Evolution of microstructure, mechanical and thermal properties with varied oxygen contents in TiAlON coatings.

In recent years, TiAlON coatings have attracted broad attention in the machining field for their excellent toughness and machining performance. Here, we explore the microstructure, mechanical properties and thermal stability of Ti-Al-O-N coatings with the O/(O + N) ratio from 0 to 1. Combined experiments and first-principle calculations show that metal vacancies induced by charge balancing is the main incentive for increased non-metal/metal ratio and decreased lattice constant with increasing O concentration. Ti 0.42 Al 0.58 N 0.95 , Ti 0.41 Al 0.59 (O 0.08 N 0.92) 1.02 and Ti 0.41 Al 0.59 (O 0.18 N 0.82) 1.13 coatings reveal a single-phase face-centered cubic (fcc) structure and almost constant hardness of ∼32 GPa. Further increasing oxygen content leads to the formation of amorphous oxides. Attributed to the negative effect of metal vacancies, weaker metal‑oxygen bond and amorphous oxides, the hardness values of Ti 0.42 Al 0.58 (O 0.57 N 0.43) 1.25 , Ti 0.40 Al 0.60 (O 0.79 N 0.21) 1.39 and Ti 0.41 Al 0.59 O 1.52 are reduced to 27.7 ± 1.1, 22.5 ± 0.6 and 10.6 ± 0.7 GPa, respectively. All coatings present the age-hardening ability during annealing, where the temperatures of hardness peak for the O-containing nitride coatings are higher than that for Ti 0.42 Al 0.58 N 0.95. This beneficial effect on the thermal stability can be attributed to the retarded wurtzite AlN formation and the precipitation strengthening of Al-containing oxides. However, for N-free Ti 0.41 Al 0.59 O 1.52 coatings, the transformation of anatase-rutile TiO 2 conduces to inferior thermal stability. [Display omitted] • Addition of O into TiAlN induces metal vacancies by charge balancing. • The transition from cubic to cubic and amorphous oxide phase occurs with increasing O-content. • The mechanical properties of TiAlON coatings scale with the O-content. • Incorporation of O has a beneficial effect on the thermal stability of TiAlN. [ABSTRACT FROM AUTHOR]