Abstract
Abstract The microhardness values in many very hard materials need to be measured at low loads to prevent cracking from unduly affecting the measurement accuracy. However, the resultant indentation-size effect (ISE) means that it is difficult to compare the results obtained by different workers and for different specimens, particularly of small crystals or coatings. The purpose of this study is to obtain and analyse Knoop microhardness data for a well-characterized material, with a view to being able to interpret the results. Consequently microhardness indentations have been performed on single-crystal MgO(001) along {110} in air for loads between 0.125 and 1.001kg and temperatures between 20 and 600°C, investigating also the influence of interfacial coatings and lubricants. The main experimental findings were that the ISE decreased with increasing test temperature and was unaffected by coating or lubrication. The data fitted equally well either the power-law or the proportional specimen resistance models. It is proposed qualitatively herein that the ISE is controlled by the extent of elastic recovery occurring on removal of the load, and should thus correlate with the hardness-to-elastic modulus ratio H/E.
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