Study of pore closure during pressure-less sintering of advanced oxide ceramics

Capsule-free hot isostatic pressing (HIP) represents a cost-effective variant to classical HIP allowing production of ceramics with top properties. For successful application of capsule-free HIP it is necessary to close all pores to prevent infiltration of the high-pressure gas into ceramics. This occurs when the sample completely transits from 2nd to 3rd stage of sintering, i.e. all open pores get closed at a density called critical density. A robust experimental study of this transition for several materials (alumina, magnesia-alumina spinel, tetragonal and cubic zirconia) has been carried out and compared with three theoretical models. From theoretical models it follows that critical density is a material parameter depending only on dihedral angle and being independent of particle size, green density and pore size distribution. It varies from 88.1% to 93.7% of theoretical density for studied materials and used models. Measured critical densities agree well with models by Beere and Carter & Glaeser for cubic systems (spinel and cubic zirconia). However, they do not match very well experimental data for alumina (hexagonal system) being in agreement with other open sources data. The sophisticated model by Svoboda et al. significantly underestimates the critical density for all systems. The reason of disagreement is analyzed in detail and several hypotheses explaining differences between the model and reality are proposed. It is recommended to use Beere's and Carter's & Glaeser's models for prediction of critical density, while the model by Svoboda et al. characterizes rather the stage of pore closing initiation. (C) 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.