Tribological Behavior of Cathode Plasma Electrolytic Deposited Al2Y4O9 Coating on Aluminum Alloy

Ceramic coatings are widely used as protective barriers on the surfaces of various metals and alloys. Herein, a novel surface ceramic treatment, i.e., cathode plasma electrolytic deposition (CPED), is proposed for the surface of an Aluminum (Al) alloy. The coating, prepared in an Y(NO3)3 aqueous solution on the surface of the Al alloy, consists of Al2Y4O9 as the major phase component, Y2O3 as a minor phase component, and amorphous Al2O3 in the grain boundaries. As the applied voltage and deposition time increased, the crystallization of the coatings was enhanced. When deposited at 130 V for 10 min, the contact angle of the ceramic coating reached 141.0° ± 2.6°, indicating an enhanced self-cleaning effect. The ceramic coating also exhibited excellent self-lubricating and anti-wear effects. The friction coefficient of the CPED-treated sample vs. ZrO2 ball or bearing steel ball decreased from 0.55–0.65 to 0.26–0.31 when the load was 3 N and the reciprocating velocity was 10 mm/s. Correspondingly, the wear rate of the CPED-treated sample vs. the ZrO2 ball or bearing steel ball was significantly reduced. Results indicated that CPED is effective for the formation of self-cleaning and anti-wear ceramic coatings on Al alloys.


Introduction
Aluminum (Al) alloys have the characteristics of low density, good mechanical properties, good processing performance, nontoxicity, easy recycling, and good corrosion resistance [1]. Al alloy materials are widely used in the marine industry [2], chemical industry [3], aerospace [4], metal packaging [5], transportation [6], automotive [7], and electronics [8]. They are generally used as structural materials and have a higher specific strength and better processing properties than those of steel. Particularly in aerospace, they are used in large quantities and are the core metal materials for important components of aircraft, such as stressed-skin construction and stringers [9,10]. The current aviation applications in harsh environments require high-performance Al alloys. However, Al-alloy-moving pairs often lead to wear failure on the surface of parts owing to several factors, such as low hardness and naturally formed oxide film on the surface that is thin and uneven. This considerably limits the service life and reliability of the entire machine as well as the application of Al-alloy substrates under extreme conditions, such as high partial load, high temperature, and severe corrosion wear [11,12]. To overcome these limitations, surface treatment technology is extremely important for solving or improving the wear resistance of Al alloys [13,14]. In recent years, further improvement in the surface properties of Al alloys has gradually become an important research direction.
To improve the service life of Al-alloy parts in an environment of wear and corrosion, numerous advanced surface engineering technologies have been developed. These include thermal spraying [15,16], remelting [17], laser cladding [18][19][20], micro-arc oxidation (MAO) [21], and nano-processing technologies [22]. Naveena et al. applied the same weight ratio of fly ash to Al 2 O 3 to the surface of an Al6061-alloy substrate using plasma-spraying technology. It was found that the structure of the composite coating was denser and had a lower porosity, and the weight loss under different conditions (load, sliding distance, sliding speed) was reduced, resulting in excellent wear resistance. Li et al. [16] prepared a 316 L stainless steel/Al composite coating on an Al-alloy substrate using arc-spraying two-way Chenxu Liu and Xiangli Wen have contributed equally.
* Yu Tian tianyu@mail.tsinghua.edu.cn 1 State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China asynchronous wire-feeding technology. In the composite coating, the addition of an Al interlayer promoted the formations of Fe 3 O 4 and an interpenetrating structure, helping to release stress and hinder crack propagation. Therefore, the composite coating increased the wear resistance of the Al alloy by a factor of more than nine. Chen et al. [17] reported that the uniform distribution of elements by laser remelting significantly improved the hardness and wear resistance of A384 Al alloy. Torres et al. [18] used laser cladding technology to prepare a nickel-based self-lubricating coating on the surface of AA6082 Al alloy. Their results showed that the coating had better tribological properties because a nickel-based sulfide layer was formed, which significantly increased the microhardness. Zhu et al. [19] prepared Co-Cu/Ti 3 SiC 2 coatings on the surfaces of Al alloys using laser cladding. Owing to the strengthening effects of dispersion, solid solution, and grain refinement, the hardness of the substrate significantly increased, and the wear resistance further improved. Wen et al. [22] also adopted mechanical grinding to achieve nanometerization on the surface of the Al alloy, which improved the wear resistance of the Al alloy.
Li et al. [21] used a bipolar-pulsed power supply to prepare a ceramic coating with high microhardness and good wear resistance on the surface of 2A50 Al alloy by MAO in a silicate electrolyte. Currently, various surface engineering technologies have a certain scope of application and technical barriers. However, these methods have certain defects and limitations in their scope of application. For example, thermal spraying can easily cause thermal stress concentration, and there is a distinct fracture layer between the substrate and coating, which eventually leads to a decrease in bonding strength between the coating and substrate. Laser cladding is not standardized, has poor repeatability, and is prone to forming cracks. As an in situ coating technique, surface ceramic treatment of Al alloys by MAO or plasma electrolytic oxidation [23] is considered promising for applications in the surface treatment of samples with complex shapes. However, the composition of MAO coatings cannot be affected by the matrix [24], which limits the design of the coating composition and structure. Moreover, the voltage of MAO often attains 400-500 V or higher [25]. The supporting equipment for MAO needs to be further improved, and large-capacity refrigeration and heat-exchange equipment is required.
Cathode plasma electrolytic deposition (CPED) is a novel surface treatment that can be designed using multifarious ceramic deposits and is applicable to most conductive substrates [26][27][28]. The basic mechanism of CPED is breaking down hydrogen or water film, leading to plasma electrochemical reactions between the cathode and electrolyte. Ions in the solution are pyrolyzed into ceramic coatings. By adding surfactants and micro-or nanoparticles into the electrolyte, the structure and composition of the coatings can be controlled [29,30]. Compared to MAO, the voltage in CPED can be decreased to approximately 100-150 V. The ceramic coatings prepared using CPED are expected to be used for oxidation resistance and thermal barrier coatings. In addition, as an intermediate layer, CPED coating composited with PTFE can be used for the friction reduction and wear resistance of titanium-alloy surfaces [31].
Herein, we introduce CPED to prepare a type of Al 2 Y 4 O 9 coating on the surface of an Al alloy. The microstructure and composition of the CPED coating on the Al-alloy surface were systematically characterized. Moreover, the selfcleaning and tribological properties of the coatings were studied. This study provides a new technical path for the surface ceramization of Al alloys.

Experimental Methods
The ceramic coatings were prepared using CPED. A platinum electrode was used as the anode with dimensions of 120 × 50 × 0.3 mm 3 . Samples of Al alloy (3.19 wt% Mg, balance Al) with dimensions of 15 × 10 × 3 mm were used as the cathode material. The hardness of the Al alloy was approximately 74.6 ± 1.2 HV. A pulsed electrical power supply (TN-KGZ01, Guangzhou Jiantong Implementor Co., Ltd., China) was used, and the voltage was increased from 0 to 120 V at a rate of 1 V/s. For sample 1, the voltage was maintained at 120 V for 5 min. For sample 2, the voltage was maintained at 120 V for the first 5 min and 130 V for another 5 min. For sample 3, the voltage was maintained at 120 V for the first 5 min, 130 V for the second 5 min, and 140 V for 10 min. The final voltages and deposition times for the different samples are listed in Table 1. The electrolyte consisted of 0.5 mol/L Y(NO 3 ) 3 (Sinopharm Chemical Reagent Co., Ltd, China) as the reactant and 25 g/L polyethylene glycol (PEG-20000, Sinopharm Chemical Reagent Co., Ltd, China) as a surface modification agent.
The surface morphology of the coatings was characterized using an optical microscope (Keyence, Japan). The chemical composition of the surfaces of the coatings was investigated using X-ray photoelectron spectroscopy (XPS, PHI Quantera II, Ulvac-Phi Inc., Japan). Phase structure was characterized by X-ray diffraction (XRD, D/max-2550, Rigaku Corporation, Japan; using a Cu Kα radiation, from 10° to 90° at a scan rate of 2°/min). The microstructure and composition of the cross section of the coating were characterized by scanning electron microscopy (SEM, Merlin, Zeiss, Germany) and energy dispersive spectrometry (EDS) employing an energy dispersive spectrometer, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) after mechanical etching using a focused ion beam (FIB, FEI QUANTA 200 FEG, USA). The contact angle of a water drop on the surface of the uncoated or coated samples was observed using a video-contact angle-measuring instrument (Dataphysics, OCA25, Germany). Pure water (3 µL) was released through a syringe onto a clean surface, and the contact angle was calculated via images and graphs. A tribometer (UMT-3, Bruker, Campbell, CA, USA) was used for friction tests. A ZrO 2 ball or a bearing steel ball was used as the upper friction pair. Samples with or without the CPED coating were used as the lower friction pair. The load applied on the ball-on-disc contact was 3 N. Tests were performed at a sliding speed of 10 mm/s over a 5 mm track. The reciprocating frequency was maintained at 1 Hz. Each friction test lasted 20 min. All tests were carried out at 25 ± 1 °C. After the friction tests, the wear-track morphology was inspected using an optical microscope (Keyence, Japan). The wear volume and rate of the lower friction pairs were measured using a 3D optical surface profiler (NewView, ZYGO, Lamda, USA). Figure 1 presents the typical surface morphology of the coatings prepared by CPED under different parameters. As shown in Fig. 1a, when the voltage is maintained at 120 V for 5 min, the obtained sample surface is relatively smooth. In fact, due to the relatively mild deposition condition, the crystallinity of the coating was poor. When the applied voltage and the deposition time increases, the crystallization of the coatings increases and the surface becomes porous, as shown in Fig. 1b and c. The crystallinity of the coatings is further characterized by XRD, as shown in Fig. 3. The difference in coating structure is reflected by the results of the hydrophobicity in Fig. 7. Figure 2 presents the Y and Al XPS profiles of coatings prepared by CPED in 20 or 30 min. In Fig. 2a for the coating prepared in 20 min, the Y 3d peaks located at 160.11 and 158.12 eV are attributed to YO x . The difference between the mentioned two peak values is 1.99 eV and their area ratio is approximately 2:3. The Y 3d peaks located at 158.92 and 157.06 eV are attributed to Y 2 O 3 . The difference between these two peaks is 1.86 eV and their area ratio is approximately 2:2.7, similar to the previously reported values of Y 2 O 3 . In Fig. 2b, the two peaks at 73.93 eV are attributed to Al 2 O 3 . For the coating prepared in 30 min (Fig. 2c, d), the Y 3d peaks located at 160.06 and 158.14 eV were attributed to YO x . The Y 3d peaks located at 158.95 and 157.04 eV were attributed to Y 2 O 3 . The two peaks at 74.34 eV were attributed to Al 2 O 3 . These results indicated that the coatings prepared by CPED were mainly made of YO x , Y 2 O 3 , and Al 2 O 3 .  Figure 3 shows the XRD patterns of the uncoated Al alloy and the coated samples prepared by CPED with different deposition times. For the substrate without coating preparation shown in Fig. 3a, the peaks correspond to Al (PDF#04-0787). For the coating prepared by CPED in 10 min, the XRD patterns show the existence of the Al (PDF#04-0787) and other phases. However, we have yet to find a phase from the available XRD cards that matches the peak at 53° in Fig. 3b. As the applied voltage and the deposition time increased, the crystallization of the coatings became better and the peaks of the substrate of Al are gradually weakened. Figure 3c and d shows that the coating samples are composed of Y 2 O 3 (PDF#43-1036) and Al 2 Y 4 O 7 (PDF-#14-0475). Figure 4 shows the TEM observations of the cross section of the coating prepared by CPED in 30 min after treatment by FIB. According to Fig. 4a, the coating is composed of heterogeneous grains. The grain sizes range from nanometers to microns. According to the EDS results in Fig. 4b, the grains are mainly composed of O and Y, while O and Al are at grain boundaries. Figure 4c and d shows that the main composition of the grains is Al 2 Y 4 O 9 , which is consistent with the XRD results in Fig. 3d. Figure 5 presents SEM and EBSD observations of the cross section of the coating. There are two phases in the coating, including Al 2 Y 4 O 9 as the major phase component and Y 2 O 3 as a minor phase component. In addition, from the XPS results in Fig. 2 and the EDS results in Fig. 4, it can be concluded that the grain boundaries in Fig. 5b are composed of amorphous Al 2 O 3 .

Results and Discussion
The results indicate that the coatings on Al alloy prepared by CPED in Y(NO 3 ) 3 solution are affected not only by the composition of the electrolyte but also by the substrate . Figure 6 shows the cross-section micrographs and elemental composition of the sample 3 measure by SEM and EDS. The coating includes two kinds of microstructure. The region mainly containing Y and O is considered to be Y 2 O 3 . Another region that contains Y, Al, and O is considered to be Al 2 Y 4 O 9 . The total thickness of the coating is about 50 μm.
The hydrophilic/hydrophobic behavior of coatings is typically used to characterize their self-cleaning capabilities. The contact angle of a water drop on the surface of the uncoated or coated samples is shown in Fig. 7. The results indicated that the contact angle of the uncoated Al-alloy surface was 87.8° ± 2.7°, much smaller than that of the coated sample surface. For the coating prepared by CPED in 5 min, the contact angle was 116.2° ± 1.4°. With a prolonged deposition time of 10 min, the contact angle of the coating increased significantly to 141.0° ± 2.6°. In general, the larger the contact angle, the better the self-cleaning effect of the coating [32]. When the deposition time was increased from 10 to 20 min, the contact angle did not increase further but decreased slightly to 132.1 ± 6.0°. Therefore, selecting Another important property of coatings was their friction behavior, which was measured with the coated or uncoated Al-alloy samples as the lower friction pair and ZrO 2 or bearing steel balls as the upper friction pair. It can be seen from Fig. 8 that when the uncoated Al alloy and ZrO 2 ball or bearing steel ball worked as friction pairs, the friction coefficient (COF) is relatively high (0.55-0.65) and fluctuates significantly. Because adhesion friction is more likely to occur between metals [33], the COF of the Al alloy vs. bearing ball was higher than that of the Al alloy vs. ZrO 2 ball. For the sample 1 that treated by CPED for 5 min, the COFs of the lower friction pair vs. ZrO 2 ball and the lower friction pair vs. bearing ball are about 0.29 and 0.37, respectively. For the sample 2 that treated by CPED for 10 min, the COF of the lower friction pair vs. ZrO 2 ball and the lower friction pair vs. bearing ball are about 0.48 and 0.35, respectively. When the Al alloy was treated via CPED for 20 min (sample 3), the COFs decreased, regardless of whether the upper sample was ZrO 2 or a bearing ball. The COFs of coated Al alloy vs. ZrO 2 ball and coated Al alloy vs. bearing ball stabilize at 0.26 and 0.31 at 1200 s, respectively, which were 55% and 51% lower than those of the uncoated Al-alloy samples. The above results indicated that the CPED coating on the Al alloy could significantly reduce the COF.
The 3D topography of the wear tracks after the friction tests was measured using a 3D optical surface profiler. From Fig. 9a and b, it can be observed that severe wear occurred in the contact area of the uncoated Al-alloy surface, and the material was considerably removed, forming deeper wear tracks. Because the hardness of ZrO 2 and the bearing ball was much higher than that of the uncoated Al-alloy samples, a significant amount of the uncoated Al-alloy material was transferred to the contact area of the ball after the friction tests. For the samples treated by CPED with the deposition time of 20 min, the wear of the coating-contact area was relatively less. No distinct wear track or material removal was observed on the surfaces of the coated samples, as shown in Fig. 9c and d. Correspondingly, there was little evidence of material transfer at the top of the ZrO 2 ball or bearing ball. These results are consistent with the friction  Fig. 8. After CPED treatment, the wear rate of the lower friction pair was markedly reduced from 6.080 × 10 -3 to 3.431 × 10 -4 mm 3 /(N m) when grinded with a bearing steel ball and from 4.344 × 10 -3 to 2.270 × 10 -4 mm 3 /(N m) when grinded with a ZrO 2 ball. Correspondingly, the wear rates of the lower friction pair were reduced by 18 and 19 times, respectively. The topographical results show that the surface of the CPED coating can significantly reduce the wear rate of the friction pairs. The stable presence of the coating provides the necessary conditions for effective antifriction and anti-wear performance.

Conclusions
In this study, a ceramic coating composed of Al 2 Y 4 O 9 , Y 2 O 3 , and amorphous Al 2 O 3 was prepared on an Al alloy by CPED. As the applied voltage and deposition time increased, the crystallization of the coatings becomes better. When deposited at 130 V for 10 min, the contact angle of the ceramic coating attained 141.0° ± 2.6°, indicating a better self-cleaning effect.
The ceramic coating also exhibited excellent self-lubricating and anti-wear effects. The COF of the CPED-treated sample vs. ZrO 2 ball or bearing steel ball stabilized at 0.26 and 0.31 at 1200 s, respectively, which was 55% and 51% lower than those of the uncoated Al-alloy sample vs. ZrO 2 ball or bearing steel ball, respectively. Correspondingly, the wear rates of the CPED-treated sample were approximately 3.431 × 10 -4 mm 3 /(N m) and 2.270 × 10 -4 mm 3 /(N m), which were 18 and 19 times lower than those of the untreated samples and ZrO 2 or bearing steel balls as friction pairs. This CPED method is considered a novel method for the preparation of wear-resistant ceramic coatings on the surface of Al alloys.