Strengths of Sapphire Crystals
THE strength of single crystals of entirely brittle materials should depend on the presence of stress raising flaws. Internally these could be formed by static dislocation arrays, voids or inclusions of foreign material. On the surface the flaws could be formed by mechanical damage or by surface steps as suggested by Marsh1.
The Ideal Strength of Solids
There is an upper limit to the mechanical strength of any material under any particular test conditions. This limit is known as the ideal strength. The present paper attempts to define the ideal strength as closely as is presently possible. To this end, a brief account is first given of some of the highest strengths recorded experimentally. Then follows a short account of the published theoretical calculations of the ideal strength. Finally, the theoretical and experimental values are compared, and it is shown that in several instances these differ by less than a factor 2x.
On the Growth of Sapphire Microcrystals
Thermodynamical considerations on the vapor‐phase growth of whisker and platelet crystals of α‐Al2O3 by the oxidation of aluminum lead to the conclusion that the vapor‐phase mass transport is probably due to the suboxide Al2O and not AlO as previously suggested. Electron microscope observations confirm the optical evidence that spiral growth steps do not occur on the (0001) surfaces of Al2O3 crystals grown in this way.