A microbridge method in tensile testing of substrate for fracture toughness of thin films

A new microbridge method is introduced in this paper in tensile testing of substrate for fracture toughness of thin films. Measurement of fracture toughness for bulk materials is a routine but extremely difficult and still not standardized for thin films (or coatings). The difficulties in clamping a freestanding thin film and the requirement of a critical delicate loading system are the main obstacles. In this paper, the film to be tested is deposited on a rectangular silicon wafer on which an edge crack is fabricated beforehand. The film is patterned and made into microbridges perpendicular to and ahead of the initial substrate crack. A displacement controlled tensile force is applied to propagate the substrate crack and fracture the microbridges. The critical fracture strain of the microbridge is measured through measuring that of the substrate at the respective location of the bridge. The fracture toughness of the film is thus obtained. This method successfully turned the tensile testing of the film into tensile loading of the substrate, avoided the delicate clamping and loading on the film itself. Two case studies are also presented.