Abstract
Silicon carbide (SiC) monofilaments are high-strength, continuous ceramic fibres manufactured using a chemical vapour deposition (CVD) process. As the reinforcement in metal matrix composites (MMCs), they significantly enhance the mechanical and thermal properties while reducing weight compared to monolithic metal alloys. UK-based TISICS produces 100 μm (SM3240) and 140 μm (SM3256) diameter SiC monofilaments for reinforcing Ti and Al alloy composites, targeting the replacement of conventional alloys in aerospace components. The high cost of SiC monofilament production challenges MMC commercialisation. This thesis investigates the characterisation, optimisation, and development of SiC monofilaments, focusing on addressing yield-limiting growth anomalies, enhancing coating layer understanding, and stabilising the tungsten core-to-SiC (W-SiC) interface within the monofilament, which compromises its high-temperature performance.
Growth anomalies causing filament fracture during quality control testing were found to result from process-induced tungsten oxide accumulation in the CVD reactor. Modifying the CVD cleaning stage significantly reduced tungsten oxide deposits and associated anomalies, resulting in a 77% decrease in monofilament breaks.
Investigations using SEM and high-speed AFM found no correlation between coating roughness and thickness with monofilament performance. The pioneering use of tip-enhanced Raman spectroscopy (TERS) successfully characterised the coatings’ nanostructures and their potential influence on performance, providing insights that could help increase the production speed of SiC monofilaments.
The development of a diffusion barrier layer to stabilise the W-SiC interface, a deposition process for coating tungsten filaments with titanium carbide (TiC), was successfully redeveloped, providing a foundation for a new high-temperature stable SiC monofilament variant.
These research findings have immediate industrial impact, including maximising yields in SiC monofilament production, eliminating coating-related uncertainties, and successfully applying TERS for SiC monofilament analysis. These advancements support the transition of TISICS’ CVD process from pilot to production scale and opens an opportunity to broaden the range of compatible matrix materials that TISICS SiC monofilaments can reinforce, expanding their use in high temperature applications.