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 d rastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition o r plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recentl y developed micro-are discharge oxidizing (MDO) technique allows for t he formation 100-200 mu m thick Al-Si-O coating on the surface of Al a lloys. 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 a nd Si supplied by an electrolyte. Al-Si-O coatings were investigated w ith 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 aluminasilica te phase. A maximum hardness of 17 GPa was found for coatings with hig hest content of Al2O3 phase. The tribological properties of Al-Si-O co atings with different composition are discussed. The lowest friction c oefficient was found for the Al0.26Si0.08O0.66 coating and was measure d 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 operati on of coated friction pairs at 1 GPa contact load.