SPS processing and superplastic deformation of silicon nitride based ceramics

Recently we have introduced a novel processing concept of sialon components implying that an extra liquid phase that is thermodynamically compatible with sialon phases is introduced by increasing the O/N ratio in the general formula (Yb+Y)(x)Si12-(3x+n)Al3x+nOnN16-n while keeping constant the Yb, Y, Si, and Al proportions. By increasing the oxygen content from its stoichiometric value of 5.16 to 15 eq%, a series of powder mixtures were prepared and their overall compositions are located slightly above the homogeneity region of the alpha-sialon phase. These compositions were consolidated to full densities by hot pressing (HP) and Spark Plasma Sintering (SPS), respectively. The sintering kinetics in the HP and SPS units is compared. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic alpha-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and in the SPS apparatus rapidly heated exceeding the temperature threshold of grain growth and using short holding times (minutes). Post heat treatment in the SPS apparatus yielded in-situ reinforced microstructures no matter if an additional liquid/glass was involved or not while corresponding microstructures could only be obtained for non-stoichiometric compositions by post heat treatment in the graphite furnace. The grain growth kinetics is discussed in terms of static and dynamic ripening mechanisms. We have recently shown that the ductility of covalent bonded silicon nitride based ceramics is dramatically enhanced in presence of a pulsed electric field. Compressive strains rates in the range of 10(-2) s(-1) can easily be achieved at T >= 1500 degrees C. The enhanced ductility is explained by that the electric field induces motion of charged species present in the grain boundary glassy/liquid phase that in turn promotes grain sliding along the grain boundaries.