Mechanical–thermal response of brittle titanium films on polymer substrates: in-situ observations and FE modelling

Abstract

The mechanical and interfacial properties of metal films on polymer substrates under combined loading directly affect the function and service life of a variety of devices. However, the temperature effects on the failure mechanisms of brittle films are still unclear. Titanium films with three different thicknesses (100, 250 and 400 nm) were deposited on polymethylmethacrylate substrates; in-situ mechanical–thermal tests were performed on these films under an optical microscope. Experimental results demonstrate that axial compression can result in buckle-delamination of thin films, while an elevated temperature can induce the formation of transverse cracks combined with further interface delamination. Moreover, it was found that the mechanical–thermal responses of the thin films are related to the film thickness, and higher buckle/crack densities can be observed in thinner films. To explore the mechanism of crack initiation, strain analyses were conducted, which reveal that the tensile strain of the polymer substrate in the non-loading direction increases under the combined mechanical–thermal loads. Tensile stresses are transferred to the film from the substrate through the film–substrate interface, contributing to the formation of transverse cracks. In addition, finite element modelling was further carried out to develop a deeper understanding of the stress transfer and crack evolution mechanisms in thin films.