A cermet composition of 70y0 alumina-30% chromium has given very promising results as a new high-strength, refractory material. Bodies of the alumina-chromium composition with a particle size of 10 p and less were prepared and fabricated into test specimens under very closely controlled conditions of milling, pressing, and sintering. The fabricated specimens were tested in the laboratory, and results indicate that the material is extremely hard and resistant to oxidation and h a s outstanding strengths both at room and at elevated temperatures. Resistance to thermal shock indicates limited application possibilities, and impact resistance is low.

Cermet b0dies.t composed of metals and oxides, are being developed for high-temperature service. Part I covers the theoretical aspects of experiments of a fundamental chemistry nature which accompanied the laboratory development of test specimens. Compacts of metal powders (Xi, Fe, Co, Cr, and Cr-B) were !ired at 2880" to 3500'F. on pure A1201 tile in various controlled atmospheres. The results indicate that a controlled layer of oxide on the surfaces of the metal-powder grains is essential for good bonding. Special furnaces, described in Part 11, were necessary for the fundamental study and for sintering cermet specimens. Equipment for determining instantaneous modulus of rupture and tensile rupture, and longtime stress-rupture in bending at temperatures up to 2400°F.. is described in Part 111. ForewordThe advent during World War I1 of new aircraftpropulsion mechanisms with optimum temperatures above the physical limits of metals aroused interest in ceramic materials as potentially capable of withstanding the severe conditions of temperature and stress encountered. Investigations of ceramic materials and compositions for such uses were under way before the war ended, and one of the first studies was the testing of the high-temperature praperties of existing commercial ceramic bodies. Outstanding among these was a composition consisting of a combination of ceramic material and metal.$ A'hen the war with Germany ended, it was learned that the Germans had been interested in combinations of iron and aluminas for gas turbine blade application and had begun an active study of this material.On the basis of this information and additional theorizing on the potentialities of such compositions, it seemed plausible that combinations of metals and ceramic oxides or carbides would incorporate desirable properties of both in the end product, which would be better suited to the service requirements than either material alone. Consequently, a full-scale investigation was undertaken.An investigation of such combinations required the development of special high-temperature equipment for fabrication purposes and for determining the properties of the material at elevated temperatures, as well as a fundamental study of the mechanics of bond developed in cermet bodies.

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Physical properties, including firing shrinkage, density, modulus of rupture from 75" to 2400"F., stress-rupture life in cross bending and in tension at 1800" and at 2000"F., modulus of elasticity, tensile strength from 75" to 2000"F., comparative impact resistance, oxidation resistance, thermal expansion, and thermal-shock resistance, for the composition 28y0 alumina-72% chromium combine to indicate the potential value of this material in high stress applications at elevated temperatures where severe thermal shock may be encountered.While the thermal-shock resistance is much better than that possessed by the low-chromium highalumina bodies discussed in Part V, the mechanical properties at elevated temperatures do not appear to suffer as a result of the highly continuous metal phase in the more recently developed body.

The properties of a cermet solid body containing 34% A1203 and 66% 8OCr-2OMo alloy (50Al2O3-50 alloy by volume) are given as firing shrinkage, density, modulus of rupture, tensile strength, stress-rupture life, modulus of elasticity, oxidation resistance, thermal-shock resistance, and thermal expansion. It is shown that substitutions of the 8OCr-2OMo alloy for Cr in a similar cermet developed earlier substantially improved thermal-shock resistance.