMC, INSA, Lyon, France). What is the influence of the boundary conditions on the pressure and film thickness results presented?

860 860 Reply by the Author Perhaps surprisingly, the likely ' P V values in MEMS are not hugely different from those in more conventional macro-devices: they may rotate or reciprocate at very high frequencies but the dimensions are very small. The role of macroboundary lubricants may well be taken on by socalled Self-Assembled-Monolayers. As at the macro-scale, these have to be slippery enough to give low friction but mechanically robust enough to survive. There is useful experience here from the magnetic storage industry which has used such protective layers for some years - although there is some question about their robustness: it might be necessary to make allowance in some way for the SAMs to be replenished from local reservoirs.

Paper III (ii) "Chain Matching Effects of Base Oils and Additives in Nanotribology of Boundary Films" by Y Akiyama, K Nakano (Yokohama National University, Japan). Dr I Minami (Iwate University, Japan). The thickness of the mobile layer increases by 7 nm if stearic acid was added to hexadecane (Fig. 11). Do you have any evidence which may explain the effect of acid concentration and/or chain length of the acid on thickness increase? Please suggest a model of the boundary film composed of mobile and immobile layers. (2) There may be some "mis-matching" effect of C-14 acid and hexadecane (Figs. 17 and 18). Do you have any additional results which may give some hints for the mechanism? Reply by the Author Dr B-O Ahrstrom (SKF Engineering and Research Centre, The Netherlands). Have you formed a theory on why the mismatch is big when the additive and base oil chain lengths almost coincide? Reply by the Author Professor H A Spikes (Imperial College, London, UK). Could you give me your thoughts on the

origin of the chain matching effect - i.e. the mechanism by which it occurs? Reply by the Author

Paper III (iii) "Lubrication Properties of Al?Oj Nanoparticles in Aqueous Suspensions" by S Radice, S Mischler (Ecole Polytechnic Federale De Lausanne, Switzerland). Professor K Ikeuchi (Kyoto University, Japan). Have you found alumina hydrates or any tribological reaction through your experiments? Reply by the Author Our experimental set-up allowed us to measure the current in the 3-electrode cell, given by electrochemical reactions taking place on the steel. The dominating reaction under passive conditions was metal oxidation. No specific chemical analyses were performed after the tests. So, we don't have evidence for generation of alumina hydrates during the tests.

SESSION IV LUBRICANTS - 1 Paper IV (ii) "Evaluation of Ionic Liquids for the Application as Lubricants" by N Doerr, E Kenesey, C Oetsch, A Ecker, A Pauschitz, F Franek. Dr I Minami (Iwate University, Japan). The authors explained "small island" on the worn surface of a ball as some boundary film, which is composed of polymers. I would like to ask three questions concerning the lubrication mechanism of ionic liquids. (1) Which parts of ionic liquids are precursors of the boundary film? - cationic, anioic/moety, or both of these? Reply by the Authors Classical tribochemistry suggests interaction of anionic moieties with metallic surfaces. The facts

861 that some of the tested ionic liquids have similar cations and that the tribometrical properties are rather influenced by the anion support these traditional findings. But we think that some cations of ionic liquids may act as precursors, e.g. for polymeric films ("small islands"), so that both anions and cations contribute to boundary films. The evaluation of the influence of cations is the object of current XPS studies. (2) Please suggest the dependence of parameters (i.e contact stress or oil temperature) on the boundary film formation. Reply by the Authors We continued our studies with tribometrical experiments at 100 °C maintaining the other test parameters given in the paper. The comparison of the results coming from 30 and 100 °C showed - as expected - different behaviour for the tested ionic liquids. Generally, it can be said that higher wear and higher friction were observed at 100°C. For IF 1, IF 4 and IF 6 wear increases and for IF 1 and IF 6 friction increases up to 100 % were observed, while the other fluids showed little dependence on temperature. (3) Does the "small island" have advantages to reduce friction and/or wear? Reply by the Authors A satisfactory answer cannot yet be given as our investigations of boundary films formed by ionic liquids are not yet finished. Having a look at figure 1 and comparing the results for IF 1 and IF 2 in figure 2 and 3 we cannot see advantages at the moment. Dr T Reg Forbus (Valvoline Technology, USA). Polyglycols with added salts (inorganic) have increased pressure-viscosity coefficients without an increase in kinematic viscosity. By using very nonnucleophilic anions in the salts that have cations which complex the polyglycol oxygen then the corrosivity of the fluids is eliminated. This might be an approach with ionic fluids to alter corrosivity characteristics.

Reply by the Author Beside the chemical composition of ionic liquids we also assume a significant influence of water and impurities from synthesis on corrosiveness. Of course, the choice of appropriate anions and cations remains a key factor.

Paper IV (iv) "Effectiveness of Boundary Lubricant Additives on Some Coated Surfaces" by R P Glovnea, A V Olver, H A Spikes. (Imperial College, Tribology Section, London, UK). Dr A Oldfield (Uniqema Ltd, Wilton, UK). In the explanation of D-OCP friction and film thickness data on steel, what is happening molecularly to the polymer? Reply by the Author The OCP-D polymer solution formed a boundary film, of about 20 nm thickness, apparently as a result of the presence on the solid surfaces of highly-viscous layers, some forty times as viscous as the bulk polymer solution. This type of boundary film-forming behaviour was found to occur only with polymers that contained polar, dispersant moieties and it is believed that this polar functionality caused the polymer to adsorb on steel surfaces to form layers of enhanced viscosity, having approximately the thickness of one molecular coil diameter.

SESSION V BIO-TRIBOLOGY - 1 Paper V (i) Invited Paper "Life Cycle Aspects of Total Replacement Hip Joints" by G H Isaac (DuPuy International, Leeds, UK). Professor D Dowson (The University of Leeds, Leeds, UK). Since the new range of hard-on-hard (metal-on-metal; ceramic-on-ceramic; ceramic-onmetal) hip replacements can exhibit very low wear rates, reliable clinical data will take a very long time to collect.

862 This emphasises the importance of 'realistic' simulator testing, particularly in relation to loading cycles and their representation of daily activity. This will make simulator design and operation more complex and more expensive. It may also be necessary to develop test fluids which more correctly simulate the boundary and rheological properties of synovial fluid present within joint replacements in the body. Would you please comment on this? Reply by the Author 1 agree that the importance of simulator testing will increase with the expanded use of hard-on-hard bearing surfaces. The next generation of simulators will be capable of applying much more sophisticated loading cycles which reflect the variety of activities undertaken during daily life. It will also be important to utilise lubricating fluids which are more realistic because hard-on-hard components are much more sensitive to variations in the properties of such fluids. This may also mean testing with a range of fluids which represent the variations from one individual to another in the fluid which surrounds the artificial joint. As a footnote it should be remembered that in-vivo this fluid is part of the living tissue and is constantly being renewed, ln-vitro the fluid will be gradually degrading. Professor J B Medley (University of Waterloo, Canada. Do you think that EHL ever completely separates the implant surfaces in the simulator studies that you cited in your presentation?

Reply by the Author In theory the wear rates could be decreased by further reducing the clearance between the components. However, practically this is not possible because another prime requirement of these surface replacement devices is that the components are as thin as possible. This means that there is significant deformation of components especially at implantation. The 100|im clearance is a compromise that optimises these conflicting requirements.

Paper V (ii) "Evaluation of Wear Resistance of Ultra-High Molecular Weight Polyethylene for Joint Prostheses in the Multi-Directional Pin-onPlate Tester" by T Sawano, T Murakami, Y Sawae, (Kyushu University, Fukouka, Japan). Professor J B Medley (University of Waterloo, Canada). Do you think that more data and a statistic treatment of this data would make your findings more certain? Reply by the Author Yes, I do. We try to get more data but in fact we have only one apparatus at the present stage and can get only two results at one time.

Paper V (iii) "The Influences of Lipid and Protein Concentration on wear of Ultra-High Molecular Weight Polyethylene" by Y Sawae, T Murakami, T Sawano.

Reply by the Author There is strong evidence in the literature, as reported by Goldsmith et al, that separation does occur for part of the articulation cycle even with relatively small diameter components. There is also evidence that, in line with lubrication theory, the percentage of the cycle time during which separation occurs increases with bearing surface diameter. Mr S Polak (Neale Consulting Engineers Ltd, Hampshire, UK). Clearances of 100 microns are large by engineering standards. Can they be reduced further to improve fluid film lubrication?

Professor J B Medley (University of Waterloo, Canada). Would it be useful to eventually test bovine synovial fluid and some of its components in your test apparatus? Professor D Dowson (The University of Leeds, Leeds, UK). In natural synovial joints it is generally thought that lipids contribute to the good lubrication system reported by many investigators. In your experiments it appears that the wear of joint replacement materials increases with lipid concentration.

863 Could you please comment on these observations? Ms Valentina Ngai (University of Waterloo, Canada). Do you know the nature of the proteins in solution and do you think denaturation has an effect on the resulting wear rates?

Paper V (iv) "Wear Process under Concentrated Contact in Alumina/Alumina Hip Joint" by K Ikeuchi, H Mackova, Y Morita (Kyoto University, Kyoto-fu, Japan). Professor D Dowson (The University of Leeds, Leeds, UK). What was the calculated value of the (very small) steady-state wear rate?

our friction and wear test measurement. In our test method, we only used one dimension of tool material (with the same area for each tool material). Our wear measurement is always dealing with one surface area (our aluminium blank or tooling piece). We measured friction coefficient and wear depth (we can also convert it to wear volume in our study). Our multi-probe sensor can be applied for different tool shapes or dimensions. In our study, we try to simplify the case by using only the same dimension tool for friction and wear measurement. Dr Y Liu (Technische Universitat, Limenau, German). Will you please elaborate your methods for measuring the wear and the strain distribution? Reply by the Authors

Reply by the Authors We obtained only one result for a couple of test pieces and the marks in Fig. 12 indicate weight loss of different couples. We could not calculate the rate of steady wear exactly because the weight loss up to 6 hours was less than the error. We suppose that a wear test with an edge-on-cy Under apparatus does not allow the steady wear to be measured because the contact pressure decreases with time.

SESSION VI COATINGS - 1 Invited Paper - "An In-situ Tribotest Method designed for Predicting Wear Life and Frictional Performance during the Aluminium Forming Process" by S C Tung, M Wenner1, Z Guo2, Z Li2, X Dong2 ('General Motors Research and Development Centre, MI, USA. 2 University of Science and Technology, China). Dr S Franklin (Philips Centre for Industrial Technology (CGT), Eindhoven, The Netherlands). How was measurement of the friction behaviour and wear rate achieved locally in different areas of the tool? Can this multi-probe sensor be applied to different tool shapes? Reply by the Author In our text, we have a very detailed description on

We apply our probe sensor and Taylor-Hobson Surface Profilometer to measure our wear depth either in-situ or after testing (Taylor-Hobson Surface Profilometer) of tool coatings or solid lubricants. Concerning the strain rate distribution, we have a built-in strain gauge (in Figure 5). The strain gauge, which is stuck on the probe, transmits the information of the bending deformation to the dynamic strain-measuring instrument. The vertical portion of the force transmitted to the horizontal probe can be also tested by the strain gauge in Figure 5 in a similar way. We can obtain stress and strain rate distribution based on the test parameters and material properties shown in our text. We have designed a 3D computer program to compute all test results. In addition, the computer system collects the data transmitted from the dynamic strain-measuring instrument, and computes the friction coefficient.

Paper VI (ii) "Durability of Perfluoropolyether Nanolubricant Film Fixed on Diamond-like Carbon Surfaces" by T Kato, M Kawaguchi (National institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan). Dr Y Liu (Technische Universitat, Limenau, German). It seems that controlling the thickness of the bonded layer is very important for achieving good tribological performance. How did you measure the thickness of the bonded layers, which were reported to be 0.5 - lnm?

864 Reply by the Author Perfluoropolyether (PFPE) molecules are fixed on disk samples by dipping the samples into the PFPE solutions followed by a heat treatment. Some PFPE molecules are chemisorbed onto the surface and form a bonded layer, but other molecules are physically adsorbed onto the surface and form a mobile layer. The bonded layer can be thickened by using a thicker PFPE solution or by making the heat treatment longer. The mobile molecules are weakly adsorbed therefore easily washed away by rinsing with a solvent. We prepared two disk samples for each test using samples from the same lot of products and through the identical process of the dipping and heat treatment. The two samples had the same thicknesses of the bonded and mobile layers, which was confirmed by preparatory experiments. One of the two samples was used for a measurement of the total film thickness and for the tribological tests. On the other hand, another sample was rinsed by a solvent and mobile molecules are washed away, and used only for the measurement of the bonded layer thickness. The thickness of the layers was measured optically by an ellipsometer (Fivelab, model MARY-102, Japan), and the results were calibrated by X-ray photoelectron spectroscopy (XPS) measurements. The thickness measurements were performed at more than two hundred points on each sample disk and the data were averaged. We must note the laser beam spot on the sample in the ellipsometer measurement was elliptical with a size of 70 u.m x 100 u.m in diameter. Thus the measured value is the spatially averaged film thickness in this elliptical area. Professor F M Borodich (Cardiff University, Wales, UK). In 1996 Suk (IBM) published a paper in J. Tribology ASME about similar wear tests. He observed abrasive wear of counterparts by full protuberances of lubricant amorphous-carbon films. Have you observed abrasive wear at initial cycles of your tests? Reply by the Authors We performed other experiments ' for the observation of the wear particles and wear traces using the same kind of lubricants and disk samples. In the experiments, we obtained the similar friction

traces with Fig. 2 of the present study. By the observations with an optical microscope, very small amounts of wear powder were observed but no wear traces were observed either on the ball and disk samples before the wear trace took the maximum (point E). On the other hand, the abrasive wear traces were observed both on the ball and disk samples and the wear powder was accumulated along the wear trace after the friction trace taking the maximum, then the amounts of wear powder increased with the friction test. 10. M. Kawaguchi and T. Kato, J. of JAST, 48, 5 (2003)410.

Paper VI (iii) "Rolling and Rolling-to-Sliding Contact Behaviour of DLC Coatings" by L Xiao1'2, B G Rosen2, P H Nilsson3, M Kalin4, J Vizintin4 ('Chalmers University of Technology, Sweden, 2 Halmstad University, Sweden, 3Volvo Technology Corporation, Sweden, 4Centre for Tribology & Technical Diagnostics, Slovenia). Dr S C Tung (General Motors R & D Centre, USA). (1) Why does the additive oil have the higher wear volume compared with the mineral oils? (2) It looks like surface interaction below the DLC coating with the addition of oils in the study. Please extend your discussion on surface interaction. Please clarify this part. Reply by the Authors (1) In our study, only for DLC coated surfaces, additivated oils have higher wear volume than the mineral oils. The effect of additives is not clear at present, but the behaviour is rather complex and less repeatable. Further detailed and dedicated studies will be necessary, similar to our other works under sliding conditions (see refs. list). (2) Surface interactions between the oils and additives and DLC coatings are completely unknown at present. Some results have been published (ref.5: M. Kalin at al., Tribol.Letters, v.17, n.14, 679-688, 2004) providing the tribological behaviour and some suggestions on mechanisms between the oils and different DLC

865 coatings (without interference of other counter materials), however, even these are based on quantitative evidence, rather than direct interaction proofs. Professor F M Borodich (Cardiff University, Wales, UK). One can see on your slide that the main wear was observed at the first several hundred cycles. Dr S Y Harris (Ford) and Dr A Weiner (GM) reported decreasing of abrasiveness of DLC:H coated surfaces as a power-law with the number of cycles. Did you try to check the applicability of the power-law relation to your tests? You have also reported that for non-lubricated DLC coatings, wear starts to increase very rapidly after 5.105 cycles, while wear of lubricated coatings is increased gradually. What physics is behind these observations? Reply by the Authors (1) Power-law relation was not applied, because any (co)relation between the wear and number of cycles depends on wear mechanism and this can change from case to case and even during the same test. However, we agree that modelling might be a good tool to understand the wear-behaviour better. (2) One of the suggestions for differences in wear of lubricated and non-lubricated tests is that lubricant provides a thin separating layer and reduces friction (see Figs. 9-11), which lowers the shear stress and thus reduces wear and fatigue phenomena. Also, oil removes any debris that possibly was generated at the surface and it provides rather "constant" contact conditions; while in non-lubricated conditions, generation and accumulation of wear debris in the contact affects and might also change the wear mechanism and stress conditions at the surface, promoting more intensive wear and/or fatigue. Dr B-O Ahrstrom (SKF Business & Technology Park, The Netherlands). A comment on the dramatic wear rate exhibited in the un-lubricated DLC case. I do not quite agree that the solution lies in chemical effects since DLC (correctly applied) is

a rather inert medium. It is not gas impermeable but still chemically rather inert. I would suggest a more straight-forward explanation; that ruptures in the coating will introduce third-body abrasion with accelerated wear as a consequence. Reply by the Authors We do not claim that the reason lies in chemical effects, but the mechanisms are not clearly understood and we believe the conclusions should not be drawn only based on the assumption of "inertness". However, our suggestion for the high wear indeed refers primarily to mechanical reasons. We think that shear stresses and surface fatigue in addition to deformation of the coating due to a rather soft substrate are the most influencing factors. In addition, wear debris, which would indicate third body abrasion were not observed (by naked eyes) in our tests.

SESSION VII TRANSMISSIONS Paper VII (i) Invited Paper. "A Full-Toroidal Traction Drive IVT for Automotive Volume Production" by I B James, D T Price, P D Winter (Torotrak Development Ltd, Leyland, UK). Professor R Coy (The University of Leeds, Leeds, UK). In the Keynote Paper (Session I) examples of flaking/pitting took place in the half Toroidal design. Have you seen similar effects and do you use specially refined bearing steels? Are the pressures in your variator similar to those in the half Toroidal design (~ 4GPa)? Reply by the Author For the Torotrak full-toroidal variator we use a standard medium heat resistant bearing steel (KUJ7). Contact pressures are in the range of 1.5 to 4.2 GPa. Dr R I Taylor (Shell Global Solutions (UK), Chester). The IVT is a similar package size to the

866 6AT. What is the likely difference in price expected to be? Reply by the Author We are confident that the cost of an IVT in volume manufacture will be comparable to that of an equivalent 6AT; preliminary costings carried out by customers already show our prototype designs prior to the application of industrial engineering and productionisation - as being within 15% of equivalent fully mature 6AT's. Importantly, however, IVT also provides a range of additional customer-oriented features when compared to conventional transmissions, and this increased functionality has considerable commercial attraction to the vehicle manufacturers.

Paper VII (ii) "Effects of the Maximum Hertzian Pressure on the Maximum Traction Coefficient Under EHL Conditions" by M Kaneta1, T Shigeta', H Hata2, and P Yang3. ('Kyushu Institute of Technology, Kitakyshu, Japan, 2Idemitsu Kosan Co. Ltd., Ichihara, Japan, 3 Qingdao Institute of Architecture and Engineering, China). Professor G W Poll (University of Hannover, Germany). What was the theory behind the "numerical method" to determine x0 and p? What is the difference to the "measured" relationship? Reply by the Author The open marks, which correspond to the Eyring stress evaluated numerically, shown in Figs. 3 and 4 were determined as follows: From a traction curve measured under a certain maximum Hertzian pressure, we can obtain the maximum traction coefficient (see Fig. 1) and the corresponding slideroll ratio (see Fig. 2). Since there are some scattering in the relationship between the slide-roll ratio and the maximum Hertzian pressure obtained experimentally, the regression line was made. That is, we can know the slide-roll ratio and the maximum traction coefficient corresponding to a certain maximum Hertzian pressure. The Eyring stress was evaluated by the thermal EHL analysis so that the maximum traction coefficient obtained numerically was consistent with the measured

value. The solid marks shown in Figs. 3 and 4 correspond to the Eyring stress obtained by the different experiments not by the thermal EHL analyses.

Paper VII (iii) "Structure and Mechanical Property of Boundary Lubricating Film formed in Belt-Type EVT Oil" by T Sato, K Kawata (Kobe Steel Ltd, Japan). Professor G Poll (University of Hannover, Germany). Did the 0.5 m/s sliding speed applied in the tests come from measurements or theoretical considerations? Reply by the Author Sliding speed was calculated by substituting the measured values of the input torque and the slip ratio of the element on the pulley into the theoretical formulas. Calculated sliding speed ranged from 0.3 to 0.7 m/s. So we determined the sliding speed into the average value of 0.5m/s. Paper VII (iv) "Efficiency Optimisation of the Full Toroidal Variator Through the Application of EHL Numerical Models" by J P Newell, S Cowperthwaite, M Hough, A P Lee (Torotrak (Development) Ltd, Leyland, UK). Dr C J Hooke (University of Birmingham, UK). The use of a load where the traction is 90% of the maximum tractive force does not allow much margin for error. Are any special design strategies being employed? Reply by the Author The current study was concerned with identification of the optimum operating point for the full toroidal variator and has shown that the optimum point calls for operation at a higher traction coefficient than previously believed. To achieve levels of traction coefficient approaching the peak traction coefficient for a given condition requires knowledge of the relationship between peak traction coefficient and the given operating condition. This is currently achieved by empirical mapping of the traction curves over the entire variator operating conditions. It has not been

867 867 possible to use numerical models for this purpose, as the accuracy in prediction of peak traction coefficient is not adequate with the current rheological models [13].

chamber, there is no flow associated with it. Reducing only the endload pressure does not influence the amount of energy required to drive the hydraulic circuit.

Since in the torque controlled full toroidal variator, traction coefficient is controlled via a hydraulic circuit, accurate control of traction coefficient is possible with suitable safety margins across the entire operating envelope.

SESSION VIII TRIBOCHEMISTRY

Professor R Coy (University of Leeds, Leeds, UK). As you change the end load pressures to maintain a constant efficiency over what range does the contact pressure vary? Reply by the Author Since traction coefficient is inversely proportional to endload, endload will generally be reduced to optimise for efficiency. This could typically be by up to 40% for some designs of variator. (For illustrative purposes Figure 11 shows the effects of reducing endload by approximately 85%.) Although a 40% reduction in endload is possible, the corresponding reduction in contact pressure will be less significant due to the one third relationship between contact pressure and normal load. Clearly this will have a very large influence on increasing the fatigue life (9th power relationship to contact stress). Typically the variator would be expected to operate at peak Hertzian stresses ranging from approximately lGPato 3.9Gpa. Professor H P Evans (Cardiff University, Wales, UK). Do the efficiency calculations include the losses associated with maintaining the end load? Reply by the Author This paper has been concerned with variator efficiency and does not look at other aspects of the transmission. Other aspects of the transmission, such as hydraulic losses are discussed elsewhere

As with any automatic transmission, there is a hydraulic system that requires energy to drive it. However, it can be said that the endload system is a very low leakage system, which consumes negligible hydraulic energy itself. This is because although there can be high pressure in the endload

Paper VIII (i) "Triboehemical Approach toward Mechanism for Synergism of Lubricant Additives on Antiwear and Friction Reducing Properties" by I Minami', T Ichihashi , T Kubo1, H Nanao', S Mori1 ('iwate University, Japan, 2 Idemitsu Kosan Corporation Ichihara, Japan). Dr R I Taylor (Shell Global Solutions, Chester, UK). Is there a particular reason why primary ZDTP was chosen for this study (an alternative could have been secondary ZDTP)? Reply by the Author In our preliminary work, both primary and secondary ZnDTP were examined. However, we found little differences in anti-wear properties between the additives. Therefore, we selected primary ZnDTP for the present work in order to have better anti-oxidation properties. Professor R Coy (The University of Leeds, Leeds, UK). Overbased Ca sulphonates are usually considered for their detergency not their antiwear and friction reducing properties. In many formulated engine oils the surface films on cams, followers, piston rings and liners are dominated by ZDDP (Zn, P, S) with some Ca or Mg. Can you explain why in your experiments Ca sulphonates dominate film formation? Reply by the Author We found certain over-based calcium sulfonates exhibit anti-wear and anti-seizure properties under boundary conditions. Therefore we paid attention to the mechanism and applicability of the additives as anti-wear agents. As a related research, wear reduction by calcium sulfonates has been reported by references 1 and 2.

868 Professor H Spikes (Imperial College, London, UK). Did you measure the surface roughness of the track after rubbing in Ca sulphonate solution, and did it smooth the surface? If so, do you think that the smoothing resulted in the observed friction reduction? Reply by the Author We measured surface roughness of the worn surface. However we could not observed any clear relationship between the roughness and tribological properties under these conditions. The authors speculate that the effect of surface morphology might be observed during pro-wear process, which will be investigated in the future.

Paper VIII (ii) "Tribochemistry of MonoMolecular Additive Films on Metal surfaces, investigated by XPS and HFRR" by R Kolm1,1 C Gebeshuber1'2, E Kenesey1, A Ecker1, A Pauschitz', W S M Werner2, H Stori2 ('Austrian Center of Competence for Tribology, Vienna, Austria, 2 Technische Universitat Wien, Vienna, Austria).

understandable. Are there any effects of (OH) position on electron density of the N atom? Reply by the Authors We did not investigate the adsorption of the various hydroxyquinoline isomers at different PH values. In any case, the hydroxyquinolines function as oxygen acids, nitrogen bases and are involved in keto-enol tautomeric equilibria. Therefore, the electron density at the N atom should be influenced by the PH value. The equilibrium constants of the various isomers cover a broad range, thus the PH may affect the different tribological behaviours. Furthermore, the tribological properties may depend on the position of the hydroxyl group with respect to mesomeric and inductive effects influencing the electron density at the heteroatoms.

Paper VIII (iii) "Water-Accelerated Chemical Reaction for Producing Gear Teeth with High Form Accuracy and Fine Surface Finish" by K Oobayashi1, K Irie', F Honda2 ('Aisinaw Co. Ltd, Aichi, Japan, 2Toyota Technological Institute, Nagoya, Japan).

Professor H Spikes (Imperial College, London, UK). Could you please explain why - in molecular terms - you think that the 2- and 8hydroxyquinolines are so much better at reducing wear than 5-hydroxyquinolines?

Professor R Coy (The University of Leeds, Leeds, UK). Have you observed any reduction of fatigue life of the gears after application of water surface treatments?

Reply by the Author

Reply by the Author

2- and 8-hydroxyquinoline have the hydroxyl group in cc-position to the nitrogen atom. Regarding the HFRR results there is a synergism between the hydroxyl group and the nitrogen atom. These molecules are able to build a bonding to the metal surface and consequently generate a stable wear protecting film. 5-Hydroxyquinoline has no synergism due to the molecular structure (the hydroxyl group and the nitrogen atom are located at opposite sides of the ring system). Therefore the 5hydroxyquinoline can not build a stable bonding to the metal surface and consequently there exists no efficient wear protecting film.

No, we have not determined the fatigue life because obviously too long a time is needed to obtain the results. We observed the SEM image of the gear surface after 24 hours of TOFF run which is much longer than the usual TOFF treating time: 20 minutes. The surfaces were over eroded in shape but the surface was exactly smooth without any etch pits or fatigued area. The TOFF treating accelerates very mild wearing using no blade that we believe micro-crack density on the TOFF surfaces is much lower than the shaved one.

Dr I Minami (Iwate University, Japan). The chelate effects of (OH) position on adsorptivity is

869

SESSION IX CONDITION MONITORING Paper IX (i) Invited Paper "Condition Monitoring as a Tool to Aid Compliance with ISO 14000" by I Sherrington, T Sperring, M Williamson (The University of Central Lancashire, Preston, UK). Professor M Fox (DeMontfort University, Leicester, UK). On the recycling of waste oil, the $40/ton rebate for burning waste oil ceased in December 2003 and this disposal route must finish by the end of 2005. This will concentrate minds onto recycling. Of the three stages, or levels, of recycling oils, my experience of the vacuum distillation/hydrogenation final product is of close to group II quality. There is a shortage of group oils at present (and at a high unit price). There is no objection to their inclusion as base oils in formulated lubricants by two major German OEM's, their emphasis is upon the overall performance of the formulation. There is a good argument that good quality recycled lubricant base oil is more robust against degradation than a virgin base oil. Replies by the Authors Professor Ian Sherrington Thank you for your comment. 1 was not aware that the rebate for burning oil would end in 2005.1 think that this step just adds more weight to the case that we have presented in the paper. That is, it is more appropriate to take measures to extend the useful life of lubricants in service than it is to take other forms of maintenance action. This is borne out of both the economics of the situation and the case for reducing environmental damage. Dr M Williamson I would agree with Prof Fox that a properly recycled base oil should be as good as a virgin base stock in principle. However, I have some concerns about the source of the recycled oil related to whether the user has maintained the levels of cleanliness necessary to avoid base stock damage. Therefore, I would question the use of recycled oils coming from a wet environment, or

from a chemical plant environment where caustic acids etc may have leaked into the product. We are always keen to suggest that users should change their oil before the depletion of the additive package occurs. This way, the base stock remains in good health and can be re-cycled. However, if the additive package is allowed to deplete, then the onset of severe oxidation will affect the quality of the base oil. Experience has shown that partial 'bleed-and-feed' of larger volume systems can extend the life of the lubricant significantly. This scenario works well in allowing the withdrawn oil to be recycled before it is harmed by extended service and also allows the bulk of the oil to be kept in service for up to twice the normal service interval through the constant refreshing of the additive package. Finally, it is my experience that the Group III oils are superior to Group II, both of which are refined by hydrogenation and hydro-cracking. In the US, Group III is sometimes referred to as a synthetic (legally, I might add). But, Group II and III oils do have lower levels of impurities of sulphurs, polar impurities and nitrogens so are generally a lighter colour than Group I. I am not sure, though, why a reclaimed oil would be better in this regard, perhaps because a re-refining will again remove any further impurities that may have remained from the first refining of the virgin base stock and any shear stressing of the oil will have stabilised its condition

"Evaluation of an Ultrasonic Method for Measurement of Oil Film Thickness in a Hydraulic Motor Piston Ring" by P Harper, R S Dwyer-Joyce, U Sjodin, U Olofsson (The University of Sheffield, Sheffield, UK). Dr H Xu (Glacier Vandervell Bearings, Rugby, UK). The transducer has certain dimensions. I presume that the measured film thickness represents an average value of the oil film thickness. However, in applications, the minimum oil film thickness is quite often one of the most important parameters. Have the authors tried to find some way to work out the minimum oil film thickness?

870 870 Reply by the Authors

Reply by the Authors

The focused transducer has a spot size of around 0.5 mm; and the film thickness returned will indeed be an average over that area. This is a disadvantage of the technique. Unfortunately, the only way to find the location of a minimum in a spatially varying film, is to use more measurement points.

Well, this is one of the conclusions of the paper. The stiffness is made of up two parts, the liquid stiffness and the solid stiffness. Both of which depend on the approach of the roughness mean lines. The idea is that we can measure the stiffness, deduce the approach, and hence deduce the liquid and solid parts separately. We have demonstrated this works, by actually measuring both with and without liquid. But we have yet to try it when we cannot do this.

Professor M Fox (DeMontfort University, Leicester, UK). Was there any evidence for ring rotation during your measurements? Reply by the Authors We did not observe any ring rotation; but even if it had occurred we were unlikely to have spotted it. The measurement frequency is low and we would not have captured this kind of transient event.

Professor R J K Wood (University of Southampton, Southampton, UK). Due to the sensitivity of measurements of plasticity - will this limit the applicability of the technology to sliding and loaded lubricated contacts? Reply by the Authors

Professor H P Evans (Cardiff University, Wales, UK). Does the method having potential to be used with moving components: Reply by the Authors Yes, it is possible. Of course we would have trailing wires or would need to take the signal out through slip rings. As it is a very high frequency signal we have the potential for introducing noise. Very high inertial loads could also be an issue for some transducers.

Paper IX (iii) "Ultrasonic Reflection from Mixed Liquid - Solid Contacts and the Determination of Interface Stiffness" by M Gonzalez-Valadez, R S Dwyer-Joyce, R Lewis (The University of Sheffield, Sheffield, UK). Dr H Xu (Glacier Vandervell Bearings, Rugby, UK). It is a very interesting piece of work. I would like to congratulate the authors. In dynamic applications, complete dry contact will take a long time to occur. In the measurement device, how can the situation of dry contact be distinguished from the case of two asperities separated by a very thin oil film?

The passage of an ultrasonic wave is elastic, even if the overall contact has taken place plastically. So it should not make a difference to the measured contact stiffness. Dr F Bucher (Goodyear, SA, Luxembourg). You measure the stiffness of a contact with and then without lubrication - to enable one to be substracted from the other to obtain the liquidstiffness. Aren't you taking some systematical error into account, since the contact geometry of the loaded rough unlubricated interface wll be different from the lubricanted one and this will influence the ultrasound reflections? Reply by the Authors This is possible; we hope that since these are purely normal contacts the presence of a lubricant will not change the rough contact significantly. This may not be the case for sliding surfaces. Professor J B Medley (The University of Waterloo, Ontario, Canada) and Professor Z-M Jin, The University of Leeds, Leeds, UK). If the lubricant is pressurised in a mixed film contact, the stiffness of the oil film and asperities may be very similar. Can your technique still handle this situation?

871 Reply by the Authors Yes, this will be the case. The speed of sound of the pressurised oil in an ehl contact is likely to be around 3 times that in the bulk. The stiffness will then go up by a factor of ~9. If we know this speed of sound then it should be possible to use the approach outlined here. Measurement of the speed of sound of pressurised oil accurately will be difficult.

Nevertheless two PhD's are currently running, done by T Elguedj and R Ribeaucourt, who are going to compare the results of the method with experimental results. MC Baietto who is leading the PhD of R Ribeaucourt is really well aware of complex rolling contact crack propagation situations. Professor I I Kudish (Kettering University, MI, USA). Is it a fair statement that cracks propagate in the direction perpendicular to the maximum tensile principal stress, whatever be the initial orientation of the crack?

SESSION X ROLLING CONTACT FATIGUE

Reply by the Authors

X (i) "Extended Finite Element Method for Numerical Simulation of 3-D Fatigue Crack Growth" by A Combescure, A Gravouil, M-C Baietto-Dubourg, E Elgudj, R Ribeaucourt, E Ferrir (INSA-LAMCOS, Lyon, France).

The statement done here is rather common for the material whose fracture is driven by the maximum principal tensile stress. This is not the case for all materials: some of them are driven by a combination of the previous stress with shear. In the latter case we only have to change in the simulation the equation which drives the direction of crack propagation.

Dr C J Hooke (University of Birmingham, Birmingham, UK). In rolling contacts there is a tendency for residual stresses to build up during running. Can the X-FEM method be extended to accommodate this type of behaviour? Reply by the Authors Yes there is no basic problem. The initial residual stress field has not been introduced in the results presented here but, provided one can compute them or have an analytical solution and that one develops interaction integrals which take into account residual stresses the method could be applied. This has never been done yet to my knowledge. Professor S Bogdanski (University of Technology, Warsaw, Poland). For experimental verification of the numerical results you used the simple specimen with a crack propagating in the perpendicular direction to the surface. Could you say something about verification for more complicated cases (for instance the RCF cracks) if you have done any? Reply by the Authors At present the method is rather new and has only been compared with "academic" experiments.

Dr M Talks (QinetiQ Ltd, Farnborough, UK). Could this modelling be used to predict the microcrack propagation during micro-pitting of gear steels, where the micro-cracks are affected by residual stresses in the case-hardened layer. Usually, these cracks initiate at the surface, propagate into the surface and then turn back towards the surface, creating micro-pits. Occasionally, however, they propagate into the bulk steel and can cause fracture of a gear tooth. Reply by the Authors There is no basic limitation in the method; nevertheless the application case you are suggesting is a complex one. The presence of initial residual stresses induces two difficulties as answered to Dr C J Hooke. The fact that the crack initiates near the surface of the gear tooth means that one starts with a very short crack loaded by a very rapidly varying stress field. Third your question implies that the propagation is rather versatile and should then be difficult to predict. This could be the field of an interesting PhD work, if there is enough funding!

872 872 Paper X (iii) "Study on Rolling Contact Fatigue in Hydrogen Atmosphere-Improvement of Rolling Contact Fatigue Life by Formation of Surface Film" by T Endo, D Dong, Y Imai, Y Yamamoto (Kyodoyushi Co. Ltd, Kanagawa, Japan).

Professor J B Medley (University of Waterloo, Ontario, Canada). What steps are taken to keep the "diatoms" happy when looking at them in the AFM? Is it possible that some images are of diatoms under abnormal conditions? Reply by the Authors

Professor G Poll (University of Hannover, Germany). Why was 440 stainless steel used as a reference material. It is known that HNS (high nitrogen content) bearing steels do much better with hydrogen! Reply by the Authors Stainless has long been known to provide longer rolling contact fatigue life and excellent resistance against hydrogen, as suggested by J. A. Ciruna et al.1 A recent study by Tamada et al.2 indicates an absence of abnormal subsurface-initiated flaking with use of a stainless bearing, and explains it as work of the passive film of stainless, which protects degradation of lubricant by newly formed surface. As just described, the excellent resistance against hydrogen of stainless has been shown. We studied stainless in order to find out if its passive film actually keeps out hydrogen. We are not familiar with an HNS bearing. 1. J.A.Ciruna & H.J.Szieleit : The Effect of Hydrogen on The Rolling Contact Fatigue Life of AISI 52100 and 440C Steel Balls, Wear, 24 (1973) 107. 2. K.Tamada & H.Tanaka : Occurrence of Brittle Flaking on Bearings Used for Automotive Electrical Instruments and Auxiliary Devices, Wear, 199(1996)245.

SESSION XI BIO-TRIBOLQGY 2 Paper XI (i) "Diatom Biotribology" by I C Gebeshuber1, H Stachelberger2, M Drack2 ('Austrian Center of Competence for Tribology, Vienna, Austria, 2Technische Universitat Wein, Vienna, Austria.

Living diatoms can be investigated in ambient conditions with the AFM (see Gebeshuber I.C., Kindt J.H., Thompson J.B., Del Amo Y., Stachelberger H., Brzezinski M., Stucky, G.D. Morse D.E. and Hansma P.K. (2003) "Atomic force microscopy study of living diatoms in ambient conditions", J. Microsc. 212(3), 292-299). In this case, the cells are surrounded by water or culture medium, and the imaging and acquisition of nanoand micromechanical parameters takes place while the cells continue to grow. The cells have to be firmly attached to the substrate (e.g. glass slide) to assure stable AFM imaging conditions. Furthermore, the force has to be kept small, to minimize mechanical stress of the cells. Imaging with too high forces destroys the organic layer around the glass case. This fact can be used to determine the thickness of the organic layer. Diatoms prepared for the acquisition of SEM images (as the diatom images in this paper) are dead and in many cases surface treated (metal sputtered, treated with bleach for the removal of organic parts, etc.).

Paper XI (ii) "Friction of Contact Lenses: Silicone Hydrogel versus Conventional Hydrogel" by V Ngai, J B Medley, L Jones, J Forrest, J Teichroeb (The University of Waterloo, Ontario, Canada). Dr S Franklin (Philips Centre for Industrial Technology, The Netherlands). Assuming hydrodynamic lubrication occurs in the contact lens-eyelid contact (with therefore a fluid film separating the surfaces), can you expect to be able to measure differences between different lens materials, bearing in mind that in reality both the lens and the eyelid are compliant surfaces?

873 873 Reply by the Authors

Reply by the Authors

This is a good question. In our apparatus, it has to be elastohydrodynamic lubrication (EHL) rather than hydrodynamic lubrication. The presence of EHL could be masking any material effect, which was consistent with our finding that there was no statistically significant difference between friction forces for the two lens types. However, a trend towards higher friction forces for the SH lenses was observed. The calculations presented in our paper gave predicted friction forces based on EHL theory that were substantially lower than the measured values, possibly because of fluid drag at the periphery of the contact zone or direct asperity micro-contacts.

The silicone hydrogel (SH) lenses, used in the present study, were specified by the manufacturer to have a water content of 24% by weight while the conventional hydrogel (CH) lenses had 38% by weight. The presence of a silicon-based polymer in the SH lenses caused a decrease in water content but an increase in oxygen permeability.

An attempt is being made to measure the fluid drag by running experiments at virtually zero load. However, we expect that small differences may still occur and, if so, some degree of asperity contact becomes a possibility. Friction forces arising from direct asperity interaction may show a material effect and thus small differences in the total measured friction forces are considered possible. Dr B Bou-Said (INSA-LAMCOS, Lyon, France). In the apparatus does the 'rigid', glass moving surface represent the eye-lid? Reply by the Authors Yes, the oscillating glass surface represents the eyelid. Eventually, this glass surface may be replaced by both a surface and a geometry that are closer to the inner surface of the upper human eyelid. However, glass has some history as a counterface surface in friction studies of articular cartilage. It has been suggested that the glass surfaces allowed realistic protein interactions in the contact. Thus, as a starting material, glass may be a good choice. Professor Z-M Jin (The University of Leeds, Leeds, UK). What are the differences between the two materials in terms of water content? Why did this difference, if any, not cause differences in friction?

The lack of statistically significant differences in friction forces for the two lens materials could be attributed to the following: 1. In their manufacture, SH lenses were given a gas plasma treatment of the surfaces to render the surfaces hydrophilic. Therefore, it was considered possible that the SH and CH lenses had very similar surface interactions in the contact and thus similar friction forces. 2. In our experiments, both lenses were tested in the presence of saline solution. In our paper, calculations were presented that suggested some EHL influence had occurred. This influence would tend to reduce direct surface contact and thus mute any material effect. 3. The differences in the friction forces obtained for our SH and CH lenses were not statistically significant. However, it is important to note that a clear trend was seen whereby SH lenses always had higher friction forces than CH lenses. Conducting more tests with more samples could clarify this situation. Dr N Emami (Lulea University of Technology, Sweden). What were the chemical compositions of the tested lenses? I think that in future, if you continue these tests, it would be beneficial to know the material composition. The scales on the SEM images were not easy to read. Reply by the Authors We agree that it would be beneficial to know the entire chemical composition of the lens materials (and the stoichiometry of their formation). This knowledge might give some insight into the tribology of the contact. As might be anticipated, the manufacturers were not willing to provide this

874 874 information in any detail. Conducting our own investigation into the chemistry of these commercial products was well beyond the scope of the present study. Some additional information can be provided. The CH lens used in our testing (B&L's Optima FW) is based on HEMA (2-hydroxethyl methacrylate). The SH lens used in our testing (CIBA Vision's Focus Night and Day) is based on TRIS (methacryloxypropyl (trimethylsiloxy) silane), DMA (N, N-dimethylacrylamide) and a siloxane macromer. A larger scale has been added to some of the SEM images in our paper.

Paper XI (iii) "Modelling the Blood Flow in an Aorta; The MPTT and Modified MPTT Models" by A Amblard, B Bou-Said (INSALAMCOS, Lyon, France). Professor J B Medley (University of Waterloo, Ontario, Canada). Some blood flow models include axial streaming of the red blood cells (driven by forces associated with red blood cell rotation). This gives a radial variation in red blood cell concentration and changes the velocity profile to a central "plug flow" with higher shear rates near the walls. Is such modelling realistic and is it relevant to your modelling? Reply by the Authors The axial streaming of the red blood cells is a consequence of purely fluid mechanics modelling. This is a global characterization of the red blood cells behaviour. In our model we are focused on microstructural observations. This naturally includes the stretching effect which is, in our sense, a more realistic modelling than those proposed from the classical fluid mechanics approaches. Dr N Doerr (AC2T Research GMBH, Austria). Does blood exert thixotropic properties? If yes, to what extent does thixotropy occur?

Reply by the Authors Thixotropy property is a continuum mechanic aspect. Our approach considers microstructural modelling which, of course, takes into account the evolution of the different fluid properties in any spatial direction. Dr F H Bucher (Goodyear Technical Centre, Luxembourg). What is the motivation to look at the true stress-distribution in an aorta plus prosthesis with respect to the mentioned leakages after operation? Reply by the Authors Some complications that might occur after an endovascular treatment are endoleaks or endoprosthesis movement. The majority of endoleaks occur upstream of the aneurysm. The stresses generated by the blood flow are believed to be the major cause of endoleaks. These stresses can lead to a separation between the prosthesis and the aorta. Then, to improve the behaviour of endoprosthesis, it is necessary to estimate the stress distribution generated by the blood flow. Professor K Ikeuehi (Institute for Frontier Medical Sciences, Japan). In what cases is your model effective? 1 2 3 4

Normal arteries Narrowed or closed arteries Aneurysms Artery-stent graft systems

Reply by the Authors Our model is effective in these 4 cases.

"A Novel Technique for Paper XI (iv) Evaluation of Articular Cartilage Lubrication Based on the Surface Plasmon Resonance" by M H Naka1, Y Arima', H Iwata1, M Hasuo1, Y Fuwa2, Y Morita, K Ikeuehi1. ('Kyoto University, Japan, 2 Toyota Motor Corporation, Japan). Dr Y Sawae (Kyushu University, Fukuoka, Japan). Why did you use collagenase? Articular cartilage surface is covered with a proteoglycan layer. Have you ever used other enzymes?

875 Reply by the Authors We use collagenase because the sensibility for recognition of substances in our surface plasmon resonance apparatus depends basically on the refractive index. Measurements of the refractive index of collagen have been reported and its value varies from 1.36 to 1.53. In the case of proteoglycan or glycosaminoglycan, it is very difficult to measure the value of refractive index. Chondroitin sulphate dissolved in PBS (phosphate buffered saline) was used in our work in order to estimate the refractive index of the proteoglycan. However, the measurement of refractive index presented negligible difference with the refractive index of PBS, which makes difficult the analysis of the presence of proteoglycans at the articular surface. On the other hand, results obtained for collagen solution present values resembling to that obtained by other researchers. At the present time, we are analysing a method for measurement of refractive index based on the volume fraction of substances present in solutions. Whether this method presents satisfactory results, the use of enzymes like hyaluronidase or chondroitinase can be applied in order to verify the presence and absence of proteoglycans at articular surface during lubrication.

substrate bias was fixed and we speculate that there was not so much change in residual stress. Professor T H C Childs (The University of Leeds, Leeds, UK). I wonder if you have information on the maximum load and speed at which the low friction and wear effects occur? Reply by the Authors The estimated contact pressure during the test was calculated from the worn area of the ball and it was about 60 MPa. As to the maximum speed, so far we only tested up to 1 m/s.

Paper XII (iii) "Tribological Characteristics of WCi.,, W,C and WC Tungsten Carbide Films" by M Gubisch, Y Liu, L Spiess, J A Schaefer, C Knedlik, (Technical University Ilmenau, Germany). Dr K Palm (The University of Leeds, UK). Carbonaceous materials are often thought to attain their lubricity due to adsorbed species and are therefore unsuitable for vacuum application. Could the authors comment on their selection of W-C materials for vacuum tribology and whether any tribological tests will be or have been carried out in vacuum? Reply by the Authors

SESSION XIICOATINGS 2 Paper XII (i) "The Effect of Si Content and Substrate Bias on Structural and Tribological Properties of (Cr, SON Coatings under Water Lubrication" by K Yamamoto, S Oomoto (Kobe Steel Ltd, Japan). Dr K Dahm (The University of Leeds, Leeds, UK). Did you assess the effects of increasing Si content on the intrinsic stress with the coatings? Reply by the Authors We did not measure the change in stress as the Si content was changed. However, the most influencing factor on the residual stress for PVD coating is substrate bias. In our experiments,

It is known that graphite ceases to lubricate in vacuo, suggesting that chemical environment and adsorbants may also play roles in lubricity. However, the recently developed amorphous diamond like carbon (DLC) films can provide extremely low friction and wear coefficients of 0.001 to 0.005 and 10"" to 10"'° mm3/Nm, respectively, when tested in inert-gas or highvacuum environments [Erdemir A., MRS Fall Meeting, Boston, 2001]. These results were obtained from DLC films grown in plasmas with higher hydrogen-to-carbon ratio, indicating the importance of surface termination and intercalation in determining the lubricity. Our intention is to develop and apply the WC/DLC multilayers on the surface of the bearings used in nano-positioning. This contribution is to compare the tribological performance of various phases in W-C system. From our results, we demonstrate that the WC

876 phase with simple hexagonal structure has the best performance in terms of both friction and wear. The tests in vacuum are in progress. Dr S E Franklin (Philips Centre for Industrial Technology, The Netherlands). How do you expect the friction and wear behaviour and the ranking between the different tungsten carbide phases to change when tribological testing in vacuum as opposed to air?

The authors are grateful to Dr. Taylor for his comments. In fact, the biodegradable ester fluid has a smaller pressure-viscosity coefficient, when compared to the mineral oil used, generating a slightly smaller film thickness. However, for the operating parameters considered the FZG gear was submitted to mixed/limit lubrication conditions and the difference in film thickness is not sufficient to explain the difference in iron content.

Reply by the Authors Normally, it is difficult to extrapolate the results tested in ambient condition to that in vacuum. However, we strongly believe that the rank of the tribological performance of the tungsten carbide films with various phases in the present study should be similar in both cases. As we have shown, the wear resistance of the W2C film was the lowest because of its highest surface roughness. When the surface roughness was at the same level, the harder one had the higher wear resistance, as compared WC to WCj.p films. Since the surface roughness and hardness of the films are not sensitive to the change of the pressure (vacuo), we expect similar tribological behaviour in vacuum. This assumption needs to be further verified by tests in vacuum.

SESSION XIIILUBRICANTS 2 Paper XIII (i) "Power Loss in FZG Gears Lubricated with Industrial Gear Oils : Biodegradable Ester vs. Mineral Oil" by R Martins', J H O Seabra2, Ch Seyfert3, R Luther3, A Igartua4, A Brito5 ('iNEGI, Portugal, 2DEMEGI, Portugal, 3FUCHS, Germany, 4Industria Portuguesa de Engrenagens, Portugal). Dr R I Taylor (Shell Global Solutions, Chester, UK). The ester probably gives lower friction in the gears due to its lower viscosity-pressure coefficient. However this probably will also lead to a lower oil film thickness between the gear teeth. Why then is the iron content of the used oil lower than that for the mineral oil? Reply by the Author

Figure 1 presents the concentration of wear particles (CPUC) vs. the maximum oil operating temperature in the power loss gear tests. At typical gear operating temperatures (90 °C - 100 °C) the CPUC values are very similar for both lubricants. At higher temperatures, around 130 °C, the mineral oil is no longer able to perform conveniently as a lubricating fluid, generating much more wear than the ester fluid. The situation becomes worse at the highest temperatures (170 °C -180°C). So the major reason for the difference in iron content is directly related to the operating temperatures used, significantly above 100cC, which the mineral oil can not stand. -Ester- CPUC

- M i n e r a l - CPUC

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n

700 600 500 400 300 200 100

70

90

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130

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170

190

Maximum operating temperature before sampling [°C]

Figure 1 - Concentration of wear particles (CPUC) vs. maximum oil operating temperature in the power loss gear tests. Professor M F Fox (DeMontfort University, Leicester, UK). Was there any evidence of increased acid number for the ester at the end of the

877 most severe tests? This may explain the low iron levels found. Reply by the Author The authors are grateful to Professor Fox for his comments. The viscosity and neutralization number measurements made at the end of the gear power loss tests, on both lubricants, did not show any evidence of significant oxidative degradation. The operating temperatures of the mineral lubricant in the most severe tests (180°C) are way beyond the recommended range for mineral oils, justifying the excessive iron content found in the mineral oil samples.

Reply by the Authors The authors are grateful to Dr. Ahrstrom for his comments. Both gears have been carefully inspected after the tests and no iron depositions have been found. They were washed using an ultrasonic bath and weighed afterwards. The mass loss results correlate very well with the ferrometric measurements and with the iron contents found in the two oils.

Paper XIII (ii) "The Effect of Dissolved Ions on Tribological Properties in Water" by A Iwabuchi, M Uchidate, H Liu, T Miura, T Schimizu (Iwate University, Japan).

Dr F Ville (INSA-LAMCOS, Lyon, France). Is the difference in density between the two oils a possible explanation of the power loss difference for the non load tests, associated with churning power loss?

Dr S Michler (Ecole Polytechnic Federate de Lausanne, Switzerland). Can you comment on the very different wear patterns observed by SEM depending on the anion added to the solution?

Reply by the Author

Do you think that anion adsorption on passivation can affect the mechanical behaviour of the metal?

The authors are grateful to Dr. Ville for his comments.

Reply by the Authors

Dr. Ville's comment is correct, but it is also necessary to consider the influence of viscosity. For an operating temperature around 80°C, the churning losses obtained with both lubricants are very similar. Below that temperature the viscosity of the mineral oil is significantly higher than that of the ester fluid, generating higher churning losses, although it has a lower density. Above 80 °C the viscosities of both fluids become very similar, and consequently the higher density of the ester oil is responsible for higher churning losses than the mineral oil. Dr B-O Ahrstrom (SKF Business & Technology Park, The Netherlands). I would like to advice you to re-investigate the measured iron content in the lubricants. Unaccounted iron depositions, in the Ester case, will most probably be found on the gears - outside the contact.

The authors think that the effect of anion on wear is the change in the composition of the boundary film formed due to the tribocorrosion. One factor is the load carrying capacity of the film. The film in NO3" solution has lower load carrying capacity and the wear moves from mild wear to severe wear at 30 N. Other anions like SO42" and Cl" added generate rather great load carrying capacity, and keep mild wear at 30 N. However, the analysis of EPMA could not show the definite composition. The latter question is substantial. The authors think that the difference in load carrying capacity, as noted above, results from the change in mechanical property of the boundary lubrication film, in which anion adsorbs and/or reacts with passivation film. However, the authors could not confirm the evidence yet.

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SESSION XIV BIO-DEGRADABILITYSUSTAINABILITY Paper XIV (i) Invited Paper "Sustainable Development Thinking in Tribology Design" by M Hadfield and C Ciantar (Bournemouth University, Poole, UK). Professor J Sugimura (Kyushu University, Japan). You insist on the importance of tribology in sustainable development, which I agree with. But we always feel that tribology comes last, i.e. the necessity for tribological improvements comes after a framework for new systems is established. It would be nice if new frameworks were made on the basis of tribological technologies - for example, the choice of refrigerant from the lubrication aspect:at an earlier stage of new system development. Do you think it is possible? And what do you think we should do to make it possible? Reply by the Author This is a reasonable point and highlights the importance of engineers to consider stakeholders at the beginning of the research or development process. In my opinion this type of design philosophy is realised by sustainable development thinking by the refrigerant manufacturers i.e. a whole life cycle approach, consideration of stakeholders and evaluation of product risks and benefits in terms of the environment, society and economics. Professor I Sherrington (Jost Institute for Tribology, University of Central Lancashire, Preston, UK). There are a number of examples of customers taking advantage of improvements in products which negates any sustainability advantage which is offered by the improvement. For example, improvements in car engine design offer longer life and more power, which should allow engines to be reduced in size and used for longer times. However, drivers tend to use these improvements to drive faster and travel longer distances at speed as a matter of routine. How do you think we can persuade the users of products to recognise improvements in products and use those advantages to contribute to sustainability rather than to use them for other purposes

Reply by the Author The social aspects of sustainability are the most complex and least understood and therefore no clear answer exists. This is a good question as it highlights the importance that tribologists consider the product use phase from a social viewpoint. As a result of increased awareness and legislation these dynamic social priorities and attitudes are changing. As tribologists we have an important role to play in quantifying sustainable development benefits.

Paper XIV (ii) "Sliding Experiments of Steels in Gaseous Hydrogen" by J Sugim, B Ono , M Hashimoto H Tanaka Y Yamamoto ('Kyushu University, Fukuoka, Japan, 22Sasebo National College of Technology, Nagasaki, Japan). C

Dr S K Sinha (National University of Singapore, Singapore). What will be the effect of hydrogen embrittlement on wear when (H2) is used for lubrication? Reply by the Authors There would be a certain effect of hydrogen embrittlement on wear of some steels. But I doubt that the effect is significant under the present sliding time and contact conditions, though we have not yet checked any crystallographic change in the test specimens.

SESSION XV MACHINE SYSTEMS 1 Paper XV (i) Invited Paper "Modern Tribology in Life Cycle Assessment" by K Kato, K Ito (Tohoku University, Japan). Dr R I Taylor (Shell Global Solutions, Chester, UK). Professor Kato has shown that for most machines, the highest costs are in use. It is not always the case that manufacturers have much incentive to reduce usage costs, and I wonder if Professor Kato could comment on how effective it would be if manufacturers were forced to have more incentives/penalties depending on the machine usage costs.

879 879 Reply by the Authors Thank you for the question about one of the most important aspects in considering the effective way of obtaining the goal of Life Cycle Assessment through the part of Triblogy. The questioning point is very suggestive and stimulative for this author to think about the advanced stage of the subject proposed in this paper. If the machine usage costs is mainly caused by the energy cost for running and is related to CO2 emission, the regulation of the total amount of CO2 emission in the manufacturing process would give strong incentives to manufacturers to reduce the costs. When such a regulation rule would be introduced, the technical contribution of Tribology would be 13% in total.

Paper XV (ii) "An Experimental Study on the Friction Behaviour of Aircraft Hydraulic Actuator Elastomeric Reciprocating Seals" by A S Rana, R S Sayles (Imperial College, Tribology Section, London, UK). Mr H J van Leeuwen (Eindhoven University of Technology, The Netherlands). Friction is studied under conditions of mixed and/or boundary lubrication. How can one be sure that these circumstances prevail in the contact between the seal and the glass shaft? Reply by the Authors The friction coefficients that have been indicatively calculated fall between 0.1 and 0.4. These relatively high values suggest that boundary/mixed conditions prevail and that a full hydrodynamic film is not set up. From experiments at Smiths Aerospace at pressures of up to 5000psi, it appears that higher pressures reduce the friction co-efficient to 0.01 typically. Under such high-pressure conditions a hydrodynamic film is more likely to exist. It is hoped that improving the optical set-up of the rig can eventually give a better insight into the lubrication conditions between the seal and glass shaft.

Paper XV (iii) "Experimental Study on Gas Transportation in Radial Shaft Seals" by S Nakaoka1, Y Sato', S Gondo2, J Sugimura2, Y Yamamoto2 (' Nok Corporation, Ibaraki, Japan, 2 Kyushu University, Fukuoka, Japan). Professor A A Lubrecht (INSA-LAMCOS, Lyon, France). Is it possible to introduce oil flow under the seal as an additional gas transport mechanism to the diffusion model. Reply by the Authors The estimated volumes of gases conveyed in pumped oil in dissolved form are 1.3 to 7 times smaller than the measured volumes. We concluded that a mechanism other than dissolution and release of gas in liquid is responsible for the behaviours of gases moving across the seal. We now think that the most possible gas transport mechanism is that gas is conveyed in the form of bubbles trapped in oil flow under the seal. Professor G Poll (University of Hannover, Germany). Did you also investigate if there is a gas migration in the opposite direction, from the "oil" side to the "air" side of the seal? This should happen in case the mechanism has to strive to balance the gas concentrations on both sides! Reply by the Authors We have only investigated the gas transportation to move from the air side to the oil side of the seal. We are interested also in the gas transportation in the opposite direction and will measure it in the future with the similar method presented in the paper.

Paper XV (iv) "Friction Coefficient between Gear Teeth in Mixed Film Lubrication" by J Castro1, A Campos, A Sottomoayor, J Seabra2 ('DEM, Portugal, 2DEMEGI, Portugal). Dr F Ville (INSA-LAMCOS, Lyon, France). Can you give more details concerning the surface roughness and surface finishing in order to explain why the friction coefficients are so low?

880 Reply by the Authors The surface finishing of type C gears is of the type MAAG (0°), without flank modification. The gear quality is DIN 4 and the surface roughness is Ra = 0.3 ± 0.1 for pinion and wheel. The quality of gears and the mixed lubricant regime observed for the operating conditions considered are responsible for the low friction coefficients.

SESSION XVI CONTACT MECHANICS, WEAR and FAILURE Paper XVI (ii) "A Model of Seizure Based on Burwell and Strang's Concept of Wear Mode Transition" by T Nakahara (Tokyo Institute of Technology, Tokyo, Japan). Dr J A Williams (Cambridge University, Cambridge, UK). Most dry or marginally lubricated tribological contacts are loaded many times so that the eventual, perhaps steady-state, contact conditions measured by the surface topography and state of surface stress - especially residual stresses - can be very different from those on the first application of the load. This is part of the process of running-in and can be modelled through the application of the ideas of shakedown mechanics. I would be interested to know how your model compares with those of Kapoor et al (Wear 200, (1996) 38-44) in the mild regime and Torrance (Wear 200 (1996), 45-54) in more severe conditions. Reply by the Authors The presented model for seizure process does not take the running-in process into consideration but shows only the estimation model of the plastic deformation mode in the apparent contact area for a given hardness at a moment. The two papers, which you have introduced, indicate the way to estimate the variation of hardness near the surface in the running-in process. If the presented model employs the way to estimate the variation of hardness as shown in the papers, this model will be modified

better as it can be applied to such dynamic process as the running-in. Professor H P Evans (Cardiff University, Wales, UK). Has the procedure described in the paper been applied to specific examples yet in order to test the model? Reply by the Authors No, it has not been applied quantitatively, yet. A qualitative application, however, has been made to a plane journal bearing composed mainly of PEEK and a little PTFE for a large turbine. The composite material has much smaller plastic deformation than white metal (Babbit metal) in scratch tests, and this property prevents the "expansion of plastic region at an asperity contact" in the figure 11. In addition, the strength of the material, i.e. hardness, is higher than that of the white metal under high temperature, and the coefficient of friction is a little lower than that of the white metal. From these properties, the procedure can indicate qualitatively that the material has a better anti-seizure property than the white metal.

Paper XVI (iii) "Theoretical Analysis for Wear Rate in Mixed Lubrication" by B Paffoni, R Robbe-Valloire, R Progri, R Gras (ISMEP, SaintOuen, France). Dr Y Liu (Technische Universitat Llmenau, Germany). I might have missed something in your talk, but you made a distinction between solid contact wear and lubricated contact wear and neglected the latter in your modelling. What will be the difference between your modelling results of lubricated wear and that of dry running wear? Reply by the Authors One may draw a distinction between the two following types of wear : solid contact (lubricated) wear and dry running wear. Dry running wear occurs without any kind of lubricant environment and thus seems unable to describe lubricated wear even in the boundary lubrication regime.

881 881 Solid contact wear, in the above text refers to the specific wear obtained with a lubricated environment but only in situations unable to provide thick film lubrication. This wear exactly corresponds to the wear measured in the boundary lubrication regime and is generally different from that obtained in dry friction. The main difference comes from adsorbed thin lubricant films. Consequently, solid contact (lubricated) wear values include the physico-chemical effects of the lubricant on both materials used in the contact. Changing the nature of the lubricant causes a change in the value of the solid contact (lubricated) wear. This wear level is used as input data in our mixed lubrication model and needs experimental measurements. We affect the obtained value for the part of asperities working with a thin lubricant film and we assume that the complementary part of asperities (those working with thick lubricant film) do not wear. Global wear of the mixed lubricated contact results from the combination of these two wear components.

SESSION XVII AUTOMOTIVE - 1 Paper XVII (iii) "A Model for Dieael Fuel Additive Lubricity" by M F Fox (De Montfort University, Leicester, UK). Dr M Talks (QinetiQ, Hants, UK). Are you aware of any changes in the effectiveness of lubricity additives that occur during prolonged periods of storage? Have you carried out any testing to evaluate the effects of storage on fuel lubricity? Reply by the Author The question raises an interesting point of important practical application. The essence of the model is a predicted equilibrium between fuel additives dissolved/emulsified in a diesel fuel and a metal surface. From this, a formulated diesel fuel placed in contact with a 'clean metal surface' will equilibrate to coat that surface with the fuel lubricity additive and necessarily deplete that

additive in the liquid. Therefore, the concentration of fuel lubricity additive put into a clean metal storage drum will be reduced when the fuel is drained from that drum due to 'plating out' of the lubricity additive on the clean inner surface of the metal drum. Further, in the pipeline transport of hydrocarbons, various commercial products are conveyed sequentially, separated by moving physical plugs. Relatively lighter unformulated hydrocarbons dissolve organic matter adsorbed on the interior surface of the steel pipeline. When diesel fuel formulated with lubricity additives is subsequently passed through the pipeline the additives 'plate out' onto the relatively clean surface of the pipeline. It is a known phenomenon for diesel lubricity additive concentrations to be depleted in pipeline transit. Formulators therefore tend to over-treat diesel fuels so that the desired concentration is delivered at the end of pipeline or from the storage drum. Dr B-O Ahrstrom (SKF Engineering Research Centre, The Netherlands). Is there a correlation between the critical additive concentration and some QSAR relations (e.g. polarizability) or does the concentration have to be measured experimentally from additive to additive? Reply by the Author The results that I presented are for one example each from a range of molecular compounds. I have results for other molecules in each structural example. However, the critical additive concentration, which I take to be to where the bottom plateau is reached in the reverse sigmoidal curve, is an additive mass/fuel mass value. This has limited utility for the perceptive question concerning the application of QSAR techniques. My view is that the current stage of understanding can be summarised as: -

-

the mass/mass relationship should be transformed into a molecular concentration value, this transformation poses some problems for the range of molecular weights of these additives as used - these are not pharmaceutical quality products with precisely defined molecular weights. However, these additives have average

882

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molecular weights with a relatively narrow spread and an average can be taken, the polymeric additive compounds pose conceptual problems of what is their effective molecular weight if they contain several active polar groups in each chain.

The way forward, I feel, is to calculate 'additive concentration' on the basis of the molecular weight associated with each polar group, which brings the polymer and non-polymeric molecules together. This would be a measure of polar group concentration. Having achieved a common measurement of concentration, then the data would be suitable for the application of QSAR techniques, certainly using polarizability properties. We need more physical tests as well. I strongly feel that QSAR will be an important technique in fuel lubricity additive studies together with many studies of other additives adsorbed onto metal surface and the tribological modification that they cause. Professor H Spikes (Imperial College, Tribology Section, London, UK). I have severe concerns about the work presented as follows: 1. The first and most fundamental is grave concern about the assumption made that the property of lubricity is colligative. Colligativeness is based on the principle that molecules of different types in a mixture are so similar in nature (shape, polarity etc.) that they can be substituted one for the other and have the same effect. If they are not then the colligative rules break down (e.g. deviations from Raoult's Law). The structures of the additives discussed in this talk are very different in shape, size, polarity etc. and would thus not be expected to be substitutable one for another at a surface. 2. In this context, the well-known phenomenon of chain matching would seem to contradict the colligative assumption. At this conference on Wednesday a paper was presented which showed that much lower wear and friction resulted in surfactant solutions when the chain length of the base fluid and the surfactant were identical than when they were just slightly different. (This has been reported many times previously). It suggests that very small

differences in the molecular shapes between pairs of co-adsorbing molecules makes a huge difference in the tribological properties of the surface film formed - i.e. lubricity is noncolligative. 3. The talk also suggested that the molecules discussed were similar molecular types with "long hydrocarbon chains". However one of the molecules examined was vinyl acetate copolymer which is a long chain, linear polyether with short (acetyl) side groups. It is very difficult to see how such a molecule could behave colligatively with an amphipathic surfactant. 4. Finally, I am confused about the statement made that surface adsorption of different types of additive in separated clusters is driven by the need to minimise entropy. I was taught that nature strives to maximise entropy. I would be grateful if the author could justify in more detail his assumption that the adsorption and consequent lubricity of lubricant additives is colligative. I would also be interested in precisely how entropic effects are involved. Reply by the Author For (1) I do not share the questioner's definition of colligative properties. Chambers Science and Technology Dictionary' derives its definition of colligative properties from the classical physical chemistry texts as: ' - those properties of solutions which depend only on the concentration of dissolved particles, ion and molecules, and not on their nature'. Zumdahl2 puts this point even more explicitly and succinctly, defining colligative properties as: '- properties of a solution that depend only on the number of, and not the identity of, the solute particles'. These definitions exclude considerations of similarity of nature, such as polarity, etc., such that they cannot be substituted one for the other and have the same effect. Breakdown of 'colligative rules' does not apply as there are none to break in this case.

883 If a mixture of dissimilar molecules is dissolved in a fluid and those molecules are adsorbed as a common effect at a metal surface because of their polar nature, then the effect of their separate and collective actions is colligative up to the limit of complete metal surface coverage. That limit is the critical additive concentration, taken as where the bottom plateau in the reverse sigmoidal curve, is reached.

subgroup structure can only be determined by HFRR testing in the first instance. For (4) -1 am pleased to explain the important role of entropy in low energy, organisational, reactions. All first year physical chemistry courses explore the relationship between Free Energy, G, Enthalpy, H, and also Entropy, S, and Temperature, T, through the Second Law of Thermodynamics, in its simplest form as:

It is agreed as self-evident that the structures of the additives discussed are very different in shape, size, polarity etc. But this is not meaningful under conditions of incomplete coverage of the metal surface where competition for metal surface sites does not exist because there is an excess of sites for the different molecular types to adsorb onto.

G = H -TS

For (2) - I well remember my interest being stirred a considerable time ago when reading the original report of 'chain matching' and reduced friction and wear, as further underlined in an earlier report of this conference. However, the relevance of this argument to diesel fuel lubricity is not clear. Until GTL (Gas-ToLiquid) diesel fuels of homogenous molecular structure become generally available, if ever that occurs, the fact is that the lubricity additives discussed are currently dissolved/dispersed into a heterogenous mixture of molecular types which comprise diesel fuel. These are the paraffinic, napthenic and aromatic structures with each structure having a range of molecular weights and attached side-chains. Therefore, 'chain matching' is not a useful concept here. It is entirely accepted that 'tuning' a match between the molecular chain length of an additive and fuel molecules could have substantial benefits of reduced friction and wear but for the reasons given, in practice this is unrealistic. For (3) - If the molecular structural backbone of a polymer is sufficiently flexible to allow the polar groups regularly arranged along it to be adsorbed onto the metal surface, then there is no distinction for surface coverage between multiples of individual molecules each with one polar group, on one hand, and a flexible polymer with multiple polar groups regularly arranged along its chain on the other. The suitability of the polymer chain/polar

A change in Free Energy is given by: AG = AH - TAS For most reactions where a high energy level is converted, as in the combustion of hydrocarbons then the AH term dominates the magnitude of AG. However, where there is a low AH term and an associated significant molecular ordering effect, then the magnitude of the second term, TAS becomes significant for the sign of the free energy change. The free energy for the adsorption process of diesel fuel lubricity additives onto a metal surface has the components of a weak adsorption enthalpy, AH, and an at least comparable TAS energy term. The latter arises from the transition of a disordered, high entropy state of the additive dispersed in the fuel to a low entropy ordered state of the additive molecules adsorbed onto the metal surface. This is an organisational process and thus a reduction in the overall entropy of the fuel/additive/metal surface system. It is possible for entropy to decrease and be minimised within specific systems. The classical review of the entropy contribution effect in organised liquid mixture systems remains that of Franks and Ives3. For the second part of (4), I refer back to the point that the proposed model for diesel fuel lubricity additives describes the progressive build-up of a surface film of additive molecules on a metal surface. The key words are 'progressive build-up' because until the limit of critical additive concentration, as stated previously taken to be where the bottom plateau in the reverse sigmoidal curve is reached, then there are vacant sites on the metal surface. For a mixture of dissimilar polar molecules dissolved in a hydrocarbon fluid, up to the critical additive concentration limit, there are

884 884 sufficient sites on the metal surface available for them to be adsorbed. It is important to note that up to the critical additive concentration limit, as defined, there is no competitive adsorption between polar but dissimilar additive molecules. What happens after the critical limit is reached is a separate consideration but by then the reduction in HFRR wear scar diameter has been achieved. The model proposes that below the critical additive concentration limit, a mixture of dissimilar polar molecules dissolved in a hydrocarbon fluid, will be adsorbed as separate clusters of similar molecules. Again, entropy will be minimised by separate cluster formation of different polar molecules on the surface as domains. It is conjectured that a surface film area composed of an adsorbed assembly of like molecules, envisaged as a 'twodimensional crystal' will be more coherent en masse against the wiping/removing action within the tribological contact than an adsorbed surface film of heterogenous molecules.

Paper XVII (iv) "The Degradation of Lubricants in Gasoline Engines: Development of a Test Procedure to Evaluate Engine Oil Degradation and its Consequences for Rheology" by P M Lee', M S Stark2, J J Wilkinson2, M Priest1, J R Lindsay Smith2, R I Taylor3, S Chung4 ('The University of Leeds, Leeds, UK, 2 The University of York, York, UK, 3 Shell Global Solutions, C Chester, UK, "infineum UK Ltd, Oxfordshire, UK). Dr I Minami (Ueda, Iwate, Japan). I am interested in the results of 'Test 2', in which an over-based calcium sulfonate shows some anti-oxidation properties. Some organic acid might be formed during the auto-oxidation. Scavenging the acid by the over-based salt is understandable. However, other organic oxides such as ketones and aldehydes would be generated during the auto-oxidation. They are also detected by the FT-IR measurement. My questions are;

From the previous arguments and also the HFRR wear scar diameter/additive concentration results presented in the proposed model, I have no difficulty in proposing that dissimilar polar molecules dissolved in a hydrocarbon fluid will be adsorbed onto a metal surface as a series of discrete film areas containing similar polar molecules.

(l)Does the over-based calcium sulfonate prevent the auto-oxidation? If your answer is 'yes', please suggest a mechanism of anti-oxidation. If your answer is 'no', please explain the reason why the intensities of carbonyl by FI-IR were decreased.

It should be possible to test this by SEM/EDAX studies of mixed molecular types adsorbed onto a metal surface from a solution of mixed additives in a hydrocarbon liquid such as diesel fuel.

(2) Do you have any additional comments for antagonism of 'dispersants' on anti-oxidation properties of the over-based calcium sulfonate (Test 3)?

References:

Reply by the Authors

1. 'colligative properties' entry in Chambers Science and Technology Dictionary, p. 176, (1988) published by W & R Chambers, Edinburgh,

The addition of the calcium sulfonate to the formulation has clearly had some preventative effect on the auto-oxidation. This sulfonate detergent was chosen so as not to affect the rate of oxidation, and the effect seen is very small when compared to an anti-oxidant additive. At the time of writing this response, the cause of the antioxidant behaviour of the sulfonate detergent additive used is still under investigation. As part of this investigation we are looking at the antagonism of 'dispersants' on anti-oxidation properties of the calcium sulfonate as observed in Test 3

2. 'colligative properties' entry in Glossary, p.A31, in S.S Zumdahl, 'Chemistry', 3rd Edn., (1993), publ. D.C Heath, Lexington, Mass. 3.

F Franks and D J G Ives, Quart Rev., 20, 1 (1966).

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SESSION XVIIIELASTOHYDRQDYNAMIC LUBRICATION - 2 Paper XVIII (i) Invited Paper "Surface Contact and Damage in Micro-EHL" by M J A Holmes, H Qiao, H P Evans, R W Snidle (Cardiff University, Wales, UK). Dr J H O Seabra (DEMEGI, Universidade de Porto, Portugal). 1. Does the fatigue damage model incorporate surface shear stresses? If Yes, how are they calculated in the film and contact zones?

Reply by the Authors The comparison with start up experiments referred to in the presentation [1] is no more than an encouraging indication that the contact calculation approach is meaningful. It does, however, relate to a situation where mesh resolution cannot be an issue and where experimental evidence is available. The Reynolds equation in the form of equation (1) is satisfied by the solution. This is the case for the trapped lubricant that you refer to. It is also subject to the elastic equation (2) as both are solved simultaneously. The liquid is compressible, so if the volume of the entrapment is increasing with time the pressure levels will fall, and vice versa.

2. Are there any correlations between the damage model results and micro-pitting in FZG gears?

Dr F H Bucher (Goodyear Technical Centre, Luxembourg).

Reply by the Authors

1. Concerning the vin-Mises-stress-like graphs for different slide-to-roll ratios you showed: are these values for a time-snap or are they already superposed results for one contact passage?

1. The fatigue damage model is very much a first attempt and the authors would be glad to receive any advice/comments on its formation. It does include surface shear. This is the lubricant shear stress at the solid surface in the film zones. In the contact zones it is the pressure multiplied by an assumed coefficient of friction of 0.1. 2. No, we have not got to the stage of looking for correlations yet. We are encouraged that the high damage areas are limited to the dimension (in x) of prominent asperities on the surfaced and that between such asperities its value falls to much lower levels. As indicated above, the fatigue analysis is under development. Professor S Bogdanski (University of Technology, Warsaw, Poland). In the PEHD liquid film analysed, some portions of liquid were temporarily entrapped between the two "dry contact points". How is the flow of this liquid controlled by the Reynolds equation if there is no flow out of or into the closed space? This case in my opinion is not comparable to the ball and plate experiment, as you suggested (used for demonstrating the features of the squeeze film) because of this experiment the continuous flow out of the liquid is possible.

2. Can you give some numbers about discretization and numerical costs of year 2-d model? How likely is it to obtain results for a full 3-d model with your approach, assuming spatial resolution fine enough or even the same as the 2-d model? Reply by the Authors 1. The damage calculations correspond to one contact passage of the profile illustrated in Figure 8. 2. The standard mesh resolution that we have adopted is a/200, for which some 8000 points are involved in the 2D model. Transient analyses for these cases take about 1.5 s/timestep to run on a PC running Linux. Point contact analyses are more time consuming with ahout 11 s/timestep required for 800x100 mesh points. These run times can potentially be reduced using the multi-grid approach developed by Venner and Lubrecht [2] that we have not implemented to date. 1. Holmes, M J A, Evans, P and Snidle, R W. "Comparison of Transient EHL Calculations

886 with Start-up Experiments", Proc. 29th LeedsLyon Tribology Symposium, Elsevier, pp 7989,2003. Venner, C H and Lubrecht, A A. Multilevel methods in lubrication, Elsevier, 2000.

is high. After cooling down some oxygen would precipitate as oxide. The oxide particles might have a detrimental influence on the tribological performance. Reply by the Authors

Paper XVIII (ii). "The Transient Elastohydrodynamic Friction Analysis of Main Bearings" by F Chu Suy, V Lagemann, H-OJ Fuesser (Daimler Chrysler AG, Germany)., G Knoll (University Kassel, Germany. Professor A Combescure (INSA-LAMCOS, Lyon, France). You compute in the same time structural response coupled with fluid film simulations for lubrication. For the structural computation you use mode superposition technique. The structural computations are transient and some of the parts (crank shaft for example) undergo large rotations and large rotational speeds. This is a very non-linear problem. How do you treat the two non-linearities in your simulations?

At the temperatures used for the BDO treatment (namely 800°C) the solubility of oxygen in Ti does indeed decrease with decreasing temperature. The decrease is however gradual. The precipitation of Ti2O might therefore be expected in the high oxygen region of the TO and BDO2 samples. In many studies (including those of the present authors) of oxidised pure titanium and near-a alloys (such as Ti-6A1-4V) only the Ti-0 solid solution and TiO2 have been found by XRD. Some groups have reported oxy-nitrides and intermetallic phases but never Ti2O. Dong and Li (ref 4 in the present paper) used transmission electron microscopy to examine the case formed on Ti-6A1-4V by thermal oxidation at 850°C for 0.5 hours followed by vacuum annealing at the same temperature for 20 hours. They found no precipitates within the diffusion hardened case. We are therefore confident that no oxide precipitates exist within the hardened cases of the samples reported in the present paper.

Reply by the Author The crankshaft and the crankcase are modelled as flexible bodies. These include rigid and elastic characteristics. The calculated motions are separated in rigid-body and elastic parts. The method of rigid-body kinematics is applied for the rigid body parts. This method is modelled as well large rotations.

SESSION XIXCQATINGS - 3 Paper XIX (i) "Sliding Wear of Oxygen Diffusion Hardened Ti-6A1—-4V in Physiological Saline" by K L Dahm, A Stanton, P A Dearnley (The University of Leeds, Leeds, UK). Dr Y Liu (Technische Universitat Llmenau, Germany). A comment on the oxygen diffusion technique. The annealing was performed at high temperature at which the solubility of oxygen in Ti

Dr S Mischler (EPFL, Lausanne, Switzerland). Can you comment on the possible effects of oxidation treatment on surface roughness? Roughness can affect initial debris formation, subsequent transfer film build-up and finally the overall wear behaviour. Reply by the Authors While we have made no direct measurement of the surface roughness of the pins, previous studies have shown that such effects can be complicated and difficult to control - especially for the duplex treatments. Oxide morphology can be a factor at high temperatures. Differential oxidation rates for different grains (utilised in thermal etching for titanium alloys) can also affect roughness. The effects of roughness on initial tribological behaviour would be most keenly felt by the oxide layer on the TO pins due to the relatively brittle nature of the oxide. We are however reasonably confident that the wear mechanisms observed were not due to the initial roughness.

887 Paper XIX (ii) "Hard Coatings Durability under Fretting Wear" by T Liskiewicz, S Fouvry', B Wendler2, ('Ecole Centrale de Lyon, France,2 (Technical University of Lodz, Poland).

cracking or debonding across the coating-substrate interface?

Professor T H C Childs (The University of Leeds, Leeds, UK). I have two questions:-

Composite, two stage damage mechanism of hard coatings is being observed in which the progressive wear is followed by a spalling phenomenon activated as the wear depth approaches the effective thickness of the coating. The identified mechanisms have not been studied in detail, however it is just one of the objectives of future study to explain the processes at the coating/substrate interface in order to control the decohesion phenomena and predict the effective thickness of the coatings. Adhesion forces between coating and substrate seem to be the most important property as the spallation of the coating can bring about accelerated wear due to hard particles generated within the contact area.

1. How is the energy density defined? Is it the energy dissipated per unit area of contact? And 2. How constant is the critical energy density, if conditions of load and displacement change? Reply by the Authors 1. Yes, it is the energy dissipated per unit area of contact. By deriving the Hertzian pressure and stress field distribution, the dissipated energy density can be determined within the contact. Usually the maximum dissipated energy density value is observed in the centre of the contact area. 2. Impact of different loading conditions on the value of the critical energy density can be easily demonstrated with use of a Wohler-like, Energy Density-Coating Endurance chart. With increasing load or displacement amplitude, the mean dissipated energy density tends to higher values. Even though a slight dispersion of the results is being observed (Fig. 5), one representative level of critical energy density can be determined for all loading conditions. Due to the additive property of the energy, this approach associates the failure of the coating with the maximum cumulated energy density and the substrate will be reached at the threshold value related to the specific energy density capacity. This energy capacity is given for a specified coating. Hence, the energy density dissipated per fretting cycle is a function of load and displacement amplitude, however the energy capacity value is constant and representative for the tribosystem. Dr P A Dearnley (The University of Leeds, Leeds, UK). Have you been able to identify any details of the pertaining wear mechanisms that cause failure of the coatings, for example, effects like micro-

Reply by the Authors

XIX (iii) 'The Effect of the Crystallographic Orientation of Ag Thin Films on their Triblogical Performance" by M Goto , K Akimoto2, F Honda3 ('Hino Motors, Tokyo, Japan, 2 Nagoya University, Nagoya, Japan, 3Toyota Technological Institute, Nagoya, Japan). Dr Y Liu (Yechnische Universitat Llmenau, Germany). You have demonstrated the large influence of the crystallographic orientations of Ag on its tribological properties. Normally the crystallographic orientation of metals is closely related to the behaviour of dislocations. Do you think that the dislocations play a role in the Ag film with a thickness of 5nm? Reply by the Authors I do not think that the dislocations play the important role in the film with the thickness of 5 nm, acting as in bulk Ag. At least, a work hardening caused by piling-up of the dislocation during deformation, and increases of shear strength of the Ag layer are not observed by lamella gliding of Ag (111) sheets, because of the lack of grain boundary in the quasi two dimensional structure of the film. STM observations of both epitaxial and polycrystalline films with 5-nm in thickness shows that adequate spaces between each crystallites, which prevent the piling-up of the dislocation.

888 Paper XIX (iv) "The Effect of Sliding History on the Steady State Friction Coefficient Between CNx Coatings Under N7-Lubrication" by K Adachi1, T Wakabatyashi2", K Kato1 ('Tohoku University, Japan, 2Ishikawajima-Harima Heavy Industries Co. Ltd, Japan). Professor T H C Childs (The University of Leeds, Leeds, UK). I have two questions: 1. If you stop supplying nitrogen, for how long do its effects last? 2. How important is the level of relative humidity as far as these results are concerned, i.e. water vapour or hydrogen levels? Reply by the Authors Thank you for the good questions about the very principal part of atmospheric effect in the N2 gas lubrication. Our answers are as follows: 1. The friction coefficient rises quickly from below 0.01 up to about 0.10-0.20 within a few cycles of friction. 2. The level of relative humidity RH of air changes friction coefficient in a wide range between CNx coatings in the stream of N2-gas. Other study in our laboratory shows that an increase of RH by a few percent in N2 gas increases friction coefficient from 0.01 level to 0.10 level. The effect of hydrogen level is not studied yet. Dr P A Dearnley (The University of Leeds, Leeds, UK). 1. The micrographs you showed provide some evidence of tribo-layer formation. Have you been able to investigate their nature or can you speculate on their composition, since they must influence the friction behaviour you observed? 2. In this talk you suggest that N2 would be a good lubricant for ceramic materials. A few years ago at the Vancouver WOM Conference you suggested H20 for the same role - which is best?

Reply by the Authors Thank you for the good questions about the basic role of tribo-layer and the compassion with the water lubrication. Our answers are as follows: 1. Yes, We believe the tribo-layer including wear debris decides the observed friction coefficient. We know that the observed low friction coefficient below 0.01 is not generated by a film of N2-gas formed hydrodynamically. The tribo-layer is observed as graphic by Raman spectra and has larger amount of C-N and C=N bonds than the initial surface in XPS analysis. Wear debris looks partly dark and partly transparent in the OM. We know those observations are not sufficient to consider the low friction mechanism, and think its necessary to know the micro structure and chemicals of wear debris in nanometer scale. 2. SiC/SiC or Si3N4/ Si3N4 sliding in water gives a friction coefficient below 0.001 by the mechanism of hydrodynamic lubrication. The thickness of water film is supposed to range in 10-100nm. It is generated by tribo-chemical wear of SiC or Si3N4 in water and the resultant smooth wear surface of surface roughness below one nanometer where a hydrodynamic water film is easily formed at a low sliding speed (<1.0m/s). High hardness (>20Gpa) of SiC or Si3N4 is another important factor to generate hydrodynamic film effect of water in low speed sliding. CNx/CNx or CNx/ Si3N4 gives a friction coefficient below 0.01 at low sliding speed (<1.0m) and relatively high contact pressure (> 500MPa) in N 2 gas even when it is supplied to the contact in air. The low friction is generated in dry solid/solid contact by removing the negative effect of H2O from the surrounding air and having adsorption of N2 on the wear surfaces which works better then vacuum So, each one of water lubrication and N2-gas lubrication has its own effective contact condition of wet or dry.

889 Professor F M Borodich (Cardiff University, Wales, UK). It seems to me that similar effects were observed in experiments on super-low friction DLC:H coatings, were they not? Reply by the Authors Thank you for the good comment of adding a paragraph 3.4 "Mechanisms of Friction". It would be nice if "Mechanisms of Friction" could be described with experimental confirmation. However the present level of our understanding of the mechanisms is low and only "Discussion on mechanisms" may be made as follow. (1) In the initial stage of running- in ( <100 friction cycles) in gas Oxygen or air, some reaction products are formed which help the reduction of friction in N2 gas during the following friction cycles (> 100 friction cycles). (2) Smoothing of contact surfaces of CNx coatings takes place in the running in process, and it can help generation of low friction (u< 0.01) obtained after sufficient running in cycles ( > 1000 friction cycles). (3) The roughness of wear scar on ball specimen is low (Ra < 1 .Onm), and this wear scar is covered by a tribo-layer which looks partly dark and partly transparent in OM. Fine wear particles are always observed around the wear scar, which may mean that wear particles in nanometer scale at the contact interface strongly effects the value of friction. (4) When N2-gas is supplied to the contact interface, it works to exclude the attack of H2O and O2 from air which increases friction. However, the role of N 2 gas is not always to exclude them. In the chamber of N2 gas, friction is lower than in vacuum. It means N2 adsorbed on the friction surface effectively reduces friction by some mechanism which is not well understood at present. It seems necessary to introduce a new idea of lubricating effect of N2 in the contact between CNx and CNx. The similar effect is observed with CNx/Si3N4.

(5) It is well confirmed by experiments that a N2gas film is not formed by the hydrodynamic effect. Dr S E Franklin (Philips CFT, Eindhoven, The Netherlands). How does the sliding history and gas atmosphere affect the tribo logical behaviour of uncoated SI3 N4? Can similar effects perhaps be generated with oxide cermics? Reply by the Authors Thank you for the good question about the uncoated Si3N4. Similar effect of the sliding history in humid air is observed with Si3N4. About 100 friction cycles in air is effective to give steady low friction (u.< 0.02) in N2-gas supplied after the running in. Oxide ceramics are not yet well tested in the view point of the history effect. However, if the chemical reaction of CNx disk and the transfer of the products to the ball surface in the mechanism of low friction observed, A12O3 or ZrO2 ball may show similar effects.

SESSION XX WEAR AND DEBRIS EFFECTS Paper XX (i) Invited Paper "The Life Cycle of a Debris Particle" by R S Dwyer-Joyce (The University of Sheffield, Sheffield, UK). Dr J Williams (Cambridge University, Cambridge, UK). You illustrate that the effect of fluid flow in a single point contact is effectively sorting the particles by size and influencing their trajectories and entrainment. A line contact, or even an extended elliptical contact, will have in effect no ends so in these cases am I correct in thinking that this mechanism cannot operate and the population of particles that goes through the gap will reflect that in the bulk? Reply by the Author We have no data for a line contact (and I have not seen any published), but I would imagine that is the case. There will be a large volume of oil and particles recirculating in the inlet region. I guess

890 this would have the effect of mixing the particles (but not concentrating them). Professor K Kato (Tohoku University, Sendai, Japan). 1. Is there any critical condition of the agglomeration of particles in relation to particle size and friction coefficient between the particle and the loaded surfaces?

On one of your slides you showed that as the Hertzian pressure increases the fatigue life increases too. Doesn't that contradict the relationship between fatigue life and stressed volume? Reply by the Authors The fatigue life is related to two main parameters: -

The stress level in the contacting bodies: this stress level depends on the maximum pressure in the dented contact and is related to the dent geometry;

-

The stressed volume, directly related to the number of dents in the contact area and the dent diameter.

2. Is there any contact size range of particles where they tend to agglomerate easily and generate high stress concentrations as a result? Reply by the Author The particles referred to in this work are usually so large that agglomeration is not an issue, with the exception of soot of course. Perhaps an agglomerated mass of particles would be entrained easier into the contact. But as soon as it experiences any shear force the agglomerate will breakdown and the constituent particles will enter the contact. This will probably happen in the inlet region remote from the contact. So, the agglomerate would not cause significant damage or high stress. But I don't know what the critical size for this to take place might be.

Paper XX (ii) "Experimental Investigations of Rolling Contact Fatigue of Dented Surfaces using Artificial Defects" by S Coulon, F Ville, A A Lubrecht (INSA-LAMCOX, Lyon, France).

Comparing the reference tests (#1-2-5) performed under 3.5 GPa to tests #3 and #4 gives the effect of stressed volume in terms of number of dents in the contact area (see table 1). Increasing the number of dents decreases the fatigue life. Comparing the tests performed with 12 dents under 3.5 GPa (test #4) to test #6 gives the effect of stress level. Increasing the stress level tends to decrease the fatigue life. Test#

Dents number

3

1

(1-2-5) 4 6

6 12 12

Professor I I Kudish (Kettering University, MI, USA). 1. What is the relationship between fatigue life and stressed volume?

Dent size

Ph (GPa)

<|>=250 urn dlh - 34 nm

3.5 2.5

Fatigue Life 40.1 (interrupted) 30 13.2 23.9

Table 1 3. The possible reason why you do not see any damage may be that you have sub-surface instead of surface cracks. What is your opinion on this?

Reply by the Authors Reply by the Authors Increasing the stressed volume either by the number of dents or the dent size leads to a decrease of the fatigue life. This effect has statistical roots. Increasing the stressed volume increases the risk of combining a high stress level due to the dent with a weak spot in the material and causing preliminary crack initiation.

Reference: Diab, Y., Coulon, S., Ville, F., Flamand, L., 2003, 'Experimental investigations on rolling contact fatigue of dented surfaces using artificial defects: subsurface analyses', Tribology series, no 41, 29th Leeds-Lyon Symposium on tribology on Tribological research and design for engineering

891 891 systems, Leeds, Amsterdam, Elsevier Science, pp. 359-366; shows that the cracks formed by the pressure peak induced by the dent, initiate just beneath the surface and propagate towards the surface. It is possible that cracks are present just beneath the surface but have not yet reached the surface and have not yet generated a spall.

residual stress increases life by a factor 2. Today, measuring the residual stress field is not easy, as the scale is very small (smaller than the dent size). The existing numerical work requires experimental measurements and tests in order to validate its predictions.

4. A number of the surface cracks you observe may be caused by the interaction between surface cracks and lubricant. Please comment.

Paper XX (iii) "Stability, Wear and Corrosion Characteristics of Rapidly Solidified High Temperature Al-8Fe-4Ce Alloy" by Y A-H Mashal1, M A Kassem2' A F Waheed2' ''Department of Production Engineering, Shebin Elkom, Egypt, 2 Department of Metallurgical Engineering, Shebin Elkom, Egypt).

Reply by the Authors The presence of additives in the lubricant has an effect on the fatigue life and is of interest but was not the principal objective of this paper. Tests were performed with two different lubricants showing that there are interactions between the steel and the lubricant. The effect of this interaction on the fatigue life is one of our current fields of interest Professor J Sugimura (Kyushu University, Fukuoka, Japan). Perhaps I do not fully understand your talk, but may I ask about residual stresses? There must be residual compressive stress generated upon indentations which may affect fatigue crack nucleation and propogation. Could you tell us if it does in your experiments, and how? Reply by the Authors Residual stresses have an effect on the fatigue life but were not investigated in this work as all the tests were conducted on AISI52100 with two dent sizes made with a Rockwell penetrator. Compressive residual stresses are probably one of the reasons why the smooth surfaces were damaged under pure rolling conditions and not the dented surfaces. In the following reference: Coulon, S., "Prediction de la duree de vie des contacts ponctuels lubrifies en presence d'indentation", These de doctorat, Institut National des Sciences Appliquees de Lyon, 2001,we have performed a test to indicate the effect of residual stress. This test consisted in using the test condition #4 and changing the smooth sample before spalling as many times as necessary until detection of damage on the dented disk. The fatigue life of the dented disk was found to be twice the fatigue life of the smooth disk. Under these operating conditions, the

Dr J Williams (Cambridge University, Cambridge, UK). Both the values of the coefficients of friction and wear rates that you report would in normal applications be considered very high and probably unacceptable. Can you comment on the possible applications of this particular material pair? Reply by the Authors The wear rates and subsequently the coefficient of friction are high because the experiments were performed in the worst case of dry (unlubricated) sliding for comparison between different alloys. The wear rates could be reduced too much in real contact by applying a suitable lubricant. The use of aluminium alloys is essential in the aerospace industry where strength to weight ratio is required.

SESSION XXI BEARINGS Paper XXI (i) - Invited Paper - "Integrated Tribological Analysis Within a Multi-Physics Approach to System Dynamics" by M Teodorescu, S Balakrishnan, H Rahnejat (Loughborough University, Loughborough, UK). Professor D Dowson (The University of Leeds, Leeds, UK). The outline of a multi-body dynamics approach to 'real' engineering systems is impressive. The marriage between multi-body dynamics and tribology is essential for the satisfactory development of machinery and the

892 authors are to be congratulated for moving us in this direction. Since the proof of the pudding is in the eating, it would be interesting to know if the authors could comment on the current limitations from both multi-body dynamics and tribology contributions to their comprehensive approach.

friction, spring coefficients, damping coefficients and stability of the bearing. When the cylindrical journal bearing has manufacturing errors of roundness as shown in my study, the cavitaion region divides into two parts in the bearing. The sizes of the cavitation depend on the value of the manufacturing error of roundness. Therefore the manufacturing error of roundness affects the bearing characteristics.

Reply by the Authors First of all we would like to than Professor Dowson for his kind words. As regards the limitations in this approach, those related to multi-body dynamics are often related to the description of the interacting phenomena at the extremes of the physics of scale both in spatial and temporal senses. In many mechanisms rigid body motions may be considered as low frequency relatively large rigid body displacements, whereas those associated with modal behaviour of components, and indeed, with load bearing surfaces are of infinitesimal nature at much higher frequencies. Thus, solution within a single multi-physics framework can lead to long computations when investigating combined short and long transience, such as impact and wear. The paper provides multi-physics example solutions with varying degree of complexity. We will be limited in holistic analyses of this kind by the available computational power and patience of the analyst. The methodology to incorporate all the physics of approach, nevertheless exists.

Paper XXI (iii) "Influence of Manufacturing Error of Roundness for Characteristics of Cylindrical Journal Bearing" by K Iwamoto, K Tanaka, (Tokyo University of Marine Science and Technology, Japan).

SESSION XXIIAUTOMOTIVE - 2 Paper XXII (i) "Additive/Additive Interactions in Boundary Lubrication - A Study of Film Formation and Tenacity" by A Morina, A Neville, J Green, M Priest (The University of Leeds, Leeds, UK). Dr B-O Ahrstrom (SKF Engineering & Research Centre, The Netherlands. ZnDTP in contact with technical steel surfaces is highly sensitive to water presence and temperature. Adding the tested additives to the base oil influences water solubility and hygroscopicity. What measures have been taken to decrease these influences? Reply by the Authors The additives in base oil were blended at the premises of our collaborators and so we are not aware of how these affect water solubility or hygroscopicity.

Dr T Hirayama (Ryukoku University, Shiga, Japan). Could you comment on the possible effects of cavitation occurring in the bearing clearance? I think that it affects considerably the bearing characteristics, especially for plain bearings.

Dr M G Talks (Qinetiq), Farnborough, Hants, UK). In your presentation, you showed a graph of the variation of load carried by asperity contacts with Llambda value - the proportion of the load carried by asperity contact falling to zero at a Llambda ratio of about 2.5 Could you explain how this graph was determined?

Reply by the Authors

Reply by the Authors

I think the size and the position of cavitation occurring in the bearing affect the minimum oil film thickness, load capacity, coefficient of

In the present paper no graph of load vs. lambda ratio is presented.

893

SESSION XXIII MACHINE SYSTEMS -2

the practical use of DLC coating, we would like to consider the influence of corrosion in future work.

Paper XXIII (i) "Seizure Tests to Select Materials for Cylinder Blocks and Valve Plates in Water Hydraulic Pumps" by H Nishida, T Hayashi, N Hayashi, A Yuge, A Matsui, Y Kogimoto (Mitsubishi Heavy Industries Ltd, Japan).

Paper XXIII (ii) "A Reduced-Scale Testing Machine for Tribological Evaluation of Brake Materials" by M Kermc1'2, M Kalin1, M J Vizintin', Z Stadler2 ('University of Ljubljama, Slovenia,2 M + S Production, Bled, Slovenia).

Professor M Priest (The University of Leeds, Leeds, UK). Clearly water removes heat very effectively from such systems. However, approaching seizure, did you observe variations in thermal behaviour between the different material combinations?

Dr B-O Ahrstrom (SKF Engineering & Research Centre, The Netherlands). Do you expect that your test equipment (by not changing the disc from both sides) may disallow the relevant dynamic and thermal distribution to form this compromising result?

Reply by the Authors

Reply by the Authors

In this study, the temperature of the test pieces was measured, and it was not confirmed that the temperature measured in a combination of steel and DLC coating was higher than that measured in a combination of copper alloy and DLC coating. So it was assumed that thermal conductivity of the material did not strongly influence the seizure condition in this study. However, it is considered that the temperature of the sliding surface is very important in preventing seizure. From this point of view, copper alloy is a better material than steel.

The design of the reduced scale testing machine was performed in order to evaluate the tribological properties of brake lining materials in the stage of their development, not the final brake evaluation, where dynamics become an additional important parameter. Therefore, our measurements are presented focused on the well-controlled material properties (including temperature-dependence) and the operating conditions at the sliding contact and less on the behaviour of the entire system.

Dr M G Talks (QinetiQ Ltd, Hants, UK). Carbon has a high potential in the electrochemical series compared with most metals. Has any work been carried out to investigate galvanic corrosion effects between DLC-coated areas and areas of uncoated metal which are in electrical contact with them? Is it possible that (galvanic) corrosion effects contribute to the wear of any uncoated areas within the contacts? Reply by the Authors In this study, severe corrosion was not observed because the test duration was short (shorter than 1 hour). However, if the DLC coating parts have slid for a long duration, galvanic corrosion may occur and influence wear or delamination of the coating. For

Professor H P Evans (Cardiff Unviersity, Cardiff, UK). Can you confirm that the results shown are for the same materials used in two different test machines and that the agreement in the results validates the (scaling) adopted? Reply by the Authors The results shown in this paper are for the same brake disc and brake lining materials. The tests were performed on two different test machines; on the Krauss machine for real scale testing and on the reduced-scale machine. The comparison of the measurements and consideration of the specific characteristics of two testers gives us promising results and confirmation of successfully performed scaling in the design process of our testing machine. Dr K L Dahm (The University of Leeds, Leeds, UK). In long-duration brake applications the

894 surface temperatures are typically dependent on power input (braking power) and convective heat transfer coefficients. Does the developed rig allow the control of air flow rates to control convective heat loss?

Reply by the Authors The main goal of the designed reduced-scale testing machine is to obtain data on tribological properties of the brake-lining materials as a function of contact conditions (speed, pressure and temperature) and at this stage of development less on the thermal response of the system. Therefore, the speed and pressure are varied as input parameters and the friction is monitored to control and evaluate braking power (energy input). However, the temperature is planned to be controlled in the next phase by the braking time adjustment and also by the additional air-blow cooling of the system, which is already installed at the system to shorten the cooling periods. Mr J Green (The University of Leeds, Leeds, UK). You have mentioned the inherently higher temperatures seen in the novel brake systems. Is it necessary to use a different grade of hydraulic fluid when compared to traditional systems? Reply by the Authors Due to regulations in automotive applications designers are limited in selection of materials to only those which are allowed for use in public transport. Therefore we are using the hydraulic fluid with the highest temperature resistance available on the market and accepted for use in public transportation, i.e. DOTS. Unfortunately, our brake systems operate at upper temperature limits even for that fluid. Hence we had to take different steps to keep the operating temperature of the hydraulic fluid in the required limits (see: Z Stadler, M Kermc, T Kosmac, A Dakskobler, A morotcycle brake system with C/C-SiC composite brake discs, Ceramic engineering & science proceedings, vol. 25, issue 4, Westerville: American Ceramic Society (2004) 179-184).

SESSION XXIV NANOTRIBOLOGY -2 Paper XXIV (i) "Effects of Ultra-Violet Irradiation on Conformation and Spreading Characteristics of Molecularly Thin Lubricant Films" by H Zhang, Y Mitsuya, M Imamura, N Fukuoka, K Fukuzawa (Nagoya University, Nagoya, Japan). Professor A Neville (Leeds University, Leeds, UK). How does the UV treatment change the nature of the DLC coating, in particular the sp3/sp2 ratio? This will in turn have an effect on the ability of species to bind to the DLC surface. Reply by the Authors At the present stage we did not investigate the change of DLC induced by UV treatment. But as shown in Fig. 3 in the paper, the thickness of lubricant bonded to the disk surface increases after UV irradiation. This indicates that more adsorption sites on the DLC surface are available for the lubricant polymers after UV treatment. Dr S K Sinha (National University of Singapore, Singapore). 1. What was the energy level of UV irradiation? 2. Do you think UV irradiation promotes bonding of PFPE with the substrate or it promotes crosslinking with PFPE molecules? Reply by the Authors 1. We used a low-pressure mercury lamp which emits UV radiation of 184.9 and 253.7 nm. The irradiances on the disk surface were approximately 3 and 5 mW/cm2 at the respective wavelengths. The irradiation time was set to be 1 min. 2. As seen in Figs. 2 and 3 in the paper, the increase of bonded lubricant thickness induced by UV irradiation is not remarkable as compared with the drastic decrease (about one fifth) of polar surface energy after UV treatment. Decreased polar surface energy is attributed to the increase in both adhesive

895 interactions (bonding of PFPE with the substrate) and cohesive interactions (crosslinking with PFPE molecules), whereas increased bonded lubricant thickness is only attributed to the increase in adhesive interactions. We thus speculate that besides adhesive interactions, cohesive interactions are also developed by UV irradiation.

Paper XXIV (ii) "Friction and Wear Life Evaluation of Silane-Based Self-Assembled Monolayers on Silicon Surface" by M Satyanarayana', S K Sinha, M R Srinivasan (National University of Singapore, Singapore).

accordingly with the change in friction. We expect that similar phenomenon as in the case of bare Si, occurs in the case of APTMS SAM also because of the presence of polar amine (NH2) terminal groups on the surface, which favours the adsorption of water at higher humidity levels. Since the relative humidity in our experiments remained between 60-70% we do not expect much effect of relative humidity on the wear performance. References 1. 2.

Dr J Williams Cambridge, UK).

(University

of

Cambridge,

1. Could you indicate the sliding speed of your tests. 2. The tribological behaviour of Si is much influenced by the availability of water vapour in the environment. Do you control the relative humidity of your test chamber? Could you comment on the effect that changes in RH might have on the levels of friction and wear that you observe with the SAMs you have looked at? Reply by the Authors

3.

J.Wei, L.Jianbin and W.Shizhu, Chinese Science Bulletin 46 (2001) 698. K.-H.Cha and D.-E.Kim, Wear 251 (2001) 1169. S.T.Patton, W.D.Cowan, K.C.Eapen and J.S.Zabinski, Tribology Letters 9 (2000) 199.

Paper XXIV (iii) "Basic Observations of Tribological Phenomena by Neutron Scattering Methods" by T Hirayama , K Inoue , T Ebisawa , S Tasaki3, M Hino4, N Torikai5, K Ito4, T Fukunaga4 ('Ryukohu University, Otsu, Japan, 2 Japan Atomic Energy Research Institute, Ibaraki, Japan, 3Kyoto University, Kyoto, Japan, 4Kyoto University Research Reactor Institute, Osaka, Japan, 5 KEK (High Energy Accelerator Research Organisation), Ibaraki, Japan)

1. Sliding speed used in our tests was 0.021m/sec. 2.

Humidity of the chamber was not controlled in our tests. The relative humidity in the chamber varied between 60-70% during the tests at a controlled room temperature of 24 °C. We expect that the tribological properties of OTS SAM are fairly insensitive to the changes in the relative humidity because of the less chance of adsorption for water due to the hydrophobic nature of OTS SAM surface [1,2]. Whereas in the case of bare Si, below a certain level of humidity friction increases as the relative humidity increases, whereas friction decreases at higher levels of relative humidity [1,3]. At lower humidity, friction rises because of the rise in capillary forces whereas the decrease in friction at higher humidity is due to the formation of a thin water film, which acts as a lubricating layer. Wear also changes

Dr I Minami (Iwate University, Iwate, Japan). Differences in rheological properties between C's Cjo H| 8 and trans - C ]0 H ]8 are known. Does your method give any clear observations for the phenomena? Reply by the Authors Theoretically, yes. A radial distribution function gives us quantitative information on a structure of the sample oil. Differences between cis- and transcan be shown if the diffraction peaks are adequately separated.

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SESSION X X V BIO-TRIBOLOGY -3

4. How can you be sure that the proteins adhered in a monolayer and not in conglomerated aggregates? Would this affect your results?

Paper XXV (i) "The Importance of Protein Physisorption in BioMEMS/NEMS Applications: A Nanotribological Study" by A Elfick1'2, K Healy3, A Unsworth4, L Pruitt2 ('University of Edinburgh, Scotland, UK, 2|3 University of California Berkeley, USA, 4 University of Durham, UK).

Reply by the Authors

Dr S E Franklin (Philips Centre for Industrial Technology, The Netherlands). How does the viscosity of the (semi-solid?) adsorbed protein layer depend on shear rate i.e. is there a shear-rate dependency? This may help to explain the observed velocity dependency of the friction behaviour. It should be possible to measure the shear-rate dependency in commercially available viscometer equipment for small sample volumes. For example, this has been done with human tear fluid. Reply by the Authors It is highly likely that the viscoelastic denatured protein plaque will possess a shear rate dependency. However, this material does not lend itself to easy assessment. That the adsorbed protein would need pressure denatured would argue against the use of many types of viscometer. Surface force apparatus experiments may provide some information to correlate with the FFM measurements. Further work considering additional sliding velocities may help to support a hypothesis for the velocity dependence of friction in this system.

To address each point in turn: allowed 1. The protein solutions were approximately 15 minutes to establish equilibrium conditions. 2. For the conditions employed adsorption was shown by QCMD to be rapid and reach saturation levels. 3. Experiments were conducted with protein adsorbed and in free solution in order to allow for 'recharging' of the adherent layer. This situation most closely emulates that found in vivo. 4. In general, protein adsorption is believed to progress to a saturated monolayer and then protein may only be adsorbed through molecular exchange. Proteins do not tend to adsorb well to protein coated surfaces. This may no longer hold true if the protein is grossly denatured at the surface e.g. pressure denatured as in the above work. There exists the possibility that the denatured plaque formed by the sliding probe may then adsorb proteins which would be denatured by the probes subsequent traverse. I would not however envisage that this would proceed to the formation of agglomerates. Professor J B Medley (University of Waterloo, Ontario, Canada). With regard to possible applications; this research is quite relevant to our work on contact lens friction and its dependence on protein adsorption.

Ms V Ngai (University of Waterloo, Ontario, Canada).

Reply by the Authors

1. How long were the samples exposed to the protein solutions prior to adhesion and friction testing? 2. Do you think the level of adhesion (proteins adhering to surfaces) could influence the results? 3. Were the samples rinsed to remove any unadhered proteins?

There are indeed many situations in which an adsorbed protein may be acting as a lubricant; these span applications as diverse as biomedical engineering, food science and even marine applications. The results presented above apply specifically to highly hydrophilic surfaces which present a very defined interface with the solvent. Whilst polymers used in contact lenses are usually hydrophilic, it is likely that proteins will also absorb into these materials. Proteins have even

897 been shown to diffuse into salt crystals. Questions relating to how proteins may behave in the volume immediately below the contact area remain unconsidered and hence I would advocate caution in extrapolating the results presented herein to any other tribosystem.

Reply by the Authors

nature of both titanium alloy and MMC surfaces. The classic, somewhat dramatic, analogy of "iceon-mud" is often mentioned in this context. The harder coatings tend to detach, fragment and act as abrasive agents thus increasing the wear when not well-supported or adhered. However, various combinations of surface treatments may help reduce wear involving ion implantation, heat treatments and subtle alloying. Investigation is in progress into such treatments and into the application of other materials with low wear and good radiographic imaging.

Paper XXV (ii) "Metal-Metal Cervical Disc Implants: Material Investigation using Pin-onPlate Wear Tests" by M L Powell, J B Medley, R Varano.

Ms S Radice (Ecole Polytechnic Federale de Lausanne, Switzerland). Please comment on the manufacturability of cervical disc implant made by CoCr-Alloy?

Dr S E Franklin (Philips Centre for Industrial Technology, The Netherlands). Is there a reason why (lubricious) coatings (e.g. DLC types) are not used in such applications, e.g. for maintaining the good radiographic (imaging) of the (MMC) but improving the wear and friction performance?

Reply by the Authors

May I conclude by thanking Dr Franklin, Ms Ngai and Professor Medley for their consideration of my work and stimulating questions.

Reply by the Authors Coatings have been tried (but not by us) and the difficulties are poor coating adhesion and the soft

Cervical disc implants could be manufactured from cobalt-based alloys. As mentioned in our paper, due to the proximity to the spinal cord, postoperative imaging is essential for effective treatment of patients. The degree of image artifact is related to the type of metal used as well as the imaging technique. Cobalt-based alloy is known to produce stronger artifact and thus has not been considered for cervical disc replacements.