Alteration of residual stress in TiN film due to heat treatment

The structure and residual stresses of TiN films deposited onto substrate of spring steel by an arc ion plating were investigated by X-ray diffraction. The films exhibited high {111} orientation. Residual stresses in the films were evaluated by the two-exposure method to obtain lattice strains for 222 diffraction at ψ = 0° and 70.5° with CuKα radiation. The results revealed very high compressive residual stresses of -7.1 to -6.3 GPa which are one order larger than the thermal residual stress expected from the thermal strain mismatch between the film and the substrate. These residual stresses decreased with increasing annealing temperature and annealing time, and finally reached a level of the thermal residual stress after annealing at temperatures above 1073 K. Vickers micro hardness tests of TiN films revealed high hardness (HV 950-2360) whose value depended on annealing treatments. No residual stress was observed in the substrate interface in an as-deposited state. Tensile residual stress was developed after the annealing above 1273 K. Vickers hardness of steel substrate was about 170HV for the annealing below 1073 K. After annealing at 1273 K, the hardness increased to about 300HV due to the structural transformation. The ratio of nitrogen to titanium (N/Ti) was observed by the X-ray photoelectron spectroscopy (XPS) before and after annealing treatments. The results of the XPS analysis showed that the initial value of N/Ti was about 0.87 in the as-deposited TiN films and that the ratio of N/Ti did not change after annealing at temperatures below 1073 K, and gradually decreased to 0.80 by annealing above 1073 K.