INFLUENCE OF ATMOSPHERE ON THE FINAL-STAGE SINTERING KINETICS OF ULTRA-HIGH-PURITY ALUMINA

The influence of sintering atmosphere on the final-stage sintering of ultra-high-purity alumina has been investigated. Model final-stage mic rostructures were tailored via a latex sphere impregnation and burnout technique. Critical experiments have been conducted to quantitatively examine the influence of the oxygen partial pressure on the final-sta ge sintering kinetics. Samples were sintered at 1850-degrees-C in eith er dry hydrogen (p(O2), almost-equal-to 3 x 10(-17) atm) or wet hydrog en p(O2) almost-equal-to 5 x 10(-10) to 2 x 10(-11) atm), and their mi crostructures were characterized as a function of sintering time. Sint ering in dry hydrogen decreased the susceptibility of the final-stage microstructure to pore/boundary breakaway. In the kinetic analysis, th e variation in the number of pores per grain, N(g), was taken into acc ount. It was found that in both atmospheres, the densification rate wa s controlled by grain boundary diffusion, and that sintering in dry hy drogen increased the densification rate by a factor of 2.25. In additi on, it was determined that the grain growth rate in both atmospheres w as controlled by the rate of surface diffusion of matter around the po res and that sintering in dry hydrogen enhanced the grain growth rate by a factor of 5.6. The overall effect of the dry hydrogen atmosphere was that it enhanced the coarsening rate relative to the densification rate by a factor of 2.5, and consequently shifted the grain size-dens ity trajectory to much lower densities for a given grain size.