Determination of young's modulus and poisson's ratio of coatings with non-destructive indentation

Coatings are now extensively used in engineering practice to modify the surfaces of mechanical components for enhanced load-bearing ability, lubricity, and service endurance. Designing thin-film coatings to meet particular engineering needs requires the knowledge of accurate mechanical properties of the coatings. Young's modulus and Poisson's ratio are two basic mechanical properties of materials, which should be conveniently measured. However, the measurement of Young's modulus of a thin film is hindered by the inevitable substrate effect when the conventional methods for a bulk material are used to obtain so called "reduced modulus". This paper presents a direct and non-destructive method for the measurement of Young's modulus and Poisson's ratio of a thin-film coating and its substrate, based on the extended-Hertz theory developed for coated bodies in contact. The theory is used to analyze load-displacement data from a spherical indentation in the elastic range, where the substrate effect is intrinsically modeled. Two sets of validation experiments are shown for coatings of a few microns thick. This new method does not need any assumption on pressure distribution and Poisson's ratio and can be easily incorporated into current indentation analysis systems.