Fabrication of dense SiSiC ceramics by a hybrid additive manufacturing process

In this work, we report the fabrication of Silicon infiltrated Silicon Carbide (SiSiC) components by a hybrid additive manufacturing process. Selective laser sintering of polyamide powders was used to 3D print a polymeric preform with controlled relative density, which allows manufacturing geometrically complex parts with small features. Preceramic polymer infiltration with a silicon carbide precursor followed by pyrolysis (PIP) was used to convert the preform into an amorphous SiC ceramic, and five PIP cycles were performed to increase the relative density of the part. The final densification was achieved via liquid silicon infiltration (LSI) at 1500°C, obtaining a SiSiC ceramic component without change of size and shape distortion. The crystallization of the previously generated SiC phase, with associated volume change, allowed to fully infiltrate the part leading to an almost fully dense material consisting of β-SiC and Si in the volume fraction of 45% and 55% respectively. The advantage of this approach is the possibility of manufacturing SiSiC ceramics directly from the preceramic precursor, without the need of adding ceramic powder to the infiltrating solution. This can be seen as an alternative AM approach to Binder jetting and direct ink writing for the production of templates to be further processed by silicon infiltration.     

Fabrication and characterization of carbon fiber reinforced SiC ceramic matrix composites based on 3D printing technology

A novel method has been developed to fabricate carbon fiber reinforced SiC (C_f/SiC) composites by combining 3D printing and liquid silicon infiltration process. Green parts are firstly fabricated through 3D printing from a starting phenolic resin coated carbon fiber composite powder; then the green parts are subjected to vacuum resin infiltration and pyrolysis successively to generate carbon fiber/carbon (C_f/C) preforms; finally, the C_f/C preforms are infiltrated with liquid silicon to obtain C_f/SiC composites. The 3D printing processing parameters show significant effects on the physical properties of the green parts and also the resultant C_f/C preforms, consequently greatly affecting the microstructures and mechanical performances of the final C_f/SiC composites. The overall linear shrinkage of the C_f/SiC composites is less than 3%, and the maximum density, flexural strength and fracture toughness are 2.83?±?0.03?g/cm³, 249?±?17.0?MPa and 3.48?±?0.24?MPa m^1/2, respectively. It demonstrates the capability of making near net-shape C_f/SiC composite parts with complex structures.     

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。     

Effect of technological parameters on densification of reaction bonded Si/SiC ceramics

Si/SiC composite ceramics was produced by reaction sintering method in process of molten silicon infiltration into porous C/SiC preform fabricated by powder injection molding followed by impregnation with phenolic resin and carbonization. To optimize the ceramics densification process, effect of slurry composition, debinding conditions and the key parameters of all technological stages on the Si/SiC composite characteristics was studied. At the stage of molding the value of solid loading 87.5% was achieved using bimodal SiC powder and paraffin-based binder. It was found that the optimal conditions of fast thermal debinding correspond to the heating rate of 10?°C/min in air. The porous C/SiC ceramic preform carbonized at 1200?°C contained 4% of pyrolytic carbon and ～25% of open pores. The bulk density of Si/SiC ceramics reached 3.04?g/cm³, silicon carbide content was 83–85?wt.% and residual porosity did not exceed 2%.     

Reaction forming of silicon carbide ceramic using phenolic resin derived porous carbon preform

SiC ceramics were prepared with porous carbon preforms derived from phenolic resin by a reaction-forming method. The effects of the structure of the preform pores and the infiltration process on the properties of SiC ceramics were investigated, and components with complex shapes were fabricated by combining this process with stereolithography (SLA). Dense SiC ceramics were obtained from carbon preforms with high apparent porosities, but SiC ceramics with many macrodefects resulted from a carbon preform with an apparent porosity of 39%. The infiltration of molten silicon into the preform pore channel was accelerated under vacuum pressure, resulting in an increase in the depth of the Si infiltration. The growth of SiC was predominantly controlled by carbon diffusion at the last stage of the reaction. The extended grain growth caused the SiC grains to coalesce and some free Si was enveloped in the SiC grains. SiC components with complex geometries were fabricated by combining reaction forming with SLA. The geometry was controlled by SLA.     

Concurrent reaction-bonded joining and densification of additively manufactured silicon carbide by liquid silicon infiltration

In this study, additive-manufactured silicon carbide preforms were joined and densified by reaction bonding via liquid silicon infiltration. The silicon carbide preforms were first printed by binder jetting additive manufacturing. To demonstrate concurrent joining and densification, two preforms with carbon or parchment papers at the interface were concurrently joined and infiltrated by liquid silicon. Results showed a robust interface with thicknesses ranging from 150 to 500 µm, depending on the paper type and the number of paper layers. High-energy synchrotron X-ray revealed that β-phase silicon carbide was formed inside the interface. Finally, two additively manufactured samples with complicated channel geometry were successfully joined. Energy dispersive spectroscopy of the interface of the channeled samples showed a consistent and robust joining. This concurrent approach of joining and densification enables efficiency improvement of fabricating silicon carbide parts with complicated geometries and widens geometry freedom for additive manufacturing of silicon carbide.     

Silicon infiltrated silicon carbide from extruded thermoplastic wood polymer composites

Silicon-infiltrated silicon carbide (SiSiC) is an important technical ceramic material for several demanding applications such as heat exchangers, nozzles or mechanical seals. However, shaping and machining tools are quickly worn down, due to the application of hard abrasive silicon carbide (SiC) particles as part of the conventional starting compounds for monolithic SiSiC ceramics. Within this work, an alternative route fabricating SiSiC without primary SiC particles and with low residual carbon contents derived from thermoplastic wood polymer composites (WPC) is described. By varying the proportions of the raw materials, the phase compositions of the SiC ceramic could be modified. A reduction in the average wood particle size from 120 to 60 µm led to a homogenous SiSiC with high SiC content. SiSiC with flexural strengths up to 230 MPa and a Weibull modulus of 16 were developed. The residual carbon content could be reduced below 1 wt%.     

Preparation of low residual silicon content Si-SiC ceramics by binder jetting additive manufacturing and liquid silicon infiltration

Complex silicon carbide (SiC) ceramic components are difficult to fabricate due to their strong covalent bonds. Binder jetting (BJ) additive manufacturing has the outstanding advantages of high forming efficiency and no thermal deformation, especially suitable for printing complex structure SiC components. This study tried to obtain low silicon content silicon carbide ceramics by binder jetting followed by phenolic resin impregnation and pyrolysis (PRIP) and liquid silicon infiltration (LSI). BJ was used for the SiC green parts fabrication, and the highest compressive strength (7.7 ± 0.3 MPa) and lowest dimensional deviations (1.2–1.6 mm) were obtained with the printing layer thickness of 0.15 mm. Subsequently, PRIP treatments were introduced to increase the carbon content for the following LSI process. As the number of PRIP cycles increased, the carbon density of SiC/C preform increased and the porosity decreased. After the LSI treatment, the final Si-SiC composites processed with 2 PIRP cycles reached the highest flexural strength (257 ± 14.26 MPa) and the best wear resistance. This was attributed to the low residual silicon content (10.2 vol%) and almost no residual carbon. Furthermore, several complex structural components were fabricated using these methods. The preparation of complex components verifies the feasibility of BJ and LSI for manufacturing high-strength and high-precision SiC ceramics. Besides, this work hopes to provide technical guidance for the preparation of complex SiC composites in the future.     

Properties of SiC-Si made via binder jet 3D printing of SiC powder,carbon addition,and silicon melt infiltration

We report the physical and mechanical properties of ceramic composite materials fabricated by binder jet 3D printing (BJ3DP) with silicon carbide (SiC) powders, followed by phenolic resin infiltration and pyrolysis (IP) to generate carbon, and a final reactive silicon melt infiltration step. After two phenolic resin infiltration and pyrolysis cycles; porosity was less than 2%, Young's modulus was close to 300 GPa, and the flexural strength was 517.6 ± 24.8 MPa. However, diminishing returns were obtained after more than two phenolic resin infiltration and pyrolysis cycles as surface pores in carbon were closed upon the formation of SiC, resulting in reaction choking and residual-free carbon and porosity. The instantaneous coefficient of thermal expansion of the composite was found to be independent of the number of phenolic IP cycles and had values of between 4.2 and 5.0 ppm/°C between 300 and 1000℃, whereas the thermal conductivity was found to have a weak dependence on the number of phenolic IP cycles. While the manufacturing procedures described here yielded highly dense, gas impermeable, siliconized SiC composites with properties comparable to those of bulk siliconized silicon carbide processed according to conventional techniques, BJ3DP enables the manufacture of objects with complex shape, unlike conventional techniques.     

Influence of Phenolic Resin Impregnation on the Properties of Reaction‐Bonded Silicon Carbide

Reaction‐bonded silicon carbide ceramics fabricated from tape casting and Si infiltration have been reported in previous studies. To reduce the residual Si content in the sintered bodies, impregnation of phenol–formaldehyde resin (PF) into the porous green preforms before Si infiltration was proposed and studied in this work. The impregnation of PF solution not only helped to reduce the porosity and increase the carbon content of the green preforms, but also improved their strength. As a result, the flexural strength of the RBSC increased a lot and reached 856 ± 161MPa, whereas the residual Si content was reduced to 10 vol%.     

Synthesis and characterization of submicron silicon carbide powders with silicon and phenolic resin

A novel three-step process is used to fabricate submicron silicon carbide powders in this paper. The commercially available silicon powders and phenolic resin are used as raw materials. In the first step, precursor powders are produced by coating each silicon powder with phenolic resin shell. Then, precursor powders are converted into carbonized powders by decomposing the phenolic resin shell. The submicron silicon carbide powders are formed in the reaction of silicon with carbon during the third step of thermal treatment. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric (TG) analyses are employed to characterize the microstructure, phase composition and free carbon content. It is found that the sintered powders consist of β-SiC with less than 0.2 wt.% of free carbon. The particle size of the obtained silicon carbide powders varies from 0.1 to 0.4 μm and the mean particle size is 0.2 μm. The silicon carbide formation mechanism of this method is based on the liquid-solid reaction between liquid silicon and carbon derived from phenolic resin. The heat generated during the reaction leads to great thermal stress in silicon carbide shell, which plays an important role in its fragmenting into submicron powders.     

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.     

Studying the wettability of Si and eutectic Si-Zr alloy on carbon and silicon carbide by sessile drop experiments

The contact angles of two different systems, molten silicon and a eutectic Si-8 at. pct Zr alloy and their evolution over timeon vitreous carbon and polycrystalline silicon carbide (SiC) substrates were investigated at 1500°C under vacuum, as well as in argon using the sessile drop technique. The contact angle and microstructure of the liquid droplet/solid substrate interface were studied to understand fundamental features of reactive wetting as it pertains to the infiltration process of silicon and silicon alloys into carbon or C/SiC preforms. Both pure Si and theeutectic alloy showed good wettability onvitreous carbon and SiC characterized by equilibrium contact angles between 29° and 39°. Theeutectic alloy showed a higher initial contact angle and slower spreading as compared to that of pure Si. On vitreous carbon bothsilicon and the eutecticalloy formed SiC at the interface, while no reaction was observed on the SiC substrates.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Gelcasting of silicon carbide ceramics using phenolic resin and furfuryl alcohol as the gel former

A new non-aqueous gelcasting system of phenolic resin and furfuryl alcohol combined with a curing catalyst was developed for casting of reaction bonded silicon carbide ceramics. This gelling system could be carried out in air, and the surface exfoliation phenomenon that seems inherent to the acrylamide gelcasting system could also be eliminated. Polymerization of the premix solutions and rheological properties of the non-aqueous silicon carbide suspensions were studied. After curing and subsequent pyrolysis of the concentrated silicon carbide suspension, homogenous silicon carbide/carbon green body with a relatively high strength of about 18 MPa could be formed. Dense complex-shaped SiC ceramic parts with flexure strength of 300±20 MPa and fracture toughness of 3.87±0.19 MPa m^1/2 can be successfully produced after reaction sintering at 1700 °C for 30 min under vacuum.     

The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP

Traditionally, SiC components with complex shapes are very difficult or even impossible to fabricate. This paper aims to develop a new manufacturing process, combining selective laser sintering (SLS), cold isostatic pressing (CIP) and polymer infiltration pyrolysis (PIP), to manufacture complex silicon carbide parts and improve the mechanical properties of silicon carbide ceramic parts. The density and porosity of SiC/SiC composites were measured. Furthermore, the mechanical properties of the specimens with cold isostatic pressing and the specimens without cold isostatic pressing were compared. The bending strength of the specimens with cold isostatic pressing was 201?MPa, and the elastic modulus was 1.27?GPa. And, the bending strength of the specimens without cold isostatic pressing was 142?MPa, and the elastic modulus was 0.88?GPa. Increasing the density of SiC/SiC can enhance the mechanical properties of SiC/SiC composites.     

Laser additive manufacturing and homogeneous densification of complicated shape SiC ceramic parts

To improve the density of SiC ceramic components with complicated shape built by laser sintering (LS), cold isostatic pressing (CIP) and reaction sintering (RS) were incorporated into the process. In the process of LS/CIP/RS, Phenol formaldehyde resin (PF)-SiC composite powder was prepared by mechanical mixing and cold coating methods, with an optimized content of PF at 18?wt%. For the purpose of obtaining improved density of the sintered body after final reaction sintering, carbon black was added into the initial mixed powder. The material preparation, LS forming and densification steps were optimized throughout the whole fabrication process. The final sintered SiC bodies with the bending strength of 292 ~ 348?MPa and the density of 2.94–2.98?g?cm^{? 3} were prepared using the PF coated SiC-C composite powder and the LS / CIP / RS process. The study further showed a positive and practical approach to fabricate SiC ceramic parts with complicated shape using additive manufacturing technology.     

Biomorphic silicon/silicon carbide ceramics from birch powder

A novel process has been developed for the fabrication of biomorphic silicon/silicon carbide (Si/SiC) ceramics from birch powder. Fine birch powder was hot-pressed to obtain pre-templates, which were subsequently carbonized to acquire carbon templates, and these were then converted into biomorphic Si/SiC ceramics by liquid silicon infiltration at 1550 °C. The prepared ceramics are characterized by homogeneous microstructure, high density, and superior mechanical properties compared to biomorphic Si/SiC ceramics from birch blocks. Their maximum density has been measured as 3.01 g/cm³. The microstructure is similar to that of conventional reaction-bonded silicon carbide. The Vicker's hardness, flexural strength, elastic modulus, and fracture toughness of the biomorphic Si/SiC were 19.6 ± 2.2 GPa, 388 ± 36 MPa, 364 ± 22 GPa, and 3.5 ± 0.3 MPa m^1/2, respectively. The outstanding mechanical properties of the biomorphic Si/SiC ceramics are assessed to derive from the improved uniform microstructure of the pre-templates made from birch powder.     

Microstructural evolution of SiC powder in molten silicon

SiC_powder/Si_matrix composites represent a new class of microstructurally toughened materials. The interactions between molten silicon and submicronic SiC powder have been considered since it could originate some limitations on the final properties of the material. Experiments putting in interaction a SiC powder and molten Si were performed while heating up to final values ranging between 1450 and 1600?°C for duration up to 8?h. The volume ratio of SiC and silicon was equal to one and SiC particles were freely dispersed within the liquid. X-ray diffraction analyses demonstrated that the apparent crystallites size increase of SiC powder followed a ripening law corresponding to a limitation either by volume diffusion or by dissolution into the liquid. Depending on the relevant mechanism, the activation energy of the crystallites’ growth has been found equal to 357?±?50?kJ?mol^?1 or 441?±?57?kJ?mol^?1. An agglomeration-coarsening process of SiC particles was also identified which promoted a quick formation of larger particles.