Reaction-bonded SiC Ceramics Research Articles

Improved mechanical properties of reaction-bonded SiC through in-situ formation of Ti3SiC2

Reaction-bonded SiC is a ceramic with excellent thermal properties, good corrosion resistance and the characteristic of near-net-shape manufacturing. However, the poor fracture toughness of free Si limits the applications of reaction-bonded SiC. In this study, TiC was added to reaction-bonded SiC and reacted with free Si to form Ti3SiC2. The effects of TiC and carbon black on the mechanical properties of reaction-bonded SiC were investigated. The results demonstrated that the in-situ formation of Ti3SiC2 and decrease in the content and size of free Si improved the mechanical properties of reaction-bonded SiC ceramics. The mechanical properties of TiC-added reaction-bonded SiC with 17.5 wt% carbon black were superior to those of TiC-added reaction-bonded SiC with 15 wt% carbon black. Moreover, increasing the TiC content of reaction-bonded SiC with 17.5 wt% carbon black from 0 to 7.5 wt% caused an increase in its bending strength from 183.92 to 424.43 MPa and an increase in fracture toughness from 3.7 to 5.24 MPa m1/2.

Effect of Soaking Time and Molding Pressure on in-situ Synthesis of β-SiC Whiskers in Reaction-bonded SiC Ceramics

Effect of Soaking Time and Molding Pressure on in-situ Synthesis of β-SiC Whiskers in Reaction-bonded SiC Ceramics

Advanced technology for fabrication of reaction-bonded SiC with controlled composition and properties

An advanced fabrication technology of reaction-bonded SiC is developed, which includes the preparation of a C/SiC preform by repeated cycles of phenolic resin impregnation and pyrolysis, followed by infiltration with silicon melt. The use of different number of impregnation stages provides control of carbon content in the preform and the corresponding SiC content in final ceramics. The effect of the impregnation number on the preform characteristics and ceramics composition, thermal and mechanical properties are investigated comprehensively. With an increase of impregnation number up to four, SiC fraction in the ceramics enlarges to 93 vol%, thermal conductivity and Young’s modulus increase to 186 W/(m∙K) and 427 GPa respectively, which are superior to most reaction-bonded SiC. Flexural strength (225 MPa) and thermal expansion coefficient (2∙10−6 K-1) are not dependent on the impregnation number. The obtained results provide an opportunity to design and fabricate reaction-bonded SiC ceramics with a given set of properties.

Oxidation behavior of Al2O3 added reaction-sintered SiC ceramics in wet oxygen environment at 1300°C

ABSTRACTThe oxidation behavior of reaction-bonded SiC ceramics with Al2O3 was investigated at 1300°C for exposure duration up to 100 h under water vapor environment. Results show that Al2O3 is enriched on the oxidation surface, reducing the volatilization of the oxide layer and preventing the reduction of macroscopic size. Raman spectra results further testified that Al2O3 obviously improves the stability of glass structure and reduces the generation of non-bridged oxygen atoms (Si–O–H), which results in a lower reaction activity with steam of the aluminosilicate melt, therefore, significantly improving the oxidation resistance of SiC in O2/H2O atmosphere.

Corrosion behaviors of porous reaction-bonded silicon carbide ceramics incorporated with CaO

Reaction-bonded SiC (RBSC) porous ceramics were fabricated at 1450 °C in air by incorporating CaO using ZrO2 as sintering aids, activated carbon as pore-forming agent, and mullite fibers as reinforcing agent. The effects of CaO content on the properties of the porous RBSC ceramics were studied. Corrosion behaviors of the prepared RBSC porous ceramics in different environments were also investigated. The optimal open porosity, bending strength, average pore size and gas permeability of the ceramics with 0.5% CaO were 40%, 22.5 MPa, 42.9 µm, and 2100 m3/m2 h kPa, respectively. A well-developed neck reaction-bonded by calcium zirconium silicate (Ca3ZrSi2O9) was identified. The porous RBSC ceramics exhibited excellent corrosion resistance in acid and basic solutions. The anti-oxidation temperature of the porous RBSC ceramics could reach 1200 °C in air. The RBSC ceramics maintained the bending strength of 17.5 MPa after 60 cold-hot cycles in air (0–800 °C). The porous RBSC ceramics also exhibited relatively good corrosion resistance in molten salts (NaCl, Na2SO4 and CaCl2). Melten NaOH can aggravate the reaction by breaking the SiO2 layers on the SiC surface. Overall, these findings offer significant insights into expanding the applications porous RBSC ceramics incorporated with CaO.

Study on grinding surface deformation and subsurface damage mechanism of reaction-bonded SiC ceramics

In order to explore the grinding surface deformation and subsurface damage mechanism for reaction-bonded SiC ceramics, the grinding experiment for reaction-bonded SiC ceramics was carried out under the condition of different grinding depths using two different kinds of grain sizes of grinding wheel. The ground surface morphology of specimen was observed using the field emission scanning electron microscope (5000 ×), and the value of surface roughness Rz was measured by the confocal microscope, which found that there were the brittle removal region and the plastic removal region on the ground surface of reaction-bonded SiC ceramics and it could improve the ground surface quality and proportion of ductile region using the fine grinding wheel and reducing the grinding depth. The specimen was polished by the ion cross section polisher and the ground subsurface was analyzed by the field emission scanning electron microscope, which found that there were transgranular fracture, intergranular fracture, crack bifurcation, ladder-shaped crack and other phenomenon in the grinding process. And it could control the subsurface damage depth using the fine grinding wheel and reducing the grinding depth. The relationship between surface roughness and subsurface damage was analyzed based on the indentation theory, which found that the simulation results were close to the experiment results when the value of m is in the range of 1/8–1/4. When m is 0.2143 calculated by genetic algorithm, the simulation results are the best.

Chemical corrosion of liquid-phase sintered SiC in NaOH aqueous solution

Corrosion behaviour of the liquid-phase sintered SiC ceramics (LPS-SiC) was studied through exploration of weight loss, strength reduction and morphology evolution of the SiC specimens etched in the room-temperature/70°C 6.12 mol/L (20 wt-%) NaOH aqueous solutions. As a comparison, corrosion of the reaction-bonded SiC ceramics (RB-SiC) was also investigated. The results show that corrosions of the SiC specimens mainly contain homogeneous dissolution of the secondary-phase oxides in the LPS-SiC and that of Si in the RB-SiC, and peel-off of the SiC particles, resulting in increase of the surface roughness, weight loss and strength reduction of the SiC ceramics. As dipped in the 70°C NaOH solution, corrosion pores and channels form in the sublayer, leading to rapid increase of weight loss and strength reduction of the LPS-SiC. No matter in the room-temperature or 70°C NaOH solution, the LPS-SiC always has a higher corrosion resistance than the RB-SiC.

Microstructures and thermal conductivities of reaction sintered SiC ceramics

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.

Random chopped fibers in reaction bonded SiC composite: Morphology, etching and reinforcing properties

Random chopped fibers were introduced into SiC/C slurry to improve the mechanical behaviors of reaction bonded SiC ceramics. The distribution, morphology and chemical properties of the siliconized carbon fiber were investigated. The effect of fiber fraction on mechanical behaviors of the reaction bonded silicon carbide composite was studied. The siliconization reaction led to a dense layer of nano and micron SiC particles on the fiber surface. Based on the morphology changes in the acid etching test, a structural model of the siliconized fiber was proposed. The fracture toughness and the flexural strength of the composite were improved by the random chopped fiber, reaching the peak values of 5.1MPam0.5 and 416MPa, respectively, at the fiber fraction of 30vol.%.

Preparation of reaction-bonded silicon carbide with well controlled structure by tape casting method

Tape casting is a reliable and cost effective method for producing thin ceramic sheets with uniform and tailored microstructures, especially for multilayered composite materials. In this paper, SiC/C tapes were prepared by tape casting method. After lamination and binder removal, porous preforms with homogeneous microstructure and narrow pore sizes distribution were developed. Then, dense reaction bonded SiC ceramics (RBSCs) were obtained by silicon infiltration into these preforms. The highest bending strength of the RBSCs can reach 410±14MPa. Moreover, impregnation of phenolic resin into the porous preforms before silicon infiltration could help to develop RBSCs with lower residual silicon content and higher flexural strength which can be as high as 598±112MPa.

Influence of uni and bi-modal SiC composition on mechanical properties and microstructure of reaction-bonded SiC ceramics

Influence of SiC composition with different particle size on mechanical properties and microstructure of reaction-bonded SiC have been investigated using infiltration process of liquid Si. SiC specimens were prepared by reaction-bonding sintering at 1620°C in vacuum using different SiC–C composition. Reaction-bonded SiC specimens show dense with 3.0–3.05 gr/cm3 of density. SiC specimens with bi-modal SiC composition show a fine microstructure than those of uni-modal SiC composition. In addition, in the specimens with bi-modal SiC composition, the microstructure of the sample with use of fine SiC starting powders is finer rather than those of use of relatively coarse SiC starting powders. The flexural strength and fracture toughness of specimens with bi-modal SiC composition are higher than those of uni-modal SiC composition. The main fracture mode of specimens with a high strength is intergranular fracture.

Fabrication of reaction-bonded SiC ceramics by slip casting of SiC/C suspension

The influence of particle size and size distribution on slip casting performance of SiC and SiC/C suspension was investigated. Two kinds of commercial grade SiC powder and petroleum coke powder were used as the starting powder. The results showed that suitable particle size distribution is helpful to obtain low viscosity and relative high packing density. Smaller particles were used to prevent sedimentation velocity of larger particles. Ternary suspension containing petroleum coke and two kinds of silicon carbide powder exhibited higher viscosity than binary suspension containing only two kinds of silicon carbide powder. The right amount of petroleum coke could lead to suitable viscosity for slip casting and high flexural strength. The maximum flexural strength of sintered products was 274 ± 15 MPa in this work.

Properties and Application of Oriented Porous SiC as Transpiration Cooling Materials

This paper describes the fabrication of porous reaction-bonded SiC ceramics with radial directed pores and the application of these materials to transpiration cooling system of rocket engines. A special mold is designed for freeze-casting process to prepare SiC cylinders with radial directed pores. Green bodies with well-oriented pore structure are obtained from slurry with solid loading up to 47vol%. The pore size is in the level of several tens micron. Green bodies with various porosities are infiltrated with different amount of liquid Si. The intrinsic permeability of each sample is measured with air as flowing media. It is concluded that permeability has relationship with not only the porosity but also the pore structure of samples.

Preparation and Properties of In~Situ Reaction Bonded Porous SiC Ceramics

An in situ reaction-bonding technique was developed to fabricate porous silicon carbide (SIC) ceramics in air from SiC and Al_2O_3,using graphite as the pore former.The main phase is SiC and the bonded phases are mullite and cristobalite in porous SiC ceramics.The reaction-bonded SiC ceramics possess connected open pores.The linear shrinkage of the specimens before and after sintering is between -1.5% and +1.5%.Open porosity decreases with the sintering temperature and forming pressure,but increases with the graphite content.Bulk density and mechanical strength increase with sintering temperature and forming pressure,but decrease with the graphite content. The pore size distribution takes on a bimodal distribution when the graphite particle size is 20.0μm. In addition,as-fabricated porous SiC ceramics exhibit low coefficient of thermal expansion,high N_2 permeability,excellent high temperature-oxidation resistance,good acid endurance and relatively bad alkaline endurance.

High-temperature strength and toughness behaviors for reaction-bonded SiC ceramics below 1400 °C

High-temperature strength and toughness behaviors of reaction-bonded SiC ceramics with 12 and 26 vol.% of free Si were investigated. The flexural strength and fracture toughness started to increase at 1000 °C and reached a maximum at 1300° and 1330 °C before a sharp drop, respectively. The transition from transgranular to intergranular fracture is considered to lead to the slight increase of strength and toughness from room temperature to 1000 °C, while the plastic deformation of free Si contributes to the great increase above 1000 °C. However, too high a temperature will result in the extreme softening of free Si and therefore decrease strength and toughness.

Development of High-Strength Si<sub>3</sub>N<sub>4</sub> Reaction-bonded SiC Ceramics

Development of High-Strength Si<sub>3</sub>N<sub>4</sub> Reaction-bonded SiC Ceramics

セラミックスの放電加工 炭化けい素焼結体について

Electro-discharge machining (EDM) of structural ceramics with relative low electrical resistivity was carried out. Some kinds of silicon carbide ceramics (hot pressed, normally sintered and reaction bonded SiC ceramics) were used as workpieces. Their removal rate, roughness of machined surface and wear ratio of copper tool electrode were examined quantitatively. Surface crackining was researched by scanning electron microscope (SEM). The remaining strength of the workpiece after EDM was measured by bending test. As the result, it was shown that any kinds of SiC ceramics with relative low electrical resistivity will be able to be machined by EDM without fatel damage. The remaining strength was mainly influenced by pulse current. The wear ratio of copper tool electrode was negligible if the pulse duration was longer than about 100μs.