Determination of growth-induced strain and thermo-elastic properties of coatings by curvature measurements

Curvature measurements on bimaterial specimens are examined for determination of residual strain/stress and thermo-elastic properties of coatings and layered materials. By using specimens with material layers of comparable thickness, large deflections and thereby high measurement accuracy are obtained. Strain-curvature relations are given for beam and plate specimens at different deformation modes. A robust fitting method for simultaneous determination of the mismatch strain and the modulus of elasticity is described. Experiments are performed on a titanium nitride (TiN) coating deposited on a thick high-speed steel substrate by PVD. The modulus of elasticity and the thermal expansion coefficient for the TiN coating are determined to 500 GPa and 7.4×10−6 K−1, respectively. The characteristic residual stress, i.e. the thin-film stress at room temperature, is −3.8 GPa, for this material combination and deposition conditions. The temperature independent growth-induced strain is determined to −0.0045, which is 75% of the mismatch strain at room temperature.