Microstructure and ablation behavior of SiC coated C/C–SiC–ZrC composites prepared by a hybrid infiltration process

In this study, C/C–SiC–ZrC composites coated with SiC were prepared by precursor infiltration pyrolysis combined with reactive melt infiltration. The pyrolysis behavior of the hybrid precursor was investigated using thermal gravimetric analysis-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy techniques. The microstructure and ablation behavior of the composites were also investigated. The results indicate that the composites exhibit an interesting structure, wherein a ceramic coating composed of SiC and a small quantity of ZrC covers the exterior of the composites, and the SiC–ZrC hybrid ceramics are partially embedded in the matrix pores and distributed around the carbon fibers as well. The composites exhibit good ablation resistance with a surface temperature of over 2300 °C during ablation. After ablation for 120 s, the mass and linear ablation rates of the composites are 0.0026 g/s and 0.0037 mm/s, respectively. The great ablation resistance of the composites is attributed to the formation of a continuous phase of molten SiO₂ containing SiC and ZrO₂, which seals the pores of the composites during ablation.     

Delayed failure of ceramic matrix composites in tension at elevated temperatures

Ultimate tensile strength of five different continuous fiber-reinforced ceramic matrix composites (CMCs), including SiC_f/BSAS (two dimensional (2D), 2 types), SiC_f/MAS (2D), SiC_f/SiC (2D), and C_f/SiC (2D, 2 types), was determined as a function of test rate at 1100–1200 °C in air. All five CMCs exhibited a significant dependency of ultimate tensile strength on test rate such that the ultimate tensile strength decreased with decreasing test rate. The dependency of ultimate tensile strength on test rate, the applicability of preload technique, and the predictability of life from one loading configuration (constant stress-rate loading) to another (constant stress loading) all suggested that the overall, phenomenological delayed failure of the CMCs would be governed by a power-law type of slow crack growth.     

Fabrication and mechanical properties of short ZrO2 fiber reinforced NiFe2O4 matrix composites

Short ZrO₂ fibers (ZrO_2(f)) reinforced NiFe₂O₄ ceramic composites were fabricated by cold pressing process. The phase composition, microstructure, mechanical properties and fiber/matrix interface of the composites were investigated by X-ray diffraction, scanning electron microscopy and mechanical testing machines. ZrO_2(f) show good thermodynamic and chemical compatibility with NiFe₂O₄ ceramic matrix and effectively enhanced the mechanical properties. The toughening mechanisms are fiber bridging, interfacial debonding, fiber pullout, phase transformation and the matrix constraint effect. By incorporation of 3 wt% fibers with the average length of 5～6 mm, the bending strength and fracture toughness of the composites reached 88.92 MPa and 4.62 MPa m^1/2, respectively, while the strength conservation ratio after thermal shock increased from 48.85% to 75.86%. The weak interface bonding built up between ZrO_2(f) and NiFe₂O₄ facilitates the reinforcing effects of the fibers to operate.     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

Influence of pyrolysis temperature on fracture response in SiOC based composites reinforced by basalt woven fabric

The fracture resistance of ceramic based composites reinforced by various ceramic fibres can be dramatically enhanced when an efficient fracture mechanism takes place during the crack propagation. Presented work shows an effect of the pyrolysis temperature of the composite matrix on the fracture behaviour of the composite. The matrix is formed from the polysiloxane resin precursor and the reinforcement is a basalt woven fabric. The temperature range under investigation was from 600 °C, where the onset of fracture properties were observed up to 800 °C. Above this temperature basalt fibres suffer by rapid degradation of the microstructure. The optimum stage of the polysiloxane resin transformation maximizing the fracture resistance of the composite was identified. The fractographic analysis of the fracture surfaces revealed the differences in the acting fracture mechanism.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

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.     

Ionic liquid assisted fabrication of high performance SWNTs reinforced ceramic matrix nano-composites

A Novel approach to fabricate high performance SiCN ceramic nanocomposites containing well dispersed single-walled carbon nanotubes (SWNTs) assisted by room temperature ionic liquids (RTILs) is developed. This method is straightforward, environment-friendly and overcome the typical challenges in the synthesis procedures of SWNTs reinforced precursor derived ceramics (PDCs). The microstructural and elemental characteristics of these ceramic matrix nanocomposites present highly-dispersed SWNTs were introduced and a profitably BN(C) interlayer could be formed in situ between the SWNTs and the ceramic matrix, which resulting in a high performance and a crack free ceramic product. Compared to the pure monolithic SiCN ceramic, the Young's modulus enhanced by ~11% and the electrical conductivities increased up to 0.06 S cm⁻¹ for ceramic composite in the case of the 1 wt% SWNTs was containing. Furthermore, the electrochemical investigation shows the potential application of these SWNTs-IL/Ceramics composites in electrochemical hydrogen storage.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

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.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Alumina fiber-reinforced silica matrix composites with improved mechanical properties prepared by a novel DCC-HVCI method

A novel direct coagulation casting via controlled release of high valence counter ions (DCC-HVCI) method was applied to prepare the alumina fiber-reinforced silica matrix composites with improved mechanical properties. In this method, the silica suspension could be rapidly coagulated via controlled release of calcium ions from calcium iodate and pH shift by hydrolysis of glycerol diacetate (GDA) at an elevated temperature. The influence of tetramethylammonium hydroxide (TMAOH) dispersant amount, volume fraction and calcium iodate concentration on the rheological properties of suspensions was investigated. Additionally, the effect of alumina fiber contents on the mechanical properties of alumina fiber-reinforced silica matrix composites was studied systematically. It was found that the stable suspension of 50 vol% solid loading could be prepared by adding 2.5 wt% TMAOH at room temperature. The addition of 0–15 wt% alumina fibers had no obvious effect on the viscosity of the silica suspension. The controlled coagulation of the suspension could be achieved by adding 6.5 g L⁻¹ calcium iodate and 1.0 wt% GDA after treating at 70 °C for 30 min. Compressive strength of green bodies with homogeneous microstructure was in the range of 2.1–3.1 MPa. Due to the fiber pull-out and fracture behaviors, the mechanical properties of alumina fiber-reinforced composites improved remarkably. The flexural strength of the composite with 10 wt% alumina fibers sintered at 1350 °C was about 7 times of that without fibers. The results indicate that this approach could provide a promising route to prepare complex-shaped fiber-reinforced ceramic matrix composites with uniform microstructure and high mechanical properties.     

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.     

LaPO4 coating on alumina-based fiber: Strength retention of fiber and improvement of interfacial performances

The characteristics of the interface are the key factors that determine the mechanical properties and fracture behavior of fiber-reinforced ceramic matrix composites. Design and preparation of coatings which can preserve fiber strength and maintain appropriate interfacial bonding strength are of great challenges. LaPO₄ coating is a promising weak interface coating for oxide fiber reinforced oxide ceramic matrix composites. Through this coating, the toughening mechanism of the composite such as fiber pulling out and fiber debonding is triggered. The LaPO₄ coating was deposited on the surface of alumina-based fibers by a solution precursor heterogeneous precipitation method. The effects of different precursors and different deposition temperatures on fiber strength were studied, and the mechanism of the strength degradation of the coated fiber was analyzed. It was found that the fibers coated with phytic acid precursor and deposited at 90 °C had the highest tensile strength compared to other coated fibers. The retention of strength is attributed to its loosely stacked coating. Besides, a single fiber pullout test was carried out to evaluate the effect of the coating on the interface of the composites. The results show that the composites coated by depositing citric acid precursor and phytic acid precursor at 90 °C can reduce the interfacial bonding strength by 32.5% and 46.7%, respectively compared to uncoated composites. This study has potential application value in the preparation of ceramic matrix composites used in oxidation and high temperature environments.     

Tough salami-inspired Cf/ZrB2 UHTCMCs produced by electrophoretic deposition

One of the biggest challenges of the materials science is the mutual exclusion of strength and toughness. This issue was minimized by mimicking the natural structural materials. To date, few efforts were done regarding materials that should be used in harsh environments. In this work we present novel continuous carbon fiber reinforced ultra-high-temperature ceramic matrix composites (UHTCMCs) for aerospace featuring optimized fiber/matrix interfaces and fibers distribution. The microstructures – produced by electrophoretic deposition of ZrB₂ on unidirectional carbon fibers followed by ZrB₂ infiltration and hot pressing – show a maximum flexural strength and fracture toughness of 330 MPa and 14 MPa m^1/2, respectively. Fracture surfaces are investigated to understand the mechanisms that affect strength and toughness. The EPD technique allows the achievement of a peculiar salami-inspired architecture alternating strong and weak interfaces.     

Tailoring carbon nanotube/matrix interface to optimize mechanical properties of multiscale composites

Pyrolytic carbon (PyC) was deposited on surfaces of carbon nanotubes (CNT) which were grown on carbon fibers to optimize the interfacial bonding between CNT/Matrix. The PyC protected CNT effectively and weakened CNT/Matrix interfacial strength, leading to long pull-out of CNT compared to brittle fracture of uncoated CNT. The well-protected CNT have more effective contributions to the improvement of mechanical properties. A “fiber-PyC/SiC-(CNT + PyC)-(CNT + SiC)” structure was formed using this process.     

Carbon fibers from dry-spinning of acetylated softwood kraft lignin

An acetylated softwood kraft lignin was dry-spun into precursor fibers and successfully processed into carbon fibers with a tensile strength exceeding most values reported in prior studies on lignin-based carbon fibers. Limited acetylation of lignin hydroxyl groups enabled dry-spinning of the precursor using acetone (solvent) followed by thermo-oxidative stabilization. Resulting carbon fibers (∼7 μm diameter) displayed a tensile modulus, strength, and strain-to-failure values of 52 ± 2 GPa, 1.04 ± 0.10 GPa, and 2.0 ± 0.2%, respectively. Because of solvent diffusion during dry-spinning, fibers displayed a crenulated surface that can provide a larger specific interfacial area for enhanced fiber/matrix bonding in composite applications.     

Ablation resistance of HfC coating reinforced by HfC nanowires in cyclic ablation environment

To improve the ablation resistance of carbon/carbon composites in cyclic ablation environment, SiC/HfC ceramic coating reinforced by HfC nanowires was prepared. The microstructure, bonding strength, coefficient of thermal expansion and cyclic ablation resistance of the as-prepared coating were investigated. After incorporating HfC nanowires, the bonding strength between inner SiC coating and outer HfC coating was increased. HfC nanowires could relieve the coefficient of thermal expansion mismatch between inner and outer coating and improve the toughness of the outer coating. By introducing HfC nanowires, the coated sample’s cyclic ablation resistance was improved. After cyclic ablation under oxyacetylene flame for 60 s × 3, the mass and linear ablation rates of the coated sample with HfC nanowires were only 0.444 mg/s and −0.767 μm/s, respectively.     

Influence of the carbonization temperature on the mechanical properties of thermoplastic polymer derived C/C-SiC composites

Carbon/Carbon (C/C) composites derived from the thermoplastic polymer polyetherimide (PEI) were pyrolized up to 1000 °C, subsequently carbonized in inert atmosphere up to 2200 °C and afterwards infiltrated with liquid silicon. The investigation of fibers and matrix with Raman microspectroscopy revealed, that an increased carbonization temperature leads to an increased carbon order as well as an incipient stress-induced graphitization of the carbon matrix close to the fiber surfaces at 2200 °C. The derived C/C-SiC samples show a maximum flexural strength of 180 MPa with C/C composites treated at 2000 °C and monotonically increasing Young’s moduli ranging from 49 GPa with C/C preforms treated at 1600 °C up to 59 GPa after carbonization at 2200 °C. The carbon fiber strength was evaluated with a single fiber tensile test, which showed a monotonically increased Young’s modulus and a decrease of the strength after carbonization at 2200 °C.     

Fabrication of hydroxyapatite/diopside/alumina composites by hot-press sintering process

Hydroxyapatite/alumina/diopside ceramic composites were fabricated by hot-pressing. The hardness, fracture toughness and bending strength of the new fabricated composites were measured. The compositions of hydroxyapatite matrix ceramic composites were discussed by XRD and FT-IR analysis. Microstructures of the composites were studied on fracture surfaces. The bending strength and fracture toughness of 58 vol.% hydroxyapatite, 40 vol.% alumina and 2 vol.% diopside sample, were 200 MPa and 2.80 MPa m^1/2, respectively.