Characterization of wear protective Al-Si-O coatings formed on Al-based alloys by micro-arc discharge treatment

The wear life of components manufactured from Al-based alloys can be drastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition or plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recently developed micro-are discharge oxidizing (MDO) technique allows for the formation 100-200 mu m thick Al-Si-O coating on the surface of Al alloys. A composite Al2O3-SiO2 coating is formed at room temperature as a result of a reactive process between Al in the alloy itself and 0 and Si supplied by an electrolyte. Al-Si-O coatings were investigated with XPS, Vickers and nanoindentation hardness tests, ball-on-disk, and block-on-ring friction and wear tests. Coatings were found to consist of at least two phases: a hard Al2O3 phase and a softer aluminasilicate phase. A maximum hardness of 17 GPa was found for coatings with highest content of Al2O3 phase. The tribological properties of Al-Si-O coatings with different composition are discussed. The lowest friction coefficient was found for the Al0.26Si0.08O0.66 coating and was measured around 0.15-0.25 depending on the test environment. The application of this coating decreased the wear rate of components fabricated from an Al-based alloy by several orders of magnitude and permitted operation of coated friction pairs at 1 GPa contact load.