Model rough surfaces in elastohydrodynamic lubrication

Thin Sohd Ftlms, 96 (1982) 1-8 METALLURGICAL AND PROTECTIVE COATINGS

M O D E L R O U G H SURFACES I N E L A S T O H Y D R O D Y N A M I C LUBRICATION * S. DE SILVA'I', J. C ANDERSON AND J A LEATHER~ Electrwal Engmeerlng Department, Impertal College, London SW7 2BT (Gt Brttam) (Received March 23, 1982, accepted Aprd 27, 1982)

Model rough surfaces have been produced on steel balls with a high (1 ~tin) surface finish by (a) sputter deposition o f c h r o m m m through a stencil-type mask and (b) laser milling of the steel surface. The technology for the chromium coatings has been developed to achieve sufficient adhesion to survive lubricated rolhng and sliding contact at a pressure of 100 000 l b f m -2. The laser milling technique was refined such that grooves of 10 lam wMth, spaced 10 Ixm apart, could be cut under microprocessor control. The main elastohydrodynamlc lubrication work has been on a machine in which the steel ball bears against a glass disc. The oil film interference colours in white hght in the hertzian contact area provide a measure of film thickness and the effect of grooves of different spacing and orientation has been examined for a range of loads, speeds and otis.

1. INTRODUCTION

The topography of engineering surfaces plays a major part in the lubrication process 1, m the fatigue life of bearings and gears and m wear and related phenomena. The adhesion of two surfaces is dependent on their ability to conform to the appropriate geometry and is therefore also a topographical effect 2' 3. The theoretical analysis of the contact of rough lubricated surfaces is limited by ignorance of the surface topography and of the behaviour of small surface features under contact conditions. In the present work methods of producing surfaces with a controlled topography have been developed and applied to elastohydrodynamic (EHD) studies using optical interferometry.

* Paper presented at the International Conference on Metallurgical Coatings and Process Technology, San Diego, CA, U S A, April 5-8, 1982 t Present address Bureau of Ceylon Standards, 53 Dharmapala Mawatha, Colombo 3, Sn Lanka :~Present address Thornton Research Centre, P.O Box 1, Chester CH1 3SH, Gt Britain 0040-6090/82/0000-0000/$02 75

© ElsevierSequola/Pnnted m The Netherlands

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s. DE SILVA, J. C. ANDERSON, J. A. LEATHER

DEPOSITED SURFACES

A technique for producing fine metal foil stencil masks was developed 4 using a laser resistor trimming machine modified by the addition of a microprocessorcontrolled x - y table. From E H D considerations the requirements for the desired surfaces were (1) the amplitude parameter, i.e. a m a x i m u m peak-to-valley height of 0 02-2.5 pm, (2) the spatial parameter, t e. a wavelength of 10-200 gm, and (3) the asperity dimensions, t.e. a m a x i m u m width at base of 10 gm or less In ad&tion the surfaces should be capable of withstanding both sliding and rolling conditions under high loading in the test machine, In order to achieve the required excellent adhesion of the deposited film to the steel balls to be used as substrates, chromium was chosen for the deposited metal and r.f. sputter deposition for the method. The steel balls were 1 In in diameter and polished to a 1 gin surface finish. They were cleaned ultrasonically in isopropyl alcohol (IPA) and vapour degreased also In IPA five or six times and finally sputter etched at 200 W for 30 min before deposition Great attention had to be paid to cleanliness in the sputtering plant, the purity and dryness of the argon used and the level of background vacuum, which was 10 _6 Torr with cold trapping. This technique produced deposits of chromium with a satisfactory adhesion, through molybdenum masks 25 jam thick. Deposition was carried out with 350 W into a target 5 in in diameter at a rate of 1-2 pm h 1 m a gas pressure of 4 mTorr. Two types of pattern were used, a two-dimensional one of parallel lines and a threedimensional one of dots. In Fig 1 is shown a parallel line deposit of chromium with a wavelength of 35 jam, a width of 4 jam and an asperity height of 1.3 jam. In Fig. 2 the lsotroplc asperities are of width 20 jam and height 8 jam and are spaced by 50 jam in one direction and 25 jam in the other

Fig 1 Two-dimensional deposited aspermes (wavelength, 35 jam, asperity width, 4 )am, asperity height, 1 3 jam) Fig 2 Pseudolsotroplc surface of three-dimensional deposited asperities (asperity width, 20 jam, spacing, 50 )am x 25 gin, height, 8 ~am)

The limitations of this technique are mainly that the curvatures and slopes of the asperities could not easily be controlled, since they depend on the uniformity of the mask-to-substrate spacing. A range of slopes could be achieved by varying the height-to-base width ratio, but m general a single well-defined slope was not achieved Figure 3 shows profile traces of the range of surfaces fabricated

3

MODEL SURFACES IN LUBRICATION

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ll/I 1/11III Illl Iil II fl lllll (b)

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(dl Fig 3. Profile traces showing the range of surfaces fabricated by sputter deposition (a) 2 = 25 p_m, m a x i m u m peak-to-valley height R, = 1 0 p_m, (b) 2 = 50 ~tm, R t = 6 pro, (c) 2 = 50 p~m, R t = 0 1 p.m, (d)) = 150pm, R, = 5 8 p m

The durabihty of the deposited films was such that no adhesion failure was seen even when the contact pressures were sufficient to cause considerable plastic deformation of them. 3. LASER-MILLED SURFACES

F o r an incident laser beam intensity of more than 1011 W m -z Ready 5 postulates a vaporization front, originating on the ~rradiated surface of the metal in a time tv, glven by tv

n Kpc

4 ~ 5 ( T v - To)2

(1)

where K, c and p are respectively the thermal conductivity, specific heat and denszty of the matertal, F is the incident laser flux density and Tv and To are the temperature of vaporization and the initial temperature. Generally t v is m u c h less than the 1 ms duration of the laser pulses p r o w d e d by the eqmpment. After the time tv the rate of material removal will a p p r o a c h a steady state g~ven by dM F dt

-

L + c ( T v -- To)

(2)

where L ~s the specific heat of vaporlzatzon per umt mass and M Is the mass of material. F o r the preparation of a surface, the steel ball was m o u n t e d on the x - y table in a m o u n t that allowed it to be rotated under the laser beam under control of a stepper

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s. DE SILVA, J. C. ANDERSON, J. A. LEATHER

motor driven from the control microprocessor. This allowed the ball surface to be kept at the focal point of the laser optical system as the ball was rotated to define the required cut. An approximate estimate of the depth of the grooves cut was initially obtained using eqns. (1) and (2). More accurate operating parameters were then determined by mllhng test specimens and calibrating by profilometry Two- and three-dimensional surfaces with r.m.s, roughness ranging from 0.05 to 2 5 Bm and minimum wavelength 10 gm were fabricated. A characteristic of laser cutting is the formation of a lip at the mouth of each groove as shown in Fig. 4(a), due to the redeposltion of melted material For lnterferometric measurements it was necessary to remove these extremely sharp features by lapping with diamond paste. Hard lapping with a 6 Bm paste followed by a 0.25 lam paste removed the asperities and produced the profile of Fig. 4(b). Soft lapping with 0.25 gm paste followed by a fine polishing fluid (Syton 30W) produced a surface of peaks which closed over the mouths of the grooves, owing to plastic flow of the lips, as shown m Fig. 4(c). An actual profile of a set of milled grooves is shown in Fig. 5 whilst a scanning electron micrograph of a pseudoisotropic set of milled holes is shown in Fig. 6 peaks removed

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Fig 4 Profiles of laser-milled grooves (a) as milled; (b) after hard lapping, (c) after soft lapping

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Fig 5 Profile of a set of laser-mdled grooves

MODEL SURFACES IN LUBRICATION

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4 INTERFEROMETRIC MEASUREMENTS Optical mterferometry is a well-established technique for determining hydrodynamic fluid film shapes 6 8, but its application to the study of relatively rough practical bearings has been hmited 9' to. For best definition of fringes the intensity of the interfering reflected rays should be nearly equal and this was achieved by coating the glass disc with a semitransparent layer of 200 ~ of chromium or chromic oxide, producing a reflectlwty of 15% in off. The reflectivity of the steel ball was about 68% in oil but that of the deposited asperities was much less and varied over the surface. To overcome this the roughened surface was coated overall with a 100/~ reflecting chromium layer. This produced an overall reflectivity of 51% and enabled continuous fringes to be obtained over the whole surface. The upper surface of the crown glass plate was coated with a quarter-wave layer (1000,~) of magnesmm fluoride as an ant~reflection coating. The best defimtion of interference fringes is obtained in monochromatic hght, but th~s limits the resolution to 2/4/~ where # ~s the refractwe index of the oil and 2 the wavelength of the source. In the present work a resolution of about 500 A was reqmred, which is too small for visible monochromatic radiation. Accordingly, white hght was used, which generates a definite

Fig 6 Scanning electron mlcrograph of a pseudolsotroplc laser-milled surface (hole diameter, 20 pm, closest spacing, 60 pro, hp height, 0 8/am)

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s. DE SILVA, J C. ANDERSON, J. A. LEATHER

sequence of closely spaced coloured fringes After about eight or ten fringes so m a n y colours are present at one point that the resultant hue ts wrtually white and the fringes are lost. Th~s hmits the off film thickness that can be observed to a m a x i m u m of 1 lam, which is adequate for observing the effects of surface roughness which are greatest where the film thickness ts less than 1 ~tm. The mechanical ng in which the lnterferometric system was mounted is shown schematically iv Ftg. 7 The ball was loaded against the plate by means of an air cyhnder and p~ston which produced hertzmn pressures of up to 720 M Pa (about 100 000 lbf an 2) In rolling experiments the ball was driven through a chuck and spindle attached to the ball surface. In shding experiments either the ball or the plate was drtven by Independent motors. Their speeds were m o m t o r e d by means of IR optocouplers The full system has been described m detad by Sayles et al.~ 5

RESULTS

Unfortunately the practical results, m the form of photographs, requtre colour printing to show the effects produced. In every case a rough and a smooth surface were photographed under the same conditions and the interference colours were used to monitor the variations m od film thickness between the two cases. A pa~r of black and white photographs ~s shown m Fig. 8 for a surface with hne asperities transverse to the direction of rolling, which gives some ~mpression of the effects of roughness

(a) (b) Fig 8 (a) Smooth surface mterferogram, (b) rough surface having transverse grooves (2 = 30 gm)

5 1 Rolhng wzth transverse roughness Ten specimens with parallel grooves oriented perpendicular to the direction of rolling were tested Wavelengths of 10-100 ~tm were available, and differing values of load and speed were used. The r.m.s surface roughness and the wavelength of the surface topography had a marked effect on the generation of the E H D oll film. Roughnesses greater than 0 2 ~tm r m,s. at a wavelength between 25 and 100 p.m increased the central thickness

7

MODEL SURFACES IN LUBRICATION

of the oil film, although no enhancement of the minimum film at the s~de constrictions was seen. The increase in thickness increases proportionately with the speed of rolhng. At wavelengths around 100 ~tm and above, no overall change m mean central film thickness was found. However, at these wavelengths the asperities acted as though they were isolated and marked m~croelastohydrodynamic effects were observed. At wavelengths less than 10 ~tm a decrease in both central and m i m m u m oil film thickness was observed.

5 2 Rolling and sliding with longitudinal roughness Rolling experiments with roughness oriented in the rolling direction were performed w~th surfaces having dominant wavelengths of 30, 60 and 100 pro. These produced a decrease in the mean central film thickness both In rolling and sliding. The effect was present for all loads for wavelengths of 30 and 60 I~m. For 100 I~m the reduction was limited to the area adjacent to the longitudinal furrows with no overall reduction in thickness A typical pair of interferograms for 2 = 50 ~m ~s shown m Fig. 9.

(a)

(6)

Fig 9 (a) Smooth surface mterferogram, (b) rough surface having longitudinal grooves (2 = 50 ~tm)

5 3 Rolling with three-dtmenstonalpseudoisotroptc roughness The surface examined consisted of dents 4 pm deep, produced by laser drilling, placed in an irregular pattern (see Fig. 6) to create an approximately isotropic surface. The holes are 20 I-tin in diameter at the mouth and their closest distance is 60 ~tm Significant lips stand to a height of 0.8 ~m around each hole and the r.m.s. surface roughness is 0.807 ~am. This surface, in rolling, does not give rise to a large increase or decrease in the mean film thickness. The film formation within the contact is governed by the deformation behavmur of the asperities and the characteristic E H D film profile is modified by the deformed shape of the asperities. Sliding experiments showed regions of reduced film bounded by the leading and trailing edges and the transverse edges of the holes to be formed at low film thicknesses These regions gradually disappeared as the central film thickness was mcreased

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S. DE SILVA, J. C. ANDERSON, J A. LEATHER

6 DISCUSSION The general features of these results can be understood in fairly simple terms. The transverse asperities provide a pumping action, forcing extra oll into the contact area by increasing the inlet pressure. This will produce an increase m the oil film thickness in the hertzian zone, provided that the load is not so large as to deform the asperities completely For very long wavelengths the pumping effect of each asperity drains away before the next one comes along For very short wavelengths the pumping action IS more than cancelled out by increased leakage via the grooves out of the sides of the contact. The longitudinal grooves do not provide a pumping action and provide leakage channels for oil, out of the contact region. As a result, the oil film thickness is reduced. The pseudolsotroplc surface provides both leakage and some pumping, but the one cancels the other. A more detailed trlbologlcal analysis of the surfaces and comparison of the results with E H D theory will be published in a separate paper. A study of elastic conformity in hertzian contacts has been made using the techniques described in this paper I 7

CONCLUSIONS

Whilst the results reported here are from a ball-and-disc testing machine they strongly suggest that improved lubrication of highly loaded bearmgs and gears could be attained, with a given oil, by depositing or milling a controlled surface roughness onto a highly polished imtlal finish. F r o m the evidence of the tests reported here the adhesion of sputtered chromium onto well-cleaned and sputteretched steel surfaces is sufficient to survive practical loadings under lubricated conditions. ACKNOWLEDGMENT

We thank the Procurement Executive of the Ministry of Defence for support of this project. REFERENCES 1 2 3 4 5 6 7

H Chrlstensen, Proc lnst Mech Eng, London, 184 (1) (1969) 1013 K N G F u l l e r a n d D Tabor, Proc R Soe London, Ser A,345(1975)327 G A D B n g g s a n d B J Bnscoe, J Phys D, 10(1977)2543 G M S deSllva, J A Leather and J C Anderson, T h m S o h d F d m s , 77(1981)341 J F Ready, J Appl Phy~ , 36 (1965) 462 M T Kirk, Nature (London), 194 (1962) 965 J F Archard and M T Kirk, Lubrteatton and Wear Cony, 1963, Institute of Mechamcal Engineers, London, 1964, Paper 15 8 A C a m e r o n and R Gohar, Proc R Soc London, Ser A, 291 (1966) 520 9 A Jackson and A Cameron, A S L E Trana , 19 (1974) 50 10 M Kaneta and A Cameron, J Lubr Technol, 102 (1980) 374 II R S Sayles, G M S de Silva, J A Leather, J C Anderson and P B Macpherson, Trtbology, (December 1981) 315