Carbon fiber reinforced carbon composites (Cf/C) have low density, high thermal conductivity, low coefficient of thermal expansion, good thermal shock resistance at high temperatures, and excellent wear resistance. They are considered as rocket shields, One of the candidates for nozzles and brake pads for spacecraft. At the same time, due to its lower neutron activity, it also has broad application prospects in structural materials for nuclear fusion/fission stacks. In practical applications, the shape of Cf/C composites is usually more complicated and the size is larger, but carbon fibers are harder and more brittle and the processing of Cf/C composites is more difficult. The use of smaller size composites to connect devices with larger and complex shapes is one of the ways to solve the difficult processing problems of Cf/C composites. At present, the connection of Cf/C composite material is mainly mechanical connection, or the connection is made with a metal material as a welding layer. However, as the application of structural materials in rocket nozzles or nuclear reactors, it is difficult to overcome the disadvantages of poor sealing of mechanical connections and poor resistance to high temperature and corrosion of metal welding layers. Titanium-silicon carbide (TiSSiC2, TSC) has excellent high-temperature and corrosion-resistant properties, and is quasi-plastic at high temperatures. It is one of candidate materials for Cf/C composite welding layers applied under high temperature conditions. It has been reported in the literature that TSC is used as the welding layer to connect Cf/C composites, but they all adopt the traditional high-temperature hot-press sintering process. This method has a high welding temperature (1600°C) and will inevitably destroy Cf/ during the welding process. The fiber structure in the C composite material made the Cf/C composite ineffective. Electric field assisted sintering technology (FAST) is an effective method for sintering high-density dense ceramics at low temperatures, and has been widely used in ultra-high temperature ceramic sintering and other fields. Recently, the Laboratory of Special Fibers and Nuclear Energy Materials of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences successfully applied FAST technology to the silicon carbide ceramics connection field, using a 60μm thick titanium-silicon-carbon cast film as the welding layer at 1300°C. The connection of SiC ceramics was successfully achieved at low temperature, the four-point bending strength was about 80.4 MPa, and the entire connection process took only 15 min. Studies have shown that the welding temperature has an important influence on the phase composition and fracture mechanism of the interface. In the low temperature range of ~1300°C, the strength of the connecting layer and interface is high, and the fracture occurs on the silicon carbide substrate. When the welding temperature is 1400 ~ 1500 °C, the connecting layer is partially decomposed, and the bridging phase such as TiSi is formed in the connecting layer. The fracture mechanism is complex, that is, part occurs at the interface and some breaks at the connecting layer. In the high temperature range ~1600 °C, the high power current density makes the Si in the connecting layer Ti3SiC2 migrate toward the interface and enrich, resulting in the interface becoming the weakest link, and the fracture mechanism is the interface type. Therefore, the conductivity of SiC ceramic increases at high temperatures, and part of the current can pass directly through the interface between SiC ceramic and the solder layer. The presence of high-power current density promotes interface electromigration and element diffusion, and generates a large amount of Joule heat at the interface. One of the main reasons for achieving SiC ceramic connections at low temperatures. This part of the research results has been published in Journal of Nuclear Materials 466 (2015): 322-327. However, silicon carbide materials are poorly conductive at low temperatures and require the use of graphite molds when connected using FAST soldering techniques. For Cf/C composites with good electrical conductivity, direct current flow can be achieved without the need for graphite die-assisted heating, allowing high power density currents to fully function. For the first time, the special fiber and nuclear energy materials engineering laboratory used the electric field assisted technology to use a 60μm thick titanium-silicon carbon cast film as the welding layer, and successfully achieved the connection of the Cf/C composite material at a low temperature of 1200°C. The entire connection process was only required. In 12 min, the shear strength of the Cf/C composite connector reaches 26.3±1.7 MPa. Studies have shown that welding temperature and holding time have important influence on the phase composition and fracture mechanism of the interface. Under the condition of high power current density and high temperature, C in Cf/C matrix diffuses into Ti3SiC2, and Si(g) in Ti3SiC2 is promoted to the interface, and Ti3SiC2 and matrix Cf/C in Ti3SiC2. The interface is enriched and reacted in-situ to produce a dense 1-2 μm thick SiC layer. The SiC transition layer formed by the in-situ reaction on the one hand prevents the C atoms in the Cf/C composite material from further diffusing to the Ti3SiC2 solder layer and inhibits further decomposition of the tie layer Ti3SiC2. At the same time, the thermal expansion coefficient of SiC is between graphite and titanium silicon carbide, which alleviates the thermal mismatch between the connecting layer and the substrate. Based on the study of microstructures and fracture modes at the connection interface, the Laboratory of Specialty Fibers and Nuclear Energy Materials Engineering proposes electric field assisted technology for connecting carbon/carbon composite materials mainly through the following stages: (I) Interfacial densification stage: During the low temperature stage (<1100°C), the densification of the connecting layer Ti3SiC2 mainly eliminates the internal pores. The atomic diffusion and chemical reaction at the interface are limited at this stage, and the sintering densification behavior of the tie layer Ti3SiC2 is only under the effect of axial pressure. Obviously, the connectors have almost no shear strength at this stage. (II) Interfacial reaction phase: In the middle temperature stage (1200~1300°C), the welding layer Ti3SiC2 further sinters, and under the action of high temperature and pressure, the Ti3SiC2 phase undergoes plastic deformation and further fills the surface defects of Cf/C. At the same time, under the action of current, C atoms in Cf/C diffuse to the C-vacancy of TiC0.67 in Ti3SiC2, Si atoms in Ti3SiC2 migrate to Cf/C, and react in situ at the interface to form SiC transition layer. At this time, if the interface reaction can be controlled and the Ti-Si brittle phase formed by the decomposition of Ti3SiC2 can be excluded, a good connection interface can be obtained. (III) Interface degradation stage: At high temperature (~1400°C) and long connection time, a large amount of C diffuses from the Cf/C substrate toward the interface, while Si in Ti3SiC2 migrates to the interface until it is completely consumed. This results in the destruction of the Cf/C matrix and even the fibers, whereas Ti3SiC2 is completely decomposed into the SiC, TiC and Ti-Si brittle phases. Therefore, the interface begins to degrade at this time, causing a large number of microcracks and defects in parallel, resulting in a decrease in shear strength. Therefore, by controlling the electric field-assisted connection temperature and connection time, the interfacial reaction between the connection layer and Cf/C and the interface phase composition can be controlled, thereby realizing the low-temperature and fast connection of Cf/C. The research results will provide experimental and theoretical support for the connection of aerospace and nuclear ceramic matrix composites and have been published in Carbon 102 (2016): 106-115. The study was supported by the National Natural Science Foundation of China (NO.91226202 and NO.91426304).
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