The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si3N4 was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase (alpha or beta) of starting Si3N4 powder, and sintering temperature. The microstructure Of Si3N4 was characterized in terms of grain size, aspect ratio of the elongated grain, and beta-to-alpha phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si3N4 were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si3N4. such as critical loads for yield (P-y) and for ring cracking (P-c). The effect of the microstructure of Si3N4 on adhesion strength evaluated by scratch test is discussed. TiN films on Si3N4 showed high adhesion strengths in the range of 80-140 N. Hardness and the P-y of Si3N4 substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si3N4, substrates with finer grain sizes and higher a phase ratios, which show high hardness and high P-y, were suitable for higher adhesion strength of TiN film. (c) 2005 Elsevier B.V. All rights reserved.