Effect of competitive deformation between grain boundary and film interface on hardness measurements of multilayer films

Ni/Al mutlialyer films with different modulated periods (L) were prepared using multi-target magnetron sputtering method. The microstructure of multilayers was studied by XRD and HRTEM. The continuous stiffness mode (CSM) was employed to measure the effect of indentation depth on the determination of hardness of thin films by means of nanoindentation. The results reveal that the film was strengthened with increasing L, and the superhardness phenomenon was observed for Ni/Al multilayers. For the film with L more than 30 nm, depth-dependent hardness increased with enhanced indentation depth. However, the hardness at the largest indentation depth showed the lowest value for the Ni/Al film with L less than 30 nm. This tendency indicates the depth-sensing hardness for multilayer at nanometer scale, which was analyzed by a competitive effect between film interface and grain boundary.