On the limits of the interfacial yield model for fragmentation testing of brittle films on polymer substrates

Abstract

Fragmentation testing is frequently used to probe film fracture strain and the interfacial properties of thin brittle films on compliant substrates. A model based upon complete yield of the film/substrate interface is frequently used to analyse data after cracking has saturated. Additionally, the film is either assumed to have a single-valued failure stress or a distribution of strengths described by Weibull statistics. Recent work by the authors showed that consideration of film thickness variations and the application of neighbour ratio analysis brought 96% of the data for an Al _x O _y /Cu film/substrate system into compliance with the predictions for a film with a single-valued failure stress. In the present work Cr/PI (polyimide) and Cr/PET (polyethylene teraphthalate) systems are analysed according to the same methodology. The Cr films on polymer substrates crack such that the neighbour ratios considerably exceed the predicted limit of 2. The influence of the relative thickness of the film and substrate and the strain rate of the test is investigated. A deviation from the idealised mechanical model due to the large difference in elastic moduli of film and substrate is put forward as a possible cause of the observed behaviour. The importance of these results to the application of the interfacial yield model is discussed.

Keywords:

• fragmentation testing
• thin films
• neighbour ratios
• statistical strength

Acknowledgements

K. Schmidegg (Hueck Folien GmbH) and J. Schalko (Institute for Integrated Sensor Systems of the Austrian Academy of Sciences and Institute of Sensor and Actuator Systems, TU Vienna) are acknowledged and thanked for providing the chromium films used in this study. In addition, A. Hohenwarter and F. Hubner of the Erich Schmid Institute are thanked for developing the screw-driven tensile frame used in this study. Lastly, MJC gratefully acknowledges funding from the Austrian Science Fund (FWF) under project P22684-N20.