Thermal stability of microstructure and mechanical properties of Ni/Ru multilayers

The microstructure, hardness and thermal stability of Ni/Ru multilayers prepared by evaporation deposition were investigated by X-ray diffraction, transmission electron microscopy, vacuum annealing and nanoindenation. The hardness values of as deposited multilayers increase, while their elastic modulus values decrease, with decreasing periodicity. After annealing at low temperature (below 450 °C), the decrease in hardness of multilayers with larger periodicity is more remarkable than that of multilayers with smaller periodicity due to coarsening of the in-plane grain size. The higher temperature (600 °C) annealing results in the breakdown of the periodical structure and a significant drop in hardness for multilayers with smaller periodicity. The results were discussed according to Orowan-type single dislocation bowing mechanism.