One-dimensional Nanomaterial-reinforced Carbide Ceramic Matrix Composites

On the one hand, since the rapid development of aerospace techniques, the advanced structural materials are required to have better performance in hightemperature and high-load corrosive environment. On the other hand, time-consuming treatment is a main disadvantage in the chemical vapor infiltration(CVI) process for the preparation of continuous fiber-reinforced carbide ceramic matrix composites(CFCCMCs), which makes the composites high-cost. Therefore, improving the mechanical properties of these materials and shortening their CVI preparation period are the key issues for the industrial application of CFCCMCs. One-dimensional nanomaterials, like carbon nanotubes(CNTs) and silicon nitride(Si3N4) nanowires, are considerated to be the ideal candidates for the matrix enhancement of carbide ceramic matrix composites, due to their outstanding mechanical properties. Moreover, the structure of fiber preform could be modified by the introduction of one-dimensional nanomaterials. As a result, the CVI process would be accelerated. However, there still exist many problems for the fabrication of one-dimensional nanomaterials reinforced ceramic matrix composites(CMCs), and the mechanical properties of these composites have not been significantly improved yet. Thus, systematic research is needed.In this thesis, CVI method was used to produce aligned carbon nanotube reinforeced silicon carbide(ACNT/SiC) and boron carbide(ACNT/B4C) CMCs. Mini and three-dimensional(3D) stitched carbon fiber reinforced silicon carbide(C/SiC) CMCs containing Si3N4 nanowires were also prepared by the CVI technique. The preparation technology, microstructure, interphase design, mechanical properties and oxidation resistance of these materials were studied systematically, which lays a solid theoretical and experimental foundation for the use of one-dimensional nanomaterials in CMCs. The main research contents and results are as follows:(1) High-quality ACNT/SiC composites were fabricated by CVI method using ACNT sheets as the preforms. The results show that: 1) The suitable process parameters can be tuned according to the diameters of the ACNT/SiC nanowires on the composite surfaces and the cross-section microstructure observations of ACNT/SiC sheets; 2) By using the CVI technique, most of the manufacturing problems previously encountered in the fabrication of CNT/CMCs composites are overcome, so that the ACNTs are well preserved and uniformly coverd by the amorphous and crack-free SiC matrix after the CVI process, and the obtained ACNT/SiC composites have a strongly bonded ACNT-matrix interface; 3) CNT pullout and bridging, and sequential breaking and slippage of the walls of the CNTs are the main reasons for the high mechanical properties of ACNT/SiC nanowires. With the increasing of infiltration time, the volume fraction of ACNT in the composite reduces, resulting in the decrease of bending properties of the composite nanowires. However, the average fracture strength of 30-min-infiltrated ACNT/SiC nanowires is still about an order of magnitude higher than that of the conventional bulk SiC; 4) The ACNT reinforcments inside the ACNT/SiC sheets are well preserved after the oxidization at 1600 °C in air.(2) The CVI technique was further used to fabricate ACNT/B4 C composites with exceptional mechanical properties. The results show that: 1) The ACNTs are well preserved and uniformly distributed in the composites after the CVI process. The obtained ACNT/B4 C composites have an amorphous, crack-free B4 C matrix and a strongly bonded ACNT-matrix interface; 2) The average fracuture strengthes of ACNT/ B4 C nanowires are about 1~2 orders of magnitude higher than those of the conventional B4 C ceramics and their composites; 3) From 700 to 900 °C, a compact B2O3 glass layer is formed on the surface of the composites via the oxidation of external B4 C matrix, protecting the internal ACNTs from oxidation.(3) Fabrication of pyrolytic carbon(PyC) interphase in ACT/B4 C comopsites. PyC was firstly deposited into ACNT sheet preforms using CVI method to form ACNT/PyC composites. Then, B4 C layer was deposited onto the ACNT/PyC composites to form ACNT/PyC/B4 C composites. The results show that: 1) The PyC interphase with smooth surface and uniform, controllable thickness can be obtained on the surface of ACNTs via the optimiziation of process parameters; 2) The as-prepared PyC interphase is highly graphitized graphene layers which roll up into seamless coaxial cylinders encapsulating the original ACNTs; 3) The self-assembly of graphene layers on the ACNT surfaces is fulfilled by the alternative occurrence of graphene nucleation and growth; 4) The strength of graphene layers and the adhesion between the graphene layers and primary ACNTs are weak due to the existence of defects and lattice misorientations; 5) ACNT/PyC/B4 C composites exhibit significant stepped fracture during the failure, indicating that the introduction of PyC interphase improves the toughness of the composites.(4) Pure α-Si3N4 nanowires were in-situ fabricated on carbon fiber bundles and fabrics via catalyst-assisted pyrolysis of polymeric precursors. The results show that: 1) The obtained α-Si3N4 nanowires are uniformly distributed and randomly oriented on carbon fibers with the length of several hundred micrometers. The nanowire surfaces are very smooth, and there are no particles on their tops; 2) The formation of Si3N4 nanowires obeys a solid-liquid-gas-solid growth mechanism; 3) A designable loose network is built by the nanowires on the surface of carbon fibers, and its thickness and nanowire density can be controlled by adjusting the process parameters.(5) Carbon fiber bundles covered by Si3N4 nanowires were densified using CVI technique to form Mini-C/SiC composites reinforced by Si3N4 nanowires(Mini-C/SiCSi3N4-NW). Two types of Mini-C/SiC-Si3N4-NW with different densities of Si3N4 nanowires were prepared. Compared with the conventional mini-C/SiC composites without nanowires, it was found that: 1) Due to the introduction of Si3N4 nanowires, rapid densification of mini-composites is achieved, and their mechanical properties are improved; 2) The density of Si3N4 nanowires and the volume fraction of carbon fibers play important roles in the matrix strength and fracture strength of mini-composites, respectively; 3) The density of Si3N4 nanowires should not be too high in order to avoid the high porosity which is harmful for the mechanical properties of mini-composites.(6) Two types of 3D stitched C/SiC composites containing Si3N4 nanowires(C/SiC-NW) were fabricated using the CVI method. The volume fractions of Si3N4 nanowires were different for these two kinds of C/SiC-NW, due to the differences between their 3D stitched preforms. Compared with the conventional 3D stitched C/SiC composites without nanowires, it was found that: 1) Rapid densification, increased density, reinforced SiC matrix, and improved mechanical properties for the composites can be achieved by the introduction of Si3N4 nanowires; 2) It is necessary to properly design the preforms in order to avoid the decreases in the mechanical properties and densification degree of composites caused by the excessive addition of Si3N4 nanowires.