In Situ Preparation of Hard Ceramic Coating Containing α-AL_2O_3 on TC4 Alloy

Ceramic coatings containing α-Al2O3 were prepared on T1-6Al-4V alloy by an alternating-current micro-plasma oxidation (MPO) method in solutions of sodium aluminate. The Knoop hardness (HK) and coating-substrate adhesion of MPO coatings were measured by the microhard-ness tester and ball-on-plate impact tester. Influences of the electrolyte concentration and cathodic to anodic current ratio Ic/Ia on the phase composition and mechanical properties of coatings were discussed. The results show that the coating formed at weak aluminate solution (0.05mol/L) consists of TiO2 and Al2TiO5; at high aluminate concentration (0.15-0.20mol/L) consists of TiO2, Al2TiO5 and α-Al2O3. The content of α-Al2O3 increases with the decreasing Ic/Ia ratio. HK of coatings containing α-Al2O3 varies within 1100-1600Hk50g which is much higher than that of Ti substrate and even the coating not containing α-Al2O3. Distribution profile of HK from interface toward the coating surface shows that HK gradually increases and reaches the maximum value around 35-45μm. 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.