Thermal stability and oxidation resistance of Ti0.22Al0.74Si0.04N coating synthesized by chemical vapor deposition

Addition of Si is known as an effective way to improve the thermal stability and oxidation resistance of TiAlN coatings. In this study, we focused on the thermal stability and oxidation resistance of a Ti0.22Al0.74Si0.04N coating which was deposited on cemented carbide substrate by low pressure chemical vapor deposition (LPCVD). According to X-ray diffraction (XRD), the phase composition of the as-deposited coating is composed of face-centered cubic (fcc-) TiN and wurtzite (w-) AlN. As the vacuum annealing temperature rises from 700 to 800 °C, the hardness increases and subsequently decreases as temperature further rises from 800 to 1100 °C, then remains basically unchanged above 1000 °C. At 900 °C and above, Co diffusion from the substrate into the coating is observed, which affects the mechanical properties of the coating. A maximum hardness of 35.6 ± 3.3 GPa is obtained after annealing in vacuum at 800 °C for 1 h. The results of transmission electron microscopy (TEM) indicate that the coating exhibits a structure similar to that of the deposited state after vacuum annealing at 1200 °C, consisting of nanocrystalline (Ti,Al)N embedded in amorphous SiNx. For the oxidation test, each sample was annealed in air with steps of 50 °C for 1 h within the range of 700–1000 °C. XRD and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the oxide layer is composed of rutile (r-) TiO2, amorphous SiO2 and a dominant amount of amorphous Al2O3 phase forms in the coating after annealing at 950 °C for 1 h. The coating exhibits excellent oxidation resistance with an oxide thickness of only 469 nm after annealing in air at 1000 °C for 1 h.