Zirconium diboride-mullite composite form mineral: Combustion synthesis,consolidation, characterizations and properties

Zirconium diboride-mullite composite powder was synthesized in-situ by combustion in argon of a zircon sand/B₂O₃/Al reactant system in a 3 : 3: 10 M ratio. Zircon sand with a particle size less than 45 μm was activated by high-energy milling for 360 min. The optimum reactant system included the addition of 0.01 mol of Si. The product of the synthesis of this system contained 34 wt% ZrB₂ and 50 wt% mullite. The obtained zirconium diboride-mullite powder was consolidated by hot pressing at 25 MPa in an argon environment, ramping at 10 °C/min to 1,450, 1550 and 1650 °C and holding for 60 min. The sintered composite hot-pressed at 1650 °C had a density of 3.39 g/cm³, flexural strength of 153.25 ± 1.19 MPa, hardness of 10.66 GPa and fracture toughness of 4.23 MPa.m^1/2. The flexural strength and hardness of the composite was significantly influenced by the grain size of the reinforced ZrB₂. The predominantly intergranular fracture observed in surface micrographs confirmed the high toughness of the composite. The coefficient of thermal expansion of the product hot-pressed at 1650 °C was 6.53 × 10^?6/°C: much lower than reported coefficients of existing Al₂O₃, ZrO₂ ZrB₂, and ZrB₂–SiC refractory ceramics.