Microstructure Characteristics and High-Temperature Oxidation Behavior of Plasma-Sprayed and Laser-Remelted MCrAlY Coatings on TiAl Intermetallics

In order to further improve the high-temperature oxidation resistance of TiAl intermetallic alloys, MCrAlY coatings were fabricated by plasma spraying and plasma spraying-laser remelting technologies. The microstructures of the as-sprayed and laser-remelted MCrAlY coatings were studied. In addition, the oxidation behaviors at 850 °C for three samples were investigated. One sample is the matrix of TiA1 intermetallic alloys, the other one is processed by plasma-spraying MCrAlY coatings, and the third one is processed by plasma-spraying and laser-remelting MCrAlY coatings. It was revealed that the oxidation resistance of TiAl intermetallics is weak due to lack of protection of Al₂O₃ film formed on the surface. The plasma-sprayed MCrAlY coatings have better oxidation resistance than the TiAl intermetallics although the plasma-sprayed MCrAlY coatings have high density of porosity and a typical layered structure. It is demonstrated that most of the holes can be eliminated by laser remelting, leading to the best oxidation resistance of the third sample with the laser-remelted coatings. The high oxidation resistance of the laser-remelted coatings is mainly attributed to three aspects: firstly, an Al enriched zone on the coating surface is formed during laser remelting, which is transformed into a protective Al₂O₃ film during oxidation process. Secondly, laser remelting eliminates most of the defects in plasma-sprayed coatings and enhances its density, thus decreases the channel of oxidation diffusion in high temperature oxidation process. Thirdly, rapid cooling of laser remelting results in a grain refinement and a preferred oxidation of Al at the initial stage, leading to a reduction of oxidation rate.