Carbon fiber reinforced multilayered pyrocarbon-silicon carbide ((PyC-SiC)n) matrix (C/(PyC-SiC)n) composites were prepared by means of layer-by-layer deposition of PyC and SiC via chemical vapor infiltration. Effects of the number of PyC-SiC sequences (n = 1, 2 and 4) on matrix microstructure, electrical conductivity, mechanical and electromagnetic interference (EMI) shielding performance of C/(PyC-SiC)n composites were investigated. The results show that with increasing the number of sequences, flexural strength and fracture toughness of the composites increase from 121 ± 17 to 193 ± 18 MPa and from 3.0 ± 0.1 to 4.2 ± 0.3 MPa m1/2, respectively. The enhanced mechanical properties of C/(PyC-SiC)n composites are attributed to the increasing number of interfaces, supplying more channels for crack deflection and propagation, which is favorable to more fracture energy dissipation. The total shielding effectiveness (SE) of the composites increases from 34 to 42 dB in the frequency range of 8.2–12.4 GHz with the increase of PyC-SiC sequences number due to the increasing electrical conductivity and polarization of the multilayered matrix. The high EMI SE combined with low density and good mechanical properties of C/(PyC-SiC)n composites exhibit great potential as lightweight and high-performance structural and functional materials.

•Carbon/carbon composites were bonded with carbon nanotube/pyrocarbon interlayer.•Carbon nanotube created mechanical interlocking at interlayer-substrate interface.•Carbon nanotube induced annularly stacking of pyrocarbon laminas.•Carbon nanotube increased additional fracture process during shear test.•Carbon nanotube/pyrocarbon-bonded composites possessed excellent shear strength.We reported a novel carbon nanotube/pyrocarbon (CNT/PyC) interlayer for joining carbon/carbon composites (C/C). Radially-aligned CNT was in-situ grown on the bonding surface of C/C and followed by the infiltration of PyC between CNT-covered C/C. Mechanical tests indicated that the average shear strength of CNT/PyC-bonded C/C was 19.2 MPa, 104% higher than that of neat PyC-bonded C/C. This great improvement results from the outstanding contributions of CNT involving enhancing interlayer-C/C interfacial interaction, inducing PyC texture to align around nanotubes instead of parallel to the bonding surface and increasing additional fracture process, which change the failure of bonded C/C from bonding-interlayer-failure dominated mode to substrate-failure dominated one.

•Both matrix and fiber/matrix interface of carbon/carbon composites were optimized.•Radial carbon nanotube (CNT) was grown on carbon fibers (CF) to strengthen matrix.•Pyrocarbon layer was introduced between CF and CNT/matrix to optimize interface.•Optimal designs endowed composite with improved strength, ductility and toughness.The direct attachment of carbon nanotubes (CNTs) on carbon fibers (CFs) always leads to a decrease of fiber-dominated properties (e.g., flexural strength) and a brittle fracture behavior of C/Cs, although the matrix-dominated properties (e.g., compressive strength and interlaminar shear strength (ILSS)) exhibit an obvious enhancement. To achieve the combination of mechanical strength, ductility and toughness in C/Cs, in this work, efforts were spent on simultaneously optimizing the matrix and fiber/matrix (F/M) interface. CNTs with radial orientation were grown onto the CFs by double-injection chemical vapor deposition to modify the microstructure of matrix. Pyrocarbon was deposited on the surface of CFs before CNT growth to protect CFs and to weaken interfacial strength between CFs and CNT/matrix. These optimal designs create strengthening and toughness mechanisms such as crack deflection and long pullout of CFs in the failure process of composites, which endow C/Cs with improved flexural strength of 31.5%, flexural ductility of 118%, compressive strength of 81.5% and ILSS of 82%, accompanied by a clear change from brittle fracture to pseudo-plastic fracture during flexural test. This work may provide a meaningful way to not only enhance both the fiber- and matrix-dominated strength but to substantially improve the ductility and toughness of C/Cs.Download high-res image (213KB)Download full-size image

To investigate the effect of preparation methods on the La-Mo-Si (LMS) coatings, we developed a new LMS coating system using pack cementation (PC) and supersonic atmospheric plasma spraying (SAPS). Microstructure analysis showed that the SAPS-LMS coating possessed a higher porosity than that of the PC-LMS coating. Higher porosity can provide more channels to the oxidative and corrosive gasses to permeate the SAPS-LMS ceramic top-coat. After static oxidation for 150 h under 1773 K, the mass loss of SAPS-LMS coating (3.12 wt%) was much higher than that of the PC-LMS coating (0.05 wt%), and the parabolic rate constants presented faster oxidation kinetic in SAPS-LMS coating with respect to the PC-LMS coating. These results revealed that the protection effectiveness of SAPS-LMS coating was inferior to PC-LMS coating. Compared with SAPS-LMS coating, microscopic pores and cracks appeared in the PC-LMS coating with a thicker oxide film, which benefits from the formed La-Si-O-Al glass oxide with an excellent ability for crack healing. The reasons for poor antioxidant performance of SAPS-LMS coating are the higher volatility of La-Si-O glass containing Mo5Si3 and a weak interfacial interaction between coatings and substrate.

To prevent the C/C composites from ablation, HfC-HfO2 protective coating was prepared by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC-HfO2 coating were characterized by X-ray diffraction and scanning electron microscopy. The ablation resistance test was carried out by an oxyacetylene torch. The results show that the as-prepared coating is dense with little pinholes and crack free. The elements Hf, C and O were uniformly distributed in the cross-section. After ablation for different time, the mass ablation rate fluctuated along with the change of ablation time. The ablation process of the surface coating could be divided into rapid oxidation and solid state sintering stages. During ablation, an HfCxOy-HfO2 transitional layer was generated in the coating, which resulted from the active oxidation of HfC. After cooling, some microcracks were observed on the surface of coating, and the structure of cross-section was broken, which were due to the phase transition of HfO2.

•The addition of ZrO2 leads to improved oxidation resistance of LM coating.•Good adherence of ZLM coating attributes to mechanical bonding with interlocking.•The formation of “inlaid structure” and Zr-La-Si-O compound glass film ensured long-term stability of ZLM coating.Plasma-sprayed ZrO2-modified LaB6-MoSi2 coating (ZLM) was firstly fabricated on SiC pre-coated carbon/carbon (C/C) composites to obtain an improved oxidation resistance at 1773 K involving long-term oxidation and short-term cyclic oxidation in an atmospheric environment. A crack-free structure in ZLM coating has been established with interlocking adhesion between the layers of coating. It is shown that the specimen with ZLM coating exhibits superior oxidation with the mass loss of 0.96% after 140 h at 1773 K and spallation resistance with the mass loss of 0.61% after 30 thermal cycles between 1773 K and room temperature. These beneficial effects can be attributed to superior integrated structure of the ZLM coating provided by the continuous Zr-La-Si-O protective scale gave rise to the suppression of oxygen diffusion in ZLM coating. Moreover, the formation of “inlaid phases” in the compound glass layer is helpful to consume the crack propagation energy and restrict the spreading of it. Dual protection of the “inlaid structure” and Zr-La-Si-O compound glass film is responsible for the excellent oxidation resistance of the ZLM coating.Download high-res image (232KB)Download full-size image

Effects of impact defects on thermal expansion behavior of 2.5D C/C composites were studied in this work. A 6 mm steel ball driven by solid explosive was used as a projectile to simulate hypervelocity impact of debris or flying stones. The coefficients of thermal expansion (CTEs) of post-impacted samples located in different areas were investigated from 850 to 2500 °C. The results show that impact defects are primarily focused on crater regions. These defects can significantly decrease the CTEs of C/C composites under 850–2350 °C and have a little influence in the range of 2350–2500 °C. Post-impacted samples near the crater have a hysteresis than that of edge samples at the minimum of CET-XY and the maximum of CET-XY. There is less thermal stress produced in samples with a large amount of damage than samples having a little damage during CTE tests. Cracks produced by the explosive impact not only offer spaces for thermal expansion but also supply channels for releasing thermal stress. Specifically, fiber breakages and matrix cracks perpendicular to fiber direction are the major factors of CTE decrements in X-Y direction. Sub-layer laminations, fiber/matrix debonding, and circumferential cracks lead to the decreases of CTEs in Z direction.

To improve the ablation resistance of C/C composites, ZrC modified composites were fabricated by precursor infiltration and pyrolysis combined with gradient chemical vapor infiltration process. The effects of ZrC precursor concentration on the microstructure, mechanical and ablation properties of the composites were studied. Results showed that with the increase of ZrC precursor concentration, the ZrC content and macroscopic uniformity of the composites increased but with obvious ZrC particle aggregation and the flexural strength decreased gradually. As the concentration of ZrC precursor improved to 60%, the fracture mode of the composites transformed from toughness to brittleness which was mainly attributed to the improved graphitization degree and reaction damage of carbon fiber in the precursor pyrolysis process. However, the ablation resistance was enhanced with the increasing precursor concentration which was resulted from the formation of ZrO2 in center ablation region and continuous ZrO2 coating in brim region serving as a barrier to heat and oxygen transfer.

To protect carbon/carbon (C/C) composites against oxidation, YSZ-La-Mo-Si (YSZ-LMS) heterogeneous coating was prepared by the plasma spraying using YSZ (Y2O3-stabilized ZrO2), MoSi2 and LaB6 as raw materials. Microstructure and surface chemistry of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Moreover, the thermogravimetric and isothermal oxidation results indicated that the YSZ-LMS coating revealed superior oxidation protective ability at elevated temperatures, which protected the C/C composites from oxidation for 50 h at 1773 K with a mass loss of 0.58%. The generation of SiO2, ZrSiO4, Y2SiO5 and La2O3 expanded the volume of solid phase and decreased the volatilization of SiO2, forming a denser Zr-Y-La-Si-O oxide glass layer. Such volume expansion promoted the formation of compressive stress within the coating at high temperature, which restrains the initiation and propagation of cracks and facilitates the oxidation resistance of the coatings for C/C composites. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 74.466 kJ mol−1.

•Effects of the simulated low earth orbit environment on C/C-ZrC-SiC composites were investigated.•Two different types of interface damage formed in C/C-ZrC-SiC composites with the increase of cycle number.•The initial 100-time thermal cycles enhanced the flexural strength (9.35%) of C/C-ZrC-SiC composites.•The defects induced by the low-temperature thermal cycling accelerated the oxidative damage of C/C-ZrC-SiC composites.Carbon/Carbon (C/C) composites modified with ZrC and SiC particles were prepared by precursor infiltration and pyrolysis (PIP) process. Effects of the simulated low earth orbit environment including the vacuum environment (under the high-vacuum state of 1.3 × 10−3 Pa) and low-temperature thermal cycling (the temperature is ranged from 153 K to 393 K for 200 times) on C/C-ZrC-SiC composites were investigated. These results show that the significantly generated microcracks, interspaces and interface debondings contributed to the increase of open porosity. The interfacial damages of C/C-ZrC-SiC composites caused by low-temperature thermal cycling in short-cut webs and non-woven webs were generated at fiber/matrix interface and fiber bundle/matrix interface, respectively. The flexural strength of C/C-ZrC-SiC composites increased by 9.35% after 100-time thermal cycles and then decreased dramatically to 83.99% of pristine strength after 200-time thermal cycles, accompanied by the transformation of fracture behavior from the brittle fracture mode into the pseudo-plastic fracture mode. In addition, the defects induced by the low-temperature thermal cycling accelerated the oxidative damage of C/C-ZrC-SiC composites during thermal shock between 1773 K and room temperature.Download high-res image (381KB)Download full-size image

Three kinds of carbon fiber reinforced multilayered (PyC–SiC)n matrix (C/(PyC–SiC)n) composites (n = 1, 2 and 4) were prepared by means of layer-by-layer deposition of PyC and SiC via chemical vapor infiltration. Thermal expansion behaviors in the temperature range of 800–2500 °C and thermal conductivity from room temperature to 1900 °C of C/(PyC–SiC)n composites with various microstructures were investigated. The results show that with increasing PyC–SiC sequences number (n), the coefficients of thermal expansion of the composites decrease due to the increase of interfacial delamination, providing room for thermal expansion. The thermal diffusivity and thermal conductivity also decrease with the increase of sequences number, which are attributed to the enhancement of phonon-interface scattering resulted from the increasing number of interfaces. Modified parallel and series models considering the interfacial thermal resistance are proposed to elaborate thermal conductivity of the composites, which is in accordance with the experimental results.

•Graphene wrapped Co3O4/NiCo2O4 DSNCs has been prepared for detection of glucose.•Sensing performance was improved by synergy between electrocatalytic activity and efficient electron transport.•The sensor has excellent sensing performance with high sensitivity and low detection limit.•The developed method was successfully applied to detect glucose in human serum.Graphene (G) wrapped porous Co3O4/NiCo2O4 double-shelled nanocages (Co3O4/NiCo2O4 DSNCs@G) were prepared by the formation of Co3O4/NiCo2O4 DSNCs using zeolite imidazole frameworks-67 as template with the subsequent calcination and package of G by hydrothermal method. The abundant accessible active sites provided by the porous structure of Co3O4/NiCo2O4 DSNCs and efficient electron transport pathways for electrocatalytic reaction offered by the high conductive G worked very well together in a ferocious synergy, which endowed Co3O4/NiCo2O4 DSNCs@G with excellent electrocatalytic behaviors for determining glucose. A comparison between Co3O4/NiCo2O4 DSNCs without G packing and Co3O4/NiCo2O4 DSNCs@G showed that former had linear response window concentrations of 0.01-3.52 mM (correlation coefficient = 0.999), detection limit of 0.744 μM (S/N = 3) and sensitivity of 0.196 mA mM−1 cm−2, whereas the latter exhibited linear response window concentrations of 0.01-3.52 mM (correlation coefficient = 0.999), detection limit of 0.384 μM (S/N = 3) and sensitivity of 0.304 mA mM−1 cm−2. The combination of Co3O4/NiCo2O4 DSNCs and G was a meaningful strategy to fabricate high-performance non-enzyme glucose sensors with low detection limit, good selectivity and high sensitivity.Download high-res image (165KB)Download full-size image

•Ca-P bio-coating was prepared by microwave hydrothermal combining supersonic atmosphere plasma spraying methods.•The shear strength of Ca-P bio-coating on C/C substrate was evaluated.•The coating shows an excellent shear strength under the Ca2+ concentration of 10 mmol/L.•The roughness of surface after adding microwave hydrothermal technique is beneficial to enhance the shear strength.•The bioactivity of Ca-P bio-coating was detected in SBF solution.Microwave hydrothermal (MH) combining supersonic atmospheric plasma sprayed (SAPS) calcium phosphorus (Ca-P) bio-coatings on carbon/carbon (C/C) composite has been widely used due to their osteoconductivity and osteoproductivity. However, the erratic shear strength between coatings prepared only by SAPS (outer coating) and C/C substrates has attached more attention over the implant failure. Adding a coating prepared by MH (inner coating) before SAPS can possess superior shear strength to conventional outer coating. The inner coating with fine Ca-P particles was prepared through a unique MH method under different concentrations (10, 500 and 1000 mmol/L). The influence of concentration on microstructure, phase composition, roughness and shear strength are investigated in this paper. In particularly, the roughness of inner coatings on C/C substrates was found to related to the morphologies and particle size.Results showed that inner coatings have higher roughness which was beneficial for the promotion of shear strength between the obtained Ca-P bio-coating and the C/C substrates. Subsequently, the specimens were immersed in a simulated body fluid (SBF) to investigate the bioactivity.

•Ca-P coating was prepared by microwave hydrothermal combined with spraying method.•Shear strength between coating and substrate was enhanced by 60% using new method.•The coating showed reactivity in stimulated body fluid.Ca-P based coatings on carbon/carbon composite (C/C) were manufactured via a combined method comprising of microwave-hydrothermal (MH) and supersonic atmospheric plasma spraying (SAPS) techniques. However, a weak mutual interaction between the coating and C/C substrate has been a critical issue for a long time. Herein, we reported a new method for shear strength enhancement without compromising the osteoconductivity and osteoproductivity. Results showed that the inner layer has a strong mechanical interlocking with C/C substrate and the failure mode of outer layer changed from the coating cohesion (within the coating) to adhesive (at the coating/substrate interface) fracture. The shear strength between Ca-P bioactive coating-C/C substrate by MH/SAPS was significantly improved as compared to that prepared by SAPS. The Ca-P bioactive coating exhibited a good bioactivity as evidenced by the formation of a uniform carbonate-apatite layer formed on coating after immersing into stimulated body fluid for a specified period of time.

•Defects in the upper side are associated with impact pressure of the projectiles.•The upper side of post-impacted sample presents a compression failure mode.•Defects in the bottom side are dominated by delamination and transverse cracks.•Delamination occurs between mat plies and other plies (0° and 90° cloth plies).•The strength decreases of post-impacted sample-R were bigger than that of sample-F.This paper describes the hypervelocity impact behavior and the residual strength of carbon fiber reinforced carbon (C/C) composites under different impact velocity. Impact tests of 2.5D C/C composite samples have been performed using one stainless steel ball driven by solid explosive to detect correlations between the impact direction and damage distribution and the residual flexural strength of the composites. Results showed that the impact resistance of the C/C composites was affected by the impact velocity. Damage modes evolved from matrix cracking and fiber breakages to spallation and delamination along the impact direction. Furthermore, delamination occurs delamination occurred in mat plies and 90° plies, resulting from poor load-bearing capability of short mat layers and weak adhesion between fiber bundles. In addition, the residual strength of the damaged C/C composites decreased by 30.6–43.5% and 47.4–48.6% in the upper side and bottom side, respectively. The flexural stress-strain curves and fracture modes of post-impacted composites were various due to the diverse damage types in different areas.

In order to improve the ablation properties of carbon/carbon composites, HfC–SiC coating was deposited on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC–SiC coating were characterized by SEM and XRD. The ablation resistance test was carried out by oxyacetylene torch. The results show that the structure of coating is dense and the as-prepared HfC–SiC coating can protect the C/C composites against ablation. After ablation for 30 s, the linear ablation rate and mass ablation rate of the coating are −0.44 μm/s and 0.18 mg/s, respectively. In the ablation center region, a Hf–Si–O compound oxide layer is generated on the surface of HfC–SiC coating, which is conducive to protecting the C/C composites from ablation. With the ablation time increasing to 60 s, the linear ablation rate and mass ablation rate are changed to −0.38 μm/s and 0.26 mg/s, respectively. Meanwhile, the thickness of the outer Hf–Si–O compound layer also increases.

A three-dimensional (3D) carbon/carbon (C/C) composite was prepared using the gaseous mixture of ethanol and methane as the precursor by isothermal chemical vapor infiltration. The preform was infiltrated at 1100 °C with the gas pressure from 2 to 10 kPa. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Texture and fracture morphology of the pyrolytic carbon matrix were observed using a scanning electronic microscope. After 150 h infiltration, the average bulk density was up to 1.72±0.02 g cm−3. 3D C/C composites with high texture pyrolytic carbon matrix were obtained by pyrolysis of ethanol and methane. The average flexure and tensile strengths of the composites are 362 MPa and 116 MPa, respectively.

•Different SiC coatings for C/C composites were prepared with β-SiC powder (FCS) or not (CS).•β-SiC powder refines SiC grain, but causes severe defects such as large open-holes and more and wider penetrating cracks.•The oxidation and thermal shock resistance of the β-SiC-containing SiC coating are inferior because of severe defects.Two kinds of SiC coatings were fabricated on C/C composites by a two-step pack cementation. The CS-SiC coating was prepared using Si, graphite and Al2O3 powder as the raw materials, while the raw materials of FCS-SiC coating were composed of Si, graphite, Al2O3 and β-SiC powder. The morphology and phase composition of the coatings were characterized by scanning electron microscopy and X-ray diffraction. The results showed that the FCS-SiC coating possessed a denser surface with smaller grain sizes. Meanwhile, some large open-holes and penetrating cracks could be found in the surface of the FCS-SiC coating. The oxidation and thermal properties of the coatings were investigated and compared. After oxidation at 1773 K for 43 h, the FCS-SiC and CS-SiC coating presented a weight-loss of 0.64% and a weight-gain of 0.44%, respectively. The FCS-SiC coating showed a weight-loss of 0.52% after 30 thermal cycles between 1773 K and room temperature, while the CS-SiC coating exhibited a weight-gain of 0.11% after 50 thermal cycles. The inferior protection ability of the FCS-SiC coating could be attributed to the severe defects. These severe defects were formed in the preparation process of the FCS-SiC coating, which might be ascribed to the effect of the β-SiC additive.

Ablation of C/C–ZrC composites in a solid rocket motor environment was investigated. The chemical compositions of the combustion gases were calculated using the principle of free energy minimisation. The chemical thermodynamics of the products of the reactions between the combustion gases and ZrC were studied using the FactSage software. The results show that H2O, CO2, and OH are the main oxidising species that consume ZrC/carbon, and the oxidation of ZrC reduces carbon consumption. In addition, melting and evaporation of ZrO2 absorb heat from the combustion gases, relieving the erosive attack on carbon. The ablation of C/C–ZrC composites is controlled by both thermal chemical ablation and mechanical breakage. The cone-shaped fibres and shell-shaped, honeycomb matrix are attributed to thermal chemical ablation, whereas the carbon fragments result from mechanical breakage.

C/C–ZrC composites with continuous ZrC matrix were prepared by precursor infiltration and pyrolysis process using zirconium-containing polymer. Ablation properties of the composites were investigated by oxyacetylene flame with heat flux of 2380 and 4180 kW/m2, respectively. The results showed that C/C–ZrC composites exhibited excellent ablation resistance under the heat flux of 2380 kW/m2 for 120 s and a tree-coral-like ZrO2 protective layer formed after ablation. However, when the heat flux increased to 4180 kW/m2, the maximum temperature of ablated surface reached 2500 °C and a strong degradation of ablation resistance was observed due to the weak bonding between the formed ZrO2 layer and the composites. The flexural strength of C/C–ZrC composites was 110.7 ± 7.5 MPa. There were a large number of carbon fiber bundles pull-out, and the composites exhibited a pseudo-plastic fracture behavior.

•The effect of vacuum thermal cycling on C/C composites was showed systematically.•An abnormal phenomenon of thermal expansion of C/C composites under cryogenic temperature was discussed by Raman spectrum.•The flexural strength showed a recovery tendency (28.84%) after 100 thermal cycles.•An interfacial damage model based on microscopic observation was proposed.The study aims to investigate the effects of vacuum thermal cycling on carbon/carbon (C/C) composites used in aerospace under simulated low earth orbit (LEO) environmental conditions. The C/C composites were tested at the temperature range from −120 °C to 120 °C for 100 cycles under the high-vacuum state of 1.3x10−3Pa, the composites were characterized through the assessment of the thermal physical and mechanical property changes. It follows from the experiment that the cyclic thermal stress derived from thermal cycling could influence the stability of C/C composites which result in an abnormal phenomenon of thermal expansion of C/C composites under cryogenic temperature. The flexural strength initially decreases from 105.51 MPa to 74.83 MPa with the thermal cycles increasing to 30, then increases to 96.41 MPa after 100 thermal cycles, showing a recovery tendency (28.84%) of flexural strength compared with 30 thermal cycles and maintains the initial flexural strength of 91.39%.The implications of these processes based on the relation between coefficients of C/C composites between vacuum thermal cycling were discussed and an interfacial damage model based on microscopic observation was proposed.

•C/C composites modified by hafnium carbide and silicon carbide were fabricated.•Precursor infiltration and pyrolysis process was used for preparing the composites.•The composites were ablated under oxyacetylene flame.•The micro-structure evolution of the composites during ablation was discussed.•The effect of ablation products on the ablation properties was analyzed.In order to investigate the ablation behavior and mechanism of carbon/carbon (C/C) composites modified by hafnium carbide (HfC) and silicon carbide (SiC), the composites were ablated by oxyacetylene flame for different time from 30 s to 120 s. Results show that linear ablation rate and mass ablation rate increased with ablation time prolonging. Two ablation regions were discussed: the center-most ablation region which was the most severely ablated and brim ablation region which was only slightly ablated. By analysis of the morphologies evolution of the two regions, it can be demonstrated that ablation products, hafnium oxide (HfO2) and silicon oxide (SiO2), acting as a protection layer for the composites, were produced. While the ablation products lost their protection gradually with ablation time going on. Moreover, mechanical denudation and thermal chemical ablation damaged more severely with ablation time prolonging.

•The mixture of ethanol and methane was used as the precursor of pyrolytic carbon.•C/C composites with high textured pyrolytic carbon and high density were obtained.•At the same condition, the high textured carbon cannot be obtained using methane.•The mixture precursor is a promising candidate for C/C composites in CVI.A high textured carbon/carbon (C/C) composite was prepared using the mixture gas of ethanol and methane as the precursor by isothermal chemical vapor infiltration. The preform was infiltrated at 1180 °C with the gas pressure from 2 to 10 kPa. For 85 h infiltration, the average bulk density is up to 1.8 ± 0.02 g cm−3. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Texture and fracture morphology of the pyrolytic carbon matrix were observed using scanning electronic microscope. C/C composites with high textured pyrolytic carbon matrix and high density were obtained by pyrolysis of ethanol and methane. This indicates the mixture of ethanol and methane is a promising candidate of the precursors for the preparation of C/C composites.

A silicon- and zirconium-containing polymer precursor (PSZC) for SiC–ZrC–C ceramic was successfully synthesized by chemical reaction of phenol, paraformaldehyde, tetraethoxysilane, acetylacetone, and ZrOCl2·8H2O. The chemical structure, pyrolysis behaviors, and pyrolysis products of PSZC were characterized by the combination of FT-IR, 1H-NMR, TG-DTG, XRD, Raman, SEM, and EDS. It indicates that the obtained PSZC might be Zr–O–Zr and Si–O–Si chain polymer connected by Si–O–Zr bond with hydroxymethyl phenol and acetylacetone as ligands. And the most probable reaction mechanism to form PSZC was proposed. Decomposition of PSZC is completed at 800 °C, and it gives amorphous carbon, SiO2, and ZrO2 with a yield of 70 %. During heat treatment, SiC forms at about 1500 °C, followed by the appearance of ZrC. Defects of the carbon are cumulated to the highest at 1500 °C. Further heating at 1800 °C induces highly crystallized ZrC, SiC ceramic, and structure-ordered carbon with a structure of ZrC and SiC particles uniformly distributed in the carbon matrix.

An electrochemical sensor based on in situ synthesized Cu2O microparticles–Cu2O nanowires–graphene (Cu2O MPs–Cu2O NWs–graphene) composite for sensitive detection of ethylenediamine (EDA) is reported. X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, field emission transmission electron microscopy, and energy-dispersive X-ray spectroscopy were utilized to characterize the composition and morphology of the composite. The electrochemical behaviors of EDA at the Cu2O MPs–Cu2O NWs–graphene composite modified electrode were investigated by electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry. The electrochemical sensor exhibited excellent analytical performance for EDA detection with low detection limit of 3.83 × 10−5 M (S/N = 3) and a reproducibility of 1.1 % relative standard deviation. The modified electrode exhibited a rapid response to EDA within 5 s and the amperometric signal showed a good linear correlation to EDA concentration in a broad range from 0.25 to 1.25 mM with a correlation coefficient of R = 0.99699. The superior electrochemical performances of Cu2O MPs–Cu2O NWs–graphene composite are attributed to their peculiar composite structure and the synergistic effects between Cu2O MPs–Cu2O NWs and graphene [Huang et al., Sens Actuators B 178:671–677, 2013; Luo et al., Anal Chim Acta 709:47–53, 2012].

The primary goal of this research was to study the flexural fatigue behavior and enhancement of residual strength in 2D cross-ply carbon-fiber-reinforced carbon (C/C) composites. Flexural fatigue behavior was simultaneously examined under two stress levels (70% and 65%) by electrical resistance change (ERC) method. Residual strength and fracture behavior of C/C composites were investigated as well. The results show that shapes of electrical resistance change rate–fatigue cycle curves could reflect applied stress levels and damage types during cyclic bending load. The enhancement of residual strength only occurs at a stress level lower than the fatigue limit, and the property of enhancement decreases with the increase of fatigue cycles. In addition, three-point bending fracture in both unfatigued and fatigued specimens is of delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time.

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were prepared by floating catalyst film boiling chemical vapor infiltration from xylene pyrolysis at 1000–1100 °C using ferrocene as a catalyst. The influence of the catalyst content on the densification behavior and matrix microstructure of the composites was studied. Results showed that the deposition rate of pyrocarbon (PyC) was enhanced remarkably by the catalyst. The density of the composites deposited at a catalyst content of 0–2.0 wt% decreased along both the axial and the negative radial directions. Rough laminar (RL) PyC matrix was formed at 0–0.8 wt% catalyst content by heterogeneous nucleation and growth. A hybrid matrix consisting of RL and isotropic (ISO) PyCs appeared at a catalyst content of 1.2–2.0 wt%. The reasons for this ISO PyC formation were attributed to the deposition of carbon encapsulated iron particles and homogeneous nucleation. A reinforcing network composed of NFCs and vapor grown carbon fibers was formed on the fiber/matrix interface and within the matrix in this floating catalyst process. The structure of NFC transformed from nanotube to nanofiber when the catalyst content was over 0.5 wt%, around which composites of a high density of 1.75 g/cm3 and uniform RL PyC matrix were produced rapidly.

A kind of zirconium modified phenolic resin (Zr-PF) was prepared by using phenol, paraformaldehyde, ZrOCl2·8H2O, acetylacetone, ethanol and H2O2 as raw materials. The structure of the Zr-PF was characterized by Fourier transform infrared (FT-IR) spectra and solution 13C nuclear magnetic resonance (13C NMR) spectra. The viscosity and thermal degradation behaviors of the Zr-PF were studied by rotary viscometer and thermogravimetric analyzer–differential scanning calorimetry (TG-DSC), respectively. The carbonized products of ordinary phenolic resin (PF) and Zr-PF were further investigated by X-ray diffraction (XRD), laser Raman spectroscopy (Raman) and scanning electron microscopy (SEM). Results show that new chemical bonds are formed through coordination reactions between zirconium atoms in Zr–OH and oxygen atoms in the acetylacetone and hydroxymethyl phenols. Viscosity of the Zr-PF is higher than that of PF during 30–80 °C. Compared with PF, thermal stability of the Zr-PF is obviously improved and its char yield is 55% at 1200 °C, 8% higher than that of PF. The d002 value of carbonized Zr-PF decreases from 0.3470 nm (carbonized PF) to 0.3329 nm and its crystallite height increases from 23.89 nm (carbonized PF) to 29.21 nm due to zirconium incorporation. In addition, the ID/(ID + IG) value of carbonized Zr-PF decreases from 0.571 (carbonized PF) to 0.364 and the crystallite size of it increases from 29.90 nm (carbonized PF) to 44.79 nm. The results prove that the incorporation of zirconium exhibits obvious effects on promoting its graphite crystallite. What is more, the morphology of the carbonized Zr-PF changes from poor structure with many pore defects into dense and uniform matrix with uniformly dispersed ZrC particles.

●The ZrO2 network nanorods are synthesized without a metal catalyst via oxyacetylene torch ablation.●A developed VLS and OAG model is discussed as the growth mechanism.●ZrO2 nanoparticles act as the growth catalyst and growth site of ZrO2 nanorods.●ZrO2 nanorods have a significant increase in length in the border region.In this work, the network structure of ZrO2 nanorods was firstly synthesized without a metal catalyst. A developed vapor–liquid–solid (VLS) and oxide-assisted growth (OAG) model is discussed as the growth mechanism of ZrO2 nanorods. ZrO2 nanoparticles, as a growth catalyst, provide zirconium and oxygen atoms for the formation of the nanorods and serve as a growth site as well. In the center and transition regions, ZrO2 nanorods with a diameter of 30–54 nm finally grow into a network of ZrO2 layer during the cooling process. In the border region, due to the appropriate growth temperature and adequate ZrO2 vapors, ZrO2 nanorods with a significant increase in length are synthesized during the ablation process.

•Graphene oxide was reduced by a novel reducing reagent sodium acetate trihydrate.•The water-dispersible properties of the reduced graphene oxide are poor.•Reduced graphene oxide formed some new and smaller average size of the sp2 domains.•A reduction mechanism of graphene oxide by sodium acetate trihydrate was proposed.•The thermal stability of reduced graphene oxide than that of graphene oxide.The synthesis of chemically reduced graphene oxide nanosheets (RGO) from graphite oxide commonly involves some harmful chemical reductants that are undesirable for most practical applications of graphene. An easy and environment friendly approach has been developed to reduce graphene oxide (GO) by using sodium acetate trihydrate as a reductant. The as-prepared RGO was characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), field emission transmission electron microscopy (FETEM), atomic force microscopy (AFM) and thermogravimetric analysis (TGA). Results show that GO was reduced to few-layers level with poor dispersion in water. A mechanism for removing of epoxy and hydroxyl groups from GO with sodium acetate trihydrate has been proposed. Considering that all the raw materials used are low cost, nontoxic and widely available, this approach may open up the new possibility for cost-effective, environment friendly and large-scale production of graphene.

•The shear strength was largest with the intermediate glass fiber content.•The compressibility increased with the increase of the glass fiber content.•The recovery decreased with the increase of the glass fiber content.•The μd increased initially and then decreased as the glass fiber content increased.•The optimized glass fiber content was 10 wt%.Five kinds of paper-based composite friction materials with different glass fiber contents were prepared by the paper-making process. The effect of glass fiber on the mechanical and tribological properties of the composites was studied. It was resulted that the shear strength increased initially and then decreased, but the compressibility increased and the recovery decreased as the glass fiber content increased. The friction torque curve of the sample with 10 wt% glass fiber was more flat during mixed asperity contact phase of the engagement, while the friction coefficient (μd) was higher and the wear rate was lower compared with other samples.

A four directional carbon/carbon (4D C/C) composite was fabricated by first using liquid phase impregnation carbonization (LPIC), followed by hot isostatic pressure impregnation and carbonization (HIPIC) at 75 MPa, and finally high temperature treatment. The flexural properties and fracture behavior of the composite were investigated in the through-thickness direction under static and fatigue loading. The critical fatigue limit of the composite was 80% of the static flexural strength for one million loading cycles at 10 Hz. The failure mechanism of the composite under static flexural loading was dependent on the orientation of the carbon fibers in the tested specimen. Cyclic fatigue loading decreased the interfacial bonding strength and released the inherent stresses in the composite, which increased fiber pull-out, enhanced pseudo-ductility and increased the residual static flexural strength at the expense of a decrease in the flexural modulus. The fatigue loading increased the number of noncritical matrix cracks, increased interfacial debonding, and caused the fracture of filaments in the surviving fatigued C/C composite. These features of the fatigued composite internally accommodated expansion in long direction as the temperature was increased, which resulted in a decrease in its residual thermal expansion.

C/C–ZrC–SiC composites were prepared by precursor infiltration and pyrolysis process using a mixture solution of organic zirconium-containing polymer and polycarbosilane as precursors. Porous carbon/carbon (C/C) composites with density of 0.92, 1.21 and 1.40 g/cm3 were used as preforms, and the effects of porous C/C density on the densification behavior and ablation resistance of C/C–ZrC–SiC composites were investigated. The results show that the C/C preforms with a lower density have a faster weight gain, and the obtained C/C–ZrC–SiC composites own higher bulk density and open porosity. The composites fabricated from the C/C preforms with a density of 1.21 g/cm3 exhibit better ablation resistance with a surface temperature of over 2400 °C during ablation. After ablation for 120 s, the linear and mass ablation rates of the composites are as low as 1.02 × 10−3 mm/s and −4.01 × 10−4 g/s, respectively, and the formation of a dense and continuous coating of molten ZrO2 solid solution is the reason for their great ablation resistance.

We reported a novel method to fabricate isotropic pyrocarbon (ISO Pc). A multi-walled carbon nanotube (MWCNT) preform (30 vol.% content, in which curved MWCNTs were randomly dispersed) was first prepared and was then densified to fabricate bulk ISO Pc by fixed-bed chemical vapor deposition (CVD) at low temperatures. Microstructure analyses show that the product is isotropic on both the micro- and nano-scale. It was found that the method can prepare ISO Pc under broad CVD conditions, which could develop a simple and reliable method to prepare ISO Pc.

Although in-situ growing carbon nanotubes (CNTs) on carbon fibers could greatly increase the matrix-dominated mechanical properties of carbon/carbon composites (C/Cs), it always decreased the tensile strength of carbon fibers. In this work, CNTs were introduced into unidirectional carbon fiber (CF) preforms by electrophoretic deposition (EPD) and they were used to reinforce C/Cs. Effects of the content of CNTs introduced by EPD on tensile property of unidirectional C/Cs were investigated. Results demonstrated that EPD could be used as a simple and efficient method to fabricate carbon nanotube reinforced C/Cs (CNT–C/Cs) with excellent tensile strength, which pays a meaningful way to maximize the global performance of CNT–C/Cs.

•The ZrC nanoparticles appear in the coating.•The ZrC nanostructure coating displays a high consistent orientation.•The ZrC nanostructure coating can prevent the coating cracking during ablation.•The dense ZrO2 layer leads to good ablation resistance.In order to improve the ablation resistance of carbon/carbon (C/C) composites, a ZrC nanostructured coating was prepared on C/C composites by chemical vapor deposition. Results show that the coating contains ZrC grains with the size of 20–30 nm and the structure displays a high consistent orientation. This nanostructured coating has improvements in linear and mass ablation rates of 50.7% and 42.1% compared with those of the conventional ZrC coating. The good ablation resistance could be attributed to the efficient prevention of the coating cracking and the formation of the dense ZrO2 layer during ablation.Figure optionsDownload full-size imageDownload as PowerPoint slide

Carbon/carbon (C/C) composite joints were successfully prepared using Ti–SiC–Si–C compound as filler by vacuum hot-press technique. The effects of interlayer thickness and SiC transition layer on the mechanical properties of C/C joints were studied. Shear test results revealed that the joints with ∼160 μm thick interlayer had the maximum shear strength (38.09±5.09 MPa), in which dense interlayer, strong interface bonding and ‘nail effect’ were responsible for the excellent mechanical properties of joints. Furthermore, the joints had high strength retention at no more than 1200 °C. The joints using SiC coating as a transition layer indicated lower shear strength (22.64±2.61 MPa), and those joints primarily fractured in SiC transition layer and the interface region between SiC layer and core interlayer due to the relatively loose SiC coating.

Straight carbon nanotubes (CNTs) were grafted radially onto carbon fibers to produce hybrid materials that were used to reinforce carbon/carbon (C/C) composites. Mechanical property tests indicated that these C/C composites have improvements in out-of-plane and in-plane compressive strengths and interlaminar shear strength of 275%, 138% and 206%, respectively. They also have a large decrease in the anisotropy of mechanical properties, compared with pure C/C composites. This great improvement is the result of the simultaneous reinforcements to the fiber/matrix interface and the matrix provided by the grafted CNTs.

A high density 3D-four directional carbon/carbon composite was fabricated by hot isostatic pressure impregnation carbonisation using coal tar pitches. The thermo-oxidative, thermophysical and ablation properties of the composite were determined. Thermo-oxidative analysis shows that the carbon matrix is oxidised faster than the fibre. The composite shows a quasi-isotropic heat capacity and coefficient of thermal expansion, whereas its thermal conductivity depends on the fibre volume fraction in the test direction. The ablation in plasma is higher and more severe than in oxyacetylene, and the resistance of carbon fibres to ablation depends on their orientation relative to the flow of ablative gasses.Highlights► 4D-c/c composite was fabricated using carbon fibre rods and coal tar pitches. ► Thermal conductivity of the 4D-c/c in-plane is higher than through-the-thickness. ► Heat capacity and CTE in the 4D-c/c composite show quasi-isotropic behaviour. ► Ablation mechanism of the composite in plasma is different than in oxyacetylene. ► The ablation rate in plasma is ∼10 times higher than in oxyacetylene environment.

Low texture and isotropic pyrocarbon were prepared by hot wall reactor chemical vapor deposition between 950 °C and 1050 °C using natural gas as the carbon source. The morphology and texture of the pyrocarbon was characterized by transmission electron microscopy, and the coefficients of thermal expansion (CTE) were measured in the directions of parallel and perpendicular to the specimen thickness direction using a Netszch Dilatometer 402C. The results showed that as the deposition temperature increased from 950 °C to 1050 °C, the fracture surface of the pyrocarbon became rougher and the length of the graphene layers was gradually increased and the pyrocarbon changed from low texture to isotropic. The CTEs in both directions were almost the same and negative below 300 °C.Highlights► The texture of the pyrocarbon was low texure and isotropic. ► The CTEs in both directions were negativ below 300 °C. ► The CTEs in both directions were almost the same.

2D carbon/carbon (C/C) composites have been successfully joined by partial transient liquid phase (PTLP) bonding using Ti–Ni–Si compound as interlayer. The shear strengths of C/C joints were found to be affected observably by joining temperature, joining pressure and holding time. The optimum process parameters were determined at 1150 °C, 30 MPa, 45 min, and the maximum joint shear strength was obtained under this optimum process condition with an average value of 23.58 MPa. Microstructures and compositions of joints were analyzed by scanning electronic microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). Results showed that element interdiffusions and chemical reactions took place in the joining zone and various compounds including Ni4Si7Ti4, TiSi2, SiC and TiC were produced, by which compact interlayer and strong interface were formed in C/C joints. The shear morphology indicated there were three fracture modes in C/C joints and the fracture was apt to occur through interlayer, interface and C/C matrix in strong joints. Thermal shock resistance experimental results revealed that joint shear strength has a rapid drop after 5 thermal cycles and then reduces gradually with the increase of thermal cycles between room temperature and 1000 °C.Highlights► First study to join C/C composites using Ti–Ni–Si compound. ► The average joint shear strength can reach up to 23.58 MPa. ► Strong joints are apt to fracture through interlayer, interfaces and C/C matrix. ► Phase compositions in joints are varied with increasing joining temperature.

Carbon/carbon composites were prepared by film boiling chemical vapor infiltration from xylene pyrolysis. Their densification behaviors such as the mass gain, the deposition rate and the density profile were investigated. The microstructure was studied by polarized light microscopy and characterized quantitatively with average extinction angle (Ae). Results showed that, under the particular experimental equipment and process, the initial deposition rate and the average Ae of pyrocarbon (PyC) increased with the increasing deposition temperature (Td). The structural transition of PyC from rough laminar (RL) to smooth laminar along both the axial and radial directions of the composites was retarded as Td increased from 900 to 1100 °C; PyC was deposited by the heterogeneous nucleation and growth. The homogeneous nucleation was generated producing isotropic PyC at the bottom of the composites for 1200–1250 °C deposition. The matrix produced at 1100–1250 °C was dominated by RL PyC, and the composites with high average density and uniform RL matrix were rapidly produced for Td around 1100 °C.

Ablation of needled carbon/carbon (C/C) composite nozzle-throats was studied by hot-fire testing in a small solid rocket motor. The composition of the combustion gases was estimated by principle of free energy minimum. The ablation morphology was investigated by scanning electron microscopy. The ablation mechanism of C/C composites was also studied. The results showed that the ablation performance of C/C composites was determined by mechanical breakage of fibers/matrix together with thermal chemical ablation from the heterogeneous reactions on the throat surface. The mechanical breakage of fibers/matrix dominated the ablation of the composites at high pressure based on the calculated ablation rate. Cone-shaped fibers were formed after ablation in high fiber density area; but in low fiber density area, the fibers were peeled off because of the weakened strength after ablation. Meanwhile, the matrix around the fiber bundles was ablated into a shell shape, while the matrix between the cone-shaped fibers might be blown away by the combustion gases. Oxidation of C/C composites led to the formation of the cone-shaped fibers and shell-shaped matrix, as well as the loss of matrix between the cone-shaped fibers. The fiber/matrix fragments on the ablation surface were caused by the mechanical breakage.

Carbon/carbon composites doped with zirconium carbide were prepared by a three-step process. Carbon fiber felts were first immersed in a zirconium oxychloride solution, followed by rapid densification using thermal gradient chemical vapor infiltration. The densified carbon/carbon composites were then graphitized at 2500 °C. The phase composition and morphology of the composites were investigated by X-ray diffraction and scanning electron microscopy. The ablation properties were tested in an oxyacetylene torch. The results show that the linear and mass ablation rates of the composites after doping with 4.14 wt.% zirconium carbide decreased by 83.0% and 77.0%, respectively. The ablated surface of the carbon matrix for pure carbon/carbon composites was very smooth and glossy, while that for doped carbon/carbon composites was honeycombed and dim. The bonding between carbon fibers and matrix decreased because of the formation of more zirconium dioxide, resulting in carbon fibers peeling off the matrix and the ablation resistance of carbon fibers could not be brought into play when the zirconium carbide contents achieved 4.14 wt.%. Although mechanical denudation does not seem to play a dominant role, the ablation was mainly controlled by heterogeneous mass transfer.

Five carbon-carbon composites were prepared with different fibre orientations in the preform and were densified by different methods. Their ablation behaviour was examined by an oxy-acetylene test and scanning electron microscopy. The densities of the composites were in the range of 1.77 to 1.85 g/cm3. Fibres having an angle of 30° with the oxy-acetylene flame turned into a sharp wedge shape, whereas fibres parallel to the flame had a needle-like shape with diameter up to 3.5–4.5 μm after ablation. The needled fibres were easily attacked and ultimately became blunt. Partially filled macropores with sizes of 1.0–1.26 mm, needle pores, interfacial cracks and gaps in non-woven cloth were easily attacked by the flame, resulting in macroscopic ablation pits that decreased with increasing density of the composites. The needled fibres around pitch carbon layers were severely denuded due to their discontinuity with the pyrolytic carbon matrix. A high density (1.85 g/cm3) composite had an excellent ablation resistance.

Effect of brake pressure and brake speed on the tribological properties of carbon/carbon (C/C) composites with a single-layered smooth laminar (SL) pyrocarbon texture and a double-layered rough laminar (RL) and SL pyrocarbon texture was investigated using a friction performance tester. The morphology of the worn surface and wear debris was observed by scanning electron microscopy (SEM) and optical microscopy (OM). The results showed that the average friction coefficient reduced and the braking stability increased when a smooth and completed friction film formed gradually on the worn surface under higher brake pressure or brake speed, whereas the mass wear and linear wear increased under these conditions due to the flake damage of friction film and high oxidation loss. Moreover, the average friction coefficient reached the maximum value under the brake pressure of 0.6 MPa and the brake speed of 15 m/s. C/C composites with the double-layered RL and SL texture had more stable friction and lower wear than those with the single-layered SL texture under the same braking condition owing to the formation of the more completed friction film and the lower oxidation loss.Research highlights▶ The average friction coefficient decreases and the braking stability increases when a smooth and completed friction film forms under higher brake pressure or brake speed. ▶ The mass wear and linear wear increase under higher brake pressure or brake speed due to the flake damage of friction film and the high temperature oxidation. ▶ C/C composites with the double-layered RL and SL texture have more stable friction and lower wear than those with the single-layered SL texture under the same braking condition.

The interface structures and fracture behavior of the two-dimensional carbon/carbon composites by isothermal vapor infiltration have been investigated. The results show that the graphene layers exhibit long-range order in high/textured pyrocarbon matrix and are curved in about 5-nm interface region of the fiber/high-textured. Some globular nanoparticles are formed on the fiber surface and the high-textured layer about 10 nm exists in the interface of the fiber/low-textured. The graphene layers stacks are scrolled and folded in the medium-textured and they are waved together in the interface of the fiber/medium-textured. The pseudo-plastic fracture behavior of the two-dimensional carbon/carbon composites is resulted from the dominant high-textured matrix and a moderate interfacial bonding force. A strong adhesion of the fiber/low-textured and the thicker fiber increased by surrounding low-textured layer result in the increasing flexural strength. The single medium-textured and a very strong bonding force of the fiber/medium-textured lead to the brittle fracture behavior.

Using hot pressing, carbon/carbon composites were joined using a Ti3SiC2/SiC interlayer which was in situ synthesized by the reaction of TiC and Si. Phase composition of the interlayer was characterized by X-ray diffraction. Morphologies of the joints before and after shear test were determined by scanning electron microscope and energy dispersive spectroscopy. The mechanical strength of the joints was assessed by shear strength test. Phase analysis reveals that the interlayer was mainly composed of ternary Ti3SiC2, SiC, and little TiC. The microstructure observation results show that the dense and uniform interlayer adheres firmly to the C/C composites. A composition gradient reaction layer was formed at the joining interface between C/C substrates and interlayer. The room temperature average shear strength of the joints is about 38.9 ± 3.6 MPa. The joining mechanism and failure behavior of the joints were also discussed.

The ablation and mechanical behavior of the carbon/carbon (C/C) and hafnium carbide (HfC) modified C/C (HfC-C/C) composites were evaluated by oxyacetylene flame and the three-point bending tests. The effect of impact damage on their mechanical behavior was also investigated. To produce the HfC-C/C composites, the refractory carbide precursor was introduced to the preforms by impregnating with HfOCl2·8H2O solution. Both the C/C and the HfC-C/C preforms were densified by thermal gradient chemical vapor infiltration. Results indicated that, although the linear and mass ablation rates exhibited by the HfC-C/C composites were lower than those for the C/C composites by 55% and 21%, respectively, the maximum flexural load for the C/C composites was significantly higher by 33% than that of HfC-C/C composites. The influence of pre-impact loading on mechanical behavior was greater for the HfC-C/C composites than for the C/C composites.

Short carbon fibers were treated at temperatures around 1100 °C in a furnace through chemical vapor infiltration technology. The fiber surface was observed by scanning electron microscopy. The reflectivity of electromagnetic radiation by the composites that were reinforced by surface-treated carbon fibers and by as-received ones was measured in the frequency range of 8.0–18.0 GHz. The reflectivity for different carbon fiber contents of 0.2%, 0.4%, 0.6% and 1.0 wt% was investigated. Results showed that the reflectivity of the composites that were reinforced by untreated carbon fibers tended to increase with the increasing fiber contents. The minimum reflectivity was −19.3 dB, far less than −10 dB, when the fiber content was 0.4% and there were wave-absorbing properties. However, after surface treatment, the minimum reflectivity was −8.1 dB for the same fiber content of 0.4%, indicating significant wave-reflecting properties. The achieved reflectivity values after surface treatment were generally greater than those without treatment.

Fluorine-containing hydroxyapatite (FHA) coatings were prepared on the surface of carbon/carbon (C/C) composites by way of combination of sonoelectrodeposition and ion exchanges. SEM, EDAX, FTIR and XRD were adopted to investigate the influence of NaF concentration on the morphology, structure and composition of the coatings. Experimental results show that treatment by NaF can promote the conversion of tricalcium phosphate (TCP) into fluor-apatite and also increase the crystallization degree of the coating. After treatment by NaF, the composition of the coating is the mixture of hydroxyapatite (HA) and F-rich apatite (or FA) in different ratios, and (3 0 0) diffraction peak shifts to a higher angle of diffraction. With the NaF concentration increasing, more and more F− in the coating will replace the OH− in the apatite, so HA content in the coating decreases while F-rich apatite (or FA) increases. After NaF treatment, dissolution phenomenon happens on the surface of the coating, and the plate-like crystals turn into larger ones along with the increasing porosities. When the concentration of NaF was up to 0.1 mol·L−1, the coating turned into amorphous powder with fluorine content of 6.31 wt%.

A thermal gradient CVI process was investigated. A graphite heater in the center of a carbon felt disk preform was heated by Joule heating to a temperature of 900 °C, the carbon felt had a low thermal conductivity, and the rapid natural gas flow cooled the exterior surface of the preform. The rate constant of the chemical vapor deposition reaction increased exponentially with increasing temperatures with pyrocarbon being formed only in the designated deposition zone. Plugging of surface pores in the preforms, which often occurs in the isothermal CVI technology was unusual in this thermal gradient CVI process. As the deposition process went on, the deposited zone moved progressively towards the outside surface of the preform. The electrical resistance between two electrodes decreased gradually while the power of the thermal gradient CVI furnace increased non-linearly with the progressive densification. The temperature distribution in the thermal gradient furnace changed non-linearly with time and position. The relationship between temperature and position in the deposition zone followed the classical Fourier law. The microstructure of pyrocarbon at different positions was discussed.

C/C–HfC–SiC composites prepared by precursor infiltration and pyrolysis process were ablated by oxyacetylene torch under two different flame conditions. The ablation performance of the composites was investigated in the heat flux of 2.38 MW/m2 (HF-L) and 4.18 MW/m2 (HF-H) for 60 s. The mechanical denudation in 4.18 MW/m2 (HF-H) was higher than that in 2.38 MW/m2 (HF-L), while the results indicated that the composites had a similar and good ablation property under two different flame conditions. C/C–HfC–SiC composites can adapt the heat flux from 2.38 MW/m2 to 4.18 MW/m2. The HfO2 was not melted completely in the heat flux of 2.38 MW/m2 (HF-L). So, both HfO2 and SiO2 layers acted as an effective barrier to the transfer of heat and oxidative gases into the underlying carbon substrate. SiO2 was severely consumed in 4.18 MW/m2 (HF-H), where the HfO2 molten layer played a more important role in protecting the inner composite.

High-purity carbon nanotubes (CNTs) with different orientation and lengths were grafted on carbon fibers (CFs) in woven fabrics by using double injection chemical vapor deposition and adjusting the growth temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman investigations reveal that the grafted CNTs change from being predominantly aligned and uniform in diameter to absolutely disordered and variable in diameter, whilst they show significantly increased crystallinity, as the growth temperature is increased from 730 °C to 870 °C. In tensile tests of fiber bundles, much more strength degradation of CFs was observed after the growth process at higher temperature than that at lower temperature. These hybrid preforms produced at different growth temperatures were used to reinforce carbon/carbon (C/C) composites. An increment of 34.4% in out-of-plane compressive strength (OCS) was obtained for the composites containing CNTs grown at 730 °C, while the OCS increment exhibits an obvious decrease with increasing the growth temperature. Compared with the higher growth temperature, the lower temperature contributes to the decrease in the strength loss of reinforcing fibers and meanwhile the growth of large extending length of CNTs, which can provide long reinforcement to the pyrocarbon matrix, and thus increase the compressive strength better.

A silicon-substituted hydroxyapatite (Si-HA) coating for SiC-coated carbon/carbon composites has been prepared by pack cementation and electrochemical deposition. The morphology, microstructure and in-vitro bioactivity of the Si-HA/SiC double-layer coatings were investigated. Results showed that the SiC inner-layer exhibited high crystallinity. The Si-HA outer-layer had a smaller crystal size than pure hydroxyapatite (HA). Simulated body fluid (SBF) immersion tests showed that the Si-HA coating was uniformly covered by a layer of apatite after 7 days of immersion, while the apatite layer formed on the HA coating was inhomogeneous. This work demonstrates that the Si-HA coating may have better bioactivity than a HA coating and shows promise for the potential application as bioactive coating.

HfC-ZrC-SiC (HZS) multiphase coating was deposited on the surface of SiC-coated carbon/carbon composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HZS coating were characterized by XRD and SEM. The as-prepared coating was composed of different carbides and oxides. The structure was dense and crack free. Each element distributed uniformly in the coating. Ablation resistance test was carried out by oxyacetylene torch. During ablation, the outer coating underwent a process of solid state sintering and formed a dense Hf-Zr-Si-O layer on the surface of coating gradually, which could prevent the oxygen diffusing inward. The inner coating was oxidized gradually with the oxygen permeation and the structure was loose. In addition, the newly formed HfSiO4 and ZrSiO4 were generated after cooling, which could play a pinning effect and prevent crack extension.

•SiC interphase was introduced into the modified C/C composites prepared by precursor infiltration and pyrolysis.•The distribution of SiC interphase enhanced the flexural strength (24%) of the modified C/C composites.•SiC interphase increased adhesion between ablation products and matrix was beneficial to ablation resistance in short term.Carbon/Carbon (C/C) composites modified with ZrC-ZrB2-SiC particles were prepared by precursor infiltration and pyrolysis (PIP) process. Effects of SiC interphase on the mechanical and ablation properties of C/C-ZrC-ZrB2-SiC composites were investigated. The SiC interphase was deposited through low pressure chemical vapor infiltration (LPCVI). It was found that as the SiC interphase was introduced into the modified C/C composites, the flexural strength increased by 24% and the failure mode showed brittle fracture. The mechanisms for the strengthening of the modified C/C composites with SiC interphase were identified as (I) the deposition of SiC interphase in short-cut webs that strengthened the bonding between fibers and matrix promoting to stress transfer, and (II) the production of weak bonding between fiber bundles and matrix in non-woven webs improved the utilization of carbon fiber strength. SiC interphase increased adhesion between the ablation products and matrix, it could enhance the ablation resistance of the modified C/C composites in short term under the action of the evaporation. However, the long-time ablation resistance was reduced since the ceramic particles could not be incorporated into the composites during preparation due to the existence of SiC interphase.Download high-res image (371KB)Download full-size image

To improve oxidation resistance of carbon/carbon (C/C) composites, a multiphase double-layer ZrB2-CrSi2-SiC-Si/SiC coating was prepared on the surface of C/C composites by pack cementation. Thermogravimetry analysis showed that the as-prepared coating could provide effective oxidative protection for C/C composites from room temperature to 1490 °C. After thermal cycling between 1500 °C and room temperature, the fracture behaviors of the as-prepared specimens changed and their residual flexural strengths decreased as thermal cycles increased. The specimen after 20 thermal cycles presented pseudo-plastic fracture characteristics and relatively high residual flexural strength (83.1%), while the specimen after 30 thermal cycles failed catastrophically without fiber pullout due to the severe oxidation damage of C/C substrate especially the brittleness of the reinforcement fibers.

Carbon fibre reinforced multilayered (PyC–SiC)n matrix (C/(PyC–SiC)n) composites were prepared by alternate deposition of pyrocarbon (PyC) and SiC inside preforms via chemical vapour infiltration. The matrix microstructures and internal friction behaviours of C/(PyC–SiC)n composites (n = 1, 2 and 4) under different testing conditions of frequency, strain amplitude and temperature were studied. The results show that with increasing the number of sequences (n), the internal friction increases due to the enhancement of interfacial internal friction. The internal friction of C/(PyC–SiC)n composites increases with the increase of frequency related to thermoelastic mechanism, but exhibits anomalous amplitude effect in the testing amplitude range. Effect of temperature on internal friction behaviours is attributed to combined effects of carbon fibres, PyC–SiC matrices and interfaces between fibres and PyC, adjoining PyC and SiC layers. It is also found that internal friction is sensitive to microstructural defects induced by damage. These results indicate that internal friction is an effective and efficient method to characterize the structural evolution and internal damage of C/(PyC–SiC)n composites non-destructively.