Effect of Substrate Orientation, Roughness, and Film Deposition Mode on the Tensile Strength and Toughness of Niobium–Sapphire Interfaces

Abstract

The strength and toughness of interfaces between sputter‐deposited polycrystalline niobium films and sapphire substrates with basal and prismatic orientations were measured. The effect of deposition parameters and substrate roughness on these interface properties also was investigated. Substrates of polycrystalline alumina with two different surface morphologies were chosen for studying the effect of interface roughness. The interface strength was measured using a previously developed laser spallation experiment in which a laser‐generated compressive stress pulse in the substrate, upon reflection into a tensile stress pulse from the coating's free surface, pulls the interface apart. The interface toughness was obtained using a controlled delamination technique, in which a residually stressed loading layer was used to buckle the underlying test layer from its substrate. The energy balance in the prebuckeled and postbuckled states provided a direct measure of the interface toughness. These values were independently obtained by another experiment in which well‐characterized, artificially generated, interfacial flaws were loaded using a stress pulse in the laser spallation assembly. The coating's free surface velocity upon crack initiation was related to the critical energy release rate via a numerical simulation. The results of the two toughness experiments conformed to each other and related fairly well to the independently obtained strength measurements.