Microstructure observation and analysis of 3D carbon fiber reinforced SiC-based composites fabricated through filler enhanced polymer infiltration and pyrolysis

3D C/SiC-BN composites were fabricated by filler enhanced polymer infiltration and pyrolysis (FE-PIP) through in situ conversion of active filler boron into h-BN in the high temperature treatment process. The bending strengths and microstructures of composites were studied here. Interphase layers deposited on the fiber surfaces can prevent the strong bonding between fiber reinforcements and composite matrix and repair the defects on the fiber surface, which can improve the bending strength and toughness of composites. The bending stress of C/SiC-BN composites without interphase layer is about 170 MPa while those of composites with PyC or PyC/SiC interphase layers are higher than 300 MPa. Some large pores were left in the interwoven zones while intra-bundle zones were relatively dense, only a small amount of micro-pores could be observed. It could also be concluded that the length of pulled-out fibers was much longer and the pulled-out fiber surface was smoother when interphase layers were deposited. Because the matrix derived from the pyrolysis of slurries adheres to the fiber bundles, some phases with layered structures could be observed in the matrix near the reinforcements. The microstructure evolution of 3D fiber reinforced ceramic matrix composites were also analyzed in this work based on the observation of both 3D C/SiC-BN composites and 3D C/SiC composites fabricated by FE-PIP, where boron and SiC particles were applied as active fillers and inert fillers respectively.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

Influence of brick pattern interface structure on mechanical properties of continuous carbon fiber reinforced lithium aluminosilicate glass-ceramics matrix composites

Continuous carbon fiber reinforced lithium aluminosilicate glass-ceramic matrix composites have been fabricated by sol-gel process and hot pressing technique. The results show that the Cf/β-eucryptite composites hot pressed at 1300 °C and Cf/β-spodumene composites hot pressed at 1400 °C form weak interface with brick pattern characteristics, leading to high mechanical performance. The maximum flexural strength and fracture toughness reach 571 ± 32 MPa and 9.8 ± 0.6 MPa m^1/2 for Cf/β-eucryptite composites and 640 ± 72 MPa and 19.9 ± 1.8 MPa m^1/2 for Cf/β-spodumene composites. On increasing the hot pressing temperature, the active chemical diffusion consumes brick pattern interface layer, which leads to the formation of strong bonding between carbon fiber and the matrix. As a result, the composites exhibit brittle fracture behavior and the mechanical properties decrease significantly.     

Carbon fiber/reaction-bonded carbide matrix for composite materials – Manufacture and characterization

The processing of self-healing ceramic matrix composites by a short time and low cost process was studied. This process is based on the deposition of fiber dual interphases by chemical vapor infiltration and on the densification of the matrix by reactive melt infiltration of silicon. To prevent fibers (ex-PAN carbon fibers) from oxidation in service, a self-healing matrix made of reaction bonded silicon carbide and reaction bonded boron carbide was used. Boron carbide is introduced inside the fiber preform from ceramic suspension whereas silicon carbide is formed by the reaction of liquid silicon with a porous carbon xerogel in the preform. The ceramic matrix composites obtained are near net shape, have a bending stress at failure at room temperature around 300 MPa and have shown their ability to self-healing in oxidizing conditions.     

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.     

Effects of copper on microstructure and mechanical properties of Cf/ZrC composites fabricated by low-temperature liquid metal infiltration

Carbon fiber-reinforced zirconium carbide matrix (C_f/ZrC) composites were fabricated by a liquid metal infiltration process at 1200 °C, using low melting Zr₇Cu₁₀, ZrCu and Zr₂Cu alloys as infiltrators. The effects of Cu on microstructure and mechanical properties of the composites were investigated. The results indicated that the products were composed of either single- or polycrystalline ZrC, C and Cu. With increasing Cu content in the infiltrators, the yield of ZrC decreased from 43.7 vol% to 27.9 vol%. When ZrCu was used as an infiltrator, the obtained composites exhibited a better bending strength of 98.2±3.1 MPa. What is more, the use of Zr₂Cu could provide the highest fracture toughness of the composites with a moderate debonding.     

High temperature bending creep behavior of a multi-cation doped α/β-SiAlON composite

SiAlON ceramics with high hardness and high toughness can be made through designing α/β-SiAlON composites. An important advantage of α-SiAlON phase is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of α-SiAlON phase during sintering. But, the thermal stability of the α-SiAlON phase is an important concern for α/β-SiAlON composites especially at high temperatures. The use of different types of single or multiple cations during fabrication directly affects resultant microstructures and mechanical behavior of α/β-SiAlON composites. In this study, the creep behavior of a multi-cation (Y, Sm and Ca) doped α/β-SiAlON composite, in which aluminum-containing nitrogen melilite solid solution phase was designed as intergranular phase, was investigated by four-point bending creep tests under stresses from 50 to 150 MPa and at temperatures from 1300 °C to 1400 °C in air. The stress exponent was determined to be 1.6 ± 0.13 at 1400 °C and the creep activation energy was calculated to be 692 ± 37 kJ/mol⁻¹. Grain boundary sliding coupled with diffusion was identified as the rate-controlling creep mechanism for the α/β-SiAlON composite.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Microstructures and mechanical properties of carbon/carbon composites reinforced with carbon nanofibers/nanotubes produced in situ

Using ferrocene as catalyst and toluene as the liquid precursor, carbon/carbon (C/C) composites were prepared by chemical liquid-vapor infiltration at 850-1100 °C. The microstructures and properties of C/C composites obtained with different ferrocene contents were studied. The results show smooth laminar and isotropic pyrocarbon are obtained after adding ferrocene to the precursor. Carbon nanofibers can be formed as the catalyst content is 0.3-1 wt.%. When the ferrocene content is 2 wt.%, multi-walled carbon nanotubes with the diameter about 20-90 nm are obtained together with carbon-encapsulated iron nanoparticles. After adding ferrocene to the precursor, the fracture modes of the composites change from brittle facture to tough fracture. The flexural strength of the composites is a maximum for 0.3 wt.% ferrocene in the precursor, higher than for ferrocene contents of 0, 0.5, 1 and 2 wt.%. The flexural modulus of the composites decreases after adding ferrocene to the precursor.     

PyC interphase growth mechanism and control models of C/SiC composites

To better understand the pyrocarbon (PyC) interphase growth mechanism, a series of experiments was conducted on the PyC deposited on T-300™ and T-700™ carbon fibers by the chemical vapor infiltration (CVI) method. Nine groups of fabrication parameters were used to analyze the effects of deposition temperature, pressure, and residence time on the PyC interphase growth mechanism. Atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), Raman spectroscopy, and nanoindentation tests were performed to characterize the microstructures of carbon fibers and PyC interphase. The PyC interphase growth mechanism was discussed, and the relationships between the fabrication parameters, R (C₂/C₆) value, texture type, and interphase thickness were established through numerical simulations. The hardness and modulus of PyC for T-300™ and T-700™ carbon fibers were measured. The tensile behaviors of C/SiC minicomposites with medium and high textures PyC interphases were analyzed. The C/SiC composite with the medium texture PyC interphase possessed the higher fracture strength and failure strain with a longer fiber pullout length at the fracture surface.     

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.     

Influence of graphitization temperature on microstructure and mechanical property of C/C-SiC composites with highly textured pyrolytic carbon

C/C-SiC composites with highly textured pyrolytic carbon (HT PyC) were prepared by a combining chemical vapor infiltration and liquid silicon infiltration. The effect of HT PyC graphitization before and after 2327 and 2723 K on C/C-SiC composites was investigated. The mechanical properties decreased with increasing graphitization temperature, but graphitization treatment changed the fracture behavior from brittle like to pseudo-ductile. The decrease in bending strength from 306.21 to 243.69 MPa resulted from the weak interfacial bonding between HT PyC and fiber, and the good orientation of graphite layers. The crack at border of fiber bundle and longitudinal crack in HT PyC shortened the path of crack propagation, resulting in fracture toughness decrease from 21.11 to 14.72 MPa·m^1/2. A more pseudo-ductile behavior was due to the longer pull-out of fibers, the better orientation of graphite layers, the sliding of sublayers, and the deflection and propagation caused by the transverse cracks.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC) _n ) multilayered interphases and a tow of SiC fibers (Hi-Nicalon) have been prepared via pressure-pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH₃SiCl₃/H₂ mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack-spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).     

Effect of PyC interphase thickness on mechanical behaviors of SiBC matrix modified C/SiC composites fabricated by reactive melt infiltration

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.     

Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Ceramic matrix composites usually utilize carbon or ceramic fibers as reinforcements. However, such fibers often expose a low ductility during failure. In this work, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibers to reach pseudo ductile behavior during failure. Tungsten and molybdenum fibers were chosen as reinforcement in SiCN ceramic matrix composites manufactured by polymer infiltration and pyrolysis process. The composites were investigated with respect to microstructure, flexural- and tensile strength. The single fiber strengths for both tungsten and molybdenum were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. The W fiber became brittle across the entire cross section, while the Mo fiber showed a superficial, brittle reaction zone but kept ductile on the inside.     

Preparation and tensile behavior of SiCf/SiC mini composites containing different types and thicknesses of interphase

SiC fiber-reinforced SiC matrix (SiC_f/SiC) composites are promising materials for high-temperature structural applications. In this study, KD-II SiC fiber bundles with a C/Si ratio of approximately 1.25 and an oxygen amount of 2.53%, were used as reinforcement. PyC interphase, PyC-SiC co-deposition interphase I and II, with different thicknesses, and SiC matrix were deposited into the SiC fiber bundles by using chemical vapor infiltration (CVI) to form SiC_f/SiC mini composites. When the thickness of the interphase is approximately 1000 nm, the ultimate tensile stress and strain of SiC_f/SiC mini composites with PyC-SiC co-deposition interphase I can reach 1120.0 MPa and 0.72%, respectively, which are significantly higher than those of SiC_f/SiC mini composites with a PyC interphase (740.0 MPa, 0.87%) and PyC-SiC co-deposition interphase II (645.0 MPa, 0.54%). The effect of thicknesses and types of interphase on tensile fracture behavior of mini composites and then the fracture mechanism are discussed in detail.     

Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process

Sub-micrometer SiC particles were firstly added to the preceramic solution in the first infiltration step to enhance the mechanical properties of 2D C_f/SiC composites fabricated via polymer infiltration and pyrolysis (PIP) process. The effects of pyrolysis temperature and SiC-filler content on microstructures and properties of the composites were systematically studied. The results show that the failure stress and fracture toughness increased with the increase of pyrolysis temperature. SiC filler of sub-micron scale infiltrated into the composites increased the mechanical properties. As a result, for the finally fabricated composite infiltrated with a slurry containing 40 wt.% SiC filler, the failure stress was doubled compared to that without SiC filler addition, and the fracture toughness reached ≈10 MPa m^1/2.     

Experimental study on the mechanical behavior and failure mechanism of 3d MWK carbon/epoxy composites under quasi‐static loading

Quasi‐static tensile, out‐of compression, in‐plane compression, three‐point‐bending and shear tests were conducted to reveal the mechanical behavior and failure mechanisms of three‐dimensional (3D) multiaxial warp‐knitted (MWK) carbon/epoxy composites. The characterization of the failure process and deformation analysis is supported by high‐speed camera system and Digital Image Correlation. The results show that tensile, bending, out‐of‐plane compression, in‐plane compression stress–strain response exhibit obvious linear elastic feature and brittle fracture characteristics, whereas the shear response exhibits a distinct nonlinear behavior and gradual damage process. Meanwhile, 3D MWK carbon/epoxy composites have good mechanical properties, which can be widely used in the fields of engineering. In addition, the failure for tension behaves as interlayer delaminating, 90/+45/−45° interface debonding and tensile breakage of 0° fibers; the damage for out‐of‐plane compression is mainly interlaminar shear dislocation together with local buckling and shear fracture of fibers; the failure pattern for in‐plane compression is 90° fiber separating along fiber/matrix interface as well as 0/+45/−45° fiber shear fracture in the shear plane. The main failure for bending is fiber/matrix interface debonding and fibers tearing on the compression surface, 0° fibers breakage on the tension surface as well as fiber layers delaminating. Although the shear behavior is characterized by a gradually growing shear matrix damage, 90/+45/−45° interface debonding, +45/−45° fibers shear fracture, and final 0° fiber compression failure. POLYM. COMPOS., 37:3486–3498, 2016. © 2015 Society of Plastics Engineers     

Interfacial optimization of SiC nanocomposites reinforced by SiC nanowires with high volume fraction

High volume fraction SiC nanowires-reinforced SiC composites (SiCNWs/SiC) were prepared by hybrid process of chemical vapor infiltration and polymer impregnation/pyrolysis in this research. SiCNWs networks are first to be made promising a high volume fraction (20 vol%), and the pyrolytic carbon (PyC) interphase with 5 nm is designed on SiCNWs surface to optimize the bonding condition between SiCNWs and SiC matrix. Nanoindentation shows a modulus of 494 ± 14 GPa of SiCNWs/SiC composites without interphase comparing to the one with PyC interphase of 452 ± 13 GPa. However, the 3-point bending test shows a higher strength of the composite with PyC interphase (273 ± 32 MPa) comparing with the one without interphase (240 ± 38 MPa). The fracture surface is observed under SEM, which shows a longer SiCNWs pullout of the composite with PyC interphase. The energy dissipation during the 3-point bending test is calculated by the length of nanowire pull-out, it demonstrates that the SiCNWs with PyC interphase possess better performance for toughening composite. Further characterization proves that the PyC interphase can give SiCNWs/SiC composites higher fracture toughness (4.49 ± 0.44 MPa·m^1/2) than the composites without interphase (3.66 ± 0.28 MPa·m^1/2).