Abstract: In Al.sub.2 O.sub.3 composites used for machining tools for difficult-cutting steels, etc., strength and toughness are increased by dispersing a Ti compound with an average grain size of not greater than 0.1 .mu.m into Al.sub.2 O.sub.3 particles with the average grain size of not greater than 1 .mu.m, and SiC whiskers are uniformly dispersed between each of Al.sub.2 O.sub.3 particles to form a nanocomposite structure.For the above-mentioned purpose, SiC whiskers containing 0.3 to 1.5% by weight of oxygen, dissolved uniformly in a solvent, are mixed with a starting Al.sub.2 O.sub.3 series powder containing Ti compounds to form a uniform mixture, which is then molded and sintered.

Abstract: By bonding a graphite sheet possessing tightness to at least one surface of carbon fiber felt sheet possessing gas permeability by means of a carbonaceous binder, a multilayer thermally insulating material suitable for use in a heating furnace employing a nonoxidizing atmosphere is obtained. This insulating material has mechanical strength which does not decrease with a long period of use and gives rise to little contamination of articles being heated in the furnace.