Oxidation mechanisms in bulk Ti2AlC: Influence of the grain size

In this work, oxidation mechanisms were studied in fine-grained (FG) and coarse-grained (CG) Ti2AlC bulk samples. Results showed that the oxidation kinetics are controlled by the grain size of Ti2AlC. Bigger are the grains, faster is the oxidation. A dense and protective Al2O3 layer forms at the surface of FG-Ti2AlC samples while for the CG-Ti2AlC samples, a thick TiO2 layer forms on top of a discontinuous Al2O3. CG-Ti2AlC was observed to simultaneously transform into Ti3AlC2 and TiC instead of being directly transformed into TiC. This transformation result in the following crystallographically sandwich-like structure: (0001) Ti2AlC // (0001) Ti3AlC2 // (111) TiC. The volume shrinkage associated to this transformation produces elongated holes that are partially filled by α-Al2O3. The stress caused by the volume shrinkage generates cracks at the surface, which makes the oxygen inwards diffusion easier and thus worsens the oxidation resistance the CG-Ti2AlC bulk.