Fiber Composition

FFF fabricates high purity fibers as a consequence of the gas decomposition process associated with LCVD. This yields material with no measurable property-robbing contaminants. For example, oxygen in polymer-derived silicon carbide fibers reduces the operating temperature of the resulting CMC parts by hundreds of degrees, thereby negatively impacting the very performance metric sought in the first place for SiC-based CMCs. SiC fiber produced via LCVD has no measurable oxygen, which means that the full temperature capability of the material is available to the designer. LCVD-derived SiC fibers subjected to long term high temperature exposure show little degradation in mechanical properties, whereas polymeric precursor-derived fibers subjected to the same exposure are typically either simply destroyed (vaporized) or so degraded that their mechanical properties render them useless as structural elements in CMCs.

Crystalline fibers produced by the LCVD process exhibit very interesting crystal properties. There is typically a region in the center of the fiber with elongated grain structure, with the grains becoming more equiaxed as you look further out towards the periphery. In some cases the morphology becomes amorphous out near the fiber periphery. In still other cases, it is possible to produce amorphous fiber with no measurable crystal structure at all. These microstructures are controllable to a large extent via LCVD process parameters.

The LCVD process also makes possible fibers that have simply never been available before. For example, there has been no polymeric precursor approach to the manufacture of boron fiber. This material has double the strength-to-weight ratio of carbon, which means that the weight of composite in a composite airframe such as the Boeing 787 Dream liner – which is on the order of 60,000 lbs – can be reduced by about 30,000 lbs and still have the same strength. This in turn creates enormous value for the operator of the airplane, either by enabling extra range or additional loading capacity or less fuel burn or a combination of all these. There is similar value to be had for “light-weighting” launch vehicles and satellites.

Finally, the LCVD process enables exquisite control over any number of elements in a fiber, opening up a vast design space for fine-tuning fiber performance in terms of tensile strength, temperature performance, creep resistance and many other performance properties, all while preventing the addition of performance-robbing elements that are often associated with polymeric precursor derived fibers.

 

 

 

 

 

 

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The LCVD-produced silicon carbide reveals several advantageous characteristics:

  • 3C (face-centered cubic) polytype crystalline structure
  • high density / very little porosity in the fiber cross-section Scanning electron microscope images of non-porous FFF silicon carbide fiber cross-section
  • (A) no oxygen contamination residual in the fiber
    (B) no secondary materials present—only Si and C
  • (A) nano-crystalline grain structure
    (B) varied grain structure across the fiber cross-section — elongated crystalline grains at the center, equiaxed grains towards the edge, amorphous structure at the fiber surface
    Transmission electron microscope composite image of non-porous FFF SiC fiber cross-section courtesy of the Air Force Research Laboratory, Materials and Manufacturing Directorate