Review: Fabrication of engineering ceramics by injection moulding. I. Materials selection

Thermodynamic analysis of the high-temperature stability of silicon nitride and silicon carbide

The isothermal oxidation behavior of commercial hot-pressed Si3N4 was evaluated for temperatures from 1300° to 1500°C Multiphase scales were formed, consisting mainly of α-cristobalite and enstatite A large increase in reaction rate above 1450°C is believed to be caused by melting in the scale and the consequent increase in the rate of oxygen transport No oxygen pressure dependence was observed at 1400°C over the oxygen pressure range 10-9 atm to 600 torr However, a small decrease in the kinetics was observed when measurements were made in reduced total pressures of oxygen as compared to O2/N2mixtures at a constant total pressure

Oxidation of Si3N4 in the Range 1300° to 1500°C

The strength of 1500 sintered alumina and reaction-bonded silicon nitride flexure specimens was measured by seven laboratories in four countries. Consistent results were obtained only with a well-defined standard test method. Older procedures were less consistent or gave erroneous results. A simple statistical procedure allowed a quick estimate of consistency, and outlier samples could be appraised for the likelihood of their occurrence.

Advanced Structural Ceramics: A Round Robin

The oxidation behavior at 1370°C of dense SiC, hot-pressed with the aid of Al2O3, has been investigated as a function of Al2O3 content. Increasing amounts of the Al2O3 hot-pressing aid increased the oxidation rate. Observations of the oxide surface show that a glassy phase (indicating formation of a liquid at the oxidation temperature) containing Si, Al, Fe, and K forms over the residual Al2O3 in the hot-pressed material. It is suggested that the oxygen transport through an impure aluminosilicate liquid is faster than that through a pure SiO2 scale, thus causing an increased oxidation rate.

Effect of Alumina Content on the Oxidation of Hot‐Pressed Silicon Carbide

: The demonstration of uncooled brittle materials in structural applications at 2500 F is the objective of the 'Brittle Materials Design, High Temperature Gas Turbine' program. Ford Motor Company, the contractor, will utilize a small vehicular gas turbine while Westinghouse, the subcontractor, will use a large stationary gas turbine. A significant achievement in the vehicular turbine project was the successful engine test, 175 hours at 1930 F, of a silicon nitride stator. Durability testing on a nose cone was extended to 246 hours, equalling the previously demonstrated durability of 245 hours on 1st and 2nd stage rotor tip shrouds. A 'Refel' silicon carbide combustor previously demonstrated 171 hours durability, crack-free, including 20 hours at 2500F. Two additional combustors of the same material were tested for 10 hours each. Eight hours of testing the stationary ceramic flowpath at 2500 F were accumulated; non-catastrophic cracks occurred in the nosecone and stator after surviving three hours crack-free. Two stator vanes survived 1000 cycles to 2500-2600 F plus 3720 cycles to 2900 F in the thermal shock rig. A concentrated effort on turbine rotor fabrication development was initiated. Improvements in the rotor fabrication processes have been made. Reduction of the MgO content increased the hot strength of the hot-pressed silicon nitride rotor hub material. Over 500 rotor blade rings were fabricated using the injection molding process and some high density rotor blade rings were also fabricated by slip-casting.

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Raymond J Bratton

Papers

Brittle Materials Design, High Temperature Gas Turbine