Substantially pore-free sintered polycrystalline articles of .alpha.-silicon carbide, boron carbide and free carbon and process for their manufacture

Abstract

The invention provides substantially pore-free sintered polycrystalline articles comprising .alpha.-silicon carbide, boron carbide and free carbon, the quantitative proportions of which, in percent by weight, are defined by the trapezoidal area having, in the ternary system B/Si/C of FIG. 1 the corner points a=89.0% B.sub.4 C, 9.9% .alpha.-Sic, 1.1% C b=9.9% B.sub.4 C, 89.0% .alpha.-SiC, 1.1% C c=9.0% B.sub.4 C, 81.0% .alpha.-Sic, 10.0% C d=81.0% B.sub.4 C, 9.0% .alpha.-SiC, 10.0% C The articles have a density of at least 99% of the theoretical density, an average structural grain size of less than 20 .mu.m and a 4-point flexural strength of at least 400 N/mm.sup.2. They are manufactured from fine-grained mixtures of .alpha.-silicon carbide, boron carbide, carbon and/or material that can be coked to form carbon, in a two-stage sintering process. In the first stage, green bodies preshaped from the powder are subjected to pressureless sintering to a density of at least 95% TD at from 1950.degree. to 2150.degree. C. In the second stage, the sintered articles are subjected to a post-densification to a density of at least 99% TD, by isostatic hot pressing, without encapsulating, at from 1850.degree. to 2150.degree. C. in a high-pressure autoclave under a gas pressure of at least 10 MPa.

Dependencies are inferred from the claim text.