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Transmission microscopy investigation of the oxidation of hard coatings for high speed machine tools

ABSTRACT: The oxidation of TiAIN based nitride hard coatings on steel substrates has been investigated as a function of time and temperature. The addition of Y during deposition changed the normal columnar growth into a fine interrupted columnar growth. XTEM examination revealed changes in the chemical composition of samples heat treated in air at 900°C. Moreover, in the Y-containing coating, grain growth due to the heat treatment was observed. FEG-STEM EDX analysis and elemental mapping showed that the nitride grain boundaries provided paths for substrate elements to diffuse rapidly to the surface. Y greatly inhibited such diffusion, probably by segregating to boundaries. At high temperatures Y diffused towards the surface to form discrete Y-rich particles within a layered polycrystalline structure of AI2O3, T1O2 and C^Cb.