Bonded Silicon Carbide Research Articles

Preparation of reaction bonded silicon carbide (RBSC) using boron carbide as an alternative source of carbon

RBSC composites are fully dense materials fabricated by infiltration of compacted mixtures of silicon carbide and carbon by molten silicon. Free carbon is usually added in the form of an organic resin that undergoes subsequent pyrolysis. The environmentally unfriendly pyrolysis process and the presence of residual silicon are serious drawbacks of this process. The study describes an alternative approach that minimizes the residual silicon fraction by making use of a multimodal particle size distribution, in order to increase the green density of the preforms prior infiltration. The addition of boron carbide provides an alternative source of carbon, thereby eliminating the need for pyrolized organic compounds. The residual silicon fraction in the RBSC composites, prepared according to the novel processing route, is significantly reduced. Their mechanical properties, in particular the specific flexural strength is by 15% higher than the value reported for RBSC composites prepared by the conventional approach.

Characterization of Silicon Carbide Films Prepared by Chemical Vapor Deposition

Silicon carbide prepared by chemical vapor deposition (CVD) is one of the important candidate materials for space mirror and high-power mirror such as laser mirror, because of its superior performances such as low density, high melting point and homogeneity. In this paper, the SiC coatings were deposited on the substrates of reaction bonded silicon carbide (RB-SiC) by CVD process. Then, the morphologies of the deposits were examined with scanning electron microscopy. The crystalline phase of the as-deposited films was confirmed with X-ray diffractometry. And the adhesion between the CVD film and the substrate was rated with scraping method. As a result, the morphologies of the deposits, i.e. whiskers at 1050°C or films at 1100°C, are different from that of the substrate. And the mean diameter of the deposits at 1100°C is larger than that at 1050°C. Furthermore, the crystalline phase of the as-deposited film is determined as β-SiC and the adhesion is firm enough not to be peeled off with the scraping test.

A New Technique for Testing Abrasion Resistance of Refractories at Elevated Temperatures

Based on an extensive comparison of test methods for abrasion resistance from home and abroad, a new technique for abrasion resistance at elevated temperature has been developed with some improvements. Two connected airtight chambers are designed to prevent compressed air from entering the sample chamber, so that heat loss of the sample chamber can be reduced and the chamber temperature is less disturbed. The sample surface temperature can reach stable within 5 min and the temperature fluctuation on sample surface can be less than 20°C in the course of inleting the compressed air at working temperatures up to1400°C. The repeatability was tested using float-glass plate as reference sample at ambient temperature and using high alumina bricks for elevated temperature. A variation coefficient under 7% at ambient temperature has been achieved. Comparison of abrasion resistance at elevated temperature was tested respectively on a high alumina brick and on a silicon nitride bonded silicon carbide brick and the results could be significantly distinguished. Nitrogen can be blown into sample chamber during heating the furnace to prevent nonoxide bearing samples from being oxidized.

Preparation of C<sub>f</sub>/SiC Composites by Liquid Silicon Infiltration

The Cf/SiC made from carbon fiber preforms infiltrated by phenol resin, pure carbon slurry and aqueous C/SiC slurry showed different binding strength between carbon fiber and SiC matrix, thus influenced the fracture behavior of the composite. The fracture toughness of the Cf/SiC composites with the value of 9.82MPa•m1/2,improved remarkably compared with reaction- bonded silicon carbide (RBSC). But the flexural strength was less than 100 MPa, because of the existence of considerable amount of pores in C/SiC composites.

The Ultra-Precision Polishing of Large Aperture Reaction Bonded Silicon Carbide Mirror

Problem statement: Silicon Carbide (SiC) optical materials have becom e the first choice for mirrors in space optical systems and large grou nd-based optical systems due to their outstanding mechanical, physical and optical properties. Compar ed with traditional optical glasses, SiC optical materials are provided with the characteristics of high hardness and multiphase, which embarrass the high efficient fabrication of SiC mirrors with ultr a-smooth surfaces and high precision. Approach: We choose some typical polishing parameters and conduct a series of experiments, trying to find out the relationship between polishing parameters and resul ting surface roughness. The polishing parameters for Reaction Bonded Silicon Carbide (RB-SiC) are optimized from the analysis of these experiments. Then we can apply these parameters to the figuring process. Computer Controlled Optical Surfacing (CCOS) is a widely used deterministic polishing met hod and features in low-cost, high-precision and large flexibility. The basic theory and process of CCOS are given in detail. Then we employ CCOS in the ultra-precision machining of RB mirrors. Results: We polish a RB SiC sample with the optimized polishing parameters and a surface roughness better than 1nm (RMS) is obtained. A 475 mm diameter sphere RB-SiC mirror with a relative aperture of 1: 1 is polished by CCOS and the surface error reduces to 0.175 λ(PV)/0.009 λ(RMS) from 0.526 λ(PV)/0.080 λ(RMS). Both results represent a very smooth surface and a very precise figure. Conclusion: The result proves the feasibility of the polishing technology and CCOS method. These works also accumulate experience for the manufacturing of aspheric SiC mirror.

Study on the aluminium phosphate bonded SiC refractory performance and how it is influenced by oxidation and chemical attack

AbstractAluminium reduction cells service life and cell efficiency are strongly influenced by the side wall refractory lining. The aluminum phosphate bonded SiC refractories developed by us, instead of the carbon ones, have increased the cell capacity and productivity by the larger anode surfaces. In order to rank performances of various SiC products were carried out specific laboratory tests. The results of our research concerning materials made from aluminum phosphate bonded SiC showed that they are refractories with a high corrosion resistance and performances close to the bricks made of silicon nitride bonded silicon carbide. By extrapolation of the present study's results we can anticipate an increased service life of electrolysis cells that have such side walls. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

다층 기공구조를 갖는 다공성 반응소결 탄화규소 다공체 제조

Reaction Bonded Silicon Carbide(RBSC) has been used for engineering ceramics due to low-temperature fabrication and near-net shape products with excellent structural properties such as thermal shock resistance, corrosion resistance and mechanical strength. Recently, attempts have been made to develop hot gas filter with gradient pore structure by RBSC to overcome weakness of commercial clay-bonded SiC filter such as low fracture toughness and low reliability. In this study a fabrication process of porous RBSC with multi-layer pore structure with gradient pore size was developed. The support layer of the RBSC with multi-layer pore structure was fabricated by conventional Si infiltration process. The intermediate and filter layers consisted of phenolic resin and fine SiC powder were prepared by dip-coating of the support RBSC in slurry of SiC and phenol resin. The temperature of

Tungsten Carbide as a Diffusion Barrier on Silicon Nitride Active- Metal-Brazed Substrates for Silicon Carbide Power Devices

Recently, silicon nitride (Si3N4) has been receiving renewed attention because of its potential use as a substrate material for packaging of silicon carbide (SiC) power devices for high temperature applications. It is an attractive material for this application because it has moderate thermal conductivity and a low coefficient of thermal expansion, which is close to that of SiC. Materials that show promise for use as a diffusion barrier on Si3N4 substrate for bonding SiC devices to a Si3N4 substrate are refractory metals such as titanium (Ti), molybdenum (Mo), tungsten (W), and their alloys. Tungsten carbide (WC) shows promise as a diffusion barrier for bonding these devices to copper metallization on Si3N4 substrates. This paper presents the results of an investigation of a metallization stack (Si3N4/Cu/WC/Ti/Pt/Ti/Au) used to bond SiC dice to Si3N4 substrates. The dice were bonded using transient liquid phase bonding. Samples were characterized using X-ray diffraction for phase identification and Auger electron spectroscopy for depth profiling of the elemental composition of the metallization stack in the as-deposited state, and immediately following annealing. The metallization remained stable following subjection to a temperature of 400°C for 100 h in air.

Process variability in honing of cylinder liner with vitrified bonded diamond tools

With the increasing use of ultra-hard abrasives and the introduction of more powerful and rigid machines, honing process is spreading its performances. In industrial honing of cylinder liners for internal combustion engines, the degree of penetration that Metallic Bonded Diamond (MBD) abrasive stones have experienced compared to conventional Vitrified Bonded Silicon Carbide (VBSC) tools is obvious. The long tool life of metal bonded diamond sticks is however balanced by its main role in producing torn and folded metal without cutting-effect finish. To simultaneously reduce folding and remove the stock with improved stone life, an experimental investigation is reported on the process variability in finish honing behavior due to stone inconsistency and nature of its properties. We consider the achievable performances by finish honing of cast iron liners with a new abrasive stone. This superabrasive consists of micro-sized diamond abrasives which are bonded with vitrified bond (VBD). By selection of finer grit abrasive while honing with VBD tool, conditions could be created that gave a cutting-effect finish to the diamond honing and minimized the folding over.

Effect of Grinding Induced Damage on Bending Strength of RBSiC

The effect of surface residual stress and crack on bending strength of reaction bonded silicon carbide(RBSiC) after grinding was investigated.The residual stress of the ground surfaces were determined using X-ray diffraction.The sizes of strength controlling cracks were assessed using fracture mechanics approach.The investigations show that mechanical load,which has relation with grinding direction,phays a dominateve role in the grinding processes.and the measured residual stresses have a direction dependency.With increase of down feed from 0.9μm/s to 1.35μm/s,the bending strength is reasonably correlated with the surface residual stress and crack size.

REVIEW AND APPLICATIONS OF THERMAL CONDUCTIVITY MODELS TO ALUMINUM CELL SIDEWALL REFRACTORIES

Silicon nitride bonded silicon carbide ( Si 3 N 4- SiC ) refractories are commonly used as the sidewall of aluminum electrolysis cells. They have to withstand an extremely corrosive molten electrolyte bath for long periods. The sidewall is normally protected with a layer of solidified electrolyte (called frozen ledge), which is sensitive to the thermal conductivity of the sidewall. In this work, through review of the literature on modeling methods for predicting the effective thermal conductivity of dense composites and porous materials, some selected methods were applied to calculate the effective thermal conductivity of Si 3 N 4- SiC refractories. The model predictions were compared with the thermal conductivity of a commercial Si 3 N 4- SiC refractory measured by using laser flash technique. The present study showed that, due to multi-phase nature and complex microstructure of Si 3 N 4- SiC refractories, most of the selected modeling methods individually do not give satisfactory predictions in one step. Recursive applications of one method or combinations of different methods are capable of giving satisfactory predictions.

Solid-State NMR Characterization of Silicon Nitride Bonded Silicon Carbide Refractories

Solid-state 29Si NMR spectroscopy and X-ray diffraction have been used to analyze commercial samples of silicon nitride bonded silicon carbide (SNBSC) refractories. Spectra of samples before and af...

용융 Si 침윤법에 의해 제조된 반응소결 탄소 섬유강화 탄화규소 복합체 제조; I. 탄소 섬유 코팅 방법에 따른 영향

Reaction bonded silicon carbide (RBSC) composite for heat-exchanger was fabricated by molten Si infiltration method. For enforcing fracture toughness to reaction bonded silicon carbide composite, the surface of carbon fiber has coating layer by SiC or pyrocarbon. For SiC layer coating, CVD method was used. And for carbon layer coating, the phenol resin was used. In the case of carbon layer coating, fracture toughness and fracture strength were enhancing to 4.4MPa ·m 1/2 and 279 MPa.

용융 Si 침윤법에 의해 제조된 반응소결 탄화규소 복합체에서 SiC 입자 크기의 영향

Reaction bonded silicon carbide(RBSC) composite for heat-exchanger was fabricated by molten Si infiltration method. The raw materials with variable particle sizes were used in this experience. The finer the particle size in sintered silicon carbide was the more increasing 3-point bending strength and fracture toughness. As the adaptable particle sizes had been occupied interstice arising from packing sample, the mechanical properties were increased. In the PCS1-1 sample, the 3-point bending strength and fracture toughness were 323㎫ and 4.9㎫·m 1/2 , respectively.

고분자 세라믹 전구체를 이용한 반응 소결 탄화규소의 접합

Reaction bonded silicon carbide (RBSiC) is an important engineering ceramic because of its high strength and stability at elevated temperatures, and it is currently fabricated using reasonably cheap manufacturing processes, some of which have been used since the 1960s. However, forming complicated shapes from these materials is difficult because of their poor workability. The purpose of this work is to join the reaction-bonded SiC parts using a preceramic polymer as joint material. The manufacturing of ceramic material in the system Si-O-C from preceramic silicon containing polymers such as polysiloxanes has attained particular interest. The mixtures of preceramic polymer and filler materials, such as SiC, Si and MoSi, were used as a paste for the joining of reaction sintered SiC parts. The joining process during the annealing in Ar atmosphere at <TEX>$1450^{\circ}C$</TEX> were described. The maximum strength of the joints was 63 MPa for the specimen joined with 10 vol.% of <TEX>$MoSi_2$</TEX> and 30 vol.% of SiC as filler materials. Fracture occurred in the joining layer. This indicates that the joining strength is limited by the strength of the joint materials.

침윤된 Si 및 성형체내 Carbon Source의 양이 반응소결 탄화규소 다공체의 기공률 및 파괴강도에 미치는 영향

A porous reaction bonded silicon carbide (RBSC) was fabricated by a molten Si infiltration method. The porosity and flexural strength of porous RBSC fabricated in this study were dependent upon the amount of carbon source used in the SiC/carbon preform as well as the amount of Si infiltrated into the SiC/carbon preform. The porosity and flexural strength of porous RBSC were in the range of 20 vol.~49 vol.% and 38~61㎫, respectively. With increase of carbon contents and molten Si for infiltration, volume fraction of the pores was gradually decreased, and flexural strength was increased. The porous RBSCs fabricated with the same amount of molten Si show less residual Si around neck with increase of carbon source, as well as a new SiC was formed around neck which resulted in the decreased porosity and improvement of the flexural strength. In addition, decrease of the porosity and increase of the flexural strength were also obtained by increase of the amount of molten Si with the same amount of carbon source. However, it was found that the flexural strength of porous RBSC depends on the porosity rather than the amount of the newly formed SiC in neck phase between SiC particles used as a starting material.

RB-SiC Ceramics Derived from the Phenol Resin Added with Starch

Reaction bonded silicon carbide (RB-SiC) was fabricated by phenol resin, starch, solidified agent and silicon powder through the following steps: first, carbonizing at high temperature for 7-9h, infiltrating silicon at 1450-1600oC for 0.5-2h, and then removing excessive silicon at 1700oC for 0.5h. Scanning electron microscopy and X-ray diffraction were employed to characterize and analyze the microstructures and phase composition of the preforms and the final RB-SiC products. In addition, the effect of carbonization temperature, the amount of starch and solidified agent on strength and apparent porosity of final RB-SiC were also discussed. The results showed that the carbonization process of phenol resin can be divided into three steps: at temperatures from 400oC to 600oC, the structure of polymer changes less; at temperatures from 600oC to 1000oC, the fundamental chain of polymer is destroyed, and inverts to glass-like carbon; at temperatures from 1000oC to 1200oC, with the increasing of carbonization temperature, the structure of glass-like carbon changes into the structure of disorder graphite. And the increased micro-pores could be obtained by adding starch.

Multi response optimization of machining parameters of drilling Al/SiC metal matrix composite using grey relational analysis in the Taguchi method

This paper presents a new approach for the optimization of drilling parameters on drilling Al/SiC metal matrix composite with multiple responses based on orthogonal array with grey relational analysis. Experiments are conducted on LM25-based aluminium alloy reinforced with green bonded silicon carbide of size 25 μm (10% volume fraction). Drilling tests are carried out using TiN coated HSS twist drills of 10 mm diameter under dry condition. In this study, drilling parameters namely cutting speed, feed and point angle are optimized with the considerations of multi responses such as surface roughness, cutting force and torque. A grey relational grade is obtained from the grey analysis. Based on the grey relational grade, optimum levels of parameters have been identified and significant contribution of parameters is determined by ANOVA. Confirmation test is conducted to validate the test result. Experimental results have shown that the responses in drilling process can be improved effectively through the new approach.

Polycarbosilane Conversion Bonded Porous SiC Ceramics

Porous silicon carbide (SiC) ceramics were fabricated by a polycarbosilane (PCS) conversion bonding technique, in which PCS was used as a binder to bond SiC particles with each other. In the preparing process, SiC particles were first coated with PCS, and then the powder compacts were heat-treated in an inert atmosphere. During the heat-treatment, the PCS decomposed and gradually converted to inorganic covalent solids composed mainly of Si-C networks. The pyrolysis process of PCS, the pore structures and flexural strength of the as-prepared specimens were analyzed and discussed. Preparing temperature as low as 1100°C was adopted in this process and the porous SiC ceramics with a flexural strength of 20 MPa at an open porosity of 43% was obtained. Since PCS was used as a binder, the critical feature of this technique was that the preparation of porous SiC body was achieved at a low temperature.

Fabrication of Large Ceramic Components with Controlled Microstructure by Powder Thermoset Molding

Thermoset molding in wet and dry state was successfully employed to fabricate high strength reaction bonded silicon carbide (RBSC) ceramics. Granule transfer molding (GCM) was developed to prevent segregation of component particles and binder phase in wet state, while granule compression molding was applied in dry state to fabricate green compact with significant variation of compaction ratio. Low-fill density granules with mixing homogeneity were critical for promoting lateral deformation of granules during consolidation. In addition, anodic performance of Ni-YSZ anode was significantly enhanced by replacing solid fugitive phase with viscoelastic fugitive phase used as binder in thermoset molding.