This Small Business Innovation Research Phase I project is designed to evaluate a novel interphase nanocoating designed for Silicon Carbide (SiC) fibers, which is intended to behave as a biomimetic immune system for high-temperature Ceramic Matrix Composites (CMCs) exposed to an oxidizing environment. High-temperature CMCs must survive oxidative and thermomechanical requirements that even the most advanced superalloys cannot withstand. Interphase coatings play a critical role in achieving these objectives. Interphase coatings represent a tiny fraction of the mass of a CMC, but are responsible for most of their outstanding properties. The coatings isolate fibers from the matrix and from each other, control failure mechanisms by regulating load transfer between fiber and matrix, and serve as an Environmental Barrier Coating (EBC) to protect the load-bearing fibers. In essence, an interphase coating is first and foremost a micromechanical system engineered for failure mitigation, oxidation prevention, and a slight self-healing. This Phase I effort aims to enhance these functions and add to them a material "immune response" capable of local detection of oxygen intrusion, followed by automatic compositional changes which resist further oxygen ingress, and finally the creation of "scar tissue" that leaves the area more resistant to future oxidation.
The broader impact/commercial potential of this project will first be felt in terms of enhanced safety and reliability of high-temperature Ceramic Matrix Composite components used, for example, in jet engines, gas turbines, or nuclear power plants. The proposed nanocoating technology is made possible by a new fiber manufacturing platform technology, which allows a single process to accomplish what currently requires two separate, very expensive, processes. The cost of Silicon Carbide (SiC) fiber-reinforced CMCs can therefore be reduced while delivering drastically improved quality. Indeed, the proposed technology finally makes possible the introduction of affordable high-quality SiC fibers to our target markets. These markets include military and aerospace (turbo machinery, rockets, advanced structures), automobile, energy and other industries that require advanced materials with exceptional strength, stiffness, heat resistance, and/ or chemical resistance. SiC fibers alone, even without considering this coating, represent a fast-growing market with great potential, the collective size of which exceeds $2 billion. The technology enabling the proposed interphase coating has an energy footprint that is 1/1000th that of competing methods and which produces a fraction of the associated waste and emissions as well. This provides a huge cost advantage as well as a dramatic improvement in quality and performance.
Achieving next generation high energy efficiencies for jet engines, with a concomitant CO2 emission reduction, will require materials that are capable of operating at temperatures that are outside the range of advanced metals, including superalloys found in today’s engines. The only known class of materials capable of surviving the extreme environment of future high-efficiency power generation is ceramics. Silicon Carbide has emerged as the ideal ceramic material for these applications. Like other ceramics, Silicon Carbide is strong, lightweight, and resistant to harsh environments; but it is also very brittle and to overcome this limitation it must be turned into an engineered composite material known as "Ceramic Matrix Composites." Like other, more familiar composite materials found in today’s consumer products, Ceramic Matrix Composites are composed of fibers reinforcing a matrix – in this case both fibers and matrix are composed of Silicon Carbide. Unlike other composite materials, however, Ceramic Matrix Composites have unique requirements that are only beginning to be met, and still at an extraordinary cost. High-temperature capable Silicon Carbide fibers used to reinforce Silicon Carbide matrix are still extremely expensive and very hard to come by. Once procured, Silicon Carbide fibers cannot be used directly but must undergo processing before they can be used in Ceramic Matrix Composites. In particular they must be coated with a thin layer of carbon or, preferably Boron Nitride. This is again an extremely costly operation that is currently performed in-situ, just prior to embedding the fibers in a Silicon Carbide matrix. The current in-situ coating process is expensive and results in imperfect coatings that can give rise to hard-to-detect flaws in the resulting Ceramic Matrix Composite material. Ceramic Matrix Composite manufacturers have long recognized the value of procuring Silicon Carbide fibers already coated with Carbon or Boron Nitride, in fact this is a feature that may represent up to a factor of 5 multiple to the value of Silicon Carbide fibers. Despite the recognition of the value, such pre-coated fibers have not been commercially available for lack of a manufacturing process capable of adequately coating commercial Silicon Carbide fibers without damaging them. The innovation supported under the present NSF SBIR phase I award stems from Free Form Fibers’ novel approach to the production of Silicon Carbide fibers, which is derived from additive manufacturing. As shown in Figure 1, Silicon Carbide fibers are effectively laser-printed and form an evenly distributed ribbon of parallel thin filaments. This unique ribbon architecture lends itself readily to coating, again by additive manufacturing methods. Figure 2 shows the result of a direct laser-write of an optically thin Boron Nitride layer onto a Silicon Carbide filament. In fact, this approach is extremely effective compared to current in-situ coating processes and far more flexible. In fact, it is so flexible that it is reasonable to envision engineering the coating’s structure and composition to expand its function and performance. The case in point under the present award was to engineer a coating that would behave like a biomimetic immune system capable of responding to mechanical and oxidation damage with an increased defense again oxidation and the buildup of a slight reinforcing "scar tissue". The present award was intended to test the capability of the direct laser-write process to coat fibers with the required level of flexibility to achieve the stated design objective. Figure 2 shows exactly such a structure. It points to the feasibility of the proposed approach, but will require additional work to actually the intended biomimetic design. At the conclusion of this award a major new capability has been demonstrated. The ability to coat Silicon Carbide fibers with a layer of Boron Nitride and/or Carbon while they are being produced represents a significant leap from the state of the art. It will eliminate the need for in-situ coating of Silicon Carbide fibers during the fabrication of Ceramic Matrix Composites, which currently represent about 20% of the production costs.
