Strengthening of Mullite by Dispersion of Carbide Ceramics Particles

Both mullite/SiC (0.27 μm and 1.20 μm) and mullite/TiC composite ceramics were prepared by hotpressing at 1650°C under 35 MPa for 4h. Room-temperature bending fracture stress, Young's modulus, Vicker's hardness and fracture toughness were investigated as functions of SiC and TiC volume fraction (0-20%). Grain growth of mullite was prevented by the existence of dispersed particles (SiC, TiC) in the matrix. As a result, bending fracture stress of both mullite/SiC and mullite/TiC composite ceramics was improved. In the case of the mullite/SiC system, bending fracture stress inceased with increasing SiC content and showed a maximum value of 604 MPa at 20 vol%, which was about 80% higher than that of monolithic mullite. On the other hand, fracture toughness of mullite/TiC ceramic composite was observed to incease from 2.65 to 3.9 MPa√m with the addition of 20 vol% TiC. Correspondingly, the bending-fracture stress increased from 330 to 410 MPa. The strengthening mechanism of thermal treatment in air was also investigated for mullite/SiC composite ceramics and it was concluded to be useful for increasing bending fracture stress. Detailed reseach on the microstructure showed that the Hall-Petch relationship was satisfied for grain size and bending fracture stress.