STRUCTURE AND ELEMENT DISTRIBUTION OF Al2O3 COATING ON ZL109 ALLOY BY PLASMA ELECTROLYSIS OXIDATION

It is thought that the PEO (plasma electrolysis oxidation) coatings are difficult to be prepared on the cast Al–Si alloys because these alloys contain lots of pure silicon phases. In the present study, oxide coatings with a thickness of about 10 μm were deposited on the substrate of the ZL109 Al–Si alloy by a PEO treatment. XRD showed that the phase composition of the coating was γ-Al2O3. SEM showed that the coating surface was porous, and the pore diameter was in the range of 1–5 μm. The cross-section view by EDS indicated that the coating matrix is quite dense, and there are few defects. Phosphorus from the applied electrolyte could be incorporated into the coating during the PEO process. A transitional zone with a thickness of about 3 μm was observed on the interface between the coating and substrate, and a high adhesive strength was expected. The average hardness of the coating was 576 HV0.025, which was 7.2 times higher than that of the substrate. High alloy element content of the substrate seems to have no remarkable and adverse influence on the structure and composition of the PEO coating.