Sintering of Covalent Solids

The sintering behavior of primarily covalently bonded β‐SiC, Si, and Si3N4 was studied using surface area and densification measurements as well as observations of microstructures developed during firing. The existence of highly dense, microscopic regions and large (≥100°) dihedral angles in fired compacts of β‐SiC and Si which experience little macroscopic densification suggests that macroscopic densification is not intrinsically limited by the effects of surface energy. The mechanism proposed to explain the microstructure that develops in unsinterable covalent solids which do not undergo a phase change is based on the existence of a high ratio of surface and/or vapor‐phase matter transport‐to‐volume and/or grain‐boundary transport. The addition of boron to both β‐SiC‐ and Si‐containing carbon retards surface and/or vapor‐phase transport and grain growth at lower temperatures, which results in enhanced densification at high temperatures. Macroscopic densification of β‐SiC and α‐Si3N4 can also be retarded by the formation of a continuous network of high‐aspect‐ratio grains of the polymorphic form that rigidities the sintering body. Finally, the sintering of pure Si depends sensitively on particle size in the submicron range. Nearly theoretical density is achieved in Si powder of ∼0.06‐μm size. This result suggests that other pure covalently bonded solids can also be sintered to high density without applied pressure.