Hot‐Pressing Alumina—Mechanisms of Material Transport

The effect of load on initial neck growth between alumina single‐crystal spheres has been measured at 1530°C. The neck areas are larger than those observed in pressureless sintering and constant for all times between 10 and 480 minutes. The neck areas are proportional to the loads; i.e. they equilibrate at equal stresses (50,000 psi). It is shown quantitatively that the initial enhanced neck growth observed cannot be attributed to enhanced sintering by diffusion under the influence of stress. The plastic flow contribution to densification during hot‐pressing is calculated from the stress inferred from the foregoing measurements, from the applied load in hot‐pressing, and from geometric relations between particles. It is shown that for aluminum oxide the contribution of plastic flow to densification at the pressures normally used in hot‐pressing is small. It is concluded that the final stage of densification of alumina during hot‐pressing occurs by enhanced diffusion under the influence of stress.