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.     

Fabrication of carbon fiber reinforced ceramic matrix composites with improved oxidation resistance using boron as active filler

Boron was introduced into C_f/SiC composites as active filler to shorten the processing time of PIP process and improve the oxidation resistance of composites. When heat-treated at 1800 °C in N₂ for 1 h, the density of composites with boron (C_f/SiC-BN) increased from 1.71 to 1.78 g/cm³, while that of composites without boron (C_f/SiC) decreased from 1.92 to 1.77 g/cm³. So when boron was used, two cycles of polymer impregnation and pyrolysis (PIP) could be reduced. Meanwhile, the oxidation resistance of composites was greatly improved with the incorporation of boron-bearing species. Most carbon fiber reinforcements in C_f/SiC composite were burnt off when they were oxidized at 800 °C for 10 h. By contrast, only a small amount of carbon fibers in C_f/SiC-BN composite were burnt off. Weight losses for C_f/SiC composite and C_f/SiC-BN composite were about 36 and 16 wt%, respectively.     

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.     

Densification,microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Enhanced mechanical and microwave-absorption properties of SiCf/AlPO4 composite with PIP–SiC interphase and the MWCNTs filler

A single-layer radar-absorbing structure in the X-band (8.2 GHz to 12.4 GHz) was designed and fabricated by blending multi-walled carbon nanotubes (MWCNTs) with the binder matrix of SiC fiber/aluminum phosphate matrix (SiC_f/AlPO₄) composite. The SiC interphase was successfully prepared on SiC fibers by a precursor infiltration and pyrolysis (PIP) method. The morphology of as-received interphase was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The effects of PIP–SiC interphase on the mechanical and dielectric properties of the composite were investigated. The influence of MWCNTs content on the dielectric and microwave-absorption properties of coated SiC_f/AlPO₄ composite was discussed. When the content of MWCNTs was between 1.5 wt% and 3.5 wt% and the composite thickness is in the range of 2.5–3.5 mm, the SiC_f/AlPO₄ composite achieved excellent absorbing wave property in X-band.     

Properties and microstructure evolution of Cf/SiC composites fabricated by polymer impregnation and pyrolysis (PIP) with liquid polycarbosilane

In the present study, a novel liquid polycarbosilane (LPCS) with a ceramic yield as high as 83% was applied to develop 3D needle-punched C_f/SiC composites via polymer impregnation and pyrolysis process (PIP). The cross-link and ceramization processes of LPCS were studied in detail by FT-IR and TG-DSC; a compact ceramic was obtained when LPCS was firstly cured at 120 °C before pyrolysis. It was found that the LPCS-C_f/SiC composites possessed a higher density (2.13 g/cm³) than that of the PCS-C_f/SiC composites even though the PIP cycle for densification was obviously reduced, which means a higher densification efficiency. Logically, the LPCS-C_f/SiC composites exhibited superior mechanical properties. The shorter length and rougher surfaces of pulled-out fibers indicated the LPCS-C_f/SiC composites to possess a stronger bonding between matrix and PyC interphase compared with the PCS-C_f/SiC composites.     

Fabrication of cylindrical SiCf/Si/SiC-based composite by electrophoretic deposition and liquid silicon infiltration

Cylindrical SiC-based composites composed of inner Si/SiC reticulated foam and outer Si-infiltrated SiC fiber-reinforced SiC (SiC_f/Si/SiC) skin were fabricated by the electrophoretic deposition of matrix particles into SiC fabrics followed by Si-infiltration for high temperature heat exchanger applications. An electrophoretic deposition combined with ultrasonication was used to fabricate a tubular SiC_f/SiC skin layer, which infiltrated SiC and carbon particles effectively into the voids of SiC fabrics by minimizing the surface sealing effect. After liquid silicon infiltration at 1550 °C, the composite revealed a density of 2.75 g/cm³ along with a well-joined interface between the inside Si/SiC foam and outer SiC_f/Si/SiC skin layer. The results also showed that the skin layer, which was composed of 81.4 wt% β-SiC, 17.2 wt% Si and 1.4 wt% SiO₂, exhibited a gastight dense microstructure and the flexural strength of 192.3 MPa.     

Preparation and characterization of ZrB2 and TaC containing Cf/SiC composites via Polymer-Infiltration-Pyrolysis process

To improve the thermo-chemical resistance of PIP–C_f/SiC composites, the SiC matrix is modified by adding ZrB₂ and Ta powder to the pre-ceramic slurry to form C_f/SiC–ZrB₂–TaC composites. Within this study the modified composites are investigated regarding their microstructure, chemical composition and physical properties (density = 2,39–2,72 g/cm³; porosity = 20,3–24,8 vol.-%; fiber volume content = 52–57 vol.-%). Mechanical properties are investigated in order to ensure that there is no negative influence by ZrB₂ and TaC matrix modification. The matrix modification is followed by an improvement in bending strength (up to 27% increase), Young’s modulus (up to 28% increase) and for interlaminar shear strength (up to 22% increase). Finally the thermo-chemical behavior of the C_f/SiC–ZrB₂–TaC composites is evaluated in a combustion chamber-like environment using the Airbus Group long-term material test facility (Environmental Relevant Burner Rig-Kerosene, ERBURIG^K). The results show that the thermo-chemical resistance of C_f/SiC–ZrB₂–TaC composites is improved and the oxygen permeability through the composite is decreased (from 5 to 1 layer).     

Influence of different matrices on the mechanical and microwave absorption properties of SiC fiber-reinforced oxide matrix composites

Fiber-reinforced ceramic matrix composites have excellent mechanical and microwave absorption properties, but still present considerable challenges. We prepared a SiC_f/mullite-SiO₂ composite (composite A) and a SiC_f/Al₂O₃-SiO₂ composite (composite B) by a precursor infiltration and sintering (PIS) process. Compared with the composite B, the composite A was easily densified. The flexural strength of the composite A reached 216 MPa, whereas that of the composite B was 159 MPa. The imaginary part of permittivity for composites A and B, which was determined by the contents of matrix and porosity, varied in the range of 2.5–3.5 and 3.6–5, respectively. The microwave absorption properties of the composite A were significantly enhanced in the range of 8.2–12.4 GHz. The results indicate that an optimal reflection loss of ?44 dB was reached at 12 GHz with a thickness of 2.9 mm for the composite A. These SiC fiber-reinforced oxide matrix materials have promising applications in microwave absorption, especially at high temperatures.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Fabrication of laminated SiCw/SiC ceramic composites by CVI

Laminated SiC_w/SiC ceramic composites were prepared by chemical vapor infiltration (CVI) and tape casting, and the advantages of this method were investigated. The results showed that this method can increase strength of the composites by reducing the damage of SiC whiskers and increasing their volume fraction, and increase toughness of the composites by controlling the interfacial bonding between whiskers and matrix and inter-laminar bonding between layers. The volume fraction of whiskers reached 40 vol.%, and the flexural strength, tensile strength and fracture toughness were 315 MPa, 158 MPa and 8.02 MPa m^1/2, respectively. Interfacial and interlaminar crack deflection and bridging were observed.     

Reactive brazing Cf/SiC to itself and to Mo using the NiPdPtAu-Cr filler alloy

The NiPdPtAu-Cr filler alloy was proposed for joining C_f/SiC composites. The wettability on C_f/SiC composite was studied by the sessile drop method at 1200 °C for 30 min. Under the brazing condition of 1200 °C for 10 min, the C_f/SiC-C_f/SiC joint strength was only 51.7 MPa at room temperature. However, when used a Mo layer, the C_f/SiC-Mo-C_f/SiC joint strength was remarkably increased to 133.2 MPa at room temperature and 149.5 MPa at 900 °C, respectively. At the interface between C_f/SiC and Mo, Mo participated in interfacial reactions, with the formation of Cr₃C₂/Mo₂C reaction layers at the C_f/SiC surface. The improvement in the joint strength should be mainly attributed to the formation of MoNiSi. The C_f/SiC-Mo joint strength was 86.9 MPa at room temperature and 73.7 MPa at 900 °C, respectively. After 10 cycles of thermal shock test at 900 °C the C_f/SiC-Mo joint strength of 71.6 MPa was still maintained.     

Mechanical and dielectric properties of SiCf/SiC composites fabricated by PIP combined with CIP process

2D KD-1 SiC fiber fabrics were employed to fabricate SiC_f/SiC composites by an improved polymer infiltration and pyrolysis (PIP) process, combined with cold isostatic pressing (CIP). The effect of CIP process on the microstructure, mechanical and dielectric properties of SiC_f/SiC composites was investigated. The infiltration efficiency was remarkably improved with the introduction of CIP process. Compared to vacuum infiltration, the CIP process can effectively increase the infiltrated precursor content and decrease the porosity resulting in a dense matrix. Thus SiC_f/SiC composites with high density of 2.11 g cm⁻³ and low porosity of 11.3% were obtained at 100 MPa CIP pressure, together with an increase of the flexural strength of the composites from 89 MPa to 213 MPa. Real part (ε′) and the imaginary part (ε″) of complex permittivity of SiC_f/SiC composites increase and vary from 11.7-i9.7 to 15.0-i12.8 when the CIP pressure reaches 100 MPa.     

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-Cf/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C_f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C_f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C_f/SiC composites fabricated with vacuum infiltration were 1.794 g cm⁻³ and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm⁻³ and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.     

Microstructure and properties of needle punching chopped carbon fiber reinforced carbon and silicon carbide dual matrix composite

Chopped carbon fiber preform reinforced carbon and SiC dual matrix composites (C/C–SiC) were fabricated by chemical vapor infiltration (CVI) combined with liquid silicon infiltration. The preform was fabricated by repeatedly overlapping chopped carbon fiber web and needle punching technique. A geometry model of the pore structure of the preform was built and reactant gas transportation during the CVI was calculated. The microstructure and properties of the C/C–SiC composites were investigated. The results indicated that the CVI time for densification of the preform decrease sharply, and the model showed the permeability of the preform decreased with the increase of its density. The C/C–SiC exhibited good mechanical characteristics, especially excellent compressive behavior, with the vertical and parallel compressive strength reached to 359(±40) MPa and 257(±35) MPa, respectively. The coefficient of friction (COF) decreased from 0.60 (at 8 m/s) with the increase of sliding velocity, and finally stabilized at ~0.35 under the velocity of 20 m/s and 24 m/s, and the variations of COF were not sensitive to the sliding distance. The wear rates were between 0.012 cm³/MJ and 0.024 cm³/MJ under different velocities. These results showed that the chopped carbon fiber preform reinforced C/C–SiC are promising candidates for high-performance and low-cost friction composites.     

Effect of multi-walled carbon nanotubes on microstructure and fracture properties of carbon fiber-reinforced ZrB2-based ceramic composite

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB₂-based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB₂ grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C_f/ZrB₂-based composite with MWCNTs were 7.0±0.4 MPa m^1/2 and 379±34 J/m², respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.     

Effects of post-sintering annealing on the microstructure and toughness of hot-pressed SiCf/SiC composites with Al2O3-Y2O3 additions

This study examined the effects of post-sintering heat treatment on enhancing the toughness of SiC_f/SiC composites. Commercially available Tyranno^® SiC fabrics with contiguous dual ‘PyC (inner)-SiC (outer)’ coatings deposited on the SiC fibers were infiltrated with a SiC + 10 wt% Al₂O₃-Y₂O₃ slurry by electrophoretic deposition. SiC green tapes were stacked between the slurry-infiltrated fabrics to control the matrix volume fraction. Densification of approximately 94% ρ_theo was achieved by hot pressing at 1750 °C, 20 MPa for 2 h in an Ar atmosphere. Sintered composites were then subjected to isothermal annealing treatment at 1100, 1250, 1350, and 1750 °C for 5 h in Ar. The correlation between the flexural behavior and microstructure was explained in terms of the in situ-toughened matrix, phase evolution in the sintering additive, role of dual interphases and observed fracture mechanisms. Extensive fractography analysis revealed interfacial debonding at the hybrid interfaces and matrix cracking as the key fracture modes, which were responsible for the toughening behavior in the annealed SiC_f/SiC composites.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.