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

Abstract

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. Ti0.42Al0.58N0.95, Ti0.41Al0.59(O0.08N0.92)1.02 and Ti0.41Al0.59(O0.18N0.82)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 Ti0.42Al0.58(O0.57N0.43)1.25, Ti0.40Al0.60(O0.79N0.21)1.39 and Ti0.41Al0.59O1.52 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 Ti0.42Al0.58N0.95. 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 Ti0.41Al0.59O1.52 coatings, the transformation of anatase-rutile TiO2 conduces to inferior thermal stability.