Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.     

Paper derived biomorphic porous titanium carbide and titanium oxide ceramics produced by chemical vapor infiltration and reaction (CVI-R)

Chemical vapor infiltration (CVI) is used for producing biomorphic porous TiC ceramics derived from paper. The paper samples are first carbonized in inert atmosphere to yield biocarbon template structures (C_b-template). Subsequently, three routes for converting the C_b-templates into TiC ceramics are studied. The first route includes CVI with TiCl₄-H₂. The effect of methane as additional carbon source is investigated on the second route (TiCl₄-H₂-CH₄). Finally, a two step CVI process (Route 3), first TiCl₄-H₂ and subsequent TiCl₄-H₂-CH₄, is performed in order to improve both the grade of conversion of the C_b-template into TiC and the mechanical properties of the resulting porous TiC ceramics. Furthermore, porous TiO₂ ceramics are produced by high temperature oxidation of the TiC ceramics in air flow.     

Microstructure and properties of 2D-Cf/SiC composite fabricated by combination of CVI and PIP process with SiC particle as inert fillers

2D-C_f/SiC composite was manufactured by chemical vapor inflation (CVI) combined with polymer impregnation and pyrolysis (PIP) with SiC particle as inert fillers. The effects of CVI processes on SiC morphologies and the properties of composite were investigated. The composites were characterized by XRD, flexural strength test and SEM. The results revealed that uniform SiC coatings and nanowires were prepared when MTS/H₂ ratio of 1:8 was employed, while gradient thick coatings were fabricated as MTS/H₂ ratio of 1:1 was employed. The flexural strength of composites varied from 156 MPa at MTS/H₂ ratio of 1:1 to 233 MPa at MTS/H₂ ratio of 1:8. All of composites exhibited toughness due to significant debonding and pullout of fibers. The laminated structure of coatings on the fibers and nanowires were manufactured by combination of above different CVI process, and the obtained composites showed flexural strength of as high as 248 MPa and impressive toughness.     

Complex geometry macroporous SiC ceramics obtained by 3D-printing,polymer impregnation and pyrolysis (PIP) and chemical vapor deposition (CVD)

We present here an original route for the manufacturing of SiC ceramics based on 3D printing, polymer impregnation and pyrolysis and chemical vapor deposition (CVD). The green porous elastomer structures were first prepared by fused deposition modeling (FDM) 3D-printing with a composite polyvinyl alcohol/elastomer wire and soaking in water, then impregnated with an allylhydridopolycarbosilane preceramic polymer. After crosslinking and pyrolysis, the polymer-derived ceramics were reinforced by CVD of SiC using CH₃SiCl₃/H₂ as precursor. The multiscale structure of the SiC porous specimens was examined by X-ray tomography and scanning electron microscopy analyses. Their oxidation resistance was also studied. The pure and dense CVD-SiC coating considerably improves the oxidation resistance.     

Synthesis of high-surface area mesoporous SiC with hierarchical porosity for use as catalyst support

Porous SiC with a hierarchical mesoporous structure is a promising material for high-performance catalytic systems because of its high thermal conductivity, high chemical inertness at high temperature, and oxidation resistance. Attempts to produce high-surface area hierarchical SiC have typically been made by using porous carbon as a template and reacting it with either Si or SiO₂ at high temperature under inert atmosphere. Because the reaction mechanism with Si involves a carbon dissolution step, and the reaction with SiO₂ is highly dependent on C-SiO₂ dispersion, the porous structure of the carbon template is not maintained, and the reaction yields nonporous SiC. In this work, mesoporous SiC has been synthesized using a novel hard-template methodology. SiC was prepared from hierarchical (mesoporous) silica which served as a solid template. Carbon deposition was done by Carbon Vapor Deposition (CVD) using CH₄ as carbon precursor, where different temperatures and reaction times were tested to optimize the carbon coating. The synthesized SiC retained 61 (118 m²/g) and 47% (0.3 cm³/g) of the BET surface area and the mesopore volume of the original SiO₂, which is 10 times higher than the retention reported for other template methods used to produce high surface area SiC.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

3D-SiC decorated with SiC whiskers: Chemical vapor infiltration on the porous 3D-SiC lattices derived from polycarbosilane-based suspensions

SiC_w/3D-SiC composites were fabricated by chemical vapor infiltration (CVI) of the 3D SiC lattices, which were prepared via direct ink writing of polycarbosilane-based suspensions. Microstructure, composition and tensile strength of the composites were investigated. Curing and pyrolysis temperature greatly affected the shrinkage, weight loss, density and composition of the 3D SiC. Although sound structure with spanning feature was achieved, cracks and pores in 3D SiC were formed during the pyrolysis owing to the large shrinkage. CVI process decreased the porosity and led to fully dense surface of the SiC_w/3D-SiC composites. After 60h of CVI, short β-SiC fibres or long SiC whiskers were deposited in the structural spacing of 3D lattices or spherical pores inside the filaments, respectively. The tensile strength of the composites by CVI increased from 3.3 MPa to 15.7 MPa (20 h) and 47.3 MPa (60 h), due to the high strength of dense CVI layers and in-situ formed SiC whiskers in pores. This work showed a way to strengthen the 3D SiC with in-situ formed whiskers via the polymer precursor routes.     

Mechanical and dielectric properties of SiCf/BN/SiBCN composites via different synthesis technologies

SiC fibers reinforced SiBCN ceramic matrix composites (SiC_f/BN/SiBCN composites) were synthesized by direct chemical vapor infiltration (CVI), polymer infiltration pyrolysis (PIP) or chemical vapor infiltration combined with polymer infiltration pyrolysis (CVI + PIP). It is shown that the insertion of a continuous and dense SiBCN matrix via the CVI process improves the flexural strength and modulus. Interface debonding and fiber pullout happened with 50–100 nm BN interface in CVI and CVI + PIP SiC_f/BN/SiBCN composites. The relative complex permittivity was measured in X-band. Higher ε′′ values in CVI-containing composites can be observed, which can be attributed to the accumulation of C and SiC phases and a multilayer matrix. Strong electromagnetic wave attenuation ability was obtained with high dielectric loss.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Sandwich structure SiCf/Si3N4–SiOC–Si3N4 composites for high-temperature oxidation resistance and microwave absorption

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) have been widely used as structural-functional materials at high temperatures. However, their mechanical and electromagnetic wave (EMW) absorbing properties will deteriorate due to high-temperature oxidation. Therefore, unique sandwich structure, consisting of inner Si₃N₄ impedance layer, middle porous SiOC loss layer and dense oxidation-resistant Si₃N₄ layer, was proposed to enhance multiple material properties in oxidation environment. Herein, SiC_f/Si₃N₄–SiOC–Si₃N₄ composites was fabricated by alternating chemical vapor infiltration (CVI) and polymer infiltration pyrolysis (PIP) methods. For these composites, SiC fiber is used as both reinforcing phase and electromagnetic (EM) absorber. CVI Si₃N₄ matrix was distributed in inner and outer layer of the SiC_f/Si₃N₄–SiOC–Si₃N₄ composites. While inner Si₃N₄ layer between BN interphase and SiOC matrix forms nano-heterogeneous interphase to consume EM energy and enhance mechanical properties of composites, outer dense and oxidation-resistant CVI Si₃N₄ coating serves to maintain properties. PIP SiOC matrix exhibits porous structure that can effectively deflect cracks and achieve multiple scattering of EMW. SiC_f/Si₃N₄–SiOC–Si₃N₄ composites with sandwich structure demonstrated excellent EMW absorbing properties and mechanical performance in high-temperature oxidation environments.     

Tensile creep properties and damage mechanisms of 2D-woven C/HfC–SiC composites in high temperature

2D-C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO₂ and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high-temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of Hf_xSi_yO_z nanowires in the samples after high-temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.     

Low-temperature preparation of porous SiC ceramics using phosphoric acid as a pore-forming agent and a binder

Porous SiC ceramics combine the properties of both SiC ceramics and porous materials. Herein, we design a facile method via pressureless sintering at relatively low temperatures for the synthesis of porous SiC ceramics. In the synthesis process, phosphoric acid was used as the sintering additive that reacted with SiO₂ on the surface of SiC to form phosphates. The formed phosphates acted as a binder to connect the SiC particles. At a fixed temperature, the phosphates were partially decomposed and released a large amount of gas. This changed the pore structure of the ceramics and greatly improved their porosity. Finally, we obtained the porous SiC ceramics with high porosity and high strength. We investigate the effects of H₃PO₄ content on the phase composition, microstructure, porosity, mechanical properties and thermal expansion coefficient of the prepared porous SiC ceramics. It was shown that at the sintering temperature of 1200 °C, the highest porosity of the samples can reach 70.42% when the H₃PO₄ content is 25 wt%, and their bending strength reaches 36.11 MPa at room temperature when the H₃PO₄ content is 15 wt%. In addition, the porous SiC ceramics show good high-temperature stability with a bending strength of 42.05 MPa at 1000 °C and the thermal expansion coefficient of 3.966 × 10⁻⁶/°C.     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.     

Effect of oxidation on the bending fatigue behavior of an advanced SiC/SiC CMC component at 1000 °C in air

The present study elucidates the effect of oxidation during static and fatigue loading in SiC/SiC CMC structured component, which shows damage in the stress-concentrated region. It is made of Tyranno SA3 fiber, BN (Boron nitride) interphase, and CVI (chemical vapor infiltration) + PIP (polymer impregnation and pyrolysis) hybrid matrix. The comparison based on strength and fracture morphology was made. After annealing, the as-received sample showed minute oxidation and slightly enhanced strength. The fatigued sample without annealing under low stress showed higher retained strength than the as-received sample due to smooth debonding. The fatigued sample with annealing under high stress showed a loss in strength than the as-received sample owing to the formation of a significant amount of borosilicates glasses, which further promoted SiO₂ formation between fiber and matrix and caused the brittle failure. However, simultaneous filling borosilicate glasses into the pores oppositely aided in maintaining the retained strength.     

Highly conductive and high-strength carbon nanotube film reinforced silicon carbide composites

Film formed by carbon nanotubes is usually called carbon nanotube film (CNT_f). In the present study, CNT_f fabricated by floating catalyst method was used to prepare CNT_f/SiC ceramic matrix composites by chemical vapor infiltration (CVI). Mechanical and electrical properties of the resulting CNT_f/SiC composites with different CVI cycles were investigated and discussed, and the results revealed that the CNT_f has a good adaptability to CVI method. Tensile test demonstrated an excellent mechanical performance of the composites with highest tensile strength of 646 MPa after 2 CVI cycles, and the strength has a decline after 3 CVI cycles for an excessively dense matrix. While, the elastic modulus of the composite increased with the CVI cycles and reached 301 GPa after 3 CVI cycles. Tensile fracture morphologies of the composites were analyzed by scanning electron microscope to study the performance change laws with the CVI cycles. With SiC ceramic matrix infiltrated into the CNT_f, enhanced electrical conductivity of the CNT_f/SiC composite compared to pure CNT_f was also obtained, from 368 S/cm to 588 S/cm. Conductivity of the SiC matrix with free carbon forming in the CVI process was considered as the reason.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

Processing of biomorphic porous TiO2 ceramics by chemical vapor infiltration and reaction (CVI-R) technique

Porous biomorphic TiO₂ ceramics were manufactured from paper preforms by chemical vapor infiltration and reaction (CVI-R) in a three-steps process. First, the cellulose fibers of the paper were converted into carbon (C_b) by pyrolysis in an inert atmosphere. Then, C_b-template was infiltrated with a precursor system consisting of TiCl₄, CH₄ and H₂ to produce porous TiC ceramics, which were oxidized in a final step with air at temperatures in the range of 400–1200 °C. Depending on the conversion degree, TiC/TiO₂ or TiO₂ ceramics were obtained. The kinetics of the oxidation process was studied by thermal gravimetric analysis (TGA) and activation energies of 63 and 174 kJ mol⁻¹ were estimated for the lower (400–800 °C) and higher (950–1200 °C) temperature regions, respectively. The TiO₂ ceramics were characterized by Raman spectroscopy (anatase/rutile ratio), SEM/EDX (morphology, composition) and nitrogen gas adsorption (pore structure). It was shown, that the anatase/rutile ratio as well as the pore structure of the resulting TiO₂ ceramics could be controlled varying the oxidation temperature. The TiO₂ samples obtained by oxidation of TiC biomorphic porous ceramics are lightweight but nevertheless have very good mechanical performances. Their bending strength varies between 30 and 40 MPa at a porosity of 65–70%. These structures have many potential applications, e.g. light structured materials, implants because of their bio-compatibility, catalyst support or catalyst for photo catalytic applications.     

Oxidation behavior of C/SiC-SiBCN composites at high temperature

In order to improve the anti-oxidation performance of C/SiC composites at high temperature, C/SiC composites should be modified by self-healing components. SiBCN ceramic is an ideal self-healing component because of excellent oxidation resistance and thermal stability. C/SiC composites were modified by PDC SiBCN ceramic (C/SiC-SiBCN) by using CVI combined with polymer infiltration and on-line pyrolysis (PI-OP). The oxidation behaviors of C/SiC composites fabricated by CVI method and C/SiC-SiBCN composites fabricated by CVI + PI-OP method and CVI + PIP method at different temperatures in air were compared. The results showed that the strength retention ratios of the composites fabricated by the three methods decreased with the increase of temperature. Compared with the samples fabricated by the other two methods, the weight loss of the samples fabricated by CVI + PI-OP method was greater, but the strength retention ratio was higher.     

A new approach for the net-shape fabrication of porous Si3N4 bonded SiC ceramics with high strength

In this study, Si₃N₄ bonded porous SiC ceramics with high strength had been net-shapely fabricated by a new approach. In this approach, we proposed a two-step processing route composed of freeze casting and carbothermal reduction reactions in which carbon aerogels, derived from sol infiltration and pyrolysis, involved. The phase components, microstructures and properties of the prepared ceramics were investigated. The results showed that carbon aerogels with high apparent surface area had been completely reacted and new SiC and Si₃N₄ grains had been produced. The porous ceramics with flexural strength of 164.3 MPa at 33% porosity and 80.5 MPa at 46% porosity were obtained, whose linear shrinkages were only 1.06% and 1.94% during the whole processing respectively.     

Fabrication and mechanical properties of SiC composites toughened by buckypaper and carbon fiber fabrics alternately laminated

SiC ceramics, for the first time, were toughened with nano scale carbon nanotubes (CNTs) buckypapers and micro scale carbon fibers within this work. The CNTs buckypapers were alternately laminated with carbon fiber fabrics (C_fb) to a preform by needle punched in Z-direction. Afterwards, the buckypaper-C_fb/SiC composites were obtained by infiltrating of SiC into the as-laminated preform via chemical vapor infiltration (CVI). Some effects of different lamination thickness and CVI times on the mechanical properties of the composites were investigated. Results showed that the maximum flexural strength and work of fracture of the buckypaper-C_fb/SiC composites reached 262.4 MPa and 4.15 kJ m⁻², respectively, when the thickness reached about 3.50 mm. Compared to C_fb/SiC composites without buckypapers, the strength and work of fracture of the buckypaper-C_fb/SiC composites increased by 19.8% and 111.7%, respectively. Densified composites can be obtained after CVI for 8 times. A main factor affecting the mechanical properties of buckypaper-C_fb/SiC composites is the degree of densification. Introducing nano scale CNTs and micro scale carbon fibers reaches a multiscale co-toughening effect. Meanwhile, a sandwich structure ceramic matrix composite with high-CNT concentration was obtained in this work.