On the evaluation of stresses in coated materials during nanoindentation with sharp indenters

It has recently been shown that the form of unloading curves obtained during indentation of elastic-plastic materials with geometrically self-similar indenters such as pyramids and cones can be understood using the concept of an “effective indenter shape”. The shape of the plastic hardness impression formed during indentation determines the geometrical form of this effective indenter shape. During unloading this concept allows one to approximately determine the elastic part of the complete stress field consisting of the residual stresses caused by the inelastic behaviour and the purely elastic one determined by the effective indenter. Applying a straightforward algorithm, this concept can be used to evaluate the distribution of the elastic field of the effectively shaped indenter even in the case of layered materials and thus provide information about changes in the stresses during the unloading process. The model is applied to a variety of single layers and even graded coating-substrate compounds and the resulting stress fields of stresses are discussed. Further, it is demonstrated how the method can be used to extract critical material parameters of the coating material like the yield stress directly from load–displacement curves of sharp indenters.