The grain boundary peaks which were observed in the internal friction of bulk and thin-film samples were compared. It was concluded that the underlying relaxation mechanism in thin films was pure grain boundary sliding while, in bulk material, grain boundary migration predominated. This was because the grain size was several orders of magnitude larger in bulk Al than in thin films. This led to differing microstructures for the grain boundaries. Also, in the larger samples a higher number of impurities was attached to the grain boundary. Impurity dragging then resulted in a retardation of grain boundary migration. This explained the observation that internal friction experiments on bulk Al yielded an activation energy of 1.4eV whereas, for thin films, a value of 0.6eV was obtained.

H.G.Bohn, M.Prieler, C.M.Su, H.Trinkaus, W.Schilling: Journal of the Physics and Chemistry of Solids, 1994, 55[10], 1157-64