Synthesis and characterization of functionally graded SiC-mullite thermal material

Abstract

SiC-mullite is considered for high temperature applications due to its high mechanical strength and thermal shock-resistance. However, maintaining these properties is challenged by the high processing temperature requirement for the formation and densification of the mullite phase which leads to abundant surface oxidation of SiC into cristobalite. Cristobalite has relatively poor mechanical strength and thermal properties compared to SiC. Herein, we report on using coal fly ash as a source of alumina (Al2O3) that reacts in situ with the silica (SiO2), oxidation product of SiC. The instantaneous mullite formation on the surface of SiC facilitated due to the presence of minor concentrations of metal oxides in coal fly ash, resulted in a strong bonding zone between the two phases at relatively low temperature. In this work, SiC was mixed with coal fly ash at weight ratios of 90SiC/10ash, 85SiC/15ash, 80SiC/20ash, and 75SiC/25ash and sintered at 1400 °C. Measurements of mechanical properties showed that the 85SiC/15ash composition had the highest mechanical strength among samples. XRD analysis showed the phase composition of thermally treated 85SiC/15ash to be 81.8 wt% SiC, 11.4 wt% mullite, and 6.8 wt% cristobalite. SEM-EDX revealed a concentration gradient of Al in the cristobalite which enhanced formation of functionally graded bonding zones between phases and resulted in SiC-mullite composite with high thermomechanical properties. The compressive strength, nanoindentation elastic modulus, and Vickers hardness were 434 ± 20 MPa, 370.9 ± 22.6 GPa, and 11.5 ± 1.2 GPa respectively. The thermal shock resistance test showed high dimensional and mechanical stabilities after quenching in liquid nitrogen (−196 °C) from 1400 °C. The SiC-mullite composite showed low thermal expansion co-efficient from 3.17 × 10−7/K to 5.615 × 10−6/K when the sample was heated from 182 K to 354 K. The specific heat capacity, thermal diffusivity, and thermal conductivity were 7.83 ± 0.0014 J/g.K, 1.04 ± 0.013 mm2/s, and 17 W/m.K at 100 °C, respectively. The SiC-mullite composite exhibited moderate electrical conductivity of 3.48 × 10−2 S/m at 1000 °C.