Novel processing approach to polymer‐derived ceramic matrix composites

Abstract

The rheological properties of commercially available polycarbosilane, SMP-10, were analyzed as a function of temperature, to guide development of thermal treatment processes for the improved yield and functionality of polymer ceramic precursors. The curing onset temperature for SMP-10 was determined to be as low as 100°C for a heating rate of 1°C/min enabling a heat treatment process at 90°C, where low molecular weight oligomers volatilize from the liquid precursor prior to curing. By driving off the low molecular weight oligomers before fabrication of a composite, the mass yield of SMP-10, from a room temperature liquid state was increased from 77% to 83%. The development of B-staging processes, or a semicure of SMP-10, were also demonstrated. B-staging processes were then applied to polymer infiltration and pyrolysis processing and compared with traditional wet layup CMC processing. It was determined that B-staging processes did not adversely affect ceramic matrix composite fabrication. B-staged processing methods were determined to result in less waste, allow ply-by-ply control of matrix compositions, and enable time independent processing when compared to traditional wet layup processing methods. K E Y W O R D Sceramic matrix composites, microstructure, pre-ceramic polymers, silicon carbide

Process modeling of the low‐temperature evolution and yield of polycarbosilanes for ceramic matrix composites

“…In this work, thermogravimetric data will be used to assess the rate at which the low molecular weight fraction volatilizes. In previous work on the same polymer, size‐exclusion chromatography showed a progressive decrease in the low molecular weight portion of the distribution at a temperature of 86°C . However, size‐exclusion chromatography is not used in this work because of the very broad molecular weight distribution observed for this polymer that exceeded the ideal range of the differential reflective index detector and laser light scattering detector individually .…”

“…This leads to a trade‐off between yield and viscosity embedded within the molecular weight distribution that can be optimized. During both infiltration and heating to the cross‐linking (curing) temperature, significant mass losses have been observed; volatilization of low molecular weight oligomers is acknowledged as the source of this mass loss . Although this type of mass loss has been observed by many, it has not been quantitatively explained.…”

“…During both infiltration and heating to the cross-linking (curing) temperature, significant mass losses have been observed; volatilization of low molecular weight oligomers is acknowledged as the source of this mass loss. [8][9][10][16][17][18][19] Although this type of mass loss has been observed by many, it has not been quantitatively explained. We suggest that the evolution of these low molecular weight species as volatiles is important to control and is why slow heating rates and/or long isothermal holds are typically required to prevent component-level void formation.…”

“…In previous work on the same polymer, size-exclusion chromatography showed a progressive decrease in the low molecular weight portion of the distribution at a temperature of 86°C. 19 However, size-exclusion chromatography is not used in this work because of the very broad molecular weight distribution observed for this polymer that exceeded the ideal range of the differential reflective index detector and laser light scattering detector individually. 20,21 Using a single detector with such a broadly distributed and highly branched polymer can introduce significant errors in the tails of the distributions, reconciling the 2 detector responses is nontrivial and has been the subject of research that goes beyond the focus of the work.…”

A new SiC precursor with high ceramic yield: Synthesis and characterization of CHxMeSiH2‐containing poly(methylsilane‐carbosilane)

“…Among them, an allylhydridopolycarbosilane (AHPCS), the HBPCS having repeating CH 2 SiH 2 unit in main chain, is commercially available from Starfire System Company (Glenville, NY) in the trade name of SMP‐10. SMP‐10 is a liquid SiC precursor with low viscosity and high ceramic yield over 72% . But one shortcoming of HBPCSs with high ceramic yield is their high cost because of using expensive LiAlH 4 in their synthesis.…”