The Roles of Alloy Elements on Self-Lubrication of Anodic Al2O3/MoS2 Composite Films on Al Alloys
Authors: Katsuta Shuji , Kure-Chu Songzhu , Liu Jiacheng , Iizuka Tateoki , Ebe Atsushi 
Published: December 2020

Aluminum alloys have good specific strength, corrosion resistance, and recyclability, so they are attracting attention from the automobile industry as a structural material to replace steel materials. Since aluminum alloys are prone to adhesive wear, resulting in poor tribological performance, it is necessary to improve the wear resistance by appropriate surface modification. It is known that filling some solid lubricants, such as MoS2 or PTFE, inside the anodic porous films is an effective method to improve the wear resistance of aluminum alloys in industry. However, few studies have been conducted on the alloy elements in aluminum alloys containing Si and Cu, especially casting alloys. The intermetallic compounds such as Si and CuAl2 contained in aluminum alloys usually cause defects inside the anodic films, which may affect the wear resistance of the anodic films. In this study, our purpose is to fabricate self-lubricating anodic Al2O3/MoS2 composite films on various aluminum materials by successive anodization and electrodeposition, to investigate the effects of alloy elements in aluminum alloys on the filling of MoS2 and wear resistance.

Three kinds of commercial aluminum alloy sheets (A1100: 99.0% Al, A2017: Al-3.85% Cu, A4045: Al-9.85% Si) and an Al-12Si-5Cu casting alloy for automotive engine parts. Anodizing was performed using a sulfuric acid-based solution in constant current mode. After anodizing, anodic electrodeposition was performed in an ammonium tetrathiomolybdate solution, to incorporate molybdenum disulphide (MoS2) into the anodic films. The surface and cross section of the prepared sample were observed by FE-SEM to investigate the influence of intermetallic compounds in aluminum alloy on MoS2 anodic electrodeposition. GD-OES was used to examine the composition depth profiles inside the anodic Al2O3/MoS2 films. XPS was used to investigate the composition and chemical bond state of the formed MoS2. Furthermore, in order to evaluate the influence of the substrate on the wear resistance, the friction coefficient of various samples was measured using a reciprocating wear tester (SRV-4; 20 N, 1.0 mm, 20 Hz). The counterpart material was a high carbon chromium steel ball (100 CR6-DIN) of φ12.7 mm.

Fig.1 a-b shows the results of GD-OES measurement of composition depth profiles within the self-lubricating anodic Al2O3/MoS2 composite films formed on (a) pure Al (A1100) and (b) Al-Si-Cu casting alloy. By making only Mo intensity the same measurement condition, it was possible to compare the amount of MoS2 filled inside the anodized films on pure Al and Al-Si-Cu casting alloy. Strong Mo peaks were found on the surface of both pure Al and Al-Si-Cu casting alloy specimens. The, with weak intensity inside the alumina film. Whereas the Mo and 0.1concentrate and Al-Si-Cu casting alloy. In case of pure Al, distribution of Mo mainly accumulates on the surface with only weak intensity inside the alumina film, and the intensity of Mo becomes almost stable toward the inside of the film. Whereas in case of the Al-Si-Cu casting alloy, the intensity Mo is much higher than that of pure Al, with a gradual distribution across the composite films, indicating that more Mo is deposited inside the anodic alumina film. The enhancement of Mo amount is ascribed to the presence of MoS2 in the defects of the anodic alumina film formed by the intermetallic compound (AlCu) contained in the Al-Si-Cu casting alloy, which can also be confirmed by cross-sectional observation by FE-SEM..

Fig.1 c shows dynamic friction coefficient vs. number of reciprocating slides of self-lubricating anodic Al2O3/MoS2 composite films. Both the pure Al and the Al-Si-Cu casting alloy maintained a low friction coefficient at the beginning and then showed a sharp rise at a certain point. It is considered that the self-lubricating film was completely worn and reached the base metal at the point where the friction coefficient sharply increased. It can be confirmed that the number of sliding times to reach the base metal is about 26000 times with pure Al and about 36000 times with the Al-Si-Cu casting alloy, and that the Al-Si-Cu casting alloy has a longer coating life. In addition, the friction coefficient of the self-lubricating films decreased from 0.15-0.25 for pure Al to around 0.1for the Al-Si-Cu casting alloy, thus indicating the excellent lubricating effect of MoS2, which can be attributed to the increase of the filling amount of MoS2 inside the anodic film, as shown in the results of GD-OES (Fig.1a-b).

Figure 1