Silicon carbide alloys

Abstract

Structure determination of a calcium ɑ'-sialon containing 1.8 calcium atoms per unit cell shows that these atoms occupy the two large interstitial sites in the structure. The carbothermal reduction of ɑ'-sialons results in silicon carhide-aluminium nitride alloys with the wurtzite-type structure. A more direct method of firing powder mixtures of silicon nitride, aluminium nitride, carhon and calcium oxide in the temperature range 1650-2100 C° is explored. Phase relationships in the silicon carbide-aluminium nitride system have been determined up to 2100°C and contrary to previous reports there is a miscihility gap extending from about 40 m/o to 84 m/o aluminium nitride at 1800°C. 2H-SiC formed at the silicon carbide-rich end of the system by the reaction of silicon nitride with carbon at 1650°C contains up to 12 a/o nitrogen which stabilizes it relative to other polytypes. At higher temperatures, 2H transforms to 3C which is itself stabilized by nitrogen and, unlike nitrogen-free 3C, does not transform to 6H below 2l00°C. The phases produced by high-temperature reaction and heat-treatment are characterized by X-ray diffraction, scanning electron microscopy and chemical analysis. High resolution electron microscopy is used to examine structural details and particularly to follow the 2H→3C transformation in silicon carbide alloys.