Blisters, blistering, blistering effect. In sealing engineering blistering refers to the dot-like spalling which occurs on the surface of face materials, notably on the sliding face. This type of damage is observed only in heterogeneous materials, whose components have unequal coefficients of expansion e.g. carbons with impregnation. The damage is caused by thermal overloading of the materials, e.g. overheating of the sliding faces in the course of dry running. The greater thermal expansion causes the impregnating agent to emerge through the surface pores, or, if the pores are situated just under the surface, causes the thin covering layer on top of the base material to flake off.