Sliding and Wear in Ionic Crystals

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Abstract
Wear tests by sliding single crystals of calcium fluoride, sodium chloride, magnesium oxide, and lithium fluoride on their natural cleavage faces beneath a weighted indenter are described. The slip systems responsible for the resultant deformation were determined and details thereof studied microscopically with the aid of etchants to reveal dislocation etch pits. In calcium fluoride, the structure of the material in the wear track is interpreted as a damage structure called ``friction damage.'' Etching revealed clusters of point defects left behind by moving screw dislocations. Subsequent annealing in inert atmospheres produced microscopically visible cavities a short distance below the wear track. Their possible origin and role in surface deterioration and wear are discussed. In calcium fluoride, surface cracks emanating from the wear track lie along ordinary cleavage faces; in rock‐salt‐type crystals, however, they are interpreted as dislocation cracks on the dodecahedral planes.

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