Knoop microhardness anisotropy and the indentation size effect on the basal plane of single-crystal alumina (Sapphire)

The Knoop microhardness anisotropy profile was determined for the basal plane of a Czochralski grown alumina single crystal for indentation test loads from 100 through 1000 g. Microhardness maxima occur at low indentation test loads for the long axis of the Knoop indenter parallel to the <2 (1) over bar(1) over bar0>. Minima exist for the long axis parallel to the <10 (1) over bar0>. This low indentation test load profile is attributed to slip on the primary slip system, the (0001)<(1) over bar(1) over bar 20>, as previously noted by Brookes and co-workers. The degree of the microhardness anisotropy decreases for higher indentation test loads. This results from the activation of multiple slip systems to accommodate the greater amounts of plastic flow required by the larger indentation sizes. The microhardness profile becomes more uniform with increasing indentation test load until the Knoop microhardness approaches a test-load-independent, orientation-independent microhardness of 1167 +/- 34 kg/mm(2). The indentation size effect (ISE) was further investigated through lubricated indentation hardness measurements. Lubrication of the test specimen surface significantly reduces the ISE. Results indicate that friction between the test specimen surface and the indenter facets is a major portion of the ISE.