Modelling of the Vickers hardness of paint coatings deposited on metallic substrates

Hardness measurements of paint coating deposited on galvanised steels used for domestic appliance housing were performed by different techniques depending on the applied load: Vickers ultra-microhardness (0.4–100 mN), Vickers microhardness (0.05–20 N) and Vickers macrohardness (50–1200 N). The Jönsson and Hogmark model based on the area law of mixture used to characterise the composite (substrate+film) response hardness is not consistent with the experimental results. The authors propose modifications, taking into consideration the very strong plastic deformation of the film under the applied load. These modifications are verified by metallographic observations of the deformed coating near the indentation print and introduced in two other models. The hardness variation with the indentation depth is studied and therefore a guideline is proposed to improve the mar resistance of organic coatings.