Cleavage of ceramic and mineral single crystals
- Univ. of Alabama, Tuscaloosa, AL (United States). Dept. of Metallurgical and Materials Engineering
The cleavage of ceramic and mineral single crystals is reviewed from the perspectives of the phenomenon that illustrate its special character relative to the crystal structure of the material. A basic geometric mineralogical classification is employed throughout the review. Criteria for the cleavage process are examined in the historical structural sense as well as in a fracture mechanics sense. It is concluded that fracture toughness is the appropriate criterion for cleavage of these materials. Summarizing experimental cleavage fracture toughnesses reveals that most ceramic and mineral crystals cleave at K{sub Ic} levels which are less than 2 MPa m{sup 1/2}, but are frequently less than 1 MPa m{sup 1/2}, with the exception of diamond. The cleavage of MgAl{sub 2}O{sub 4} spinel on different crystal planes, where the crack has been restricted to extend on a plane through specimen and crack configuration control illustrates the brittle, elastic nature of the cleavage toughness of that structure. When the fracture toughness of the MgAl{sub 2}O{sub 4} spinel is measured for the (100) cleavage plane at increasing temperatures, it is observed that the cleavage toughness initially decreases with increasing temperature until the brittle-ductile transition occurs. Plastic flow causes a rapid toughness increase with further temperature increases above the brittle-ductile transition. Finally, experimental cleavage toughnesses for sapphire are compared with theoretical calculations of the surface energies for the same crystalline planes. From this consideration it is further confirmed that the phenomenon of cleavage in these types of materials on their preferred cleavage planes occurs with very little, or perhaps no accompanying dislocation plastic flow. It also suggests that energies for cleavage on non-preferred cleavage planes may differ substantially from theoretical calculations of the surface energy.
- OSTI ID:
- 293153
- Report Number(s):
- CONF-970980--; ISBN 0-87339-381-3
- Country of Publication:
- United States
- Language:
- English
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