Microstructure and phase evolution in pyrolysed short fibre reinforced polymethylsilsesquioxane-phenolic interpenetrating networks

Simultaneous interpenetrating networks (IPNs) comprising of polymethylsilsesquioxane (PMSQ) and phenolic resole (PF) systems at various PMSQ to PF ratios (0-75% w/w PF) which formed the matrix resin for 60% w/w short silica fibre reinforced composites (SR-PMSQ-PF) are subjected to a controlled heating regime in argon atmosphere at 1500 °C to investigate the microstructure and phase evolution based on IPN composition. This study aims at giving an insight into thermal, mechanical and chemical stability of silicone-phenolic IPN composites, particularly for high temperature thermal protection applications as in aerospace. The morphology and microstructure of the pyrolysed composites are well characterized using Raman and FTIR spectroscopy, XRD and SEM-EDS and based on this a mechanism for microstructure evolution is proposed. It is inferred that the pyrolysis of SR-PMSQ-PF composites results exclusively in the formation of cristobalite silica. In addition to that, the formation of SiC is also facilitated particularly in nano dimension with increase in phenolic content, by carbothermal reduction of silica derived from PMSQ and silica fibre predominantly through solid-vapour phase reaction along the (111) crystalline plane. The systematic evaluation of microstructure and morphology of pyrolysed SR-PMSQ-PF IPN composites is useful to define specific thermal protection applications for these composites.