Electron Microscopic Observations of Microcracking about Indentations in Aluminum Oxide and Silicon Carbide.

摘要

Transmission electron microscopy was used to examine the nature of microcracking about small-scale indentations in two highly brittle solids,sapphire and carborundum. The observed crack geometry is discussed in terms of an earlier model of indentation fracture beneath a point force,in which both loading and unloading half-cycles contribute to the crack growth. The residual interfaces are characterized mainly by moire patterns,sometimes by dislocation networks. These observations are discussed in relation to spontaneous closure and healing mechanisms,and the 'lattice mismatch'necessary for their production estimated at about one part in a thousand. It is shown that cleavage steps comprise the main source of obstruction to lattice restoration across the interfaces. Mechanical and thermal treatments of the indented surfaces are found to influence the extent of the residual cracking. Some practical implications of the observations are discussed.