Investigation on the Tribological Behaviors of as-sprayed Al2O3 Coatings With the Effects of MoS2 Lubricant and External Loads


 The Al2O3-MoS2 ceramic-based lubricating coating was prepared via introducing MoS2 dry film lubricant into the pores and cracks of thermal-sprayed Al2O3 coating by facial paint spraying method. The microstructure, mechanical properties and tribological behaviors of the as-received Al2O3-MoS2 coating were thoroughly evaluated. The results illustrated that MoS2 was mainly concentrated on the shallow surface of the Al2O3 coating, and thus more uniform, more compact and smoother Al2O3-MoS2 coating was obtained. Meanwhile, the mechanical properties of the Al2O3 coating did not change significantly after the introduction of MoS2. The tribological experiments illustrated that compared with the Al2O3 coating, the friction coefficient and specific wear rate of the Al2O3-MoS2 coating under different loads were greatly reduced due to the generation of the lubricating layer. Especially under the load of 5 N, the friction coefficient was as low as 0.36, and the wear rate (1.49×10-5 mm3×(N×m)-1) was almost 17 times lower than that of Al2O3 coating (2.53×10-4 mm3×(N×m)-1). This research established a new and simple way to prepare ceramic-based self-lubricating coatings by using temperature-sensitive solid lubricants.


Introduction
With the continuous development and improvement of science and technology, traditional materials in some emerging industries have been far from satisfying the needs of the requirements. Especially in the eld of electronics and aerospace, the materials are required to have the properties of oxidation resistance, high-temperature resistance and wear resistance [1][2][3][4]. Although metal and alloy materials possess good toughness, machinability and conductivity, their wear resistance, corrosion resistance and As a protective surface coating of mechanical parts, plasma-sprayed Al 2 O 3 coating has attracted more and more attention. However, there are essential differences between the covalent/ionic bond of ceramic and the metal bond of metal. Therefore, the plasticity of ceramic material is limited, and its ductility is far lower than that of metal. The instinctive brittleness of ceramic coatings is the main factor causing abrasive wear, which seriously inhibits the widely application of the ceramic coating in frictional parts [15,18,22]. Therefore, the study and evolution of self-lubricating ceramic coating with long service life is extremely important to solve the tribological and lubrication problems of mechanical parts under harsh working conditions. At present, most of the studies were reported that the solid lubricants were mixed with the feedstock powders to prepare ceramic-based self-lubricating wear resistant coatings [23][24][25].
However, some commonly used solid lubricants (such as C, MoS 2 , PTFE) have a low melting point and are apt to oxidize or fail in the high-temperature ame of plasma spraying (up to 15000K) [26,27]. It is possible to provide the ceramic-based coatings with lubricating properties by depositing lubricants covered with metal Ni, but owing to the bad compatibility between ceramics and metals as well as the introduction of soft lubricating phase. As a result, the prepared coating presents more cracks and pores, resulting in poor mechanical properties (hardness, bonding strength, etc.) [28,29]. It has been reported that the tribological properties of ceramic coatings under dry friction environment could be improved via introducing lubricating phases into the pores or textures holes of the as-sprayed ceramic coatings. Li et al. [30] have prepared micro-textured holes on 8YSZ coatings by a pulsed laser, and then the modi ed polytetra uoroethylene (M-PTFE) emulsion solution was injected to the textured holes by vacuum impregnation. Ultimately, the self-lubricating ceramic-based coating with the low friction coe cient and ultra-high wear resistance was acquired. Besides, Zhao et al. [31] in-situ synthesized Ag in the micro defects of as-sprayed zirconia coating by chemical synthesis process, which also endows the ceramic coating excellent lubrication performance. However, these post-treatment methods were cumbersome, and the high-temperature process also has an impact on the coating structure.
Based on the above analysis, in the present paper, in consideration of the inherent defects such as pores and cracks in the as-sprayed ceramic coating, MoS 2 dry lm lubricant was introduced into the micro defects of the Al 2 O 3 coating by facial paint spraying method. The Al 2 O 3 -MoS 2 coating with lubricating characteristics was obtained without affecting any performances of the original ceramic coating. The friction and wear experiments were conducted to study the tribological behaviors of the Al 2 O 3 -MoS 2 coating under different loads. Meanwhile, the wear mechanism of Al 2 O 3 -MoS 2 coating was clari ed to realize the long-term lubrication of ceramic coating and endowed the ceramic coating with excellent lubrication performance.

Preparation of Al 2 O 3 coating
The Al 2 O 3 coatings were deposited onto 316L stainless steel substrates (Φ25mm × 7.8 mm) by an APS-2000A system (Beijing, China). The Al 2 O 3 powders (purity > 99.9 wt%), of which a particle size distribution of 5-45 µm, were purchased from Sulzer Metco (USA) used as the feedstock in this study. The plasma gun was xed by a six-axis robot (Switzerland) to realize the uniform coatings by nely dominating the spraying distance or passes. Before depositing Al 2 O 3 coatings, the substrates were grit-blasted to roughen their surfaces with a GS-943 sandblasting machine (Beijing, China) followed by ultrasonically clean with acetone to eliminate residual ne silica powders, oils and other impurities. To reduce the residual thermal stress between ceramic coating and metal substrate, a NiCrAlY bonding layer with a thickness of ~ 100 µm was fabricated before depositing Al 2 O 3 coating. The thickness of the Al 2 O 3 coating was kept within the limits of 300 ~ 350 µm, which was measured using a digital micrometer with a high-resolution of 1 µm. The detailed plasma spray parameters used in this study were described in the previous report [32].

Preparation of Al 2 O 3 -MoS 2 coating
Commercially available MoS 2 dry lm lubricant was mainly applied to improve the lubrication performance of the Al 2 O 3 coating while lling its pores. The detailed spraying process was referred as

Characterization of Al 2 O 3 powders and as-received coatings
The morphologies of powders, as-received coatings and their worn surfaces were characterized by a scanning electron microscope (SEM, JSM-5600LV) with an energy dispersive X-ray spectrometer (EDS). An OLYCIA m3 image microscope has been contracted to examine the porosity of as-sprayed coatings. Xray diffraction (XRD, Cu-Kα radiation) analysis was con rmed to measure the crystal structures and phase composition of powders and coatings. The hardness and elastic modulus of the polished coatings were determined by nano-indentation equipment (Switzerland). The applied load was 40 mN with a dwelling time of 10 s. Then, an average of 5 times was reported for each specimen to evaluate mechanical properties. Raman spectroscopy (LabRAM HR Evolution) was used to determine the phase analysis of the wear scars under different frictional conditions. The 3D morphologies, the roughness and section pro le of the wear scars were measured by Micro-XAM-3D non-contact surface pro ler (ADE Corporation, Massachusetts, USA). The worn surfaces of counterparts were characterized using an optical microscopy.

Friction and wear test
Friction and wear behaviors of the coatings sliding against Al 2 O 3 ceramic balls (Φ 6 mm, commercially available) were conducted by a ball-on-disc con guration (CSM, Switzerland) with a linear reciprocating mode at room temperature. The sliding tests were executed in an air environment with a sliding velocity of 10 cm/s, the amplitude of 2.5 mm and a total sliding distance of 300 m. The friction coe cients were continuously recorded by a connected computer. The applied normal loads of 5 N and 10 N were selected. The speci c wear rates of coatings were estimated as: K = V/FL, where K indicates the wear rate (mm 3 /Nm), V indicates the wear volume (mm 3 ) that determined by a Micro-XAM-3D non-contact surface pro ler, F means the external load (N) and L implies the sliding distance (m). It is worth emphasizing that three separate measurements were observed under the same situation, and the average values were cited in this study.

Characterization of powders and coatings
The morphology of Al 2 O 3 feedstock powder has doubtless considerable performance implications on microstructure, mechanical properties and tribological behaviors of Al 2 O 3 coating. Therefore, it is of great importance to analyze the size and shape of Al 2 O 3 powder. Figure 1 showed the typical micrographs of Al 2 O 3 powders used in this study. Obviously, the Al 2 O 3 feedstock powders exhibited an irregular blocky or angular shape resulted from the crushing and fusing process or its intrinsic brittleness. However, the powders were complete with the particle size range was about 5-45 µm. Due to the narrow particle distribution and small particle size, the powders can be full-melted during the spraying process, thus ensuring the compact coating.
It is generally known that the surface topography (such as microscopic appearance and surface roughness) and composition analysis are important parameters to measure the whole properties of the ceramic coating. Figure 3 exhibited the SEM morphologies of the spreading behavior of a single molten particle as well as the top surface and cross-section morphologies of as-sprayed Al 2 O 3 coating. As shown in Fig. 3(a), the spreading of the molten particle was discontinuous and many cracks existed in the ceramic splat, which was mainly attributed to the stress in splat during the rapid cooling process. In fact, the sprayed particles were stacked and deposited in successively, resulting in incomplete overlaps along with cracks and pores in the Al 2 O 3 coating. This could be explained by SEM morphologies of the top surface and cross-section of Al 2 O 3 coating ( Fig. 3(b) and (c)). As exhibited in Fig. 3(b), the top surface of the as-sprayed Al 2 O 3 coating was very rough with a surface roughness of about 5.49 µm. Meanwhile, many protrusions, pores, semi-molten particles and cracks presented on the surface of the Al 2 O 3 coating, while most ceramic particles were completely melted and spread in the form of a pancake. The defects in ceramic coating mainly came from incomplete in ltration between ceramic splats, cracks caused by stress change during rapid cooling and pores formed by air ow during spraying [33,34]. Besides, from the cross-section of Fig. 3(c) that the coating has compact structure and the porosity was about 11.6%.
The pores observed from the cross-section mainly came from two aspects: one is the inherent pores in the coating, the other is the grain pulling out or brittle spalling caused by mechanical force during the mechanical polishing process. Therefore, it can be inferred that the real porosity of the coating was less than 11.6%. Additionally, as shown in Fig. 3(d), the ceramic coating, the bonding layer and the metal substrate were tightly integrated, thereby greatly increasing the bond strength between ceramic coating and metal substrate, which is conducive to the enhancement of general properties of the ceramic coating. powder showed that the original powder was composed of stable α-Al 2 O 3 without other visible characteristic peaks. Additionally, it can be found that metastable γ-Al 2 O 3 was the principal phase of the Al 2 O 3 coating, which is mainly due to the fact that the nucleation energy of γ-Al 2 O 3 was lower than that of α-Al 2 O 3 . The molten Al 2 O 3 droplets impacted on the substrate, γ-Al 2 O 3 formed priority in the rapid cooling process with a rate of ~ 10 5 -10 6 K/s [35]. However, it should be noted that α-Al 2 O 3 was still contained in the coating after spraying, which mainly came from semi-molten or un-molten ceramic particles.

Mechanical properties
The mechanical properties of the materials are intimately related to the preparation method, the selected parameters as well as the pre-treatment or post-treatment process, which have a great impact on the tribological behaviors of the coating. It is absolutely interesting and extremely signi cant to take a further examination of the mechanical properties of the coatings. Hardness is a kind of indentation hardness that re ected the ability of a measuring goal to resist another hard goal. Besides, due to the indentation area was small and the depth was shallow, the values of hardness acquired can further re ect the mechanical strength of the coatings. Therefore, nano-indentation technology was used to test the mechanical properties of the polished surface of the Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating. The typical loading-unloading curve was shown in Fig. 8. During the testing, the hardness and elastic modulus of the coating was directly recorded by the system, and the corresponding elastic failure strain (H/E) and plastic deformation resistance (H 3 /E 2 ) were calculated. At the same time, the elastic recovery rate (w rev .%) of each coating was calculated according to the loading-unloading curve, and the calculation formula is as follows [36]: where w rec is the elastic recovery rate, d max is maximum displacement and d res is the residual displacement. The calculation results are shown in Table 1. The hardness of the Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating was similar. This can be ascribed to the two reasons: on the one hand, the depth of nanoindentation is between 300-500 nm, indicating that the curve obtained by the nano-indentation test has little correlation with coating defects; on the other hand, if the diamond Berkovich indenter pressed into the defects, the standard curves cannot be obtained. coating has higher toughness, which corresponding to the high wear resistance.

Friction and wear behavior
To Under different sliding conditions, the variation tendency of speci c wear rate of Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating was consistent with that of the average friction coe cient ( Fig. 9 (b)). It is found that In order to obtain insight into the in uence of MoS 2 on the wear mechanism of Al 2 O 3 coating, the morphology of worn surfaces was investigated with SEM. Figure 10 exhibited the SEM morphologies and EDS mapping of the wear tracks of Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating under various sliding conditions. As can be seen clearly from Fig. 10(a), there is a great number of ne debris on the worn surface of the Al 2 O 3 coating that caused slight abrasive wear. This can be explained by the fact that the interfacial bonding among the deposited splats is relatively weak in the thermal-sprayed ceramic coatings [29,40]. As a result, the cracks induced by the sliding friction would propagate along the interfaces among the splats, which consequently lead to the brittle micro-peeling. The ake spalling marks or pits on the worn surface further proved that the Al 2 O 3 coating undergone the serious splat delamination sliding against the Al 2 O 3 ball, which is the important factor for the large wear rate of the Al 2 O 3 coating. Contrastively, the worn surface of Al 2 O 3 -MoS 2 coating under 5 N was relatively smooth ( Fig. 10(b)), which was mainly attributed to the generation of the MoS 2 lubricating layer. Furthermore, uniformly distributed Mo and S elements were detected in the EDS mapping, implying the formation of evenly MoS 2 tribo-lm on the worn surface. Besides, it is pointed out that some Mo and S elements were accumulated in the pores on the worn surface, which mainly came from two aspects: one is the original MoS 2 introduced into the pores of the coating; and the other is the MoS 2 lubricating layer squeezed into the pores. As a result, the friction coe cient and wear rate of the Al 2 O 3 -MoS 2 coating was greatly reduced. Under 10 N, the worn surface of Al 2 O 3 -MoS 2 coating was smooth, while micro-cracks and spalling defects emerged due to the brittle fracture induced by fatigue under high load (Fig. 10(c)). Although the MoS 2 tribo-lm was formed, the contact stress under high load was large. This means the cyclic stress on the worn surface of Al 2 O 3 -MoS 2 coating under 10 N was larger than that of at 5 N.
Therefore, the generation of ceramic abrasive would destroy the lubricating layer, leading to the increase of friction coe cient and wear rate.
The three-dimensional morphologies (denoted as 3D) and cross-section pro le are more instinctive parameters to estimate the wear loss of the coatings. Figure 11 displayed the 3D topographies as well as the cross-section pro les of the wear scars of the Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating under different sliding conditions. Evidently, the comparison indicated that the 3D morphologies of the two coatings well corresponded to their respective wear rates. To be speci c, the size of the wear tracks of Al 2 O 3 -MoS 2 coating was small but with obvious parallel grooves or scratches presented on the wear tracks, especially at 10 N ( Fig. 11(b) and (c)). This fact should be attributed to the destruction of the MoS 2 lubricating layer by ceramic debris, resulting in the slightly abrasive wear characteristics. Different from the Al 2 O 3 -MoS 2 coating, the wear tracks of Al 2 O 3 coating was wider and deeper under the same friction condition without obvious scratches or grooves. It is mainly due to the serious peeling off of ceramic coating during the friction process. Ultimately, a more extreme wear loss was caused by splat delamination. These peeled ceramic debris were pressed into ne wear debris, of which were removed from the wear tracks, and some were lled into the pores of the coating, eventually, leading to a smooth worn surface. This is one of the reasons that the friction coe cient curve could keep stable ( Fig. 9 (a)).
For  [43,44]. According to the tribological applications in the atmospheric environment, the failure of the MoS 2 lubricating layer was partly due to oxidation, which is the reason of many wear mechanisms [43]. Combined with the previous SEM analysis (Fig. 10(c)), the MoS 2 lubricating layer of Al 2 O 3 -MoS 2 coating was damaged or peeled off under the load of 10 N, leading to the increase of the friction coe cient and speci c wear rate. Except for the damage of ceramic abrasive, part of the reason was the oxidation failure of the MoS 2 lubricating layer. It is reassuring that MoS 2 peak was still detected in the Raman spectrum of Fig. 12(d), indicating that MoS 2 was not completely oxidized. The friction coe cient of Al 2 O 3 -MoS 2 coating was lower than that of Al 2 O 3 coating that con rmed the existence of the MoS 2 lubricating layer (Fig. 9). It is concluded that the superior friction and wear properties of Al 2 O 3 -MoS 2 coating were mostly given to the formation of the MoS 2 lubricating layer.
The tribological properties of the coatings can also be further explored by comparing and analyzing the morphologies of their counterparts. Figure 13 exhibited the optical micrographs of the worn surface of the counterparts coupled with Al 2 O 3 coating and Al 2 O 3 -MoS 2 coating under the corresponding frictional conditions. It was obvious that the worn morphologies of the counterparts were in good agreement with the wear degree of the coatings (Fig. 11). The worn surface of the Al 2 O 3 ball presented an oval shape sliding against the Al 2 O 3 coating ( Fig. 11(a)), which corresponded to the large wear rate and deeper wear track of the coating. Besides, the worn surface of the counterpart presented scratches and some attachments, it can be inferred that the attachments were the debris formed by the peeling off of the Al 2 O 3 coating. However, due to excellent friction-reducing ability, the worn surface of the Al 2 O 3 ball was regular circular coupled with Al 2 O 3 -MoS 2 coating (Fig. 11(b)). The adhesion substances were found on the Al 2 O 3 ball, suggesting that the MoS 2 lubricating layer transferred to counterpart. The existence of the lubricating layer reduced the direct contact between the friction pair and the coating, thus endowing the Al 2 O 3 -MoS 2 coating with a lower friction coe cient and wear rate. However, the lubricating layer was not continuous which is probably due to the damage of ceramic debris produced during the friction. That is the major reason that the friction coe cient of Al 2 O 3 -MoS 2 coating was higher than that of pure MoS 2 under the same conditions [45]. Besides, it was found that there is a lot of wear debris around the wear track of counterpart, which means wear debris was removed in the tribological test. The wear morphology of the Al 2 O 3 ball under the load of 10 N was similar to that of at 5 N, but the wear area was larger than that of 5 N. However, the slight scratches presented on the counterpart (Fig. 13 (c)), which also corresponded to the wear track morphology of the Al 2 O 3 -MoS 2 coating at the load of 10 N. It is also proved that the existence of lubricant not only reduced the wear damage of the coating but also greatly reduced the destruction to the counterpart. (c) The formation of the MoS 2 lubricating layer reduced the friction coe cient and wear rate of the ceramic coating, meanwhile, the wear degree of the counterpart was also alleviated. The lubricating layer formed in the friction process transferred to the counterpart. The existence of the lubricating layer reduced the immediate contact between the friction pair and the coating, thus optimizing the tribological properties.