本科毕业设计（论文）
外文翻译
A modified dip-coating method to prepare BN coating on SiC fiber by introducing the sol–gel process
作者：Jianggao Liu ，Shubin Wang,Pengyang Li，Mengjie Feng，XinwangYang
国籍：China
出处：a School of Materials Science and Engineering, Beihang University, Beijing 100191, China
b Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
原文正文：Abstract：Boron nitride (BN) coatings were synthesized on silicon carbide (SiC) fibers by a modified dip-coating methodusing boric acid and urea as BN precursors and acrylamide as gel-casting monomer heated at 850 °C for 3 h under N2 atmosphere. For comparison, a conventional dip-coating method was also adopted to prepare BN coatings under the same conditions. The structure of BN coatings was characterized by X-ray diffraction, Fouriertransform infrared spectroscopy and transmission electron microscopy. The morphology of the fibers after im-pregnation and heat treatment was observed by the electron micrographs. Fiber tensile strength was evaluatedby single fiber tensile test. The effects of polyacrylamide on the formation of BN coatings were also investigated. The results show that the BN coatings, prepared by two cycles of impregnation in the modified dip-coating process, covered the SiC fibers well, and exhibited a compact turbostratic–amorphous hybrid structure with a thick-ness of 550 nm. The tensile strength of the coated SiC fibers increased by ~22% after the conventional dip-coating method was modified. The introduction of the sol–gel process enabled the BN precursors to form a film on the surface of SiC fibers with an even distribution in the impregnation process and to retain the evenness in the heat treatment process. Based on the two effects, the conventional dip-coating method was improved.
Keywords:Boron nitride，Fiber coatings，Dip-coating，Sol–gel，Polyacrylamide
1. Introduction
Silicon carbide (SiC) fibers are widely used as reinforcement in ce-ramics matrix composites (CMCs) because of their excellent properties, such as high strength, high modulus, high temperature resistance, high oxidation resistance, and high chemical corrosion resistance [1]. However, under high temperature preparation, these fibers easily form a strong adhesion with ceramic matrix, leading to a brittle composite with poor mechanical properties [2]. Thus, an interphase is necessary to make the fiber–matrix bonding weak enough to allow crack deflection along the interface, yet strong enough to transfer load from the matrix to the fibers [3]. Numerous studies have shown that pyrolytic carbon (PyC) [4–8] and hexagonal boron nitride (h-BN) [9–13] can be used to solve the interface problem in CMCs reinforced with SiC fibers. However, h-BN is reported to be a more suitable SiC fiber coating than PyC for the reason that h-BN has higher antioxidative property [14].Currently, two methods are mainly adopted to prepare h-BN coatings on fibers, namely, chemical vapor deposition (CVD) method [15–17] and dip-coating method [18–20]. Compared with the CVD method, the dip-coating method is simple in operation, inexpensive and able to obtain coatings on both surface and interior fibers of fabrics, which are widely used in CMCs [21]. However, to get further applications, the dip-coating method still needs to be improved in the aspects of controllability, uniformity and integrity. In our previous work, two key deficiencies were found in the conventional dip-coating method limiting the uniformity and integrity of BN coatings. One deficiency is that the precipitation of BN precursors from the residual solution on the surface of impregnated fibers is such a random process that BN precursors can hardly be evenly distributed. The other one is that the existence of gravity and capillary force greatly weakens the ability of the liquid intermediate products, formed in the heat treatment process, to cover the fibers completely and evenly. As a result, poor-quality BN coatings are formed with many elongated defects, which were also reported by other researchers [18,19].
To overcome the two deficiencies, the sol–gel process was considerably adopted in this study. Ease of controlling the homogeneity and physical characteristics during synthesis steps and the ability to form coating materials are the most suitable advantages to overcome the two deficiencies. Besides, the sol–gel method can be easily introduced into the dipping process. As for the gel-casting monomer used in a sol–gel method, acrylamide is inexpensive and its gel-casting process is fast reproducible and easily scaled up [22]. Furthermore, acrylamide has been used in preparing h-BN particles [23]. Therefore, acrylamide is chosen as the gel-casting monomer for the modified dip-coating method.
In this study, BN was coated onto SiC fibers via a modified dip-coating method by introducing the sol–gel method into the dipping process with acrylamide as the gel-casting monomer. The effects of the sol–gel process on the morphology and chemical composition of the fiber coatings and the tensile strength of the SiC fibers were investigated.
2. Experiment
2.1. Sizing removal of SiC fiber
The KD-I SiC fiber bundles (provided by National University of Defense Technology, China) were used as samples for dip coating in this study. These fibers were obtained with sizing; hence, surface treatment was required for sizing removal. The as-received fibers were cleaned in acetone solution for 24 h. Through this pr
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