Nicalon-SiC fibers were coated with C, SiC at 1200°C and 1300°C by chemical vapor deposition to improve the thermal stability of the fibers and to modify fiber-matrix interface. A coating layer having thickness of 0.1-25μm was obtained by controlling the CVD conditions. According to the AES and SEM analyses, a carbon rich layer (0.1-0.5μm) was deposited on the fiber surface initially. Nuclei of silicon carbide were formed on the surface of the carbon rich layer and finally whiskers or domed grains of silicon carbide were grown. The deposition rates of the carbon rich layer and silicon carbide layer were about 0.02μm/min and 0.65μm/min, respectively. It was observed by XRD and TEM that domed grains, which were the mixtures of β-SiC and small amount of α-SiC, consisted of subgrains and contained stacking faults in high density.
The effects of CVD conditions, thickness and composition of CVD-C, SiC coating on the mechanical properties, microstructure and thermal stability of Nicalon-SiC fibers were studied. Tensile strength of fibers coated at 1300°C was lower than that of as-received fiber, because large flaws were formed on the fiber surface during the heating stage of the CVD process. When a fiber was treated at elevated temperatures, the coating layer acted as an effective fiber-coating interfacial diffusion barrier, and resisted the gas (CO, SiO) evolution from the fiber surface, which improved thermal stability of coated fibers. Thicker coating layer was more effective to prevent decomposition of Nicalon fiber at higher temperatures. The fibers coated at 1200°C possessed high values of strength, though it decreased due to the interface reaction in the heat treatment process.
The effect of fiber coating on the residual thermal stresses, interface, and mechanical properties of CVD-(C, SiC) coated Nicalon fiber-reinforced alumina matrix composite were studied. Using a tri-cylindrical model, the residual thermal stresses caused by the mismatch of their thermal expansion coefficients and the mechanical behavior were calculated. The results indicated that when the fibers coated with carbon, which has lower Young's modulus, the residual thermal stresses at fiber-coating interface and coating-matrix interface were reduced compared with uncoated fiber reinforced composite materials, and these stresses decrease with increase in the thickness of carbon coating. In the CVD-(C, SiC) coated fiber-reinforced alumina composite, the reduction of the residual interfacial stress decreased the shear strength required for fiber pullout, so that the work of fracture was increased compared with that of uncoated Nicalon fiber-reinforced alumina composite.
