Investigation of the origin of compressive residual stress in CVD TiB_2 hard coatings using synchrotron X-ray nanodiffraction

摘要

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 substratematerial. However, CVD TiB_2 coatings have been reported to show considerable compressive residual stresses, with lacking information about their origin.Within thiswork, TiB_2 coatings were depositedwith andwithout 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 TiB_2 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 TiNbase layer showed tensile stresses,while the compressive stresses in the TiB_2 layer even further increased with increasing layer thickness.No pronounced orientation could be determined for the early growth stage of the TiB_2 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 TiB_2 top layer. Using the sin~2ψ method, compressive stresses of ~2.9 GPa could be determined for the TiB_2 layers, which significantly relaxed after annealing at 1000 °C. Nanoindentation experiments revealed a hardness of ~44 GPa. In conclusion, the nanocrystalline nature of the coatings seems to be the dominating origin of the compressive stresses of CVD TiB_2 coatings.