Deactivated sintering by particle coating: the significance of static and dynamic surface phenomena

The suppression of bonding of micron-sized host particles due to the application of heat at temperatures below their melting point (deactivated sintering) when coated with submicron guest particles is theoretically predicted and confirmed by experiments. It was possible to preserve individual particles of polymethylmethacrylate (PMMA) at 150 degreesC, about 45 degreesC above the minimum sintering temperature (MST) of the uncoated particles, and individual glass beads at 750 C, about 175 degreesC above the minimum sintering temperature of the uncoated glass beads, by applying a surface coating of silicon cat-bide. A dense monolayer of submicron particles coated onto the surfaces of much larger host particles prevents contact of their surfaces (host-host contact) and suppress the dilation of their mobile surfaces. At the temperature at which the host particle material starts to flow, the coated particles behave as solid colloid particles sitting on the surface of a liquid. If the adhesion energy at the interface between the host and guest particles is not small in comparison to the cohesion energy, i.e., when the contact angle is not very large, the decrease in bonding energy due to the coating is small and sintering deactivation is weak. If the contact angle is very large, i.e., the work of adhesion is very small compared to the work of cohesion, the decrease in bonding energy is large resulting in appreciable sintering deactivation. This deactivation is only temporary, however, because of the forced intrusion of the host fluid into the pores of the coating layer (CL). This process is similar to the intrusion of mercury, which occurs in mercury porosimetry. Models for the intrusion dynamics are developed for both a dense coating monolayer and polylayers with account for different pore size distributions. (C) 2004 Elsevier B.V. All rights reserved.