In situ observations of indentation damage in single crystals of MgO

In situ optical microscope observations of the indentation plasticity and ensuing fracture of MgO single crystals of a (100) orientation have been made at room temperature using a Vickers diamond pyramid. Special attention has been paid to the evolution of the square-shaped "picture frame'' of slip bands and the {110}(90) cracks around the indentations as the indenter load was gradually increased from zero. These {110}(90) cracks form due to dislocation interactions on adjacent {110}(45) slip planes and it has been shown that there is a minimum indenter load of similar to 160 to 185 gf below which these cracks do not form during indenter loading and, in fact, not all the four {110}(90) cracks form simultaneously. The number of these cracks has been shown to increase from 2 to 3, and then to 4, as the indenter load goes up to 500 gf. It has also been shown that the size of the picture frame as well as the size of the {110}(90) cracks increase linearly with increasing indenter load. Moreover, it has been found that further growth of the picture frame and the size of the {110}(90) cracks occurs gradually during indenter unloading. These results have been explained on a previous model relating the displacement of material and the formation of the {110}(90) cracks. (C) 1999 Elsevier Science S.A. All rights reserved.