The composition of these new porcelains is characterized by the use of such materials as alumina, beryllia, zirconia, and thoria, with minor additions of other metallic oxides, and by the absence of silica. A representative high-beryllia porcelain contains 84 per cent. beryllia and 8 per cent. zirconia, with small amounts of lime and alumina; a typical high-zirconia composition is 80 per cent. zirconia, 10 per cent. beryllia, and 10 per cent. magnesia.
In 1940 the Bureau began a series of investigations to develop glassfree ceramic bodies of the porcelain type which would have simple compositions favorable to high mechanical strength, particularly at elevated temperatures, and good resistance to thermal shock, properties not found in the conventional porcelains. The usual porcelain composition includes feldspar, which reacts as a flux with clay and silica, dissolving them in part to produce a viscous liquid. The liquid fills the interstices between the crystalline phases of the mixture, and upon cooling, becomes a matrix of glass upon which the strength of the porcelain is largely dependent. This glass will soften and deform under stress at temperatures much lower than the temperatures at which the crystalline phases liquefy.
The work, initially undertaken as a fundamental contribution in the field of ceramics, was accelerated by the war-time demands for a material that would resist deformation at temperatures up to 2,000 ยฐ F. for use in jet propulsion and gas turbine engines. A decided advantage of ceramic materials for this purpose, if other properties are satisfactory, is their low density as compared with the high-temperature metallic alloys. Accordingly, the investigation was extended under the sponsorship of the NACA to include strength and creep characteristics in pure tension at temperatures ranging from 1,500 ยฐ to 2,000 ยฐ F. On the basis of preliminary tests, six compositions were chosen for further * Communicated by the Director.