Indentation of elastic solids with a rigid Vickers pyramidal indenter

Experimental investigations of the normal loading of a rigid Vickers pyramidal indenter on to several blocks of elastic solids, namely neoprene, rubber, and optically clear polydimethylsiloxane (PDMS) containing 10%, 5%, and 2.5% by volume of the curing agent have been described. An instrumented indentation machine was used and several types of measurement were made. These included (1) indentation load versus indenter penetration behaviour, (2) in situ photography of the contact area between the indenter and the substrate, (3) the depth of the points of contact where a plane going through an indenter diagonal and containing the indenter tip intersects the surface of the specimen, and the depth of the contact points lying along a direction at an angle of 45° to the planes containing the diagonals. The measurements were compared with the predictions of the theory of a rigid cone indenting an elastic half space and by assuming that the rigid pyramid could be likened to a cone of a semi-included angle of 70.3°. It is shown that in all cases there were significant discrepancies between the predictions of the theory and the experimental measurements. It is concluded that a rigid pyramidal indenter normally loading on to an elastic solid cannot be likened to a conical indenter for such studies. It is suggested that it is the high friction at the ridges of the indenting pyramid, which gives rise to the discrepancies between the experimental data and the theory for a frictionless indentation with a rigid cone. This conclusion has very significant implications for a commonly used method of nanoindentation data analysis. These implications are also discussed.