Processing and impact behavior of Al/SiCp composites fabricated by the pressureless melt infiltration method

In the current investigation, pressureless melt infiltration was applied to fabricate the Al/SiC composites based on the SiC porous preforms. The process was conducted by introducing the aluminum melt into the SiC preforms at 950 °C under the nitrogen atmosphere, without the aid of pressure. To explore development of melt infiltration, initial preforms were produced with variable SiC fractions (40, 50, and 60 vol.%) using three different SiC powders with the mean particle size of 20, 50, and 90 μm. While the infiltration of aluminum melt into the preforms with 40 vol.% initial SiC volume fraction (SiC particle size of 90 μm) resulted to the composites with final density of 0.94 theoretical density (TD), this value drops down to ～0.9 TD for the composites produced by preforms with the SiC (90 μm) volume fraction of 60 vol.%. On the other hand, composites fabricated by 50 μm SiC powder (SiC volume fraction of 40 vol.%) demonstrated the final density of ～0.91 TD. The impact resistance tests performed on the composites demonstrated an enhancement in the value of impact energy with an increase of SiC powder particle size. Results, additionally, revealed a significant superiority of impact energy for the composites fabricated by a combined melt infiltration and sintering (MIS) procedure compared to those produced by infiltration at 950 and 1350 °C.     

Fabricating 2.5D SiCf/SiC Composite Using Polycarbosilane/SiC/Al Mixture for Matrix Derivation

2.5D SiC_f/SiC composites were produced by a modified polymer infiltration and pyrolysis process. Fine Al and SiC powders were first infiltrated into large inter-bundle pores. During pyrolytic decomposition of the polymer, the active Al filler reacts with small carbon-bearing polymer fragments, and reactive nitrogen atmosphere to form new phases of carbide or nitride. The volume expansion compensates for polymer shrinkage to a certain degree. The microstructural evolution and mechanical performances were characterized. The result indicates that the addition of Al fillers has significant influence on the mechanical properties of the composites. For the composite with Al loading, a proportional-limit stress of 380 MPa and a maximum stress of 441 MPa are achieved.     

Effect of monomers content in enhancing solid-state densification of silicon carbide ceramics by aqueous gelcasting and pressureless sintering

The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.     

Effect of impregnated phenolic resin on the properties of Si–SiC ceramic matrix composites fabricated by SLS-RMI

Selective laser sintering (SLS) combined with reaction melt infiltration was used to fabricate Si–SiC ceramic matrix composites, and the effects of different concentrations of phenolic resin (PF) on the properties of the SLS green body and carbonized and final Si–SiC samples were investigated. The results showed that the impregnation with PF can increase the bulk density, reduce the porosity of the samples at all stages, and improve the mechanical properties of the reactive bonded samples. The degree of densification and mechanical properties of the sample gradually enhanced with an increase in PF concentration. The main phases of the Si–SiC composites were free Si, α-SiC, β-SiC, plus an extremely small amount of Al–Si alloy, and the SiC and the Si phase contents increased and decreased, respectively, as the concentration of PF increased when measured using Rietveld refinement and image analysis software. The macroscopic properties of the samples improved greatly after precursor infiltration pyrolysis (PIP) treatment with 66.7%vol PF-ethanol solution twice. According to the crystal nucleation-growth theory, it was inferred that the infiltrated PF could provide a certain amount of pyrolytic carbon in the carbonized specimen. During the reaction bonded process, the carbon formed by carbonization pyrolysis first dissolves into the molten Si and reaches saturation. With the further dissolution of carbon, [C] and [Si] in the liquid phase contact each other to form β-SiC nuclei, the nuclei that precipitate at the pore wall position and gradually form a continuous interfacial layer of β-SiC. The β-SiC layer prevents the liquid Si from direct contact with C inside the prefabricated body, therefore, further reactants diffuse through the layer. Finally, the fine crystalline β-SiC grains were fabricated inside the specimen.     

Microstructures and thermal conductivities of reaction sintered SiC ceramics

Abstract

Reaction sintered SiC ceramics were prepared by the silicon melt infiltration method over temperatures of 1450?1550°C. The effects of the carbon and silicon contents of the starting materials as well as the sintering temperature and time on the thermal conductivities and microstructures of the ceramic materials were studied. The thermal conductivities and microstructures of the samples were characterised using thermal conductivity measurements, X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy and mercury injection porosimetry. The results showed that sintering temperature and time as well as the carbon and silicon contents of the green specimens are the main factors affecting the microstructure and porosity of reaction bonded SiC ceramics. Increasing the reaction temperature and time decreased the porosity of the ceramics. This was due to the infiltration of the silicon melt into the ceramic specimens. The thermal conductivity and porosity of the sample sintered at 1550°C for 3 h in an argon atmosphere were 102·5 W m K^?1 and 0·3% respectively.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part II of the study, the effects of Y₂O₃/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.     

Phase formation in porous liquid phase sintered silicon carbide: Part I:: Interaction between Al2O3 and SiC

The structure and phase formation of porous liquid phase sintered silicon carbide (porous LPS-SiC), containing yttria and alumina additives have been studied. The present paper is focused on the system Al–Si–C–O, which is part of the system describing the interactions with sintering additives.The influence of different sintering atmospheres, namely argon and Ar/CO, and different temperatures on structure and composition was investigated by XRD and SEM. Additionally, reaction products were calculated from thermodynamic data and correlated with experimentally determined reaction products. Alumina and SiC reacted at 1950 °C in an argon atmosphere, forming a metal melt of aluminium and silicon. No reduction of Al₂O₃ was observed in a CO-containing argon sintering atmosphere.In the second and third parts of this paper the interactions between Y₂O₃–SiC and Y₂O₃–Al₂O₃–SiC are analysed [J. Eur. Ceram. Soc. (in press), parts II and III].     

逆反应烧结制备碳化硅/氮化硅复合材料的工艺

制备Si3N4／SiC复合材料的常规反应烧结是以Si和SiC为原料进行氮化烧结,而逆反应烧结是以Si3N4和SiC为原料,首先使Si3N4反向反应为活性氧化物后再进行烧结。建立逆反应烧结工艺制备Si3N4／SiC复合材料的热力学基础。确定了Si3N4先于SiC氧化;氧化产物可以是SiO2,也可以是Si2N2O;形成的SiO2氧化膜不会与基体材料反应;在膜与基体之间可能生成Si2N2O。论证了逆反应烧结的热力学可行性。通过6个烧结实验,证实了其热力学分析的正确性,并从工艺参数与密度变化、残氮率和比强度等关系筛选出最佳的烧结工艺参数。     

微波烧结SiC-Cu/Al复合材料的工艺及机理

用微波烧结工艺成功制备了铜包裹碳化硅颗粒增强铝基(SiC-Cu/Al)复合材料,利用扫描电镜和X射线衍射分析仪对烧结样品进行表征,并讨论了烧结过程及机理.研究表明:采用多晶莫来石纤维棉、硅碳棒和氧化铝坩锅组合设计的保温结构能很好地促进烧结.烧结温度为720℃时,SiC-Cu/Al复合材料的密度取得最大值为2.53g/cm3.SiC-Cu/Al复合材料的硬度随烧结温度的升高的变化成马鞍状.烧结温度对样品显微结构的影响较大,随着烧结温度的升高,相分布的均匀性降低,在较高的烧结温度下会出现SiC颗粒的偏聚.涡流损耗和界面极化损耗是促进微波烧结的主要动力.     

Processing of silicon carbide–boron carbide–aluminium composites

The aim of this work was to shed light on the wetting mechanism in the SiC–B₄C–Al system and to explore processing routes that enable infiltration of Al alloys into these ceramic powder mixtures without the formation of the deleterious reaction product Al₄C₃. For this purpose, powder mixtures consisting of SiC and pre-treated B₄C were pressureless infiltrated with Al alloys at relatively low temperatures under an inert gas atmosphere. Depending on the characteristics of the starting powders fully infiltrated composites were achieved in the temperature range of 935–1420 °C. It was observed that addition of pre-treated B₄C to SiC enabled complete infiltration of the ～0.6 cm thick preforms. The bulk density of all produced composites was >98% of the X-ray density. By controlling the surface chemistry and particle size of the starting powders as well as the processing conditions, the wetting behaviour and reaction kinetics of this system could be tailored so as to render fully dense SiC–B₄C–Al composites devoid of Al₄C₃.     

Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Al2O3–SiC composites prepared by infiltration of pre-sintered alumina with a poly(allyl)carbosilane

The Al₂O₃/SiC nanocomposites containing 3–8 vol.% SiC were prepared through infiltration and in situ thermal decomposition of a preceramic polymer SiC precursor (poly(allyl)carbosilane) in pre-sintered alumina matrix. The volume fraction of SiC, and the microstructure of composites were adjusted by concentration of the polymer solution, and by the conditions of pyrolysis and sintering. The specimens were densified by pressureless sintering at temperatures between 1550 and 1850 °C in flowing argon. The use of powder bed producing SiO, CO and other volatile species suppressed decomposition reactions in the composites and was vital for their successful densification. The experimental results are discussed against thermodynamic analysis of the system Al₂O₃/SiC/SiO₂ in an inert Ar atmosphere.     

Effect of Al addition on pressureless sintering of B4C

The effect of Al addition on pressureless sintering of B₄C ceramic was analyzed in the present work. Different amounts of Al, from 0 to 5 wt.% were added to the base material and pressureless sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystal phases and density evolution were studied and correlated to Al additions and firing temperature. Density and grain size of sintered samples, increased significantly with Al load while less evidence is the effect of sintering temperature; 94% dense material was obtained by adding 4 wt.% Al regardless of the maximum firing temperature.     

Fabrication of Continuous-SiC-Fiber-Reinforced SiAlON-Based Ceramic Composites by Reactive Melt Infiltration

A technique for fabrication of β'-SiAlON-based ceramics in three-dimensional woven fabrics of BN-coated SiC (Hi-Nicalon™) fibers was developed by reactive melt infiltration in a controlled N₂ atmosphere. β'-SiAlON was produced in situ by the reaction of β-Si₃N₄, AlN, and Y-Al-Si-O molten glass. The wettability of the fibers with the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane, resulting in fully dense matrix composites. The reaction between the fiber and molten glass could be depressed by increasing the N₂ partial pressure during the melt infiltration. The inhibition of the interfacial reaction may be related to the formation of carbon and oxynitride on the SiC fiber, in agreement with thermodynamic calculations as a function of N₂ partial pressure. The fabricated composites had a high ultimate flexure strength and a large work of fracture at room temperature. Degradation of the mechanical performance of the composites was small, even at 1773 K in an argon atmosphere.     

Fine-Grained Silicon Carbide Ceramics with Oxynitride Glass

By using an oxynitride glass composition from the Y-Mg-Si-Al-O-N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid-phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β-SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine-grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room-temperature strength and toughness. The best results were obtained when the ceramics were hot-pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room-temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m^1/2, respectively.     

Alumina/silicon carbide composites fabricated via in situ synthesis of nano-sized SiC particles

Alumina/silicon carbide composites have been fabricated by a new technique involving the in situ synthesis of nano-sized SiC particles. A mixture of alumina powder and silicon carbide precursors was prepared in an aqueous suspension. Green bodies were formed by cold isostatic pressing of granules obtained by freeze granulation, and pressureless sintered at 1750 °C for 4 h in an argon atmosphere. Mullite (10–20 vol%) formed in addition to SiC during sintering. The SiC particles were located predominantly to the interior of the mullite and alumina matrix grains.     

Superior bending and impact properties of SiC matrix composites infiltrated with an aluminium alloy

A simple process based on melt infiltration was used to modify a silicon carbide (SiC) ceramic and thus improve its mechanical properties. SiC ceramics infiltrated with an Al alloy for 2 h, 4 h, 6 h, and 8 h exhibited outstanding mechanical performance. The three-point bending strength, four-point bending strength, and impact toughness of the SiC ceramics increased by 125–135%, 170–180%, and 140%, respectively, after infiltration with the Al alloy at 900 °C for 4–6 h. The maximum three-point bending strength, four-point bending strength, and impact toughness achieved were 430 MPa, 360 MPa, and 3.5 kJ/m², respectively. Analysis of the processing conditions and microstructure demonstrated that the molten Al alloy effectively infiltrated the gaps between the SiC particles, forming a compact structure with the particles, and some of the Al phases reacted with Si to form Al-Si eutectic phases. Moreover, the results showed that a reaction layer is present on the surface of the SiC sample, which mainly contains the Ti₃SiC₂ phase. Both complete infiltration with the Al alloy and the formation of the Ti₃SiC₂ phase contributed to the improvement of the mechanical properties.     

Rapid Reactive Synthesis and Sintering of Submicron TiC/SiC Composites through Spark Plasma Sintering

Submicrometer TiC/SiC composites were fabricated by a rapid reactive sintering process through spark plasma sintering (SPS) technique using the carbon, titanium, and nanosized-SiC powders without any additive. It was found that the composite could be sintered in a relatively short time (8 min at 1480°C) to 97.9% of theoretical density. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction techniques and observed by scanning electron microscopy. The effect of nanosized and microsized SiC additives on the microstructure of TiC/SiC composites was investigated.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.