Measurement of fracture strength in brittle thin films

Many coatings and films of interest for engineering applications and microelectromechanical systems (MEMSs) are brittle, and their use under applied stress is ultimately limited by fracture. Safe design and material selection therefore require detailed knowledge of the fracture strength. The present work reviews the state-of-the-art in its characterization and analysis. First, we review the experimental methods for strength measurement in small-scale specimens. The more conventional nanomechanical testing of micromachined free-standing films is compared with alternative techniques in which testing is conducted directly on as-deposited films. While the former is preferred for the analysis of MEMS components (e.g.: silicon, silicon oxide, silicon carbide), we argue that the latter is advantageous in order to accurately measure the strength of protective coatings containing residual stress (e.g.: carbon based films; titanium oxide, nitride and nanocomposites; thermal barrier coatings) and of ultra-thin films, and to assess microstructural effects. Given the stochastic nature of brittle fracture, we summarize the fundamentals of Weibull theory for the probabilistic analysis of fracture strength. Special attention is paid to critical variables which affect the measured strength, such as loading geometry and specimen size. Finally, extrapolation from laboratory tests to actual loading configurations during service is briefly discussed.