Fabrication of 2D C/ZrC–SiC composite and its structural evolution under high-temperature treatment up to 1800 °C

Two-dimensional C/ZrC–SiC composites were fabricated by chemical vapor infiltration (CVI) process combined with a modified polymer infiltration and pyrolysis (PIP) method. Two kinds of ZrC slurries (ZrC aqueous slurry and ZrC/polycarbosilane slurry) were employed to densify composites before the PIP process. The as-produced C/ZrC–SiC composites exhibited better mechanical properties than the C/SiC composites densified only by CVI and PIP process. Structural evolution for C/ZrC–SiC composites treated in the range 1200–1800 °C mainly consisted of the change of SiC whiskers and the decomposition of polymer derived ceramic.     

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.     

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.     

Tension–tension fatigue behavior and residual strength evolution of SiC/(PyC/SiC)2/SiC minicomposites prepared by CVI+MI and CVI+PIP processes

The residual tensile strength (RTS) evolution of SiC/(PyC/SiC)₂/SiC samples, prepared by chemical vapor impregnation (CVI) combined with either melting reaction sintering (MI) or polymer impregnation and pyrolysis (PIP), was investigated after 10⁶ cycles as the pre-fatigue stress increased. The fatigue limits of the two tested specimen types, CVI + MI and CVI + PIP, were 760 and 660 MPa, respectively, corresponding to 95% and 75% of the tensile strength, respectively. Although the RTS of the two specimen types first increased and then decreased, it was interesting that after pre-fatigue, the maximum RTS of the CVI + PIP-prepared sample at 680 MPa, was 16 MPa higher than that of the CVI + MI-prepared sample at 590 MPa. The nanoindentation results indicate that the matrix prepared by CVI could protect the fiber from heavy damage in subsequent preparation, and the matrix modulus and hardness were higher in samples prepared by PIP than those prepared by MI. According to the microstructure observations and hysteresis characteristics, it was concluded that the differences mainly came from the internal stress state, matrix, and fiber properties.     

Fabrication of highly dense three-layer SiC cladding tube by chemical vapor infiltration method

Three-layer silicon carbide (SiC) cladding architectures are considered to be promising materials for current light-water nuclear reactors. Herein, a novel processing approach was proposed to fabricate dense three-layer SiC tubes by introducing SiC nanowires (NWs) on the graphite rod, which resulted in change in the valley-peak structure of SiC_f tubular preform. A dense three-layer-NWs SiC cladding tube, consisting of a chemical vapor infiltration (CVI)-SiC inner layer, a CVI-SiC_f/SiC composite layer, and a CVI-SiC outer layer, was obtained through CVI process. Microstructure and hoop strength of the as-obtained three-layer-NWs SiC cladding tube were systematically investigated. By avoiding delamination of the layers and reducing the pores, the three-layer-NWs SiC cladding tube exhibited an average hoop strength of 316.6 MPa with a Weibull modulus of 11.55.     

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.     

Growth mechanism of β-SiC nanowires in SiC reticulated porous ceramics

Polycarbosilane (PCS) was used as a precursor to prepare SiC reticulated porous ceramics (RPCs) with in situ growth of β-SiC nanowires at 1000–1300 °C. The nanowires in diameters of ∼50 nm exist on the surface of the strut and in the fracture surface of strut in SiC RPCs. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) indicate that the nanowire consists of a twinned β-SiC, which grows along the 〈1 1 1〉 direction. Field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) reveal that β-SiC nanowire grows by the vapor–liquid–solid (VLS) process at low temperature. The morphologies of the nanowire formed at different temperatures testify the process. As the heat-treated temperature increased, the growth mechanism of the nanowire changes from VLS to vapor–solid (VS).     

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.     

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.     

Effect of SiC coating and heat treatment on the thermal radiation properties of C/SiC composites

The effects of SiC coating and heat treatment on the emissivity were investigated for 2D C/SiC composites prepared by CVI in the 6–16 μm range. SiC coating had an obvious effect on the spectral emissivity of the composites but caused just 5% difference in the total emissivity. A radiation transport model was applied to explain those changes caused by SiC coating. Heat treatment affected the thermal radiation properties of the composites through the microstructure evolution. Base on the complementary analytical techniques, the changes in the emissivity were attributed to a good graphitization degree of carbon phases, large β-SiC grain sizes and high α-SiC content resulting in high emissivity.     

Multifunction properties of SiOC reinforced with carbon fiber and in-situ SiC nanowires

Herein, the SiC nanowires were successfully fabricated via chemical vapor infiltration (CVI) into carbon fiber felts (CFs) and then the SiOC/SiCnws/CFs composites were synthesized by precursor infiltration and pyrolysis (PIP) processes. Results indicated that the lightweight composites possessed enhanced mechanical performance, low thermal conductivity, and excellent electromagnetic wave absorption properties. Detailedly, the compressive strength reached to 22.0 MPa and 9.6 MPa after two PIP processes cycles in z and x/y directions, respectively. Meanwhile, the composites exhibited tailored electromagnetic wave absorption performance with the effective absorption bandwidth of 3.06 GHz, and the minimum reflection loss (RL_min) was -48.2 dB with a thickness of 3.6 mm. The present work has a guidance to prepare and design multifunction properties for application in harsh environment.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Fabrication of a 3D4d braided SiCf/SiC composite via PIP process assisted with an EPD method

In order to improve the thermal conductivity and full-fill the gaps between the fiber bundles for three-dimensional four-directional (3D4d) braided SiC_f/SiC composites, 500?nm submicron-sized β-SiC particles were introduced into the 3D4d preform by an electrophoretic deposition (EPD) method. ζ-potential of the KD-Ⅱ SiC fibers and the aqueous suspension of the β-SiC particles were analyzed, as well as the efficiency of the deposition. After densified via PIP process, microstructure, three-point bending strength and thermal conductivity of the composite were investigated. The results showed that, SiC particles filled the gaps between the SiC fiber bundles efficiently, and thermal conductivity of the composites fabricated through PIP process assisted by EPD was 2.3 times that of the composites fabricated via PIP only. The bending strength of the EPD-composites was 647.08?±?69.53?MPa, which decreased to 2/3 of that of the composites manufactured only by PIP, owing to the reduction of fiber volume fraction and the damages to the interface coatings and fibers under the action of the electric field.     

Morphology and anti-ablation properties of PIP Cf/C–SiC composites with different CVI carbon content under 4.2 MW/m2 heat flux oxy-acetylene test

In this work, the needled carbon fiber preforms were used to make seven groups of carbon/carbon composite billets with different matrix carbon contents by controlling the processing time of chemical vapor infiltration (CVI). C_f/C–SiC composites were prepared by infiltration of SiC into these C/C composites billets using polycarbosilane (PCS) through precursor infiltration and pyrolysis (PIP). After oxy-acetylene torch testing (heat flux of 4.2 MW/m²) for 200s, 300s and 400s, respectively, it revealed that the anti-ablation properties of the C_f/C–SiC composite samples were enhanced by a higher content of SiC matrix. Additionally, specimens bearing longer duration tests showed a trend of lower average ablation rates. The lowest linear ablation rate is 0.008 mm/s and the mass ablation rate is 0.0019 g/s for those high SiC content samples tested for 400s. The SEM images of the tested samples showed the mechanism and the non-linear process of ablation resistance progression.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Microstructure and tribological properties of PIP-SiC modified C/C–SiC brake materials

A combination method of precursor infiltration and pyrolysis (PIP), chemical vapor infiltration (CVI) and liquid silicon infiltration (LSI) was proposed to prepare PIP-SiC modified C/C–SiC brake materials. The SiC ceramic matrix pyrolyzed by polymethysilane (PMS) homogeneously dispersed in the fiber bundles region, which improved the plough resistance of local C/C region and the wear resistance of C/C–SiC brake materials. When the braking speed rises to 28 m/s, the fluctuation range of friction coefficient was limited to 0.026. The linear wear rate of the as-prepared composites was could be ~50% less than that of C/C–SiC, when the braking speed was above 15 m/s (for instance, the wear rate of 1.02 μm/(side·cycle) at 28 m/s less than 2.02 μm/(side·cycle) of traditional C/C– SiC). The fading ratio D of CoF under wet conditions was ~11%. The results showed that introducing PIP-SiC could stabilize the braking process and effectively prolong the service life of C/C–SiC brake materials.     

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.     

Porous SiCnw/SiC ceramics with unidirectionally aligned channels produced by freeze-drying and chemical vapor infiltration

The current study introduces a methodology for the fabrication of porous silicon carbide nanowire/silicon carbide (SiC_nw/SiC) ceramics with macroscopic unidirectionally aligned channels and reports on their microstructural and mechanical properties. The material was produced by freezing of a water-based slurry of β-SiC nanowires (SiC_nw) with control of the ice growth direction. Pores were subsequently generated by sublimation of the columnar ice during freeze-drying. Chemical vapor infiltration (CVI) of SiC into the open pore network of the SiC_nw aerogel with unidirectionally aligned channels, resulted in the formation of highly porous SiC_nw/SiC ceramics which exhibited a unique microstructure as identified by scanning electron microscopy. The pore size distribution and the mechanical properties of the as-fabricated porous ceramics were examined by mercury intrusion porosimetry and three-point bending and compression tests, respectively, while phase composition was investigated through X-ray diffraction.     

Chemical vapor deposition derived ultralong SiC nanofibers: structural characterization and growth mechanism

In this study, the ultralong SiC nanofibers (SiC NFs) were synthesized through the sol–gel method assisted the chemical vapor deposition technique. The scanning electron microscope, transmission electron microscopy, X-ray diffraction, Raman, Fourier transform infrared, and X-ray photoelectron spectroscopy techniques were systematically employed to investigate the microstructure, morphology, and phase composition of the as-prepared products. The results demonstrated that the as-obtained products were β-SiC nanofibers with face-centered cubic crystal structure. Meanwhile, the ultralong SiC NFs present an average diameter of about 18 nm and a length up to several hundreds of micrometers and grew along the [1 1 1] direction with a planar stacking faults. In addition, we also investigated the formation mechanism and growth process of the ultralong SiC NFs. The successful preparation of such ultralong SiC NFs provides new idea for fabricating of other silicon-based ultralong nanofibers.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.