SHEAR THICKENING CREEP IN SUPERPLASTIC SILICON-NITRIDE

A novel shear-thickening phenomenon has been observed in superplastic silicon nitrides compression tested between 1500-degrees and 1600-degrees-C. Liquid-enhanced creep of SiAlONs undergoes a transition from Newtonian behavior to shear-thickening behavior at a characteristic stress, with the strain rate sensitivity increasing from unity to around 2. The transition stress is always around 20 MPa, even though the Newtonian flow stress is very sensitive to temperature, grain size, and phase composition. Rheopexic hysteresis, manifested as a slow stress relaxation to a steady-state value after a strain rate decrease, was also observed in the shear-thickening regime. We attribute the cause for shear thickening to a repulsive force between initially wetted SiAlON grains, which form a "dry" and "rigid" bridge in between when pressed above a characteristic stress, possibly due to the contact of the residue Stern layers on the opposing grain/liquid interfaces. A micromechanical model, which takes into account the stress variation among differently oriented grain boundaries, has been formulated to assess the effect of "rigid" grain boundaries. A continual stochastic rearrangement of grain configurations and a relatively thick Stern layer are suggested as the necessary prerequisites for shear thickening in liquid-enhanced creep.