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-poro sity MgO powder compacts were investigated with regard to the dependen ce of creep rate on the applied stress. This dependence was found to b e non-linear, obeying a power law with an exponent n < 1, in contrast with the behaviour of dense compacts which exhibit linear Nabarro-Herr ing creep under the same type of loading. The nature of the creep expo nent, expected to be in relation to mechanisms of particle disconnecti on and rearrangement, frequently observed in a high-porosity compact d uring the early stage of neck formation, has been explored using an ap propriate physico-mathematical model. The relevant point concerning hi gh-porosity compacts is that, owing to the loosely packed microstructu re, the necks must resist not only normal forces, but also bending mom ents. It is the action of such bending moments which is supposed to dr ive 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 th e model explain the experimental results for n < 1 (high-porosity gree n compacts), with a smooth transition to the case of n = 1 (low-porosi ty green compacts).