A new gradient pore structure in porous SiC ceramics was fabricated by low pressure chemical vapor infiltration (LPCVI). Effects of deposition duration on the mechanical properties and permeability of porous SiC ceramics were investigated. Results demonstrated that pore diameter and shapes decreased from the surface to the interior along with LPCVI duration. Porous SiC ceramics with deposition duration of 160 h exhibited flexural strength of 48.05 MPa and fracture toughness of 1.30 MPa m1/2, where 221% and 189% improvements were obtained compared to porous SiC ceramics without LPCVI, due to CVI-SiC layer strengthening effect. Additionally, at the same gas velocity, pressure drop increase rate was faster due to apparent porosity and pore size change.

Si-B-C ceramic is an effective self-healing component to improve the oxidation resistance of C/SiC composites for long-life applications in aircraft and aerospace fields. To clarify protection mechanisms, oxidation behaviors and mechanisms of CVD Si-B-C ceramic in wet oxygen from 700 °C to 1400 °C were investigated in this work. Microstructure, bonds transformation and phase evolution of Si-B-C ceramic after oxidation were analyzed. The weight loss of Si-B-C ceramic increased linearly with time between 700 °C and 1000 °C, while it changed in a non-linear way above 1200 °C. The oxidation production of Si-B-C ceramic in wet oxygen is mainly composed of Si(OH)4, SiO2, H3BO3, B2O3 and B2O. The oxidation resistance of Si-B-C in wet oxygen is better than that of single-phase B4C ceramic because borosilicate glass is rapidly formed, which can effectively resist the wet oxygen. No fiber oxidation is found up to 1400 °C.

SiC nanowires were synthesized by LPCVI using different catalysts, and the influences of input gas ratio (α) and catalysts were investigated. The average diameter firstly decreased and then increased with increasing α. Under Ni-based catalysis, SiC nanowires were long and thin, and increased with increasing concentration; under Fe-based catalysis, they were short and thick, and the influence of concentration could be neglected. The growth of SiC nanowires was controlled by vapor-liquid-solid (VLS) growth mechanism and the liquid-solid interface between nanowire and metal droplet was the growth plane. At same concentration, the diameter grown under Ni-based catalyst decreased with decreasing diameter of catalyst droplet, while under Fe-based catalyst, the diameters were not affected by catalyst droplet because of the high concentration. SiC nanowires were synthesized in 2D C/SiC composites and could enhance the mechanical properties effectively because of energy consuming from the fracture, pulled up and debond of SiC nanowires.

In order to investigate fatigue behavior of 2D C/SiC composites modified with Si-B-C ceramic, different kinds of modified specimens were fabricated and fatigued at desired temperatures in static air. Fatigue lifetime and load-displacement hysteresis loop of C/(SiC-SiBC)m modified with Si-B-C matrix and SBS-C/SiC modified with Si-B-C coating were analyzed after fatigue respectively. Results show that Si-B-C ceramic can improve obviously fatigue lifetime of C/SiC composites in static air. Meanwhile, fatigue lifetime of sample modified with Si-B-C coating is superior to that of Si-B-C matrix. Quantity and formation rate of borosilicate glass formed from Si-B-C ceramic directly affect crack-healing and fatigue lifetime of specimens. Oxidation damage dominates for the specimens, and damage caused by fatigue stress accelerates the oxidation damage during fatigue process. The damage is controlled by oxidation of carbonaceous material under 1000 °C, and oxidant diffusion from borosilicate glass above 1000 °C.

Picosecond laser machining is an important modern technology for materials with high hardness. In this paper, micro-holes with several hundred micrometer diameter were drilled in SiC/SiC composite using picosecond laser, and the quality of the micro-holes under different machining parameters was investigated in detail. The results indicated that energy density and feeding speed had remarkable effect on the micro-hole quality. The roundness of the micro-holes on the laser entry side was rarely affected by energy density and feeding speed. However, the roundness of the micro-holes on the laser exit side and micro-hole diameters along processing direction were quite sensitive to the energy density and feeding speed. Feeding speed had little influence on the quality of drilling holes, except for more debris on the entry side with 11.2 μm/s feeding speed.

SiCf-CNTs/SiC composites with about 25–30 vol% carbon nanotubes were successfully prepared by vacuum filtration combined with chemical vapor infiltration method. Through this method, the problems of agglomeration and limited amount of introduced CNTs in SiC/SiC composites were solved. The microstructure, thermal conductivity, bending strength and fracture toughness of SiCf-CNTs/SiC were observed. Results showed that the composites were composited by 7 layers of SiCf/SiC and 6 layers of CNTs/SiC, each layer was bonded to another by CVI SiC quite well, which presented the morphology of laminated structure. SiCf-CNTs/SiC had outstanding thermal conductivity as 23.9 W/(m K) at room temperature which was 2.9 times higher than traditional SiC/SiC composites due to the extremely high thermal conductivity and large amount of CNTs. The bending strength and fracture toughness of SiCf-CNTs/SiC composites were 240±7 MPa and 14.1±0.5 MPa m1/2, respectively. The bending strength-displacement curve of SiCf-CNTs/SiC presented zigzag and multi-step-like drop of the strength after reaching the maximum value. The fracture morphologies showed that, cracks defected in the interface of SiCf/SiC matrix, inner SiCf/SiC layer/CNTs/SiC layer and there were pull out of both CNTs and SiC fibers in SiCf-CNTs/SiC composites.

•SiBN coating is deposited by low pressure chemical vapor deposition.•The deposition temperature is important on deposition rate, morphologies, chemical composition and bonding states.•SiBN coating shows an amorphous phase.•The deposition reactions were mainly controlled by BCl3 + NH3 under 900 °C and by SiCl4 + NH3 over 900 °C.The effect of deposition temperature on deposition kinetics and mechanism of silicon boron nitride (SiBN) coating was investigated from SiCl4–BCl3–NH3–H2–Ar mixture using low pressure chemical vapor deposition (LPCVD). Results showed that the deposition rates increased from 700 °C to 1030 °C, and then decreased above 1030 °C. The relative content of silicon increased with increasing deposition temperature. The SiBN coating was of amorphous phase and its surface morphology showed cauliflower-like. The bonding states of SiBN coating were the B–N and Si–N bonding at all deposition temperatures, which demonstrated that the SiBN coating is composed of very small Si–N and B–N particles and the main deposition mechanisms refer to two parallel reaction systems of BCl3 + NH3 and SiCl4 + NH3. The deposition reactions were mainly controlled by BCl3 + NH3 under 900 °C, and by SiCl4 + NH3 over 900 °C.

•The machining depth was essentially proportional to the laser power.•The well patterned microgrooves and ripple structures with nanoparticles were formed distinctly in the channels. And the number of nanoparticles increased with the processing power as well.•It revealed a conversion from amorphous carbon to nanocrystalline graphite after laser treated with increasing laser power.•It showed that a great decrease of sp3/sp2 after laser treatment.Femtosecond laser is of great interest for machining high melting point and hardness materials such as diamond-like carbon, SiC ceramic, et al. In present work, the microstructural and chemical bond evolution of diamond-like carbon films were investigated using electron microscopy and spectroscopy techniques after machined by diverse femtosecond laser power in air. The results showed the machining depth was essentially proportional to the laser power. The well patterned microgrooves and ripple structures with nanoparticles were formed distinctly in the channels. Considering the D and G Raman band parameters on the laser irradiation, it revealed a conversion from amorphous carbon to nanocrystalline graphite after laser treated with increasing laser power. X-ray photoelectron spectroscopy analysis showed a great decrease of sp3/sp2 after laser treatment.

The thermodynamic phase stability area diagrams of BCl3-NH3-SiCl4-H2-Ar system were plotted via Factsage software to predict the kinetic experimental results. The effects of parameters (i e, partial pressure of reactants, deposition temperature and total pressure) on the distribution regions of solid phase products were analyzed based on the diagrams. The results show that: (a) Solid phase products are mainly affected by deposition temperature. The area of BN+Si3N4 phase increases with the temperature rising from 650 to 900 °C, and decreases with the temperature rising from 900 to 1 200 °C; (b) When temperature and total pressure are constants, BN+Si3N4 phase exists at a high partial pressure of NH3; (c) The effect of total system pressure is correlated to deposition temperature. The temperature ranging from 700 to 900 °C under low total pressure is the optimum condition for the deposition. (d) Appropriate kinetic parameters can be determined based on the results of thermodynamic calculation. Si–B–N coating is obtained via low pressure chemical vapor deposition. The analysis by X-ray photoelectron spectroscopy indicates that B–N and Si–N are the main chemical bonds of the coating.

•We found that the helical line width and the helical line spacing, machining time and the scanning speed on the surface morphology of machined holes had remarkable effects on the qualities of micro-holes such as shape and depth.•The debris consisted of C, Si and O was observed on the machined surface. The SiC bonds of the SiC matrix transformed into SiO bonds after machined.Picosecond laser is an important machining technology for high hardness materials. In this paper, high power picosecond laser was utilized to drill micro-holes in C/SiC composites, and the effects of different processing parameters including the helical line width and spacing, machining time and scanning speed were discussed. To characterize the qualities of machined holes, scanning electron microscope (SEM) was used to analyze the surface morphology, energy dispersive spectroscopy (EDS) and X-ray photoelectric spectroscopy (XPS) were employed to describe the element composition change between the untreated and laser-treated area. The experimental results indicated that all parameters mentioned above had remarkable effects on the qualities of micro-holes such as shape and depth. Additionally, the debris consisted of C, Si and O was observed on the machined surface. The SiC bonds of the SiC matrix transformed into SiO bonds after machined. Furthermore, the physical process responsible for the mechanism of debris formation was discussed as well.

Carbon nanotubes (CNTs) were introduced into the interface of SiC/SiC composites via electrophoretic deposition to improve their mechanical and thermal properties. SiC/SiC composites with pyrocarbon (PyC) and CNTs–PyC interfaces were denoted as SiC/SiC–P and SiC/SiC–CP, respectively. The results show that the flexural strength, fracture energy, interfacial shear strength and thermal conductivity increased upon the introduction of CNTs (SiC/SiC–CP) and were 1.174, 1.257, more than 2, and 2.158 times those of SiC/SiC–P, respectively. It was demonstrated that meshed CNTs on SiC fibres can strengthen the interfacial bonding between the fibre and matrix and improve the thermal conductivity of the interface. The meshed CNTs had positive effects on the mechanical and thermal properties of the SiC/SiC composites.

The effect of energy density and feeding speed on micro-holes was investigated, which were machined in C/SiC composites by picosecond laser. Morphologies and elemental compositions of the machined holes in 2 mm and 3 mm thickness specimens were analyzed. The results indicated that both energy density and feeding speed had remarkable effect on the quality of micro-holes, especially on the exit side and cross-section of micro-holes. While the circularity of drilling holes on the entry side was affected by the energy density and feeding speed slightly. Additionally, the machining debris also played an important role in the quality of micro-holes. The micro-holes were consisted with three elements C, Si, and O, and those predominantly attributed to CC (sp2), CC (sp3) and SiO bonds. Moreover, the laser machining process was discussed, which was responsible for the formation of the debris.

Boron nitride (BN) was prepared by nitriding pure boron (B) deposited on carbon substrates by chemical vapor deposition (CVD). Thermodynamic analysis of preparing BN by nitriding CVD B at 1200–1550 °C was firstly performed. And then, the effects of nitridation conditions, including temperature, nitridation atmosphere and CVD B microstructure, on the conversion of B to BN were analyzed by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Results show that the conversion degree of B to BN firstly increased and then slightly decreased with rising temperature. The nitridation degree was controlled by mutual actions between the nitridation of B and consumption of the effective nitrogen source (NH3). The morphology of products and the reaction mechanism between B and N were influenced by nitridation temperature. At high temperatures (1400–1500 °C), BN with highly ordered microstructure was produced. On using N2–H2 as nitridation atmosphere instead of NH3–H2–N2, no BN was obtained in the studied temperature range. The microstructure and component of BN obtained in nitridation process were little affected by the microstructure of CVD B.

Propylene was used to fabricate boron-doped carbon coatings by low-pressure chemical vapor deposition. The effects of carbon/boron (C/B) ratio in reactants on the deposition rate, morphologies, and bonding states of the deposits were investigated. Deposition rate increased with increasing C/B ratio, when C/B ratio was less than 4.0. Then, deposition rate decreased with increasing C/B ratio. The maximum rate was 500 nm/h. SEM results showed that cross section morphologies and thickness of deposits were influenced by C/B ratio. Morphologies were compact and not-delaminated with a low C/B ratio, however nanoscale delamination occurred in the deposits with a high C/B ratio. The infiltration characteristic was also influenced by the C/B ratio. The suitable C/B ratio was 1.0–2.0 for infiltration in a T300 carbon bundle. XPS results showed that carbon content is major in the deposits with all C/B ratios. The boron contents decreased and carbon contents increased with increasing C/B ratio. B-sub-C and BC2O were main bonding states. The total contents of B-sub-C and BC2O were above 60.0 at.% with all C/B ratios.

•The Diamond/SiC composites with large diamond particles were fabricated by slip casting and CVI method.•The thermal conductivity of Diamond/CVI-SiC composites was improved remarkably by using large diamond particles.•The fracture modes of Diamond/SiC composites with different diamond particle sizes were investigated.The Diamond/SiC composites were fabricated via slip casting and chemical vapor infiltration (CVI) approaches. The effects of diamond particle size (50–500 μm) on microstructure, flexural strength, density, fracture toughness and thermal conductivity of Diamond/SiC composites were discussed. By using large-sized diamond particles, thermal conductivity of composites can be improved and the maximum value could reach up to 241 W/(m·k), which is 2.1 times higher than that of the Diamond/SiC composites prepared by tape-casting and CVI process. With increasing of diamond particle size, the density of Diamond/SiC composites increased, and the mechanical properties decreased. The maximal density, flexural strength and fracture toughness were 3.18 g/cm3, 287 MPa and 5.0 MPa·m1/2 respectively. The fracture mechanism of the composites transferred from diamond particles transgranular fracture to interfacial debonding.