- Email: [email protected]
Strength measurement of thin lubricating films
Wear 237 Ž2000. 155–162 www.elsevier.comrlocaterwear
Strength measurement of thin lubricating films Larry Y. Wang b
a,)
, Z. Frank Yin b, Jun Zhang b, Chun-I Chen b, Stephen Hsu
b
a HMT Technology, 1055 Page AÕenue, Fremont, CA 94538, USA Ceramics DiÕision, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
Received 27 January 1999; received in revised form 9 July 1999; accepted 15 September 1999
Abstract A new method has been developed to measure the film-failure maximum shear stress at surface tsm of thin films in sliding. A ball-on-inclined plane scratch test was employed in these measurements. The tested balls were 52100 steel and the tested plane samples were 52100 steel, sapphire and a silicon wafer. Films on the plane samples were dip-coated to different thickness Ž0.5–200 nm.. Paraffin, oleyl alcohol, octadecanol, octadecamide, n-octadecyl mercaptan, 1,2-octadecanediol, octadecylamine, oleic acid, stearic acid, ZDDP, etc. were used to deposit different films. The test results indicate that the film-failure maximum shear stress at surface tsm increases with an increase of film thickness. Roughness and substrate differences are also found to have an influence on the measured tsm . A smoother surface gives higher tsm . Among the studied films, stearic acid possesses the highest tsm while octadecanol has the lowest. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Film-failure; Film; Shear stress; Scratch; Strength; Lubrication; Measurement
1. Introduction Film-failure is widely believed to be a cause of wear and scuffing, a fatal failure of mechanical systems. Practically, the main parameters governing film-failure are sliding speed and normal load. Blok w1x suggested that a critical temperature at the contact surface dominates the film-failure and cause scuffing, and numerous investigations on scuffing followed this criterion w2–4x. Some studies w5–9x ascribed the phenomenon of film breakdown to the break up of the physical or chemical bonds between the lubricant molecules or atoms and the substrate. From the energy transfer point of view, the substraterfilm bonding can be broken up not only by thermal energy due to high temperature, but also by other kinds of energy, such as distortion energy due to shear even though the temperature is low. Unfortunately, there are no previous investigations into film-failure as a result of shear strain Ždistortion energy. at normal temperatures. Since many mechanical systems work at low speed but under fairly high loads, it is necessary to study the film-failure at low speeds without temperature effects. To predict and eventually prevent failure of mechanical systems, it is useful to know the strength of a thin film which will prevent the two rela)
Corresponding author. E-mail: [email protected]
tively moving surfaces from directly contacting and resulting in wear. To solve this problem, a test method is developed in this paper to measure the film-failure maximum shear stress at contact surface Ždefined as tsm in this work. of a thin film undergoing shear Žsince tsm is directly related to the surface distortion energy. and tsm values for different films are measured and compared. The reason why a concept of largest maximum shear strength is used to define failure rather than simpler parameters such as the distance of slide before failure or a failure normal load is presumably that the latter parameters depend on experimental conditions. Because a monolayer or multi-layers film could have good strength in protecting the substrate w10,11x, and in order to control the film thickness so that it has an even distribution on the substrate during the film deposition, the film thickness in this study is controlled mostly in monolayer or multilayer range.
2. Experimental details 2.1. Ball-on-inclined plane tester The ball-on-inclined plane tester shown in Fig. 1 was used to measure the critical film-failure load and its corresponding friction coefficient. A spherical ball slides on an
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 3 1 2 - 9
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L.Y. Wang et al.r Wear 237 (2000) 155–162
inclined plane so that the contact pressure and stress increases with sliding distance. A three-dimensional KISTLER 9251A quartz force transducer and a Labview data acquisition system are employed to display the vertical and horizontal forces continuously on the screen and record them. The loading range for normal load is 1–800 N with precision up to 0.01 N. The data sampling rate can be changed from less than 1 scanrs to more than 5000 scansrs. The speed and position of moving samples are controlled by x, y and z direction stages with precisions of 1 mmrs for speed and 1 mm for position. A high speed Žup to 500 picturesrs. digital camera is mounted to observe the contact interface during sliding. The inclination angle of the plane sample can be adjusted from y2.58 to 2.58 with a tilting table. 2.2. Film deposition The films were deposited onto samples of different materials Ž52100 steel, sapphire and silicon wafer. with
Fig. 2. Dip-coating diagram.
dimensions 14 mm = 14 mm = 4 mm with a dip coating technique illustrated in Fig. 2. The 14 mm = 14 mm surface was used for film deposition and testing. The plane sample is placed vertically in the dilute lubricant. The dilute lubricant flows down through the sample and leaves
Fig. 1. Ža,b. Ball-on-inclined test apparatus: Ža. overview, and Žb. contact diagram.
L.Y. Wang et al.r Wear 237 (2000) 155–162
a very thin layer Žnano-scale. on the sample surface. Hexane was used to dilute the lubricants. The flow speed of the liquid level can be changed, but was kept at 0.5 mmrs in this study. All the specimens were cleaned ultrasonically by hexane Ž2 min., acetone Ž2 min., detergent ŽMicro. solution Ž2 min. and rinsed in deionized water 10 times and dried with nitrogen gas before dip coating. The polished steel plane sample surface was repolished to remove any possible oxide layer immediately before above cleaning and dip coating. Different films and substrates used in this study are listed below: Substrates: 52100 steel with hardness Rc 62 and roughness Ra s 85 " 5 nm and Ra s 1.5 " 0.5 nm, Sapphire, Ž100.-plane, Ra s 2 nm, Silicon Wafer, Ž100.-plane, Ra - 2 nm Films: Paraffin Žlight, saybolt viscosity 125r135. films: Oleyl alcohol Ž55%ŽGC.. Stearic acid ZDDP Oleic acid 1,2-Octadecanediol Octadecamide Octadecanol Chlorooctadecane n-Octadecyl Thiol Octadecylamine n-Octadecyl Thiol Benzyl Phenyl Sulfide 2.3. Film thickness measurement Each film thickness data point is an averaged value from three measurements at different locations of a sample surface using 43603-200E Ellipsometer, manufactured by Rudolph Research, NJ, USA. The laser source was a 1-mW continuous-wave heliumrneon laser with a wavelength of 632.8 nm. The angle of incidence was 708, the compensator was set at 458 and the spot size was 2–3 mm in diameter. Measurements were performed in air at room temperature. The signal of sample substrates was deducted from the one after film deposition so that the effects of substrate parameters such as surface roughness could be eliminated.
157
the scratch tests. All the ball specimens were cleaned ultrasonically by hexane Ž2 min., acetone Ž2 min., detergent ŽMicro. solution Ž2 min. and rinsed in deionized water 10 times and dried with nitrogen gas before testing. The test ball was held firmly with a screw-tighting holder without rotating. The inclined plane sample was driven by the high precision stage to move against the stationary ball a distance of 5 mm at a speed 0.2 mmrs. The incline angle was adjusted between 0.18 and 1.08 in order to get a better resolution for the location of the onset of film-failure. The sampling speed of data acquisition used was 20 scansrs. Both vertical and horizontal force were monitored on computer screen and recorded. Two to five scratches were made on the same plane sample 2 mm apart from each other, and their values were averaged to obtain a data point. A clean ball was used for each scratch. All the tests were conducted in atmosphere at temperature 228C with relative humidity of 50%.
3. Film-failure determination and film-failure max. surface shear stress 3.1. Film-failure determination In order to determine a failure load for films, a base line of dry scratch must be established. Fig. 3 gives the vertical force ŽFz., horizontal force ŽFx. and friction coefficient Ft Žtangential force.rFn Žnormal force. curves vs. scratch distance under dry conditions for a 52100 steelr52100 steel contact. The tangential force Ft and normal force Fn are calculated based on Fx and Fz, respectively with an incline angle correction. The friction coefficient varied from 0.6 to 0.9 and the two contacting surfaces were severely torn.
2.4. Roughness measurement The roughness of plane samples was measured using a MultiMode Scanning Probe Microscope made by Digital Instruments in USA and a Perthometer S5P profilemeter made by Perthen in Germany. 2.5. Test procedures and conditions 52100 steel balls of 3.175 mm Ž1r8Y . diameter with hardness Rc 63 and roughness Ra 0.012 mm were used in
Fig. 3. Vertical force Fz, horizontal force Fx and friction coefficient FtrFn curves of 52100 steelr52100 steel vs. scratch distance under dry condition.
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One typical graph for a lubricated scratch showing Fz, Fx and friction coefficient, FtrFn, vs. scratch distance is shown in Fig. 4a. The film for this test was dip-coated to be 6 nm thick. It is noted that there is location where friction coefficient FtrFn starts to rise substantially. The onset of the friction coefficient rise indicates the start of film-failure. To determine a consistent onset failure location, two straight lines are drawn along the friction coefficient curve before and after the point of increase as shown in Fig. 4a. The intersection of the two lines is defined as the film-failure critical point. The corresponding optical micrograph of the scratch is given in Fig. 4b. The filmfailure location in Fig. 4b was identified by measuring the scratch trace according to the film-failure location determined in Fig. 4a. The optical micrograph in Fig. 4b shows
no damage on the surface at the onset location of filmfailure, but surface damage follows soon. Film-failure takes place initially in the elastic regime in this study according to the surface observation and stress calculation. 3.2. Film-failure maximum shear stress at surface tsm Film-failure in our scratch tests is believed to be a result of energy absorption by the bonding between the substrate atoms and the molecules of the lubricant. In the case of local high temperature due to high speed sliding, the thermal energy can break the bonds between the film and the substrate and result in wear or scuffing. In this study, thermal energy is not a concern since the sliding speed Ž0.2 mmrs. is very low, but distortion energy does exist and
Fig. 4. Film-failure determination from the measured Fx, Fz, FtrFn-scratch distance curves and corresponding optical micrograph for film dip-coated with 1% oleyl alcohol in hexane: Ža. Fx, Fz, FtrFn curves, and Žb. corresponding micrograph.
L.Y. Wang et al.r Wear 237 (2000) 155–162
Fig. 5. The film-failure maximum shear stress at surface tsm of paraffin films vs. paraffin film thickness.
could dominate the break up process for bonds. For the concept of film strength, it is better for it to have an unit of stress. Since Von Mises max. shear stress is a square root of distortion energy of the material and also has a stress unit, and since distortion energy at the contact surface may dominate the film break-up process in this study Žcontact temperature is not a concern., we take the highest one among Von Mises maximum shear stress values at surface in the contact area at the onset of film-failure as a value of film strength and define it as tsm . A tsm value can be calculated from the measured critical film-failure normal load and its corresponding friction coefficient FtrFn. The Von Mises maximum shear stress takes the following formula:
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there is a ‘‘maximum shear stress’’ value. Different positions will have different ‘‘maximum shear stress’’ values. The film-failure maximum shear stress at surface tsm , which is defined in this work as a critical value to the film-failure, is the highest of all the ‘‘maximum shear stress’’ values at surface Ž z s 0.. Since the maximum shear stress distribution is symmetrical relative to y s 0 in the contact area, we only need to calculate the ‘‘maximum shear stress’’ values under the conditions z s 0 and y s 0. In this study, the central elastic contact line Žfrom ya to a, where a is the radius of the elastic contact circle. in x direction at z s 0 and y s 0 was divided as 30 grids for calculating the ‘‘maximum shear stress’’ values. Then the highest one among the 30 ‘‘maximum shear stress’’ values was chosen as the ‘‘film-failure maximum shear stress at surface tsm ’’. A computer program was used for the calculation. Based on the calculated results, film-failure maximum shear stress at surface tsm occurs at the location Ž x s ya, y s 0 and z s 0.. Mouginot pointed out some typing errors in Hamilton’ solutions. Those errors were corrected in our calculation based on Mouginot’ correction w13x. The procedures to calculate the largest maximum shear stress are as follows. Ža. Calculate critical normal load Fn and friction coefficient based on the tested Fx, Fz curves as those illustrated in Fig. 4 and contact geometry Žthe angle between Fx and Fn.. Žb. Divide the central elastic contact line Žfrom ya to a, where a is the radius of elastic contact circle. in x direction as 30 grids; calculate stress components sx , sy ,
Von Mises maximum shear stress
½
2
½
2
s Ž 1r6 . Ž sx y sy . q Ž sx y sz . . q Ž sy y sz .
2
5 qt
2 2 2 x y q t y z q tz x
1r2
5
The calculation for the above six stress components is based on Hamilton’s solution w12x. In Hamilton’s solution, the sliding direction was defined as ‘‘ x’’, vertical direction Žnormal load direction. as ‘‘ z’’, and the direction perpendicular to x–z plane as ‘‘ y’’. Hamilton gave the above six stress components explicit expressions as a function of normal load, friction coefficient, coordinate Ž x, y, z . value, Young’s module and Poisson’s ratios of the contact materials, as well as radius of the contact curvature w12x. For each position Ž x, y, z . beneath the contact surface,
Fig. 6. The film-failure maximum shear stress at surface tsm of oleic acid films vs. oleic acid film thickness.
L.Y. Wang et al.r Wear 237 (2000) 155–162
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4.1. Effects of film thickness on film-failure maximum shear stress at surface tsm
Fig. 7. The film-failure maximum shear stress at surface tsm of n-octadecyl thiol films vs. film thickness.
The film-failure maximum shear stress at surface tsm of paraffin films is plotted against film thickness as shown in Fig. 5. It is noted that the tsm value increases with the increase of paraffin film thickness. It is clear that the repeatability of the tests is fairly good and the error-bar range is relatively small. The effect of film thickness of oleic acid films on tsm value is illustrated in Fig. 6. The tsm value of oleic acid films seems to increase linearly with the increase of the film thickness. It is noted that the strength of an oleic acid film is much more sensitive to film thickness than the strength of a paraffin film. Fig. 7 presents the film strength of n-octadecyl thiol against film thickness. The increase pattern of film strength of n-octadecyl thiol is similar to that for an oleic acid film in Fig. 6. The film strength increases with the increase in film thickness and, for most of the thin films in this study, follows the patterns of Figs. 6 and 7. 4.2. Effect of roughness on tsm
sz , t x y , t y z , tz x as a function of x based on the express in Refs. w12,13x at z s 0 and y s 0; Žc. Calculate Von Mises maximum shear stress
½
2
½
2
s Ž 1r6 . Ž sx y sy . q Ž sx y sz . . q Ž sy y sz .
2
5 qt
2 2 2 x y q t y z q tz x
1r2
The surface roughness value may have an influence on the measured tsm values. Fig. 8 compares measured tsm values for two groups of 52100 steel samples with roughness values 2 nm and 85 nm, respectively. It is noted that smoother surface has a higher tsm value. This may result from the fact that, for a given normal load, the real local contact stress of a smoother surface will be lower than that of a surface with higher roughness value.
5
as a function of x. Žd. Choose the largest maximum shear stress value as the film-failure maximum shear stress. In calculating the film-failure maximum shear stress at surface tsm through the critical film-failure normal load and corresponding friction coefficient, the following parameters were used: Steel plane and steel ball: Young’s module E s 210 GPa, Poisson’s ratios n s 0.3, ball radius R s 1.5875 mm. Sapphire plane: E s 372 GPa, Poisson’s ratios, n s 0.22 Silicon Wafer: E s 113 GPa, Poisson’s ratios n s 0.42
4. Experimental results According to the criteria to determine the critical filmfailure normal load described in Section 3, the values of film-failure maximum shear stress at surface tsm for different kinds of films were measured. To minimize the error in the measurement, each data point is from an average of 2–5 scratch measurements.
Fig. 8. The influence of surface roughness on film-failure maximum shear stress at surface tsm .
L.Y. Wang et al.r Wear 237 (2000) 155–162
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4.3. Influence of substrate on tsm Different substrates may have different surface energy values. This will certainly result in different bonding strengths between the substrates and the films. Hence, the tsm values of different substrates for a given film are expected to be different. The experimental tsm values of substrates 52100 steel, Sapphire and silicon wafer deposited with a 8-nm thick paraffin film supported the expectation and the results are shown in Fig. 9. The theoretical surface energy of Si is 1232 ergrcm2 and the theoretical surface energy of alpha-Fe is in the range of 1450–1600 ergrcm2 w14x. The surface energy data for steel and Sapphire are not available. But the surface energy value for steel might be close to that of alpha-Fe. 4.4. tsm lubricant
Õalue comparison among seÕeral different
Fig. 10 compares the tsm values of several lubricants and additives with a film thickness of 2 nm. It is seen that octadecanol has the lowest tsm while stearic acid and ZDDP possess the highest tsm among the lubricants listed in the figure. It is predicted, from practical experience, that stearic acid film would provide the highest film strength. This is consistent with the test data in this study. It is found from Fig. 10 that polarity has strong influence on the film strength. It is understandable since polarity would change the bonding strength between a substrate and a film. It is not surprising that the film strength of a paraffin film is fairly low. To fully understand why different
Fig. 10. Comparison of film-failure maximum shear stress at surface tsm for different films with film thickness of 2 nm.
lubricants give the ranking of tsm in Fig. 10, more study needs to be done and is beyond the scope of this paper.
5. Discussions 5.1. Precision The data have a certain degree of scatter for the scratches on the same sample. The standard deviation displayed as error bars in the figures is within 10% of the average tsm values. The scattering may come from two sources. One is the possible uneven distribution of the film thickness due to the plane surface roughness ŽRa s 85 nm for most samples.. The reason to choose a roughness of Ra 85 nm for most plane samples is that it is practical, and the test results can be used directly in practical applications. The other error source may be the determination of initial film-failure position. In order to minimize the scattering, scratch tests were repeated two to five times for each test plane sample and the results were averaged to obtain a data point. 5.2. The possible effect of incline angle
Fig. 9. The effect of substrate on film-failure maximum shear stress at surface tsm .
In order to measure the film-failure load, appropriate angles should be chosen to give a desired load range which includes the failure point. If the load range is too large the resolution will be reduced and this will introduce additional errors in the failure load determination. If the load range is too small, the film may not fail over the attainable range. Therefore, based on the strength of the film, the plane incline angle needs to be adjusted in the range of 0.258–1.08. Theoretically, when the incline angle is very small Ž- 18., the measured tsm should be independent of
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the value of the incline angle. To verify this, the tsm values of a paraffin film with thickness of 8 nm were measured with three different angles 0.258, 0.508 and 0.758. The average tsm values of the three incline angles are very close with a variation within 5% of the measured value. 5.3. Influence of film thickness on measured tsm Õalues Conventionally it is believed that the first layer of a lubricant film has the strongest bond to a substrate and the subsequent layers have weaker bonds between each other. Based on this consideration, the film-failure strength is expected to increase non-linearly with increasing film thickness. The experimental results for paraffin in Fig. 5 support this argument. However, Figs. 6 and 7 seemed to have a nearly linear increase of tsm value with increasing film thickness in linear coordinate. There may be two reasons for this. First, the films used in this study were prepared with a dip-coating technique. The molecules on the surface were not supposed to stand up perpendicularly to the surface. Hence there were more interaction between layers than the case of Langmuir–Blodgett layers where moleculars are believed to stand up perpendicularly to the surface. Second, failure of film may not be a static process. In the case of multilayers, once the bonds between the first layer and the substrate break up due to shear, the second layer might initiate strong bonds with the substrate because all lubricant layers are highly pressured against the substrate surface during sliding. Thus a multilayer film may have higher tsm value than a single layer film may have.
6. Conclusions The following are conclusions that can be drawn based on the work described above. – A method has been developed to measure film-failure maximum shear stress tsm at surface of a thin film. This will permit the possibility of generating a database of tsm for various films and eventually guide film design in practical applications to prevent wear or scuffing.
– tsm generally increases with an increase of film thickness. – Different substrates with different surface energy values or chemical activity would result in different tsm values for a given film. – A smoother surface will give a higher tsm value. – Among the studied films, octadecanol gives the lowest tsm while stearic acid and ZDDP possess the highest one.
References w1x H. Blok, Surface temperatures under extreme pressure conditions, Congres Mondial du Petrole, Paris Ž1937. 471–486. w2x J.C. Enthoven, P.M. Cann, H.A. Spikes, Temperature and scuffing, Tribology Transaction, STLE 36 Ž2. Ž1993. 258–266. w3x J.H. Horng, J.F. Lin, K.Y. Li, Scuffing as evaluated from the viewpoint of surface roughness and friction energy, Journal of Tribology, ASME 118 Ž1996. 669–675. w4x S.M. Hsu, M.C. Shen, E.E. Klaus, H.S. Cheng, P.I. Lacey, Mechano-chemical model: reaction temperatures in a concentrated contact, Wear 175 Ž1994. 209–218. w5x H.A. Spikes, A. Cameron, A comparison of adsorption and boundary lubricant failure, Proc. R. Soc. Lond. A 336 Ž1974. 407–419. w6x A. Camera, Basic lubrication theory, Ellis Horwood, London, UK, 1981. w7x S.M. Hsu, E.E. Klaus, Some chemical effects in boundary lubrication: Part I. Base oil–metal interaction, ASME Trans. 22 Ž2. Ž1979. 135. w8x E.E. Klaus, R. Nagarajan, J.L. Duda, K.M. Shah, The adsorption of tribochemical reaction products at solid surfaces, Friction and Wear — 50 years on, Proc. Inst. Mech. Eng., International Conference on Tribology 370 Ž1987.. w9x S.M. Hsu, X.H. Zhang, Lubrication: Traditional to nano-lubricating films, to be published. w10x V. Novotny, Swalen, J.P. Rabe, Tribology of Langmuir–Blodgett layers, Langmuir 5 Ž1989. 485–489. w11x V. Depalma, N. Tillman, Friction and wear of self-assembled trichlorosilane monolayer films on silicon, Langmuir 5 Ž1989. 868– 872. w12x G.M. Hamilton, Explicit equations for the stresses beneath a sliding spherical contact, Proc. Inst. Mech. Engrs. 197C Ž1983. 53–59. w13x R. Mouginot, Crack formation beneath sliding spherical punches, J. Mater. Sci. 22 Ž1987. 989–1000. w14x D.H. Buckley, Surface Effects in Adhesion, Friction, Wear and Lubrication, Elsevier, Amsterdam, 1981.
Strength measurement of thin lubricating films Larry Y. Wang b
a,)
, Z. Frank Yin b, Jun Zhang b, Chun-I Chen b, Stephen Hsu
b
a HMT Technology, 1055 Page AÕenue, Fremont, CA 94538, USA Ceramics DiÕision, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
Received 27 January 1999; received in revised form 9 July 1999; accepted 15 September 1999
Abstract A new method has been developed to measure the film-failure maximum shear stress at surface tsm of thin films in sliding. A ball-on-inclined plane scratch test was employed in these measurements. The tested balls were 52100 steel and the tested plane samples were 52100 steel, sapphire and a silicon wafer. Films on the plane samples were dip-coated to different thickness Ž0.5–200 nm.. Paraffin, oleyl alcohol, octadecanol, octadecamide, n-octadecyl mercaptan, 1,2-octadecanediol, octadecylamine, oleic acid, stearic acid, ZDDP, etc. were used to deposit different films. The test results indicate that the film-failure maximum shear stress at surface tsm increases with an increase of film thickness. Roughness and substrate differences are also found to have an influence on the measured tsm . A smoother surface gives higher tsm . Among the studied films, stearic acid possesses the highest tsm while octadecanol has the lowest. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Film-failure; Film; Shear stress; Scratch; Strength; Lubrication; Measurement
1. Introduction Film-failure is widely believed to be a cause of wear and scuffing, a fatal failure of mechanical systems. Practically, the main parameters governing film-failure are sliding speed and normal load. Blok w1x suggested that a critical temperature at the contact surface dominates the film-failure and cause scuffing, and numerous investigations on scuffing followed this criterion w2–4x. Some studies w5–9x ascribed the phenomenon of film breakdown to the break up of the physical or chemical bonds between the lubricant molecules or atoms and the substrate. From the energy transfer point of view, the substraterfilm bonding can be broken up not only by thermal energy due to high temperature, but also by other kinds of energy, such as distortion energy due to shear even though the temperature is low. Unfortunately, there are no previous investigations into film-failure as a result of shear strain Ždistortion energy. at normal temperatures. Since many mechanical systems work at low speed but under fairly high loads, it is necessary to study the film-failure at low speeds without temperature effects. To predict and eventually prevent failure of mechanical systems, it is useful to know the strength of a thin film which will prevent the two rela)
Corresponding author. E-mail: [email protected]
tively moving surfaces from directly contacting and resulting in wear. To solve this problem, a test method is developed in this paper to measure the film-failure maximum shear stress at contact surface Ždefined as tsm in this work. of a thin film undergoing shear Žsince tsm is directly related to the surface distortion energy. and tsm values for different films are measured and compared. The reason why a concept of largest maximum shear strength is used to define failure rather than simpler parameters such as the distance of slide before failure or a failure normal load is presumably that the latter parameters depend on experimental conditions. Because a monolayer or multi-layers film could have good strength in protecting the substrate w10,11x, and in order to control the film thickness so that it has an even distribution on the substrate during the film deposition, the film thickness in this study is controlled mostly in monolayer or multilayer range.
2. Experimental details 2.1. Ball-on-inclined plane tester The ball-on-inclined plane tester shown in Fig. 1 was used to measure the critical film-failure load and its corresponding friction coefficient. A spherical ball slides on an
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 3 1 2 - 9
156
L.Y. Wang et al.r Wear 237 (2000) 155–162
inclined plane so that the contact pressure and stress increases with sliding distance. A three-dimensional KISTLER 9251A quartz force transducer and a Labview data acquisition system are employed to display the vertical and horizontal forces continuously on the screen and record them. The loading range for normal load is 1–800 N with precision up to 0.01 N. The data sampling rate can be changed from less than 1 scanrs to more than 5000 scansrs. The speed and position of moving samples are controlled by x, y and z direction stages with precisions of 1 mmrs for speed and 1 mm for position. A high speed Žup to 500 picturesrs. digital camera is mounted to observe the contact interface during sliding. The inclination angle of the plane sample can be adjusted from y2.58 to 2.58 with a tilting table. 2.2. Film deposition The films were deposited onto samples of different materials Ž52100 steel, sapphire and silicon wafer. with
Fig. 2. Dip-coating diagram.
dimensions 14 mm = 14 mm = 4 mm with a dip coating technique illustrated in Fig. 2. The 14 mm = 14 mm surface was used for film deposition and testing. The plane sample is placed vertically in the dilute lubricant. The dilute lubricant flows down through the sample and leaves
Fig. 1. Ža,b. Ball-on-inclined test apparatus: Ža. overview, and Žb. contact diagram.
L.Y. Wang et al.r Wear 237 (2000) 155–162
a very thin layer Žnano-scale. on the sample surface. Hexane was used to dilute the lubricants. The flow speed of the liquid level can be changed, but was kept at 0.5 mmrs in this study. All the specimens were cleaned ultrasonically by hexane Ž2 min., acetone Ž2 min., detergent ŽMicro. solution Ž2 min. and rinsed in deionized water 10 times and dried with nitrogen gas before dip coating. The polished steel plane sample surface was repolished to remove any possible oxide layer immediately before above cleaning and dip coating. Different films and substrates used in this study are listed below: Substrates: 52100 steel with hardness Rc 62 and roughness Ra s 85 " 5 nm and Ra s 1.5 " 0.5 nm, Sapphire, Ž100.-plane, Ra s 2 nm, Silicon Wafer, Ž100.-plane, Ra - 2 nm Films: Paraffin Žlight, saybolt viscosity 125r135. films: Oleyl alcohol Ž55%ŽGC.. Stearic acid ZDDP Oleic acid 1,2-Octadecanediol Octadecamide Octadecanol Chlorooctadecane n-Octadecyl Thiol Octadecylamine n-Octadecyl Thiol Benzyl Phenyl Sulfide 2.3. Film thickness measurement Each film thickness data point is an averaged value from three measurements at different locations of a sample surface using 43603-200E Ellipsometer, manufactured by Rudolph Research, NJ, USA. The laser source was a 1-mW continuous-wave heliumrneon laser with a wavelength of 632.8 nm. The angle of incidence was 708, the compensator was set at 458 and the spot size was 2–3 mm in diameter. Measurements were performed in air at room temperature. The signal of sample substrates was deducted from the one after film deposition so that the effects of substrate parameters such as surface roughness could be eliminated.
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the scratch tests. All the ball specimens were cleaned ultrasonically by hexane Ž2 min., acetone Ž2 min., detergent ŽMicro. solution Ž2 min. and rinsed in deionized water 10 times and dried with nitrogen gas before testing. The test ball was held firmly with a screw-tighting holder without rotating. The inclined plane sample was driven by the high precision stage to move against the stationary ball a distance of 5 mm at a speed 0.2 mmrs. The incline angle was adjusted between 0.18 and 1.08 in order to get a better resolution for the location of the onset of film-failure. The sampling speed of data acquisition used was 20 scansrs. Both vertical and horizontal force were monitored on computer screen and recorded. Two to five scratches were made on the same plane sample 2 mm apart from each other, and their values were averaged to obtain a data point. A clean ball was used for each scratch. All the tests were conducted in atmosphere at temperature 228C with relative humidity of 50%.
3. Film-failure determination and film-failure max. surface shear stress 3.1. Film-failure determination In order to determine a failure load for films, a base line of dry scratch must be established. Fig. 3 gives the vertical force ŽFz., horizontal force ŽFx. and friction coefficient Ft Žtangential force.rFn Žnormal force. curves vs. scratch distance under dry conditions for a 52100 steelr52100 steel contact. The tangential force Ft and normal force Fn are calculated based on Fx and Fz, respectively with an incline angle correction. The friction coefficient varied from 0.6 to 0.9 and the two contacting surfaces were severely torn.
2.4. Roughness measurement The roughness of plane samples was measured using a MultiMode Scanning Probe Microscope made by Digital Instruments in USA and a Perthometer S5P profilemeter made by Perthen in Germany. 2.5. Test procedures and conditions 52100 steel balls of 3.175 mm Ž1r8Y . diameter with hardness Rc 63 and roughness Ra 0.012 mm were used in
Fig. 3. Vertical force Fz, horizontal force Fx and friction coefficient FtrFn curves of 52100 steelr52100 steel vs. scratch distance under dry condition.
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One typical graph for a lubricated scratch showing Fz, Fx and friction coefficient, FtrFn, vs. scratch distance is shown in Fig. 4a. The film for this test was dip-coated to be 6 nm thick. It is noted that there is location where friction coefficient FtrFn starts to rise substantially. The onset of the friction coefficient rise indicates the start of film-failure. To determine a consistent onset failure location, two straight lines are drawn along the friction coefficient curve before and after the point of increase as shown in Fig. 4a. The intersection of the two lines is defined as the film-failure critical point. The corresponding optical micrograph of the scratch is given in Fig. 4b. The filmfailure location in Fig. 4b was identified by measuring the scratch trace according to the film-failure location determined in Fig. 4a. The optical micrograph in Fig. 4b shows
no damage on the surface at the onset location of filmfailure, but surface damage follows soon. Film-failure takes place initially in the elastic regime in this study according to the surface observation and stress calculation. 3.2. Film-failure maximum shear stress at surface tsm Film-failure in our scratch tests is believed to be a result of energy absorption by the bonding between the substrate atoms and the molecules of the lubricant. In the case of local high temperature due to high speed sliding, the thermal energy can break the bonds between the film and the substrate and result in wear or scuffing. In this study, thermal energy is not a concern since the sliding speed Ž0.2 mmrs. is very low, but distortion energy does exist and
Fig. 4. Film-failure determination from the measured Fx, Fz, FtrFn-scratch distance curves and corresponding optical micrograph for film dip-coated with 1% oleyl alcohol in hexane: Ža. Fx, Fz, FtrFn curves, and Žb. corresponding micrograph.
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Fig. 5. The film-failure maximum shear stress at surface tsm of paraffin films vs. paraffin film thickness.
could dominate the break up process for bonds. For the concept of film strength, it is better for it to have an unit of stress. Since Von Mises max. shear stress is a square root of distortion energy of the material and also has a stress unit, and since distortion energy at the contact surface may dominate the film break-up process in this study Žcontact temperature is not a concern., we take the highest one among Von Mises maximum shear stress values at surface in the contact area at the onset of film-failure as a value of film strength and define it as tsm . A tsm value can be calculated from the measured critical film-failure normal load and its corresponding friction coefficient FtrFn. The Von Mises maximum shear stress takes the following formula:
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there is a ‘‘maximum shear stress’’ value. Different positions will have different ‘‘maximum shear stress’’ values. The film-failure maximum shear stress at surface tsm , which is defined in this work as a critical value to the film-failure, is the highest of all the ‘‘maximum shear stress’’ values at surface Ž z s 0.. Since the maximum shear stress distribution is symmetrical relative to y s 0 in the contact area, we only need to calculate the ‘‘maximum shear stress’’ values under the conditions z s 0 and y s 0. In this study, the central elastic contact line Žfrom ya to a, where a is the radius of the elastic contact circle. in x direction at z s 0 and y s 0 was divided as 30 grids for calculating the ‘‘maximum shear stress’’ values. Then the highest one among the 30 ‘‘maximum shear stress’’ values was chosen as the ‘‘film-failure maximum shear stress at surface tsm ’’. A computer program was used for the calculation. Based on the calculated results, film-failure maximum shear stress at surface tsm occurs at the location Ž x s ya, y s 0 and z s 0.. Mouginot pointed out some typing errors in Hamilton’ solutions. Those errors were corrected in our calculation based on Mouginot’ correction w13x. The procedures to calculate the largest maximum shear stress are as follows. Ža. Calculate critical normal load Fn and friction coefficient based on the tested Fx, Fz curves as those illustrated in Fig. 4 and contact geometry Žthe angle between Fx and Fn.. Žb. Divide the central elastic contact line Žfrom ya to a, where a is the radius of elastic contact circle. in x direction as 30 grids; calculate stress components sx , sy ,
Von Mises maximum shear stress
½
2
½
2
s Ž 1r6 . Ž sx y sy . q Ž sx y sz . . q Ž sy y sz .
2
5 qt
2 2 2 x y q t y z q tz x
1r2
5
The calculation for the above six stress components is based on Hamilton’s solution w12x. In Hamilton’s solution, the sliding direction was defined as ‘‘ x’’, vertical direction Žnormal load direction. as ‘‘ z’’, and the direction perpendicular to x–z plane as ‘‘ y’’. Hamilton gave the above six stress components explicit expressions as a function of normal load, friction coefficient, coordinate Ž x, y, z . value, Young’s module and Poisson’s ratios of the contact materials, as well as radius of the contact curvature w12x. For each position Ž x, y, z . beneath the contact surface,
Fig. 6. The film-failure maximum shear stress at surface tsm of oleic acid films vs. oleic acid film thickness.
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4.1. Effects of film thickness on film-failure maximum shear stress at surface tsm
Fig. 7. The film-failure maximum shear stress at surface tsm of n-octadecyl thiol films vs. film thickness.
The film-failure maximum shear stress at surface tsm of paraffin films is plotted against film thickness as shown in Fig. 5. It is noted that the tsm value increases with the increase of paraffin film thickness. It is clear that the repeatability of the tests is fairly good and the error-bar range is relatively small. The effect of film thickness of oleic acid films on tsm value is illustrated in Fig. 6. The tsm value of oleic acid films seems to increase linearly with the increase of the film thickness. It is noted that the strength of an oleic acid film is much more sensitive to film thickness than the strength of a paraffin film. Fig. 7 presents the film strength of n-octadecyl thiol against film thickness. The increase pattern of film strength of n-octadecyl thiol is similar to that for an oleic acid film in Fig. 6. The film strength increases with the increase in film thickness and, for most of the thin films in this study, follows the patterns of Figs. 6 and 7. 4.2. Effect of roughness on tsm
sz , t x y , t y z , tz x as a function of x based on the express in Refs. w12,13x at z s 0 and y s 0; Žc. Calculate Von Mises maximum shear stress
½
2
½
2
s Ž 1r6 . Ž sx y sy . q Ž sx y sz . . q Ž sy y sz .
2
5 qt
2 2 2 x y q t y z q tz x
1r2
The surface roughness value may have an influence on the measured tsm values. Fig. 8 compares measured tsm values for two groups of 52100 steel samples with roughness values 2 nm and 85 nm, respectively. It is noted that smoother surface has a higher tsm value. This may result from the fact that, for a given normal load, the real local contact stress of a smoother surface will be lower than that of a surface with higher roughness value.
5
as a function of x. Žd. Choose the largest maximum shear stress value as the film-failure maximum shear stress. In calculating the film-failure maximum shear stress at surface tsm through the critical film-failure normal load and corresponding friction coefficient, the following parameters were used: Steel plane and steel ball: Young’s module E s 210 GPa, Poisson’s ratios n s 0.3, ball radius R s 1.5875 mm. Sapphire plane: E s 372 GPa, Poisson’s ratios, n s 0.22 Silicon Wafer: E s 113 GPa, Poisson’s ratios n s 0.42
4. Experimental results According to the criteria to determine the critical filmfailure normal load described in Section 3, the values of film-failure maximum shear stress at surface tsm for different kinds of films were measured. To minimize the error in the measurement, each data point is from an average of 2–5 scratch measurements.
Fig. 8. The influence of surface roughness on film-failure maximum shear stress at surface tsm .
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4.3. Influence of substrate on tsm Different substrates may have different surface energy values. This will certainly result in different bonding strengths between the substrates and the films. Hence, the tsm values of different substrates for a given film are expected to be different. The experimental tsm values of substrates 52100 steel, Sapphire and silicon wafer deposited with a 8-nm thick paraffin film supported the expectation and the results are shown in Fig. 9. The theoretical surface energy of Si is 1232 ergrcm2 and the theoretical surface energy of alpha-Fe is in the range of 1450–1600 ergrcm2 w14x. The surface energy data for steel and Sapphire are not available. But the surface energy value for steel might be close to that of alpha-Fe. 4.4. tsm lubricant
Õalue comparison among seÕeral different
Fig. 10 compares the tsm values of several lubricants and additives with a film thickness of 2 nm. It is seen that octadecanol has the lowest tsm while stearic acid and ZDDP possess the highest tsm among the lubricants listed in the figure. It is predicted, from practical experience, that stearic acid film would provide the highest film strength. This is consistent with the test data in this study. It is found from Fig. 10 that polarity has strong influence on the film strength. It is understandable since polarity would change the bonding strength between a substrate and a film. It is not surprising that the film strength of a paraffin film is fairly low. To fully understand why different
Fig. 10. Comparison of film-failure maximum shear stress at surface tsm for different films with film thickness of 2 nm.
lubricants give the ranking of tsm in Fig. 10, more study needs to be done and is beyond the scope of this paper.
5. Discussions 5.1. Precision The data have a certain degree of scatter for the scratches on the same sample. The standard deviation displayed as error bars in the figures is within 10% of the average tsm values. The scattering may come from two sources. One is the possible uneven distribution of the film thickness due to the plane surface roughness ŽRa s 85 nm for most samples.. The reason to choose a roughness of Ra 85 nm for most plane samples is that it is practical, and the test results can be used directly in practical applications. The other error source may be the determination of initial film-failure position. In order to minimize the scattering, scratch tests were repeated two to five times for each test plane sample and the results were averaged to obtain a data point. 5.2. The possible effect of incline angle
Fig. 9. The effect of substrate on film-failure maximum shear stress at surface tsm .
In order to measure the film-failure load, appropriate angles should be chosen to give a desired load range which includes the failure point. If the load range is too large the resolution will be reduced and this will introduce additional errors in the failure load determination. If the load range is too small, the film may not fail over the attainable range. Therefore, based on the strength of the film, the plane incline angle needs to be adjusted in the range of 0.258–1.08. Theoretically, when the incline angle is very small Ž- 18., the measured tsm should be independent of
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the value of the incline angle. To verify this, the tsm values of a paraffin film with thickness of 8 nm were measured with three different angles 0.258, 0.508 and 0.758. The average tsm values of the three incline angles are very close with a variation within 5% of the measured value. 5.3. Influence of film thickness on measured tsm Õalues Conventionally it is believed that the first layer of a lubricant film has the strongest bond to a substrate and the subsequent layers have weaker bonds between each other. Based on this consideration, the film-failure strength is expected to increase non-linearly with increasing film thickness. The experimental results for paraffin in Fig. 5 support this argument. However, Figs. 6 and 7 seemed to have a nearly linear increase of tsm value with increasing film thickness in linear coordinate. There may be two reasons for this. First, the films used in this study were prepared with a dip-coating technique. The molecules on the surface were not supposed to stand up perpendicularly to the surface. Hence there were more interaction between layers than the case of Langmuir–Blodgett layers where moleculars are believed to stand up perpendicularly to the surface. Second, failure of film may not be a static process. In the case of multilayers, once the bonds between the first layer and the substrate break up due to shear, the second layer might initiate strong bonds with the substrate because all lubricant layers are highly pressured against the substrate surface during sliding. Thus a multilayer film may have higher tsm value than a single layer film may have.
6. Conclusions The following are conclusions that can be drawn based on the work described above. – A method has been developed to measure film-failure maximum shear stress tsm at surface of a thin film. This will permit the possibility of generating a database of tsm for various films and eventually guide film design in practical applications to prevent wear or scuffing.
– tsm generally increases with an increase of film thickness. – Different substrates with different surface energy values or chemical activity would result in different tsm values for a given film. – A smoother surface will give a higher tsm value. – Among the studied films, octadecanol gives the lowest tsm while stearic acid and ZDDP possess the highest one.
References w1x H. Blok, Surface temperatures under extreme pressure conditions, Congres Mondial du Petrole, Paris Ž1937. 471–486. w2x J.C. Enthoven, P.M. Cann, H.A. Spikes, Temperature and scuffing, Tribology Transaction, STLE 36 Ž2. Ž1993. 258–266. w3x J.H. Horng, J.F. Lin, K.Y. Li, Scuffing as evaluated from the viewpoint of surface roughness and friction energy, Journal of Tribology, ASME 118 Ž1996. 669–675. w4x S.M. Hsu, M.C. Shen, E.E. Klaus, H.S. Cheng, P.I. Lacey, Mechano-chemical model: reaction temperatures in a concentrated contact, Wear 175 Ž1994. 209–218. w5x H.A. Spikes, A. Cameron, A comparison of adsorption and boundary lubricant failure, Proc. R. Soc. Lond. A 336 Ž1974. 407–419. w6x A. Camera, Basic lubrication theory, Ellis Horwood, London, UK, 1981. w7x S.M. Hsu, E.E. Klaus, Some chemical effects in boundary lubrication: Part I. Base oil–metal interaction, ASME Trans. 22 Ž2. Ž1979. 135. w8x E.E. Klaus, R. Nagarajan, J.L. Duda, K.M. Shah, The adsorption of tribochemical reaction products at solid surfaces, Friction and Wear — 50 years on, Proc. Inst. Mech. Eng., International Conference on Tribology 370 Ž1987.. w9x S.M. Hsu, X.H. Zhang, Lubrication: Traditional to nano-lubricating films, to be published. w10x V. Novotny, Swalen, J.P. Rabe, Tribology of Langmuir–Blodgett layers, Langmuir 5 Ž1989. 485–489. w11x V. Depalma, N. Tillman, Friction and wear of self-assembled trichlorosilane monolayer films on silicon, Langmuir 5 Ž1989. 868– 872. w12x G.M. Hamilton, Explicit equations for the stresses beneath a sliding spherical contact, Proc. Inst. Mech. Engrs. 197C Ž1983. 53–59. w13x R. Mouginot, Crack formation beneath sliding spherical punches, J. Mater. Sci. 22 Ž1987. 989–1000. w14x D.H. Buckley, Surface Effects in Adhesion, Friction, Wear and Lubrication, Elsevier, Amsterdam, 1981.