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Microstructure and wear mechanisms of thermal-sprayed alumina coatings
January 2001
Materials Letters 47 Ž2001. 77–82 www.elsevier.comrlocatermatlet
Microstructure and wear mechanisms of thermal-sprayed alumina coatings P.P. Psyllaki a,) , M. Jeandin b, D.I. Pantelis c a
c
Materials Science and Engineering Section, Department of Chemical Engineering, National Technical UniÕersity of Athens, Zografou, Greece b Ecole des Mines de Paris, Centre des Materiaux Pierre-Marie Fourt, EÕry, France ´ Laboratory of Shipbuilding Technology, Department of NaÕal Architecture and Marine Engineering, National Technical UniÕersity of Athens, Zografou, Greece Received 28 February 2000; received in revised form 15 June 2000; accepted 19 June 2000
Abstract Alumina coatings, deposited on metallic substrates by two thermal spraying techniques, were tested against sintered alumina in a pin-on-disc tribometer. The wear mechanics involved were investigated with respect to the microstructural characteristics of the coatings. The denser Al 2 O 3 coatings presented higher wear resistance than the more porous ones. q 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.Rs; 81.40.Pq Keywords: Atmospheric plasma spraying; Detonation gun spraying; Thermal-sprayed coatings; Alumina coatings; Pin-on-disc testing; Wear mechanisms
1. Introduction Thermal spraying belongs to the class of semimolten state coating techniques. It is a general term used for the description all the techniques consisting of the injection of the selected powder into an area of high temperature, where they are melted, accelerated and directed onto the substrate surface. The coatings are formed by the immediate solidification of the molten droplets on the surface of the substrate,
) Corresponding author. Current address: Laboratoire de Recherches sur la Reactivite ´ ´ des Solides, UMR 5613 CNRS, Universite´ de Bourgogne, UFR Sciences et Techniques, BP 47 870, 21078 Dijon cedex, France. Tel.: q33-3-80396137; fax: q33-3-80396132. E-mail address: [email protected] ŽP.P. Psyllaki..
which is, in general, of much lower temperature Žambient temperature.. Ceramic coatings produced by thermal spraying are widely used in a range of industrial applications to provide wear and erosion resistance, corrosion protection and thermal insulation onto metallic substrates. Especially, alumina coatings are commonly used to resist wear by solid particle erosion w1x and friction w2–6x. Among thermal spraying techniques, atmospheric plasma spraying ŽAPS. is a rather simple process from a practical point of view; thus, it has become well-established as a commercial process for the realisation of ceramic coatings. However, the coatings obtained in this way present microstructures with typical types of defects, i.e. interlamellar microcracks, un-molten particles, somewhat weak interfaces and pores primarily between solidified splats
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 1 5 - 9
78
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
w7x. On the other hand, the detonation gun ŽDG. spraying technique w8x is known to promote, for some materials, excellent densification, but its use for industrial applications is still limited. The present work is a comparative study on the influence of the microstructural characteristics of APS and DG alumina coatings on the wear mechanics taking place during sliding friction. 2. Experimental Alumina coatings were achieved by APS and DG techniques on cast-iron and Hastelloy X substrates, respectively. The former had a thickness of 320 mm, obtained in 8 successive passes, while a 60-mm-thick Ni–5%Al bond layer was previously deposited by the same technique, in order to ameliorate the adhesion to the metallic substrate w9x. The latter was directly deposited on Hastelloy X and reached its final thickness Ž200 mm. in two successive passes. The characterisation of the examined coatings comprised: S.E.M. observations ŽJEOL 3700., X-ray
diffraction analysis ŽPHILIPS diffractometer., roughness ŽHOMMER TESTER T-500. and microhardness ŽLEITZ. measurements. Sliding friction tests were carried out using a pin-on-disc apparatus ŽCentre Suisse d’Electrotechnique et de Microtechnique, C.S.E.M... All tests were performed using a normal load of 10 N, a sliding speed of 0.1 m sy1 and an alumina ball Ždiameter: 6 mm, hardness: 1900 HV. as a counterbody. During testing, the relative humidity of the environment was kept at 25% while the sliding. Radius was 5 mm. Interrupted tests every 20.000 sliding cycles permitted to estimate both the wear lifetime and rate of the coatings, as well as the wear mechanisms involved in the degradation of the ceramic coatings. 3. Results and discussion 3.1. Coatings’ characterisation The alumina coatings realised by the APS technique were characterised by the coexistence of both
Fig. 1. SEM micrographs of the examined coatings: Ža. cross-section and Žb. surface splats of the APS Al 2 O 3 coatings, Žc. cross-section and Žd. surface splats of the DG Al 2 O 3 coatings.
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
79
the splats’ borders. The microhardness presented no fluctuation, tending to a constant value of 1200 HV0.3 . The average surface roughness Ž R a . was found to be 3.5 mm. X-ray diffraction measurements indicated that both APS and DG alumina coatings consisted of a mixture of a- and g-Al 2 O 3 ŽFig. 2.. Quantitative analysis showed that, in the case of DG coatings, the percentage of a-Al 2 O 3 was almost double Ž7%. than that determined in the case of APS coatings Ž3%.. 3.2. Friction coefficient and wear rates
Fig. 2. X-ray diffraction diagrams of Ža. APS and Žb. DG Al 2 O 3 coatings Ž1: g-Al 2 O 3 , 2: a-Al 2 O 3 ..
un-molten and semi-molten particles, as well as of pores with a diameter of 2–5 mm ŽFig. 1a.. Their porosity, determined by image analysis means, was about 10%. Microscopic observations of the in-depth splats’ morphology indicated a progressive variation from splash-like, for the first, to disc-like geometry, for the last layers deposited. The splash-splats near the Al 2 O 3rbond layer interface had a mean diameter of 35 mm, while that of the disc-like surface splats was about 55 mm ŽFig. 1b.. This variation was due to the progressive increase of the temperature of the deposition surface, on which the solidification of the melted droplets is taking place, during multi-pass spraying w9x. During cooling, the elevated stresses developed in the coating led to the extensive cracking of the surface splats. The high microstructural heterogeneity influenced the microhardness of the coatings, which presented a wide fluctuation, ranging from 820 to 1150 HV0.3 . Finally, the average surface roughness Ž R a . was found to be 6.0 mm. The alumina coatings realized by DG technique ŽFig. 1c. presented a porosity of 4%. The surface splats were disc-like ŽFig. 1d., with a mean diameter of 30 mm, while some random cracks were limited at
For both the coatings examined, regardless of their microstructural differences, the friction coefficient of the contact Al 2 O 3 coatingrsintered Al 2 O 3 ball was 0.78 and remained constant until the coating total was worn ŽFig. 3.. The wear lifetime was estimated by interrupted tests with a step of 20.000 sliding cycles and was found to be ; 110,000 and ; 85,000 sliding cycles for the APS and the DG coatings, respectively. Taking into account that the thickness of the former was about 50% higher than that of the latter, the higher wear resistance exhibited by the DG Al 2 O 3 coatings should be noticed. The wear rates were calculated as the volume loss per unit of applied load and unit of sliding distance. The volume losses were determined by measurements on micrographs of cross-sections of the specimens worn by interrupted tests. For both coatings, the wear rate was found constant during testing: 10y3 mm3 Ny1 my1 , in the case of APS Al 2 O 3
Fig. 3. Friction coefficient evolution against a sintered alumina ball Žapplied load: 10 N, sliding speed: 0.1 m sy1 ..
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coating, and four times lower Ž0.25 = 10y3 mm3 Ny1 my1 ., in the case of the denser DG coating. 3.3. Wear mechanisms For both the coatings examined, microscopic observations of the worn surfaces indicated that the wear was taking place mainly by adhesion mechanism ŽFig. 4. and was enhanced by the exfoliation of the surface splats ŽFig. 5a.. The latter mechanism was also observed by other researchers w10x, working under fretting wear conditions. Taking into account the differences in testing parameters, the exfoliation mechanism is presented in Fig. 5b. According to this model, friction can result in the initiation of a crack between two splats of the same lamella. During the cyclic loading of the surface, the crack is propagating following the splat boundary, leading to its final exfoliation from the worn surface. In the case of APS coatings, exfoliation was facilitated by the presence of pre-cracked surface splats. Observations of sections perpendicular to the wear traces showed the activation of two more degradation mechanisms. In the case of the APS Al 2 O 3 coatings, the weak interfaces between the successive lamellae failed, leading to the delamination of the coating, which was extended for long distance from the axis of loading ŽFig. 6a.. Similar observations concerning delamination phenomena due to static loading of plasma-sprayed ceramic coatings were also referred to in Refs. w11,12x. This degradation mechanism induced the rapid wear of the ceramic
Fig. 4. SEM micrograph of the worn surface ŽDG Al 2 O 3 coating, after 40,000 sliding cycles..
Fig. 5. Ža. SEM micrograph, indicating exfoliation of splats from the worn surface ŽDG Al 2 O 3 coating, after 40,000 sliding cycles.. Žb. Exfoliation mechanism during sliding friction testing w10x.
coating and the increase of the wear rate. In the case of the dense DG Al 2 O 3 coatings, the stress field developed during sliding led to the initiation of cracks perpendicular to the coatingrsubstrate interface and to their propagation through the coating’s thickness ŽFig. 6b.. The presence of these cracks does not seem to have significant role on the acceleration of the coatings wear. The above-mentioned mechanisms were confirmed by the observation of the wear debris remaining on the worn surfaces. In the case of APS coatings, wear debris presented a wide distribution from 0.12 to 1.00 mm ŽFig. 7a.. Debris with mean size of 0.50 mm were present in a percentage of 40%. The co-action of the adhesionrexfoliation wear mechanisms and the delamination of the coating led to a mixture of Ža. big-sized and irregular-shaped debris
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and Žb. small-sized and regular-shaped debris. The former were debris produced during the last stages, while the latter were debris produced during the earlier stages of testing. Their entrapment at the contact area between tested materialrcounterbody led to the diminution of their size, due to the developed shear stresses. In the case of DG coatings, wear debris were almost spherical with a narrow distribution around a mean diameter of 0.25 mm ŽFig. 7b.. The denser microstructure, the smaller size of the splats, the absence of extensive pre-cracking and
Fig. 7. SEM micrographs of wear debris remained on the worn surface of Ža. APS and Žb. DG Al 2 O 3 coatings, after 80,000 sliding cycles.
delamination failure of these coatings did not favour the formation of large-sized debris.
4. Conclusions
Fig. 6. Ža. SEM micrograph, indicating delamination of the successive lamellae ŽAPS Al 2 O 3 coating, after 40,000 sliding cycles.. Žb. SEM micrograph, indicating cracking perpendicularly to coatingrsubstrate interface ŽDG Al 2 O 3 coatings, after 90,000 sliding cycles..
The alumina coatings deposited by DG technique were denser and of higher wear resistance, compared to those deposited by APS. Although the microstructural characteristics of the thermal sprayed coatings had no influence on the value of friction coefficient, they were the principal factor, which determined the wear rate and the wear lifetime, as well as the wear mechanisms of the coatings examined. In the case of a punctually applied load, besides adhesion mechanism and cracking along the splats boundaries, which were observed for both coatings, the failure of the APS ones was enhanced by the delamination of the weakly adhered successive lamellae. On the con-
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trary, the propagation of cracks perpendicular to the coatingrsubstrate interface, which were observed in the case of DG coatings, did not seem to have any negative influence on their wear resistance.
Acknowledgements The authors would like to express their thanks to the CE.RE.CO. ŽChalkida, Greece., for the preparation of the APS coatings.
References w1x D.A.J. Ramm, T.W. Clyne, A.J. Sturgeon, S. Dunkerton, in: C.C. Berndt, S. Sampath ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 239. w2x S.E. Hartfield-Wunsch, S.C. Tung, in: C.C. Berndt, S. Sam¨ path ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 19.
w3x K. Niemi, P. Sorsa, P. Vuoristo, T. Mantyla, ¨ ¨ in: C.C. Berndt, S. Sampath ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 533. w4x R. Kingswell, D.S. Rickerby, K.T. Scott, S.J. Bull, Proc. 3rd National Thermal Spray Conference, ASM International, 1991, p. 179. w5x K. Kamachi, M. Magome, K. Ueno, G. Ueno, T. Yoshioka, Proc. 3rd National Thermal Spray Conference, ASM International, 1991, p. 497. w6x Y. Naerheim, C. Coddet, P. Droit, Proc. 8th Surface Modification Technologies ŽSMT8., The Institute of Materials, London, 1995, p. 734. w7x D.I. Pantelis, P. Psyllaki, N. Alexopoulos, Wear 237 Ž2000. 197. w8x E. Kadyrov, in: C.C. Berndt ŽEd.., Proc. 9th National Thermal Spray Conference, ASM International, 1996, p. 835. w9x D.I. Pantelis, I. Kyriopoulou, P. Psyllaki, M. Vardavoulias, Proc. 11th Surface Modification Technologies ŽSMT11., The Institute of Materials, London, 1998, p. 306. w10x S.E. Hartfield-Wunsch, S.C. Tung, 7th National Thermal ¨ Spray Conference, ASM International, 1994, p. 19. w11x A. Pajares, L. Wei, B. Lawn, J. Am. Ceram. Soc. 79 Ž1966. 1907. w12x A. Pajares, L. Wei, B. Lawn, N. Padtura, C. Berndt, Mater. Sci. Eng., A 208 Ž1996. 158.
Materials Letters 47 Ž2001. 77–82 www.elsevier.comrlocatermatlet
Microstructure and wear mechanisms of thermal-sprayed alumina coatings P.P. Psyllaki a,) , M. Jeandin b, D.I. Pantelis c a
c
Materials Science and Engineering Section, Department of Chemical Engineering, National Technical UniÕersity of Athens, Zografou, Greece b Ecole des Mines de Paris, Centre des Materiaux Pierre-Marie Fourt, EÕry, France ´ Laboratory of Shipbuilding Technology, Department of NaÕal Architecture and Marine Engineering, National Technical UniÕersity of Athens, Zografou, Greece Received 28 February 2000; received in revised form 15 June 2000; accepted 19 June 2000
Abstract Alumina coatings, deposited on metallic substrates by two thermal spraying techniques, were tested against sintered alumina in a pin-on-disc tribometer. The wear mechanics involved were investigated with respect to the microstructural characteristics of the coatings. The denser Al 2 O 3 coatings presented higher wear resistance than the more porous ones. q 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.Rs; 81.40.Pq Keywords: Atmospheric plasma spraying; Detonation gun spraying; Thermal-sprayed coatings; Alumina coatings; Pin-on-disc testing; Wear mechanisms
1. Introduction Thermal spraying belongs to the class of semimolten state coating techniques. It is a general term used for the description all the techniques consisting of the injection of the selected powder into an area of high temperature, where they are melted, accelerated and directed onto the substrate surface. The coatings are formed by the immediate solidification of the molten droplets on the surface of the substrate,
) Corresponding author. Current address: Laboratoire de Recherches sur la Reactivite ´ ´ des Solides, UMR 5613 CNRS, Universite´ de Bourgogne, UFR Sciences et Techniques, BP 47 870, 21078 Dijon cedex, France. Tel.: q33-3-80396137; fax: q33-3-80396132. E-mail address: [email protected] ŽP.P. Psyllaki..
which is, in general, of much lower temperature Žambient temperature.. Ceramic coatings produced by thermal spraying are widely used in a range of industrial applications to provide wear and erosion resistance, corrosion protection and thermal insulation onto metallic substrates. Especially, alumina coatings are commonly used to resist wear by solid particle erosion w1x and friction w2–6x. Among thermal spraying techniques, atmospheric plasma spraying ŽAPS. is a rather simple process from a practical point of view; thus, it has become well-established as a commercial process for the realisation of ceramic coatings. However, the coatings obtained in this way present microstructures with typical types of defects, i.e. interlamellar microcracks, un-molten particles, somewhat weak interfaces and pores primarily between solidified splats
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 1 5 - 9
78
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
w7x. On the other hand, the detonation gun ŽDG. spraying technique w8x is known to promote, for some materials, excellent densification, but its use for industrial applications is still limited. The present work is a comparative study on the influence of the microstructural characteristics of APS and DG alumina coatings on the wear mechanics taking place during sliding friction. 2. Experimental Alumina coatings were achieved by APS and DG techniques on cast-iron and Hastelloy X substrates, respectively. The former had a thickness of 320 mm, obtained in 8 successive passes, while a 60-mm-thick Ni–5%Al bond layer was previously deposited by the same technique, in order to ameliorate the adhesion to the metallic substrate w9x. The latter was directly deposited on Hastelloy X and reached its final thickness Ž200 mm. in two successive passes. The characterisation of the examined coatings comprised: S.E.M. observations ŽJEOL 3700., X-ray
diffraction analysis ŽPHILIPS diffractometer., roughness ŽHOMMER TESTER T-500. and microhardness ŽLEITZ. measurements. Sliding friction tests were carried out using a pin-on-disc apparatus ŽCentre Suisse d’Electrotechnique et de Microtechnique, C.S.E.M... All tests were performed using a normal load of 10 N, a sliding speed of 0.1 m sy1 and an alumina ball Ždiameter: 6 mm, hardness: 1900 HV. as a counterbody. During testing, the relative humidity of the environment was kept at 25% while the sliding. Radius was 5 mm. Interrupted tests every 20.000 sliding cycles permitted to estimate both the wear lifetime and rate of the coatings, as well as the wear mechanisms involved in the degradation of the ceramic coatings. 3. Results and discussion 3.1. Coatings’ characterisation The alumina coatings realised by the APS technique were characterised by the coexistence of both
Fig. 1. SEM micrographs of the examined coatings: Ža. cross-section and Žb. surface splats of the APS Al 2 O 3 coatings, Žc. cross-section and Žd. surface splats of the DG Al 2 O 3 coatings.
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
79
the splats’ borders. The microhardness presented no fluctuation, tending to a constant value of 1200 HV0.3 . The average surface roughness Ž R a . was found to be 3.5 mm. X-ray diffraction measurements indicated that both APS and DG alumina coatings consisted of a mixture of a- and g-Al 2 O 3 ŽFig. 2.. Quantitative analysis showed that, in the case of DG coatings, the percentage of a-Al 2 O 3 was almost double Ž7%. than that determined in the case of APS coatings Ž3%.. 3.2. Friction coefficient and wear rates
Fig. 2. X-ray diffraction diagrams of Ža. APS and Žb. DG Al 2 O 3 coatings Ž1: g-Al 2 O 3 , 2: a-Al 2 O 3 ..
un-molten and semi-molten particles, as well as of pores with a diameter of 2–5 mm ŽFig. 1a.. Their porosity, determined by image analysis means, was about 10%. Microscopic observations of the in-depth splats’ morphology indicated a progressive variation from splash-like, for the first, to disc-like geometry, for the last layers deposited. The splash-splats near the Al 2 O 3rbond layer interface had a mean diameter of 35 mm, while that of the disc-like surface splats was about 55 mm ŽFig. 1b.. This variation was due to the progressive increase of the temperature of the deposition surface, on which the solidification of the melted droplets is taking place, during multi-pass spraying w9x. During cooling, the elevated stresses developed in the coating led to the extensive cracking of the surface splats. The high microstructural heterogeneity influenced the microhardness of the coatings, which presented a wide fluctuation, ranging from 820 to 1150 HV0.3 . Finally, the average surface roughness Ž R a . was found to be 6.0 mm. The alumina coatings realized by DG technique ŽFig. 1c. presented a porosity of 4%. The surface splats were disc-like ŽFig. 1d., with a mean diameter of 30 mm, while some random cracks were limited at
For both the coatings examined, regardless of their microstructural differences, the friction coefficient of the contact Al 2 O 3 coatingrsintered Al 2 O 3 ball was 0.78 and remained constant until the coating total was worn ŽFig. 3.. The wear lifetime was estimated by interrupted tests with a step of 20.000 sliding cycles and was found to be ; 110,000 and ; 85,000 sliding cycles for the APS and the DG coatings, respectively. Taking into account that the thickness of the former was about 50% higher than that of the latter, the higher wear resistance exhibited by the DG Al 2 O 3 coatings should be noticed. The wear rates were calculated as the volume loss per unit of applied load and unit of sliding distance. The volume losses were determined by measurements on micrographs of cross-sections of the specimens worn by interrupted tests. For both coatings, the wear rate was found constant during testing: 10y3 mm3 Ny1 my1 , in the case of APS Al 2 O 3
Fig. 3. Friction coefficient evolution against a sintered alumina ball Žapplied load: 10 N, sliding speed: 0.1 m sy1 ..
80
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
coating, and four times lower Ž0.25 = 10y3 mm3 Ny1 my1 ., in the case of the denser DG coating. 3.3. Wear mechanisms For both the coatings examined, microscopic observations of the worn surfaces indicated that the wear was taking place mainly by adhesion mechanism ŽFig. 4. and was enhanced by the exfoliation of the surface splats ŽFig. 5a.. The latter mechanism was also observed by other researchers w10x, working under fretting wear conditions. Taking into account the differences in testing parameters, the exfoliation mechanism is presented in Fig. 5b. According to this model, friction can result in the initiation of a crack between two splats of the same lamella. During the cyclic loading of the surface, the crack is propagating following the splat boundary, leading to its final exfoliation from the worn surface. In the case of APS coatings, exfoliation was facilitated by the presence of pre-cracked surface splats. Observations of sections perpendicular to the wear traces showed the activation of two more degradation mechanisms. In the case of the APS Al 2 O 3 coatings, the weak interfaces between the successive lamellae failed, leading to the delamination of the coating, which was extended for long distance from the axis of loading ŽFig. 6a.. Similar observations concerning delamination phenomena due to static loading of plasma-sprayed ceramic coatings were also referred to in Refs. w11,12x. This degradation mechanism induced the rapid wear of the ceramic
Fig. 4. SEM micrograph of the worn surface ŽDG Al 2 O 3 coating, after 40,000 sliding cycles..
Fig. 5. Ža. SEM micrograph, indicating exfoliation of splats from the worn surface ŽDG Al 2 O 3 coating, after 40,000 sliding cycles.. Žb. Exfoliation mechanism during sliding friction testing w10x.
coating and the increase of the wear rate. In the case of the dense DG Al 2 O 3 coatings, the stress field developed during sliding led to the initiation of cracks perpendicular to the coatingrsubstrate interface and to their propagation through the coating’s thickness ŽFig. 6b.. The presence of these cracks does not seem to have significant role on the acceleration of the coatings wear. The above-mentioned mechanisms were confirmed by the observation of the wear debris remaining on the worn surfaces. In the case of APS coatings, wear debris presented a wide distribution from 0.12 to 1.00 mm ŽFig. 7a.. Debris with mean size of 0.50 mm were present in a percentage of 40%. The co-action of the adhesionrexfoliation wear mechanisms and the delamination of the coating led to a mixture of Ža. big-sized and irregular-shaped debris
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
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and Žb. small-sized and regular-shaped debris. The former were debris produced during the last stages, while the latter were debris produced during the earlier stages of testing. Their entrapment at the contact area between tested materialrcounterbody led to the diminution of their size, due to the developed shear stresses. In the case of DG coatings, wear debris were almost spherical with a narrow distribution around a mean diameter of 0.25 mm ŽFig. 7b.. The denser microstructure, the smaller size of the splats, the absence of extensive pre-cracking and
Fig. 7. SEM micrographs of wear debris remained on the worn surface of Ža. APS and Žb. DG Al 2 O 3 coatings, after 80,000 sliding cycles.
delamination failure of these coatings did not favour the formation of large-sized debris.
4. Conclusions
Fig. 6. Ža. SEM micrograph, indicating delamination of the successive lamellae ŽAPS Al 2 O 3 coating, after 40,000 sliding cycles.. Žb. SEM micrograph, indicating cracking perpendicularly to coatingrsubstrate interface ŽDG Al 2 O 3 coatings, after 90,000 sliding cycles..
The alumina coatings deposited by DG technique were denser and of higher wear resistance, compared to those deposited by APS. Although the microstructural characteristics of the thermal sprayed coatings had no influence on the value of friction coefficient, they were the principal factor, which determined the wear rate and the wear lifetime, as well as the wear mechanisms of the coatings examined. In the case of a punctually applied load, besides adhesion mechanism and cracking along the splats boundaries, which were observed for both coatings, the failure of the APS ones was enhanced by the delamination of the weakly adhered successive lamellae. On the con-
82
P.P. Psyllaki et al.r Materials Letters 47 (2001) 77–82
trary, the propagation of cracks perpendicular to the coatingrsubstrate interface, which were observed in the case of DG coatings, did not seem to have any negative influence on their wear resistance.
Acknowledgements The authors would like to express their thanks to the CE.RE.CO. ŽChalkida, Greece., for the preparation of the APS coatings.
References w1x D.A.J. Ramm, T.W. Clyne, A.J. Sturgeon, S. Dunkerton, in: C.C. Berndt, S. Sampath ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 239. w2x S.E. Hartfield-Wunsch, S.C. Tung, in: C.C. Berndt, S. Sam¨ path ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 19.
w3x K. Niemi, P. Sorsa, P. Vuoristo, T. Mantyla, ¨ ¨ in: C.C. Berndt, S. Sampath ŽEds.., Proc. 7th National Thermal Spray Conference, ASM International, 1994, p. 533. w4x R. Kingswell, D.S. Rickerby, K.T. Scott, S.J. Bull, Proc. 3rd National Thermal Spray Conference, ASM International, 1991, p. 179. w5x K. Kamachi, M. Magome, K. Ueno, G. Ueno, T. Yoshioka, Proc. 3rd National Thermal Spray Conference, ASM International, 1991, p. 497. w6x Y. Naerheim, C. Coddet, P. Droit, Proc. 8th Surface Modification Technologies ŽSMT8., The Institute of Materials, London, 1995, p. 734. w7x D.I. Pantelis, P. Psyllaki, N. Alexopoulos, Wear 237 Ž2000. 197. w8x E. Kadyrov, in: C.C. Berndt ŽEd.., Proc. 9th National Thermal Spray Conference, ASM International, 1996, p. 835. w9x D.I. Pantelis, I. Kyriopoulou, P. Psyllaki, M. Vardavoulias, Proc. 11th Surface Modification Technologies ŽSMT11., The Institute of Materials, London, 1998, p. 306. w10x S.E. Hartfield-Wunsch, S.C. Tung, 7th National Thermal ¨ Spray Conference, ASM International, 1994, p. 19. w11x A. Pajares, L. Wei, B. Lawn, J. Am. Ceram. Soc. 79 Ž1966. 1907. w12x A. Pajares, L. Wei, B. Lawn, N. Padtura, C. Berndt, Mater. Sci. Eng., A 208 Ž1996. 158.