Robocasting of reaction bonded silicon carbide structures

A novel shaping method for the fabrication of reaction bonded silicon carbide structures was investigated in this work. A paste consisting of silicon carbide as inert filler and carbon powder was developed and printed by robocasting technology. Layer by layer deposition of the ceramic paste facilitates the printing of complex shaped structures. Different structures such as lattices, hollow cylinders, bending bars and gyroids were printed using nozzles with diameter of 0.5 mm and 1.5 mm. After pyrolysis at 700 °C and further heat treatment at 1850 °C the samples were infiltrated using the liquid silicon infiltration technique to obtain dense near-net shape RBSC structures. The robocasted structures showed a hardness of approximately 20 GPa, a thermal conductivity of ～112 W/m*K, Young’s modulus of ～356 GPa, flexural strength of ～224 MPa and an amount of residual silicon of approximately 23%. These measured properties are comparable with those of traditionally fabricated RBSC.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

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.     

反应烧结碳化硅材料的抗热震性能研究

通过对比不同温差热震后材料的残余强度 ,对反应烧结碳化硅材料的抗热震性能进行了研究。结果表明 :反应烧结碳化硅材料的抗热震性能与显微组织密切相关 ,低游离硅含量与小粒径的反应烧结碳化硅材料具有较好的抗热震断裂性能 ,而高游离硅含量或大碳化硅粒径的材料具有相对优异的抗热震损伤性。对反应烧结碳化硅材料的抗热震性与显微组织的关系进行了探讨。     

氧氮化硅结合碳化硅制品的生产与使用

以工业用黑色碳化硅砂、硅粉为主要原料，研制出了导热性能优良、抗热震性好、耐高温、耐侵蚀及耐磨损，且生产工艺较简单、成本较低的氧氮化硅结合的碳化硅制品．该产品已广泛应用于冶金炉、化工设备及发电用锅炉的内衬，并取得了较满意的效果。     

Open-cell reaction bonded silicon nitride foams: Fabrication and characterization

In this study, a newly designed fabrication procedure was utilized to produce silicon nitride foams. The main goal of the present study was to obtain Si₃N₄ foams with high levels of porosity and pore interconnectivity via an economical fabrication procedure including sacrificial template technique, gel-casting and reaction bonding processes. The fabrication procedure was studied and optimized in terms of suspension preparation and rheology, gel-casting parameters, and reaction bonding conditions. The produced foams have a precisely controlled level of porosity which can be varied up to 87 vol%. BET analysis showed that the surface area of the foam is of the order of 2.01 m²/g. The pore interconnectivity of the foam was investigated via polyester resin infiltration. Based on XRD and SEM analysis, the dominant nitriding reactions are the gas-phase reactions which lead to α-Si₃N₄ in the form of whiskers.     

Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite

This paper deals with the microstructure and mechanical properties of reaction bonded silicon carbide reinforced with random chopped carbon fibers of 3 mm length. The composites were fabricated by dispersing chopped carbon fibers into bimodal SiC/C suspension, forming green body through slip casting, and then reaction sintering at 1700 °C. The effect of the chopped fiber fraction on microstructure and mechanical properties was evaluated. A significant increase of fracture toughness was obtained as the carbon fiber fraction approaches 30 vol.%. The chopped fibers had reacted with liquid silicon during reaction sintering, so little fiber pullout was observed. Crack deflection and bridging is the predominant mechanism for the composite toughening.     

Combination of direct ink writing and reaction bonded for rapid fabrication of SiCw/SiC composites

Silicon carbide ceramic matrix composites are widely used in aerospace field due to their advantages of high temperature resistance, high strength and corrosion resistance. However, its application is greatly limited because of the difficulty in preparing complex shape structures by traditional machining methods. Here, a new strategy for preparing SiC_w/SiC complex structure by combining direct ink writing with reaction bonding is proposed. A water-based slurry consisting of silicon carbide, carbon powder and silicon carbide whisker was developed. The influence laws of C content and SiC_w content in slurry on sintering properties of direct-written samples were studied. The reaction bonding mechanism and whisker reinforcing and toughening mechanism were analyzed by means of microstructure and phase composition. The results show that the slurry exhibits shear thinning behavior with stress yield point, and its flow behavior and plasticity meet the requirements of direct writing. When the carbon content is 6.4 wt%, the maximum flexural strength is 239.3 MPa. When 15 wt% SiC_w was added, the flexural strength of the composite reached 301.6 MPa, and when 20 wt% SiC_w was added, the fracture toughness of the composite reached 4.02 MPa m^1/2, which was increased by 26% and 18% compared with single-phase SiC, respectively. The reinforcing and toughening mechanisms of the whiskers mainly include whisker pullout, crack deflection and whisker bridging. After direct ink writing and reaction bonded, the whole process shows good near net forming ability. 3D printed SiC_w/SiC composites have great application prospects in aerospace field.     

Direct laser sintering of reaction bonded silicon carbide with low residual silicon content

Additive manufacturing (AM) techniques are promising manufacturing methods for the production of complex parts in small series. In this work, laser sintering (LS) was used to fabricate reaction bonded silicon carbide (RBSC) parts. First, silicon carbide (SiC) and silicon (Si) powders were mixed in order to obtain a homogeneous powder. This powder mixture was subsequently laser sintered, where the Si melts and re-solidifies to bind the primary SiC particles. Afterwards, these SiSiC preforms were impregnated with a phenolic resin. This phenolic resin was pyrolysed yielding porous carbon, which was transformed into secondary reaction formed SiC when the preforms were infiltrated with molten silicon in the final step. This resulted in fully dense RBSC parts with up to 84?vol% SiC. The optimized SiSiC combined a Vickers hardness of 2045?HV, an electrical conductivity of 5.3?×?10³?S/m, a Young's modulus of 285?GPa and a 4-point bending strength of 162?MPa.     

激光闪射法测试氮化硅结合碳化硅材料导热系数的研究

激光闪射法在材料导热系数的测量方面已得到广泛研究和应用。利用激光闪射法测试了氮化硅结合碳化硅材料的导热系数,分析了影响结果准确性的因素,并提出了相应措施。     

Microstructure and mechanical properties of reaction bonded B4C-SiC composites: The effect of polycarbosilane addition

Reaction bonded B₄C-SiC composites were prepared by infiltrating silicon melt into porous B₄C-SiC green preforms at 1500 °C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B₄C after pre-sintering at 1600 °C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B₄C-SiC composites. For the B₄C-SiC composite with 5 wt% PCS added, a flexural strength of 319±12 MPa, and an elastic modulus of 402±18 GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B₄C-SiC composites are further discussed in our work.     

Relaxation of residual microstress in reaction bonded silicon carbide

High temperature annealing reduces the residual microstress in the silicon phase and silicon carbide phase in monolithic reaction bonded silicon carbide and in the matrix of melt-infitrated composites of silicon carbide reinforced with silicon carbide fibers. Stress relaxation is related to creep of the silicon carbide with power-law creep exponents similar to tensile creep in reaction bonded silicon carbide.     

热处理温度对反应烧结碳化硅材料组织与性能的影响

研究了真空热处理温度对反应烧结碳化硅材料显微组织和断裂强度的影响.结果表明反应烧结碳化硅中的游离硅在1600℃、1800℃真空热处理过程中已全部去除;经过1800℃真空热处理材料的强度均高于1600℃真空热处理材料的强度.在1800℃真空热处理过程中发生的碳化硅再结晶以及气孔形状的变化,是其强度较高的主要原因.     

氮化硅结合碳化硅耐火材料在钢水中的腐蚀行为

运用扫描电镜（ＳＥＭ）技术，研究了氮化硅结合碳化硅耐火材料在钢水中的腐蚀行为。结果表明，金属与氮化硅结合碳化硅材料之间的界面清晰，基体内部无任何金属渗入，但氧化的材料表面有氧化物粘附。     

Effect of multi-walled carbon nanotubes and silicon carbide nanoparticles on the deleterious influence of water absorption in adhesively bonded joints

In this study, the deleterious influence of hot deionized water on adhesively bonded joints was reduced with silicon carbide (SiC) nanoparticles and multi-walled carbon nanotubes (MWCNTs). A gravimetric method was used to study the kinetics of water ingress into the neat and nanocomposite epoxy adhesives. Then, joints were manufactured using the same neat and nanocomposite adhesives and aged for different periods according to the results obtained from the bulk sample tests and finite element modeling. The results showed that the reinforcing effect of nanofillers on the strength was about three times higher for the wet epoxy adhesive compared to the dry one. Moreover, it was found out that introducing 4.4 wt% of SiCs or 0.52 wt% of MWCNTs to the adhesive can compensate the degrading influence of aging under near-saturated condition. Furthermore, the scanning electron microscope (SEM) fractography was used to assess the fracture surfaces of the neat and reinforced samples.     

The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient,and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si₃N₄ porous ceramics (Si₃N₄-PC) have attracted attention due to their excellent intrinsic properties of Si₃N₄ and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si₃N₄-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si₃N₄-PC while it has been neglected in recent years. In this study, RB and SRB Si₃N₄-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si₃N₄-PC in wave-transparent materials. The results show that RB Si₃N₄-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si₃N₄-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σ_F and γ_wof. As a result, the RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σ_F of 109.94 MPa and 119.56 MPa as well as maximum γ_wof of 990.74 J m^-2 and 1167.88 J m^-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10^-3 and 22.90 × 10^-3, respectively. It is concluded that RB Si₃N₄-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.     

Synthesis and characterization of mesoporous silicon carbide

A modified sol–gel method is proposed for the preparation of mesoporous silicon carbide. In this method, tetraethoxysilane (TEOS) and phenolic resin are used for preparing a binary carbonaceous silicon xerogel, and nickel nitrate is employed in the sol–gel process as a pore-adjusting reagent. SiC was produced in the carbothermal reduction of the binary xerogel at 1250 °C in an argon flow (40 cm³ min⁻¹) and purified by removing excess silica, carbon and other impurities. The purified SiC sample was characterized by XRD, SEM, TEM, and N₂ adsorption, and the results showed that the SiC sample had a surface area of 112 m² g⁻¹ (BET) and an average pore diameter of about 10 nm.     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

Interfacial zone surrounding the diamond in reaction bonded diamond/SiC composites: Interphase structure and formation mechanism

Diamond/SiC composites have attracted considerable research interests due to their outstanding properties sought for a wide range of applications. Among a few techniques used for the fabrication of diamond/SiC composites, molten Si infiltration is an approach highly favored due to its cost-effectiveness and process flexibility. This study critically evaluated the interfacial zone surrounding the diamond in a reaction bonded (RB) diamond/SiC composite. XRD suggests that the composite consists of diamond, α-SiC, β-SiC, Si, and graphite. TEM reveals that a thin layer of graphite surrounds the diamond grain and it appears to form through a process of diamond graphitization and amorphous carbon transformation during the fabrication. In addition, a carbon dissolution and saturation process is proposed as a predominant mechanism for the formation of nano-crystalline SiC near the interface as well as the defects inside the SiC grits. A minor Al₄C₃ phase is occasionally detected near the interface region.     

环氧树脂/碳化硅复合材料的热性能及韧性

采用碳化硅作为增强剂制备了环氧树脂/碳化硅复合材料,考察了复合材料的热学及力学性能。实验结果表明,碳化硅的添加使环氧树脂的玻璃化温度提高。当碳化硅添加质量分数为3%时,复合材料的韧性与纯环氧树脂相比提高了35%。