A model for the mechanical interaction between a deposited surface film and its substrate during indentation is investigated for a number of material combinations. Linear relationships between the indentation microhardness and the inverse of the indentation diagonal are found for indentations larger than a limiting size, for both coated and uncoated materials. These relationships are used for measuring the hardening effect of a harder coating on a softer substrate and for finding separate hardness values characteristic of the film and the substrate materials.