Rheological behavior of dilute SiAlON with or without intergranular X-phase

Dense nearly single-phase beta'-SiAION materials (with substitutional level z approximate to 1) have been prepared by hot isostatic pressing and their high-temperature deformation behavior has been investigated using low-freq uency damping and torsional creep experiments. Addition of a small fraction of AlN (approximate to0.5 wt.%) to the starting (nominally z = 1) SiAlON p ow der enabled us to "balance" the excess SiO2 which likely arises from sur face contamination of the starting SiAlON ponder upon exposure to atmospher e. As a result, a fine-grained beta'-SiAlON polycrystal free of residual (g lassy) X-phase segregated to grain boundaries could be prepared. This micro structure is in contrast with that found for an "unbalanced" composition pr epared from the same raw beta'-SiAlON powder but without the corrective AIN addition. In this latter case, residual glass (X-phase), consisting of Al- rich SiO2, was entrapped at multiple grain junctions. The presence of such a low-melting intergranular glass dominates the high-temperature deformatio n behavior of the dilute SiAlON material, involving marked degradation of c reep resistance and significant damping relaxation due to grain-boundary sl iding. "Balancing" the SiAlON microstructure with a small addition of AIN e nabled us to suppress anelastic relaxation by grain-boundary sliding and to increase the creep resistance of the material by more than 1 order of magn itude.