Densification of liquid-phase-sintered silicon carbide

Abstract

There has been growing interest in pressureless liquidphase sintering (LPS) of SiC, with Al2O3 additions [1] or simultaneous Al2O3 and Y2O3 (or rare earth oxides) additions [2–6], as it allows sintering at lower temperatures (1850 to 1950 ◦C) relative to solid-state sintering. A particularly important aspect of this class of materials is that they can be significantly toughened through careful microstructure design, wherein elongated SiC grain-reinforcements are grown in situ during sintering [7–10]. The elongate nature of the SiC grains and the weakness at the grain boundaries promote crack-wake bridging in these ceramics, resulting in a significant increase in the toughness [8, 10, 11]. Although in situ reinforced LPS SiC ceramics represent a new class of potentially important structural materials, the mechanisms of densification and subsequent microstructural evolution in these ceramics are not well understood. Some advances have been made in the understanding of the microstructural evolution in LPS SiC [7, 12–16], however, reliable processing of these ceramics is still plagued with two important shortcomings, namely: (i) inadequate and irreproducible densification; and (ii) significant weight loss during sintering. In the context of pressureless sintering of LPS SiC with simultaneous additions of Al2O3 and Y2O3, prior studies have demonstrated that sintering in a contained crucible and embedding the specimens in a packing bed consisting of coarse Al2O3 and SiC powders dramatically improves the densification [4–7, 17]. However, these results could not be reproduced by other workers (see, e.g., Refs. [7, 9, 18, 19]). Thus, the broad objective of this study was to understand the role of the packing bed on the densification of LPS SiC, with the specific goal of being able to reproducibly sinter these materials to near-full densities and with minimum weight loss. A long elusive parameter that significantly influences the densification in this system, namely the composition of the packing-bed powder, was identified. In particular, it was found that an optimum concentration of Al2O3 in the packing-bed powder is essential for consistent, near-complete densification (1% porosity), accompanied with negligible weight loss. While the absence of Al2O3 in the packing bed has been known to result in incomplete densification, here it is shown that excess Al2O3 in the packingbed powder also adversely affects the densification.