Synthesis and characterization of a novel silica nanowire-reinforced SiC thermal material

Abstract

When forming a 3D SiC object by thermal oxidation of compacted particles, the resultant solid is often found to have limited strength. A major reason for this lack of structural integrity is believed to be due to poor oxygen diffusion and this, in turn, limits the thickness of the silica bonding layer at the interface between SiC particles. In the present study it is demonstrated that SiC particles treated with NaOH will create a silica gel layer that enhances the bonding during thermal oxidation. The surface modified particles were compacted into discs at an applied pressure of 0.47 ​MPa and then heated at temperatures ranging from 500 to 1000 ​°C. FTIR confirmed the presence of the characteristic peaks for Si–O and Si–O–Si on the surface of the discs characteristic of the formation of a silica gel layer that bonded the particles at room temperature. SEM-EDX analysis revealed the growth of silica nanowires that bridged the walls of the pores between SiC particles during heat treatment. XRD peaks characteristic for α-SiC phase were persistent at all temperatures. After oxidation XRD showed the phase transformation of the silica gel into quartz which was followed by further transformation to cristobalite solid solution at higher temperature. In conjunction with the increased growth of the silica nano wires, the density, strength, toughness and percent weight gain increased significantly without any measurable change in the dimensions of the discs. FTIR analysis of the condensed volatile species released from surface modified SiC discs during heating revealed the involvement of SiO gas released from the silica gel surface layer in the mechanism of nanowires growth. Preliminary thermal shock resistance tests showed stability of the mechanical properties after water quenching of samples treated at 900–1000 ​°C.