Performance indices and selection of thin hard coatings on soft substrates for indentation and scratch resistance

A generic mechanical framework has been developed for assessing the indentation and scratch resistance of hard films on soft substrates. Analytical expressions for the critical loads leading to filmcracking or substrate yielding are proposed based on closed form plate bending models and on finite element simulations. These models lead to the definition of performance indices for the ranking of “hard-film-on-soft-substrate” systemswith respect to the resistance to indentation and scratch failure under the constraint of minimizing film thickness. These performance indices show that the hardest coating is not always the best choice and that other material properties of the film and the substrate have to be taken into account. An illustrative material property chart is proposed in order to compare the protective ability of some usual thin coatings. These results constitute a guide for the development of layered systems in order to avoid time consuming and expensive trial and error campaigns