Effects of annealing temperature on the microstructure and hardness of TiAlSiN hard coatings

TiAlSiN hard coatings were synthesized on high-speed steel using an arc ion enhanced magnetic sputtering hybrid system. The microstructure and hardness of the coatings at different annealing temperatures were explored by means of XRD, TEM, EDAX and Vickers indentation. The as-deposited TiAlSiN coatings were confirmed to be amorphous due to high depositing rate and low deposition temperature during the film growth. The transformation from amorphous to nanocomposites of nano-crystallites and amorphousness were observed after the annealing treatment, the microstructure of TiAlSiN coatings annealed at 800°C and 1000°C were consisted of crystalline hcp-AlN, fcc-TiN and amorphous phase, however, the coatings were only consisted of fcc-TiN and amorphous phase when annealing at 1100°C and 1200°C. Meanwhile, the formation of Al₂O₃ was detected on the coating surface after annealing at 1200°C and it indicated the excellent oxidation resistance of the TiAlSiN coatings under the present experimental conditions. Furthermore, the average grain size of the TiAlSiN coatings after high temperature annealing even at 1200°C was less than 30 nm and the size increased with the increasing temperature. However, the hardness of the so-deposited coatings with HV_0.2N=3300 dramatically decreased with the increase of temperature and reached nearly to the hardness of TiN coatings with HV_0.2N=2300.