Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions

Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions Kai Yang, Xiaming Zhou, Chenguang Liu, Shunyan Tao, and Chuanxian Ding (Submitted January 4, 2013; in revised form May 26, 2013) Al2O3, Cr2O3, and Al2O3-Cr2O3 composite coatings were produced by plasma spraying. Their tribo- logical properties were evaluated at high load conditions. The average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs were measured. Compared with the single coating/graphite pairs, the friction coefficients of composite coating/graphite pairs are more stable. The corresponding wear rates and worn surface temperatures are lower, which may be conducive to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. Especially, 10wt.%Al2O3-90wt.%Cr2O3 (AC90) composite coating shows better anti-wear performance, which may be attributed to its higher thermal conduction. Keywords Al2O3-Cr2O3 composite coatings, friction coeffi- cient, plasma spraying, thermal conductivity, wear rate 1. Introduction Oxide ceramics exhibit high strength, high hardness, and anti-wear performance, as well as high temperature and good oxidation resistances (Ref 1-3). The corre- sponding coatings fabricated by thermal spraying show excellent potential in the dynamic seal system in the tur- bine pumps of the rocket engines with the combined working conditions of high temperature, high specific pressure, strong oxidation, and thermal shock. However, low toughness of oxide ceramics restricts their practical applications. It is difficult to combine conventional toughening methods, such as particle toughening (Ref 4), whisker and fiber toughening (Ref 5, 6), ZrO2 transfor- mation toughening (the low thermal conductivity of ZrO2 easily results in the large thermal stress in the coating under the severe wear conditions) (Ref 7, 8), and gradient structure toughening (Ref 9) with plasma spraying technology. Al2O3 and Cr2O3 coatings, as typical thermal sprayed oxide ceramic coatings, have attracted increasing interests for developed industrial applications like in turbine pump field (Ref 10). Cr2O3 and a-Al2O3 possess the same crys- talline structure. Cr3+ and Al3+ have the approximate ionic radiuses. Accordingly, Al2O3-Cr2O3 solid solutions are easily formed. In our previous work, phase compositions, microstructures, and mechanical properties of the plasma- sprayed Al2O3-Cr2O3 composite coatings were investi- gated (Ref 11). The correlated coating strengthening- toughening mechanisms were further discussed (Ref 12). These experimental results show that heterogeneous nucleation and partial solid solution in Al2O3-Cr2O3 composite coatings are conducive to the coating mechan- ical performance improvement. Therefore, an interest has been taken in Al2O3-Cr2O3 composite coating as a plasma-sprayed coating for severe condition application against graphite. It is generally considered to believe that the mechanical properties of the coating have the important effects on its wear resistance. Distinct tribological behavior may be displayed under severe conditions. In the present study, the dry sliding wear characteristics of the plasma-sprayed Al2O3-Cr2O3 composite coatings against graphite under the high loads of 500 and 1000 N were evaluated to exploit the rela- tionship of tribological performance, microstructure and thermo-mechanical properties. 2. Experimental Procedure 2.1 Materials and Preparation Al2O3-Cr2O3 composite coatings were deposited onto the stainless steel (1Cr18Ni9Ti) substrates using a Sulzer- Metco F4 plasma spray gun under an atmospheric condi- tion (APS-2000). Commercially fused and crushed Al2O3 and Cr2O3 powders (their average sizes are 17.5 and 16.7 lm, respectively) were mechanically mixed to pro- duce the composite powders. According to our previous Kai Yang, Xiaming Zhou, Chenguang Liu, Shunyan Tao, and Chuanxian Ding, The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050 People�s Republic of China; and Kai Yang, Xiaming Zhou, Chenguang Liu, Shunyan Tao, and Chuanxian Ding, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People�s Republic of China. Contact e-mail: [email protected]. JTTEE5 DOI: 10.1007/s11666-013-9959-y 1059-9630/$19.00 � ASM International Journal of Thermal Spray Technology P e e r R e v ie w e d work (Ref 11, 12), the 50wt.%Al2O3-50wt.%Cr2O3 (AC50) and 10wt.%Al2O3-90wt.%Cr2O3 (AC90) composite coat- ings possess better comprehensive mechanical properties. Therefore, their tribological performance should be fur- ther studied. For performance comparison, pure Al2O3 and Cr2O3 coatings were also deposited. Prior to spraying, the stainless steel substrates were degreased ultrasonically in acetone and then grit blasted with corundum. A mixture of argon and hydrogen was used as plasma gas. In addi- tion, NiCr powder was used to deposit as a bond coating prior to spraying ceramic coatings. The thicknesses of NiCr bond coating and top ceramic coatings were around 60 and 450 lm, respectively. The corresponding plasma spraying parameters are shown in Table 1. 2.2 Coating Characterization The cross-sectional and worn surface morphology of the coatings were characterized by an EPMA-8705 QH2 elec- tron probe microanalyzer (Shimadzu, Tokyo, Japan). Porosity of the coatings was estimated by image analysis method. Knowledge of the thermal diffusivity (a), together with the specific heat (cp) and bulk density (q), allows the determination of thermal conductivity (k) from Ref 13. a ¼ 1:38L 2 p2t1=2 ðEq 1Þ k ¼ acpq ðEq 2Þ where t1/2 is the time required for the rear face of the coating sample to reach the half-maximum of the tem- perature rise and L is the specimen thickness. Coating specimens with dimensions of U 10.2 9 1.6 mm2 were prepared to measure the thermal diffusivity using the laser-flash technique (Ref 14). Specific heat capacity measurements were carried out using a modulated differential scanning calorimeter (Diamond DSC, PE, USA). Measurements were performed in the temperature range from 50 to 400 �C with a heat rate of 5 �C/min. The bonding strength of the coatings was examined by pull-off test (shown in Fig. 1, dimension unit is mm) using Instron material tester (Model 5592) based on ISO 4624: 2002 with a stretching rate of 1 mm/min. The photos of testing coating bonding strength are presented in Fig. 2. The adhesive used in the test was epoxy adhesive (Adbest, E-7 type) produced by Shanghai Research Institute of Syn- thetic Resins. The adhesive was cured at 120 �C for 3 h. To reduce the influence of random error, the bonding strengths were all obtained from quintuplicate tests. 2.3 Tribological Testing Friction and wear tests of the coatings were executed on a MM-200 (Shanghai University, China) tribological tester using a block-on-ring arrangement under different load conditions at room temperature. The schematic view of tribological tester is shown in Fig. 3. The dimensions of stainless steel ring and graphite block were Uouter 40 9 Uinner 16 9 10 mm 3 and 30 9 7 9 6 mm3, respec- tively. The thickness of top ceramic coatings deposited onto the stainless steel ring surfaces was about 320 lm after grinding and polishing. XXXPrior to wear testing, the sur- face roughness (Ra), measured by a TK300 HOMMEL WERKE roughness tester (Wave, Germany) with 0.5 mm/s traverse speed at 4.8 mm length, of block and coated ring treated by grinding or polishing was 0.8 and 0.2 lm, respectively. The wear tests were conducted at following Table 1 The plasma spraying parameters for NiCr bond coating and top ceramic coatings Parameters NiCr bond coating Top ceramic coatings Arc current, A 600 660 Primary plasma gas (Ar), slpm 57 49 Secondary plasma gas (H2), slpm 8 12 Carrier gas (Ar), slpm 3.5 3.5 Powder feed rate, g/min 18 35 Spray distance, mm 120 110 Nozzle diameter, mm 6 6 Powder injector diameter, mm 1.5 1.8 Fig. 1 Schematic illustration testing coating bonding strength Fig. 2 The photos of testing coating bonding strength Journal of Thermal Spray Technology P e e r R e v ie w e d conditions: relatively high loads of 500 and 1000 N; a rota- tional speed of 400 rpm (equivalent to a sliding velocity of 0.84 m/s). Friction coefficient was obtained from friction torque, which was directly displayed on the tester, being divided by load and radius of the ring. Wear rate was ac- quired by wear mass loss, which was measured by weighing the samples before and after each of the wear tests with a TG328 analytical balance, being divided by wear time (h). Prior to weighing, the specimens were cleaned in an ultra- sonic bath with acetone for 15 min and then dried in an oven at 120 �C for 60 min. During the wear tests, a Mini IR Thermometer, as indicated in Fig. 3, was employed to measure the friction temperature, which could be applied to enforce a sound comparison of the tribological heat gen- erated and concentrated on the contact area for the differ- ent frictional pairs. To insure the reproducibility of the measurements, three repeated tests were performed under the same test conditions for each friction pair. 3. Results and Discussion 3.1 Two Important Factors for the Coating Anti-wear Performance The thermal conduction property and interface bond- ing strength have great effects on the coating anti-wear performance under the severe conditions. The severe wear process easily generates large friction heat and external stress, which may result in the coating failure due to the thermal expansion coefficient difference between the coating and the metal substrate. The coatings showing higher thermal conductivity usually possess greater wear resistance (Ref 15). High interface bonding strength con- tributes to avoiding the coating spalling (Ref 16). Based on our previous work (Ref 11, 12), namely considering Vickers microhardness, fracture toughness, and bending strength, the tribological properties of AC50 and AC90 composite coatings were further studied. With regard to AC50 and AC90 composite coatings, the Cr2O3 weight fractions in the corresponding composite powders are 50 and 90%, respectively. Figure 4 shows the thermal conduction properties of several coatings. It can be seen that the thermal diffusivity and thermal conductivity of composite coatings increase with the increment of Cr2O3 content. In 200-800 �C, the average thermal conductivity values of AC50 and AC90 composite coatings are 3.64 and 3.90 W/m K, respectively. Especially, AC90 composite coating exhibits better thermal conduction performance than other coatings. The thermal conductivity improve- ment for Al2O3-Cr2O3 composite coatings may be attrib- uted to lower porosity, larger intersplat adhesion, and better interlamellar cohesion, which result chiefly from heterogeneous nucleation and partial solid solution in the coatings (Ref 11, 12). Fig. 3 The schematic view of tribological tester Fig. 4 Thermal diffusivity (a) and thermal conductivity (b) of the coatings Fig. 5 The bonding strength of the coatings Journal of Thermal Spray Technology P e e r R e v ie w e d The coating/substrate interface bonding strength is the other important factor for anti-wear behavior of the coating. Figure 5 presents the bonding strength of Al2O3, Cr2O3, AC50, and AC90 coatings (the corre- sponding average values are 30.3, 32.3, 30.5, and 27.8 MPa, respectively). Spray efficiency, density, ther- mal expansion coefficient, wettability, and internal stress of Al2O3 and Cr2O3 are different. Consequently, the composite coatings do not show better interface bonding properties than the single-component coatings. However, these bonding strength values are compara- ble. This kind of phenomenon benefits from heteroge- neous nucleation, partial solid solution, and low porosity in composite coatings (Cr2O3 and a-Al2O3 possess the same crystalline structure. Simultaneously, Cr3+ and Al3+ have the approximate ionic radiuses). 3.2 The coating Tribological Behaviors 3.2.1 500 N Load. Figure 6 shows the Al2O3, Cr2O3, AC50, and AC90 coating wear rings. Various coating wear rings were cut, inlayed, ground, and polished to fabricate the metallographic specimens. The corre- sponding cross-sectional morphology is shown in Fig. 7. It could be observed that the bonding at NiCr bond coating/steel substrate and top ceramic coating/NiCr bond coating interfaces is good. No obvious coating defects, including crack, pore, and inclusion, are found in the above-mentioned binary interfaces. The average friction coefficients and wear rates of the coating/ graphite pairs are exhibited in Fig. 8. The related wear tests were performed at following conditions: a load of 500 N and a rotational speed of 400 rpm (equivalent to a Fig. 6 The photo of coating wear rings (A: Al2O3; B: Cr2O3; C: AC50; D: AC90) Fig. 7 Cross-sectional morphology of the coating wear ring samples Journal of Thermal Spray Technology P e e r R e v ie w e d sliding velocity of 0.84 m/s). The friction coefficients of all coating/graphite pairs increase first, following de- crease and then gradually stabilize (shown in Fig. 8a). The worn surface features are unstable in the initial wear process, which leads to the increment of the fric- tion coefficients. During sliding process, multiple reac- tions occurred on the real contact areas of the friction pairs under the combined actions of the contact stress and tribological heat (Ref 17, 18). The fragmentation and detachment of the worn surface in the initial process lead to the instability of tribological behaviors. With increasing the wear time, the formation of graphite transfer films on the coating wear ring surfaces contrib- utes to the lubrication effect. Accordingly, the corre- sponding friction coefficients display the descending trend. Transfer film phenomenon becomes ubiquity in the process of wearing (Ref 19, 20). The transferred film firmly attaches to the coating surface to decrease the wear loss of coating. It can be conspicuously seen that the Cr2O3 coating/graphite pair shows the largest friction coefficient instability under severe conditions. Compared to the Al2O3 coating/graphite pair, the composite coat- ing/graphite pairs possess greater stability in the friction coefficient (the values are between 0.10 and 0.15), particularly with respect to AC90 composite coating/ graphite pair. Simultaneously, AC90 composite coating/ graphite pair reveals the lowest wear rates of ring and block (shown in Fig. 8b). The corresponding values are 0.0032 and 0.0015 g/h, respectively. Therefore, the con- tinuous graphite transfer films might be easily formed onto the composite coating surfaces. In order to explore the coating failure process and mechanism, the worn surfaces of Al2O3, Cr2O3, AC50, and AC90 coatings were individually analyzed. It can be seen from Fig. 9 that the stripe-type wear tracks appear on the Al2O3 and Cr2O3 coating surfaces subjected to abrasion; however, no stripe-type wear topography is found on the worn surfaces of AC50 and AC90 com- posite coatings. More residual graphite phases might be transferred onto the composite coating surfaces in the wear process (the distribution densities of black phases on the composite coating surfaces appear larger; the black phases are probably transferred graphites), which indicates that the graphite may supply the effective lubrication and alleviate the abrasion. Energy spectrum analysis further proved that the graphite was trans- ferred onto the coating surface. It seems that the graphites on the worn surfaces of the composite coat- ings possess larger areas than those on the single coating surfaces. The variations of peak intensity ratio of C/Al or C/Cr reflect (shown in Fig. 9; Table 2) the different relative contents of C element in the regions analyzed by energy dispersive spectrometer (EDS). The relative contents of C element increase in sequence of Spectrum A (or Spectrum B), Spectrum C, and Spec- trum D. This indicates that the thickness of graphite phases on the worn surfaces of the composite coatings may be larger. Based on the premise that the EDS analyses for each sample have the same area and depth, the above-mentioned conclusion should be tenable. According to the above analysis, graphite transfer film may be effectively formed on the worn surfaces of the composite coatings, which is conducive to decreasing the corresponding friction coefficients and wear rates. In addition, the porosities of Al2O3, Cr2O3, AC50, and AC90 coatings are 2.01, 2.15, 0.96, and 0.77%, respec- tively. The composite coatings possess more dense structure than Al2O3 or Cr2O3 coating. To exclude the pore factor, the graphite has better adhesion property for the Al2O3-Cr2O3 composite coatings, which may be conducive to the formation of more residual graphite phases transferred onto the composite coating surfaces. Moreover, the size of the graphite phases is much lar- ger than that of pores (shown in Fig. 7, 9). 3.2.2 1000 N Load. The above-mentioned wear test results show that the tribological properties of Al2O3 coating/graphite, AC50 composite coating/graphite, and AC90 composite coating/graphite pairs are better than that of Cr2O3 coating/graphite pair (namely, lower fric- tion coefficient and wear rate). In an attempt to further verify the anti-wear performance of the composite coatings, wear tests were carried out under a larger load Fig. 8 Average friction coefficients and wear rates of the coat- ing/graphite pairs (500 N, 400 rpm) Journal of Thermal Spray Technology P e e r R e v ie w e d of 1000 N (the rotational speed was still 400 rpm). Figure 10 displays the average friction coefficients, wear rates and worn surface temperatures of three kinds of coating/graphite pairs (the wear time was 60 min). Compared to Fig. 8(a), increasing the load leads to the decrease of the friction coefficients of coating/graphite pairs (exhibited in Fig. 10a). Particularly, the AC90 composite coating/graphite pair shows the minimum of stable friction coefficient, which is between 0.07 and 0.08. Comparing Fig. 8(a) with Fig. 10(a), it is apparent that the increase of the load is beneficial to the stabilization of friction coefficient. Moreover, the wear time that the coating/graphite pair initially obtains the stabilizing friction coefficient shrinks. It is speculated that the increment of the load contributes to more effective and Fig. 9 Worn surface morphology of the coatings (500 N, 400 rpm, 75 min) Table 2 The C/Al or C/Cr peak intensity ratio in the different spectra Spectrum number C/Al C/Cr A 0.49 ÆÆÆ B ÆÆÆ 0.47 C 1.05 0.93 D ÆÆÆ 1.35 Journal of Thermal Spray Technology P e e r R e v ie w e d faster formation of graphite transfer film on the surface of the coatings subjected to wear. Similarly, comparing Fig. 8(b) with Fig. 10(b), with increasing the load, the wear rates of the coatings decrease and those of graphite blocks increase, which may indicate that more graphite phases are transferred onto the coating surfaces and the correlated friction-reducing lubrication effect is enhanced. The composite coating/graphite pairs still present lower wear rates of the coating and graphite than the Al2O3 coating/graphite pair. The AC90 com- posite coating possesses excellent tribological perfor- mance. In order to further elucidate the reason for better wear resistance of the Al2O3-Cr2O3 composite coatings, the worn surface temperatures were recorded by a Mini IR Thermometer in the wear process (re- vealed in Fig. 10c). With the increase of the wear time, worn surface temperatures of the coating/graphite pairs tend to be stable, which denotes the formation of steady graphite transfer film. In the wear tests, the worn sur- face temperature follows the sequence: AC90 composite coating/graphite pair < AC50 composite coating/graphite pair < Al2O3 coating/graphite pair, which is consistent with the sequence of thermal conduction performance of the coatings (shown in Fig. 4). In wear process, friction heat on the surface of the coating possessing better thermal conduction property would be more conve- niently transferred to metal substrate, which is condu- cive to lower worn surface temperature. Low worn surface temperature may facilitate the formation of effective and stable graphite transfer film on the surface of the coating subjected to wear. Consequently, thermal conductivity is the significant factor for anti-wear per- formance of the coating under the severe conditions. In section 3.1, interface bonding strength is considered as other important factor for the wear-resistant perfor- mance of the coating. For the wear conditions with the load 500 and 1000 N, all of the coatings were not stripped from coating/substrate interface. Therefore, the tested bonding strength values should be enough for the coatings to bear with the abrasion in our experiments. That is to say, in the present work, the conclusion that the higher the thermal diffusivity of a coating, the better is its wear resistance could be drawn. Figure 11 exhibits the worn surface morphology of Al2O3, AC50, and AC90 coatings. No stripe-type wear topography appears on the worn surfaces of AC50 and AC90 composite coatings. Moreover, some residual graphite phases on their sur- faces may present greater areas (the distribution densi- ties of graphite phases on the composite coating surfaces appear larger), which would contribute to better tribo- logical property of the coating. 4. Conclusions Plasma-sprayed Al2O3, Cr2O3, and their composite coatings were fabricated in the present work. Tribological properties of the coatings were investigated. The following conclusions can be drawn: (1) Compared with Al2O3 coating/graphite and Cr2O3 coating/graphite pairs, the friction coefficients of AC50 composite coating/graphite and AC90 composite coating/graphite pairs are more stable. The corre- sponding wear rates of the coating and graphite are Fig. 10 Average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs (1000 N, 400 rpm) Journal of Thermal Spray Technology P e e r R e v ie w e d lower. Especially, AC90 composite coating/graphite pair shows outstanding tribological properties and AC90 composite coating has good wear-resistant performance. (2) The worn surface temperature follows the sequence: AC90 composite coating/graphite pair < AC50 com- posite coating/graphite pair < Al2O3 coating/graphite pair. Lower worn surface temperature may be bene- ficial to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. The related coatings show better anti-wear property. (3) The AC90 composite coating/graphite pair presents better tribological performance, which may be attrib- uted to greater thermal conductivity of the coating. 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Technol., 2000, 123(2-3), p 278-287 Journal of Thermal Spray Technology P e e r R e v ie w e d Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions Abstract Introduction Experimental Procedure Materials and Preparation Coating Characterization Tribological Testing Two Important Factors for the Coating Anti-wear Performance The coating Tribological Behaviors 500 N Load 1000 N Load Conclusions Acknowledgments References