Fabrication of short carbon fibre reinforced SiC multilayer composites by tape casting

Silicon carbide multilayer composites containing short carbon fibres (C_sf/SiC) were prepared by tape casting and pressureless sintering. C fibres were dispersed in solvents and then mixed with SiC slurry to make green C_sf/SiC tape. Triton X-100 was found to be the best one for Toho Tenax HTC124 fibres (with water soluble coating) among BYK-163, BYK-410, BYK-2150, BYK-9076, BYK-9077 and Triton X-100 dispersants. C_sf/SiC multilayer composites containing 5 vol.% fibre (mean fibre length of 3, 4.5, and 6 mm) were obtained. Addition of short C fibres seems to worsen the densification process in the C_sf/SiC multilayer composites, whereas anisotropy shrinkage in C_sf/SiC was also observed. Open pores size was increased slightly after the addition of C fibre but it decreased with the mean fibre length. Mechanical properties were affected by high residual porosity. The addition of short C fibre has not changed the crack deflection at weak interfaces. C_sf/SiC multilayer composites containing longer fibres (4.5 and 6 mm) presented higher elastic modulus, bending strength and Vickers hardness as compared to shorter fibres (3 mm). Improved sintering performance and fibre content are necessary to improve mechanical properties.     

Microstructure control of highly oriented short carbon fibres in SiC matrix composites fabricated by direct ink writing

A novel method is proposed for fabricating highly oriented carbon fibre reinforced SiC ceramic composites (C_f/SiC) by direct ink writing (DIW). For the first time, the control of carbon fibers’ orientation in DIW was studied by numerical simulation. An interfacial layer was prepared by chemical vapor infiltration (CVI). The microstructure and phase composition of C_f/SiC were studied by scanning electron microscopy and X-ray diffraction, respectively. The results showed that fibers of different interfacial thicknesses could be obtained effectively by varying the CVI time. The breakage of short fibres remarkably improved the fracture toughness of the parts. The specimens showed excellent mechanical properties with bending strength of 274 ± 13 MPa and fracture toughness of 5.82 ± 0.25 MPa m^1/2. This method could be extended to the preparation of other resin and ceramic composites.     

Thermophysical Properties of Short Carbon Fiber/SiC Multilayer Composites Prepared by Tape Casting and Pressureless Sintering

C_sf/SiC multilayer composites for thermal conductivity (TC) test in three directions were successively prepared by tape casting and pressureless sintering. After 1500°C/5 h oxidation treatment, short carbon fibers were oxidized which produced many pores. However, a core area, which was composed by short carbon fiber, SiC, and few SiO₂, was still observed. TC properties of C_sf/SiC multilayer composites were highly anisotropic. The TC was decreased with the increase in fiber amount. C_sf/SiC multilayer composites demonstrated the highest TC along the tape casting direction and the lowest TC through the thickness direction, which is favorable for thermal protection purpose.     

Mechanical and dielectric properties of short carbon fiber reinforced Al2O3 composites with MgO additive

A series of short-carbon-fiber/Al₂O₃ composites with MgO as sintering additive were fabricated by pressureless sintering process. The effects of short carbon fiber (C_sf) content on the mechanical, dielectric and microwave absorbing properties of the composite were investigated. The results show that the addition of MgO enhances the density, hardness and the flexural strength of the alumina ceramic. However, these mechanical properties of the C_sf/Al₂O₃–MgO composite decrease with increasing C_sf content. Both the real and imaginary parts of the complex permittivity increase with increasing C_sf content in the frequency range of 8.2–12.4 GHz, which is attributed to the increasing electron polarization and associated polarization relaxation, respectively. When the C_sf content is 0.3 wt%, the reflection loss less than −10 dB and the minimum value of −27 dB are obtained with the coating thickness being 1.4 mm. The results indicate that the C_sf/Al₂O₃ with MgO is an excellent candidate for microwave absorbing material with favorable mechanical property.     

Effects of random chopped short carbon fibers on microstructure and mechanical properties of joints for Cf/SiC composites by reaction bonding process

Random chopped short carbon fibers (C_sf)/phenol-formaldehyde resin (PF)/SiC powder mixtures are used as filler for the joining of C_f/SiC composites to obtain SiC interlayer at the joining region. The influences of C_sf on the microstructure and mechanical properties have been investigated. Research shows that the introduction of C_sf can improve the microstructure uniformity of the joint and reduce residual silicon content in the interlayer. The joint achieve a high flexural strength of 232?±?33?MPa as the carbon fiber content is 30?wt.%, which is similar to that of the C_f/SiC composites (220?±?21?MPa). The decrease in residual silicon content and the formation of nano-sized SiC particles are the main reasons for high joining strength.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior

The effects of processing variables on densification behavior of hot pressed ZrB₂-based composites, reinforced with SiC particles and short carbon fibers (C_sf), were studied. A design of experiment approach, Taguchi methodology, was used to investigate the characteristics of ZrB₂–SiC–C_sf composites concentrated upon the hot pressing parameters (sintering temperature, dwell time and applied pressure) as well as the composition (vol% SiC/vol% C_sf). The analysis of variance recognized the sintering temperature and SiC/C_sf ratio as the most effective variables on the relative density of hot pressed composites. The microstructural investigations showed that C_sf can act as a sintering aid and eliminate the oxide impurities (e.g. B₂O₃, ZrO₂ and SiO₂) from the surfaces of raw materials. A fully dense composite was achieved by adding 10 vol% C_sf and 20 vol% SiC to the ZrB₂ matrix via hot pressing at 1850 °C for 30 min under a pressure of 16 MPa. Moreover, the in-situ formation of interfacial ZrC, which also improves the sinterability of ZrB₂-based composites, was studied by energy-dispersive X-ray spectroscopy analysis and verified thermodynamically.     

Fabrication of CNT-SiC/SiC composites by electrophoretic deposition

Due to the outstanding mechanical and thermal properties of carbon nanotubes (CNTs), they are considered suitable reinforcement for structural materials. In this study, for the first time, electrophoretic deposition (EPD) was used to deposit (multi-walled) CNTs onto SiC fibres (SiC_f) to form an effective CNT interphase layer for SiC_f/SiC composites. This deposition was followed by electrophoretic infiltration of the CNT-coated SiC fibre mats with SiC powder to fabricate a new CNT-SiC-fibre-reinforced SiC-matrix (SiC_f/SiC) composite for fusion applications. In these EPD experiments, a commercial aqueous suspension of negatively charged CNTs and an optimized aqueous suspension of negatively charged SiC particles were used. The CNT-coatings on the SiC fibres were firm and homogenous, and uniformly distributed nanotubes were observed on the fibre surfaces. In a following step of EPD, a thick SiC layer was formed on the fibre mat when the CNT-coated SiC fibres were in contact with the positive electrode of the EPD cell; however, spaces between the fibres were not fully filled with SiC. Conversely, when CNT-coated SiC fibres were isolated from the electrode, the SiC particles were able to gradually fill the fibre mat resulting in relatively high infiltration, which leads to dense composites.     

Investigations on the thermal behaviours of SiC-ZrC continuous ceramic fibres

In this study, nanoscale composite SiC-ZrC ceramic fibres, derived from polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam crosslinking, pyrolysis and sintering were investigated in detail. Compared with several commercial products of second-generation SiC fibres, the produced composite fibres exhibit improved thermal stability, mechanical properties and oxidation resistance. SiC grains in the fibre grew from 9.8 nm to 33.9 nm after annealing in an inert atmosphere at 1800 °C for 1 h, as well as decomposition of the SiC_xO_y phase and the growth of SiC grains affected the mechanical properties of the fibres, and the mechanical properties of the fibres were maintained at 1.1 GPa, accompanied by an increase in the modulus. After the fibres were oxidized at 1100～1400 °C for 1 h, a dense oxide layer of SiO₂-ZrO₂ was formed on the surface of the fibres, which slowed down the rate of further fibre oxidation, thus, the fibres exhibited excellent oxidation resistance.     

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.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth

The synergetic effects SiC particles and short carbon fibers (C_sf) as well as hot pressing parameters (sintering temperature, dwell time and applied pressure) on the grain growth of ZrB₂-based composites were investigated. Taguchi methodology was employed for the design of experiments to study the microstructure and grain growth of ZrB₂–SiC–C_sf ceramic composites. Three hot pressing parameters and SiC/C_sf ratio were selected as the scrutinized variables. The sintering temperature and SiC/C_sf ratio were identified by ANOVA as the most effective variables on the gain growth of ZrB₂-based samples. Removal of oxide impurities from the surface of starting particles by the reactant C_sf, not only hindered the extraordinary grain growth of ZrB₂ matrix, but also improved the sinterability of the ceramics. A fully dense ceramic with an average grain size of 8.3 µm was obtained by hot pressing at 1850 °C for 30 min under 16 MPa through adding 20 vol% SiC and 10 vol% C_sf to the ZrB₂ matrix. SEM observations and EDS analysis verified the in-situ formation of ZrC which can restrain the growth of ZrB₂ particles, similar to the role of SiC, by the pinning of grain boundaries as another stationary secondary phase.     

Effect of short carbon fibre concentration on microstructure and mechanical properties of TiCN-based cermets

Short carbon fibre (C_f) reinforced TiCN-based cermets (C_f/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of C_f/TiCN composites were investigated. The results showed that TiC, TiN, WC, Cr₃C₂ and Mo phases disappeared gradually and diffused into core and rim phases by dissolution–reprecipitation process, finally formed new hard TiCN core phases and complex compound (Cr, W, Mo, Ti)(CN) rim phases, with the sintering temperature increasing. The added C_f did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The C_f/TiCN composite containing 3?wt-% C_f achieved the best comprehensive mechanical properties, with fracture toughness and bending strength increasing by about 14.4% and 30.8%, respectively, when compared with the composite without C_f. Toughening and strengthening mechanisms of C_f/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.     

Characteristics of SiCf/SiC hybrid composites fabricated by hot pressing and spark plasma sintering

Abstract

Abstract

SiC fibre reinforced SiC–matrix ceramic composites were fabricated by electrophoretic deposition (EPD) combined with ultrasonication. Fine β-SiC powder and Tyranno-SA fabrics were used as the matrix and fibre for reinforcement, respectively. Different amounts of fine Al₂O₃–Y₂O₃ were added for liquid phase assisted sintering. For EPD, highly dispersed slurry was prepared by adjusting the zeta potentials of the constituent particles to ?+40 mV for homogeneous deposition. The composite properties were compared after using two different consolidation methods: hot pressing for 2 h at 20 MPa and spark plasma sintering (SPS) for 3 min at 45 MPa at 1750°C to minimise the damage to the SiC fibre. The maximum flexural strength and density for the 45 vol.-% fibre content composites were 482 MPa and 98% after hot pressing, respectively, whereas those for SPS were 561 MPa and 99·5%, indicating the effectiveness of SPS.     

Processing and properties of Cfibre/SiCfillers/SiOC composites using solvent free liquid polysiloxane as matrix source

Abstract

Carbon fibre reinforced SiOC composites (denoted as C_fibre/SiC_fillers/SiOC) were prepared by slurry coating and polymer infiltration pyrolysis (PIP) process. Low viscosity liquid polysiloxane (PSO) and SiC powder were combined at a 1∶1 weight ratio to produce a blend (S-PSO), which was employed as matrix source. Heat treated carbon fibre fabric was adopted as the reinforcement. The lamination process was determined on the basis of cure and rheology investigations on S-PSO. The effects of PIP cycles and temperature of heat treatment of the carbon fibre on the mechanical properties of C_fibre/SiC_fillers/SiOC were examined. The results indicate that composites using carbon fibres annealed at 1700°C as reinforcement reached a maximum flexural strength of 300 MPa after six PIP cycles. The resistance of the C_fibre/SiC_fillers/SiOC composite to oxidation was also evaluated. Without any protective coatings, the composite retained 60% of its strength after oxidation at 800°C for 3 h in a static air environment.     

Effect of PAN-based and pitch-based carbon fibres on microstructure and properties of continuous Cf/ZrB2-SiC UHTCMCs

In this paper the microstructure and mechanical properties of two different C_f/ZrB₂-SiC composites reinforced with continuous PyC coated PAN-derived fibres or uncoated pitch-derived fibres were compared.Pitch-derived carbon fibres showed a lower degree of reaction with the matrix phase during sintering compared to PyC/PAN-derived fibres. The reason lies in the different microstructure of the carbon. The presence of a coating for PAN-derived fibres was found to be essential to limit the reaction at the fibre/matrix interface during SPS. However, coated bundles were more difficult to infiltrate, resulting in a less homogeneous microstructure.As far as the mechanical properties are concerned, specimens reinforced with coated PAN-derived fibres provided higher strengths and damage tolerance than uncoated pitch-derived fibres, due to the higher degree of fibre pull-out. On the other hand, the weaker fibre/matrix interface resulted in lower interlaminar shear, off-axis strength and ablation resistance.     

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.     

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     

Is spark plasma sintering suitable for the densification of continuous carbon fibre - UHTCMCs?

For the first time we show that spark plasma sintering can efficiently replace hot pressing for the densification of UHTCMCs, in the present case ZrB₂/SiC composites reinforced with continuous carbon fibres. To this purpose, the same materials were first produced by hot pressing as baseline samples and then by spark plasma sintering (SPS) to compare microstructure and basic mechanical properties. A special emphasis was given to the study of interfaces, in case of both coated and uncoated carbon fibres.SPS allowed for faster sintering but required an adjustment of the temperature to avoid fibre degradation compared to hot pressing. With similar porosity levels, we observed a slight decrease of flexural strength (300 vs 470 MPa), and an improvement of fracture toughness (15 vs 10 MPa√m) for SPSed samples. SPS was proved to be an effective method for the consolidation of continuous fibre reinforced UHTC composites.     

Properties,morphology and structure of BPDA/PPD/ODA polyimide fibres

Abstract

A family of random co-poly(amic acid)s containing 4,4′-oxydianiline (ODA) moiety were synthesised in N,N′-dimethylacetamide. The co-poly(amic acid) solutions were used as spinning dope for dry jet wet spinning process into as spun poly(amic acid) (PAA) fibres. The polyimide (PI) fibres were obtained from PAA fibres after being imidised and drawn in furnace. The processability and mechanical properties of the fibres were notably improved by incorporating ODA into 3,3′,4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PPD) backbone. The best strength and modulus of BPDA/PPD/ODA PI fibre (diamine mole ratio of PPD/ODA?=?85∶15) attained 2·25 and 96·5 GPa respectively, which were approximately three times the tenacity of the BPDA/PPD PI fibre. The SEM image showed that the cross-section of each stage fibres was round and void free. In addition, ‘skin–core’ and microfibrillar structure were not observed. The thermal properties of PI fibres were also investigated. The results showed that the PI fibres have excellent thermal stability; moreover, the dimensional stability and structural homogeneity of the fibres were significantly improved by heat drawn stage. T_g was found to be ～290°C by thermomechanical and dynamic mechanical analyses. The X-ray (wide angle X-ray diffraction and small angle X-ray scattering) experiments indicated that the ordering degree of longitudinal and lateral stacks, as well as the molecular orientation of PI fibre, was improved in the preparation process of fibres. Furthermore, the mechanical properties of fibres are profoundly affected by the heat drawn conditions.     

Fiber size effects on mechanical behaviours of SiC fibres-reinforced Ti3AlC2 matrix composites

In this study, different diameters SiC fibres-reinforced Ti₃AlC₂ matrix composites were fabricated via hot-pressing. The mechanical behaviours of the obtained composites were evaluated at room temperature by using 4-point bending testing. The results show that the flexural strength and damage tolerance are significantly higher for smaller diameter fibres-containing composites than for large diameter fibres-containing ones. In addition, the mechanical properties of the composites were improved when smaller diameter fibres partially replaced large diameter fibres and the improvement increased with increase of the amount of smaller diameter fibre. Furthermore, the fracture fashion of the composites depended on fibre diameters used.     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.