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A systems approach to the tribological testing of coated materials
gmIl? ilt ImW! ELSEVIER
Surface and Coatings Technology 74-75 (1995) 15 22
A systems approach to the tribological testing of coated materials J.P. Celis Dept. MTM, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium
Abstract This paper is an attempt to illustrate how laboratory wear tests combined with an in-depth material characterization and interpretation based on models present in literature can support the use of advanced wear protection coatings. Therefore it is necessary to define what is meant by tribology and what the definitions are of wear and friction. The following step is a structured approach for determining the tribological characteristics of coated materials. Some considerations are made about the choice of the experimental set-up for testing coated materials. A framework for situating and describing the wear modes noticed is discussed based on existing wear models. This is illustrated by a discussion of laboratory wear test results on electrolytic hard gold, ceramic TiN-based coatings and hard carbon-based coatings.
Keywords: Tribology; Coatings; Laboratory tests; Wear; Tribochemistry
I. Tribology: a multidiseiplinary approach The tribological characteristics of a material are determined by properties related to its sliding and rolling behaviour against any other material. In general, friction and wear processes occur jointly when two surfaces undergo sliding or rolling under load. Wear is defined as the progressive loss of a substance from the operating surface of a b o d y occurring as a result of the relative m o t i o n between surfaces [ 1 ] . This progressive loss of substance can result from a mechanical interaction at a n d / o r a chemical reaction induced between two contacting surfaces. D y n a m i c friction is defined as the resistance to a relative m o t i o n of contacting bodies and can be quantitatively represented as the coefficient of friction (COF), being the ratio of the resulting tangential and applied n o r m a l force (as far as stick or partial slip conditions are not prevailing). The basic laws for friction and wear were developed some centuries ago by L e o n a r d o da Vinci (1495), A m o n t o n s (1706), Euler (1750) and C o u l o m b (1785). Tribology, being the unified a p p r o a c h of friction, adhesion, wear and lubrication, is by contrast quite a y o u n g science, born in 1966 when the O E C D recognized the important economic savings related to a reduction of friction and wear. An important point out of these definitions is the fact that wear is not a materials property, but a systems property! Tribology definitely needs a multidisciplinary approach, which is best reflected by the type of information that people with 0257-8972/'95/$09.50© 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(95)08211-5
different interests require when dealing with friction and wear (see Table 1). Focusing now our interest on films and coatings, it is worth remembering the nice statement of the late Professor Godet: "A coating without thermomechanical characteristics is as useless as a c o m p u t e r without a manual" [-2]. Indeed, if one z o o m s in on what happens in the contact zone between two bodies, the existence of Table 1 Tribology: a multidisciplinary approach Information required by the mechanical engineer Coefficient of friction, wear rate Young's modulus (evaluated from 4-point bend test) Poisson's ratio (evaluated from 4-point bend test) Coefficient of thermal expansion Thermal conductivity, density, specific heat Tensile strength, fatigue limit Information required by the tribologist Stress and temperature fields Load-carrying process - used to separate the first bodies as characteristics of third bodies Information required by the materials scientist Composition, hardness Matrix-to-second phase particles adhesion Crystallographic texture, internal stress profile Information required by the surface scientist Deposition process, thickness coating Hardness coating/substrate system Coating-to-substrate adhesion, interface
J. JR Celis/Surjitce and Coatings" Technology 74 75 (1995) 15 22
16
a dynamic system with high complexity becomes evident. From the materials point of view, the mechanical and chemical reactivity induce large modifications in the starting materials, as shown schematically in Fig. 1, while the contact conditions also evolve with the formation of a third body resulting from an interaction between the first two bodies and the surrounding atmosphere (Fig. 2). This last observation has been the starting point for a recent evolution from wear considered as a matter of mechanics to a matter of materials and surface science. Indeed the preponderant role of tribochemistry is being recognized more and more. The definition of tribochemistry is the study of the occurrence, acceleration or modification of chemical reactions caused by the friction between two surfaces. Aspects of dissipation of frictional energy in the contacts, the exposure and reactivity of bare metal surfaces, and the elastic and plastic deformation in sliding bodies play a large role in that respect. The challenge of tribochemistry in the near future is surely to indicate ways to turn material losses into a material protection. Optimization of wear behaviour can indeed be based on a closer look at the chemical surface conditions of materials, as was done by Gardos [3] investigating the effect of oxygen content of non-stoichiometric titanium oxides on the COF, and by Mohrbacher [-4] demonstrating the effect of relative humidity on the COF of TiN sliding under unlubricated conditions against steel. The importance of these findings is high in view of the trend towards avoiding lubrication by oils as imposed by recent environmental legislation and the related need for
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2. Wear testing is more than "wear" and "wear rate"
People involved with any of the different aspects of tribology are always puzzled by the problem of evaluating the wear performance of a given combination of first-body materials being operated under a set of environmental and machining conditions. Wear testing on a laboratory scale is certainly the most attractive solution, but it contains a lot of limitations, and can be performed at different levels of sophistication as discussed by Czichos [5,6]. The basic question is, in fact, what do people expect from wear testing? One could be interested in a comparative investigation of different kinds of materials, such as metals, polymers, ceramics, cermets, and coated materials; or in the investigation of the aggressiveness of a system; or in an attempt to extrapolate to lifetime and behaviour under field conditions. In any case, as long as a full simulation of the field practice cannot be done, the use of laboratory data will always be subjected to a risky exercise of extrapolation! However, this does not mean that laboratory testing is completely useless. Lab testing becomes fully relevant when used for unravelling the active wear mechanisms, for understanding the wear process dependence on testing parameters as was done by Ashby [7] in terms of wear maps, and finally for developing trivalent relationships between the production of materials, their structural characteristics, and their functional behaviour. In that respect, wear testing requires two types of information: that which can be obtained as on-line information, namely the COF, the surface contact temperature, the acoustic noise, etc.; and that from an off-line investigation of the surfaces after testing. With respect to wear loss, one can differentiate between simple material loss (by direct measurement of the weight change or indirectly by profilometry and metallography), investigation at a macroscale of the surface topography (by e.g. optical microscopy, atomic force microscopy, scanning and transmission electron microscopy), and an investigation at the microscale of the atomic arrangements (by e.g. X-ray or electron diffraction, Auger electron spectroscopy, Rutherford backscattering). All this information may not result simply in a "wear rate", which is in most cases almost useless for practical situations, as wear is a dynamic process continuously evolving with time. It should essentially result in a better understanding of the mechanisms of friction and wear. The challenge of laboratory testing lies then in turning the "'trial and error approach", that is still nowadays frequently the best way to solve industrial wear problems, into a "knowing without testing" that would allow
ZP. Celis/Surface and Coatings Technology 74-75 (1995) 15 22 Inforrnadon on Indhddual Materials
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an optimized design of parts and selection of materials (see Fig. 3). With the introduction of coated parts in wear systems, the complexity of the system to be analysed increases markedly. The combination of materials having totally different mechanical and thermal properties, as well as the presence of an interface between coating and substrate, results in complex mechanical systems. The evolution of the mechanical response under cycling contact conditions for such a system is difficult to analyse even when using currently best-performing finite element analysis methods. The appeal to laboratory testing and the scientifically based analysis of test data are thus necessary working tools for the near future to ensure the enhancement of engineering systems in terms of performance and safety.
3. Selection of the wear testing mode
The evaluation of materials in view of wear applications can be done either by destructive or by nondestructive methods. The non-destructive method is clearly the less accessible one, as it requires much input and know-how to develop trivalent relationships between the production, structure and functionality of materials. Two prerequisites have to be fulfilled: the basic understanding of the relevant wear mechanisms, and the determination of the relevant structural properties of the materials. An illustration of that approach is given in Fig. 4 for the case of the development of cobalt-hardened gold coatings on electrical connectors [8,9]. Based on an intensive test programme, the wear mechanism was identified: the coating composition and the testing parameters selected must allow for the formation of a lubricious polymeric cobalt compound in the contact zone. Depending on the
Fig. 4. Illustration of the principle of non-destructive evaluation of the wear performance of cobalt-hardened gold layers on connectors
[8].
ratio between polymeric cobalt and metallic cobalt, either brittle layers (at low ratio of polymeric to metallic cobalt content) are obtained, which suffer from abrasive wear, or too soft layers (at low content of metallic cobalt) are obtained exhibiting an adhesive wear. The differentiation between these material[ states could be easily done based on the level of internal stress present in the coatings. This has opened the way to an on-line quality control and to the optimization of the production of coatings based on the determination of the lattice parameter by X-ray diffraction [ 10]. In the destructive testing of coated systems one can differentiate between single-pass and multiple-pass tests. The first allow, for example, the determination of the C O F and the bond strength, while the second result in information on the evolution of the C O F and the wear process over the testing time. The selection of a specific type of multiple-pass testing condition should be dictated by the contact conditions in the given application. The three testing modes that can be differentiated are summarized in Table 2. A good selection and control of the "real" testing conditions is of major importance to guarantee a valuable evaluation of the test data. In the destructive testing mode, the evaluation of wear data generated on-line or off-line must in any case justify the effort and cost done to test materials combinations. The open literature offers much information on wear mechanisms, wear models, microstructural and macrostructural modelling of materials. The following examples are intended to illustrate how a judicious use of such a scientifically based systems approach can result
18
J.P. Celis/Surjace and Coatings Technology 74 75 (1995) 15-22
Table 2 Overview of the destructive wear testing modes Mode 1 Characteristics of the contact conditions constant speed unidirectional constant load Possible testing procedures pin-on-disk, ball-on-disk permanent loading (in case of coated pin or ball ) periodic loading (in case of coated disk) block-on-wheel permanent loading (in case of coated block) Mode II Characteristics of the contact conditions varying speed reciprocating constant load Possible testing procedure pin-on-flat, ball-on-flat permanent loading (in case of coated pin or ball) periodic loading (in case of coated flat) fretting (small displacement amplitude) permanent loading Mode III Characteristics of the contact conditions impact load constant impinging rate varying Impinging rate Possible testing procedures erosion test dry or wet
in an added value of simple laboratory wear testing programmes for field practice.
4. Pin-on-disk testing of ceramic coatings on steel in view of their use for cutting operations Typical on-line data obtained from pin-on-disk tests are the evolution of the coefficient of friction and the total displacement at the contacting zone. Fig. 5 shows the case of TiN-based coatings sliding against steel balls. A characterization of the wear track by microscopic methods revealed the occurrence of two well-defined wear modes: high loads applied to rough coatings result in brown iron oxide debris covering the whole track (referred as here mode 1), while low loads applied to smooth coatings lead to a shiny surface and a kind of polishing wear with the creation of titanium oxide (referred to as mode 2). The understanding of why these two modes occur was derived from applying the Greenwood-Williamson model of stress fields of rough surfaces in contact [11]. A "plasticity index" 7, was proposed by these authors as a parameter combining the material and topographic properties of two rough solids in contact. The index merely determines the critical
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load at which the deformation in the contact zone changes from elastic to plastic. Extension of this model for coated system can be done by introducing the hardness area mixture model of JOnsson Hogmark [ 12]. In this way, a roughness-load diagram can be derived for each asperity radius, giving ~u isolines for a coating of a given thickness (see Fig. 6) [ 13,14]. This leads to the condition that if the plasticity index is bigger than 1, a transfer layer from the steel ball is formed. This is then a "failure test" rather than a wear test, and corresponds to mode 1 wear. If the plasticity index is smaller than 0.6, the contact on the TiN asperities is elastic and the coating will fail due to fatigue-induced delamination. The characteristics of this type of wear are polishing of the TiN coating, and fast oxidation of the fine wear particles, resulting in titanium oxides. This is a situation resulting in mode 2 wear. Based on this understanding, test parameters for pinon-disk experiments can now be chosen more adequately in order to evoke only coating wear. For mode 1 wear, a modelling of the displacement curve in the pin-on-disk test can also be done based on the particle flux equation proposed by Pendlebury [ 15]. Such a model is largely empirical. It is based on a mechanistic approach of wear particles created in the contact (formation rate F), residing there for a while, and escaping from the contact zone (escape rate E) or adhering to the rotating disk (adhesion rate A). Fig. 7 visualizes in these terms the mode 1 wear process. Application of this model to the displacement curve of steel against steel (Fig. 8) is a way of assessing the reproducibility of the pin-on-disk test, and allows one to quantify the F A - E parameters. Obviously, a great number of sampled points increase the amount of available information, so that the F - A - E equation values better represent the displacement curve. A sampling rate corresponding to 1024 points per 50min (=0.34Hz) appears as a reasonable compromise between file size considerations and measuring accuracy.
19
J P. Celis/Surface and Coatings Technology 74-75 (1995) 15 22
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oriented approach making use of the principles of tribochemistry has proven its validity in field tests linked to laboratory wear tests [ 13]. The introduction of predictions from phase diagrams, such as e.g. a quaternary diagram A 1 - T i - N - O as calculated by Singer [ 16], is in that respect an interesting support towards the understanding of surface layer formation and the estimation of real contact conditions from a materials point of view.
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This example shows how an in-depth analysis of experimental data easily obtainable in laboratory wear tests provides added value, going much further than a simple "wear rate" approach. The load/roughness dependence shown can now be the central guideline for selecting load and roughness conditions so that either a transfer layer builds up or substantial coating wear is produced. In the case of cutting tools, a materials-
A frequently occurring source of unexpected failures in industrial parts is material loss or cracking in vibrating contacts. Such vibrations can be induced by cyclic accelerations, cyclic stresses, acoustical noise, or by thermal cycling. It has been identified that 20% of failures in aerospace turbine engines are linked to a material degradation in such oscillating contact zones, of which the amplitude can even be in the submicron range. The introduction of hard coatings was expected to be the ideal solution to that problem. Field experience has, however, shown the occurrence of unexpected failure under such conditions. Actual insight in the deterioration mechanisms of hard coatings used as protective layers in oscillating contacts is so limited that a modelling in view of a knowledge-based design of tribological components with respect to improved vibrational resistance is yet far from being available.
,L P. Celis/Surface and Coatings Technology' 74-75 (1995) 15-22
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Fretting-wear lab testing can help in identifying the major parameters influencing the deterioration of hard coatings in such vibrating contacts, and in developing in-situ monitoring techniques for the detection of such wear initiation and changes in wear processes at different stages of the failure development. As in the case of pinon-disk testing, one can rely on some valuable modelling work related to fretting available in the literature [-17,18]. Fretting has been shown to be very sensitive to normal load, relative displacement, frequency, and chemical environment as well as to materials couples. Laboratory simulation of fretting relies then on applying a simplified, and thus more controllable, combination of fretting parameters to a given contact. In that respect two testing modes can be used, namely displacement and load-induced fretting [-19]. In the first case, fretting vibrations are generated by oscillating a linear relative displacement of constant stroke between the contacting bodies or, as in the second case, by oscillating the applied contact load, resulting in a cyclic radial expansion of the contact. In this second case, the measurement of the tangential force is not possible but the analysis of the contact mechanics relies on hertzian theory. Most actual testing equipment is based on displacement induced fretting• Under such testing conditions, on-line
measurements are generally the tangential force and the real displacement amplitude. The analysis of the fretting wear consists then in the interpretation of fretting maps obtained by displaying for two given fretting parameters the variation of the fretting-wear mode. With respect to fretting-wear mode, three regimes can be identified: stick, partial slip, and gross slip conditions. Where partial slip results in a fatigue-linked crack formation, gross slip will, in contrast, cause a progressive material loss by abrasive wear. This two concepts of fretting maps, namely running condition fretting maps and material response fretting maps, are at present the basis of a thorough analysis of the fretting mechanisms by which surface degradation occurs. The following examples are intended to illustrate this. The tribological behaviour of CVD diamond coatings in oscillating contact with a corundum ball is illustrated in Fig. 9 [20]. In this material couple, the three wear modes appear successively: during the running-in period the tangential force versus displacement displays a closed loop indicative of a partial slip fretting condition. The high friction forces are partly due to the high roughness of the coating and result in high localized stresses, sufficient to induce some by a surface fatigue process at
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the contacting asperities. This running-in period is followed by a transition period of asperity fracturing, during which gross slip is progressively developed. This gross slip results from the creation of debris trapped in the remaining surface valleys. Finally the amount of debris becomes large enough to separate the contacting bodies, and we enter in the third, body-controlled, regime where low tangential forces are noticed. In view of this fretting process, one can start developing diamond coatings exhibiting the desired fretting wear performance. So, for example, one can expect that high compressive
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stresses in CVD diamond coatings can have a beneficial influence in preventing the initiation and propagation of fatigue cracks. The progress of gross slip fretting wear in material couples consisting of PVD TiN coatings and 100Cr steel balls can be easily quantified in a fretting test as shown in Fig. 10 [21]. The fretting wear is in the first stage confined to the TiN coating and extends mainly into the depth. During that stage, the wear volume increases linearly with the number of cycles, and fine white debris is formed. Structural analysis of this debris can easily be done in situ by Raman spectroscopy, and reveals the presence of non-crystalline titanium oxides, being a mixture of anatase and futile. Once the interface with the steel substrate is reached, a lateral extension of the wear track due to abrasive wear is noticed. This progressive extension of fretting wear of hard coatings is interesting, as it allows a quantitative comparison between different types of coating or for a given coating but deposited using different conditions [Fig. 11]. From this figure it is obvious that diamondlike carbon (DLC) coatings are more resistant to fretting than the PVD TiN coating, although their hardness is not always superior. The reason for the slow progress of fretting wear on DLC compared with TiN should be understood in terms of the operative wear mechanisms: on TiN sliding causes tribooxidation of TiN to TiO2-x; on DLC the wear debris formed consists of lubricating, graphitic forms of carbon. Based on such information, it thus becomes possible to develop the trivalent relationship production-structure functionality, and optimize the performance of coated parts.
6. Conclusion
The full benefit of laboratory wear testing can be obtained when an analysis of experimental data based
22
J.P. Celis/SuiJbee and Coatings Technology 74 75 (1995) 15 22
of Texas (Dallas) in providing samples is highly appreciated. This research was supported by the Ministry of Science Policy under JUAP Contract No. 041.
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Fig. 11. Comparative fretting wear progress for TiN coatings and different diamond-like carbon coatings produced by different techniques. The tests were done under gross slip conditions, with a corundum counterbody of 10mm diameter, and in ambient air of 70%-80% relative humidity.
[7] [8]
on comprehensive models is done. It becomes then part of a global strategy to understand the failure mechanism of wear-retarding coatings for dedicated field applications. It gives a thinking framework that allows a structured approach to the problem by relating field tests to laboratory tribological tests. A more complete understanding of mechano-chemical interactions of two sliding bodies with their environment should be one of the challenges and major issues in modern tribology concepts.
[10]
Acknowledgments The author would like to thank his Ph.D. coworkers, who contributed to the different research topics briefly referred to in this text, and especially B. Blanpain, W. Van Vooren, E. Vancoille and H. Mohrbacher. The assistance of Siemens (Oostkamp), Gt~hring (Sigmaringen), Hauzer (Venlo), WTCM (Diepenbeek), RWTH (Aachen), VITO (Mol), and the University
[9]
Ell] [12]
[13] [14] [15] [16] [17] [18]
[19]
[20] [21]
D. Dowson, History of Tribology, Longman, London, 1979. M. Godet, Surf. Coat. Technol., 45 (1991) 1--8. M.N. Gardos, MRS Syrup. Proc., 140 (1989) 325. H. Mohrbacher, B. Blanpain, J.-P. Celis and J.R. Roos, Wear, 180(1995) 43 52. H. Czichos, Tribology, a Systems Approach to the Science and Technology qf Friction, Lubrication and Wear, Tribology Series, 1, Elsevier, Amsterdam, 1978. H. Czichos, Systematic diagnosis of friction and wear test data, in P. Blau (ed.), ASM Handbook, vol. 18, 1992, pp. 473 493. S. Lim and M. Ashby, Acta Metall., 35 (1987) 1 24. M. De Bonte, J.-P. Celis, J.R. Roos and J. Vanhumbeeck, in Berichtsband EAST Precious Metals: Modern Technologies and Applications, E. Leuze Verlag, Saulgau, 1992, pp. 81 87. J.R. Roos, J.-P. Celis and M. De Bonte, Electrodeposition of metals and alloys, in R.W. Cahn (ed.), Materials Science and Technology: a Comprehensive Treatment, 1991, pp. 482-537. K. Van Acker, L. De Buyser, J.-P. Celis and P. Van Houtte, J. Appl. Crystallogr., 27 (1994) 56-66. J. Greenwood and J. Williamson, Proc. R. So~. London, A295 (1966) 300 319. O. Vingsbo, S. Hogmark, B. J6nsson and A. Ingemarsson, in ASTM STP 889, American Society for Testing and Materials, Philadelphia, 1986, pp. 257 271. E. Vancoille, Ph.D. thesis, Katholieke Universiteit Leuven, Dept. MTM, 1993. B. Matliet, J.-P. Celis, J.R. Roos, L. Stals and M. Van Stappen, Wear, 142 (1991) 151 170. R. Pendlebury, Wear, 125 (1988) 3-23. I. Singer, Surf. Coat. Technol., 49 (1991) 474--481. M. Godet, Y. Berthier, J. Lancaster and L. Vincent, Wear, 149 ( 1991 ) 325 340. O. Vingsbo, Fretting and Contact Fatigue studied with the Aid of Fretting Maps, American Society for Testing and Materials, Philadelphia, PA, in press. H. Mohrbacher, J.-P. Celis and J.R. Roos, On the laboratory testing of displacement and load induced fretting, Tribology Int., in press. B. Blanpain, H. Mohrbacher, J.-P. Cells and J.R. Roos, Diamond Films Technol., 3, (1994) 177 188. H. Mobrbacher, Brite/Euram project No. BRE2-CT92-0224, internal progress report, July 1994.
Surface and Coatings Technology 74-75 (1995) 15 22
A systems approach to the tribological testing of coated materials J.P. Celis Dept. MTM, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium
Abstract This paper is an attempt to illustrate how laboratory wear tests combined with an in-depth material characterization and interpretation based on models present in literature can support the use of advanced wear protection coatings. Therefore it is necessary to define what is meant by tribology and what the definitions are of wear and friction. The following step is a structured approach for determining the tribological characteristics of coated materials. Some considerations are made about the choice of the experimental set-up for testing coated materials. A framework for situating and describing the wear modes noticed is discussed based on existing wear models. This is illustrated by a discussion of laboratory wear test results on electrolytic hard gold, ceramic TiN-based coatings and hard carbon-based coatings.
Keywords: Tribology; Coatings; Laboratory tests; Wear; Tribochemistry
I. Tribology: a multidiseiplinary approach The tribological characteristics of a material are determined by properties related to its sliding and rolling behaviour against any other material. In general, friction and wear processes occur jointly when two surfaces undergo sliding or rolling under load. Wear is defined as the progressive loss of a substance from the operating surface of a b o d y occurring as a result of the relative m o t i o n between surfaces [ 1 ] . This progressive loss of substance can result from a mechanical interaction at a n d / o r a chemical reaction induced between two contacting surfaces. D y n a m i c friction is defined as the resistance to a relative m o t i o n of contacting bodies and can be quantitatively represented as the coefficient of friction (COF), being the ratio of the resulting tangential and applied n o r m a l force (as far as stick or partial slip conditions are not prevailing). The basic laws for friction and wear were developed some centuries ago by L e o n a r d o da Vinci (1495), A m o n t o n s (1706), Euler (1750) and C o u l o m b (1785). Tribology, being the unified a p p r o a c h of friction, adhesion, wear and lubrication, is by contrast quite a y o u n g science, born in 1966 when the O E C D recognized the important economic savings related to a reduction of friction and wear. An important point out of these definitions is the fact that wear is not a materials property, but a systems property! Tribology definitely needs a multidisciplinary approach, which is best reflected by the type of information that people with 0257-8972/'95/$09.50© 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(95)08211-5
different interests require when dealing with friction and wear (see Table 1). Focusing now our interest on films and coatings, it is worth remembering the nice statement of the late Professor Godet: "A coating without thermomechanical characteristics is as useless as a c o m p u t e r without a manual" [-2]. Indeed, if one z o o m s in on what happens in the contact zone between two bodies, the existence of Table 1 Tribology: a multidisciplinary approach Information required by the mechanical engineer Coefficient of friction, wear rate Young's modulus (evaluated from 4-point bend test) Poisson's ratio (evaluated from 4-point bend test) Coefficient of thermal expansion Thermal conductivity, density, specific heat Tensile strength, fatigue limit Information required by the tribologist Stress and temperature fields Load-carrying process - used to separate the first bodies as characteristics of third bodies Information required by the materials scientist Composition, hardness Matrix-to-second phase particles adhesion Crystallographic texture, internal stress profile Information required by the surface scientist Deposition process, thickness coating Hardness coating/substrate system Coating-to-substrate adhesion, interface
J. JR Celis/Surjitce and Coatings" Technology 74 75 (1995) 15 22
16
a dynamic system with high complexity becomes evident. From the materials point of view, the mechanical and chemical reactivity induce large modifications in the starting materials, as shown schematically in Fig. 1, while the contact conditions also evolve with the formation of a third body resulting from an interaction between the first two bodies and the surrounding atmosphere (Fig. 2). This last observation has been the starting point for a recent evolution from wear considered as a matter of mechanics to a matter of materials and surface science. Indeed the preponderant role of tribochemistry is being recognized more and more. The definition of tribochemistry is the study of the occurrence, acceleration or modification of chemical reactions caused by the friction between two surfaces. Aspects of dissipation of frictional energy in the contacts, the exposure and reactivity of bare metal surfaces, and the elastic and plastic deformation in sliding bodies play a large role in that respect. The challenge of tribochemistry in the near future is surely to indicate ways to turn material losses into a material protection. Optimization of wear behaviour can indeed be based on a closer look at the chemical surface conditions of materials, as was done by Gardos [3] investigating the effect of oxygen content of non-stoichiometric titanium oxides on the COF, and by Mohrbacher [-4] demonstrating the effect of relative humidity on the COF of TiN sliding under unlubricated conditions against steel. The importance of these findings is high in view of the trend towards avoiding lubrication by oils as imposed by recent environmental legislation and the related need for
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2. Wear testing is more than "wear" and "wear rate"
People involved with any of the different aspects of tribology are always puzzled by the problem of evaluating the wear performance of a given combination of first-body materials being operated under a set of environmental and machining conditions. Wear testing on a laboratory scale is certainly the most attractive solution, but it contains a lot of limitations, and can be performed at different levels of sophistication as discussed by Czichos [5,6]. The basic question is, in fact, what do people expect from wear testing? One could be interested in a comparative investigation of different kinds of materials, such as metals, polymers, ceramics, cermets, and coated materials; or in the investigation of the aggressiveness of a system; or in an attempt to extrapolate to lifetime and behaviour under field conditions. In any case, as long as a full simulation of the field practice cannot be done, the use of laboratory data will always be subjected to a risky exercise of extrapolation! However, this does not mean that laboratory testing is completely useless. Lab testing becomes fully relevant when used for unravelling the active wear mechanisms, for understanding the wear process dependence on testing parameters as was done by Ashby [7] in terms of wear maps, and finally for developing trivalent relationships between the production of materials, their structural characteristics, and their functional behaviour. In that respect, wear testing requires two types of information: that which can be obtained as on-line information, namely the COF, the surface contact temperature, the acoustic noise, etc.; and that from an off-line investigation of the surfaces after testing. With respect to wear loss, one can differentiate between simple material loss (by direct measurement of the weight change or indirectly by profilometry and metallography), investigation at a macroscale of the surface topography (by e.g. optical microscopy, atomic force microscopy, scanning and transmission electron microscopy), and an investigation at the microscale of the atomic arrangements (by e.g. X-ray or electron diffraction, Auger electron spectroscopy, Rutherford backscattering). All this information may not result simply in a "wear rate", which is in most cases almost useless for practical situations, as wear is a dynamic process continuously evolving with time. It should essentially result in a better understanding of the mechanisms of friction and wear. The challenge of laboratory testing lies then in turning the "'trial and error approach", that is still nowadays frequently the best way to solve industrial wear problems, into a "knowing without testing" that would allow
ZP. Celis/Surface and Coatings Technology 74-75 (1995) 15 22 Inforrnadon on Indhddual Materials
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an optimized design of parts and selection of materials (see Fig. 3). With the introduction of coated parts in wear systems, the complexity of the system to be analysed increases markedly. The combination of materials having totally different mechanical and thermal properties, as well as the presence of an interface between coating and substrate, results in complex mechanical systems. The evolution of the mechanical response under cycling contact conditions for such a system is difficult to analyse even when using currently best-performing finite element analysis methods. The appeal to laboratory testing and the scientifically based analysis of test data are thus necessary working tools for the near future to ensure the enhancement of engineering systems in terms of performance and safety.
3. Selection of the wear testing mode
The evaluation of materials in view of wear applications can be done either by destructive or by nondestructive methods. The non-destructive method is clearly the less accessible one, as it requires much input and know-how to develop trivalent relationships between the production, structure and functionality of materials. Two prerequisites have to be fulfilled: the basic understanding of the relevant wear mechanisms, and the determination of the relevant structural properties of the materials. An illustration of that approach is given in Fig. 4 for the case of the development of cobalt-hardened gold coatings on electrical connectors [8,9]. Based on an intensive test programme, the wear mechanism was identified: the coating composition and the testing parameters selected must allow for the formation of a lubricious polymeric cobalt compound in the contact zone. Depending on the
Fig. 4. Illustration of the principle of non-destructive evaluation of the wear performance of cobalt-hardened gold layers on connectors
[8].
ratio between polymeric cobalt and metallic cobalt, either brittle layers (at low ratio of polymeric to metallic cobalt content) are obtained, which suffer from abrasive wear, or too soft layers (at low content of metallic cobalt) are obtained exhibiting an adhesive wear. The differentiation between these material[ states could be easily done based on the level of internal stress present in the coatings. This has opened the way to an on-line quality control and to the optimization of the production of coatings based on the determination of the lattice parameter by X-ray diffraction [ 10]. In the destructive testing of coated systems one can differentiate between single-pass and multiple-pass tests. The first allow, for example, the determination of the C O F and the bond strength, while the second result in information on the evolution of the C O F and the wear process over the testing time. The selection of a specific type of multiple-pass testing condition should be dictated by the contact conditions in the given application. The three testing modes that can be differentiated are summarized in Table 2. A good selection and control of the "real" testing conditions is of major importance to guarantee a valuable evaluation of the test data. In the destructive testing mode, the evaluation of wear data generated on-line or off-line must in any case justify the effort and cost done to test materials combinations. The open literature offers much information on wear mechanisms, wear models, microstructural and macrostructural modelling of materials. The following examples are intended to illustrate how a judicious use of such a scientifically based systems approach can result
18
J.P. Celis/Surjace and Coatings Technology 74 75 (1995) 15-22
Table 2 Overview of the destructive wear testing modes Mode 1 Characteristics of the contact conditions constant speed unidirectional constant load Possible testing procedures pin-on-disk, ball-on-disk permanent loading (in case of coated pin or ball ) periodic loading (in case of coated disk) block-on-wheel permanent loading (in case of coated block) Mode II Characteristics of the contact conditions varying speed reciprocating constant load Possible testing procedure pin-on-flat, ball-on-flat permanent loading (in case of coated pin or ball) periodic loading (in case of coated flat) fretting (small displacement amplitude) permanent loading Mode III Characteristics of the contact conditions impact load constant impinging rate varying Impinging rate Possible testing procedures erosion test dry or wet
in an added value of simple laboratory wear testing programmes for field practice.
4. Pin-on-disk testing of ceramic coatings on steel in view of their use for cutting operations Typical on-line data obtained from pin-on-disk tests are the evolution of the coefficient of friction and the total displacement at the contacting zone. Fig. 5 shows the case of TiN-based coatings sliding against steel balls. A characterization of the wear track by microscopic methods revealed the occurrence of two well-defined wear modes: high loads applied to rough coatings result in brown iron oxide debris covering the whole track (referred as here mode 1), while low loads applied to smooth coatings lead to a shiny surface and a kind of polishing wear with the creation of titanium oxide (referred to as mode 2). The understanding of why these two modes occur was derived from applying the Greenwood-Williamson model of stress fields of rough surfaces in contact [11]. A "plasticity index" 7, was proposed by these authors as a parameter combining the material and topographic properties of two rough solids in contact. The index merely determines the critical
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load at which the deformation in the contact zone changes from elastic to plastic. Extension of this model for coated system can be done by introducing the hardness area mixture model of JOnsson Hogmark [ 12]. In this way, a roughness-load diagram can be derived for each asperity radius, giving ~u isolines for a coating of a given thickness (see Fig. 6) [ 13,14]. This leads to the condition that if the plasticity index is bigger than 1, a transfer layer from the steel ball is formed. This is then a "failure test" rather than a wear test, and corresponds to mode 1 wear. If the plasticity index is smaller than 0.6, the contact on the TiN asperities is elastic and the coating will fail due to fatigue-induced delamination. The characteristics of this type of wear are polishing of the TiN coating, and fast oxidation of the fine wear particles, resulting in titanium oxides. This is a situation resulting in mode 2 wear. Based on this understanding, test parameters for pinon-disk experiments can now be chosen more adequately in order to evoke only coating wear. For mode 1 wear, a modelling of the displacement curve in the pin-on-disk test can also be done based on the particle flux equation proposed by Pendlebury [ 15]. Such a model is largely empirical. It is based on a mechanistic approach of wear particles created in the contact (formation rate F), residing there for a while, and escaping from the contact zone (escape rate E) or adhering to the rotating disk (adhesion rate A). Fig. 7 visualizes in these terms the mode 1 wear process. Application of this model to the displacement curve of steel against steel (Fig. 8) is a way of assessing the reproducibility of the pin-on-disk test, and allows one to quantify the F A - E parameters. Obviously, a great number of sampled points increase the amount of available information, so that the F - A - E equation values better represent the displacement curve. A sampling rate corresponding to 1024 points per 50min (=0.34Hz) appears as a reasonable compromise between file size considerations and measuring accuracy.
19
J P. Celis/Surface and Coatings Technology 74-75 (1995) 15 22
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oriented approach making use of the principles of tribochemistry has proven its validity in field tests linked to laboratory wear tests [ 13]. The introduction of predictions from phase diagrams, such as e.g. a quaternary diagram A 1 - T i - N - O as calculated by Singer [ 16], is in that respect an interesting support towards the understanding of surface layer formation and the estimation of real contact conditions from a materials point of view.
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This example shows how an in-depth analysis of experimental data easily obtainable in laboratory wear tests provides added value, going much further than a simple "wear rate" approach. The load/roughness dependence shown can now be the central guideline for selecting load and roughness conditions so that either a transfer layer builds up or substantial coating wear is produced. In the case of cutting tools, a materials-
A frequently occurring source of unexpected failures in industrial parts is material loss or cracking in vibrating contacts. Such vibrations can be induced by cyclic accelerations, cyclic stresses, acoustical noise, or by thermal cycling. It has been identified that 20% of failures in aerospace turbine engines are linked to a material degradation in such oscillating contact zones, of which the amplitude can even be in the submicron range. The introduction of hard coatings was expected to be the ideal solution to that problem. Field experience has, however, shown the occurrence of unexpected failure under such conditions. Actual insight in the deterioration mechanisms of hard coatings used as protective layers in oscillating contacts is so limited that a modelling in view of a knowledge-based design of tribological components with respect to improved vibrational resistance is yet far from being available.
,L P. Celis/Surface and Coatings Technology' 74-75 (1995) 15-22
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Fretting-wear lab testing can help in identifying the major parameters influencing the deterioration of hard coatings in such vibrating contacts, and in developing in-situ monitoring techniques for the detection of such wear initiation and changes in wear processes at different stages of the failure development. As in the case of pinon-disk testing, one can rely on some valuable modelling work related to fretting available in the literature [-17,18]. Fretting has been shown to be very sensitive to normal load, relative displacement, frequency, and chemical environment as well as to materials couples. Laboratory simulation of fretting relies then on applying a simplified, and thus more controllable, combination of fretting parameters to a given contact. In that respect two testing modes can be used, namely displacement and load-induced fretting [-19]. In the first case, fretting vibrations are generated by oscillating a linear relative displacement of constant stroke between the contacting bodies or, as in the second case, by oscillating the applied contact load, resulting in a cyclic radial expansion of the contact. In this second case, the measurement of the tangential force is not possible but the analysis of the contact mechanics relies on hertzian theory. Most actual testing equipment is based on displacement induced fretting• Under such testing conditions, on-line
measurements are generally the tangential force and the real displacement amplitude. The analysis of the fretting wear consists then in the interpretation of fretting maps obtained by displaying for two given fretting parameters the variation of the fretting-wear mode. With respect to fretting-wear mode, three regimes can be identified: stick, partial slip, and gross slip conditions. Where partial slip results in a fatigue-linked crack formation, gross slip will, in contrast, cause a progressive material loss by abrasive wear. This two concepts of fretting maps, namely running condition fretting maps and material response fretting maps, are at present the basis of a thorough analysis of the fretting mechanisms by which surface degradation occurs. The following examples are intended to illustrate this. The tribological behaviour of CVD diamond coatings in oscillating contact with a corundum ball is illustrated in Fig. 9 [20]. In this material couple, the three wear modes appear successively: during the running-in period the tangential force versus displacement displays a closed loop indicative of a partial slip fretting condition. The high friction forces are partly due to the high roughness of the coating and result in high localized stresses, sufficient to induce some by a surface fatigue process at
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the contacting asperities. This running-in period is followed by a transition period of asperity fracturing, during which gross slip is progressively developed. This gross slip results from the creation of debris trapped in the remaining surface valleys. Finally the amount of debris becomes large enough to separate the contacting bodies, and we enter in the third, body-controlled, regime where low tangential forces are noticed. In view of this fretting process, one can start developing diamond coatings exhibiting the desired fretting wear performance. So, for example, one can expect that high compressive
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stresses in CVD diamond coatings can have a beneficial influence in preventing the initiation and propagation of fatigue cracks. The progress of gross slip fretting wear in material couples consisting of PVD TiN coatings and 100Cr steel balls can be easily quantified in a fretting test as shown in Fig. 10 [21]. The fretting wear is in the first stage confined to the TiN coating and extends mainly into the depth. During that stage, the wear volume increases linearly with the number of cycles, and fine white debris is formed. Structural analysis of this debris can easily be done in situ by Raman spectroscopy, and reveals the presence of non-crystalline titanium oxides, being a mixture of anatase and futile. Once the interface with the steel substrate is reached, a lateral extension of the wear track due to abrasive wear is noticed. This progressive extension of fretting wear of hard coatings is interesting, as it allows a quantitative comparison between different types of coating or for a given coating but deposited using different conditions [Fig. 11]. From this figure it is obvious that diamondlike carbon (DLC) coatings are more resistant to fretting than the PVD TiN coating, although their hardness is not always superior. The reason for the slow progress of fretting wear on DLC compared with TiN should be understood in terms of the operative wear mechanisms: on TiN sliding causes tribooxidation of TiN to TiO2-x; on DLC the wear debris formed consists of lubricating, graphitic forms of carbon. Based on such information, it thus becomes possible to develop the trivalent relationship production-structure functionality, and optimize the performance of coated parts.
6. Conclusion
The full benefit of laboratory wear testing can be obtained when an analysis of experimental data based
22
J.P. Celis/SuiJbee and Coatings Technology 74 75 (1995) 15 22
of Texas (Dallas) in providing samples is highly appreciated. This research was supported by the Ministry of Science Policy under JUAP Contract No. 041.
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References
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Fig. 11. Comparative fretting wear progress for TiN coatings and different diamond-like carbon coatings produced by different techniques. The tests were done under gross slip conditions, with a corundum counterbody of 10mm diameter, and in ambient air of 70%-80% relative humidity.
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on comprehensive models is done. It becomes then part of a global strategy to understand the failure mechanism of wear-retarding coatings for dedicated field applications. It gives a thinking framework that allows a structured approach to the problem by relating field tests to laboratory tribological tests. A more complete understanding of mechano-chemical interactions of two sliding bodies with their environment should be one of the challenges and major issues in modern tribology concepts.
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Acknowledgments The author would like to thank his Ph.D. coworkers, who contributed to the different research topics briefly referred to in this text, and especially B. Blanpain, W. Van Vooren, E. Vancoille and H. Mohrbacher. The assistance of Siemens (Oostkamp), Gt~hring (Sigmaringen), Hauzer (Venlo), WTCM (Diepenbeek), RWTH (Aachen), VITO (Mol), and the University
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