On the evaluation of the ductility of thin films

Abstract

A new tensile test procedure has been developed to evaluate the ductility of thin films deposited on a substrate. The tensile sample has a continuously variable cross-sectional area, resulting in a continuous strain gradient along the sample after deformation. The films present cracks where the strain imposed exceeds their ductility. The strain attained at the boundary of the region where cracks appear characterises the film's ductility. This tensile test procedure has been used to evaluate the ductility of TiAl films deposited on an AISI 304 steel tensile sample. Cracks were observed by optical microscopy. The results of the evolution of the mean distance between cracks as a function of the deformation value are presented and discussed. A three-dimensional finite element code was used to simulate the deformation of this tensile sample. Special attention is devoted to the analysis of the state of stress and strain in the composite film/substrate. A similar study was made of a conventional tensile test sample to confirm and validate the results obtained from the modified sample. Moreover, the influence of the presence of cracks on the stress and strain distributions was also studied by numerical simulation. The experimental tests on conventional samples need to be interrupted at several strain values in order to follow the evolution of the cracks during the deformation. The ductility results obtained from conventional samples do not differ from those obtained with the modified sample. These conclusions and the simplicity of the method demonstrate the advantage of using the continuously variable cross-sectional area sample to study the ductility of thin films.