Properties regulation of SiC ceramics prepared via stereolithography combined with reactive melt infiltration techniques

Stereolithography(SLA) combined with reactive melt infiltration (RMI) is an effective way to fabricate silicon carbide(SiC) ceramic components with complex shapes and high precision. The purpose of this paper is to increase the content of SiC in the sintered body and improve the properties of SiC ceramics prepared by SLA/RMI technologies by the impregnation of the precursor of carbon source after debinding. The effects of the concentration of phenolic resin solution on the strength of sintered body were studied. The results show that this method can reduce the coefficient of thermal expansion and improve the thermal conductivity of the final body. At the same time, when the concentration of phenolic resin solution is 40 wt%, the final body obtained the best comprehensive properties. The value of bulk density, flexural strength and elastic modulus were 2.89 g/cm³, 244.17 ± 5.13 MPa and 402.39 GPa, respectively. This strategy provides a promising prospect for the preparation of space optical mirrors with complex shapes and high strength by the SLA/RMI method.     

Microstructure and flexural strength of C/HfC-ZrC-SiC composites prepared by reactive melt infiltration method

C/HfC-ZrC-SiC composites were fabricated via reactive melt infiltration (RMI) of the mixed HfSi₂ and ZrSi₂ alloys. The microstructure, infiltration behavior of the hybrid silicide alloys infiltrating C/C composites, and flexural strength of C/HfC-ZrC-SiC composites was studied. Inside composites, there were more Hf-rich (Hf, Zr)C phases distributed in the exterior region, while more SiC and Zr-rich (Zr, Hf)Si₂ in the interior region. There was compositional segregation in (Hf, Zr)C, with the HfC content decreasing from the exterior region to interior region. The RMI process was performed at different temperatures to investigate the structural evolution, and a model for the reactive melt infiltration of the mixed HfSi₂ and ZrSi₂ alloys into C/C composites was established. Compared with C/HfC-SiC and C/ZrC-SiC prepared by same process, C/HfC-ZrC-SiC had the highest flexural strength of 247Mpa and 213Mpa after oxidation at 1200 ℃ for 15 min. Both the unoxidized and oxidized samples presented a pseudo-plastic fracture behavior.     

Comparative ablation behaviors of C/SiC-HfC composites prepared by reactive melt infiltration and precursor infiltration and pyrolysis routes

Carbon fiber reinforced silicon carbide-hafnium carbide (C/SiC-HfC) composites were prepared by reactive melt infiltration (RMI) and precursor infiltration and pyrolysis (PIP) routes. The ablation behaviors of the two composites were investigated and compared under an oxyacetylene torch flame. The C/SiC-HfC composites prepared by PIP showed a better ablation resistance than those synthetized by RMI. Microstructural observations revealed an island distribution of HfC for the sample prepared by RMI, which resulted in SiC being directly oxidized during the ablation process. In contrast, the PIP-prepared sample showed a uniform distribution of HfC, which resulted in SiC being oxidized via the Knudsen diffusion mechanism under ablation. The Knudsen diffusion of oxidants retarded the oxidation process, thereby increasing the ablation resistance of the C/SiC-HfC composites prepared by PIP.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Evolution of microstructure and properties of Si3N4 whisker reinforced composites during densification by polymer infiltration and pyrolysis

Si₃N₄ whisker (Si₃N_4w) reinforced composites were prepared by a near-net shaping process, i.e., gel-casting of the Si₃N_4w preform followed by polymer infiltration and pyrolysis (PIP) densification using polysilazane as precursor. The densification process by PIP was described mathematically, after which several key parameters affecting densification efficiency were discussed. The small pore size (0.04 ～ 1 μm) of Si₃N_4w preform can cause filtration effect (low permeability of precursor with a molecular size bigger than pore size), which resulted in the density gradient of the composites. Porosity (P) dependence of flexural strength and elastic modulus of Si₃N_4w/Si₃N₄ followed a power law of (1 – P). With increasing density, the response of Si₃N_4w when confronting cracks transformed from whisker debonding to whisker fracture, which was supposed to be due to the increase of whisker/matrix interface strength. The Si₃N_4w/Si₃N₄ developed by us achieved a good balance between high strength and low dielectric constant, making it promising for high-temperature wave-transparent application.     

B4C reinforced NiTi-based composites: Microstructure and wear performance

In this study, NiTi–x wt.% B₄C (x = 0, 2, and 4) composites were consolidated with spark plasma sintering method, and the effects of boron carbide reinforcement addition on the microstructure and wear behavior of samples were investigated. Identification of the constituent phases of samples by the X-ray diffraction method plus Rietveld analysis revealed that the stability of the martensite phase increased in the composite samples because of mismatch stresses between the NiTi matrix phase and the reinforcing particles, which increases the density of the dislocations and facilitates the diffusion process that subsequently leads to the formation of stable intermetallics. The results of hardness test indicated that the hardness value increased from 3.67 GPa for pure NiTi to 10.99 GPa for NiTi–4 wt.% B₄C. Results of wear test revealed that boron carbide reinforced composite specimens had higher wear resistance, whereas wear rate of NiTi sample was 3.6 × 10⁻³ mm³/N m, and it reached to .21 × 10⁻³ mm³/N m for NiTi–4 wt.% B₄C. Investigation of microstructure by scanning electron microscopy images and EDS analysis revealed that the wear mechanism in NiTi samples was abrasive and the addition of B₄C to NiTi changed the wear mechanisms from abrasive to a combination of oxidation, adhesive, and delamination mechanisms.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

Preparation and properties of epoxy/BN highly thermal conductive composites reinforced with SiC whisker

A simple method is reported to increase the thermal conductivity and improve the poor mechanical properties caused by high filler loadings of epoxy composites, simultaneously. Epoxy composites were prepared with micro‐boron nitride (BN) and silicon carbon whisker (SiCw) chemically treated by 3‐aminopropyltriethoxysilane (KH550) and 3‐glycidyloxypropyltrimethoxysilane (KH560), respectively. Effects of surface modification of BN particles on the thermal conductivity and flexural strength of epoxy/BN composites were investigated. About 3% SiCw particles grafted with KH560 were incorporated into composites with BN grafted with KH550, which led to about 13.8–17.8% increase of the flexural strength as well as a marginal improvement of the thermal conductivity of composites, and they possessed good dielectric properties. In addition, dynamic mechanical analysis results showed that the storage modulus of composites increased significantly with the addition of fillers, while the glass transition temperature exhibited a slight decrease. POLYM. COMPOS., 37:2611–2621, 2016. © 2015 Society of Plastics Engineers     

Reaction-bonded B4C/SiC composites synthesized by microwave heating

The reaction-bonding technique was used to synthesize boron carbide (B₄C) - silicon carbide (SiC) composites by microwave heating. Preforms of porous B₄C were obtained by compaction followed or not by partial densification. Then, the material was infiltrated by molten silicon under a microwave heating. The influence of the thermal cycles (T: 1400-1500°C, t: 5-120 minutes) is low. The hardness of boron carbide is comparable to that of alumina (15-19 GPa) for a much lower density (≈2.5 g/cm³ for B₄C-based material instead of 3.95 g/cm³ for alumina). These properties make this composite, obtained by microwave heating, a good candidate for ballistic applications.     

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50?nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400?µm, and the outer two layers are about 200?µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773?K for 361?h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80?mm) after erosion at 1873?K for 130?h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

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.     

HfC-based coating prepared by reactive melt infiltration on C/C composite substrate

Reactive melt infiltration based on alloy design is proposed in the present work for preparing HfC-based coating on C/C composite substrate. A 50Hf10Zr40Si alloy ingot was prepared and infiltrated into a C/C preform at temperatures much lower than the melting point of the alloy to obtain the HfC-based coating. An obvious layered microstructure of the coating was formed. The carbonization reactions occurring between Hf and carbon of the surface layer of the C/C composite is the primary reason for the reactive melt infiltration process to proceed at relative low temperatures. Acetylene flame test showed that the HfC-based coating protected the C/C composite from serious oxidation.     

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m^?1 K^?1) and low density (3.33 g cm^?3) of the composites were beneficial to thermal management applications.     

Effects of zirconium addition on microstructure and ablation resistance of carbon fibre reinforced carbon and SiC ceramic matrix composite prepared by reactive melt infiltration

Abstract

Carbon fibre reinforced C and SiC binary ceramic matrix composites (C/C–SiC) were fabricated by a quick and low cost reactive melt infiltration (RMI) method with Si–Zr25 and Si melts. Effects of zirconium addition in infiltrated Si melt on microstructure and ablation resistance of the composite were investigated. The composite by Si–Zr25 melt infiltration was composed of SiC, ZrC, C and a little amount of ZrSi₂ without residual silicon, overcoming the problem of residual silicon in C/C–SiC composite by Si RMI. Compared with the composite by Si melt infiltration, the ablation resistance of the composite by Si–Zr25 was greatly improved by zirconium addition due to ZrO₂ and SiO₂ protecting layer formed during ablation.     

Fabrication and properties of SiCnw-reinforced SiC composites by reactive melt infiltration with BN and PyC interface

To tailor the fiber–matrix interface of SiC nanowires-reinforced SiC (SiC_nw/SiC) ceramic matrix composites (CMCs) for improved mechanical properties, SiC nanowires were coated with BN and pyrolytic carbon (PyC) compound coatings prepared by the dip-coating process in boric acid and urea solution and the pyrolysis of phenolic resin. SiC_nw/SiC CMC with PyC/BN interfaces were fabricated by reactive melt infiltration (RMI) at 1680°C for 1 h. The influences of phenolic resin content on the microstructure and mechanical properties of the CMC were investigated. The results showed that the flexural strength and fracture toughness reach the maximum values of 294 MPa and 4.74 MPa m^1/2 as the phenolic resin content was 16 and 12 wt%, respectively. The displacement–load curve of the sample exhibited a gradient drop with increasing phenolic resin content up to 12 wt%. The results demonstrated that the PyC/BN compound coatings could play the role of protecting the SiC_nw from degradation as well as improving the more moderate interfacial bonding strengths during the RMI.     

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.     

Low-surface-temperature jump behavior of C/SiC composites prepared via precursor impregnation and pyrolysis in high-enthalpy plasma flows

The response of C/SiC composites prepared via precursor impregnation and pyrolysis was investigated in a 1 MW plasma wind tunnel. Under a considerable aero heating of up to 26.2 MJ/kg of specific total enthalpy, the samples were exposed to heat fluxes exceeding 5.7 MW/m² and low pressures of 4.5–6.6 kPa. The samples were able to withstand low heat fluxes and low stagnation pressures, and their carbon-rich nature improved the thermal conductivity, presenting a low steadystate surface temperature. However, a spontaneous jump in the surface temperature at around 1700 °C was observed at high heat fluxes and high stagnation pressures. The jump temperature was lower compared with that reported in previous studies, and was found to increase rapidly to temperatures above 2000 °C. This low-temperature jump phenomenon was associated with the evolution of microstructure during testing, and the underlying mechanism was revealed through the use of thermodynamics analysis.     

B4C/SiC ceramic hollow microspheres prepared by slurry-coating and precursor conversion method

In this work, a new approach to fabricate B₄C/SiC ceramic hollow microsphere was established through combination of slurry-coating and precursor conversion method. Firstly, different slurries were prepared using modified B₄C powder with addition of PCS and LPCS. Subsequently, slurry was coated on a POM microsphere, followed by crosslinking, POM decomposition and heat-treatment at 1100 °C. Finally, the B₄C/SiC hollow microspheres with average diameter of 1.6–1.9 mm and thickness of 10–60 μm were obtained. The roughness (Ra) of outer surface was as low as 40.3 nm with high sphericity of 99.6%. Deoxidation was also explored by heat treatment at 1700 °C. The oxygen content was decreased from 11.06 wt.% to 0.37 wt.%, and crush load was promoted from 1.73 N to 4.50 N. The preparative method of B₄C/SiC ceramic hollow microspheres in this work can be easily extended for large-scale production.     

Effect of acid treatment on the formation of SiC whiskers from raw rice husks

Raw rice husks have been treated by boiling for 1 h in 5 N hydrochloric acid. The washed and dried acid-treated raw rice husks (TRRH) and untreated raw rice husks (RRH) were directly pyrolysed (without precoking) in argon atmosphere at different temperatures between 1050 and 1600 °C. Silica obtained from TRRH has a lower level of impurities than that obtained from RRH. Acid treatment has been found to decrease the degree of crystallization of silica and carbon in rice husks. The whisker formation has been decreased in TRRH. As the carbon loss due to the formation of CO was high, the SiC content in the pyrolysed TRRH was higher than that in pyrolysed RRH.