A combinatorial X-ray sub-micron diffraction study of microstructure, residual stress and phase stability in TiAlN coatings

The understanding of the relationship between structure and properties of thin coatings is one of the key requirements for their further improvement. Especially for metastable TiAlN, commonly used as a protective coating for cutting applications, detailed knowledge about the apparent microstructure, the resulting residual stresses and the accompanied decomposition behavior at elevated temperatures are of vital importance, as the subsequent high-temperature formation of the thermally stable wurtzite phase results in deteriorated mechanical properties. In this study, the recently introduced cross-sectional scanning X-ray diffraction method is applied as a combinatorial tool to investigate the depth evolution of the local microstructure and residual stresses and their effect on phase stability of a TiAlN coating synthesized by cathodic arc evaporation at stepwise increased bias voltages. The decomposition behavior of the coating during annealing is obviously given by the initial microstructure and stress state, which control the local stress relaxation as well as the formation of precipitates. Thus, the phase transformation is favored in the coating regions that developed under moderate ion-irradiation conditions, characterized by low stress and well-developed columnar grains, and retarded under the presence of high compressive stress. The observed non-linear dependency of phase decomposition on the initial microstructure and stress is interpreted by ab initio calculations.