HIGH-TEMPERATURE OXIDATION AND WEAR-RESISTANCE OF TI-AL-N HARD COATINGS FORMED BY PVD METHOD

An extensive study was carried out on the phase relations, microhardness and oxidation properties of hard coatings formed in the Ti-Al-N system using the cathodic arc ion plating method. When the ratio of the partial pressure of N2 to the evaporation rate of titanium was reduced, the resulting phases in the Ti-N system changed from cubic TiN through tetragonal Ti2N to h.c.p. alpha-Ti. When the AlN content in the TiN-AlN solid solution was increased, the films, from TiN up to (Ti0.3Al0.7)N, exhibited a cubic B1 structure. As the AlN content increased, the lattice parameter monotonically decreased from 0.426 nm for TiN, to 0.416 nm for (Ti0.3Al0.7) N. With a further increase in the AlN content, (Ti0.15Al0.85) N films were found to have a wurtzite structure. It was concluded that the crystal structure of the ternary Ti-Al-N system along with the wide-ranging solubility of AlN in TiN is typical for the isostructural group of cubic titanium-based nitrides. The (Ti1-xAlx) N films with a cubic structure were stable in air at the temperatures ranging from 710-degrees-C for x = 0.25 to 830-degrees-C for x = 0.6, whereas the TiN films began to oxidize at 550-degrees-C. It was found that protective layers of aluminum oxides formed on top of the (Ti, Al)N films during elevated temperature oxidation tests protected the films from further oxidation. The crystallized TiO2 that was formed on the TiN film did not give the same protection. Tool-life tests on the (Ti, Al) N coated carbide inserts revealed the superior wear resistance, compared with a variety of nitride-coatings including TiN.