Abstract
Silicon carbide (SiC) monofilaments are high strength, continuous ceramic fibres produced through chemical vapour deposition (CVD) and used to reinforce metal matrix composites. Such composites have excellent mechanical properties. However, they are expensive to manufacture and the monofilaments must be highly reproducible to ensure reliability of the resulting composite. TISICS Ltd are the sole producers of the material outside of the United States of America and have recently developed two new monofilaments, SM3256 (140 µm diameter) and SM3240 (100 µm diameter) with enhanced mechanical properties and reduced cost of production. These monofilaments and composite panels have been evaluated through tensile testing. They have been found to be highly reproducible over three years of production with the monofilaments possessing an average tensile strength of 4.0±0.2 GPa with a Weibull modulus of 50±10. Recent advances in plasma focussed ion beam (PFIB) milling techniques and scanning transmission electron microscopy (STEM) have been exploited to produce specimens revealing the interior of the monofilaments with unparalleled detail and precision. Raman spectroscopy and Auger spectroscopy have been used to characterise the microstructure and composition of the monofilaments and inform their development. The process for depositing a protective coating on the monofilaments has been improved, resulting in a 17% decrease in the total cost of CVD feedstock chemicals required. Previously unobserved nanoscale voids in the tungsten filament substrate have been identified as a critical process variable potentially responsible for the narrow strength distribution of the monofilaments. Analysis of the monofilament microstructures has indicated the potential for increasing the production speed of SM3256. Experimental trials have resulted in up to 75% faster production however a resulting decrease in performance demonstrates that further work is necessary. This research has resulted in significant cost reductions and has improved the economic viability of the monofilaments. The demonstration of reproducibility of the material properties has contributed to ongoing qualification for their use in aerospace components. The potential for further fundamental improvements to the process has been identified.