Dense shaped articles of polycrystalline α-silicon carbide and process for the manufacture thereof by hot-pressing

Claim 1

A process for the manufacture of shaped articles of polycrystalline silicon carbide having a density of at least 99% of the theoretical density of SiC, consisting essentially of: at least about 98.8% by weight of α-silicon carbide; from 0.1 to 0.4% by weight of aluminum; up to about 0.4% by weight of nitrogen and/or phosphorus; up to about 0.2% by weight of oxygen; and up to about 0.2% by weight of carbon, the α-silicon carbide being in the form of a homogeneous microstructure having grain sizes of not more than 12 μm and the other constituents being substantially in the form of a solid solution in the α-SiC lattice, and having a flexural strength of at least 600 N/mm 2 , up to at least 1450° C., measured according to the four-point method at a stress rate of at least 3 N.mm -2 .sec -1 , and a predominately transgranular fracture mechanism, said process comprising: (a) forming a homogeneous mixture of pulverulent silicon carbide substantially in the form of the α-modification and having a particle size distribution of 3 μm and finer with from 0.1 to 0.4% by weight of the mixture calculated as free aluminum based on the total weight of solid constituents of an aluminum-containing additive as a sintering aid, and (b) hot-pressing the mixture in a die at a temperature of from 1850° C. to 2300° C. and a die pressure of at least 100 bar in an atmosphere inert to silicon carbide or in a vacuum for a sufficient length of time to form the desired product.