Process for making silicon nitride articles

Claim 1

A process for making a silicon nitride article, said process including the steps of: (a) providing a homogeneous slurry comprising a mixture of about 40-60 parts by weight silicon powder, about 60-40 parts by weight water, and from about 0.5 to about 3 parts by weight of at least one nitriding agent; (b) comminuting the slurry of step (a) in a comminutation device to activate the silicon powder and enhance a chemical reaction between the silicon powder and the water; (c) aging the activated silicon slurry of step (b) for a period of time effective to enable the chemical reaction between the silicon powder and the water to occur; (d) processing the aged silicon slurry of step (c) to reduce the water content sufficiently to form the article; (e) forming the processed slurry of step (d) into the article; (f) heating the article of step (e) from ambient temperature to a temperature of about1000° C. in a hydrogen atmosphere at a rate non-damaging to the article whilst removing volatiles therefrom; exposing the article of step (f) to gas; (g) heating the article of step (f) from the temperature of step (f) to a temperature from about 1400° C. to about 1450° C. at a rate of from about 15° C. to about 25 ° C. per hour in said gas mixture of from about 40 parts to about 60 parts helium and from about 1 part to about 4 parts hydrogen and the balance nitrogen, whilst replenishing consumed nitrogen with fresh nitrogen so as to maintain a substantially constant nitriding gas composition while converting the silicon powder therein to silicon nitride; and (h) cooling the article of step (g) to ambient temperature.