Indentation-size effects in single-crystal MgO

The microhardness values in Man. 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 or 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 1.001 kg and temperatures between 20 and 600degreesC, 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.