Preparation of mullite/NbN composites through spark plasma sintering

The main objective of this article is to obtain reaction sintered mullite based ceramics through spark plasma sintering process. 5, 10 and 15 wt% NbN particles were mixed with kaolinite and alumina powders as mullite precursor through a wet milling process. The mullite ceramic and composites were fabricated by spark plasma sintering (SPS) method at 1350 ? with initial and final applied pressure of 10 and 50 MPa, respectively. All of prepared composites showed relative density higher than 99% of theoretical density as well as mullite ceramic. The XRD patterns obtained from sintered composites demonstrated dominant crystalline phase of mullite and NbN and also some low-intensity of alumina peaks were observed. The FESEM images of sintered composites illustrated uniform distribution of reinforcement particles for 5 and 10 wt% addition of NbN. While using 15 wt% NbN reinforcement phase lead to form some agglomerates areas. The optimum mechanical properties were obtained for composites containing 10 wt% NbN as bending strength of 441 +/- 36 MPa, Vickers hardness of 17.89 +/- 0.61 GPa and fracture toughness of 3.79 +/- 0.12 MPa m(1/2).