In-situ Grown Cnts For Sic_f/sic Composites

SiC fiber reinforced SiC matrix composites (SiC_f/SiC) have been proposed as promising high temperature structural material for a variety of advanced applications in aeronautic & aerospace and fusion systems, but their mechanical performance still needs to be improved to satisfy those application requirements. Carbon nanotubes (CNTs) were incorporated in the matrices to change the surface microstructure of SiC fiber and ameliorate the interfacial bonding between SiC fiber and SiC matrix, as well as acting as a bridge to secondly reinforce SiC_f/SiC composites. Unfortunately, the agglomeration problem of CNTs poses obstacles to realizing the potential benefits, thus CNTs–SiC_f/SiC composites were successfully fabricated by growing CNTs directly in the composite performs followed by PIP routes densification for the first time. In this work, the approach to load catalyst nanoparticles into the fabric was studied; then the technics and mechanism of CNTs growth in the CVD process were investigated; finally the effort of in–situ grown CNTs introduction on the mechanical properties of the CNTs–SiC_f/SiC composite was studied, and some reinforcement mechanisms of CNTs were also discussed. The main results have been obtained:The agglutinant over the fibers could be wiped off by calcining in air at 400℃, which was convenient for catalysts loading and CNTs growth. The influence of different catalysts, preparation methods and the concentration of precursor on the CNTs growth were investigated, and the experimental results showed that the suitable catalyst particles could be produced by vacuum–impregnation process, with a Ni–La–Al precursor solution concentration of 0.1 mol/L.In CVD process for growing CNTs, different CNTs were produced with the changing of the C₂H₂/H₂/N₂ ratio, reaction temperature and time. N₂ and H₂, which served as carder gas, controlled the reaction course and played an etching role in the process, and the growth time was prolonged as a result of H₂ intruction which could reduce the poisoned catalyst particles; the C₂H₂ flux determined the yield of raw CNTs and reaction temperature affected the yield and morphologies of CNTs by changing the activity of catalyst. The yield of CNTs increased with reaction time and decelerated after a certain time. The optimized parameters for CNTs growth were: reaction temperature at 750℃, reaction time of 60 min, the C₂H₂/H₂/N₂ ratio at 1/1/3 and the C₂H₂ flux at 100–150 sccm.The CNTs were grown following the top–model and the motive force was supposed to be from the grads of temperature and carbon concentration between the front and back of catalysts particles. During the CVD progress, most catalyst occupied the condition that the solid liquid coexisted and the different diffusion rate of carbon atoms in the surface and inside of catalyst particle lead to the bamboo–like structure of CNTs. The anisotropy of catalyst particles, the van der Waals bonding among CNTs and the flux inside the fabric during the growth resulted in the"forest–like"CNTs over the SiC fiber.The mechanical properties of SiC_f/SiC composites were elevated as a result of the introduction of in–situ grown CNTs. Flexural strength, flexural modulus and fracture toughness raised at the ratio of 16.3%, 90.4% and 106.3% when the volume fraction of CNTs was 5.21%, respectively. The composite was reinforced by mechanisms such as CNTs pullout, crack propagate suffocation and crack deflexion. However, the introduction of CNTs decreased the efficiency of impregnation during the PIP routes, which lead to a high volume of micropores in the matrix, resulting in a limited improvement in flexural strength. The CNT/SiC fiber joint was generally so weak that the majority of the CNTs were fractured at the joints when the matrix began to crack, which restricted the effect of CNTs reinforcement.