Measurement of Hardness of Very Hard Materials

In the last twenty years or so, the development of hard thin coatings has progressed to the state that hardness testing based on an instrumented technique has become very popular since for this application, the depth of penetration has to be kept within a small percentage of the overall coating thickness and the resulting impressions are too small for an accurate traditional optical measurement. Despite the well-known methods of analyzing instrumented indentation data, considerable problems arise when this test is applied to very hard materials. The underlying boundary conditions for instrumented indentation analysis are often ignored by practitioners who are sometimes accepting of the results at face value, since often they provide a very pleasing and desirable estimation of hardness of their samples. This chapter reviews the essential features of instrumented indentation analysis and points out the significance of those issues that can affect the computed values of both hardness and elastic modulus. In particular, the significance of the geometry factor ε, the indenter area function, and the mean pressure elastic limit. These interrelated factors can conspire to increase the computed value of hardness by up to a factor of 2 if not properly taken into account. This chapter educates and informs the reader so that results of hardness for very hard materials may be properly interpreted when either viewed in the literature or obtained experimentally so as to avoid incorrect conclusions and results.