Measurement of thin film interface toughness by using laser-generated stress pulses

A new technique to measure the toughness of interfaces is presented. In this experiment a laser-generated compressive stress pulse on the back surface of a substrate disc is made to propagate towards a thin him interface containing a population of well characterized microcracks. The compressive stress pulse reflects into a tensile pulse from the coating's free surface and leads to the dynamic initiation of one of the cracks. The coating's free surface velocity at crack initiation is measured by using an optical interferometer, and related to the local energy release rate (equal to interface toughness) for a prescribed flaw geometry by using a numerical simulation based on dynamic fracture. Both penny-shaped and line interfacial microcracks were generated by combining photolithography and encouraging layer buckling from selected spots under the influence of their own residual compressive stresses. The experimental strategy is demonstrated on interfaces between sapphire substrates with (0001) plane orientations and coatings of Al, Cr, No and Sb for which the interface toughnesses of 1.3, 0.8, 0.6 and 0.04 J/m(2) were obtained, respectively. (C) 1998 Elsevier Science Ltd. All rights reserved.