PREPARATION OF ZIRCONIA AND SILICON-CARBIDE WHISKER BIPHASIC POWDER MIXTURES BY CARBOTHERMAL REDUCTION OF ZIRCON POWDERS

Carbothermic reduction of zircon powders has been studied under argon and nitrogen gas pressures of 0.15 MPa in order to obtain biphasic com posite powder mixtures containing zirconia and silicon carbide whisker s. The reduction has been carried out using different mole ratios of c arbon and zircon. Carbon was used in the form of activated charcoal (s pecific surface area similar to 1000 m(2) g(-1)) or carbon black (spec ific surface area similar to 300 m(2) g(-1)). Whilst complete decompos ition to m-ZrO2 was obtained in the argon atmosphere at 1700 degrees C , under the nitrogen atmosphere the conversion was incomplete even at 1700 degrees C. However, the extent of conversion to zirconia at 1650 degrees C under nitrogen was found to be more than that under argon ga s. In a few cases, particularly under the nitrogen atmosphere, minor a mounts of other forms of zirconia e.g., tetragonal (t)-ZrO2 or orthorh ombic (o)-ZrO2 were formed along with the major monoclinic (m)-ZrO2 ph ase. The rate of reaction was found in general to increase with an inc rease in the carbon content. The studies particularly indicate that ac tivated charcoal is a better reducing agent than carbon black owing pr obably to its enormous surface area. Further, it was also noted that c obalt chloride and sodium chloride act as a catalyst and a space formi ng agent respectively. They aid silicon carbide whisker formation and growth and hence the reaction is appreciably accelerated and reaches c ompletion at 1650 degrees C in the argon atmosphere.