Deformation of Magnesium Oxide Crystals by Softer Indenters and Sliders

Abstract

WHEN a scratch or indentation hardness measurement is made one is generally more concerned with gross changes in shape, due to macro-deformation or fracture, than in any aspects of small scale or micro-deformation which might also form a part of the permanent deformation process. Consequently, certain guide lines have been established¹. The possibility of micro-deformation in a relatively hard crystal due to point contact through a significantly softer indenter or slider may have been assumed but there seems to be little published information on its nature and extent. We have recently completed some preliminary experiments on this aspect of deformation in the indentation and scratch hardness processes and the results indicate that the cumulative effects, due to repeated loading, may well play an important part in subsequent macro-deformation. Most of the experiments were carried out on magnesium oxide because the resultant distribution of dislocations could be readily observed, using the etchant perfected by Gilman and Johnston², on a (001) plane prepared by cleavage and then chemically polished. Lead, copper and diamond cones, having apical angles of 136°, were used as indenters and sliders. The relevant diamond cone indentation hardness values for the magnesium oxide, copper and lead were 6,000 MN m⁻², 980 MN m⁻², and 50 MN m⁻² respectively and can be assumed to be between 69,000 to 96,000 MN m⁻² for diamond³.