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The effect of crystal orientation on the indentation cracking and hardness of MgO single crystals

Indentation cracking behaviour and hardness of (100), (110) and (111) surfaces of single crystals of MgO have been studied using a Vickers diamond pyramidal indenter and a 0.4 mm diameter tungsten carbide spherical indenter. Also, the plastically deformed zone around the indentations has been investigated with the technique of cathodoluminescence. It is shown that the crystal orientation has a very marked influence on the indentation cracking patterns. In the case of the Vickers indentations on (100) and (110) surfaces, radial cracks, four and two in number respectively, form for indenter loads as low as 50 gf. On the other hand, no such cracking is observed when indentations are made on a (111) surface even for indenter loads of up to 0.5 kgf. These differences have been explained in terms of the material flow and dislocation interactions on the various slip systems. The hardness is also found to be dependent upon both the indenter geometry and the indenter crystallographic plane. The Vickers hardness of all three crystal planes decreases with increasing indenter load in the load range of 0.025 to 1 kgf, whereas the Meyer's hardness (for the spherical indenter) increases with increasing load in the range of 0.5 to 13 kgf for the (111) and (110) surfaces, but shows a monotonic decrease with increasing load for the (100) surface. It has been suggested that this is related to the indentation cracking. The cathodoluminescence studies in the scanning electron microscope have been shown to be very successful in revealing the distribution of dislocations around the indentations. This information has been valuable in the interpretation of the cracking patterns. Finally it is concluded that the cathodoluminescence technique is a sensitive and non-destructive one for detecting contact-induced plasticity and cracking of ceramics.