Microstructure-flexural strength correlation and efficacy of rare earth nitrates on the sintering of SiCf/SiC composites

A vacuum infiltration technique was used to infiltrate a SiC-based matrix phase, containing rare earth (Sc, Er and Yb) nitrate (10 wt.%) as a sintering additive, into a 2D-woven Tyranno SiC fabric preform coated with pyrolytic carbon (PyC) for the fabrication of a dense SiCf/SiC composite. By inserting a SiC green tape, a high composite density >95% was achieved after hot pressing at 1750 °C for 2 h under a pressure of 20 MPa. The effects of the addition of each rare earth nitrate on the density, microstructure and mechanical properties of the SiCf/SiC composites were investigated. In particular, Sc-nitrate was found to be the most effective sintering additive in terms of retaining an ultrafine grain size distribution and the retention of structural integrity by preserving the weak PyC interface. Finally on the basis of identified key failure mechanisms, a correlation between microstructure and variation of flexural strength was proposed.