Room temperature fracture and high temperature creep characteristics of 20 vol.% Nb particulate reinforced alumina

Composites consisting of an alumina matrix with 0.05 wt.% MgO and 20 vol.% Nb with an average particle size of 30 and 100 μm, respectively, were produced by dry mixing and sintering to near their theoretical densities. Fracture toughness tests were carried out in three point bending on chevron notched samples. The results indicate that the R-curve of the composites exhibited a more than 300% increase in crack growth resistance compared with the crack growth resistance of the alumina produced by the identical procedure. The crack growth resistance curve of the composites increased with increasing Nb particle size. The metallographic investigations indicated that the failure of the Nb particles in the crack path ranged from full interface separation, without any significant deformation of Nb particles, to cleavage failure, without any evidence of interface separation. Although the creep rates of the composites were about an order of magnitude higher than those seen for the unreinforced matrix material with a grain size 9–14 μm, they were similar to those seen for the unreinforced alumina having smaller grain sizes. In the creep regime, while grain boundary facet cracks, resulting from grain boundary cavitation, was widespread in the unreinforced alumina, localization of creep damage only to regions of the Nb particles was seen in the composites.