A Study on Absolute Evaluation of Fracture Strength of Thin Films by Indentation

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In this research, a method was proposed to evaluate absolutely fracture strength of a thin film coated on a substrate by indenting a spherical indenter, and feasibility and usefulness of the method were studied. The term "absolutely" was used to intend that the proposed method was not for mere relative evaluation of different films. When a spherical indenter is indented to a brittle film coated on a substrate and the indentation load is increased, a ring crack is formed in the film. By measuring the ring crack initiation load and analying the stress field in the film, it is expected that fracture strength of the film can be obtained. The results of the present research are summarized as follows.1. In this research, a SiO_2 film of 2mum thickness coated on a SUS440C substrate by ion beam spattering was indented using s SIC spherical indenter of 750mum radius. Evaluation of its fracture strength required stress anlysis as well as indentation experiments. An elastic finite element stress … More analysis was first carried out. An approximate expression was derived for the stress field of the film when the film thickness was much smaller than the radius of the contact circle.2. For taking into account plastic deformation of the SUS440C substrate under indentation, an elasto-plastic finite element stress analysis was then carried out. It was found that the radial tensile stress appears in the film outside the contact circle in this case too.3. Before evaluation of fracture strength of the film, the proposed method was applied to evaluation of fracture strength of a bulk material. An indentation experiment was carried out on a sodalime glass plate. It was confirmed that fracture strength could be evaluated properly using the proposed method.4. The fracture strength of a SiO_2 film coated on a SUS440C substrate was evaluated by indenting a SiC spherical indenter. In order to take into account the size effect, a probabilistic treatment using a Weibull distribution was employed. The residual compressive stress in the film was also taken into account. A reasonable value was obtained for the fracture strength of the film. Less