Strength measurement of a brittle coating with a trilayer structure using instrumented indentation and in situ observation techniques

Abstract

A mechanical-testing method, devised to measure the strength of brittle thin coatings with a trilayer structure, was investigated. A semi-empirical relationship among radial cracking and associated parameters was derived by introducing a buffer and coating layer with an effective thickness concept, extending the established bilayer equation. Adjustable parameters of the proposed trilayer equation were determined for various ratios of coating thicknesses and moduli. The validity of the FEA-based relationship was analyzed by an experimental method utilizing a graphite/glass/substrate structure. An in situ observation of radial cracks through the transparent substrate during sphere indentation enabled the determination of the strength of the thin coating layer. Chemical vapour-deposited silicon carbide films were used to evaluate the strength and Weibull modulus using the present technique, in a case study format. The validity of the trilayer equation in terms of critical load for radial cracking and the strength of the thin brittle coating are discussed. This study can contribute to knowledge in the area of evaluating brittle thin coating systems on a compliant substrate.