Measurement of the interface tensile strength at elevated temperatures

Abstract

A new experimental procedure to measure the tensile strength of thin film interfaces. in iii, at elevated temperatures is presented. In this experiment, the substrate with coating of interest is placed inside a quartz furnace tube evacuated to 10 -3 torr and heated under a continuous flow of argon. A Nd:YAG laser pulse is then focussed on the back surface of the substrate, which due to surface melting and rapid cooling generates a compressive stress wave directed towards the test coating interface. The compressive stress wave reflects as a tensile wave from the free surface of the coating and leads to its spallation (complete removal) at a sufficiently high wave amplitude The coating's free surface velocity during interface decohesion is measured using an optical interferometer and used to calculate the peak interfacial tensile stress. The technique is quite general and applicable to interlaces between polymeric metallic and ceramic coatings and engineering substrates. Here, it is demonstrated on a model Nb/sapphire interface system, for which an average tensile strength of 400 MPa at 1100°C was obtained. This value is remarkably different than the value of 1.9 GPa obtained under ambient conditions. This large difference suggests the importance of this research because interfaces in tribology, multilayer devices. and composite materials undergo local temperature excursion during service and their design should consider the possibility of such strength degradations.