Non-cavitation tensile creep in Lu-doped silicon nitride

The tensile creep behavior of a Lu-doped silicon nitride was studied in the temperature range 1400 1550 degreesC with test periods of up to 10200 h. Strain rates were 3-4 orders of magnitude less than those for Yb-doped grades of silicon nitride under the same conditions, suggesting a potential for prolonged operation of this material at temperatures up to 1470 degreesC. The stress exponent, n, and the activation energy, Q, for creep are 5.3 +/- 2.0 and (757 +/- 117) kJ/mol, respectively. Precise density and ultra-small-angle X-ray scattering measurements revealed that, in contrast to other grades of silicon nitride, cavitation could not be detected in the material studied. Redistribution of the secondary phases via solution-precipitation combined with grain boundary sliding is discussed as a possible creep mechanism. A discussion of the effect of Lu on viscosity indicates that replacement of Y by Lu may explain the improvement of creep behavior. (C) 2002 Published by Elsevier Science Ltd.