Effect of the specimen size on strength of Si3N4 + SiC composite

Bending and contact strength of a recently developed carbon derived Si3N4 + SiC micro/nanocomposite have been investigated in four-point and opposite sphere modes using specimens with two different sizes of 3 mm × 4 mm × 45 mm and 1 mm × 2 mm × 20 mm. The fracture origins during bending test were clusters of pores and large SiC grains with different size, shape and location, which resulted in relatively low strength (675 MPa and 832 MPa) and Weibull moduli (6.4 and 8.6). The fracture origins in specimens tested in contact mode were cone cracks, originated and arised during the loading, and their similar size and shape, together with the preexisting stress field resulted in high strength (1167 MPa and 1997 MPa) and relative high Weibull moduli (17 and 15). In agreement with the theory, bending test specimens with smaller effective volume exhibited higher strength. In contact test, interaction of cone cracks occured in the small specimens at lower load as in larger one, which resulted in the lower strength of small specimens.