Effect of silane grafted h-BN fillers on microstructure and mechanical properties of CVI-based C/C-BN composites

Abstract

2.5D carbon fiber (Cf)-reinforced carbon and boron nitride dual matrix composites (C/C-BN) were prepared by a combined process of C/C silanization, hexagonal boron nitride (h-BN) slurry impregnation, and chemical vapor infiltration (CVI). The surface morphology of Cf after each step of modification, the fracture surface and microstructure of the C/C-BN composites were systematically characterized by scanning electron microscopy. Through the hydrolysis of silane, a polysiloxane network was successfully formed inside the porous C/C composites, which provided more attachment sites for the subsequent grafting of h-BN filler and the deposition of pyrocarbon (PyC). The microstructure and mechanical properties of the C/C-BN composites containing three different of h-BN fillers content were studied in comparison with pure C/C composites. The TEM characterization of the h-BN filler/PyC interface confirmed that the different crystal planes of the h-BN grains affect the structure of the PyC deposited thereon. The results show that the uniformly distributed h-BN filler reduces the size, thermal shrinkage anisotropy, and texture of the PyC deposited around it, resulting in the disappearance of the interlayer cracks in the matrix and the enhancement of the cohesion of the composite. The tight physical bond at the interface is due to the radial compressive stress caused by the thermal mismatch between the fiber, matrix, and h-BN fillers. Therefore, as the h-BN fillers content increases, the flexural strengths, compressive strength of the C/C-BN composites increase, which are both greater than that of C/C composite, and the compression properties gradually tended to be isotropic. However, when the h-BN content in the C/C composite is excessive (such as the volume content of 15 vol%), the densification efficiency of the subsequent CVI is reduced. As a result, the compactness of the transition interface and mixed matrix composed of h-BN and PyC decreased, which led to the decrease of mechanical properties of the composites.