Preparation mullite/Si3N4 composites by reaction spark plasma sintering and their characterization

In the present study, in-situ mullite/Si3N4 composites were prepared successfully by reaction spark plasma sintering. For this purpose, 5, 10 and 15 wt% of Si3N4 were added to stoichiometric mullite made of mechanically milled mixture of alumina and kaolin clay to investigate the effect of reinforcement content on the final properties of the prepared composites. The sintering processes were performed at 1400 degrees C under the initial and final applied pressures of 10 and 30 MPa and the vacuum condition of 17 Pa. The XRD patterns revealed the mullite and Si3N4 peaks as the dominant crystalline phases. Microstructural investigations demonstrated a uniform distribution of Si3N4 inside mullite matrix for the composites containing 5 and 10 wt% of the reinforcement particles. Meanwhile, some agglomerates of Si3N4 were observed in the microstructure of the mullite-15 wt%Si(3)N(4 )composite. Moreover, no evidence of reaction between the starting materials was detected through XRD and FESEM analyses. The highest values of hardness, bending strength, and fracture toughness obtained for the composite containing 15 wt% of Si3N4 were 19.14 GPa, 481 MPa and 3.85 MPa m(-1/2), respectively. The fracture toughness mechanisms were detected as crack branching, breaking and deflection, as well as particles pulling-out, all of which were observed in the mullite-15 wt%Si(3)N(4 )composite.