A SiC whisker-toughened SiC-CrSi2 oxidation protective coating for carbon/carbon composites

A SiC whisker-toughened SiC-CrSi₂ oxidation protective coating was prepared on the surface of C/C composites by a two-step technique of slurry and pack cementation. The oxidation protective ability and thermal stress resistance of the coating exhibit the trend of increase first and decrease afterwards as the SiCw content increases from 0 to 20 wt.%. The compound effect of SiCw and CrSi₂ on the oxidation protective ability of SiC coating is better than their individual ones.     

Microstructure and anti-oxidation property of CrSi2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a new type of oxidation protective coating has been produced by a two-step pack cementation technique. XRD and SEM analysis show, the coating obtained by the first step pack cementation was a porous β-SiC structure, and a new phase of CrSi₂ was generated in the porous SiC coating after heat-treatment according to the second step pack cementation process. Oxidation test shows that, the weight loss of the SiC coated C/C is up to 11.26% after 5 h oxidation in air at 1773 K, and the weight loss of the CrSi₂-SiC coated C/C composites is only 4.15% after oxidation in air at 1773 K for 34 h. The oxidation of C/C composites was primarily due to the reaction of C/C matrix and oxygen diffusing through the penetrable cracks in the coating.     

Interphase formation and fiber matrix adhesion in carbon fiber reinforced epoxy resin: influence of carbon fiber surface chemistry

The chemistry and morphology of the carbon fiber surface are important parameters which govern the properties of the interfacial region and the adhesion between carbon fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented paper the surface chemistry of the fibers is varied while the surface morphology is left unchanged. We analyze chemical functionality and morphology of carbon fiber surfaces showing different degrees of activation, together with the adhesion of these fibers to an epoxy matrix and the width of the interfacial region between fiber and matrix. An increase of the oxygen and nitrogen concentration of the fiber surface, in particular in form of carboxyl functional groups, results in a significant increase of interfacial shear strength. Also the width of the interphase, as determined by scanning force microscopy in nanomechanical mode, depends on the activation degree of the carbon fibers. However, no direct correlation between interphase width, surface chemistry and fiber matrix adhesion is found, suggesting no direct influence of interphase width on adhesion properties.     

Interfacial mechanical properties of carbon nanotube-deposited carbon fiber epoxy matrix hierarchical composites

Functionalized multiwalled carbon nanotubes were successfully deposited on carbon fibers using four different techniques including dip coating, hand layup, spray up and electrophoretic deposition (EPD). A uniform coating of nanotubes was achieved from EPD in comparison to other coating techniques. Later nanotube-coated fibers by EPD were introduced in epoxy resin to investigate interfacial mechanical properties of the developed hierarchical composites by vacuum bagging technique. The increases in flexural and interlaminar shear properties up to 15% and 18% were observed in composites containing nanotube-coated carbon fibers than composites with virgin carbon fibers, respectively. Microscopic observation revealed the proper impregnation of multiscale reinforcements, i.e., carbon fibers and carbon nanotubes, in resin along with the modification of fiber/matrix interface due to the presence of nanotubes at interface. Finally, the mechanisms for improved mechanical properties were identified along with the presentation of a schematic model for better understanding of the improved performance of hierarchical composite after depositing uniformly dispersed nanotubes on carbon fibers.     

Microstructure of interfaces between a magnesium matrix and preoxidized silicon carbide particles

An amorphous SiO₂ layer, 100–150 nm thick, has been produced on the surface of -SiC particles by controlled oxidation in air. The as-treated particles were immersed for 1.5h at 1,000 K in a large excess of liquid magnesium, which resulted in the formation by pressureless infiltration of a composite region with a particle volume fraction about 50%. Conventional transmission electron microscopy (CTEM) and high-resolution TEM (HRTEM) were used to characterize and localize the different species present in the transition zone between the Mg matrix and the -SiC particles. The results are discussed in terms of the physico-chemical processes occurring during formation of the composite region or during cooling of the Mg ingot. A reaction mechanism based on local equilibria and liquid phase diffusion is proposed. Four steps are involved in this mechanism: (i) rapid conversion of SiO₂ into MgO and Si; (ii) formation of a true Mg/SiC interface; (iii) partial decomposition of SiC into carbon and silicon; and (iiii) precipitation of -SiC and Mg₂Si on cooling.     

Effect of the surface roughness on interfacial properties of carbon fibers reinforced epoxy resin composites

The effect of the surface roughness on interfacial properties of carbon fibers (CFs) reinforced epoxy (EP) resin composite is studied. Aqueous ammonia was applied to modify the surfaces of CFs. The morphologies and chemical compositions of original CFs and treated CFs (a-CFs) were characterized by Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). Compared with the smooth surface of original CF, the surface of a-CF has bigger roughness; moreover, the roughness increases with the increase of the treating time. On the other hand, no obvious change in chemical composition takes place, indicating that the treating mechanism of CFs by aqueous ammonia is to physically change the morphologies rather than chemical compositions. In order to investigate the effect of surface roughness on the interfacial properties of CF/EP composites, the wettability and Interfacial Shear Strength (IFSS) were measured. Results show that with the increase of the roughness, the wettabilities of CFs against both water and ethylene glycol improves; in addition, the IFSS value of composites also increases. These attractive phenomena prove that the surface roughness of CFs can effectively overcome the poor interfacial adhesions between CFs and organic matrix, and thus make it possible to fabricate advanced composites based on CFs.     

Effect of arc spraying power on the microstructure and mechanical properties of Zn-Al coating deposited onto carbon fiber reinforced epoxy composites

This paper investigates the effect of arc spraying power on the microstructure and mechanical properties of Zn-Al coatings deposited on carbon fiber reinforced epoxy composites (CFRE composites). The bond strength between the Zn-Al coatings and the substrates was tested on a RGD-5 tensile testing machine. The microstructures and phase composition of the as-sprayed coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results showed that both the melting extent of Zn-Al particles and the bond strength of the coatings were evidently improved by increasing the spraying power. Moreover, the content of crystalline Zn-Al coatings was slightly changed. Observation of fracture surfaces showed that the Zn-Al coatings could bond well with the carbon fiber bundles using 40 kW spraying power.     

Dynamic mechanical characterization of PMR polyimide/carbon fiber composites modified by fiber coating with silicones

Viscous and elastomeric silicones have been applied as interlayers to carbon fibers in order to develop a tougher, micro-crack resistant, thermally stable polyimide (PMR-15) composite. Carbon fiber is continuously coated with very high molecular weight polydimethylsiloxane (PDMS) and polyvinyl-methylsiloxane (PVMS). Dynamic mechanical properties of the composites have been determined and compared with uncoated carbon fiber reinforced PMR-15 polyimide composites. The presence of the interlayer is shown by the appearance of a new relaxation peak. The peak temperature is found to be a good indication of the degree of the cure of the silicone elastomer. Comparison of the storage moduli of uncoated and coated carbon fiber composites at the service temperature range of the composites indicates that the presence of the silicone interlayer affects the shear moduli of the composites. Apparent activation energy of the α transition of the matrix in the modified composites varies with the amount of interlayer and composition in concert with the impact strength.     

Structure and impact resistance of short carbon fiber reinforced polyamide 6 composites

Short carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by homogenization of the components in a twin-screw extruder and by injection molding. Fiber content was varied between 0 and 16 vol%, while specimens were injection molded at rates between 2.0 and 22.6 cm/s. Average fiber length and orientation were measured to characterize structure. Average fiber length decreased with increasing fiber content and processing rate. The observed structure is contradictory to those reported in the literature for short glass fiber reinforced composites. Fibers were oriented randomly relative to the mold fill direction in the skin layer, while they were oriented parallel to this direction in the middle of the specimen. The thickness of the skin decreased with increasing injection rate and decreasing fiber content. Although instrumented impact testing indicated brittle failure at all combinations of the variables, the strain energy release rate could not be determined by the usual technique using varying notch depths because of the different properties of the skin and the core. Also, the mechanism of failure seems to be different in the two layers. A minimum appears in the fracture toughness and impact resistance at low fiber contents, indicating that fibers might promote fracture initiation at such compositions. Fiber length changed in a narrow range in the studied composites; thus, properties are determined mainly by orientation. As a consequence, both increased fiber content and injection rate lead to an increase of stiffness and toughness.     

Composite electroplating of Cu-SiO2 nano particles on carbon fiber reinforced epoxy composites

The main purpose of this work is to co-deposit nano-SiO₂ particles into the copper coatings on carbon fiber reinforced epoxy (C/EP) composite surface by electrodeposition method in order to improve the micro hardness of coatings. C/EP composites are copper plated with sulfuric acid based solution, and the effects of nano-SiO₂ and C₆H₁₂O₆ in the electrolyte contents on the copper coatings are investigated. It is found that crystalline grains of coatings are markedly refined by nano-SiO₂ in the acidic sulfate copper plating bath and the ceramic particles cause an increase in hardness of coatings though nano-SiO₂ results in a decline of deposition rate and a decrease in electrical conductivity of electroplating layers. Otherwise, C₆H₁₂O₆ in the plating bath is indispensable to the layer formation even though nano-SiO₂ added. These results demonstrate that the hardness of coatings will be increased with appropriate contents of co-deposited SiO₂ and C₆H₁₂O₆ in the plating bath.     

Effect of the silicone interlayer on mechanical properties of carbon fiber reinforced PMR-15 polyimide composites

Carbon fibers coated with various types and amounts of very high molecular weight silicones (780000 g/mol) are used to make unidirectional PMR-15 polyimide composites. Coating conditions have been found to affect the fiber arrangement within the interlayered composites which consequently has a strong effect on composite properties. The effect of variation of the type and the amounts of the silicone on the impact resistance, toughness and mechanical properties of the composite is determined. Retention of properties of the thermally aged composites has also been studied. Finally, the interlayered composites are checked for improvement of microcracking resistance.     

Preparation and ablation properties of ZrC-SiC coating for carbon/carbon composites by solid phase infiltration

A novel ZrC-SiC coating was prepared on carbon/carbon (C/C) composites surface by solid phase infiltration and the ablation properties of the ZrC-SiC coated C/C composites under oxyacetylene flame were studied. The results show that the coating prepared on the condition of optimum process parameters exhibits dense surface and outstanding anti-ablation ability. After ablation for 20 s, the mass ablation rates of the coated C/C composites can be lowered to 2.36 × 10⁻³ g/s, 37.1% reduction compared with uncoated C/C composites. The oxide layer composed of ZrO₂ and SiO₂ acts as oxygen diffusion barrier and the evaporation of ZrO₂ and SiO₂ absorbs a great amount of heat from the flame and reduces the erosive attack on the coating.     

Role of curing conditions and silanization of glass fibers on carbon nanotubes (CNTs) reinforced glass fiber epoxy composites

Curing behavior of amino-functionalized carbon nanotubes (ACNT) used as reinforcing agent in epoxy resin has been examined by thermal analysis. Experiments performed as per supplier’s curing conditions showed that modification of the curing schedule influences the thermo-mechanical properties of the nanocomposites. Specifically, the glass transition temperature (T_g) of ACNT-reinforced composites increased likely due to the immobility of polymer molecules, held strongly by amino carbon nanotubes. Further, a set of composites were prepared by implementing the experimentally determined optimal curing schedule to examine its effect on the mechanical properties of different GFRP compositions, while focusing primarily on reinforced ACNT and pristine nanotube (PCNT) matrix with silane-treated glass fibers. From the silane treatment of glass fibers in ACNT matrix composition it has been observed that amino silane is much better amongst all the mechanical (tensile and flexural) properties studied. This is because of strong interface between amino silane-treated glass fibers and modified epoxy resin containing uniformly dispersed amino-CNTs. On the other hand, PCNT GFRP composites with epoxy silanes demonstrated enhanced results for the mechanical properties under investigation which may be attributed to the presence of strong covalent bonding between epoxy silane of glass fiber and epoxy–amine matrix.     

Hierarchical hollow urchin-like structured MnO2 microsphere/carbon nanofiber composites as anode materials for Li-ion batteries

In this work, novel hollow urchin-like MnO₂ microspheres (u-MnO₂), consisting of a hollow core with nanotubes, are synthesized by a simple hydrothermal process. The morphology of the MnO₂ structures could be tuned from round particles to a hierarchical hollow urchin structure by controlling the hydrothermal reaction time, with no need for surfactant or templates. The nanostructures of the obtained u-MnO₂ are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The X-ray diffraction (XRD) pattern of the u-MnO₂ reveals a tetragonal structure of α-MnO₂. The carbon nanofibers (CNFs) are uniformly deposited on u-MnO₂ to improve the electrical conductivity and to utilize the hierarchical architecture of u-MnO₂. As the anode electrode of Li-ion batteries, the u-MnO₂/CNFs nanocomposites exhibit discharge capacity of 988 mAh·g⁻¹ after 100 cycles with a good rate capability. The superior electrochemical performances of the u-MnO₂/CNFs nanocomposites can be attributed to the hierarchical urchin-like structures and the superior electrical conductivity of the nanocomposites, which can facilitate fast electron and ion transport and accommodate a large volume change during charge/discharge.     

Optical characterisation and modelling for oblique near-infrared laser heating of carbon fibre reinforced thermoplastic composites

The optical behaviour of a carbon fibre reinforced thermoplastic composite material is investigated for a near infrared laser heating process applied to automated composite tape placement. A nip point heating strategy in laser tape placement results in a shadow before the nip point on both the incoming tape and substrate. The moderate laser angle relative to the surface of the composite leads to reflections in the cavity formed by the tape and the substrate, reducing the shadow. An optical ray tracing model can provide valuable insight to the interaction of the laser with the composite, as well as detailed estimation of the irradiance distributions. This paper provides the foundations for such a model, describing an optical characterisation process and formulation of appropriate models to capture the composite surface and laser source behaviour. A micro-half-cylinder surface treatment was shown to give a good approximation of the anisotropic scattering behaviour of the composite. Angular dependent reflectance was described well by Fresnel equations. An approximation of the laser beam profile and propagation is also presented.     

SiC/SiO2 coating for improving the oxidation resistive property of carbon nanofiber

The SiC/SiO₂ deposition was performed to improve the oxidation resistive properties of carbon nanofiber (CNF) from electrospinning at elevated temperatures through sol-gel process. The stabilized polyacrylonitrile (PAN) fibers were coated with SiO₂ followed by heat treatment up to 1000 and 1400 °C in an inert argon atmosphere. The chemical compositions of the CNFs surface heat-treated were characterized as C, Si and O existing as SiC and SiO₂ compounds on the surface. The uniform and continuous coating improved the oxidation resistance of the carbon nanofibers. The residual weight of the composite was 70-80% and mixture of SiC, SiO₂ and some residual carbon after exposure to air at 1000 °C.     

基于碳纤维复合材料的月基极紫外相机照准架结构设计

为了减少月基极紫外相机的质量并保证相机的二维转动机构在卫星发射、地月变轨及月表着陆过程中受到大量级振动冲击以及月表超大温差环境下能正常工作,设计并研制了基于碳纤维复合材料（CFRP）的照准架结构。首先,设计了基于金属材料和CFRP的不同照准架结构,通过有限元法对不同材料的照准架进行分析对比,证明了CFRP照准架的优越性。温度和力学验证试验表明：基于CFRP的照准架质量小于其它材料的照准架,其刚度和热稳定性能满足极紫外相机环境适应性的要求。     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.