Microstructure and mechanical properties of (Ti,Al,Nb)N hard films with N-gradient distributions

• (Ti,Al,Nb)N films with nitrogen gradient distribution are manufactured.
• Nitrogen distribution in film corresponds to N2 partial pressure in deposition.
• Hardness lager than 30 GPa and adhesive strength larger than 200 N are reached.
• Thermal shock resistance in terms of 11–13 cycles at 650 °C is achieved.

(Ti,Al,Nb)N hard reactive films with N-gradient distribution are deposited on high-speed steel substrates by multi-arc ion plating technology using Ti-50Al (at%) and Ti-25Nb (at%) alloy targets. The partial pressure of N2 was raised by two steps from 0 to 1.0 × 10− 1 Pa and then to 2.5 × 10− 1 Pa in each deposition process. The surface morphology, the cross-sectional morphology and the phase structure of the (Ti,Al,Nb)N films are investigated by means of scanning electronic microscope and X-ray diffraction. The dense columnar microstructure was obtained in each of the (Ti,Al,Nb)N films. The micro-hardness of film surface, the adhesive strength of film/substrate and the thermal shock resistance were also investigated. The results reveal the effects of the deposition process and the addition of Nb on the composition, phase structure, and mechanical properties. The improved properties include micro-hardness lager than 30 GPa, adhesive strength larger than 200 N, and thermal shock resistance to sustain 11–13 thermal shock cycles of 650 °C. The present (Ti,Al,Nb)N hard films with N-gradient distribution may be an actual substitution of TiN, (Ti,Al)N, and (Ti,Nb)N hard films.