Mechanical behavior of (Ti,Al)N coatings exposed to elevated temperatures and an oxidative environment

TiAlN coatings were deposited in an ion beam-assisted deposition process on silica substrates with variations in the deposition parameters, particularly the ion energies. For these coatings the residual stress after deposition and its temperature dependence was determined. In order to study the influence of oxidation, the residual stress was also analyzed after exposing specimens to air at temperatures up to 650°C. For the characterization of the stability and the mechanical behavior, scratch tests were performed on coatings having different initial stresses, before and after the exposure treatments. It was found that for coatings with compressive stress between 1000 and 6000 MPa, the critical loads determined in the scanning scratch tests exceeded the values observed for tensile-stressed coatings by more than three-fold. Exposing the coated specimens to hot air led to changes in the residual stress, e.g. to a decrease in the compressive stress from 5700 to 3300 MPa after 1 h at 600°C, and also to a decrease in the mechanical stability; for such coatings for example, a drop in the critical load by up to 50% after 1 h at 600°C in air was observed.