Measurement of Interface Strength, Intrinsic Toughness and Their Dependence onInterfacial Segregants

摘要

This report discusses a novel laser spallation technique for measuring thetensile strength of planar thin film interfaces. In this technique, a laser-produced compressive stress pulse in the substrate, reflecting from the coating's free surface pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. The interface stress is determined by recording the coating or substrate free-surface velocities using a Doppler interferometer. Interface strengths of several metal/ceramic, ceramic-ceramic and ceramic/polymer systems are summarized from our recent efforts. In addition, two breakthroughs, the first of a novel interferometer to record velocities from rough surfaces and the second of a technique to produce subnanosecond rise-time stress pulses with no asymptotic post-peak decay, are discussed which further allows the technique to be applied to rough thermal spray coatings and also to films as thin as 0.1 um. This technique is used to establish a fundamental strength-structure chemistry relationship for Nb/sapphire interfaces, with and without the interlayers of Cr and Sb. This allows the interface strengths to be controlled over a wide range, as required for realizing the strategy of deflecting impinging matrix cracks along the fiber/matrix interfaces so as to impart toughness in otherwise brittle composites. The required values of fiber/matrix interface toughness needed for crack deflection were obtained by using the method of dual singular integral equations. Finally, in a separate study, the short stress pulses are used to determine the dynamic response of laminates, and preliminary results are presented that show their potential in evaluating the damage in composites in a nondestructive mode.