Investigation of the origin of compressive residual stress in CVD TiB2 hard coatings using synchrotron X-ray nanodiffraction

Abstract

CVD hard coatings deposited on cemented carbide substrates typically exhibit tensile residual stresses, which are mainly due to differences in the thermal expansion coefficients of coating and substrate material. However, CVD TiB2 coatings have been reported to show considerable compressive residual stresses, with lacking information about their origin. Within this work, TiB2 coatings were deposited with and without TiN base layers on cemented carbide substrates using thermally activated CVD. The coating composition was examined using glow discharge optical emission spectroscopy yielding a slight sub-stoichiometry of the nanocrystalline TiB2 layers. Slices of the coating were prepared and synchrotron X-ray nanodiffraction experiments were performed in transmission geometry. Thus, gradients of stresses and texture could be determined as a function of the coating thickness. The TiN base layer showed tensile stresses, while the compressive stresses in the TiB2 layer even further increased with increasing layer thickness. No pronounced orientation could be determined for the early growth stage of the TiB2 layers, but with increasing thickness, a distinct (101) fiber texture develops. Pole figure measurements revealed no pronounced texture for the TiN base layer and corroborated the (101) texture for the TiB2 top layer. Using the sin2ψ method, compressive stresses of ~2.9GPa could be determined for the TiB2 layers, which significantly relaxed after annealing at 1000°C. Nanoindentation experiments revealed a hardness of ~44GPa. In conclusion, the nanocrystalline nature of the coatings seems to be the dominating origin of the compressive stresses of CVD TiB2 coatings.