Different thicknesses of smooth and uniform carbon coatings composed of disordered carbon and micro-crystallite graphite were prepared on KD-II silicon carbide (SiC) fibres by atmospheric pressure chemical vapour deposition. The effects of carbon coatings on the mechanical property and thermal stability of SiC fibres were investigated. Carbon coatings have both favourable and unfavourable influences on SiC fibres strength. The fibres with up to 0.3 µm carbon coatings showed higher strength than the uncoated fibres due to surface flaw healing effect of the thin coatings, in contrast, the 1 µm carbon-coated fibres showed lower strength than uncoated fibres owing to effect of soft nature of the thick coatings. Furthermore, the carbon coatings effectively improved the thermal stability of SiC fibres by suppressing thermal decomposition of SiCXOY and SiCXOYNZ phase.

KD-II SiC fibers, a new type of SiC fibers, were heat treated at elevated temperatures for 1h in an Ar atmosphere. The microstructure and mechanical properties of the fibers before and after annealing were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and tensile tests, et al. The results show that the as-received fibers consisted of β-SiC nano-crystals, free carbon and a small amount of amorphous intergranular SiCxOy phase. With increasing heat treatment temperature, significant growth of β-SiC grains, ordering of free carbon and decomposition of SiCxOy phase occurred and the fiber surfaces became coarser with development of new defects, such as pores and large SiC particles. It was also found that KD-II SiC fibers maintained high strength with heat treatment at temperatures up to 1500°C, exceeding which a sharp strength degradation was observed. The strength degradation and microstructural evolution of the fibers due to heat treatment were correlated, and it can be concluded that the β-SiC grains growth, residual tensile stresses as well as surface flaws were the dominating factors for the fiber strength degradation after exposure at high temperatures.

To prepare SiC fibers with different free carbon contents, polycarbosilane (PCS) fibers cured with unsaturated hydrocarbons were pyrolyzed at 1000 degrees C under controlled hydrogen/nitrogen atmosphere and subsequently heat treated at 1500 degrees C under nitrogen atmosphere. The process of carbon removal during pyrolysis was investigated using chemical elemental analysis, FTIR, and AES analysis. The microstructure and properties were examined by SEM, TEM, XRD, density measurements, tensile tests and resistivity measurements. The results show that the carbon content in SiC fibers decreases with H-2 concentration increasing. The hydrogen atmosphere suppresses H2 evolution and helps to remove excess carbon as CH4 during pyrolysis. Although thin carbon-enriched films are present on the fiber surfaces, the distribution of silicon and carbon is uniform in the fiber cores. The microstructure and properties of the resulting SiC fibers are very dependent on their C/Si chemical compositions. The beta-SiC grain size increases with a decrease in the carbon content because the excess carbon aggregates at the grain boundary and impedes the grain growth. Moreover, the removal of free carbon also results in fiber densification, decrease of porosity and improvement of fiber specific resistivity, tensile strength and tensile modulus. Therefore, the nearly stoichiometric SiC fiber has good comprehensive performance.

To provide oxidation protection and/or to act as an interfacial coating, titanium oxide (TiO2) coatings were deposited on KD-II SiC fibers by employing atomic layer deposition (ALD) technique with tetrakis(dimethylamido)titanium (TDMAT) and water (H2O) as precursors. The average deposition rate was about 0.08 nm per cycle, and the prepared coatings were smooth, uniform and conformal, shielding the fibers entirely. The as-deposited coatings were amorphous regardless of the coating thickness, and changed to anatase and rutile crystal phase after annealing at 600 degrees C and 1000 degrees C, respectively. The oxidation measurement suggests that the TiO2 coating enhanced the oxidation resistance of SiC fibers obviously. SiC fibers coated with a 70-nm-thick TiO2 layer retained a relatively high tensile strength of 1.66 GPa even after exposition to air at 1400 degrees C for 1 h, and thick silica layer was not observed. In contrast, uncoated SiC fibers were oxidized dramatically through the same oxidation treatment, covered with a macro-cracked thick silica film, and the tensile strength was not measurable due to interfilament adhesion. The above results indicate that TiO2 films deposited by ALD are a promising oxidation resistance coating for SiC fibers. (C) 2016 Elsevier B.V. All rights reserved.

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N2 at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N2. In particular, the fibers heat treated in N2 at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC x O y N z and SiC x O y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N2 on the decomposition of the SiC x O y N z phase in the fiber surfaces and nitrided case on the decomposition of the SiC x O y phase in the fiber cores, led to higher SiC fiber temperature stability in N2 rather than Ar.

Three low oxygen-containing SiC fibers with different surface compositions were heat-treated at high temperature in argon, and large SiC particles were observed on the surfaces of SiC fibers with oxygen-containing surface layers. The SiC particles were identified as single crystal β-SiC. The growth process, formation mechanism of these large SiC grains and their influence on fiber strength were investigated. Nano-sized SiC particles were precipitated from the decomposition of SiCxOy in the fiber surface that acted as nuclei, and these nuclei grew gradually to form large grains because of the gaseous reaction of SiO and CO. The formation of large SiC grains was related closely to the SiC fiber surface composition, which affected SiC nucleation and SiO diffusion from the core to the fiber surface. The large grains influenced the fiber strength degradation significantly and should be avoided to retain high fiber strength.

A liquid polycarbosilane with vinyl groups was synthesized via Grignard coupling reaction and the subsequent reduction reaction by the core dilution/slow addition technique, with 1,1,3,3-tetrachloro-1,3-disilabutane as the core molecules and chloromethyltrichlorosilane, vinyl-magnesium chloride as the monomers. The as-synthesized VHPCS and its ceramic product were investigated by means of FT-W, NMR, GPC, TG XRD and SEM. Results show that VHPCS possesses a hyperbranched structure and could convert to non-porous ceramic product after pyrolysis. The ceramic yield of cross-linked VHPCS at 1000 degrees C in N-2 atmosphere is 85.9wt%, and beta-SiC crystallite appeares in the ceramic product at 1200 degrees C.

As a polymer precursor of SiC ceramics,liquid hyperbranched polycarbosilane has drawn a great attention for its excellent fluidity,self-cross-linking,high ceramic yield and near-stoichiometric pyrolysis product.According to the characteristics of liquid hyperbranched polycarbosilane,the preparation methods are reviewed,including the ring-opening polymerization,Grignard coupling method,Wurtz coupling method and hydrosilysition reaction.In addition,the research progress of its application in silicon carbide based ceramic composites,silicon carbide fiber and silicon carbide membrane is summarized.Finally,the prospect of further research,large-scale synthesis and modification of liquid hyperbranched polycarbosilane,is also addressed.