Fiber reinforced composites have great potential for use in aircraft and gas turbines. This is due to the high strength to weight ratio of such materials because fibers, such as silicon carbide fibers, provide toughness. The strength, toughness, and other properties of silicon carbide fibers are determined by their microstructure which is controlled by the manner in which they are made.
Polycrystalline fibers can be prepared by extrusion and spinning of plasticised mixtures of powder with subsequent sintering. The spinning step, which extrudes the fibers, causes the fibers to exhibit a rough surface. During the deformation, particles in the fiber are displaced, causing the formation of pits and asperities. The particles often have an appreciable aspect ratio which due to rotation results in the formation of pits. Also, the elongated particles tend to orient themselves by shear forces with their long axis at a small angle to the axis of the fiber which leads to the formation of bumps.
The surface roughness generated during the spinning operation is essentially preserved in sintering because of the short firing cycle applied to the fiber. There is little or no smoothing of the fiber surface by mass transport. Consequently, the surface pits are sharp edged to their root. This makes the surface pits on the fibers undesirable stress risers which limit tensile strength.
Surface roughness degrades fiber properties in at least two ways. Pits and uneven surfaces limit the silicon carbide fiber strength. The surface defects also inhibit the sliding of fiber surfaces pass each other which is required in the manipulation of fiber strands. Asperities bring about high friction and the fiber breaks.
There is a need to provide a silicon carbide fiber having a surface with reduced pits and asperities and improved strength.