Deriving mechanical properties of soft coatings using nanoindentation: an application of mechanism-based strain gradient plasticity

In this paper, a dislocation model based on MSG theory has been used to analyse the indentation process on soft coating systems. It is shown that the indentation process can be divided into two stages, according to the critical penetration depth for every soft coating. For penetration depth smaller than the critical depth, the soft coating behaves the same as the bulk material and the deformation behaviour can be well defined by the dislocation model, while for penetration depths larger than the critical depth, the influence of the substrate is triggered and deviation from the dislocation model occurs. The ratio of the critical penetration depth to the coating thickness depends on the coating types, but not on the coating thickness. The ratio is approximately 0.2 for Au/Ni coatings and approximately 0.4 for Al/BK7 coatings. The coating-only mechanical properties can be determined from the indentation data with penetration depth smaller than the critical value using the MSG theory.