Evaluation of the effect of PyC coating thickness on the mechanical properties of T700 carbon fiber tows

Nanostructured uniform pyrolytic carbon (PyC) coatings with different thicknesses were prepared on surface of T700 carbon fiber by chemical vapor deposition (CVD). The effect of coating on tensile strengths of the carbon fibers was studied by tow tensile strength test. The morphological change, roughness, microstructures, and thermal stability of carbon fibers with PyC coating (PyC-Cf) were investigated by SEM, AFM, XRD, Raman, and Tg. It was found that with increase of the deposition time, the thickness of coating varied from 100 to 320 nm with increase in coating surface roughness. The coating adhered well with the carbon fiber substrates and the coating thickness is uniform. The oxidation resistance of PyC-Cf was significantly higher than uncoated carbon fibers. However, the fiber became brittle after the coating and the coating is very sensitive to tensile stresses, which decreased the tensile strength of the fiber tows. The tensile strength measurement revealed that the carbon fiber with a 100 nm thickness of PyC coating maintained ∼69.4% of its original strength, and the 320 nm thickness of PyC-coated fibers showed the lowest strength because of the embrittlement caused by the PyC coating.