High Temperature Creep Behavior of High Purity Hot-Pressed Silicon Nitride

Abstract

The high-temperature compressive creep behavior of different hot-pressed silicon nitride ceramics, without any additives, with very small amounts of alumina/yttria additives and with silicon carbide platelet reinforcement, is investigated with respect to microstructural evolution, additive role and platelet reinforcement.

Creep experiments are conducted at temperatures ranging from 1773K to 1873K and under stresses from 100 to l000MPa. The creep behavior of the studied materials points out the remarkable high temperature creep resistance of these pure materials (10⁻⁷ to 10⁻⁸s⁻¹) compared to conventional silicon nitride ceramics. It shows the destructive effect of already small amounts of additives on the creep resistance: the stationary creep rate of the material containing only 1% additives is by a factor 10 faster, eventhough its grain size is much larger! Silicon carbide platelets do not change the creep behavior.

Creep of the investigated materials is described by a power law with a stress exponent slightly above 1, basically, for all experiments. Activation energies are determined as 421 kJ/mole, 425 kJ/mole and 520 kJ/mole for the pure material, the additive containing one and the one with 15% SiC platelets, respectively. At low stresses, the materials deform by grain boundary sliding associated to diffusion along the grain boundaries, and to a lesser extent by cavitation in triple junctions (for additive containing materials) or along the grain boundaries (for pure materials). With increasing stresses, the contribution of cavitation and microcracking increases and yields finally to failure of the materials.