Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from β-SiC (3C) to the various α-SiC polytypes in the presence of AlN and Al2O3 additives after hot pressing from 1700 to 2100 ◦C. The 2Hand 6H-polytypes of α-SiC were the predominate polytypes with additions of only AlN or Al2O3, respectively. The amount of 2Hand 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al2O3. Improvements in fracture toughness to ∼9 MPa√ m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al2O3 additions. C © 1999 Kluwer Academic Publishers