In situ preparation of hard ceramic coating containing α-Al₂O₃ on TC4 alloy

Ceramic coatings containing α-Al₂O₃ were prepared on Ti-6Al-4V alloy by an alternating-current micro-plasma oxidation (MPO) method in solutions of sodium aluminate. The Knoop hardness (H_Κ) and coating-substrate adhesion of MPO coatings were measured by the microhardness tester and ball-on-plate impact tester. Influences of the electrolyte concentration and cathodic to anodic current ratio I_c/I_a on the phase composition and mechanical properties of coatings were discussed. The results show that the coating formed at weak aluminate solution (≤0.05 mol/L) consists of TiO₂ and Al₂TiO₅; at high aluminate concentration (0.15-0.20 mol/L) consists of TiO₂, Al₂TiO₅ and α-Al₂O₃. The content of α-Al₂O₃ increases with the decrease of I_c/I_a ratio. H_Κ of coatings containing α-Al₂O₃ varies within 1100-1600 H_k50g which is much higher than that of Ti substrate and even the coating not containing α-Al₂O₃. Distribution profile of H_Κ from interface toward the coating surface shows that H_Κ gradually increases and reaches the maximum value around 35-45 μm. In addition, there is no spallation after impact tests observed by SEM, which indicates MPO coatings have the excellent interfacial adhesion with the titanium substrate. The cycle thermal shock tests show that, the thinnest film possesses the highest number of cycles.