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cylinder wall test environments
Tribological Research and Design for Engineering Systems D. Dowson et al. (Editors) 9 2003 Published by Elsevier B.V.
511
The wear response of ceramic matrix nano-composite coatings in simulated pistonring/cylinder wall test environments K.L. Dahm, K. Panagopoulos, and P.A. Dearnley a a School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
A series of piston rings with surface coatings of alumina-matrix nano-composite materials have been evaluated in lubricated high-speed sliding contact with cast-iron cylinder liners at 200~ Comparisons were made with standard CKS-36 (a commercial coating comprising chromium plating reinforced with alumina particles) coated cast-iron piston rings. Two types of wear mechanism were observed for the nano-composite coatings: (i) micro-scale cohesive failure, and (ii) smooth wear. The latter mechanism resulted in lower wear rates. The predominance of a given wear mechanism was found to depend on the coating composition and type of grinding operation used to finish the nano-composite coated piston rings.
1.2. Alumina-matrix nanocomposites 1. BACKGROUND AND RATIONALE Increasing performance demands placed on the piston ring-cylinder liner couple are increasing demand for highly wear-resistant piston ring coatings. There is additional environmental impetus to replace techniques such as electroplating (of chromium in particular) with environmentallyfriendly plasma-based coating techniques.
1.1. Plasma-sprayed ceramic coatings Plasma spraying represents a versatile technique for deposition of metallic, ceramic and cermet coatings onto metallic substrates. Molybdenum-base coatings are widely used for piston ring coatings due to their resistance to scuffing. These coatings are also nearing the limits of operation [ 1]. Several authors have investigated plasma Sprayed alumina (A1203) coatings as an alternative to both electrodeposited chromium and thermally sprayed molybdenum coatings [1-3]. Tribological testing of such coatings has shown that while they are resistant to wear and scuffing, long running times leads to coating fracture (or "break-out") [1 ]. A ceramic coating that combines the inherent tribological advantages of A1203 but increases its resistance to fracture is therefore highly desirable.
Ceramic nano-composites of A1203 and silicon carbide (SIC) have been reported to possess enhanced fracture toughness compared with pure A1203 [4]. These nano-composites comprise nanosized (typically <200nm) SiC particles embedded within A1203 grains. The mechanism by which this enhanced fracture toughness is achieved and the magnitude of the increase are currently unclear [5,6]. Wet erosive wear testing showed that A1203-SiC nano-composites containing 5 vol% SiC exhibited wear rates at least two times lower than pure A1203
[7]. To date AI203-SiC nano-composites have been mainly investigated in bulk form. This paper reports an investigation into the production of wear resistant and fracture-resistant plasma sprayed AI203-SiC nano-composite coatings. 2. COATING OPTIMISATION Nano-composite A1203-SiC powders containing 2.5, 5 and 10 % (by volume) SiC were obtained. Initial x-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses of the powders confirmed the presence of nano-sized (10-20 nm) crystalline SiC particles within the larger tI-A1203 grains.
512
2.1. Physical Powder Characteristics Considerable study was devoted to the physical properties (the size distribution and flow characteristics) of the powders. Eventually a set of powders with adequate flowability for plasma spraying was obtained.
2.2. Spray Trials 2.2.1
Procedure
Spray trials were conducted using a commercial Sulzer-Metco spray gun. Spray parameters were based initially on recommended spray parameters for commercial fine alumina powders. For comparison, pure alumina coatings were also deposited. For the optimisation trials all of the coatings were deposited onto plain carbon steel substrates which had been grit-blasted and coated with a commercial Ni-AI bond layer. Metallographic cross-sections of the coatings were prepared. These were examined using a Leitz Laborlux metallggraphic optical microscope in conjunction with a JVC camera. Using commercial image analysis software (Image Pro Plus) running on a PC the apparent porosity of the coatings was assessed from at least six micrographs taken from different areas of the coatings at a nominal magnification of 250x. The microhardness of the coatings was measured on the same cross-sections using a Shimadzu HMV2000 microhardness testing machine. At least five hardness indentations were made on each cross-section at a load of 0.98N (0.100 kgf) and a 15 second dwell time.
2.2.2
alumina coating produced at the same distance, Fig 1. The 5% SiC nano-composite coating, although improved by the optimised spray parameters, showed higher porosity and lower hardness than the other nano-composite coatings. The reasons for this difference remain unclear. XRD of the resultant coatings confirmed the retention of crystalline SiC in the coatings although the alumina matrix for the coatings was 7-A1203 rather than the ot-A1203 phase present in the feed powders. TEM also showed the presence of 1020nm SiC crystals within 7-A1203 coating grains. 3. TRIBOLOGICAL TESTING PROCEDURES
3.1. Sample preparation Two types of piston ring sample were fabricated: (i) plasma spray coated spheroidal graphite cast iron rings and; (ii) plasma spray coated plain carbon steel rings. The cast iron rings (supplied by AE Goetze)
Results
The initial spray trials yielded highly porous coatings with a low indicated hardness compared to commercial alumina coatings. Subsequent coatings were deposited using a proprietary plasma gun. These coatings showed a lower porosity and higher indicated hardness than the previous coatings but still inferior (higher porosity and lower indicated hardness) than the commercial alumina coatings, Fig 1. Further optimisation of the spray parameters led to improved nano-composite coatings. At a small working distance (50mm) both the 2.5% and 10% SiC nano-composite coatings showed higher hardnesses and lower porosity than the commercial
Fig 1. Porosity and microhardness (HV0,100) of plasma sprayed A1203-SiC nano-composite coatings deposited with trial parameters (open bars) and optimised parameters (shaded bars) using a proprietary plasma torch. For comparison, properties for a plasma sprayed pure alumina coating are included.
513
were coated whilst mounted on steel mandrels. The coated mandrels were then returned to AE Goetze for finishing using production-scale SiC grinding procedures. The cast iron piston rings were produced in all three nano-composite coating compositions. The plain carbon steel rings were produced by spray coating a solid plain carbon steel cylinder. The coated cylinder surface was then diamond ground and bored out. The ring samples were parted off from the cylinder and a bevel or "land" was applied to the coating edges to prevent premature coating fracture. Only the 2.5% SiC powder was used for these rings. The rings are listed in Table 1. In all cases the substrates were grit blasted and then plasma sprayed with a commercial Ni-AI bond coat prior to plasma spraying with the nanocomposite coating. The optimal spray parameters previously identified were used for all of the coatings. The total coating thickness was approximately 250#m and the outside diameter of all the rings was 90mm. Table 1. Piston ring samples Ring Coating Subst. D02 2.5% SiC steel S02 2.5% SiC cast iron S05 5% SiC cast iron S10 10% SiC cast iron CKS CKS-36 cast iron
tested. Finish diamond ground commercial (SIC) commercial (SIC) commercial (SIC) commercial (SIC)
A commercial chromium coated ring, CKS-36 (supplied by AE Goetze) was also tested. The CKS36 comprises a multilayer electroplated chromium coating in which alumina particles are embedded
[8].
3.2. Test Parameters
High-speed reciprocating wear tests were performed using a Plint TE-77 test machine. Small sections (approximately. 25-30 mm long) of the uncoated and coated piston rings were slid against sections from a 130mm i.d. grey cast iron liner. All tests were conducted using an additive free base oil (Rocol, UK). The wear test parameters are listed in Table 2. Tribological testing of stainless steel piston rings with and without sputter-deposited glassy alumina-based coatings has been reported previously
[9]. The use of 90mm diameter rings sliding against a 130mm internal diameter liner produced a small elliptical wear scar. This simplified wear measurement and microscopy considerably. The 40N load gave average contact pressures at the end of the wear tests of between 4 and 8 MPa. Table 2. Reciprocating wear test parameters. Applied load (N) 40 Stroke (mm) 15 40 Frequency (Hz) 1.2 Average sliding speed (m/sec) 200 Temperature (~ Duration (hours)
3.3. Characterisation of worn surfaces
Contacting-2D profilometry was used to quantify the wear of both the rings and the liners. In the case of the liners the profiles were measured parallel to the reciprocation direction while for the rings the profiles were measured perpendicular to the
Fig 2. Wear of ring and liner samples after 6 hours wear testing. For sample codes see Table 1.
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Fig 3. Worn surfaces of plasma sprayed nano-composite coatings after 6 hours testing: a) S05 ring exhibiting pull-out (fine particles) with only a small number of smoothly worn grains; b) D02 ring - smoothly worn. For sample codes see Table 1. reciprocation direction. The wear (in /tm 2) was calculated from the area of the trace after subtracting any background slope and curvature using unworn portions of the profile. The worn surfaces were examined using scanning electron microscopy (SEM). 4. TRIBOLOGICAL TESTING- RESULTS The wear of both ring and liner samples for the plasma spray coatings was higher than for the CKS-36 coating, Fig 2. While the increase in liner wear due to the adoption of plasma sprayed ceramic coatings on piston rings has been observed previously [1] the high wear of the coatings themselves was not expected since they had a higher hardness than the CKS-36 coating (879+_77 HV0,100). Examination (by SEM) of the worn plasma sprayed coatings revealed two distinct types of behaviour. One gave a rough surface and was attributed to pull out of material from the coating surface, Fig 3a. The second gave a smooth surface due to a much finer-scale wear response of the coating, Fig 3b. The former was more prevalent in coatings that exhibited high wear. Although there was considerable spread, the S05 rings (5% SiC) exhibited higher wear than the S02
and S 10 rings. Additionally there was lower coating wear for the D02 ring compared to the S02 ring. The high points of the honed liner surface had been worn fiat with small grooves parallel to the sliding direction. These latter grooves were more pronounced for the liner samples coupled with the plasma sprayed A1203-SiC coatings than for the liners paired with the commercial CKS coating. Also visible within the wear tracks were deeper groves remaining from the original (honed) surface topography. 5. DISCUSSION AND FUTURE WORK The plasma sprayed AI203-SiC coatings performed poorly in piston-ring simulation tests due
Fig 4. A large partially-melted particle observed in the coating on one of the S02 rings.
~1~
to their poor cohesive strength. This allowed large amounts of the coating to be removed during wear testing. This wear mechanism was more prevalent in the higher SiC content coatings and was also sensitive to the surface finishing technique. Metallographic examination of cross-sections of the coatings revealed the presence of Fe-rich particles (probably formed during the powder production) and unidentified small black inclusions. Most detrimental to the cohesive strength however was the presence of large (up to 30#m in diameter) partially-melted particles within the coatings, Fig 4. As a result of the present tribological study, significant attention has been paid to improving the coating density (lowering porosity) and in particular to the elimination of unmelted or partially-melted particles within the coatings. Current powders have a much narrower size distribution and the thermal spray parameters have been optimised still further. Tribological testing of this next generation of A1203SiC nano-composite coatings is presently underway. 6. CONCLUSIONS Wear tests have been conducted with plasma sprayed coatings prepared from AI203-SiC nanocomposite powders. The powders contained 2.5, 5 and 10 percent (by volume) SiC. The following conclusions were drawn. 1. Following satisfactory powder characterisation, A1203-SiC nano-composite coatings were successfully deposited using the nano-composite feed powders. Optimisation of spray parameters gave microhardness behaviour equivalent to or superior to commercial plasma sprayed pure alumina coatings. 2. The plasma sprayed nano-composite coatings showed poor wear resistance in simulated pistonring tests compared to an existing commercial CKS-36 coated piston ring. Two types of wear mechanism were observed for the sprayed coatings: (i) cohesive failure (grain pull-out due to cohesive failure) and (ii) smooth wear. Cohesive failure was related to the presence of unmelted particles (approximately 30#m in diameter) in the coatings.
3. The wear of the plasma spray coatings was highest for the 5% SiC coatings and lowest for the 2.5% SiC coatings. The use of diamond lapping procedures was also found to give lower coating wear than when the commercial SiC finishing process was used. REFERENCES
1. U. Buran et al, in Surface Modifications and Coatings, R.D. Sisson ed., ASM (1985) 255-264. 2. J.C. Bell & K.M. Delargy, in Mechanics of Coatings, Proceedings of the 16th Leeds-Lyon Symposium on Tribology, D. Dowson et al eds, Elsevier, Amsterdam, (1989) 371-378. 3. M. Dela~t, in Surface Modification Technologies XI, T.S. Sudarshan et al eds, The Institute of Materials, London (1998) 256-273. 4. K. Niihara, in Advanced Structural Inorganic Composites, P. Vincenzini Ed., Elsevier Science Publishers, London (1990) 637-664. 5. M. Sternitzke, Journal of the European Ceramic Society 17 (1997) 1061-1082. 6. H.K. Schmid et al, Journal of the European Ceramic Society 18 (1998) 39-49. 7. R.W. Davidge et al, Journal of the European Ceramic Society 16 (1996) 799-802. 8. W.J. Griffiths & F.-G. Cantow, T&N Technical Symposium 1995 (1995), paper 23. 9. K.L. Dahm & P.A. Dearnley in Boundary and
Mixed Lubrication; Science and Applications, Proceedings of the 28th Leeds-Lyon Symposium on Tribology, D. Dowson et al eds, Elsevier, Amsterdam, (2002) 243-246. ACKNOWLEDGEMENTS This research (BRST-CT98-5490) was funded by the European Commission under the Innovative Manufacturing Technologies Programme of Framework IV. The authors are grateful to all partners in the project, especially Federal Mogul Wiesbaden, Plasma Coatings Ltd, the University of Siegen and Zoz GmbH.
511
The wear response of ceramic matrix nano-composite coatings in simulated pistonring/cylinder wall test environments K.L. Dahm, K. Panagopoulos, and P.A. Dearnley a a School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
A series of piston rings with surface coatings of alumina-matrix nano-composite materials have been evaluated in lubricated high-speed sliding contact with cast-iron cylinder liners at 200~ Comparisons were made with standard CKS-36 (a commercial coating comprising chromium plating reinforced with alumina particles) coated cast-iron piston rings. Two types of wear mechanism were observed for the nano-composite coatings: (i) micro-scale cohesive failure, and (ii) smooth wear. The latter mechanism resulted in lower wear rates. The predominance of a given wear mechanism was found to depend on the coating composition and type of grinding operation used to finish the nano-composite coated piston rings.
1.2. Alumina-matrix nanocomposites 1. BACKGROUND AND RATIONALE Increasing performance demands placed on the piston ring-cylinder liner couple are increasing demand for highly wear-resistant piston ring coatings. There is additional environmental impetus to replace techniques such as electroplating (of chromium in particular) with environmentallyfriendly plasma-based coating techniques.
1.1. Plasma-sprayed ceramic coatings Plasma spraying represents a versatile technique for deposition of metallic, ceramic and cermet coatings onto metallic substrates. Molybdenum-base coatings are widely used for piston ring coatings due to their resistance to scuffing. These coatings are also nearing the limits of operation [ 1]. Several authors have investigated plasma Sprayed alumina (A1203) coatings as an alternative to both electrodeposited chromium and thermally sprayed molybdenum coatings [1-3]. Tribological testing of such coatings has shown that while they are resistant to wear and scuffing, long running times leads to coating fracture (or "break-out") [1 ]. A ceramic coating that combines the inherent tribological advantages of A1203 but increases its resistance to fracture is therefore highly desirable.
Ceramic nano-composites of A1203 and silicon carbide (SIC) have been reported to possess enhanced fracture toughness compared with pure A1203 [4]. These nano-composites comprise nanosized (typically <200nm) SiC particles embedded within A1203 grains. The mechanism by which this enhanced fracture toughness is achieved and the magnitude of the increase are currently unclear [5,6]. Wet erosive wear testing showed that A1203-SiC nano-composites containing 5 vol% SiC exhibited wear rates at least two times lower than pure A1203
[7]. To date AI203-SiC nano-composites have been mainly investigated in bulk form. This paper reports an investigation into the production of wear resistant and fracture-resistant plasma sprayed AI203-SiC nano-composite coatings. 2. COATING OPTIMISATION Nano-composite A1203-SiC powders containing 2.5, 5 and 10 % (by volume) SiC were obtained. Initial x-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses of the powders confirmed the presence of nano-sized (10-20 nm) crystalline SiC particles within the larger tI-A1203 grains.
512
2.1. Physical Powder Characteristics Considerable study was devoted to the physical properties (the size distribution and flow characteristics) of the powders. Eventually a set of powders with adequate flowability for plasma spraying was obtained.
2.2. Spray Trials 2.2.1
Procedure
Spray trials were conducted using a commercial Sulzer-Metco spray gun. Spray parameters were based initially on recommended spray parameters for commercial fine alumina powders. For comparison, pure alumina coatings were also deposited. For the optimisation trials all of the coatings were deposited onto plain carbon steel substrates which had been grit-blasted and coated with a commercial Ni-AI bond layer. Metallographic cross-sections of the coatings were prepared. These were examined using a Leitz Laborlux metallggraphic optical microscope in conjunction with a JVC camera. Using commercial image analysis software (Image Pro Plus) running on a PC the apparent porosity of the coatings was assessed from at least six micrographs taken from different areas of the coatings at a nominal magnification of 250x. The microhardness of the coatings was measured on the same cross-sections using a Shimadzu HMV2000 microhardness testing machine. At least five hardness indentations were made on each cross-section at a load of 0.98N (0.100 kgf) and a 15 second dwell time.
2.2.2
alumina coating produced at the same distance, Fig 1. The 5% SiC nano-composite coating, although improved by the optimised spray parameters, showed higher porosity and lower hardness than the other nano-composite coatings. The reasons for this difference remain unclear. XRD of the resultant coatings confirmed the retention of crystalline SiC in the coatings although the alumina matrix for the coatings was 7-A1203 rather than the ot-A1203 phase present in the feed powders. TEM also showed the presence of 1020nm SiC crystals within 7-A1203 coating grains. 3. TRIBOLOGICAL TESTING PROCEDURES
3.1. Sample preparation Two types of piston ring sample were fabricated: (i) plasma spray coated spheroidal graphite cast iron rings and; (ii) plasma spray coated plain carbon steel rings. The cast iron rings (supplied by AE Goetze)
Results
The initial spray trials yielded highly porous coatings with a low indicated hardness compared to commercial alumina coatings. Subsequent coatings were deposited using a proprietary plasma gun. These coatings showed a lower porosity and higher indicated hardness than the previous coatings but still inferior (higher porosity and lower indicated hardness) than the commercial alumina coatings, Fig 1. Further optimisation of the spray parameters led to improved nano-composite coatings. At a small working distance (50mm) both the 2.5% and 10% SiC nano-composite coatings showed higher hardnesses and lower porosity than the commercial
Fig 1. Porosity and microhardness (HV0,100) of plasma sprayed A1203-SiC nano-composite coatings deposited with trial parameters (open bars) and optimised parameters (shaded bars) using a proprietary plasma torch. For comparison, properties for a plasma sprayed pure alumina coating are included.
513
were coated whilst mounted on steel mandrels. The coated mandrels were then returned to AE Goetze for finishing using production-scale SiC grinding procedures. The cast iron piston rings were produced in all three nano-composite coating compositions. The plain carbon steel rings were produced by spray coating a solid plain carbon steel cylinder. The coated cylinder surface was then diamond ground and bored out. The ring samples were parted off from the cylinder and a bevel or "land" was applied to the coating edges to prevent premature coating fracture. Only the 2.5% SiC powder was used for these rings. The rings are listed in Table 1. In all cases the substrates were grit blasted and then plasma sprayed with a commercial Ni-AI bond coat prior to plasma spraying with the nanocomposite coating. The optimal spray parameters previously identified were used for all of the coatings. The total coating thickness was approximately 250#m and the outside diameter of all the rings was 90mm. Table 1. Piston ring samples Ring Coating Subst. D02 2.5% SiC steel S02 2.5% SiC cast iron S05 5% SiC cast iron S10 10% SiC cast iron CKS CKS-36 cast iron
tested. Finish diamond ground commercial (SIC) commercial (SIC) commercial (SIC) commercial (SIC)
A commercial chromium coated ring, CKS-36 (supplied by AE Goetze) was also tested. The CKS36 comprises a multilayer electroplated chromium coating in which alumina particles are embedded
[8].
3.2. Test Parameters
High-speed reciprocating wear tests were performed using a Plint TE-77 test machine. Small sections (approximately. 25-30 mm long) of the uncoated and coated piston rings were slid against sections from a 130mm i.d. grey cast iron liner. All tests were conducted using an additive free base oil (Rocol, UK). The wear test parameters are listed in Table 2. Tribological testing of stainless steel piston rings with and without sputter-deposited glassy alumina-based coatings has been reported previously
[9]. The use of 90mm diameter rings sliding against a 130mm internal diameter liner produced a small elliptical wear scar. This simplified wear measurement and microscopy considerably. The 40N load gave average contact pressures at the end of the wear tests of between 4 and 8 MPa. Table 2. Reciprocating wear test parameters. Applied load (N) 40 Stroke (mm) 15 40 Frequency (Hz) 1.2 Average sliding speed (m/sec) 200 Temperature (~ Duration (hours)
3.3. Characterisation of worn surfaces
Contacting-2D profilometry was used to quantify the wear of both the rings and the liners. In the case of the liners the profiles were measured parallel to the reciprocation direction while for the rings the profiles were measured perpendicular to the
Fig 2. Wear of ring and liner samples after 6 hours wear testing. For sample codes see Table 1.
514
Fig 3. Worn surfaces of plasma sprayed nano-composite coatings after 6 hours testing: a) S05 ring exhibiting pull-out (fine particles) with only a small number of smoothly worn grains; b) D02 ring - smoothly worn. For sample codes see Table 1. reciprocation direction. The wear (in /tm 2) was calculated from the area of the trace after subtracting any background slope and curvature using unworn portions of the profile. The worn surfaces were examined using scanning electron microscopy (SEM). 4. TRIBOLOGICAL TESTING- RESULTS The wear of both ring and liner samples for the plasma spray coatings was higher than for the CKS-36 coating, Fig 2. While the increase in liner wear due to the adoption of plasma sprayed ceramic coatings on piston rings has been observed previously [1] the high wear of the coatings themselves was not expected since they had a higher hardness than the CKS-36 coating (879+_77 HV0,100). Examination (by SEM) of the worn plasma sprayed coatings revealed two distinct types of behaviour. One gave a rough surface and was attributed to pull out of material from the coating surface, Fig 3a. The second gave a smooth surface due to a much finer-scale wear response of the coating, Fig 3b. The former was more prevalent in coatings that exhibited high wear. Although there was considerable spread, the S05 rings (5% SiC) exhibited higher wear than the S02
and S 10 rings. Additionally there was lower coating wear for the D02 ring compared to the S02 ring. The high points of the honed liner surface had been worn fiat with small grooves parallel to the sliding direction. These latter grooves were more pronounced for the liner samples coupled with the plasma sprayed A1203-SiC coatings than for the liners paired with the commercial CKS coating. Also visible within the wear tracks were deeper groves remaining from the original (honed) surface topography. 5. DISCUSSION AND FUTURE WORK The plasma sprayed AI203-SiC coatings performed poorly in piston-ring simulation tests due
Fig 4. A large partially-melted particle observed in the coating on one of the S02 rings.
~1~
to their poor cohesive strength. This allowed large amounts of the coating to be removed during wear testing. This wear mechanism was more prevalent in the higher SiC content coatings and was also sensitive to the surface finishing technique. Metallographic examination of cross-sections of the coatings revealed the presence of Fe-rich particles (probably formed during the powder production) and unidentified small black inclusions. Most detrimental to the cohesive strength however was the presence of large (up to 30#m in diameter) partially-melted particles within the coatings, Fig 4. As a result of the present tribological study, significant attention has been paid to improving the coating density (lowering porosity) and in particular to the elimination of unmelted or partially-melted particles within the coatings. Current powders have a much narrower size distribution and the thermal spray parameters have been optimised still further. Tribological testing of this next generation of A1203SiC nano-composite coatings is presently underway. 6. CONCLUSIONS Wear tests have been conducted with plasma sprayed coatings prepared from AI203-SiC nanocomposite powders. The powders contained 2.5, 5 and 10 percent (by volume) SiC. The following conclusions were drawn. 1. Following satisfactory powder characterisation, A1203-SiC nano-composite coatings were successfully deposited using the nano-composite feed powders. Optimisation of spray parameters gave microhardness behaviour equivalent to or superior to commercial plasma sprayed pure alumina coatings. 2. The plasma sprayed nano-composite coatings showed poor wear resistance in simulated pistonring tests compared to an existing commercial CKS-36 coated piston ring. Two types of wear mechanism were observed for the sprayed coatings: (i) cohesive failure (grain pull-out due to cohesive failure) and (ii) smooth wear. Cohesive failure was related to the presence of unmelted particles (approximately 30#m in diameter) in the coatings.
3. The wear of the plasma spray coatings was highest for the 5% SiC coatings and lowest for the 2.5% SiC coatings. The use of diamond lapping procedures was also found to give lower coating wear than when the commercial SiC finishing process was used. REFERENCES
1. U. Buran et al, in Surface Modifications and Coatings, R.D. Sisson ed., ASM (1985) 255-264. 2. J.C. Bell & K.M. Delargy, in Mechanics of Coatings, Proceedings of the 16th Leeds-Lyon Symposium on Tribology, D. Dowson et al eds, Elsevier, Amsterdam, (1989) 371-378. 3. M. Dela~t, in Surface Modification Technologies XI, T.S. Sudarshan et al eds, The Institute of Materials, London (1998) 256-273. 4. K. Niihara, in Advanced Structural Inorganic Composites, P. Vincenzini Ed., Elsevier Science Publishers, London (1990) 637-664. 5. M. Sternitzke, Journal of the European Ceramic Society 17 (1997) 1061-1082. 6. H.K. Schmid et al, Journal of the European Ceramic Society 18 (1998) 39-49. 7. R.W. Davidge et al, Journal of the European Ceramic Society 16 (1996) 799-802. 8. W.J. Griffiths & F.-G. Cantow, T&N Technical Symposium 1995 (1995), paper 23. 9. K.L. Dahm & P.A. Dearnley in Boundary and
Mixed Lubrication; Science and Applications, Proceedings of the 28th Leeds-Lyon Symposium on Tribology, D. Dowson et al eds, Elsevier, Amsterdam, (2002) 243-246. ACKNOWLEDGEMENTS This research (BRST-CT98-5490) was funded by the European Commission under the Innovative Manufacturing Technologies Programme of Framework IV. The authors are grateful to all partners in the project, especially Federal Mogul Wiesbaden, Plasma Coatings Ltd, the University of Siegen and Zoz GmbH.