Hydrothermal Construction Of Carbon Layer On Carbon Fiber And Performance Optimization Of C_f/ZrB₂ Composite

Nowadays,novel military technology has gradually become the dominant factor in the battlefield.As an epochal near-space vehicle,the"hypersonic aircrafts"will play a vital role in the field of national defense.Ultra-high temperature ceramics（UHTCs）with excellent oxidation resistance are important thermal protection materials for hypersonic aircrafts.However,the extreme brittleness of UHTCs has been severely restricting its practical engineering applications.The combination of continuous carbon fibers（CFs）and ultra-high temperature ceramics is considered to be the most promising approach to enhance the toughness of composites.However,the applications of carbon fibers in UHTCs are limited owing to its low specific surface area,poor high temperature oxidation resistance and hydrophilicity.Therefore,the CFs should be appropriately coated by a function layer or protective layer to improve its properties.Due to the excellent designability and compatibility of carbon materials,it is possible to obtain carbon coated carbon fibers with desirable properties.Herein,our objective in this work is to construct uniform and controllable hydrothermal carbon coating（HTCC）on CFs via the Fe Cl₃-catalyzed hydrothermal carbonization（HTC）method with sustainable carbohydrates.A possible mechanism is proposed to disclose the growth process of HTCC.Moreover,the influence of HTC coating on the mechanical and oxidation properties of CF/ZrB₂-based composites is analyzed in detail.The fiber damage behavior has also been studied systematically to demonstrate the protecting role of the HTC coating.Combining with the renewable carbohydrate and the universal HTC synthesis route,the uniform and controllable HTCC on the CFs are successfully prepared for the first time.The thickness of the HTCC could be effectively controlled by the concentration of glucose to satisfy the requirements of diverse applications.If the glucose concentration was as low as 5 wt.%,the average thickness of the HTCC is about 70 nm.The thickness of HTCC is in the nano or sub-micron scale in which the concentration of glucose was lower than 15 wt.%.At higher concentrations,the HTCC shows a broad thickness distribution（micro-scale）.A possible mechanism is proposed to disclose the growth process of HTCC,in which the glucose would be transformed rapidly to 5-hydroxymethylfurfural（HMF）with the assist of Fe Cl₃,and then converted into carbonaceous structure to form HTCC.The HTCC acted as a critical role in regulating the interface strength and noticeably increased the specific capacitance of the carbon fibers.Slurry injection assisted by vibration technology is adopted during the preparation of high-performance fiber toughened ultra-high-temperature ceramics UHTC for the advantages of short production cycle and facility.The wettability of CF is significantly enhanced by the abundant oxygen-containing groups of the HTC coating.The surface energy of hydrothermal carbon coated carbon fiber（HTC/CF）is 43.1 m J/m²,increasing by around 4.6 times of CF（9.3 m J/m²）.The improvement of the fiber surface energy could efficiency optimize the injection process,ultimately contributing to the formation of the compact and uniform ceramic green body.The obtained composite generated from the HTC/CF fabric reveals the flexural strength and fracture toughness of 206±16 MPa and 6.14±0.91 MPa·m^1/2,which are evidently better than the product without HTC coating(the fracture toughness was 153±18 MPa and fracture toughness was 3.78±0.78MPa·m^1/2).Moreover,the HTC coating could act as a sacrifice layer to prevent the fibers from damage in oxidation environment.The influence of the thickness of the hydrothermal coating on the properties of materials with different fiber content is studied respectively.The mechanical properties of the CF/UHTC have been greatly improved after the introduction of HTC coating.If the fiber volume content was 21%,the flexural strength of the composites reaches the highest（264±48 MPa）when the thickness of HTC is about 360 nm.The composites with the fiber volume content of 8%would display the largest flexural strength（181±20 MPa）when the HTC coating is about 200nm.The regulatory mechanism of HTC coating on the interface between fiber and substrate is revealed.The bonding force between HTC and CF is 71 m N,which is much lower than that between CF and UHTC matrix（≥171 m N）.Therefore,the CF without coating would be bond tightly with the UHTC matrix during the preparation process.When the crack propagates from the matrix to the fiber surface,the crack would directly penetrate the fiber and lead a sudden breakage of the CF.When the HTC is introduced in the composites system,it could prevent the carbon fiber from reacting directly with matrix,and inhibit the performance degradation of carbon fibers,which enables the carbon fiber to fully exert its toughening effect.Although a strong interface would be formed between the coating and matrix,while the fibers also could pull out from the HTC coatings,which is attributed to the weak interface between fiber and coating.