Abstract: The present invention discloses a composition for a valve of an advanced heat engine. The valve comprises grains of aluminum-doped silicon carbide. The grains are of two main types: (1) a matrix of equi-axed grains of alpha and beta SiC for optimum distribution of mechanical properties; and (2) elongated grains of alpha SiC which are distributed within the matrix for reinforcement. The invention also comprises a process for making such valves. The process begins with a powder of aluminum-doped beta-silicon carbide. A binder and a solvent are then added to the powder, and the mixture is agitated while being heated. The mixture is then transferred to an injection mold which is used to form a molded component. The molded component is then sintered and machined into desired dimensions. An annealing step is then performed so that the component exhibits the characteristics of improved strength and toughness.

Abstract: A thermally stable, oxidation resistant ceramic Si.sub.3 N.sub.4 material is disclosed. The material has a substantially pure primary phase formed of substantially pure silicon nitride. A substantially pure secondary phase of the material is formed of a material selected from the group consisting essentially of Si.sub.3 N.sub.4 /SiO.sub.2 /4Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 /SiO.sub.2 /2Y.sub.2 O.sub.3 and Si.sub.3 N.sub.4 /4SiO.sub.2 /5Y.sub.2 O.sub.3. The primary phase may be in the form of individual grains and the secondary phase may be in the form of grain boundaries between the individual grains.

Abstract: A method is disclosed in this specification for forming a densified silicon nitride article. The method is initiated by forming a reaction bonded article of moderate density. This moderate density reaction bonded article has a primary phase of substantially pure grains of silicon nitride surrounded by a secondary grain boundary phase. The secondary grain boundary phase contains silicon nitride, silicon dioxide and a densification aid incorporated in the reaction bonded article of moderate density. The compounds forming the secondary grain boundary phase are present either in their pure forms or interacted with one another. The reaction bonded article is packed in a packing powder which contains portions of the pure compounds found in or interacted with one another to define the secondary grain boundary phase.