Knoop microhardness profiles were determined on the (100), (110) and (111) planes for indentation test loads of 50 to 300g. The profiles were analyzed with regard to possible slip systems to explain the Knoop microhardness anisotropy by applying the effective resolved shear stress concept of Brookes et al. The predicted slip systems were {100}<011>,{110}<111> and {111}<110>. The load dependence of the microhardness was initially addressed in terms of the classical Meyer law, P = Adn, for which the two parameters, A and n, were observed to be related. The peculiar dimensionality of the classical Meyer law coefficient A was addressed by applying the concepts of a load-independent so-called microhardness, critical indentation load and characteristic indentation size. Subsequently the development of a normalized Meyer law was directly demonstrated; yielding, P = 2Pc n d d0*η, where Pc was a critical test load indicative of the region where hardness was independent of the indentation test load and d0* was a characteristic indentation dimension, below which the indentation size effect was significant. It was suggested that this normalized Meyer law might have universal applicability to various hardness tests and materials.

Knoop Microhardness Anisotropy of Single-Crystal LaB6. Li, H., Bradt, R.C.: Materials Science and Engineering A, 1991, 142[1], 51-61