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The analysis of the friction and wear mechanisms of plasma-sprayed ceramic coatings at 450 °C
Wear, G!8 (1988) 265 - 276
265
Summary In this paper, thy! friction and wear mechanisms of plasma-sprayed ceramic coatings (~r~Us~~iU~3~TiU~, ~r~~~l~%~i3%Cr~TiG,, Al&.&(Metco), WCl$%Co, PSZ, Al& (made in China)) at 450 “C have been studied. It has been determinedthat the friction azzdwear behaviour,such as fatigue sp~g, plastic deformation and adhesive transfer, of ceramic coatings is similar to that of metals. Under high ~rnpe~~~ (450 “61, the dry friction coefficients of ceramic coatings were much higher than the lubricated fiction coefficients of ceramic coatings. At the same time the wear me~h~isms and the frictional.surface ~po~aphy under dry lotion condition were much different from those under lub~~a~ conditions. Adhesive transfer occurred easily under dry friction conditions. However, it was ratherdifficult for adhesivefriction to take place when lubricated. 1(I Introduction With good resistanceto high ~ernpe~~re~wear and corrosion, plasma sprayed ceramic coatings have found their uses in many applicationssuch as tail exhaust pipes in rockets, liners and rings in adiabatic engines, bearings etc. The friction and wear behaviour of ceramic coatings is very important in their pr~ti~ applications; however, there are few papers pub~~~ on this subject. Because the temperaturesof the top ring reversalposition of the liner and the top ring on the piston are about 350 - 400 “c 111, the friction and wear mechanisms at high ternpe~~~s need to be studied. The aim of this paper is to study the friction and wear mechanisms of plasma-sprayed ceramic coatings at high temperatures(460 “Cl. 2. Test rig
The high temperaturefriction and wear tester used in the experiments is shown in Fig. 1 [Z] e One roller specimenand one block specimen formed the linearcontact fsiction couple. The roller specimenwas rotated by a 90 W
Fig. 1. High temperature friction tester.
electrical motor at a speed of 1450 rev mm-‘. The outer diameter of the roller specimen was 50 mm. The block specimen was driven by an eccentric to move back and forth. The average velocity of the block specimen could be varied between 0 and 1 mm s-l. The block specimen was 14 mm long, 6 mm wide and 5 mm thick. The linear contact length was 6 mm and the stroke of the block specimen was 10 mm. The velocity of the roller specimen was 3.8 m s-l, When the friction coefficient was 0.1, the Hertz load could reach 0.6 GPa. The block specimen was heated using local heating and the temperature could be varied between room temperature and 500 “C. The temperature was controlled using a 1 mm diameter thermocouple which was mounted 0.2 mm under the friction surface of the block specimen and a temperature controller. The lubricant was pumped into the entrance of the linear contact region. The rate of supply of lubricant could be varied between 0 and 1600 ml h-r. The friction torque and speed were measured using a torque-speed sensor and a torque-speed measuring instrument. 3. Test results and SEM analysis
The substrates of the roller and block specimens were made of 0.45 C plain common steel (Fig. 2). Ceramic coatings were sprayed onto the friction surfaces of the roller and block specimens using plasma spraying technology.
Fig. 2. Roller and block specimens.
267
Plasma spraying was done at the Beijing Spraying Centre. The spraying equipment came from Meteo Inc. and the spraying parameters used were chosen from a plasma spraying handbook of Metcu Inc. The sprayed coating had a thickness of 0.30 mm before grinding and a thickness of 0.20 mm after grinding. All the coating surfaces of the specimens were ground before friction testing to a roughness R, of less than 0.40 p, The specimens were cleaned for 10 min ultr~onic~ly using pure alcuhol before friction testing. The following ceramic powders were used to make the coatings: Cr,O,* (Metco 136F, Cr2035%Si023%TiOz) Cr&#
(M&co 30 NS, Cr~~*l2%Ni3%~r)
TiOz (M&co 102) A1,03 (Metco 105) WC18%Co (Metco 75F) PSZ (Metco 202NS, ~~~~O%Y~U~~ Al&
(made in China)
3.2. Lubricated test results 3.2.1. Test conditions
The results given in Table 1 were obtained under the following conditions: lubricant, mineral oil and calcium, sulphur, phosphorus and zinccontaining additives; velocity of roller specimen , 3.8 m C’; average velocity of block specimen, 1 mm s-l; quantity of supplied lubricant, 80 ml h-i; temperature of block specimen, kept at 450 “C. 3.2.2. Test procedure
Firstly, heat the block specimen and keep its temperature at 450 “C. Secondly, pump lubricant to the entrance of the contact region. Then, run
Block specimens
Friction coefficient 0.08 GPa load
0.20 GPa load
Cr2U3*
0.08 0.09 0.07 0.12 0.07 0.11
0.09 0.09 0.08 0.14 0.08 0.11
f&w+
TiUz WClEI%Co Al203 (China) PSZ
Total wear depth of block specimens (Mm) . . 2.6 1.0 3.2 3.3 10.0 18.0
268
for 10 min at a Hertz load of 0.08 GPa, measure the friction torque; after that, run for 20 min at a Hertz load of 0.20 GPa, measure the friction torque. 3.2.3. Test results The wear depth of the roller specimen was too small to measure. The lubricated test results are given in Table 1. It is found in Table 1 that the friction coefficients varied between 0.07 and 0.14. The couples of TiOz, Al,Os (China) blocks against Cr2031crollers gave the lowest friction coefficient and the couple of the Cr&# block against the Cr,Os+ roller showed the least wear. The pair of PSZ block us. Cr,O,* roller possessed the highest wear rate. 3.3. The observation of the lubricated friction surface using scanning electron microscopy and optical microscopy 3.3.1. Cr,03* Figure 3 shows the surface of the Cr,Os* block after friction testing. For the test conditions see Table 1. Plastic deformation is evident on the surface; the wear mechanism seems to be plastic smearing. Since the plasmasprayed coating has many large apertures in which oil can collect (the surface in Fig. 3 is relatively rough) this will be of benefit to lubrication. Figure 4 also shows the surface of the Cr*Os* block after friction testing. Dropping lubricant was employed and the other test conditions were the same as in Table 1. It is obvious in Fig. 4 that the surface has been smeared flat. The mechanism may be that, since only a little oil (drops) was used, the cooling effect of the lubricant had little effect; moreover, the coating has a rather low thermal conductivity. Both of these factors resulted in a very high surface temperature (melted drops of Cr20s* were found on some parts of
Fig. 3. The surface of C&03* after friction testing, conditions see Table 1. Fig. 4. The surface of Cr103* after friction testing, dropping oil, conditions see Table 1.
269
Fig, 5. The surface of C-q&* after frktion testing, drop k&&ant, 0.33 GPa Hertz load. Fig. 6. The surface of Cr$$# after friction testing, conditions see Table 1.
the surface), so that the surface was softened and severely deformed. In this way, the ap&tures were filled up and a smooth surface was formed. Figure 5 also shows the Cr,Os* surface after friction testing. Ilsopping lubricant was still used and the Hertz load was increased to 0.33 GPa. It is clear in Fig. 5 that severe deformation has taken place. More heat was generated and a higher temperature was reached. Therefore an adhesian junction, which had a higher strength than the substrate, was formed locally and caused severe deformation. 3.3.2. Cr&# Figure 6 shows the surface of the CrsC,# block after friction testing. This coating was rather hard but no obvious brittle spa&g was observed. This indicates that the coating exhibited a rather high cohesive strength. Many protuberances on the surface appeared to be plastically deformed but the extent of deformation had not reached that of Fig, 3. Figures 7 and 8 show the surface of Cr&#, dropping lubrication was used and the Hertz load reached 0.6 GPa, the other test conditions were the same as those in Table 3.. Remarkable plastic deformation had occurred on the surface but the degree of deformation was not as severe as that in Fig, 4 and no traces of adhesion were found, Several small circu&r holes were observed clearly on the deformed plane. These may have resulted from the melting and eruding of local nickel (its melting point is 1450 “C, lower than the melting point 1890 “C of CrsC$). From this point of view the surface temperature was far higher than the substrate temperature (450 “C) (the thermocouple head was 0.2 mm away from the friction surface and the coating has a low conductivity). The wear mechanism was still plastic smearing. Comparing the surface of Cr&,# with the surface of CrzOaa after friction testing, it can be seen that the Cr&# coating is more seizure
270
Fig. 7. The surface of Cr3C2#after friction testing, drop lubricated, 0.60 GPa Hertz load, the other conditions were the same as those of Fig. 6. Fig. 8. The surface of Cr&# Fig. 7.
after friction testing, conditions were the same as those of
resistant than the Cr,O,* coating. In addition, the Cr3C2# coating possesses relatively high cohesive strength; although it is rather hard, brittle spalling still did not occur. 3.3.3. TiO, The surfaces of the TiO, coatings after friction testing are illustrated in Figs. 9 and 10. For the test conditions see Table 1. It is seen that the TiOz coating surface deformed more severely than the CrzOs* coating surface (Fig. 3) under the same testing conditions. No brittle fatigue spalling or adhesion were observed. The wear mechanism is also plastic smearing.
Fig. 9. The surface of TiOz after friction testing, conditions see Table 1. Fig. 10. Partial amplification of Fig. 9.
271
Fig. 11. The surface of
(WCl$%Co) after testing, conditions
Fig. 12. Optical photo of A1103 (China) 168~). Conditions see Table 1.
see Table 1.
surface after friction
testing (magnification
The surface of WC18%Co after friction testing is shown in Fig. 11. Although WC is very hard, there were still many areas of plastic deformation distributed over the surface. This indicates that the local temperature approached the melting point of WC (2700 “C) and caused many protuberances to melt and conjugate under the repeated action of friction. The reason why the surface had a very high temperature was that, since WC is very hard, the contact area in reality was rather small, this caused a high local pressure and resulted in a large frictional force and more frictiond heat. Therefore the temperature rose and approached the melting point of WC. The WC18%Co coating had strong cohesive strength since no obvious brittle spa&g was observed on the surface. 3.3.5. A120, (made in China) Figure 12 is an optical micrograph of the Al&& coating after friction testing. For the test conditions see Table 1. There are many large and small pits on the surface. In the region without large pits there is a distribution of small pits. This indicates that the wear rne~h~~rn of the Al@s coating was fatigue spa&g. No plastic deformation is observed. This may be explained, since Al@s has a high elastic modulus, it is difficult to reduce the stress concentration through plastic deformation. This resulted in the fatigue spalling mechanism. Figure 13 shows the surface of PSZ after friction testing. For the test conditions see Table I. No large pits are observed on the surface. There was a little plastic defo~ati~n locally. For most regions, the adhesion between particles was not strong. The wear mechanism was fatigue spalling. Maybe
Fig. 13. The surface of PSZ after friction testing, conditions see Table 1.
the adhesion between the particles was the weakest, since this caused the highest spalling rate and the highest wear of all the materials in this paper. 3.4. The dry friction test resulk Table 2 gives the results under dry friction. The test conditions were as follows: velocity of roller specimen, 3.8 m s-l; average velocity of block specimen, 1 mm s-r; temperature of block specimen, kept at 450 “C; specimen preparation, cleaned ultrasonically for 10 min with clean alcohol; test procedure, run for 10 min at a Hertz load of 0.08 GPa, measure the Eriction torque, then run for 20 min at a Hertz load of 0.20 GPa, measure the friction torque. It is found from Table 2 that the dry friction coefficients of the tested couples were in the range 0.4 - 0.66 (seizure occurred with the Cra03*-Al,O,(Metco) couple when a 0.20 GPa Hertz load was applied for 3 mm). The Cr,O,*-Cr,O,* couple gave the lowest wear depth, Su~r~~giy, it was TABLE 2 The dry friction results of some couples Friction pairs
Friction
Roller coating
Block coating
y;ii
Cr203*
Cr20j*
0.52
0.47
c&03*
cr,c,#
Cr,O,* CrzO3*
TiOz AlpO3(Metco)
0.66 0.53 0.61
Cr&,#
Cr#&#
0.53
0.48 0.40 > 0.64 (seized) 0.46
Gpa
~oef~c~e~t Lfi
‘Pa
Roughness of biock Total wear depth of block specimen surface after specimen (/&II) test R, (/AXI) Too small to measure 40.0 38.0 Not measured
0.9
24.0
0.9
0.1. 0.3 0.8
Fig. 14. The surface of C&03* after dry friction testing, conditions see Table 2. Fig. 16. The surface of Cr3Cz# rubbed against Cr&*, Table 2.
dry friction, conditions see
discovered, using a topography instrument (Talysurf 5), that the outline of the Cr,Os* block specimen rose after friction testing. This was caused by adhesive transfer. From Table 2, the high wear corresponds to a low R, value after testing and low wear corresponds to a high R, value after testing. The R, value of the surface after friction testing reflects the wear rne~h~~rn. Figure 14 shows the block specimen surface of the ~r*~~*-~r~~~~ couple after testing at high temperature and under dry friction conditions. It can be seen from Fig. 14 that severe plastic flow and tearing had taken place. The high frictional force generated a large amount of frictional heat. This resulted in a very high temperature, plastic flow and tearing under high stress. Figure 15 shows the friction surface of the Crs&# block against the Cr&$* roller. In a relatively high region, a thin plastic deformation layer wa6 formed. The relatively low region was pressed smooth and no severe defo~ation was observed. No traces of adhesion were evident. Evidence of microcutt~~ was observed on the thin deformation layer. The wear mechanisms were micro~utt~g and fatigue spalling. The binding between the thin defo~ation layer and the substrate was not strong, therefore the thin deformation layer could undergo fatigue spalling along the edge of the layer by repeated circular stress. The difference in the test conditions between Figs. 15 and 6 was only with respect to lubrication (dry friction compared with lubrication with oil) but the topographs and wear mechanisms of the two types of friction were remarkably different, Fig. 15 was fatigue spalhng and Fig. 6 was plastic smearing, Figure 16 shows the surface of the Cr3C2# blocks US.C!rsC& roller after dry friction testing. The surface was covered with torn pits and traces of plastic flow. Brittle fatigue spalling had still occurred in the local region.
274
Fig. 16. Table 2.
The surface of Cr&&
rubbed against Cr&*#,
dry friction,
conditions
see
Fig. 17. The surface of TiOz rubbed against Crz03*, dry friction, conditions see Table 2.
The reason why adhesive transfer took place might be attributed to wear itself since no adhesion occurred with the Cr203* roller-Cr&!,# block couple. Figure 17 shows the surface of the Ti02 block against Cr,Os* roller after dry friction testing. The wear behaviour was similar to that shown in Fig. 15. Obviously, a thin plastic flow layer on the substrate is observed. The wear indicated a strain fatigue mechanism. Figure 18 shows the surface of the A120s(Metco) block us. Cr,O,* roller after dry friction testing. Severe plastic flow and tearing had taken place. The couple seized after running for 3 min at a Hertz load of 0.20 GPa. It had the poorest antiseizure capacity of the couples studied in this paper. For all the test conditions for the graphs in Figs. 14 - 18 see Table 2.
Fig. 18. The surface of AlzO@4etco) see Table 2.
rubbed against Crz03*,
dry friction,
conditions
215
The general ~~~sid~~t~onis that, since cemnics are of high slosh and b~~iene~, it is di~~cu~t for both plastic def~~a~u~ and ~hesj~~ to take place dting the &ktion process. However, in pmtice this is not so. Sintered MgCI, Al& and Sic have been observed to be deformed at the eontact interface in the friction process [3 * S]* In @Iis paper, ~~~~~-sp~ay~ ceramic coatings were obscmed to be severely ~~~t~c~~~ deformed and tom up, Acco~g to bang elected ~ic~~o~~ tryst of the action surface af ceramic clangs, the fiction and weax bebav~uur of ceramic ~at~gs was very &miIap to that of rtletals, such as fatigue wear; pkstic defo~~~o~ and ~hes~ve wear. ~~we~e~~ the c~~d~ti~~s reed for Celtics and metals to reach Ike same wear ~o~~t~o~s wem nut the same. rhea metals begin to peform ~~~t~c~y (at a cam bad, vekxity and te~~e~~re~ ceramics are still not deformed. Howe~eq if the load, velocityS te~pe~~t~~ are irxmmed further, phstie d~f~~a~~~~~even tangy can mxur on the friction surface of the ceramics. An absolute brittle material its have a low ~~~d~ct~t~ generally does not exist, ~o~eove~~ since c it is diffkzult for ~~cti~n~ heat to disperse. Therefore local contacts will have very hi& te~p~t~~es and be s~f~e~~* This makes t;krephstic d~f~~~~~~~ happen more easily. ~0~ this point of view, pkstic deformation in the friction pr~%ss cm be ach~evad. ~ec~~~y~ through the study in this paper, it is found that fur ceramic coat~gs in dry friction, h&b fkktion coeff~c~e~~ axe obsess and adhesive wear mxms easy@. Mrhena ~~b~~~~ (oil) is ~~p~~~~~~~t~~~and wear decrease subst~ti~y feven under 450 “C). The wear ~ech~is~ and the t~po~aph after testing differ greatly from those for dry friction. This suggests that for general ceramic coatings dry friction is unusable.
(1) The friction and wear bebav~~u~ of peaks-sprayed ceramic c~at~gs is safe to that of metals, such as fat~e s~~i~g~ pfastk d~fo~~t~o~ and e&m* a ce~~cs bas s high ~~c~~o~c~ef~c~e~t (2$ The oiBess ~~t~o~ of (about 0.5) and adhesive weax occurs easily. When the ~~e~~ oil (with calcium, s~phur~ phosphors and ~~c-c~n~~~g ~ditives~ is used, even at high t~~~e~~~s and in the presenceof a smallq~~t~ty of oil, friction and wear decrease subst~~~y (friction inefficient is about 0.1) and it is not easy tcr@ameadhesion, (3) The ~~U~~Cr*~s~ couple gives a low wear r&e and the Cr&Q+A~~~s~~e~~ cou@e has a poor resj. ce to seizure. (4) The ape~~~es on ~l~~a-s~~ay~ ceramic ~oat~gs give rise to a ~ech~is~ involving acc~u~atio~ af oil in the allures. This makes lub~cat~~~ easier.
276
References 1 Tribology at high temperature for uncooled heat insulated engine, SAE Paper 840429. 2 Wang Yinglong, Jin Yuansheng and Wen Shizhu, The design of a friction and wear tester for high and low temperature uses, Scientific Report, Tsinghua University, China, July 1987. 3 K. F. Dufrane and W. A. Glaeser, Study of rolling-contact phenomena in magnesium oxide, NASA Tech. Rep, CR-72295, 1967 (National Aeronautics and Space Administration). 4 R. P. Steijn, On the wear of sapphire, J. Appl. Phys., 32 (10) (1961) 1951 - 1958. 5 K. Miyoshi and D. H. Buckley, Friction, deformation and fracture of single-crystal silicon carbide, ASLE Trans., 22 (1) (1979) 79 - 90. 6 A. K. Chattopadhyay and A. B. Chattopadhyay, Wear characteristics of ceramic cutting tools in machining steel, Wear, 93 (1984) 347 - 359.
265
Summary In this paper, thy! friction and wear mechanisms of plasma-sprayed ceramic coatings (~r~Us~~iU~3~TiU~, ~r~~~l~%~i3%Cr~TiG,, Al&.&(Metco), WCl$%Co, PSZ, Al& (made in China)) at 450 “C have been studied. It has been determinedthat the friction azzdwear behaviour,such as fatigue sp~g, plastic deformation and adhesive transfer, of ceramic coatings is similar to that of metals. Under high ~rnpe~~~ (450 “61, the dry friction coefficients of ceramic coatings were much higher than the lubricated fiction coefficients of ceramic coatings. At the same time the wear me~h~isms and the frictional.surface ~po~aphy under dry lotion condition were much different from those under lub~~a~ conditions. Adhesive transfer occurred easily under dry friction conditions. However, it was ratherdifficult for adhesivefriction to take place when lubricated. 1(I Introduction With good resistanceto high ~ernpe~~re~wear and corrosion, plasma sprayed ceramic coatings have found their uses in many applicationssuch as tail exhaust pipes in rockets, liners and rings in adiabatic engines, bearings etc. The friction and wear behaviour of ceramic coatings is very important in their pr~ti~ applications; however, there are few papers pub~~~ on this subject. Because the temperaturesof the top ring reversalposition of the liner and the top ring on the piston are about 350 - 400 “c 111, the friction and wear mechanisms at high ternpe~~~s need to be studied. The aim of this paper is to study the friction and wear mechanisms of plasma-sprayed ceramic coatings at high temperatures(460 “Cl. 2. Test rig
The high temperaturefriction and wear tester used in the experiments is shown in Fig. 1 [Z] e One roller specimenand one block specimen formed the linearcontact fsiction couple. The roller specimenwas rotated by a 90 W
Fig. 1. High temperature friction tester.
electrical motor at a speed of 1450 rev mm-‘. The outer diameter of the roller specimen was 50 mm. The block specimen was driven by an eccentric to move back and forth. The average velocity of the block specimen could be varied between 0 and 1 mm s-l. The block specimen was 14 mm long, 6 mm wide and 5 mm thick. The linear contact length was 6 mm and the stroke of the block specimen was 10 mm. The velocity of the roller specimen was 3.8 m s-l, When the friction coefficient was 0.1, the Hertz load could reach 0.6 GPa. The block specimen was heated using local heating and the temperature could be varied between room temperature and 500 “C. The temperature was controlled using a 1 mm diameter thermocouple which was mounted 0.2 mm under the friction surface of the block specimen and a temperature controller. The lubricant was pumped into the entrance of the linear contact region. The rate of supply of lubricant could be varied between 0 and 1600 ml h-r. The friction torque and speed were measured using a torque-speed sensor and a torque-speed measuring instrument. 3. Test results and SEM analysis
The substrates of the roller and block specimens were made of 0.45 C plain common steel (Fig. 2). Ceramic coatings were sprayed onto the friction surfaces of the roller and block specimens using plasma spraying technology.
Fig. 2. Roller and block specimens.
267
Plasma spraying was done at the Beijing Spraying Centre. The spraying equipment came from Meteo Inc. and the spraying parameters used were chosen from a plasma spraying handbook of Metcu Inc. The sprayed coating had a thickness of 0.30 mm before grinding and a thickness of 0.20 mm after grinding. All the coating surfaces of the specimens were ground before friction testing to a roughness R, of less than 0.40 p, The specimens were cleaned for 10 min ultr~onic~ly using pure alcuhol before friction testing. The following ceramic powders were used to make the coatings: Cr,O,* (Metco 136F, Cr2035%Si023%TiOz) Cr&#
(M&co 30 NS, Cr~~*l2%Ni3%~r)
TiOz (M&co 102) A1,03 (Metco 105) WC18%Co (Metco 75F) PSZ (Metco 202NS, ~~~~O%Y~U~~ Al&
(made in China)
3.2. Lubricated test results 3.2.1. Test conditions
The results given in Table 1 were obtained under the following conditions: lubricant, mineral oil and calcium, sulphur, phosphorus and zinccontaining additives; velocity of roller specimen , 3.8 m C’; average velocity of block specimen, 1 mm s-l; quantity of supplied lubricant, 80 ml h-i; temperature of block specimen, kept at 450 “C. 3.2.2. Test procedure
Firstly, heat the block specimen and keep its temperature at 450 “C. Secondly, pump lubricant to the entrance of the contact region. Then, run
Block specimens
Friction coefficient 0.08 GPa load
0.20 GPa load
Cr2U3*
0.08 0.09 0.07 0.12 0.07 0.11
0.09 0.09 0.08 0.14 0.08 0.11
f&w+
TiUz WClEI%Co Al203 (China) PSZ
Total wear depth of block specimens (Mm) . . 2.6 1.0 3.2 3.3 10.0 18.0
268
for 10 min at a Hertz load of 0.08 GPa, measure the friction torque; after that, run for 20 min at a Hertz load of 0.20 GPa, measure the friction torque. 3.2.3. Test results The wear depth of the roller specimen was too small to measure. The lubricated test results are given in Table 1. It is found in Table 1 that the friction coefficients varied between 0.07 and 0.14. The couples of TiOz, Al,Os (China) blocks against Cr2031crollers gave the lowest friction coefficient and the couple of the Cr&# block against the Cr,Os+ roller showed the least wear. The pair of PSZ block us. Cr,O,* roller possessed the highest wear rate. 3.3. The observation of the lubricated friction surface using scanning electron microscopy and optical microscopy 3.3.1. Cr,03* Figure 3 shows the surface of the Cr,Os* block after friction testing. For the test conditions see Table 1. Plastic deformation is evident on the surface; the wear mechanism seems to be plastic smearing. Since the plasmasprayed coating has many large apertures in which oil can collect (the surface in Fig. 3 is relatively rough) this will be of benefit to lubrication. Figure 4 also shows the surface of the Cr*Os* block after friction testing. Dropping lubricant was employed and the other test conditions were the same as in Table 1. It is obvious in Fig. 4 that the surface has been smeared flat. The mechanism may be that, since only a little oil (drops) was used, the cooling effect of the lubricant had little effect; moreover, the coating has a rather low thermal conductivity. Both of these factors resulted in a very high surface temperature (melted drops of Cr20s* were found on some parts of
Fig. 3. The surface of C&03* after friction testing, conditions see Table 1. Fig. 4. The surface of Cr103* after friction testing, dropping oil, conditions see Table 1.
269
Fig, 5. The surface of C-q&* after frktion testing, drop k&&ant, 0.33 GPa Hertz load. Fig. 6. The surface of Cr$$# after friction testing, conditions see Table 1.
the surface), so that the surface was softened and severely deformed. In this way, the ap&tures were filled up and a smooth surface was formed. Figure 5 also shows the Cr,Os* surface after friction testing. Ilsopping lubricant was still used and the Hertz load was increased to 0.33 GPa. It is clear in Fig. 5 that severe deformation has taken place. More heat was generated and a higher temperature was reached. Therefore an adhesian junction, which had a higher strength than the substrate, was formed locally and caused severe deformation. 3.3.2. Cr&# Figure 6 shows the surface of the CrsC,# block after friction testing. This coating was rather hard but no obvious brittle spa&g was observed. This indicates that the coating exhibited a rather high cohesive strength. Many protuberances on the surface appeared to be plastically deformed but the extent of deformation had not reached that of Fig, 3. Figures 7 and 8 show the surface of Cr&#, dropping lubrication was used and the Hertz load reached 0.6 GPa, the other test conditions were the same as those in Table 3.. Remarkable plastic deformation had occurred on the surface but the degree of deformation was not as severe as that in Fig, 4 and no traces of adhesion were found, Several small circu&r holes were observed clearly on the deformed plane. These may have resulted from the melting and eruding of local nickel (its melting point is 1450 “C, lower than the melting point 1890 “C of CrsC$). From this point of view the surface temperature was far higher than the substrate temperature (450 “C) (the thermocouple head was 0.2 mm away from the friction surface and the coating has a low conductivity). The wear mechanism was still plastic smearing. Comparing the surface of Cr&,# with the surface of CrzOaa after friction testing, it can be seen that the Cr&# coating is more seizure
270
Fig. 7. The surface of Cr3C2#after friction testing, drop lubricated, 0.60 GPa Hertz load, the other conditions were the same as those of Fig. 6. Fig. 8. The surface of Cr&# Fig. 7.
after friction testing, conditions were the same as those of
resistant than the Cr,O,* coating. In addition, the Cr3C2# coating possesses relatively high cohesive strength; although it is rather hard, brittle spalling still did not occur. 3.3.3. TiO, The surfaces of the TiO, coatings after friction testing are illustrated in Figs. 9 and 10. For the test conditions see Table 1. It is seen that the TiOz coating surface deformed more severely than the CrzOs* coating surface (Fig. 3) under the same testing conditions. No brittle fatigue spalling or adhesion were observed. The wear mechanism is also plastic smearing.
Fig. 9. The surface of TiOz after friction testing, conditions see Table 1. Fig. 10. Partial amplification of Fig. 9.
271
Fig. 11. The surface of
(WCl$%Co) after testing, conditions
Fig. 12. Optical photo of A1103 (China) 168~). Conditions see Table 1.
see Table 1.
surface after friction
testing (magnification
The surface of WC18%Co after friction testing is shown in Fig. 11. Although WC is very hard, there were still many areas of plastic deformation distributed over the surface. This indicates that the local temperature approached the melting point of WC (2700 “C) and caused many protuberances to melt and conjugate under the repeated action of friction. The reason why the surface had a very high temperature was that, since WC is very hard, the contact area in reality was rather small, this caused a high local pressure and resulted in a large frictional force and more frictiond heat. Therefore the temperature rose and approached the melting point of WC. The WC18%Co coating had strong cohesive strength since no obvious brittle spa&g was observed on the surface. 3.3.5. A120, (made in China) Figure 12 is an optical micrograph of the Al&& coating after friction testing. For the test conditions see Table 1. There are many large and small pits on the surface. In the region without large pits there is a distribution of small pits. This indicates that the wear rne~h~~rn of the Al@s coating was fatigue spa&g. No plastic deformation is observed. This may be explained, since Al@s has a high elastic modulus, it is difficult to reduce the stress concentration through plastic deformation. This resulted in the fatigue spalling mechanism. Figure 13 shows the surface of PSZ after friction testing. For the test conditions see Table I. No large pits are observed on the surface. There was a little plastic defo~ati~n locally. For most regions, the adhesion between particles was not strong. The wear mechanism was fatigue spalling. Maybe
Fig. 13. The surface of PSZ after friction testing, conditions see Table 1.
the adhesion between the particles was the weakest, since this caused the highest spalling rate and the highest wear of all the materials in this paper. 3.4. The dry friction test resulk Table 2 gives the results under dry friction. The test conditions were as follows: velocity of roller specimen, 3.8 m s-l; average velocity of block specimen, 1 mm s-r; temperature of block specimen, kept at 450 “C; specimen preparation, cleaned ultrasonically for 10 min with clean alcohol; test procedure, run for 10 min at a Hertz load of 0.08 GPa, measure the Eriction torque, then run for 20 min at a Hertz load of 0.20 GPa, measure the friction torque. It is found from Table 2 that the dry friction coefficients of the tested couples were in the range 0.4 - 0.66 (seizure occurred with the Cra03*-Al,O,(Metco) couple when a 0.20 GPa Hertz load was applied for 3 mm). The Cr,O,*-Cr,O,* couple gave the lowest wear depth, Su~r~~giy, it was TABLE 2 The dry friction results of some couples Friction pairs
Friction
Roller coating
Block coating
y;ii
Cr203*
Cr20j*
0.52
0.47
c&03*
cr,c,#
Cr,O,* CrzO3*
TiOz AlpO3(Metco)
0.66 0.53 0.61
Cr&,#
Cr#&#
0.53
0.48 0.40 > 0.64 (seized) 0.46
Gpa
~oef~c~e~t Lfi
‘Pa
Roughness of biock Total wear depth of block specimen surface after specimen (/&II) test R, (/AXI) Too small to measure 40.0 38.0 Not measured
0.9
24.0
0.9
0.1. 0.3 0.8
Fig. 14. The surface of C&03* after dry friction testing, conditions see Table 2. Fig. 16. The surface of Cr3Cz# rubbed against Cr&*, Table 2.
dry friction, conditions see
discovered, using a topography instrument (Talysurf 5), that the outline of the Cr,Os* block specimen rose after friction testing. This was caused by adhesive transfer. From Table 2, the high wear corresponds to a low R, value after testing and low wear corresponds to a high R, value after testing. The R, value of the surface after friction testing reflects the wear rne~h~~rn. Figure 14 shows the block specimen surface of the ~r*~~*-~r~~~~ couple after testing at high temperature and under dry friction conditions. It can be seen from Fig. 14 that severe plastic flow and tearing had taken place. The high frictional force generated a large amount of frictional heat. This resulted in a very high temperature, plastic flow and tearing under high stress. Figure 15 shows the friction surface of the Crs&# block against the Cr&$* roller. In a relatively high region, a thin plastic deformation layer wa6 formed. The relatively low region was pressed smooth and no severe defo~ation was observed. No traces of adhesion were evident. Evidence of microcutt~~ was observed on the thin deformation layer. The wear mechanisms were micro~utt~g and fatigue spalling. The binding between the thin defo~ation layer and the substrate was not strong, therefore the thin deformation layer could undergo fatigue spalling along the edge of the layer by repeated circular stress. The difference in the test conditions between Figs. 15 and 6 was only with respect to lubrication (dry friction compared with lubrication with oil) but the topographs and wear mechanisms of the two types of friction were remarkably different, Fig. 15 was fatigue spalhng and Fig. 6 was plastic smearing, Figure 16 shows the surface of the Cr3C2# blocks US.C!rsC& roller after dry friction testing. The surface was covered with torn pits and traces of plastic flow. Brittle fatigue spalling had still occurred in the local region.
274
Fig. 16. Table 2.
The surface of Cr&&
rubbed against Cr&*#,
dry friction,
conditions
see
Fig. 17. The surface of TiOz rubbed against Crz03*, dry friction, conditions see Table 2.
The reason why adhesive transfer took place might be attributed to wear itself since no adhesion occurred with the Cr203* roller-Cr&!,# block couple. Figure 17 shows the surface of the Ti02 block against Cr,Os* roller after dry friction testing. The wear behaviour was similar to that shown in Fig. 15. Obviously, a thin plastic flow layer on the substrate is observed. The wear indicated a strain fatigue mechanism. Figure 18 shows the surface of the A120s(Metco) block us. Cr,O,* roller after dry friction testing. Severe plastic flow and tearing had taken place. The couple seized after running for 3 min at a Hertz load of 0.20 GPa. It had the poorest antiseizure capacity of the couples studied in this paper. For all the test conditions for the graphs in Figs. 14 - 18 see Table 2.
Fig. 18. The surface of AlzO@4etco) see Table 2.
rubbed against Crz03*,
dry friction,
conditions
215
The general ~~~sid~~t~onis that, since cemnics are of high slosh and b~~iene~, it is di~~cu~t for both plastic def~~a~u~ and ~hesj~~ to take place dting the &ktion process. However, in pmtice this is not so. Sintered MgCI, Al& and Sic have been observed to be deformed at the eontact interface in the friction process [3 * S]* In @Iis paper, ~~~~~-sp~ay~ ceramic coatings were obscmed to be severely ~~~t~c~~~ deformed and tom up, Acco~g to bang elected ~ic~~o~~ tryst of the action surface af ceramic clangs, the fiction and weax bebav~uur of ceramic ~at~gs was very &miIap to that of rtletals, such as fatigue wear; pkstic defo~~~o~ and ~hes~ve wear. ~~we~e~~ the c~~d~ti~~s reed for Celtics and metals to reach Ike same wear ~o~~t~o~s wem nut the same. rhea metals begin to peform ~~~t~c~y (at a cam bad, vekxity and te~~e~~re~ ceramics are still not deformed. Howe~eq if the load, velocityS te~pe~~t~~ are irxmmed further, phstie d~f~~a~~~~~even tangy can mxur on the friction surface of the ceramics. An absolute brittle material its have a low ~~~d~ct~t~ generally does not exist, ~o~eove~~ since c it is diffkzult for ~~cti~n~ heat to disperse. Therefore local contacts will have very hi& te~p~t~~es and be s~f~e~~* This makes t;krephstic d~f~~~~~~~ happen more easily. ~0~ this point of view, pkstic deformation in the friction pr~%ss cm be ach~evad. ~ec~~~y~ through the study in this paper, it is found that fur ceramic coat~gs in dry friction, h&b fkktion coeff~c~e~~ axe obsess and adhesive wear mxms easy@. Mrhena ~~b~~~~ (oil) is ~~p~~~~~~~t~~~and wear decrease subst~ti~y feven under 450 “C). The wear ~ech~is~ and the t~po~aph after testing differ greatly from those for dry friction. This suggests that for general ceramic coatings dry friction is unusable.
(1) The friction and wear bebav~~u~ of peaks-sprayed ceramic c~at~gs is safe to that of metals, such as fat~e s~~i~g~ pfastk d~fo~~t~o~ and e&m* a ce~~cs bas s high ~~c~~o~c~ef~c~e~t (2$ The oiBess ~~t~o~ of (about 0.5) and adhesive weax occurs easily. When the ~~e~~ oil (with calcium, s~phur~ phosphors and ~~c-c~n~~~g ~ditives~ is used, even at high t~~~e~~~s and in the presenceof a smallq~~t~ty of oil, friction and wear decrease subst~~~y (friction inefficient is about 0.1) and it is not easy tcr@ameadhesion, (3) The ~~U~~Cr*~s~ couple gives a low wear r&e and the Cr&Q+A~~~s~~e~~ cou@e has a poor resj. ce to seizure. (4) The ape~~~es on ~l~~a-s~~ay~ ceramic ~oat~gs give rise to a ~ech~is~ involving acc~u~atio~ af oil in the allures. This makes lub~cat~~~ easier.
276
References 1 Tribology at high temperature for uncooled heat insulated engine, SAE Paper 840429. 2 Wang Yinglong, Jin Yuansheng and Wen Shizhu, The design of a friction and wear tester for high and low temperature uses, Scientific Report, Tsinghua University, China, July 1987. 3 K. F. Dufrane and W. A. Glaeser, Study of rolling-contact phenomena in magnesium oxide, NASA Tech. Rep, CR-72295, 1967 (National Aeronautics and Space Administration). 4 R. P. Steijn, On the wear of sapphire, J. Appl. Phys., 32 (10) (1961) 1951 - 1958. 5 K. Miyoshi and D. H. Buckley, Friction, deformation and fracture of single-crystal silicon carbide, ASLE Trans., 22 (1) (1979) 79 - 90. 6 A. K. Chattopadhyay and A. B. Chattopadhyay, Wear characteristics of ceramic cutting tools in machining steel, Wear, 93 (1984) 347 - 359.