Mechanical characterisation of the protective Al2O3 scale in Cr2AlC MAX phases

MAX phases have great potential under demands of both high-temperature and high-stress performance, with their mixed atomic bonding producing the temperature and oxidation resistance of ceramics with the mechanical resilience of metals. Here, we measure the mechanical properties up to 980C by nanoindentation on highly dense and pure Cr2AlC, as well as after oxidation with a burner rig at 1200 degrees C for more than 29 h. Only modest reductions in both hardness and modulus up to 980 degrees C were observed, implying no change in deformation mechanism. Furthermore, micro-cantilever fracture tests were carried out at the Cr2AlC/Cr7C3 and Cr7C3/Al2O3 interfaces after the oxidation of the Cr2AlC substrates with said burner rig. The values are typical of ceramic-ceramic interfaces, below 4 MPa root m, leading to the hypothesis that the excellent macroscopic behaviour is due to a combination of low internal strain due to the match in thermal expansion coefficient as well as the convoluted interface.