Thermal shock and fracture in crystals of magnesium oxide

Crystals of magnesium oxide have been thermally shocked by down-quenching. A single crystal in the as-cleaved condition is shattered by a moderate thermal shock and it is shown that the complex fracture pattern resulting can often be traced back to a single crack nucleus on the crystal surface. If such crack nuclei be removed by chemical polishing the crystal can withstand severe thermal shock without fracturing; the heavy slip produced does not result in crack nucleation by dislocation interaction and reasons for this are discussed. Similar experiments on bi-crystals have shown that the intercrystalline boundary presents a source of weakness which cannot be removed by chemical polishing, and such crystals fracture under moderate thermal shock whatever the state of the surface. Preliminary experiments on repeated low level shocks suggest that crack nucleation may occur under these conditions but the pattern of slip introduced is not the same as that produced by single shocks.