Effect of Nano-Si3N4 Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al2O3-8YSZ Ceramic Coatings

Journal of Thermal Spray Technology, Feb 2017

In this paper, the effect of nano-Si3N4 additives and plasma treatment on the wear behavior of Al2O3-8YSZ ceramic coatings was studied. Nano-Al2O3, nano-8YSZ (8 wt.% Y2O3-stabilized ZrO2) and nano-Si3N4 powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al2O3-8YSZ coatings with and without Si3N4 additives was studied. Nano-Si3N4 additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al2O3-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si3N4 and plasma treatment could improve the wear resistance of Al2O3-8YSZ coatings.

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Effect of Nano-Si3N4 Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al2O3-8YSZ Ceramic Coatings

J Therm Spray Tech Effect of Nano-Si3N4 Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al2O3-8YSZ Ceramic Coatings Junfeng Gou 0 1 2 Jian Zhang 0 1 2 Qiwen Zhang 0 1 2 You Wang 0 1 2 Chaohui Wang 0 1 2 0 Laboratory of Nano Surface Engineering, School of Materials Science and Engineering, Harbin Institute of Technology , Harbin 150001 , People's Republic of China 1 College of Materials Science and Engineering, Qiqihar University , Qiqihar 161006 , People's Republic of China 2 School of Materials Science and Engineering, Liaocheng University , Liaocheng 252000 , People's Republic of China In this paper, the effect of nano-Si3N4 additives and plasma treatment on the wear behavior of Al2O3-8YSZ ceramic coatings was studied. Nano-Al2O3, nano-8YSZ (8 wt.% Y2O3-stabilized ZrO2) and nano-Si3N4 powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al2O3-8YSZ coatings with and without Si3N4 additives was studied. Nano-Si3N4 additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al2O318 wt.% 8YSZ-10 wt.% Si3N4-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al2O3-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si3N4 and plasma treatment could improve the wear resistance of Al2O3-8YSZ coatings. bonding strength; hardness; nano-Si3N4 plasma treatment; wear resistance Introduction Metallic materials play an important role in human life (Ref 1-3). The main forms of failure of metallic materials include wear, corrosion and fatigue. Wear occurs on the surfaces of components, which is the most common form of damage (Ref 4, 5). Wear causes heavy economic loss in the manufacturing, construction, trade and service industries. The component failure caused by wear occupied about 80% of all component failure all over the world. Researchers have proposed two methods to improve the wear resistance of metallic materials. One way is to prepare composite materials composed by different materials to exploit their excellent performances and avoid their shortcomings. This method improves the functional performances of single material or single phase material. The other way is to directly improve the wear resistance of material surfaces, extending the service life of components. In recent years, surface engineering technology emerges for the purpose of improving the surface performances of metallic parts (Ref 6, 7). This technology improves the comprehensive performances of materials by reinforcing the performances of material surfaces and preserving their original performances simultaneously. Plasma spray technology is an important surface engineering technology, which is widely used to prepare ceramic coatings with excellent properties (Ref 8, 9). An important advantage of ceramic coatings is high wear resistance. Alumina ceramics are commonly used as wear-resistant coatings because of their high hardness and wear resistance (Ref 10, 11). The addition of partially stabilized ZrO2 (8 wt.% Y2O3stabilized ZrO2) into alumina improves the wear resistance of alumina ceramic coatings, which is widely studied (Ref 12). In recent years, new Al2O3-YSZ (Y2O3-stabilized ZrO2) composite coatings are developed. Cano et al. added SiO2 particles into Al2O3-YSZ coatings (Ref 13). Pan et al. added SiC particles into Al2O3-YSZ coatings (Ref 14). Their results showed that the performances of the Al2O3YSZ ceramic coatings were improved remarkably by adding SiO2 and SiC particles. Si3N4 ceramic materials possess high hardness, high strength, high heat resistance, excellent corrosion resistance and wear resistance (Ref 15-17). Si3N4 has been widely used in ceramic composites (Ref 18). Submicron and nano-Si3N4 can improve the mechanical properties of alumina matrix nanocomposites (Ref 18, 19). In the light of the excellent properties and practical application of Si3N4, it will have attractive application prospect in ceramic coatings. At present, nanostructured thermal spray coatings prepared by using nanostructured feedstocks attract researchers’ attention because of their excellent performances (Ref 14, 20). Therefore, researches on application of Si3N4 in nanostructured coatings are innovative and necessary. In this paper, nano-Al2O3, nano-8YSZ (8 wt.% Y3O3stabilized ZrO2) and nano-Si3N4 were used to prepare sprayable feedstocks. Plasma treatment was used to improve the properties of the sprayable feedstocks. Al2O38YSZ ceramic coatings with and without Si3N4 additives were prepared by using plasma spray technology. The effect of nano-Si3N4 additives and plasma treatment on the surface morphologies and properties of the coatings was studied. The wear behavior of the ceramic coatings was studied and discussed in detail. Materials and Methods Experimental Materials Feedstocks Preparation The raw materials were nano-Al2O3, nano-8YSZ (8 wt.% Y3O3-stabilized ZrO2) and nano-Si3N4. The basic data of the raw powders are shown in Table 1. Nanopowders were used to prepare sprayable feedstocks. But a detrimental problem was that nanopowders are unsuitable for thermal spraying. Nanopowders cannot be deposited effectively on matrix surface during the spraying process. Therefore, nanopowders were granulated to meet the demands of sprayable feedstocks. The granulation processes for making sprayable feedstocks included ball milling process, spray drying process, calcination process and plasma treatment process. Details of the granulation processes were introduced. During the ball milling process, zirconia ceramic balls, raw powders, deionized water and polyvinyl alcohol (PVA) were put into milling pot. The volume ratio of the grinding balls to the raw powders was about 1:1. PVA was used as binder. The weight ratio of PVA to the raw powders was about 2%. Tributyl phosphate (TBP) was used as defoamer. After the ball milling process, the raw powders were made into slurry. The slurry was spray-dried in a spray drying tower (Changzhou Pinxin Dust Exclusion Equipment Co., Ltd., China). During the spray drying process, the process parameters (including inlet temperature, outlet temperature, rotate speed of atomizer and feeding rate) should be controlled strictly in order to prepare the particles with good morphology and size. The particles prepared by spray drying process had high porosity and organic additives. Therefore, the spray-dried particles should be treated by calcination. The calcination temperature was determined by referring to the thermogravimetric and differential thermal analysis results (Ref 21). The PVA and TBP are decomposed and volatilized at 224.8 and 319.5 C. The spray-dried powders were calcined at 450 C for 120 min in order to decompose the organics. Then the calcination temperature was heated to 800 C for 60 min to eliminate the residual organics, and the water molecules bound to the oxide molecules. High temperature calcination was in favor of increasing the adhesive strength of the agglomerated nanoparticles at the contact point among grains. But the calcination temperature should be lower than 1181 C to avoid the phase transformation of ZrO2. The detail calcination process is shown in Fig. 1. The nanopowders undergoing the first granulation still cannot be used as sprayable feedstocks because of their small size, low flowability and low tap density. Therefore, the granulated powders were subjected to the second granulation processes including ball milling process, spray drying process, calcination process. The second granulation process was similar to the first granulation process. Materials Particle size distribution, nm Supplier Al2O3 8YSZ Si3N4 After the second granulation, the powders can be used as sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The plasma treatment process was that the feedstocks were sprayed into the deionized water by means of plasma spraying. The spray current and spray voltage were lower than those used for coating preparation. The plasma treatment parameters are shown in Table 2. The primary gas was argon. The secondary gas was hydrogen. The schematic of the plasma treatment is shown in Fig. 2. Four types of feedstocks were prepared by the methods mentioned above. The feedstocks included AZ (Al2O3-20 wt.% 8YSZ), AZS (Al2O3-18 wt.% 8YSZ10 wt.% Si3N4), AZP (Al2O3-20 wt.% 8YSZ-plasma treatment) and AZSP (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4-plasma treatment). AZP and AZSP meant that AZ and AZS feedstocks were treated with plasma flame. Nano-Si3N4 was added during the first granulation process. The morphologies of the feedstocks after the second granulation and plasma treatment are shown in Fig. 3. Obvious hollows exist in the particles of AZ feedstocks. But no hollows can be seen in the AZS feedstocks. Nano-Si3N4 additives decrease the defects of feedstocks. The particles in the AZP and AZSP feedstocks are smooth. No voids exist on the particles of AZSP feedstocks. Plasma treatment not only increases the compactness and degree of sphericity of the feedstocks, but also decreases the voids. Coating Preparation Metco 9MC spraying system (Sulzer Metco, Switzerland) was used to prepare the ceramic coatings. The substrate was ASTM 1045 steel with a size of U25 mm 9 6 mm, which met the specifications for ASTM A29/A29M-04. The chemical composition of the substrate is shown in Table 3. The detail spray processes mainly included four steps, which are shown in Fig. 4. The steel substrates should be roughened in order to improve the adhesion strength between coatings and substrates. The specimens were roughened with 24# black corundum by using a JZB sandblaster (China, Beijing Jinjiu Zhuoer Technology Co., Ltd.). After roughening, the substrate surfaces were ultrasonically cleaned with acetone. Then the substrates were preheated by using plasma flame. After preheating, the feedstocks were fed into plasma flame to prepare ceramic coatings. NiCoCrAlY2O3 bond coating (chemical composition (wt.%): 18Cr, 15Al, 2Y2O3, 20Co, Ni: balance; Beijing General Research Institute of Mining & Metallurgy, China) was prepared on the substrate before preparing ceramic coating. The bond coating can reduce the thermal mismatch between the steel substrate and ceramic coating. Four types of ceramic coatings including AZ coating (Al2O3-20 wt.% 8YSZ), AZS coating (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4), AZP coating (Al2O3-20 wt.% 8YSZ-plasma treatment) and AZSP (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4-plasma treatment) coating were prepared on the surfaces of the bond coatings. Plasma spraying process parameters and coating thickness are shown in Table 4. 56.64 Fig. 1 Calcination process of the spray-dried powders Hollows Void HXA-1000 micro-hardness tester (Shanghai Precision Instrument and Meter Co., Ltd., China) was used to measure micro-hardness of the plasma sprayed ceramic coatings. Equation 1 was used to calculate the micro-hardness of ceramic coatings (Ref 22). HV denotes the hardness. f is the force (kgf). d is the average value of indentation diagonal (mm). The load was 500 g. Holding time was 15 s. The final hardness was the average value of at least five measurements. Porosity of the ceramic coatings was calculated by using Image-Pro Plus software. Cross-sectional SEM pictures of the coatings were used to calculate the porosity. f HV ¼ 1:8544 d2 Bonding strength was measured by using a universal testing machine (Instron-5569, Instron Corporation, America). The bonding strength testing followed the standard ASTM C633 (Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings). Before testing, cylindrical specimens with a dimension of U25 mm 9 40 mm were prepared. Two cylindrical specimens were glued together for each test. The front face of one cylindrical specimen had ceramic coating. The other cylindrical specimen had no coatings. Epoxy resin was smeared on the coating surface of the cylindrical specimen. Then the opposite surfaces of the two specimens were glued together. The pair must be placed in dry place for enough time before tensile testing. The final bonding strength was the average value of three measurements. Wear Tests The dry sliding wear tests were conducted on a pin-ondisk wear testing machine (Art-Teer Coatings Co., Ltd., China). The counterparts were Si3N4 ceramic balls with a diameter of 5 mm. The frictional radius was 4 mm. The sliding speed was 0.25 m/s. The normal loads were 5N, 10N and 15N. The wear time was 15 min. The environmental temperature was 26 ± 2 C. The humidity was 50 ± 5%. Friction coefficient and friction force were recorded by the transducer attached to the wear testing machine controlled by a computer. Wear track width of each wear specimen was measured by using Image-Pro Plus software in order to calculate wear volume loss. The final wear track width was the average value of five measurements on different locations. The calculation of the wear volume loss followed the ASTM standard G9905 (Ref 23). Equation 2 was used to calculate the wear volume loss of wear specimen. V is the wear volume loss (mm3). R is the wear track radius (mm). r is the radius of counter ball (mm). d is the wear scar width (mm). Wear Results and Discussion Surface Morphologies of the Coatings rate of wear specimens was calculated according to Eq 3 (Ref 24). W is the wear rate (mm3/N m). V is the wear volume loss. F is the normal load (N). L is the sliding distance (m). The wear tracks and wear debris were analyzed immediately after the wear tests. V ¼ 2pRhr2 sin 1ðd=2rÞ ðd=4Þ 4r2 Figure 5 shows the surface morphologies of the ceramic coatings. Figure 5(a) shows the morphology of AZ coating. Micro-defects including voids and micro-cracks exist on the coating surface. Some unmelted particles distribute on the surface, which embed in the molten splats or splats interface. The hollow sphere observed on the surface of the AZ coating implies that AZ feedstock particles have hollow structure. The coating cracks because of thermal stress in the plasma spraying process (Ref 14). Figure 5(b) show the morphology of AZP coating. The coating is mainly constituted by pancakelike splats. The unmelted splats decrease. Besides it, some voids and micro-cracks exist on the coating. Figure 5(c) shows the morphology of AZS coating. The morphology characteristics of the AZS coating are similar to those of the AZ coating. But the micro-cracks and hollow spheres are not evident. Figure 5(d) shows the morphology of AZSP coating. The morphology of the AZSP coating is similar to that of the AZP coating. The spreading of the molten particles is good. The unmelted particles and micro-cracks are not evident. It can be concluded from Fig. 5 that plasma treatment is in favor of reducing the unmelted particles. Nano-Si3N4 can prohibit the initiation of micro-cracks. It can be attributed to the formation of SiO2, which will be discussed in the following section. Rajendran and Karthik have found that SiO2 in Al2O3-ZrO2 coatings can prohibit cracks (Ref 25, 26). Besides it, their results show that SiO2 in the Al2O3-ZrO2 coatings would form mullite, zirconium silicate and yttrium silicate. The mullite has low thermal expansion coefficient, low thermal conductive and low creep deformation. The mullite fibers can hinder the propagation of cracks and make the micro-cracks selfheal (Ref 27). Another reason that nano-Si3N4 additives prohibit the initiation of micro-cracks is that SiO2 can stabilize ZrO2 and avoid initiating of cracks caused by phase transformation (Ref 25, 26). Cracks Cracks Cracks Figure 6 shows the x-ray diffraction patterns of the different ceramic coatings. AZ coating is composed by a-Al2O3, c-Al2O3, h-Al2O3 and t-ZrO2. Both c-Al2O3 and h-Al2O3 are metastable phases. a-Al2O3 and c-Al2O3 are typical phases in the plasma sprayed alumina coating (Ref 28). The formation of the metastable phases depends on the cooling rate (Ref 29). AZP coating is composed by a-Al2O3, c-Al2O3 and Al0.1Zr0.9O1.95. A new solid solution phase Al0.1Zr0.9O1.95 forms in the coating. Al0.1Zr0.9O1.95 is a ZrO2 solid solution containing Al3? (Ref 30, 31). The ionic radius of Al3? is small, which can dissolve into ZrO2 at high temperature. The high temperature of plasma flame leads to the formation of Al0.1Zr0.9O1.95 solid solution. The high cooling rate of plasma sprayed particles avoids decomposing of Al0.1Zr0.9O1.95 solid solution. The particles were sprayed into the deionized water during the plasma treatment process. Therefore, plasma treatment promotes the formation of ZrO2 solid solution containing Al. AZS coating is composed by a-Al2O3, h-Al2O3, t-ZrO2 and SiO2. AZSP coating is composed by a-Al2O3, c-Al2O3, SiO2 and Al0.1Zr0.9O1.95. The formation of SiO2 indicates that Si3N4 transformed into SiO2 during the feedstock and coating preparation processes. The oxidation reaction of Si3N4 and the standard Gibbs free energy of reaction are shown in Eq 4 (Ref 32, 33). When the temperature is higher than 426.2 C, the standard Gibbs free energy of reaction becomes negative. The calcination temperature during the feedstock preparation was higher than 450 C. It means that nano-Si3N4 might be oxidized. But the oxidation of Si3N4 was limited because nanopowders were agglomerated into micrometer particles before calcination process. The diffraction peaks of t-ZrO2 and Al0.1Zr0.9O1.95 are very close. But the number of diffraction peaks of t-ZrO2 observed is more than that of Al0.1Zr0.9O1.95. The diffraction peaks of AZ coating at about 50 and 60 degrees are accompanied by some other diffraction peaks belonging to t-ZrO2. So, t-ZrO2 is calibrated on the diffraction peaks of AZ coating. But the diffraction peaks of AZP coating at about 50 and 60 degrees have no obvious accompanying peaks. The diffraction peaks of AZP coating at about 30, 50 and 60 degrees are close to those of Al0.1Zr0.9O1.95. Al0.1 Zr0.9O1.95 is calibrated on the diffraction peaks of AZP coating. The similar methods have been used to calibrate the diffraction peaks of AZS and AZSP coatings. Si3N4ðsÞ þ 3O2ðgÞ ¼ 3SiO2ðsÞ þ 2N2ðgÞ DGh ¼ 220247 315T Properties of the Coatings ðEq 4Þ Figure 7 shows the properties of four types of ceramic coatings. The hardness of AZ and AZS coatings is similar. But the hardness of AZP and AZSP coatings is much larger than that of AZ and AZS coatings. The hardness of AZSP coating increases by 79.2% compared to that of AZ coating. It indicates that plasma treatment and nano-Si3N4 additives can significantly increase the hardness of the plasma sprayed Al2O3-8YSZ coating. Plasma treatment has a more obvious effect on the hardness of the coatings than nano-Si3N4. The bonding strength of AZP coating increases by 22.2% compared to that of AZ coating. The bonding strength of AZS coating is lower than that of AZ coating. The bonding strength of AZSP coating increases by 44% compared to that of AZ coating. It implies that the addition of nano-Si3N4 has no remarkable effect on the bonding strength of the plasma sprayed Al2O3-8YSZ coating without plasma treatment. But plasma treatment significantly improves the bonding strength of the plasma sprayed Al2O3-8YSZ coatings with and without nano-Si3N4 additives. The porosities of AZP and AZS coatings are lower than that of AZ coating. The porosity of AZSP coating decreases by 85.4% compared to that of AZ coating. Plasma treatment and nano-Si3N4 can decrease the porosity of the plasma sprayed Al2O3-8YSZ coating, respectively. The synergistic effect of nano-Si3N4 and plasma treatment can effectively reduce the pores of Al2O3-8YSZ coating. The defects had a deteriorating effect on the mechanical properties of the coatings (Ref 34). The similar surface morphologies and defects of AZ and AZS coatings lead to their similar hardness and bonding strength. The porosity of AZS coating decreases by 62.3% compared to that of AZ coating. It can be attributed to the formation of SiO2. SiO2 can promote the densification of Al2O3-ZrO2 thermal barrier coatings (Ref 25). Plasma treatment increased the compactness of the feedstocks. The overlap and connection of the pancake-like splats in the coatings were good, which was in favor of improving the hardness and bonding strength. The unmelted particles in the AZP coating decreased. These possible factors lead to the high hardness and bonding strength of AZP coating. The porosity decreases with the decrease in defects. As for AZSP coating, plasma treatment of the feedstocks and formation of SiO2 simultaneously improve its mechanical properties and decrease its porosity. Y3? was a more stable substitutional solute in ZrO2 compared with Al3? and dissolved in ZrO2 preferentially over Al3? (Ref 35). Al3? can be also in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and c-ZrO2 (Ref 30). A large quantity of Al3? promoted the formation of ZrO2 solid solution (Al0.1Zr0.9O1.95) containing Al3?, which can be demonstrated by the XRD diffraction results. The formation of solid solution containing Al3? avoided the transformation of ZrO2. On one hand, solid solution strengthening improved the strength of the coatings to some extent. Besides it, the stabilization of ZrO2 restrained the growth of ZrO2 grain, avoiding strength decrease in the coatings resulted from grain growth. Besides it, the transformation of t-ZrO2 or c-ZrO2 to m-ZrO2 would lead to the transformation strain. The increase in hardness of AZP and AZSP coatings can reflect the strength improvement in the coatings. Destabilization of ZrO2 solid solution containing Al3? was accompanied by Al depletion (Ref 35). The Fig. 7 Properties of the coatings outward diffusion of the Al that was in solid solution with ZrO2 led to the formation of Al2O3. The expansion mismatch between Al2O3 and ZrO2 caused micro-cracks in the coatings (Ref 30). In other words, the existence of solid solution containing Al was in favor of decreasing microcracks. Wear Test Results Friction Coefficient Figure 8 shows the friction coefficients of the different ceramic coatings under different normal loads. It can be seen from Fig. 8 that variation of friction coefficients with wear time includes running-in stage and steady stage. During the running-in stage, the micro-protuberances on the surfaces of the coatings and the counter balls contacted and collided. The contact stress was high because of low contact area. The high contact stress resulted in high friction coefficients and wear rates (Ref 36, 37). With the extension of wear time, the micro-protuberances on surfaces were broken. And the rough surfaces of the coatings were worn smooth. The real contact areas between the friction pairs increased. The wear rates of the ceramic coatings decreased (Ref 37). The friction coefficients decreased and tended to be steady. Most importantly, the friction coefficients of AZ and AZS coatings increase with the increase in normal load during the steady stage. But the friction coefficients of AZP and AZSP coatings have a maximum value when normal load is 10N during the steady stage. The variation of friction coefficient with normal load is correlated with the hardness and microstructure of the coatings. Wear Volume Loss and Wear Rate Figure 9 shows the wear volume losses of the coatings. The variation of wear volume losses of the coatings with normal load is remarkable. The wear volume losses of the different coatings are similar under normal load of 5N. The wear volume losses of the coatings under normal load of 10N increase nearly more than two times than their wear volume losses under normal load of 5N. But when normal load increases to 15N, the wear volume losses of the coatings are much higher than those under normal load of 10N. It indicates that the coatings were subjected to serious wear when normal load increased to 15N. Furthermore, the wear volume losses of AZS and AZSP coatings are much lower than those of AZ and AZP coatings when normal load is 15N. It implies that nano-Si3N4 additives increase the high stress wear resistance of the coatings. Figure 10 shows the wear rates of the different coatings. The wear rate of AZ coating is higher than the wear rates of AZP, AZS and AZSP coatings under normal load of 5N. The wear rates of AZS and AZSP coatings are lower than those of AZ and AZP coatings under normal load of 10N. When normal load increases to 15N, the wear rates of AZ and AZP coatings are 4.85 9 10-6 mm3/(N m) and 6.68 9 10-6 mm3/(N m). The wear rates of AZS and AZSP coatings decrease by 42.7 and 67.6% compared to the wear rate of AZ coating under normal load of 15N. Plasma treatment can improve the wear resistance of AZ coating under low normal load. But when normal load was high, the plasma treatment has no effect on the wear resistance of AZ coating. Plasma treatment is not in favor of improving the high stress wear resistance of AZ coating. When normal load increases from 10N to 15N, the wear rates of AZ and AZP coatings increase remarkably. It 1.0 0.9 0.8 ten0.7 i ifc 0.6 feo0.5 cn0.4 o tic 0.3 i rF0.2 0.1 5N 10N 15N (a) implies that the wear mechanism may change. The wear of the coatings changes from mild wear to severe wear. But nano-Si3N4 additives can improve the wear resistance of the coatings under low and high normal loads. The wear mechanism for AZS and AZSP coatings may not change. Their wear volume losses increase with the normal load under the same wear mechanism. The synergistic effect of nano-Si3N4 additives and plasma treatment can improve the wear resistance of the coating remarkably. Wear Track and Wear Debris of the Coatings Figure 11 shows the wear tracks of the different coatings under different normal loads. Smooth regions and rough regions can be observed on the wear tracks. The smooth regions are characterized by plastic deformation, which was called mild wear (Ref 38). But rough regions indicate a large quantity of micro-fractures, which mean severe wear (Ref 38). The ratio of smooth area to rough area on coating is correlated with the normal load. Under normal load of 5N, the wear tracks of the different coatings are basically (f) AZP, (g) AZS, (h) AZSP under normal load of 10N, (i) AZ, (j) AZP, (k) AZS, (l) AZSP under normal load of 15N similar. The micro-protuberances on the coating surfaces were deformed and broken under the condition of repeated contact and collision. These fractured micro-protuberances transferred on the surfaces of frictional pairs and adhered on the coatings. Thus, the smooth area formed on the coating surfaces (Ref 38). When normal load increased, the transfer of the fractured micro-protuberances became serious. The adhered transfer layers on the coatings are large and thick. It can be seen from Fig. 11 that a large quantity of smooth regions with large area exists on the coating surfaces under normal load of 10N. When normal load increases to 15N, the wear characteristics on the surfaces of AZS and AZSP coatings are different from those on the surfaces of AZ and AZP coatings. The smooth regions on the surfaces of AZS and AZSP coatings seemingly increase, especially the smooth regions on the surface of AZSP coating. But smooth regions on the surfaces of AZ and AZP coatings under normal load of 15N are less than those on the surfaces of AZ and AZP coatings under normal load of 10N. The composition is a key factor influencing the wear behavior of the coatings. The wear characteristics of AZS and AZSP coatings with SiO2 are different from those of AZ and AZP coatings without SiO2. It can be speculated that SiO2 played an important role in the wear process. The adhesion and transfer between AZ coating, AZP coating and their counterparts were serious. The transfer layers spalled under relatively low normal load or in relatively short sliding time. When normal load was 15N, the transfer layers reached the critical thickness and spalled in short wear time. This explains that the smooth regions on the surfaces of AZ and AZP coatings under normal load of 15N are less than those under normal load of 10N. Comparatively, the adhesion layer on the wear surfaces of AZS and AZSP coatings did not reach the critical spall thickness in 15 min. This explains that the smooth regions on the surfaces of AZS and AZSP coatings increase when normal load increases from 10N to 15N. Figure 12 shows the SEM images of wear debris under normal load of 15N. It can be seen that the wear debris consists of large slices and small granules. During the wear process, the coatings were extruded by repeatedly loading. Tensile stress and compressive stress formed in the subsurface of coatings (Ref 39). The cracks initiated and propagated. Then the coatings spalled and broke into debris. Large slice debris was easy to form when the thickness of adhesion layers reached the critical spall thickness. Taking the morphology of wear tracks into consideration, this slice debris is the spall products of adhesion layers. During the wear process, the slice debris was broken into small granules under the repeated contact stress. The hard debris or work-hardening debris could act as hard bodies between the coating surfaces and surfaces of counter balls, which aggregated the wear on the surfaces of friction pairs (Ref 33). It indicates that abrasion wear may occur. But in our study, the characteristics of abrasion wear are unobvious. Wear Mechanism of the Coatings Friction and wear of the coatings occurred under different normal loads. The micro-protuberances on the surfaces of frictional pairs contacted and micro-deformations occurred on the surfaces of coatings. During repeated sliding, the coating surface spalled. The spalling micro-protuberances transferred on the surfaces of the coating and the counter ball or left the coating surface. Thus, the coating was damaged. The hardness, porosity and bonding strength of the coatings influenced the wear mechanisms and wear resistance (Ref 11, 12). The different ceramic coatings in our study slid against Si3N4. The main wear mechanism was adhesive wear. The adhesive wear was the adhesion and transfer of materials between coating surfaces and Si3N4 surfaces. The adhesions cemented, welded and smeared on the coating surfaces. The ceramic coatings had high hardness and brittleness. When they slid against counter balls, microprotuberances on their surfaces contacted and initiated cracks under shear stress and normal stress. The cracks propagated, resulting in brittle fracture of the layered splats in the coatings. The detail process was introduced here. The rough surfaces contacted and deformed plastically under low load. Then micro-cracks formed. The cracks initiated in the micro-defects of the coating surfaces or subsurfaces. These micro-cracks propagated and connected at the boundary of the layered splats in the coatings with low bonding strength, resulting in the spalling of surface materials. The wear resistance and wear mechanism of the coatings should be analyzed by taking friction coefficient, wear volume, wear rate and morphology of wear track into consideration simultaneously (Ref 40, 41). At the beginning of the wear process, the surface roughness of friction pairs was high. Their surfaces intertwined during the wear process. This led to the increase in the friction coefficient. Regarding Al2O3-8YSZ coatings without nano-Si3N4 additives, slight adhesive wear occurred on the coating surfaces because of low contact stress under normal load of 5N. The adhesion layer of coating materials was broken into slices under repeatedly loading and then left the coating surfaces. Therefore, the wear rate was large. When normal load was 15N, the contact stress between the coatings and counter balls was large. The coatings spalled seriously, resulting in large wear volume losses of the coatings. The wear mechanism was serious adhesive wear. When normal load was 10N, the adhered and transferred materials of frictional pairs formed relatively smooth cover layer, which was in favor of alleviating the wear of Al2O38YSZ coatings. Regarding Al2O3-ZrO2 coatings with nanoSi3N4 additives, Si3N4 transformed into SiO2. Their wear mechanism was the same with that of Al2O3-ZrO2 coatings without nano-Si3N4 additives under low load. Their variation trends of wear volume losses were the same. When normal load was large, a large amount of friction energy was produced. The poor thermal conductivity of the ceramic coatings resulted in high temperature in the contact areas (Ref 42). The phase transformations of Al2O3 and ZrO2 in the coatings might occur. The extent of phase transformations was different because of the different properties of Al2O3-8YSZ coatings and Al2O3-8YSZ coatings containing SiO2 (Ref 25, 26). SiO2 improved the wear resistance of the coatings by improving their properties (Ref 25). The humidity during the wear process was high. The ceramic coatings containing SiO2 formed a certain amount of hydroxide surface layer in the humid environment (Ref 43, 44). The possible reaction involving SiO2 during the wear process is shown in Eq 5. The formation of Si(OH)4 improved the wear resistance of the coating to some extent. Furthermore, plasma treatment and nano-Si3N4 additives increased the hardness and bonding strength of the coatings synergistically. The porosity of coatings was also improved. The high bonding strength, high hardness and low porosity of the coatings meant high wear resistance. ðEq 5Þ SiO2 þ 2H2O ¼ SiðOHÞ4 Conclusions ( 1 ) ( 2 ) ( 3 ) Nano-Si3N4 additives prohibited the initiation of micro-cracks. Plasma treatment reduced the unmelted particles in the coatings. The synergistic effect of nano-Si3N4 and plasma treatment improved the morphology of the coating. Plasma treatment increased the hardness and bonding strength of AZ (Al2O3-20 wt.% 8YSZ) and AZS (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4) coatings. Nano-Si3N4 additives decreased the porosity of AZ coating. 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Junfeng Gou, Jian Zhang, Qiwen Zhang, You Wang, Chaohui Wang. Effect of Nano-Si3N4 Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al2O3-8YSZ Ceramic Coatings, Journal of Thermal Spray Technology, 2017, 764-777, DOI: 10.1007/s11666-017-0540-y