Evaluation of nano-mechanical properties of hard coatings on a soft substrate

Abstract

The present study focuses on the manifestation of substrate effect while characterizing films of different hardness using nanoindentation technique. Nanoindentation tests were carried out on hard TiN and (Ti, Al)N coatings deposited on soft D9 steel substrates. Hardness of coatings was extracted from different indentation depths for revealing the substrate effect. For both coatings, hardness was found to decline with increasing indentation depths indicating the severity of substrate effect at higher depths. However, the decline in hardness was much steeper in case of (Ti, Al)N film than TiN film. Also, discontinuities (pop-ins), signature of film cracking, were observed on the load–displacement curves. Using a scanning electron microscope, surface cracks were observed near the indentation edge and inside indentation zone in both coatings. To understand the observed behavior, 2-D Finite element analysis was used to simulate the indentation process. Plastic deformation in the substrate was found to take place in the early stages in both cases resulting in an earlier decline of hardness. However, the critical depth, at which substrate started deforming plastically, was 8% and 11% of the total film thickness for (Ti, Al)N and TiN films, respectively. Interestingly, pop-ins were found to appear on the load–displacement curves only after the occurrence of yielding of the substrate in both cases. Film cracking is explained on the basis of stress distribution in the coatings during the indentation process. It is concluded that substrate effect becomes more pronounced in the case of harder films and manifests itself at early stages of indentation.