Microstructure evolution and thermal shock properties of PEO coatings on a TiAl alloy

Abstract

In this paper, plasma electrolytic oxidation (PEO) coatings were produced on Ti-48Al-2Cr-2Nb alloy in a silicate electrolyte. During the PEO process, four coating growth stages were revealed, which were anodic oxidation (I), breakdown of the anodized film (II), micro-arcing growth (III) and formation of glass phase with coating fusing (IV). It was found that the porous aluminium silicate phase preferentially grew at the Ti3Al regions of the substrate in stage II and the amount of Al2TiO5 phase increased rapidly in stage III. In stage IV, a Si-rich glass phase layer grew rapidly leading to a large increase in coating thickness. The thermal shock tests were conducted for both the substrate and PEO-coated samples from 1123 K to room temperature. The thermal shock resistance study showed that the oxide layer kept growing with the thermal shock cycles on the uncoated TiAl substrate. PEO coatings with predominate Al2TiO5 and aluminium silicate phases can withstand 55 thermal shock cycles without changing. The samples with a thin amorphous anodized layer cannot withstand the internal stress generated during thermal shock cycles due to the weak bond between the outmost layer and the substrate. The coating with thick glass outmost layer also peeled off because of the weak thermal shock resistance of the glass phase. The wear test results showed that, after thermal shock cycles, the PEO coating with suitable thickness and roughness has good wear resistance and potential application in wear protection.