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The isothermal oxidation behaviour of Cr 2 AlC-MAX phase (ternary alloy with general formula M nz1 AX n : M5early transition metal, A5A-group element, mostly IIIA or IVA, X5C or N, n51-3) films on alumina substrates was investigated at temperatures between 700 and 1200uC for hold times of 1 to 30 h. The influence of the annealing temperature and time on the structure, surface morphology and microstructure evolution was studied. It was found that two processes occur simultaneously in these layers. These are the transformation of the disordered solid solution (Cr,Al) 2 C x to the ordered Cr 2 AlC-MAX phase and the oxidation of the MAX phase. In this work, a detailed discussion of these processes is given. Moreover, a schematic model of the associated structural and chemical changes in the annealed Cr 2 AlC layers based on the X-ray diffraction (XRD), Energy Dispersive X-ray (EDX), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and magnetic results was developed.

The isothermal oxidation behaviour of Cr 2 AlC films deposited with 0?1-0?3 at.%Y addition on alumina substrates was investigated at temperatures between 700 and 1200uC for hold times of 1 to 30 h. It was found that the Y atoms impede the bulk diffusion of Al and Cr atoms and, thus, the initial transformation of the disordered solid solution (Cr, Al) 2 C x to the ordered Cr 2 AlC-MAX phase. It also reduces the oxidation rate of the ordered Cr 2 AlC-MAX phase and promotes the a-Al 2 O 3 phase formation relative to the other alumina on the surface and leads to better adhesion of the layers. 0?3 at.%Y was found to promote the formation of a-Al 2 O 3 relative to the other possible phases most strongly for 700-900uC. However, 0?2 at.%Y addition was found to cause the best oxidation resistance at 1200uC. The model introduced in part I was developed further.

Self-healing materials facilitate a design concept which includes damage management, where damage that is inflicted during operation can be healed autonomously. It has been shown that the M n+1 AX n phases Ti 3 AlC 2 , Ti 2 AlC and Cr 2 AlC exhibit such behaviour. Cracks are filled and hence healed by oxidation products of the M and A elements in the MAX phase at the high operating temperatures in which they are used. After crack healing the fracture strength is recovered to the level of the virgin material. In this work, the Cr 2 AlC-MAX phase was shown to exhibit excellent self-healing properties. Cracks of maximum widths of approximately 500 nm and 4 μm could be healed after a 3-5 h anneal at 900 and 1200°C, respectively. In this system, the addition of Y on the rate of oxidation and on the self-healing behaviour was investigated.