Shameel Farhan

Effect of density and fibre orientation on the ablation behaviour of carbon-carbon composites

Abstract 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/cm 3 . 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/cm 3 ) composite had an excellent ablation resistance.

A novel thermal gradient chemical vapor infiltration process for carbon-carbon composites

Abstract Solid cylindrical carbon-carbon composites were processed using conventional thermal gradient chemical vapor infiltration. High thermal conductivity (55 W/m·°C) carbon fibers (48 k) were inserted in the center of a cylindrical low thermal conductivity (0.15 W/m·°C) needle punched carbon felt preform, to create a thermal gradient because of the difference in thermal conductivities. The hottest portion (900–1200 °C) was along the inserted carbon fibers, where the pyrolytic reaction of natural gas occurred. The densification radially moved outwards and ultimately a density of 1.778 g/cm 3 was obtained after 67 h. The process parameters such as the electric power of the furnace, electrical resistance of the sample, densification time, and the position of the deposition layer were studied. A densified sample having a volume fraction of carbon fibers of 10% was tested for ablation and erosion. The microstructure of the pyrolytic carbon matrix of the as-prepared sample was investigated by polarized light microscopy and scanning electron microscopy.

Ca-P bioactive coating prepared by combining microwave-hydrothermal and supersonic atmospheric plasma spraying methods

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.

Optimization and preparation of an allyl phenoxy-modified bismaleimide resin

A thermosetting resin system has been developed by the copolymerization of allyl phenoxy, bismaleimide (BMI), and diallyl bisphenol A and optimized using response surface methodology. An optimized modified resin system with enhanced properties was achieved based on empirical second-order models expressing the relationship between the modifier contents and the mechanical properties. Dicumyl peroxide (DCP) was selected as initiator to further improve the curing behavior and mechanical properties of the optimized resin system. The effect of initiator contents on impact, flexural strength, and heat distortion temperature was also investigated. The curing behavior, morphology, and thermal stability of the optimized resin were carefully characterized using differential scanning calorimeter, scanning electron microscope, and thermogravimetric and dynamic mechanical analyzers, respectively. For evaluating the efficiency of modified BMI resin system, laminated composites using glass fiber cloth were fabricated using a hot press and tested for mechanical properties. The results showed that the DCP reduced the curing temperature significantly, improved the curing process, and proved to be very effective in heat resistance. Meanwhile, the laminated composite with initiator showed 13–27% higher mechanical properties and 5–7% higher retention rate at high temperature when compared with the neat resin composite system. The optimized resin system with higher mechanical properties, good heat resistance, and better manufacturability can be used as matrix resin for making advanced fiber-reinforced composites.

Curing kinetics, thermal and mechanical properties of TDE-85 modified by bicyclo-benzoxazine

To reduce the curing temperature and also to enhance the mechanical properties and the heat resistance of diglycidyl-4,5-epoxy-cyclohexane-1,2-dicarboxylate (TDE-85), it was crosslinked with 4,4-diamino diphenyl sulphone (DDS) using a synthesized bicyclo-benzoxazine (BOZ) crosslinker. The kinetic parameters of the curing reaction were evaluated using Flynn–Wall–Ozawa and Kissingers methods. The gelation time and differential scanning calorimetry (DSC) of non-isothermal testing were performed to determine the curing process of BOZ/DDS/TDE-85 systems. Fourier transform infrared spectroscopy (FTIR) was used to follow the major changes in functional groups during the curing process. The absorbance of the oxazine ring showed a significant reduction and the peak of epoxy group became very weak, indicating that the curing reaction was almost complete. The thermal properties were evaluated by heat distortion temperature (HDT) and thermogravimetric analysis (TGA), exhibiting a higher initial decomposition temperature, decreased rate of decomposition and higher char yield compared to the neat epoxy resin system. BOZ enhanced the stability of the epoxy blend, which restricted the mobility of the chain and hindered the decomposition process. The fractured surface of the cured product was observed by scanning electron microscopy (SEM). The fracture form of BOZ/DDS/TDE-85 systems belongs to typically ductile fracture compared to the neat DDS/TDE-85 system. It was found that the thermal and mechanical properties of BOZ/DDS/TDE-85 systems increased with the addition of a certain amount of BOZ. The impact, flexural, and compressive strengths increased by 43.4%, 13.1%, and 8.5%, respectively. The addition of BOZ lowered the activation energy on account of the reduced viscosity, allowing better contact of the resin with the curing agent. The reaction order and activation energy were found to be 0.92 and 63.15 kJ mol−1, respectively.

By-product processing of Si3N4 saw-tooth nanoribbons during carbon foam processing using pyrolysis–nitridation reactions

Si3N4 saw-tooth nanoribbons (SNSNs) have been synthesized via a novel approach involving a by-product pyrolysis–nitridation process during carbon foam manufacturing at 1450 °C. The SNSNs formed are ribbon shaped, 80–750 nm wide, 70–80 nm thick and several micrometres in length. The process simply involved thermal pyrolysis of a powdered mixture containing carbon foam precursors and silicon powder under flowing high-purity nitrogen. Pyrolysis gases rich in silicon, silicon oxide and active nitrogen vapours promoted the subsequent synthesis of the SNSNs over the outer surface of the carbon foams via a vapour–solid mechanism. The crystal structure, morphology, chemical composition, growth mechanism and photoluminescence (PL) properties have been studied. The infrared adsorption of SNSNs exhibited two absorption bands with all the peaks related to the Si–N bonds of the α-Si3N4 crystalline structure. X-ray photoelectron spectroscopy measurements further confirmed the chemical composition, with minor impurities such as oxygen and carbon. A single nanoribbon has the same width-to-thickness ratio, suggesting a stable morphology resulting from the reduction of the overall surface energy. Intense PL was observed centred at 2.03, 2.48, 2.62, and 3.01 eV, which resulted from the recombination between the intrinsic conduction band edges and silicon dangling bonds with deep-level or trap-level states.

Impact behavior of different cross-ply laminated carbon fiber-reinforced silicon carbide composites under the low velocity

Carbon fiber-reinforced silicon carbide composites (C/SiC) were fabricated by a chemical vapor infiltration process using various ply orientations of 2D carbon fabric in the preform. The effect of ply angles (0°/90°, 45°/−45°, and 0°/45°/90°/−45°) on the low-velocity impact damage of the C/SiC composites was investigated externally and internally. Different depths of pits were formed in the impacted side with fibers sheared off, whereas fiber bundles breakage and matrix cracking appeared on the back side. The sample with 0°/90° ply angles experienced a serious fiber fracture and stratification damage, while the other two samples only showed a slight stratification. The samples with 0°/45°/90°/−45° and 0°/90° ply angles had nearly the same strength loss rate, which was smaller than that of the sample with 45°/−45° ply angle. In 45°/−45°, the ply angle had a major influence on the residual strength as the fibers were at 45° to the loading direction. Comparison of compression strength of the samples before and after impact test showed that the quasi-isotropic laminate improved the impact damage resistance due to the prevention of the devastating crack extension.

Effect of raw materials on the pore morphologies of carbon foams prepared through templating method

The present paper reports about the effects of raw materials, processed through templating method, on the pore microstructures and pore size distribution of carbon foams (CFs). Biomaterials along with polyurethane and phenolic resin were employed to prepare CFs. To further investigate the adjustability of the pore microstructures and pore size distribution, several kinds of chemical additives/fillers, including activated charcoal, NaCl, and silicon, were used. Surface morphological studies were carried out using scanning electron microscope (SEM), and pore size distribution was analysed by high-pressure Hg porosimetry. The results showed that the structures of the precursor played a dominant role in determining the final pore microstructures. Chemical additives/fillers affect the average pore sizes, pore size distributions, and the pore walls. High amount of macropores are found in all the samples with the radius ranging from 2 to 4 μm. It is practical to adjust the pores using different raw materials and chemical additives/fillers.