Oxidative durability of TBCs on Ti2AlC MAX phase substrates

Air plasma spray (APS) and plasma-spray-physical vapor deposition (PS-PVD) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC), ~ 80-100 μm thick, were produced on a commercial Ti2AlC MAX phase compound. They were oxidized in interrupted furnace tests for 500 h each, at five successive temperatures from 1100°-1300 °C. The APS coating survived 2400 accumulated hours, failing catastrophically after 500 h at 1300 °C. Porosity, large cracks, sintering, and high monoclinic YSZ phase contents were seen as primary degradation factors. The PS-PVD coating remained completely intact over 2500 total hours (65 cycles) including 500 h at 1300 °C, exhibiting only fine porosity and microcracking, with less monoclinic. These Ti2AlC systems achieved a minimum α-Al2O3 scale thickness of 29 and 35 μm, respectively, as compared to ~ 6 ± 2 μm on average at failure for conventional bond coats on superalloys. Accordingly, times predicted from thermogravimetric analyses (TGA) of oxidation kinetics project an improvement factor of ~ 25-50 × for the time to achieve these scale thicknesses at a given temperature. Extreme oxidative TBC durability is achieved because the thermal expansion coefficient of Ti2AlC is only slightly different than those for α-Al2O3 and YSZ. The strain energy term driving scale and TBC failure is therefore believed to be fundamentally diminished from the large compressive stress produced by higher expansion superalloys.