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1049-AA03-07

Strength Measurement in Brittle Thin Films Oscar Borrero-Lopez1,2, Mark Hoffman1, Avi Bendavid3, and Phil J Martin3 1 University of New South Wales, Sydney, NSW 2025, Australia 2 Universidad de Extremadura, Badajoz, 06071, Spain 3 Materials Science and Engineering, CSIRO, Lindfield, NSW 2070, Australia ABSTRACT In this work we have investigated the strength variability of brittle thin films (thickness ≤ 1 µm) utilising a simple test methodology. Nanoindentation of as-deposited tetrahedral amorphous carbon (ta-C) and Ti-Si-N nanocomposite films on silicon substrates followed by cross-sectional examination of the damage with a Focused Ion Beam (FIB) Miller allows the occurrence of cracking to be assessed in comparison with discontinuities (pop-ins) in the loaddisplacement curves. Strength is determined from the critical loads at which cracking occurs using the theory of plates on a compliant foundation. This is of great relevance, since the fracture strength of thin films ultimately controls their reliable use in a broad range of functional applications. INTRODUCTION Brittle thin films (thickness ≤ 1µm) are one of the most commonly used means of overcoming materials-based limitations to attain increased functionality in components. Examples of their use include tribological films for wear parts, magnetic drives, biomedical applications, or MEMS devices. The mechanical properties of thin films that have been most widely studied are elastic modulus and hardness, using nanoindentation probe techniques. However, the fracture strength of the films, which ultimately limits their structural integrity and reliable application, has received much less attention. While in bulk specimens the procedure for strength measurement is well-established ⎯i.e., three- or four-point bending, biaxial flexure tests, and so forth⎯, direct application of these experiments at the sub-micron scale is not straightforward. Current methodologies for strength measurement in brittle thin films and small systems relevant to MEMS consist in nanomechanics testing of micromachined free-standing films [1-6]. However, these methods lack simplicity and present limitations. Namely (i) only one single datum per specimen can be obtained, (ii) micromachining techniques may induce defects in the resulting specimens, which in turn can influence the measured strength, and (iii) the use of free-standing films is limited to nearly stress-free systems, as residual stresses hinder machinability of microbeams. Based on the above, the aim of this work is to extrapolate the use of a simple procedure developed by Lawn et al. for thick coatings to the realm of sub-micron films [7, 8]. Using this methodology, films can be tested as-deposited in the substrate avoiding micromachining. Furthermore, it allows films with high residual stresses to be tested, and several tests to be performed within the same specimen.

EXPERIMENTAL Materials Filtered-arc deposited tetrahedral amorphous carbon (ta-C) and arc-magnetron deposited Ti-Si-N nanocomposite films (thickness