Promotion of deactivated sintering by dry-particle coating

Dry-particle coating was used to examine the feasibility of increasing the sintering temperatures of different particulate materials (hosts) by applying a discrete coating of a highly refractory material (guest). Increasing the sintering temperature reduces the sintering rate of the material, and hence the phenomenon is called "deactivated sintering." Several host particles, such as glass beads, PMMA, gamma-alumina, and an alumina-silica composite, were coated with submicron-size silicon carbide in three different dry-particle coating devices: magnetically assisted impaction coating, mechanofusion, and hybridizer. The coated products were characterized using several techniques, including particle-size analysis, SEM, EDX, BET, optical microscopy, and dilatometry studies to compare the coating performance of each device. By coating the host particles with silicon carbide, it was possible to appreciably increase the sintering temperatures of all four of the particulate host materials. Simple phenomenological sintering models based on volume diffusion for crystalline materials and viscous flow for amorphous materials were combined with dilatometry data to explain the phenomenon of deactivated sintering. For gamma-alumina, there was a significant reduction in the effective diffusivity due to the addition of a surface coating of SiC, whereas for SiC-coated glass beads and PMMA, an increase in the effective viscosity was postulated as the cause of deactivated sintering.