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Internal friction of high-purity single-crystal sapphire and three polycrystalline aluminas with different grain sizes has been measured up to very high temperature at the frequency of 10 Hz. Only an exponential background curve was found in the single-crystal while polycrystalline specimens showed a broad anelastic peak, superposed on the background component, whose nature was presumably related to the presence of internal grain boundaries. The overall internal friction curve of the polycrystalline specimens were markedly shifted to lower temperatures as compared to sapphire. The intensity of the grain-boundary peak was decreasing with increasing the grain size. A series of alloys, prepared by doping with increasing amounts of Cr2O3, were also tested in comparison with ruby single-crystals. The Cr addition led to both an increase in intensity of the grain-boundary peak and a shift of its maximum towards lower temperatures. This characteristic anelastic behavior was clearly dictated by a lowered coherency of grain boundaries upon Cr alloying

Anelastic Grain-Boundary Sliding in Ceramics. I. Directly Bonded Polycrystals