Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Environment-friendly deposition of SiCN interlayer for reinforcing carbon fibers in C/SiC composites

Several C/SiC composites with no interlayer, single pyrocarbon (PyC) interlayer and PyC/SiCN interlayer were fabricated by polymer infiltration and pyrolysis process. The microstructure and mechanical properties were investigated. The results verified that SiCN interlayer was formed on carbon fibers. Both bulk density and flexural stress of C/SiC composite with PyC/SiCN interlayer were slightly higher than composite fabricated with single PyC interlayer. When the weight fraction of SiCN interlayer in the composite was about 18 wt%, the flexural stress of the composite was enhanced to 416 MPa from 352 MPa for composite with single PyC interlayer. The observations of pulled-out fibers on fracture surfaces revealed non-catastrophic fracture features for PyC/SiCN deposited C/SiC composite.     

Microstructure and properties of ablative C/ZrC–SiC composites prepared by reactive melt infiltration of zirconium and vapour silicon infiltration

C/ZrC-SiC composites with a density of 3.09 g/cm³ and a porosity of 4.8% were prepared by reactive melt infiltration and vapour silicon infiltration. The flexural strength and modulus were 235 MPa and 18.3 GPa, respectively, and the fracture toughness was 7.0 MPa m^1/2. The formation of SiC and ZrSi₂ during vapour silicon infiltration, at the residual cracks and pores in the C/ZrC, enhanced the interface strength and its mechanical properties. The high flexural strength (223 MPa, c. 95% of the original value) after oxidation at 1600 °C for 10 min indicated the excellent oxidation resistance of the composites after vapour silicon infiltration. The mass loss and linear recession rate of the composites were 0.0071 g/s and 0.0047 mm/s, respectively and a fine ablation morphology was obtained.     

Electromagnetic interference shielding and dielectric properties of SiCf/SiC composites containing pyrolytic carbon interphase

The SiC_f/SiC composites containing various thickness of pyrolytic carbon (PyC) interphase were prepared and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2–12.4 GHz. The composites containing 310 nm thickness (3.3 vol%) PyC interphase show an about 25 dB shielding effectiveness in the whole frequency band. Interestingly, the contribution of reflection to the EMI shielding effectiveness increases and the contribution of absorption decreases as the PyC interphase thickness increases.     

Mechanical properties and oxidation resistance of SiCf/CVI-SiC composites with PIP-SiC interphase

SiC coatings were successfully synthesized on SiC fibers by precursor infiltration and pyrolysis (PIP) method using polycarbosilane (PCS) as precursor. The morphology of as-fabricated coatings was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The SiC fiber reinforced chemical vapor infiltration SiC (SiC_f/CVI-SiC) composites with PIP-SiC coatings as interphase were fabricated. And, the effects of PIP-SiC interphase on mechanical properties of composites were investigated. The experimental results point out that the coating is smooth and there is little bridging between fibers. The coating is amorphous with SiC and carbon micro crystals. The flexural strength of composites with and without PIP-SiC interphase is 220 and 100 MPa, respectively. And the composites with PIP-SiC interphase obviously exhibit a toughened fracture behavior. The oxidation resistance of composites with PIP-SiC interphase is much better than that of composites with pyrolytic carbon (PyC) interphase.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

The effect of heat treatment on the strength and toughness of carbon fiber/silicon carbide composites with different pyrolytic carbon interphase thicknesses

The effect of heat treatment on the strength and toughness of carbon fiber/silicon carbide composites (C/SiC) with different pyrolytic carbon (PyC) interphase thicknesses was investigated. It was found that as the heat treatment temperature (HTT) increases to 1900 °C, the strength and toughness of a low strength specimen (LSS, thin PyC ≈40 nm) increase by as much as 43.2% and 274.0%, while those of a high strength specimen (HSS, thick PyC ≈140 nm) show decreases of 25.1% and 14.8%, respectively. The elastic moduli of both LSS and HSS monotonically decreased with increasing HTT while the failure strains always became larger regardless of the initial interfacial bonding strengths (IBS). The mechanisms involved in the heat treatment of the C/SiCs were identified as (I) partial graphitization of the PyC that weakens the IBS, and (II) production of defects such as matrix cracks/delamination, interfacial debonding and fiber fracture/pull-out that lead to thermal residual stress relaxation. Thus heat treatment improves the strength and toughness of LSS with a relatively high IBS, but has a negative impact on both properties of HSS with a moderate IBS because the stress transfer efficiency onto the fibers is hindered by the too low IBS and the excessive stress relief.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrC composites

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C.     

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

A new method to weaken the interfacial bonding and increase the strength of 3D mullite fiber reinforced mullite matrix (Mu_f/Mu) composites is proposed and tested in this paper. Firstly, Mu_f/Mu composites were fabricated through sol–gel process with varied sintering temperature. Then, the effects of sintering temperature on mechanical properties of the composites were tested. As sintering temperature was raised from 1000 °C to 1300 °C, the three-point flexural strength of the composites firstly decreased from 66.17 MPa to 41.83 MPa, and then increased to 63.17 MPa. In order to explain the relationship between composite strength and sintering temperature, morphology and structure of the mullite fibers and mullite matrix after the same heat-treatment as in the fabrication conditions of the composites were also investigated. Finally, it is concluded that this strength variation results from the combined effects of matrix densification, interfacial bonding and fiber degradation under different sintering temperatures.     

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.     

Interface controlled micro- and macro- mechanical properties of aluminosilicate fiber reinforced SiC matrix composites

Novel Nextel™ 440 aluminosilicate fiber reinforced SiC matrix composites, with/without chemical vapor deposited carbon interphase were fabricated by polymer derived ceramic process, and they were studied by a combination of micro- and macro- mechanical techniques such as nanoindentation, micropillar splitting, fiber push-in, digital image correction and high temperature three point bend tests. Specifically, micropillar splitting test was firstly employed to measure in-situ the localized fracture toughness. The results revealed that the carbon interphase can effectively hinder the interfacial reactions between Nextel™ 440 fiber and SiC matrix, thus remarkably weakening the composite interfacial shear strength from ∼293 MPa to ∼42 MPa, and enhance the composite fracture toughness from ∼1.8 MPa√m to ∼6.3 MPa√m, respectively. This is mainly a consequence of weak interface that triggers crack deflection at the fiber/interphase interface. Finally, this novel composite showed stable mechanical properties in vacuum at temperature range from 25 °C to 1000 °C.     

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.     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

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.     

Development of SiC–SiC joint by reaction bonding method using SiC/C tapes as the interlayer

SiC/C tapes with different compositions and thicknesses were used to join pressureless sintered silicon carbide ceramics by reaction bonding method. The microstructure of the joints and the influences of joint thickness and residual silicon content in joint layer on the 4-point flexural strength of as joined SiC ceramics have been investigated. Specimens with high flexural strength can be achieved through the control of the composition and the thickness of the joint layer. The highest flexural strength of the joined specimens with the joint thickness of 13 μm can reach 346 ± 35 MPa and 439 ± 31 MPa at room temperature and 1250 °C, respectively. The microstructure development and the reaction bonding mechanism were also studied.     

Fabrication of tough SiCf/SiC composites by electrophoretic deposition using a fabric coated with FeO-catalyzed phenolic resin

A hybrid processing route based on vacuum infiltration, electrophoretic deposition, and hot-pressing was adopted to fabricate dense and tough SiC_f/SiC composites. The as-received Tyranno SiC fabric preform was infiltrated with phenolic resin containing 5 wt.% FeO and SiC powders followed by pyrolysis at 1700 °C for 4 h to form an interphase. Electrophoretic deposition was performed to infiltrate the SiC-based matrix into the SiC preforms. Finally, SiC green tapes were sandwiched between the SiC fabrics to control the volume fraction of the matrix. Densification close to 95% ρ_theo was achieved by incorporating 10 wt.% Al₂O₃-Sc₂O₃ sintering additive to facilitate liquid phase sintering at 1750 °C and 20 MPa for 2 h. X-ray diffraction and Raman analyses confirmed the catalytic utility of FeO by the formation of a pyrolytic carbon phase. The flexural response was explained in terms of the extensive fractography results and observed energy dissipating modes.     

Enhanced mechanical and dielectric properties of SiCf/SiC composites with silicon oxycarbide interphase

SiC_f/SiC composites with silicon oxycarbide (SiOC) interphase were successfully prepared using silicone resin as interphase precursor for dip-coating process and polycarbosilane as matrix precursor for PIP process assisted with hot mold pressing. The effects of SiOC interphase on mechanical and dielectric properties were investigated. XRD and Raman spectrum results show that SiOC interphase is composed of silicon oxycarbide and free carbon with a relatively low crystalline degree. The surface morphology of SiC fibers with SiOC interphase is smooth and homogeneous observed by SEM. The flexural strength and failure displacement of SiC_f/SiC composites with SiOC interphase vary with the thickness of interphase and the maximum value of flexural strength is 289 MPa with a failure displacement of 0.39 mm when the thickness of SiOC interphase is 0.25 µm. The complex permittivity of the composites increases from 8.8-i5.7 to 9.8-i8.3 with the interphase thicker.     

Effects of 1600 °C annealing atmosphere on the microstructures and mechanical properties of C/SiC composites fabricated by precursor infiltration and pyrolysis

Effects of 1600 °C annealing atmosphere on microstructures and mechanical properties of the C/SiC composites fabricated by PIP route were remarkable. Due to carbothermic reductions, the ratios of weight loss of the C/SiC composites were all above 7 wt% in 1 h. Consequently, the mechanical properties all had a significant drop during the first hour of annealing because of the bonding between the fibers and matrix remarkably weaken by cracks and pores. And then the flexural strengths gradually decreased with the annealing time increasing, when the flexural moduli slightly changed within the range of 44.2–49.7 GPa. However, the fracture behaviors of the C/SiC composites annealed under Ar faster became brittle than the C/SiC composites annealed under vacuum. The C/SiC composites annealed under Ar for 5 h and under vacuum for 10 h both became brittle mainly due to the sensitive to annealing of the weak carbon interphase, while the C/SiC composites annealed under Ar for 7 h became brittle mainly due to the chemical bonding between the fibers and matrix. And these phenomena were confirmed by the post densification and the stress-releasing annealing.     

Reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks and SiC powder

A reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks (RHs) and SiC powder in different ratios is reported, in which FeSi₂ alloy was used as infiltrant. The preforms were heat-treated at 1550 °C for 6 h prior to the infiltration. The coked RHs, which are composed of SiO₂ and C, were converted to SiC and poorly crystallized C by carbothermal reduction during the heat treatment. The study of the microstructure and mechanical properties of the composites shows that molten Fe–Si alloy had good wetting of the heat-treated preforms and adequate infiltration properties. Free carbon in the preform reacted with Si in the molten FeSi₂ during infiltration forming new SiC, the composition of the intermetallic liquid being moved towards that of FeSi. As a result, the infiltrated composites are composed of SiC, FeSi₂ and FeSi phases. Vickers hardness, elastic modulus, three-point flexural strength and indentation fracture toughness of the composites are found to increase with SiC additions up to 30% w/w in the preforms, reaching the values of 18.2 GPa, 290 GPa, 213 MPa and 4.9 MPa m^1/2, respectively. With the SiC addition further raised to 45% w/w, the elastic modulus, flexural strength and fracture toughness of the composite turned down probably due to high residual stress and hence the more intense induction of microcracks in the composite. De-bonding of SiC particles pulled out of the Fe–Si matrix, transgranular fracture of part of the SiC particles and in the Fe–Si matrix, and crack bridging all exist in the fracture process of the composites.