NiCoCrAlY coatings with and without an Al2O3/Al interlayer on an orthorhombic Ti2AlNb-based alloy: Oxidation and interdiffusion behaviors

The mechanism of oxidation protection of NiCoCrAlY overlay coatings on the orthorhombic Ti(2)AINb-based alloy (O alloy) Ti-22Al-26Nb (at.%) is described. While the bare alloy exhibited poor oxidation resistance at 800 degrees C, adding NiCoCrAlY coatings significantly improved the oxidation resistance. However, serious interdiffusion between the coatings and the substrate resulted in rapid degradation of the coating system. Several reaction layers were formed at the coating/substrate interface by interdiffusion, and nonprotective scales mainly of Cr2O3 and TiO2 were formed due to the degradation of the coating. In order to solve this problem, an Al2O3/Al interlayer was sandwiched into the coating system as a diffusion barrier. The isothermal and cyclic oxidation protection of the multilayer coating system on the Ti-22Al-26Nb substrate was evaluated at 800 and 900 degrees C. The results indicated that the interdiffusion was much suppressed, and the duplex coating system demonstrated improved oxidation resistance on the Ti-22Al-26Nb substrate, with a thin and adherent protective alpha-Al2O3 scale forming on the surface. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.