Increasing the oxidation resistance of γ-TiAl by applying a magnetron sputtered aluminum and silicon based coating

Abstract

In this study, the suitability of a magnetron sputtered Al–18Si (at.%) coating as oxidation protection for a γ-TiAl based TNB-V2 alloy was investigated. The coating microstructure and its changes as well as the evolution of the different phases during the different steps, deposition process, post heat treatment and isothermal oxidation test, was studied using x-ray diffraction as well as scanning and transmission electron microscopy. The well adherent coating provides a homogenous, dense and crack free crystalline microstructure of Si grains in an Al matrix. The phase evolution analysis during a 20 h post-heat treatment at 550 °C with in situ x-ray diffraction reveals the complete formation of the TiAl3 phase due to the reaction of Ti from the TNB-V2 substrate and Al from the coating. During oxidation at 850 °C for 300 h, a thermally grown α-Al2O3 layer was formed providing excellent oxidation resistance. The formation of α-Al2O3 relies on the presence of TiAl3 whereas the silicon acted as a getter for Ti and thus could prevent the formation of TiO2. Furthermore, the finally formed Ti5Si3 phase reduces the inwards diffusion of Al into the substrate and prevents fast Al depletion in the coating. Moreover, the hierarchical coating microstructure, consisting of a TiAl3 matrix with a network-like structure of the Ti5Si3 phase, could be beneficial in terms of mechanical properties.