Structural transformations in TiAlSiCN coatings in the temperature range 900–1600 °C

Abstract Thermal stability, a characteristic that describes the ability of a material to maintain its structure and properties in a wide temperature range, is a critical property for hard and wear-resistant protective coatings designed for use at elevated temperatures. The present paper gives a detailed description of structural changes in TiAlSiCN coatings with a “comb”-like nanocomposite structure in a wide temperature range of 900–1600 °C. The structure, elemental and phase composition of the coatings were studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. Three distinct stages were identified. (i) Below 1200 °C, only a change in face-centered cubic (fcc) phase stoichiometry and phase transformations inside the amorphous phase were observed to occur because of a short-range redistribution of elements inside each phase component. (ii) The “comb” like nanocomposite morphology remained mainly unchanged up to a temperature of 1300 °C, but additional processes took place between 1200 and 1300 °C: Al atoms started to diffuse towards the coating surface and defect density in the fcc phase decreased. (iii) Above 1300 °C, diffusion processes were activated, resulting in the recrystallization of the structure, which completed at 1400 °C. When annealing temperature was further raised, the structural changes in the fcc grains were controlled by long-range Si and Al diffusion. At 1500 °C, intensive Al diffusion towards the surface resulted in the formation of a thin h-AlN layer, which, however, completely decomposed at 1600 °C. The main cubic (Ti, Al)(C, N) phase was stable in a wide temperature range between 900 and 1600 °C, although its elemental composition was changed during heat treatments. The TiAlSiCN coatings with a “comb”-like nanocomposite structure were characterized by high hardness above 37 GPa in the temperature range 25–1300 °C.