Mechanisms of hot pressing of magnesium oxide powders

Abstract

Estimation of the rate controlling mechanisms of mass transport during the initial, Intermediate and a portion of the final stage of hot pressing of powder compacts by an adequate application of the theory of creep of non-porous polycrystals at elevated temperature is first discussed and their significance for understanding of the relations between powder characteristics and their densification during hot pressing stressed. Review of recent developments of the creep theory indicates that if intraparticle (intracrystallite) i.e. dislocation mechanisms are acting to a negligible extent only, the steady-state strain rate ϵ, should be controlled entirely either by boundary reactions (emission/absorption of point defects at sources and sinks, e.g. at boundary line-defects) or by diffusion between sources and sinks (Nabarro-Herring and/or Coble creep). An analysis has been made in these terms of ϵ observed with well characterised MgO powders, having a rather uniform crystallite size, during hot pressing at 775–975K under loads of P_A = 60–295 MPa. Two types of powders, obtained by thermal decomposition of Mg(OH)₂ at different temperatures, have been studied, namely i. powders constituted by well-annealed fine crystallites (d = 28–56 nm) showing no lattice microstrains, and ii. powders constituted by fine crystallites (25–45 nm) showing appreciable microstrains of the lattice and, consequently, a high density of the line defects. Experimental determination of the stress sensitivity, n, particle sensitivity, m, and true activation energy for creep, Q_c, has shown that the data correspond very closely to $\text{ϵ}{}_{\mathrm{<mtext>s</mtext>}}\text{∝}\text{P}{}_{\mathrm{A}}{}^2\text{d}$ in the first, and to $\text{ϵ}{}_{\mathrm{<mtext>s</mtext>}}\text{∝}\text{P}{}_{\mathrm{A}}\text{d}{}^3$ in the second case, respectively. The first situation is expected for strain rates controlled by boundary reactions while the second for the ones controlled by Coble creep, respectively. The different rate-controlling mechanisms enable to explain rationally the different densifications obtained under comparable conditions of hot pressing with the two types of MgO powders.