Microstructure and thermal expansion behavior of diamond/SiC/(Si) composites fabricated by reactive vapor infiltration

Diamond/SiC/(Si) composites were fabricated by Si vapor vacuum reactive infiltration. The coefficient of thermal expansion (CTE) of composites have been measured from 50 to 400 °C. With the diamond content increasing, CTE of composite decreased, simultaneously, the microstructure of the composites changed from core–shell particles embedded in the Si matrix to an interpenetrating network with the matrix. The CTEs of composites versus temperature matched well with those of Si. The Kerner model was modified according to the structural features of the composites, which exhibited more accurate predictions due to considering the core–shell structure of the composites. The thermal expansion behavior of the matrix was constrained by diamond/SiC network during heating.     

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

Oxidation resistance of SiC/SiC composites containing SiBC matrix fabricated by liquid silicon infiltration

The mechanical behavior and oxidation resistance of SiC/SiC-SiBC composites were studied in this work. According to the debonding criterion of He and Hutchinson, the debonding could occur at the BN interphase, which insures that the fibers can well play the strengthening and toughening performance. The oxidation resistance of SiC/SiC-SiBC composites consisting of SiC fibers with thermal expansion coefficients (CTE) of 5.1 × 10^?6 K^?1 and 4.0 × 10^?6 K^?1 was compared. The composites consisting of SiC fibers with higher CTE show slight weight changes at 800, 1000, and 1200 °C, and the corresponding strength retention ratios are 109.6%, 103.2% and 102.9%, exhibiting excellent oxidation resistance. The CTE of composites consisting of SiC fibers with higher CTE matches well with the CTE of SiC coating, so rarely no cracks can be formed in the coating, which inhibits the inward diffusion of oxidizing medium and leads to high strength retention ratios after oxidation tests.     

Effect of PyC interphase thickness on mechanical behaviors of SiBC matrix modified C/SiC composites fabricated by reactive melt infiltration

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.     

Thermal transport characteristics in diamond/SiC composites via molten Si infiltration

Heat dissipation remains a critical challenge in optical and electronic devices and diamond/SiC composite is the premiere material solution because of its outstanding thermal and mechanical properties. Si liquid infiltration is one of the most promising techniques to fabricate fully dense diamond/SiC composites with desired phase structures and exceptional properties. In this study, the thermal conductivity from room temperature to 1000 °C was investigated for the diamond/SiC composites fabricated by a liquid Si infiltration method. The experimental thermal measurement shows a good agreement with the computational solution obtained by solving the Boltzmann transport equation. The results suggest a strong correlation between the composite thermal conductivity and diamond volume percentage. A level-off of the thermal conductivity at high diamond content reflects increased thermal resistance. In addition, the annealing effect on the composite thermal conductivity as well as the thermal stability were evaluated.     

Reaction kinetics and ablation properties of C/C-ZrC composites fabricated by reactive melt infiltration

Carbon/carbon-zirconium carbide (C/C-ZrC) composites were prepared by reactive melt infiltration. Carbon fiber felt was firstly densified by carbon using chemical vapor infiltration to obtain a porous carbon/carbon (C/C) skeleton. The zirconium melt was then infiltrated into the porous C/C at temperatures higher than the melting point of zirconium to obtain C/C-ZrC composites. The infiltration depth as a function of annealing temperature and dwelling time was studied. A model based on these results was built up to describe the kinetic process. The ablation properties of the C/C-ZrC were tested under an oxyacetylene torch and a laser beam. The results indicate that the linear and mass ablation rates of the C/C-ZrC composites are greatly reduced compared with C/SiC-ZrB₂, C/SiC, and C/C composites. The formation of a dense layer of ZrC and ZrO₂ mixture at high temperatures is the reason for high ablation resistance.     

Compressive strength and wear properties of SiC/Al6061 composites reinforced with high contents of SiC fabricated by pressure-assisted infiltration

Pressure-assisted infiltration was used to synthesize SiC/Al 6061 composites containing high weight percentages of SiC. A combination of PEG and glass water was used to fabricate SiC preforms and the effect of the presence of glass water on the microstructure and mechanical properties of the preforms was evaluated by performing compression tests on the preforms. Also, the compressive strength and the hardness of the SiC/Al composites were investigated. The results revealed that the glass water improved the compressive strength of the preforms by about five times. The microstructural characterization of the composites showed that the penetration of the aluminum melt into the preforms was completed and almost no porosity could be seen in the microstructures of the composites. Moreover, the composite containing 75 wt% SiC exhibited the highest compressive strength as well as the maximum hardness. The results of the wear tests showed that increasing the SiC content reduces the wear rate so that the Al-75 wt% SiC composite has a lower wear rate and a lower coefficient of friction than those of Al-67 wt% SiC composite. This indicated higher wear resistance in these composites than the Al alloy due to the formation of a tribological layer on the surface of the composites.     

Mechanical and dielectric properties of SiCf/SiC composites fabricated by PIP combined with CIP process

2D KD-1 SiC fiber fabrics were employed to fabricate SiC_f/SiC composites by an improved polymer infiltration and pyrolysis (PIP) process, combined with cold isostatic pressing (CIP). The effect of CIP process on the microstructure, mechanical and dielectric properties of SiC_f/SiC composites was investigated. The infiltration efficiency was remarkably improved with the introduction of CIP process. Compared to vacuum infiltration, the CIP process can effectively increase the infiltrated precursor content and decrease the porosity resulting in a dense matrix. Thus SiC_f/SiC composites with high density of 2.11 g cm⁻³ and low porosity of 11.3% were obtained at 100 MPa CIP pressure, together with an increase of the flexural strength of the composites from 89 MPa to 213 MPa. Real part (ε′) and the imaginary part (ε″) of complex permittivity of SiC_f/SiC composites increase and vary from 11.7-i9.7 to 15.0-i12.8 when the CIP pressure reaches 100 MPa.     

Microstructure and mechanical properties of Cf/ZrC composites fabricated by reactive melt infiltration at relatively low temperature

Carbon fiber-reinforced zirconium carbide matrix composites (C_f/ZrC) were prepared by vacuum infiltrating porous carbon/carbon preforms with molten Zr₂Cu alloy at 1200 °C. X-ray diffraction, scanning electron microcopy and transmission electron microscopy analysis were used to characterize the composition and microstructure of the final composites. It was found that the matrix of the composites were composed of the Cu–Zr–C amorphous phase dispersed with either single- or polycrystalline ZrC. Based on the microstructural analysis, the formation mechanism of the matrix was proposed to be a solution-precipitation and grain coalescence process. The influence of the heat treatment at 1800 °C was also investigated. Results indicated that at very high temperature the volatilization of residual metal somewhat deteriorated the flexural strength and the elastic modulus, but the fracture toughness of the composites was improved due to the sintering of ZrC grains.     

Fabrication of C/SiC composites by combining liquid infiltration process and spark plasma sintering technique

Carbon fibre-reinforced silicon carbide composites (C-SiC) were fabricated combining, for the first time, a liquid infiltration process (LI) of a mesophase pitch doped with silicon carbide nanoparticles followed by reactive liquid silicon infiltration using Spark Plasma Sintering (SPS) technique. A graphitization step was applied in order to improve the effectiveness of the processing. Up to three different morphologies of SiC particles were identified with a noticeable influence on the preliminary oxidation tests carried out. The presence of SiC nanoparticles added to the carbon matrix affects the morphology of the SiC obtained by in situ reaction of silicon and carbon during the LI process by SPS and it leads to an improvement of the material oxidation resistance. The results show that SPS is a promising method to develop C-SiC composites in a short time and with a high efficiency in the liquid silicon infiltration process.     

Thermal properties of diamond/Ag composites fabricated by eletroless silver plating

Thermal management in microelectronic technology has become an important issue due to the increase of device power and integration levels. Diamond and silver were selected for the fabrication of composites with high thermal conductivity and low coefficient of thermal expansion (CTE). Diamond reinforcement powders with varied types, shapes and sizes were electroless plated by silver. Then these powders were hot-pressed in air at 600 °C, 500 MPa for 30 min to produce bulk silver matrix composites. The thermal conductivity and the CTEs of the composite at 20 vol.% are 420 W/m K and 12 ppm/K, respectively. These diamond/Ag composites have potential applications for the high integration electronic devices.     

Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process

Sub-micrometer SiC particles were firstly added to the preceramic solution in the first infiltration step to enhance the mechanical properties of 2D C_f/SiC composites fabricated via polymer infiltration and pyrolysis (PIP) process. The effects of pyrolysis temperature and SiC-filler content on microstructures and properties of the composites were systematically studied. The results show that the failure stress and fracture toughness increased with the increase of pyrolysis temperature. SiC filler of sub-micron scale infiltrated into the composites increased the mechanical properties. As a result, for the finally fabricated composite infiltrated with a slurry containing 40 wt.% SiC filler, the failure stress was doubled compared to that without SiC filler addition, and the fracture toughness reached ≈10 MPa m^1/2.     

Effects of the single layer CVD SiC interphases on the mechanical properties of the SiCf/SiC composites fabricated by PIP process

Owing to the degradation of the mechanical properties of the SiC fiber reinforced SiC matrix (SiCf/SiC) composites with the pyrocarbon (PyC) and BN interphases under oxidation environment and neutron irradiation, single layer SiC interphases prepared by chemical vapor deposition (CVD) process were employed to substitute for them. Effects of the CVD SiC interphases on the mechanical properties and interfacial characteristics of the SiCf/SiC composites fabricated by precursor infiltration and pyrolysis (PIP) process were investigated. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with the single layer CVD SiC interphases exhibit an obvious toughened fracture behavior, the flexural strength of which is about 4 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the SiC interphases play a key part in protecting the fibers from damage during composite preparation and weakening interfacial bonding, which can provide high in situ fiber strength and appropriate interfacial bonding strength for the SiCf/SiC composites.     

Laser-supported joining of SiC-fiber/SiCN ceramic matrix composites fabricated by precursor infiltration

SiC-fiber/SiCN ceramic matrix composites were manufactured by means of polymer infiltration and pyrolysis. The fiber preform was made by slurry infiltration and winding using a computer-controlled winding module. Multiple infiltration steps using a Si–C–N precursor were included to increase the density. The influence of the sintering conditions on the microstructure of the CMC was demonstrated.Pipe sections made of the CMC materials were joined using a laser-supported heating technology with an Y–Al–Si–O glass–ceramic filler. The thermal response of the CMC components was controlled by the anisotropic thermal conductivity. Fast heating by laser beam was achieved for elements rotating in the direction of the fiber winding. SEM micrographs of the joints showed the good wettability of the CMC by the glass–ceramic filler. Nearly defect-free joints were obtained using a nitrogen process atmosphere. The laser-supported technology was shown to be promising for the joining of CMC components.     

High-performance 3D SiC/PyC/SiC composites fabricated by an optimized PIP process with a new precursor and a thermal molding method

To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process and the mechanical properties for SiC/SiC composites, 3-dimensional (3D) SiC/SiC were fabricated by a PIP process with a new precursor polymer and the thermal molding method. Liquid polyvinylcarbosilane (LPVCS) with active Si–H and –CHåCH₂ groups was adopted as the SiC matrix precursor. The SiC/SiC composites with superior mechanical properties were efficiently fabricated. The fiber volume of the SiC/SiC was 50.4%. The bulk density and porosity of the SiC/SiC composites were 2.16 g cm⁻³ and 15.4% respectively. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m^1/2 respectively. The influences of LPVCS and molding pressure on the performances of the SiC/SiC composites were discussed in-depth.     

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si₃N₄/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si₃N₄/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si₃N₄/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.     

Ring compression properties of SiCf/SiC composites prepared by chemical vapor infiltration

SiC fiber reinforced SiC matrix (SiC_f/SiC) composites prepared by chemical vapor infiltration are one of promising materials for nuclear fuel cladding tube due to pronounced low radioactivity and excellent corrosion resistance. As a structure component, mechanical properties of the composites tubes are extremely important. In this study, three kinds of SiC_f preform with 2D fiber wound structure, 2D plain weave structure and 2.5D shallow bend-joint structure were deposited with PyC interlayer of about 150–200?nm, and then densified with SiC matrix by chemical vapor infiltration at 1050?°C or 1100?°C. The influence of preform structure and deposition temperature of SiC matrix on microstructure and ring compression properties of SiC_f/SiC composites tubes were evaluated, and the results showed that these factors have a significant influence on ring compression strength. The compressive strength of SiC_f/SiC composites with 2D plain weave structure and 2.5D shallow bend-joint structure are 377.75?MPa and 482.96?MPa respectively, which are significantly higher than that of the composites with 2D fiber wound structure (92.84?MPa). SiC_f/SiC composites deposited at 1100?°C looks like a more porous structure with SiC whiskers appeared when compared with the composites deposited at 1050?°C. Correspondingly, the ring compression strength of the composites deposited at 1100?°C (566.44?MPa) is higher than that of the composites deposited at 1050?°C (482.96?MPa), with a better fracture behavior. Finally, the fracture mechanism of SiC_f/SiC composites with O-ring shape was discussed in detail.     

Oxidation behavior of carbon/carbon-boron nitride composites fabricated by additives and chemical vapor infiltration

Carbon/carbon-boron nitride (C/C-BN) composites were manufactured by adding hexagonal boron nitride (h-BN) powders into carbon fiber preform and a subsequent chemical vapor infiltration (CVI) process for deposition of pyrolytic carbon (PyC). Microstructure and oxidation behavior of carbon/carbon composites with 9?vol% h-BN (C/C-BN9) were studied in comparison to carbon/carbon (C/C) composites. Results showed that with the addition of h-BN powders, a regenerative laminar (ReL) PyC with higher texture was achieved. Note that the introduction of h-BN powder make great contributes to graphitization degree of PyC, leading to larger oxidation activation energy. Moreover, under an air atmosphere, h-BN started to oxidize above 800?°C, and generated molten boron oxide (B₂O₃) which prohibited oxygen diffusion by filling in pores, cracks and other defects. As these reasons mentioned above, after oxidation tests under an air atmosphere, mass losses of C/C-BN9 composites were lower than that of C/C composites at all test temperatures (600–900?°C), indicating that the oxidation resistance of C/C-BN9 composites is better than that of C/C composites.     

Ablation behavior of C/C-ZrC and C/SiC-ZrC composites fabricated by a joint process of slurry impregnation and chemical vapor infiltration

C/C-ZrC and C/SiC-ZrC composites were fabricated by a joint process of slurry impregnation and chemical vapor infiltration, in which ZrC matrix was obtained by slurry impregnation process, while C or SiC matrix was introduced by chemical vapor infiltration process. The as fabricated C/C-ZrC and C/SiC-ZrC composites have densities of 1.67 g cm^?3 and 1.91 g cm^?3 respectively. Tensile strength is 89.4±8.4 MPa and 182.2±14.0 MPa respectively for the as prepared C/C-ZrC and C/SiC-ZrC. Ablation behavior of the C/C-ZrC and C/SiC-ZrC composites under air plasma was studied and compared in detail. Due to different oxidation resistance and heat transfer capacity of the matrix, these two ZrC based composites showed various ablation behavior. The linear erosion rate is 48 µm s^?1 and 39 µm s^?1 respectively for C/C-ZrC and C/SiC-ZrC composites.