Fracture in Thin Oxide Films

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FRACTURE IN THIN OXIDE FILMS D.F. Bahr, A.L. Olson, K.R. Morasch, M.S. Kennedy, D. Rodriguez Marek, and A. Alamr Mechanical and Materials Engineering, Washington State University, Pullman WA 99164 ABSTRACT In thin film systems, failure often occurs via fracture mechanisms, with either through thickness cracking or interfacial delamination leading to failure of the device or layer. Measuring the stress at which fracture occurs in these thin film systems requires testing methods amicable to both the small scale of the films as well as the complex relationship between the mechanical properties of the film and the substrate. This paper will cover two oxide film systems, a piezoelectric ceramic (PZT) on platinum and passive oxide films (primarily Cr2O3) on stainless steels. Both will be tested with a bulk method (bulge testing in the case of PZT and circumferentially notched tensile bars for the stainless steel) and then with a nanoindentation method developed for testing fracture for hard coatings on soft substrates. The differences in stresses required for failure between the bulk and the nanoscale tests will be discussed in terms of differences in flaw population. INTRODUCTION Much of the work in measuring the mechanical behavior of thin films has been carried out using nanoindentation experiments [1,2]. These tests are primarily used to measure either the elastic or plastic properties of the material being examined. However, when thin films that are significantly harder than the underlying materials are strained, they rarely fail via mechanisms of plastic deformation. Instead, complex fracture behavior is the likely failure mechanism. As outlined by Page and Hainsworth [3], there are several possible mechanisms of failure which can be observed by examining the load – depth curve during nanoindentation, leading to their description of using this curve as a “mechanical fingerprint” of a material. Previous studies have been carried out to demonstrate that it is possible to use nanoindentation as a tool to cause through thickness fracture events for thin oxide films on metals [4]. Titanium [5] , tungsten oxide [6], aluminum [7] and stainless steels [8] have been shown to undergo cracking patterns around indentations that are due to tensile stretching of the film as it elastically deforms over a plastically deforming substrate. These fracture morphologies have been characterized using atomic force microscopy, and the radial stress which develops during the stretching of the film corresponds with the observed fracture patterns. Figure 1 shows cracks which develop after indentation into a titanium oxide film on a bulk titanium substrate, while Figure 2 demonstrates schematically the process which leads to these cracks running along the surface of the oxide at the circumference of the plastic zone. Nanoindentation is not the only mechanism by which film fracture can be achieved in thin film systems. Bulge testing has been used in previous studies to examine the elastic and plastic properties of thin films, as well as

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