Heat-resistant silicon carbide with aluminum nitride and scandium oxide

The effect of glassy-phase, using AlN and Sc2O3 as sintering additives, on mechanical properties of liquid-phase-sintered, and subsequently annealed SiC ceramics was investigated. The homophase and heterophase boundaries of the SiC ceramics were completely crystalline in all specimens. The room temperature (RT) strength decreased with increasing Sc2O3/(AlN + Sc2O3) molar ratio whereas the RT toughness showed a maximum (similar to 6.5 MPa m(1/2)) at the molar ratio of 0.5. The flexural strength of the ceramics with the molar ratio of 0.2 was similar to 750 MPa at RT. The high temperature strength was critically affected by the chemistry, especially the content of Al in the intergranular phase (IGP). The best strength at temperatures >= 1500 degrees C was obtained when the molar ratio was 0.8. Flexural strengths of the ceramics at 1500 and 1600 degrees C were similar to 670 and similar to 620 MPa, respectively. The beneficial effect of the new additive compositions (Sc2O3-AlN) on high-temperature strength of SiC ceramics was attributed to the crystalline nature of the IGP and the introduction of Al into SiC, i.e., removal or reduction of Al content from the IGP, resulting in an improved refractoriness of the IGP. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.