Gravitational Effects on Distortion in Liquid Phase Sintering

During sintering, powder compacts gain strength through low-temperature interparticle bonding, usually induced by solid-state surface diffusion, followed by further strength contributions from high-temperature densification. In cases where a liquid phase forms, sintering densification is accelerated and shape retention is sustained while the open pores contribute capillary forces that give strength to the compact to resist distortion. Unfortunately, sintering densification requires the compact to become thermally softened to a point where creep strain rates often reach levels near 10-2 s-1 (1 % per second) when the liquid forms. Our measurements show the in situ viscosity during rapid sintering densification often falls below 1 GPa-s. Thermal softening of the solid which occurs at high temperatures further substantially reduces the strength. Therefore, the in situ strength provides a means to separate compact densification (as required for high performance) from distortion (as required for net-shaping). There are two points of substantial weakness during heating- prior to significant interparticle bonding and during final pore closure. This research examines the interplay that allows densification without distortion. Most surprising is the beneficial role of gravity, where the deviatoric stress acting on the powder structure induces skeletal formation to reduce distortion in contrast to sintering in the absence of gravity