Stress measurements in thermal loaded (Ti,Al)N hard coatings

Extreme thermal load of (Ti,Al)N coated WC–Co tools during the dry high-speed cutting process results in large thermal stresses superimposing the residual stresses in the films. This can cause layer damage by cracking and delamination. Therefore a stress determination in (Ti,Al)N coatings can help to understand these damaging processes. Fine crystalline (Ti,Al)N hard coatings with an Al/Ti ratio of 1.33 and a thickness of 1, 3, 6 and 14 μm, respectively, were deposited onto WC–6% Co hard-metal substrates or Si(100) wafers, respectively, from TiCl4/AlCl3/N2/Ar/H2 gas atmosphere using the plasma-assisted CVD (PACVD) process. Thermal load was applied either by annealing or by thermal laser shock. Stress state of the coatings before and after thermal load was analysed using X-ray diffraction methods or the substrate curvature method. In the as-deposited state of the (Ti,Al)N films compressive stress values between 300 MPa and some GPa at the film surface were measured caused by deposition mechanism and different thermal expansion coefficients of the film and the substrate. Stress values decrease towards the substrate and change to tensile stress up to 2.5 GPa near the film–substrate interface. At temperatures above 450°C stress relaxation mechanisms occur during substrate curvature experiments, which result in tensile stresses up to approximately 200 MPa after cooling. In laser shock experiments with homogenised laser beam profiles (ΔE′/E<10% across the laser spot) typical crack networks were generated inside the laser spot region when the energy density reached a critical value, corresponding to a surface temperature of approximately 750°C for irradiation times between 1 and 10 ms. However, there are remaining questions for the explanation of the latter fact by means of the stress measurements.