- Email: [email protected]
Multi-technique surface analytical studies of automotive anti-wear films
Applied Surface Science 144–145 Ž1999. 222–227
Multi-technique surface analytical studies of automotive anti-wear films G.C. Smith ) , J.C. Bell Shell Research, Shell Research and Technology Centre Thornton, P.O. Box 1, Chester, CH1 3SH, UK
Abstract A multi-technique study of model automotive anti-wear films using SEM with quantitative electron microprobe analysis, reflection–absorption IR spectroscopy, SIMS imaging and XPS depth profiling is described. Constrained simulation of the XPS depth profiles allowed model cross-sections of the film structures to be constructed. With zinc dialkyl dithiophosphate as an anti-wear additive, the anti-wear films were found to have a thin inorganic mixed sulphideroxide layer immediately above the Fe substrate. This formed the base for a thicker Zn-containing polyphosphate-like overlayer. Addition of detergent and dispersant to the lubricant formulation resulted in thicker, more patchy films, with a clearer differentiation between film and substrate. q 1999 Shell Research Ltd. Published by Elsevier Science B.V. All rights reserved. Keywords: Automotive anti-wear films; Zinc dialkyl dithiophosphate additives; Lubricant
1. Introduction Modern automotive lubricants incorporate additives to reduce friction and protect against engine wear through the formation and renewal of protective films on critical engine components. The increasing emphasis on friction reduction, for improved fuel economy, coupled with higher temperature operation and longer oil drain intervals in modern passenger car engine design results in increasingly stressed lubricant. Traditionally, zinc dialkyl dithiophosphate ŽZDDP. additives have been used to give protection under extreme pressure conditions, however, their effectiveness may depend on the pre-
)
Corresponding author. Tel.: q44-151-373-5611; Fax: q44151-373-5406; E-mail: [email protected]
cise engine conditions, and the precise lubricant formulation. ZDDP-based protective anti-wear films are known to be chemically complex multi-layer structures of thickness typically 10–100 nm and with lateral variations on the 1–10 mm scale w1,2x. They have been subject to investigation by surface analysis since the early days of the techniques w3a,3bx. However, because of their lateral and vertical inhomogeneity, the results of earlier investigations were often ambiguous. The structure and chemistry of these films can only be understood through a coordinated multi-technique approach probing a range of depth and resolution scales. Here, a multi-technique approach using XPS and SIMS, together with infra-red spectroscopy and SEMrEPMA, is used to build up full three-dimensional schemes of film structures produced using typical lubricant additives.
0169-4332r99r$ - see front matter q 1999 Shell Research Ltd. Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 0 1 - 0
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
2. Experimental
223
3. Results 3.1. RAIRS
2.1. Specimen preparation Real engine components are often unsuitable for surface analysis, on grounds of size and uniformity. Here, films were generated on 8 mm square hardened steel blocks in a modified Reciprocating Amsler machine, known to accurately simulate the conditions found in internal combustion engine valvetrain systems w4x. To prevent further reaction with additives or degradation products, the specimens were stored in fresh mineral oil, and rinsed in 3 aliquots of n-heptane before analysis. Specimens were generated using two simple model lubricant formulations consisting of mineral base oil with Ža. ZDDP additive, and Žb. ZDDP in combination with a succinimide dispersant and a calcium salicylate detergent. For convenience, in this paper these are referred to as oils A and B, respectively.
2.2. Analytical techniques One pair of specimens was analysed by reflection absorption infra-red spectroscopy ŽRAIRS. w5x followed by SEM and electron probe microanalysis ŽEPMA., whilst a duplicate pair was analysed by SIMS imaging followed by XPS depth profiling. A Perkin Elmer 1760-X FTIR spectrometer with a Specac grazing angle accessory and a KRS 5 polarizer was used for the RAIRS measurements. SEMrEPMA was performed using a JEOL Superprobe 733 electron microanalyser. EPMA data acquired with a 10-kV beam at 10 nA current were quantified to give estimates of film thickness and composition using JEOL thin film on substrate ŽTFOS. software based on the method of Yakowitz and Newbury w6x. SIMS measurements were made using a VG SIMSLAB instrument with 10 keV Gaq ions and a 0–1200 amu range quadrupole mass spectrometer. XPS spectra and depth profiles were acquired with an SSI M-Probe small-spot monochromatic XPS instrument analysing a 300-mm diameter spot in the centre of a 2 = 2 mm crater produced with a rastered 4.5 keV Arq beam.
The spectrum from the film formed using the Oil A had a broad, strong absorbance peaking between 1235 cmy1 , attributable to P5O and P5O-metal bonds, and minor absorbances between 1000 and 940 cmy1 , attributable to P–O–P, P–OH and PO4 structures. Oil B gave a stronger main peak, with a maximum at 1201 cmy1 . These observations are consistent with earlier anti-wear film experiments and with IR spectra of reference phosphate compounds w1,7x. Inspection of the contributing spectral components showed the longer chain phosphate content of the anti-wear films was greater for Oil A than for Oil B. 3.2. SEM r EPMA analysis A representative backscattered electron image and Zn and P element maps of the film formed at the centre of the wear track with Oil A are shown in Fig. 1. The darker areas in the BSE image correlate with the main features of the Zn and P maps. Oxygen Žnot shown. was closely correlated with Zn and P, indicating a dense coverage of patches of phosphate anti-wear film, typically 5–15 mm in lateral extent. S was strong in the same areas as Zn and P, but also in areas where Zn and P were weaker, suggesting that more than one type of film was present. With Oil B similar spatial correlations between Zn, P, S were apparent, but with lower surface coverage and size range of film. The film was primarily in patches a few mm in size. The Zn, P and S images showed high spatial correlation, and there was a weak Ca signal, which correlated loosely with the other elements. TFOS surveys were performed along a line of 50 points 20 mm apart perpendicular to the sliding direction at the centre of the wear track. The method gives the average thickness and composition of the film, assuming it contains no substrate material. Data were corrected for the O K arZn L a 2 overlap, the change in O K a line shape with chemical environment, and the presence of carbon as contamination and from the underlying steel. Thicknesses of 83 and
224
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
Fig. 1. Representative BSE and ZnrP maps for oil A.
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
225
66 nm were obtained for the films generated using oils A and B, respectively, calculated using a density of 3.29 g cmy3 estimated from the XPS film compositions. Average compositions in at.% were Zn 18.5%, P 16.5%, S 26.3% and O 38.8% ŽOil A., and Zn 15.1%, P 17.1%, S 12.1%, Ca 8.3% and O 47.5% ŽOil B.. The use of Oil B increased the relative O content, indicating a change in the structure of the phosphate. The S content was drastically reduced and Ca was incorporated into the film. Zn and P were not greatly affected. 3.3. Imaging SIMS Both specimens showed peaks corresponding to y POq, POy 2 and PO 3 , confirming the presence of phosphates indicated by SEMrEPMA and RAIRS. For the film generated with Oil A, Feq was only found outside areas giving POq whereas Znq and Sy were obtained from both types of area. Overall, the images indicated the presence of a phosphate film over much of the surface. The film contained Zn and S, but there existed other areas where Zn, Fe and S were found but the phosphate was not. With oil B the positive ion images for Caq, Znq and POq were strongly correlated, indicating a phosphate film containing Zn and Ca. POy 2 was correlated with POq. However, the Sy signal covered the phosphate film areas, and also showed intensity in the region between the phosphate. The CNy ion intensity, originating from the amine head groups of the succinimide dispersant, seemed uniformly distributed across the specimen surface. Overall, these data show that with ZDDP alone, the film is relatively uniform and consists primarily of phosphate-like species together with Zn and possibly Fe. This part of the film was associated with a low level of S, but S also appeared to be present on the surface in patches between the phosphate areas. With Oil B, Ca was incorporated into the film, to some extent displacing Zn. There was incorporation into or adsorption onto the phosphate film of dispersant-related species. 3.4. XPS depth profiling Fig. 2 shows XPS depth profiles for the two films, corrected for carbon recontamination during
Fig. 2. XPS depth profiles for films generated using oil A and oil B.
the extended periods of ion beam exposure required for the measurements. For Oil A, the concentrations of P, Zn and O fell uniformly with time, whereas S initially increased with time and only fell when the P had diminished almost to zero. Zn and O persist below the P-containing part of the structure. This suggests a Zn-containing phosphate over a sulphide film, both probably over a metal oxide layer. Fe appeared near the start of the depth profile and is therefore either incorporated into the film, or exposed at the surface between patches of film. With Oil B the film appeared to have a well defined constant composition over a significant proportion of its depth, with all film species falling in concentration at similar rates once substrate material was revealed. Fe was not incorporated into the film, although Ca was. Nitrogen was not detected in any of the films. Throughout the films Ca and Zn were found mainly in highly ionic states, with binding energies
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G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
higher than those expected for simple inorganic sulphides or oxides. P was at a binding energy consis. tent with metaphosphates ŽPOy 3 , corresponding well to the main component of the O peak. The results are consistent with a phosphate glass-like structure containing interstitial metal ions. S appeared to be present in inorganic sulphide and organic forms. The XPS data indicate that there was no substrate material in the film formed using oil B, whereas this was probably not the case using oil A. Therefore, the TFOS estimate of film thickness using oil B could be used to give an estimate of the sputter rate. A mean value of 1.24 nm miny1 under the conditions of these measurements was found. This estimate was applied to the depth profile for oil A, for which the TFOS analysis was not valid. Film thickness estimates of 38 and 66 nm were obtained for the films generated using oils A and B, respectively. The XPS results show that with Oil B, the film was significantly thicker relative to that for Oil A, and a phosphate film containing Ca, Zn and S was produced. Oil A gave a Zn-containing phosphate film over a sulphide layer, with possible Fe incorporation.
4. Anti-wear film structures Hypothetical film models were constructed that, while probably not unique, are consistent with all the experimental data. Such models are shown as schematic vertical sections through the films in Fig. 3. They were partially validated through computer simulations of the expected XPS depth profiles from the model structures, and subsequent optimisation. The models are of course gross simplifications of the true structures. The effects of surface roughness are ignored, and real films will show a range of compositions and thicknesses. Nevertheless, they give insights into how the additives may function. With a simple ZDDPrmineral oil solution it is likely that initially a sulphide film is formed as a result of high temperatures and pressures during asperity contacts. This is rapidly coated with a protective phosphate film, although some sulphide film remains. The composition of the sulphide film gradu-
Fig. 3. Schematic structures for films generated using oils A and B.
ally varies, to include more Fe and more oxide, by ionic exchange and diffusion, with increasing depth. When both detergent and dispersant are used in the ZDDP solution, the effect is to produce a clear separation between the substrate and the phosphate film. There is no underlying sulphide layer, probably because of competition for surface sites between the wider range of surface-active species present in the more complete formulation. Fe is not found in the phosphate film, although relatively high levels of Ca and Zn are, and the surface is almost completely covered. Dispersant does not form part of the film structure; with only a thin layer of nitrogen-containing material found on the outer surface, detectable only by SIMS.
5. Conclusions The systematic combination of techniques used in this study enabled the structural models of ZDDP films developed from previous work to be extended, revealing the origins of their anti-wear properties. The essential constituent is a phosphate film bonded to the ferrous substrate by interfacial sulphides. The presence of detergent and dispersant in the oil increases the film thickness but give a more patchy film with no clear sulphidic bonding layer.
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
Acknowledgements We are grateful to D. Park, S. Garner, G.W. Roper, S.J. Smith ŽShell Research., S. Hibbert ŽCSMA. and D. Johnson ŽLiverpool John Moores University. for experimental assistance and stimulating discussions. References w1x P.A. Willermet, D.P. Bailey, R.O. Carter III, P.J. Schmitz, W. Zhu, J.C. Bell, D. Park, Tribol. Int. 28 Ž3. Ž1995. 163.
227
w2x I.R. Barkshire, M. Prutton, G.C. Smith, Appl. Surf. Sci. 84 Ž1995. 331. w3ax R.J. Bird, G.D. Galvin, Wear 37 Ž1976. 143. w3bx B.A. Baldwin, Lub. Eng. 32 Ž1975. 125. w4x G.W. Roper, J.C. Bell, SAE paper 952473, Society of Automotive Engineers Fuels and Lubricants Meeting and Exposition, Toronto, Ontario, 16–19 October 1995, reprinted from Recent Snapshots and Insights into Lubricant Technology, SAE Special Publication SP-1116, 1995. w5x R.G. Greenler, J. Chem. Phys. 44 Ž1966. 310. w6x H. Yakowitz, D.E. Newbury, SEM I, Proc. 9th Ann. SEM Symp., 1976, p. 151. w7x P.A. Willermet, R.O. Carter III, E.N. Boulos, Tribol. Int. 25 Ž6. Ž1992. 371.
Multi-technique surface analytical studies of automotive anti-wear films G.C. Smith ) , J.C. Bell Shell Research, Shell Research and Technology Centre Thornton, P.O. Box 1, Chester, CH1 3SH, UK
Abstract A multi-technique study of model automotive anti-wear films using SEM with quantitative electron microprobe analysis, reflection–absorption IR spectroscopy, SIMS imaging and XPS depth profiling is described. Constrained simulation of the XPS depth profiles allowed model cross-sections of the film structures to be constructed. With zinc dialkyl dithiophosphate as an anti-wear additive, the anti-wear films were found to have a thin inorganic mixed sulphideroxide layer immediately above the Fe substrate. This formed the base for a thicker Zn-containing polyphosphate-like overlayer. Addition of detergent and dispersant to the lubricant formulation resulted in thicker, more patchy films, with a clearer differentiation between film and substrate. q 1999 Shell Research Ltd. Published by Elsevier Science B.V. All rights reserved. Keywords: Automotive anti-wear films; Zinc dialkyl dithiophosphate additives; Lubricant
1. Introduction Modern automotive lubricants incorporate additives to reduce friction and protect against engine wear through the formation and renewal of protective films on critical engine components. The increasing emphasis on friction reduction, for improved fuel economy, coupled with higher temperature operation and longer oil drain intervals in modern passenger car engine design results in increasingly stressed lubricant. Traditionally, zinc dialkyl dithiophosphate ŽZDDP. additives have been used to give protection under extreme pressure conditions, however, their effectiveness may depend on the pre-
)
Corresponding author. Tel.: q44-151-373-5611; Fax: q44151-373-5406; E-mail: [email protected]
cise engine conditions, and the precise lubricant formulation. ZDDP-based protective anti-wear films are known to be chemically complex multi-layer structures of thickness typically 10–100 nm and with lateral variations on the 1–10 mm scale w1,2x. They have been subject to investigation by surface analysis since the early days of the techniques w3a,3bx. However, because of their lateral and vertical inhomogeneity, the results of earlier investigations were often ambiguous. The structure and chemistry of these films can only be understood through a coordinated multi-technique approach probing a range of depth and resolution scales. Here, a multi-technique approach using XPS and SIMS, together with infra-red spectroscopy and SEMrEPMA, is used to build up full three-dimensional schemes of film structures produced using typical lubricant additives.
0169-4332r99r$ - see front matter q 1999 Shell Research Ltd. Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 0 1 - 0
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
2. Experimental
223
3. Results 3.1. RAIRS
2.1. Specimen preparation Real engine components are often unsuitable for surface analysis, on grounds of size and uniformity. Here, films were generated on 8 mm square hardened steel blocks in a modified Reciprocating Amsler machine, known to accurately simulate the conditions found in internal combustion engine valvetrain systems w4x. To prevent further reaction with additives or degradation products, the specimens were stored in fresh mineral oil, and rinsed in 3 aliquots of n-heptane before analysis. Specimens were generated using two simple model lubricant formulations consisting of mineral base oil with Ža. ZDDP additive, and Žb. ZDDP in combination with a succinimide dispersant and a calcium salicylate detergent. For convenience, in this paper these are referred to as oils A and B, respectively.
2.2. Analytical techniques One pair of specimens was analysed by reflection absorption infra-red spectroscopy ŽRAIRS. w5x followed by SEM and electron probe microanalysis ŽEPMA., whilst a duplicate pair was analysed by SIMS imaging followed by XPS depth profiling. A Perkin Elmer 1760-X FTIR spectrometer with a Specac grazing angle accessory and a KRS 5 polarizer was used for the RAIRS measurements. SEMrEPMA was performed using a JEOL Superprobe 733 electron microanalyser. EPMA data acquired with a 10-kV beam at 10 nA current were quantified to give estimates of film thickness and composition using JEOL thin film on substrate ŽTFOS. software based on the method of Yakowitz and Newbury w6x. SIMS measurements were made using a VG SIMSLAB instrument with 10 keV Gaq ions and a 0–1200 amu range quadrupole mass spectrometer. XPS spectra and depth profiles were acquired with an SSI M-Probe small-spot monochromatic XPS instrument analysing a 300-mm diameter spot in the centre of a 2 = 2 mm crater produced with a rastered 4.5 keV Arq beam.
The spectrum from the film formed using the Oil A had a broad, strong absorbance peaking between 1235 cmy1 , attributable to P5O and P5O-metal bonds, and minor absorbances between 1000 and 940 cmy1 , attributable to P–O–P, P–OH and PO4 structures. Oil B gave a stronger main peak, with a maximum at 1201 cmy1 . These observations are consistent with earlier anti-wear film experiments and with IR spectra of reference phosphate compounds w1,7x. Inspection of the contributing spectral components showed the longer chain phosphate content of the anti-wear films was greater for Oil A than for Oil B. 3.2. SEM r EPMA analysis A representative backscattered electron image and Zn and P element maps of the film formed at the centre of the wear track with Oil A are shown in Fig. 1. The darker areas in the BSE image correlate with the main features of the Zn and P maps. Oxygen Žnot shown. was closely correlated with Zn and P, indicating a dense coverage of patches of phosphate anti-wear film, typically 5–15 mm in lateral extent. S was strong in the same areas as Zn and P, but also in areas where Zn and P were weaker, suggesting that more than one type of film was present. With Oil B similar spatial correlations between Zn, P, S were apparent, but with lower surface coverage and size range of film. The film was primarily in patches a few mm in size. The Zn, P and S images showed high spatial correlation, and there was a weak Ca signal, which correlated loosely with the other elements. TFOS surveys were performed along a line of 50 points 20 mm apart perpendicular to the sliding direction at the centre of the wear track. The method gives the average thickness and composition of the film, assuming it contains no substrate material. Data were corrected for the O K arZn L a 2 overlap, the change in O K a line shape with chemical environment, and the presence of carbon as contamination and from the underlying steel. Thicknesses of 83 and
224
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
Fig. 1. Representative BSE and ZnrP maps for oil A.
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
225
66 nm were obtained for the films generated using oils A and B, respectively, calculated using a density of 3.29 g cmy3 estimated from the XPS film compositions. Average compositions in at.% were Zn 18.5%, P 16.5%, S 26.3% and O 38.8% ŽOil A., and Zn 15.1%, P 17.1%, S 12.1%, Ca 8.3% and O 47.5% ŽOil B.. The use of Oil B increased the relative O content, indicating a change in the structure of the phosphate. The S content was drastically reduced and Ca was incorporated into the film. Zn and P were not greatly affected. 3.3. Imaging SIMS Both specimens showed peaks corresponding to y POq, POy 2 and PO 3 , confirming the presence of phosphates indicated by SEMrEPMA and RAIRS. For the film generated with Oil A, Feq was only found outside areas giving POq whereas Znq and Sy were obtained from both types of area. Overall, the images indicated the presence of a phosphate film over much of the surface. The film contained Zn and S, but there existed other areas where Zn, Fe and S were found but the phosphate was not. With oil B the positive ion images for Caq, Znq and POq were strongly correlated, indicating a phosphate film containing Zn and Ca. POy 2 was correlated with POq. However, the Sy signal covered the phosphate film areas, and also showed intensity in the region between the phosphate. The CNy ion intensity, originating from the amine head groups of the succinimide dispersant, seemed uniformly distributed across the specimen surface. Overall, these data show that with ZDDP alone, the film is relatively uniform and consists primarily of phosphate-like species together with Zn and possibly Fe. This part of the film was associated with a low level of S, but S also appeared to be present on the surface in patches between the phosphate areas. With Oil B, Ca was incorporated into the film, to some extent displacing Zn. There was incorporation into or adsorption onto the phosphate film of dispersant-related species. 3.4. XPS depth profiling Fig. 2 shows XPS depth profiles for the two films, corrected for carbon recontamination during
Fig. 2. XPS depth profiles for films generated using oil A and oil B.
the extended periods of ion beam exposure required for the measurements. For Oil A, the concentrations of P, Zn and O fell uniformly with time, whereas S initially increased with time and only fell when the P had diminished almost to zero. Zn and O persist below the P-containing part of the structure. This suggests a Zn-containing phosphate over a sulphide film, both probably over a metal oxide layer. Fe appeared near the start of the depth profile and is therefore either incorporated into the film, or exposed at the surface between patches of film. With Oil B the film appeared to have a well defined constant composition over a significant proportion of its depth, with all film species falling in concentration at similar rates once substrate material was revealed. Fe was not incorporated into the film, although Ca was. Nitrogen was not detected in any of the films. Throughout the films Ca and Zn were found mainly in highly ionic states, with binding energies
226
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
higher than those expected for simple inorganic sulphides or oxides. P was at a binding energy consis. tent with metaphosphates ŽPOy 3 , corresponding well to the main component of the O peak. The results are consistent with a phosphate glass-like structure containing interstitial metal ions. S appeared to be present in inorganic sulphide and organic forms. The XPS data indicate that there was no substrate material in the film formed using oil B, whereas this was probably not the case using oil A. Therefore, the TFOS estimate of film thickness using oil B could be used to give an estimate of the sputter rate. A mean value of 1.24 nm miny1 under the conditions of these measurements was found. This estimate was applied to the depth profile for oil A, for which the TFOS analysis was not valid. Film thickness estimates of 38 and 66 nm were obtained for the films generated using oils A and B, respectively. The XPS results show that with Oil B, the film was significantly thicker relative to that for Oil A, and a phosphate film containing Ca, Zn and S was produced. Oil A gave a Zn-containing phosphate film over a sulphide layer, with possible Fe incorporation.
4. Anti-wear film structures Hypothetical film models were constructed that, while probably not unique, are consistent with all the experimental data. Such models are shown as schematic vertical sections through the films in Fig. 3. They were partially validated through computer simulations of the expected XPS depth profiles from the model structures, and subsequent optimisation. The models are of course gross simplifications of the true structures. The effects of surface roughness are ignored, and real films will show a range of compositions and thicknesses. Nevertheless, they give insights into how the additives may function. With a simple ZDDPrmineral oil solution it is likely that initially a sulphide film is formed as a result of high temperatures and pressures during asperity contacts. This is rapidly coated with a protective phosphate film, although some sulphide film remains. The composition of the sulphide film gradu-
Fig. 3. Schematic structures for films generated using oils A and B.
ally varies, to include more Fe and more oxide, by ionic exchange and diffusion, with increasing depth. When both detergent and dispersant are used in the ZDDP solution, the effect is to produce a clear separation between the substrate and the phosphate film. There is no underlying sulphide layer, probably because of competition for surface sites between the wider range of surface-active species present in the more complete formulation. Fe is not found in the phosphate film, although relatively high levels of Ca and Zn are, and the surface is almost completely covered. Dispersant does not form part of the film structure; with only a thin layer of nitrogen-containing material found on the outer surface, detectable only by SIMS.
5. Conclusions The systematic combination of techniques used in this study enabled the structural models of ZDDP films developed from previous work to be extended, revealing the origins of their anti-wear properties. The essential constituent is a phosphate film bonded to the ferrous substrate by interfacial sulphides. The presence of detergent and dispersant in the oil increases the film thickness but give a more patchy film with no clear sulphidic bonding layer.
G.C. Smith, J.C. Bell r Applied Surface Science 144–145 (1999) 222–227
Acknowledgements We are grateful to D. Park, S. Garner, G.W. Roper, S.J. Smith ŽShell Research., S. Hibbert ŽCSMA. and D. Johnson ŽLiverpool John Moores University. for experimental assistance and stimulating discussions. References w1x P.A. Willermet, D.P. Bailey, R.O. Carter III, P.J. Schmitz, W. Zhu, J.C. Bell, D. Park, Tribol. Int. 28 Ž3. Ž1995. 163.
227
w2x I.R. Barkshire, M. Prutton, G.C. Smith, Appl. Surf. Sci. 84 Ž1995. 331. w3ax R.J. Bird, G.D. Galvin, Wear 37 Ž1976. 143. w3bx B.A. Baldwin, Lub. Eng. 32 Ž1975. 125. w4x G.W. Roper, J.C. Bell, SAE paper 952473, Society of Automotive Engineers Fuels and Lubricants Meeting and Exposition, Toronto, Ontario, 16–19 October 1995, reprinted from Recent Snapshots and Insights into Lubricant Technology, SAE Special Publication SP-1116, 1995. w5x R.G. Greenler, J. Chem. Phys. 44 Ž1966. 310. w6x H. Yakowitz, D.E. Newbury, SEM I, Proc. 9th Ann. SEM Symp., 1976, p. 151. w7x P.A. Willermet, R.O. Carter III, E.N. Boulos, Tribol. Int. 25 Ž6. Ž1992. 371.