Effect of weak uniaxial loads on creep strain rate in high-porosity MgO compacts during early sintering stages

Creep phenomena accompanying the early stage of sintering of high-porosity MgO powder compacts were investigated with regard to the dependence of creep rate on the applied stress. This dependence was found to be non-linear, obeying a power law with an exponent n < 1, in contrast with the behaviour of dense compacts which exhibit linear Nabarro-Herring creep under the same type of loading. The nature of the creep exponent, expected to be in relation to mechanisms of particle disconnection and rearrangement, frequently observed in a high-porosity compact during the early stage of neck formation, has been explored using an appropriate physico-mathematical model. The relevant point concerning high-porosity compacts is that, owing to the loosely packed microstructure, the necks must resist not only normal forces, but also bending moments. It is the action of such bending moments which is supposed to drive the particles rearrangement. In this framework, the nature of the creep exponent appears to be related mainly to the green density, but it is substantially constant with densification. The predictions of the model explain the experimental results for n < 1 (high-porosity green compacts), with a smooth transition to the case of n = 1 (low-porosity green compacts).