Refractory materials formed from refractory grains bonded by a sialon matrix containing dispersed graphite and/or boron nitride particles and a process for the preparation of these materials

A process for the production of a refractory material, which comprises

• (1) preparing a mixture of the following constituents in the indicated proportions:
• a) 33 to 88% by weight of grains, at least 90% of which are between 50 .mu.m and 5 mm in size, these grains consisting of a refractory material whose melting point and, where appropriate, thermodecomposition temperature are higher than 1700.degree. C;
• b) 9 to 40% of a mixture of reactive powders comprising:
• (i) 23 to 90% of silicon powder, at least 90% of the particles of which have a diameter smaller than 150 .mu.m,
• (ii) 9 to 62% of calcined alumina, at least 90% of the particles of which have a diameter smaller than 20 .mu.m, and
• (iii) 0 to 24% of aluminum powder, at least 90% of the particles of which have a diameter smaller than 80 .mu.m, the sum of the constituents (i) to (iii) representing 100% and the ratio of the proportions of aluminum and calcined alumina being less than 0.7,
• c) 3 to 45% by weight of hexagonal boron nitride particles or of crystalline graphite flakes or of a mixture of these, at least 80% of the graphite flakes being greater than 100 .mu.m in size;
• d) 0 to 3% of a dried and ground refractory clay, the sum of the ingredients (a) to (d) making up 100%, and
• (2) shaping the resulting mixture by pressing;
• (3) drying the shaped mixture;
• and
• (4) firing the shaped and dried mixtured under a nitrogen atmosphere at a temperature of 1300.degree. C. to 1600.degree. C.