Formation of dumbbell-shaped β-SiC whiskers in Al₂O ₃-ZrO₂-C composite refractories

Based on the traditional processing route of carbon- bonded materials, alumina powder, silicon powder, graphite flakes, carbon black as well as phenolic resin are mixed, pressed to samples and fired in a coke bed at between 1200 °C and 1600 °C. Their phase composition and microstructure are determined and observed with the aid of XRD, SEM, EDX and TEM. The microstructure evolution as a function of the temperature influences the bulk density, the apparent porosity and the mechanical strength. The formation of SiC whiskers and cristobalite balls at temperatures between 1200 °C and 1300 °C influence primarily the mechanical properties. The amount of SiC whiskers increases as a function of the temperature. However, the morphology of SiC produced at different temperatures is remarkably diverse. In the range of 1200 °C to 1400 °C, most of the β-SiC whiskers are nano-sized in diameter and micrometer-sized in length, some of them are dumbbell-shaped and well distributed in the matrix. At temperatures above 1500 °C SiC whiskers become curled, coarser and congregated. The structure of the samples is loosened due to the escape of a great amount of gaseous silicon and/or silicon monoxide. The dumbbell-shaped β-SiC whiskers can be produced in situ in functional components such as nozzles or slide gate systems or can be prefabricated and incorporated as fine material in the carbon bonded recipes for advanced composite carbon- bonded materials in the system Al2O 3-ZrO2-C with superior thermo- mechanical properties.