Indentation induced film fracture in hard film-soft substrate systems

Nanoindentations were made into oxide films on aluminum and titanium substrates for two cases; one where the metal was a bulk (effectively single crystal) material and the other where the metal was a 1 mum thick film grown on a silica or silicon substrate. In both cases indentation was used to produce discontinuous loading curves, which indicate film fracture after plastic deformation of the metal. The oxides on bulk metals fractures occurred at reproducible loads, and the tensile stresses in the films at fracture were approximately 10 and 15 GPa for the aluminum and titanium oxides, respectively. Fine grained films only exhibited discontinuous loading under cyclic indentations, and demonstrated decreased load at catastrophic film fracture as the amplitude of cycling was decreased. A hypothesis regarding film fracture in these film systems is presented based on plastic deformation in the substrate and the superposition of the load carried by the elastic stretching of the film and the plastic deformation of the substrate.