The effect of pyrolysis on the mechanical properties and microstructure of carbon fiber-reinforced and stabilized fiber-reinforced phenolic resins for carbon/carbon composites

Carbon/carbon composites were prepared with phenol-formaldehyde resin, one kind of commercial carbon fiber, and a stabilized fiber that was developed in our laboratory. The effect of pyrolysis on the microstructure, fracture behavior, and flexural strength of the composites during the carbonization process was studied. During the pyrolysis of the composites a chemical reaction at the fiber/resin interface apparently took place. A thermogravimetry (TG) study indicated that the use of stabilized fiber reinforced composites inhibited decomposition reactions and thermal fragmentation in the matrix resin, and reduced the weight loss of the final composites. The X-ray reflection of the resin and the two composites showed a reflection appearing at 2θ ≈ 12° when the samples were carbonized above 600°C. The intensity of this reflection in the composites made with stabilized fiber was higher than that of the composite made with carbon fiber. Because of the formation of strong bonding in the fiber-matrix interface, the composites made with stabilized fiber showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with an increase in the carbonization temperatures, even if the fracture behaviors showed catastrophic failure. The flexural strength of the composites made with carbon fiber showed pseudo-plastic patterns and debonding with very little fiber pullout. Above 800°C, these composites showed a catastrophic failure and smooth failure surfaces. During pyrolysis the flexural strength decreased with an increase in the carbonization temperature.     

The effect of pyrolysis on the properties of stabilized PAN fabric reinforced phenolic resins for 2D carbon/carbon composites

Two-dimensional (2D) carbon/carbon composites were prepared with phenol-formaldehyde resin and a commercial stabilized PAN fabric. The effect of pyrolysis on the microstructure and flexural strength of the composites during the carbonization process was studied. The interaction between fabric and matrix inhibited the decomposition and the thermal fragmentation, leading to a higher carbon yield for the final composition. Because of the formation of strong bonding in the fiber/matrix interface, the composites made with stabilized PAN fabric showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with the increase in the carbonization temperatures, even when the fracture behavior showed catastrophic failure.     

Modification of a carbon/carbon composite with a thermosetting resin precursor as a matrix by the addition of carbon black

Carbon/carbon composites were made through the pyrolysis of stabilized PAN felt and phenolic resin with the addition of 5 or 10 wt % carbon black to the matrix and then heat treatment at 600–2500°C. The effects of adding carbon black to the matrix precursor on the physical properties, microstructure, and mechanical properties of the resultant composites were investigated. Adding carbon black not only reduced the weight loss but also limited the shrinkage of the resultant composites. Adding carbon black also accelerated the formation of carbon basal planes in the matrix. At 2500°C, the crystalline stacking height in the composite with 10 wt % added carbon black was 200% greater than that with no additive. The flexural strength of the composite also increased from 15 to 42 MPa (almost 300%). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 333–337, 2006     

Effect of quasi‐carbonization processing parameters on the mechanical properties of quasi‐carbon/phenolic composites

In this work, quasi‐carbon fabrics were produced by quasi‐carbonization processes conducted at and below 1200°C. Stabilized polyacrylonitrile (PAN) fabrics and quasi‐carbon fabrics were used as reinforcements of phenolic composites with a 50 wt %/50 wt % ratio of the fabric to the phenolic resin. The effect of the quasi‐carbonization process on the flexural properties, interfacial strength, and dynamic mechanical properties of quasi‐carbon/phenolic composites was investigated in terms of the flexural strength and modulus, interlaminar shear strength, and storage modulus. The results were also compared with those of a stabilized PAN fabric/phenolic composite. The flexural, interlaminar, and dynamic mechanical results were quite consistent with one another. On the basis of all the results, the quasi‐static and dynamic mechanical properties of quasi‐carbon/phenolic composites increased with the applied external tension and heat‐treatment temperature increasing and with the heating rate decreasing for the quasi‐carbonization process. This study shows that control of the processing parameters strongly influences not only the mechanical properties of quasi‐carbon/phenolic composites but also the interlaminar shear strength between the fibers and the matrix resin. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mechanical and electrical properties of aligned carbon nanotube/carbon matrix composites

To synthesize carbon nanotube/carbon matrix (CNT/C) composites rivaling or exceeding the mechanical and electrical properties of current carbon fiber/carbon matrix composites, it is essential to align carbon nanotubes in the composite. In this work, we fabricated CNT/polyacrylonitrile (PAN) precursor composites with high degree of CNT alignment, and carbonized and graphitized them at high temperatures. Carbonizing the precursor composites significantly improved their elastic modulus, strength, and electrical conductivity. The matrix was uniformly carbonized and highly graphitized. The excellent mechanical and electrical properties make the CNT/C composites promising for many high temperature aerospace applications.     

Mechanical properties of phenolic composites reinforced with jute/cotton hybrid fabrics

Mechanical properties (tensile, flexural, impact, and dynamic mechanical thermal analysis) of novolac type phenolic composites reinforced with jute/cotton hybrid woven fabrics were investigated as a function of fiber orientation and roving/fabric characteristics. Scanning electron microscopy (SEM) was carried out to investigate the fiber‐matrix adhesion. Results showed that the composite properties are strongly influenced by test direction and rovings/fabric characteristics. The anisotropy degree was shown to increase with test angle and to strongly depend on the type/architecture of fabric used, i.e., jute rovings diameter, relative fiber content, etc. It was possible to obtain composites with higher mechanical properties and lower anisotropy degree by producing cross‐ply laminates. Best overall mechanical properties were obtained for the composites tested along the jute rovings direction. Composites tested at 45° and 90° with respect to the jute roving direction exhibited a controlled brittle failure combined with a successive fiber pullout, while those tested in the longitudinal direction (0°) exhibited a catastrophic failure mode. Our results indicate that jute promotes a higher reinforcing effect and cotton avoids catastrophic failure. Therefore, this combination of natural fibers is suitable to product composites for lightweight structural applications. POLYM. COMPOS., 26:1–11, 2005. © 2004 Society of Plastics Engineers.     

Compressive damage in filament‐wound carbon/carbon composites

The processing characteristics and the induced compressive damage in filament‐wound carbon/carbon composites were examined. Tubular carbon/phenolic composites were made by the filament winding method. The inside diameter was 25.4 mm and the nominal thickness was 2.5 mm. The cured composites were then carbonized. Three more cycles of matrix densification were repeated. After densification, the composites were graphitized at 2600°C. Compressive tests were then carried out for the cured, carbonized, and graphitized composites. The load was applied directly on the end of the tube. The loading response and the induced damage were studied. Their microscopic fracture behaviors were examined by optical and scanning electron microscopes. With the heating temperature increased, the damage modes shifted from a ductile fracture in the cured composites to a more brittle fracture in the graphitized composites. The resulting strength and stiffness were also decreased significantly with the temperature. Kinking of fibers was the major mode in the cured specimens, while separation of bundles became more prominent in the graphitized specimens. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

The influence of post‐cure on properties of carbon/phenolic resin cured composites and their final carbon/carbon composites

Two‐dimensional (2D) carbon/carbon (C/C) composites were prepared with phenol‐formaldehyde resin and graphite fabric. After curing, polymer composites were post‐cured in air at 160°C and 230°C for several hours and then all polymer composites were carbonized up to 1500°C. The effect of post‐cure on the microstructure and fracture behavior of the resultant carbon/carbon composites was studied. The post‐cure process was characterized by weight loss. This process promoted the crosslinking and condensation reactions and led to the formation of long‐chain, cross‐linked polymeric structures in the matrix. The post‐cured composites had a greater density than the unpost‐cured composite. This study indicates that a longer post‐curing time and higher post‐curing temperature would limit the shrinkage for the post‐cured composites during carbonization. The improvement in linear shrinkage was 22% to 44%. This process also limited the formation of open pores and decreased the weight loss of the resultant C/C composites. The resultant C/C composites developed from post‐cured composites had a greater flexural strength by 7 to 26% over that developed from unpost‐cured composite.     

Changes in the microstructure and characteristics of carbon/carbon composites with mesophase mesocarbon microbeads added during graphitization

Carbon/carbon (C/C) composites were prepared from oxidative PAN fiber felts, a resol‐type phenolic resin, and mesophase pitch derived from coal tar. In this study, the effects on mesocarbon microbeads (MCMBs), flexural strength, flexural moduli, electric conductivity, and thermal conductivity of C/C composites with a mesophase content ranging from 0 to 30 wt % were examined during pyrolysis. The results show that the C/C composite with the addition of 10–30 wt % mesophase had a higher density, greater stacking size, and higher preferred orientation than the C/C composites without any mesophase during heat treatment. These composites also exhibited an improvement in flexural strength from 19.7 to 30.3%. The flexural moduli of these composites with mesophase added increased by 15.1 to 31.3% compared to that with no mesophase added. These composites also showed improved electric conductivity, from 15.1 to 43.7%, and thermal conductivity, from 12 to 31.3%. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2178–2190, 2005     

Effect of fiber weight fraction on mechanical properties of carbon–carbon composites

This article presents the synthesis of carbon–carbon (C/C) composites by preformed yarn (PY) method, by varying the percentage of carbon fiber weight fraction. The PY used was carbon fiber bundle surrounded by coke and pitch which was enclosed in nylon‐6. Three types of samples with fiber weight fractions of 30, 40, and 50%, respectively, are fabricated and mechanical properties were studied. In each case, the PY was chopped and filled into a die of required shape and hot pressed at 500°C to get the preform composite. To obtain the carbonized and graphitic structure, the specimen was heat treated at 2500°C followed by soaking for 10 to 12 hrs. Further, two cycles pitch impregnation was done by hot isostatic pressing, to eliminate the voids and to increase the density hence to obtain good mechanical properties. The characteristics such as hardness, flexural strength, and impact strengths were studied. It is observed that, as the carbon fiber percentage increases, the properties also get improved, provided sintering is done at fairly higher temperatures such as 2700°C. The superiority of the new class of C/C composites made by the proposed PY technique over those obtained by the conventional methods is also demonstrated. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Comparison of mechanical properties of epoxy composites reinforced with stitched glass and carbon fabrics: Characterization of mechanical anisotropy in composites and investigation on the interaction between fiber and epoxy matrix

The primary purpose of the study is to evaluate and compare the mechanical properties of epoxy‐based composites having different fiber reinforcements. Glass and carbon fiber composite laminates were manufactured by vacuum infusion of epoxy resin into two commonly used noncrimp stitched fabric (NCF) types: unidirectional and biaxial fabrics. The effects of geometric variables on composite structural integrity and strength were illustrated. Hence, tensile and three‐point bending flexural tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in NCF. In this article, an important practical problem in fibrous composites, interlaminar shear strength as measured in short beam shear test, is discussed. The fabric composites were tested in three directions: at 0°, 45°, and 90°. In addition to the extensive efforts in elucidating the variation in the mechanical properties of noncrimp glass and carbon fabric reinforced laminates, the work presented here focuses, also, on the type of interactions that are established between fiber and epoxy matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the failure mechanisms in the composite laminates broken in tension. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

Preparation and properties of an oxide fiber‐reinforced silica matrix composite

Oxide (Nextel? 440) fiber‐reinforced silica composites, with the density and porosity of 1.97 g/cm³ and 21.8%, were prepared through sol‐gel. Their average flexure strength, elastic modulus, shear strength, and fracture toughness at room temperature were 119.7 MPa, 25.6 GPa, 10.8 MPa, and 4.0 MPa·m^1/2, respectively. The composites showed typical toughened fracture behavior, and distinct pullout fibers were observed at the fracture surface. Their mechanical properties were performant up to 1000°C, with the maximum flexural strength of 132.2 MPa at 900°C. Moreover, the composites showed good thermal stability, even after thermal aging and thermal shock at elevated temperatures.     

Thermoplastic composites based on poly(ethylene 2,6‐naphthalate) and basalt woven fabrics: Static and dynamic mechanical properties

Composites based on poly(ethylene 2,6‐naphthalate) and basalt woven fabrics have been investigated with the aim to develop composites with a minimum service temperature of 100°C. Laminates have been manufactured by using the film‐stacking technique. A very low void content and a good fabric impregnation has been obtained as confirmed by the morphological analysis performed with scanning electron microscopy. Static flexural modulus and strength have been measured at 20, 60, and 100°C and compared with the dynamic mechanical behavior, evaluated from −100 to 220°C. A very good agreement has been detected between static and dynamic tests, proving that the dynamic mechanical analysis can be used to estimate the flexural modulus in a wide temperature range. Poly(ethylene 2,6‐naphthalate)/basalt composites have exhibited (at 20°C) a flexural modulus and strength as high as 20 GPa and 320 MPa, respectively. The flexural modulus and the flexural strength at 100°C have been found to be equal to 18 GPa and 230 MPa, confirming that this system can retain very good mechanical properties at a service temperature of 100°C. POLYM. COMPOS., 37:2549–2556, 2016. © 2015 Society of Plastics Engineers     

Mechanical behavior of two-dimensional carbon/carbon composites with interfacial carbon layers

Effect of interfacial carbon layers on the mechanical properties and fracture behavior of two-dimensional carbon fiber fabrics reinforced carbon matrix composites were investigated. Phenolic resin reinforced with two-dimensional plain woven carbon fiber fabrics was used as starting materials for carbon/carbon composites and was prepared using vacuum bag hot pressing technique. In order to study the effect of interfacial bonding, a carbon layer was applied to the carbon fabrics in advance. The carbon layers were prepared using petroleum pitch with different concentrations as precursors. The experimental results indicate that the carbon/carbon composites with interfacial carbon layers possess higher fracture energy than that without carbon layers after carbonization at 1000°C. For a pitch concentration of 0.15 g/ml, the carbon/carbon composites have both higher flexural strength and fracture energy than composites without carbon layers. Both flexural strength and fracture energy increased for composites with and without carbon layers after graphitization. The amount of increase in fracture energy was more significant for composites with interfacial carbon layers. Results indicate that a suitable pitch concentration should be used in order to tailor the mechanical behavior of carbon/carbon composites with interfacial carbon layers.     

Carbonized electrospun polyacrylonitrile nanofibers as highly sensitive sensors in structural health monitoring of composite structures

Electrospun polyacrylonitrile (PAN) nanofibers were stabilized at 280°C for 1 h in an ambient condition, and then carbonized at 850°C in inert argon gas for additional 1 h in order to fabricate highly pure carbonous nanofibers for the development of highly sensitive sensors in structural health monitoring (SHM) of composite aircraft and wind turbines. This study manifests the real‐time strain response of the carbonized PAN nanofibers under various tensile loadings. The prepared carbon nanofibers were placed on top of the carbon fiber pre‐preg composite as a single layer. Using a hand lay‐up method, and then co‐cured with the pre‐preg composites in a vacuum oven following the curing cycle of the composite. The electric wires were connected to the top surface of the composite panels where the cohesively bonded conductive nanofibers were placed prior to the tensile and compression loadings in the grips of the tensile unit. The test results clearly showed that the carbonized electrospun PAN nanofibers on the carbon fiber composites were remarkably performed well. Even the small strain rates (e.g., 0.020% strain) on the composite panels were easily detected through voltage and resistance changes of the panels. The change in voltage can be mainly attributed to the breakage/deformation of the conductive network of the carbonized PAN nanofibers under the loadings. The primary goal of the present study is to develop a cost‐effective, lightweight, and flexible strain sensor for the SHM of composite aircraft and wind turbines. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43235.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

芳纶针织物复合材料力学性能研究

为探究芳纶针织物复合材料的力学性能,用电子织物强力机对平板硫化机制得的芳纶针织物复合材料进行拉伸、弯曲、压缩性能测试,结果表明:芳纶针织物增强复合材料为非脆性破坏;经硅烷偶联剂处理有效地提高了芳纶针织物增强复合材料的抗拉、抗弯、抗压强度;罗纹空气层组织的抗拉、抗弯及抗压性能优于满针罗纹组织。     

Preparation of multiscale graphene oxide‐carbon fabric and its effect on mechanical properties of hierarchical epoxy resin composite

Graphene oxide (GO) was used to modify the surface of carbon fiber layers through electrophoretic deposition, forming a multiscale reinforcement fabric. By adjusting the experimental parameters, the resulting GO‐carbon fabric showed productive and homogenous distribution of thin and less‐agglomerate GO platelets on carbon fiber surface, remarkably enlarging the surface area and roughness of carbon fabric. To investigate the effect of GO sheets on composites, GO‐carbon fabric and carbon fabric‐reinforced hierarchical epoxy resin composites were respectively manufactured. Mechanical tests demonstrated that after introducing GO flakes on carbon fabric, both the flexural strength and interlaminar shear strength of composite had achieved an increase, especially the interlaminar shear strength rising by 34%. Through fractography analysis, it was found that in pure carbon fabric‐reinforced epoxy composite, the fiber/matrix debonding fracture mechanism predominated, while after the GO decoration on carbon fiber surface, the composite featured a stronger interfacial bonding, leading to the enhancement in mechanical properties of hierarchical epoxy resin composite. POLYM. COMPOS., 37:1515–1522, 2016. © 2014 Society of Plastics Engineers     

Dynamic oxidation resistance and residual mechanical strength of ZrB2-CrSi2-SiC-Si/SiC coated C/C composites

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.