Mechanical properties of ceramic multilayers: TiN/CrN, TiN/ZrN, and TiN/TaN

Polycrystalline TiN/ZrN, TiN/CrN, and TiN/TaN multilayers were grown by reactive magnetron sputtering on WC/Co sintered hard alloy substrates. Hardness and elastic modulus were measured by nanoindentation testing. Hardness of TiN/ZrN multilayers decreased rapidly with increasing bilayer thickness (Lambda), peaking at hardness values = 30% lower than rule-of-mixtures values at Lambda = 30 Angstrom, and increased with further increases in Lambda. A comparison with other lattice mismatched systems showed a similar hardness variation, but the sign was negative. The results suggest that coherency strains are responsible for the greater hardness change. Nanoindenter elastic modulus results showed the same behavior with hardness dependence on Lambda, i.e., elastic softening at Lambda = 30 Angstrom. TiN/CrN system showed no hardness and elastic anomalies. In TiN/TaN systems, hardness are lower than rule-of-mixtures value of individual single layers for Lambda > 80 Angstrom, and increased rapidly with decreasing Lambda, peaking at Lambda = 43 Angstrom. As a result of analysis on the inclination of applied load for indenter displacement (Delta P/Delta h), this paper demonstrates that the enhancement of the resistance to dislocation motion and elastic anomaly due to coherency strains are responsible for the hardness change.