Hillock formation in lead films by grain boundary sliding

Abstract

Lead films varying in thickness and grain size were deposited from the vapour onto substrates varying in their coefficient of thermal expansion. This way both the yield stress of the films and the thermal stress arising from temperature change could be varied. Hillock formation brought about by various numbers of thermal cycles between R.T. and 4.2 ? was observed in these films by SEM and the quantitative dependence of hillock volume on stress and grain size determined. These dependences together with detailed observations suggested that, under thermal stresses larger than the yield stress of the film, hillocks form by grain boundary sliding in the superplastic mode. A model is proposed in which the sliding rate is given as a product of slip band density and the rate of climb of absorbed slip dislocations in the grain boundary. Since both quantities depend linearly on stress—with the yield stress as the threshold stress—the experimentally observed second‐power dependence of sliding rate on stress is derived without evoking piled‐up dislocations. The conditions for superplastie deformation are outlined. © 1980 Taylor & Francis Group, LLC.