Mechanism of epitaxial growth of silica nanowires reinforcing agent in porous SiC scaffold

Abstract

Processing 3D SiC objects by thermal oxidation is cumbersome due to poor oxygen diffusion in SiC and the limited thickness of the silica layer at the interface between particles. In the present study we treated SiC particles with NaOH to create a silica gel layer. The surface modified particles were pressed manually at 0.47 MPa into discs and subjected to thermal treatment in the temperature range 500-1000 oC. FTIR confirmed the presence of the characteristic peaks for Si-O and Si-O-Si on the surface of the discs confirming the formation of a silica gel layer that bonded the particles at room temperature. SEM-EDX analysis demonstrated the growth of silica nanowires that bridged the walls of the pores during heat treatment. XRD showed the phase transformation of the silica gel into quartz and then cristobalite solid solution at higher temperature. In conjunction with the increased growth of the silica nano wires, the 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 SiC discs during heating revealed the involvement of SiO gas 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 oC. The new mechanism of SiC densification by silica nanowires growth while maintaining dimensional stability opens the door for new SiC manufacturing technology.