Structure, Mechanical Properties, Thermal Stability, and Chemical Stability of Metastable Ti1-xAlxN (x=0.03-0.05) Solid Solutions Prepared as Arc PVD Coatings on WC-Co Alloys

We report a comparative study of the chemical and thermal stability and mechanical properties of arc PVD TiN and Ti0.97Al0.03N coatings. Heating the coatings in vacuum to 600 and 700 degrees C has been shown to cause an increase in crystallite size and a decrease in biaxial macrostress, lattice parameter, and lattice strain. These effects are due to thermally activated structure restoration processes associated with annihilation of defects generated during the growth of the coatings. The stress relaxation rate in the Ti0.97Al0.03N coating is higher because it contains a higher defect density. At 700 degrees C, the Ti0.97Al0.03N solid solution undergoes spinodal decomposition into TiN and AlN (FCC). Unlike those of the TiN coating, the hardness and the parameters H-3/E-2 and H/E of the Ti0.97Al0.03N coating remain essentially unchanged as the annealing temperature is raised to 700 degrees C, which is due to dispersion hardening as a result of the spinodal decomposition. The coatings exhibit similar behavior in acidic and alkaline media, but Ti0.97Al0.03N has a somewhat higher oxidation resistance in air at 550 degrees C.