4 - Hot-Pressed SiC (HPSC)

Abstract

Hot-pressed SiC (HPSC) was invented in 1956 and was published not patented, making it a public domain technology since 1956. HPSC is densified by hot-pressing SiC powder in inductively heated graphite dies at temperatures approximately 2000°C and pressures of tens of MPa. Hot-isostatically pressed SiC (HIP-SiC) is densified by hot isostatic pressing. Sintering aids are essential. Although HPSC is increasingly a boutique technology today, it formed a vital step along the pathway over the last half a century towards contemporary direct-sintered (pressureless sintered) SiC (DSSC). On the plus side: (1) HPSC is able to attain 100% density without exaggerated grain growth, for DSSC 100% density is associated with exaggerated grain growth; (2) HPSC attains full densification at lower temperatures than DSSC, and by association finer grain size than DSSC; (3) HPSC is essentially net shape in the lateral plane, all shrinkage is axial, whereas DSSC is not a net shape process, it undergoes firing shrinkage; (4) ultrafine SiC particle size is not essential for a successful hot pressing process, whereas for DSSC ultrafine particle size is essential, and such ultrafine powders are costly to produce. On the minus side: (1) HPSC is a more expensive and costly process than DSSC; (2) HPSC has limited shape complexity compared to DSSC; (3) HPSC is inherently much less amenable to cost-effective large-scale mass production than DSSC. Nonetheless, HPSC remained a leading process for commercial manufacture of high-quality dense SiC for much of the second half of the 20th century, and in the armour realm, HPSC continued to be significantly commercially available right up until the present day. This chapter represents the most comprehensive update on the HPSC technology to date, and includes HIP-SiC. It overviews the invention and evolution of HPSC technology, from 1956, up to the present day, both conventional graphite-die-pressed HPSC, as well as the later evolution HIP-SiC.