Increase In Interfacial Shear Strength Research Articles

Microstructure, interfacial and mechanical properties of SiC interphase modified C/C-SiC composites prepared by reactive melt infiltration

Interfacial modification can effectively regulate the interfacial properties of ceramic matrix composites. This paper reports on the effect of SiC interphase thickness on the microstructure, interfacial and mechanical properties of C/C-SiC composites. As the SiC interphase thickness increased to 2 μm, the flexural strength and modulus of the C/C-SiC composites increased up to 252 MPa and 40 GPa, an increase of 36 % and 40 %, respectively. Because the SiC interphase protected the carbon fibers from the corrosion of liquid silicon, thereby increasing the effective fiber volume fraction. Furthermore, the increased interfacial shear strength (IFSS) measured by the fiber push-in test improved the load transfer efficiency. Raman spectrum and transmission electron microscopy indicated that the damage process of the interface with high IFSS after the fiber push-in test showed fiber/pyrolytic carbon interface debonding, release of thermal residual stress, and larger upward rebound protrusion of carbon fibers without interface crushing.

Micro- and macro- mechanical properties of an Al2O3/Al2O3 ceramic matrix composite after thermal aging at 1400 °C

The evolution mechanism of mechanical properties at elevated temperature is particularly crucial to the oxide/oxide composites. In this work, an Al2O3/Al2O3 ceramic matrix composite was prepared by laminating with high solid content slurry. The microstructure, micro- and macro- mechanical properties of the composites before and after thermal aging at 1400 °C were examined. The density and porosity of the original composite were ∼2.60 g/cm3 and ∼27.2 %, respectively. The sintering phenomenon of matrix in the composite after thermal aging was clearly visible, resulting in the matrix modulus increased by 83.5 % and the interfacial shear strength increased by 154.9 % compared with those of the original composite. The fundamental mechanics theory of composites stated that the significant increase of interfacial shear strength was the main reason that the flexural strength retention ratio of the composite decreased to ∼55 % after thermal aging.

Hierarchical Polyimide‐Covalent Organic Frameworks Carbon Fiber Structures Enhancing Physical and Electrochemical Properties

The multifunctionality of carbon fiber (CF) is being extensively explored. Herein, polyimide covalent organic frameworks (PI‐COFs) are grafted bound to CF to enhance their mechanical and electrochemical properties. Here, a range of COF scaffolds are grafted to the surface of CFs via a two‐step functionalization. First, melamine is tethered to the fiber surface to provide an anchoring point for the COFs followed by a “graft from” approach to grow three different sized PI‐COFs utilizing three differently sized dianhydride, PMDA to form MA‐PMDA, NTCDA to form MA‐NTCDA, and PTCDA to form MA‐PTCDA COFs. These COFs increase the capacitance of CF by a maximum of 2.9 F g−1 (480% increase) for the MA‐PTCDA, this coincides with an increase in interfacial shear strength by 67.5% and 52% for MA‐NTCDA and MA‐PTCDA, respectively. This data represents that the first‐time CF has been modified with PI‐COFs and allows access to COF properties including their porosity and CO2 capture ability while being attached to a substrate. This may lead to additional high‐value recyclability and second‐life applications for CFs.

Strengthening sandwich composites by laminating ultra-thin oriented carbon nanotube sheets at the skin/core interface

Strong, tough, and lightweight composites are increasingly needed for diverse applications, from wind turbines to cars and aircraft. These composites typically contain sheets of strong and high-modulus fibers in a matrix that are joined with other materials to resist fracture. Coupling these dissimilar materials together is challenging to enhance delamination properties at their interface. We herein investigate using a trace amount of carbon nanotube sheets to improve the coupling between composite skins and core in a composite sandwich. Ultra-thin (∼100 nm) forest-drawn multi-walled carbon nanotube (MWNT) sheets are interleaved within the skin/core interphase, with MWNTs aligned in the longitudinal direction. The mechanical behavior is characterized by end-notched flexural testing (ENF). With two MWNT sheets placed in the skin/core interphase, the following performance enhancements are achieved: 36.8 % increase in flexural strength; 127.3 % and 125.7 % increases in mode I & II fracture toughness values, respectively; and 152.8 % increase in interfacial shear strength (IFSS). These are achieved with negligible weight gain of the composite sandwich (0.084 wt% increase over the baseline sandwich without MWNT sheets). The finite element simulation results show that MWNT sheets enhance the skin/core coupling by reducing stress concentration, enabling the transition from catastrophic brittle failure to a stable ductile failure mode. The MWNT sheets interleaved sandwich composites are thus demonstrated to be stronger and tougher while providing electrical conductivity (4.3 × 104S/m) at the skin/core interface for potential de-icing, electromagnetic interference shielding, and structural health monitoring.

Scalable electrochemical grafting of anthraquinone for fabrication of multifunctional carbon fibers

Carbon fiber electrodes were prepared by grafting anthraquinone molecules via a scalable electrochemical approach which simultaneously increased interfacial and electrochemical capacitance properties. In this work, anthraquinone diazonium salts were synthesized and grafted onto carbon fiber tows at various concentrations. These modified fibers were subsequently evaluated mechanically and electrochemically to analyze their suitability in structural supercapacitors. Compared to control fibers, the grafted anthraquinone groups resulted in a 30% increase in interfacial shear strength (IFSS) and 6.6× increase in specific capacitance. Industry application was also a focus thus carbon fibers were also modified with in-situ generated diazonium salts to determine the applicability to an in-line industrial process. Specifically, potentiostatic functionalization of fibers with in-situ generated diazonium salts AQ-1 and AQ-2, showed 3× and 4.3× increase in specific capacitance, respectively, relative to unmodified carbon fiber (CF). We expect that implementing a scalable method to introduce a conductive and electrochemically active covalently bound surface chemistry layer onto carbon fiber exhibits a higher specific capacitance than carbon fiber grafted with most other small molecules reported in literature. This will open new avenues for manufacturing multifunctional and high-performance fibers with tailored properties for specific/targeted applications.

Synergistic interfacial effects of sizing agent containing gradient curing agent and pre-heating treated carbon fibers

This investigation focuses on the use of sizing agent containing gradient curing agent from two directions for carbon fibers coupled with pre-heating treatment before fibers combined into composites to improve interfacial adhesion. Growth in interfacial shear strength (IFSS) up to 92.4 % was noticed for fibers functionalized with sizing agent of high-to-low curing agent from fiber-to-resin (CFH→L), relative to control fibers. This is related to the greatly enhanced interactions at both fiber/sizing and sizing/resin interfaces accompanied by increased interfacial curing degree that generate a stronger interphase. Further optimization of 4.1 % increase in IFSS was achieved for CFH→L via pre-heating treatment before being composites, while larger IFSS increment of 16.7 % after pre-heating treatment was observed for fibers coated with sizing agent of low-to-high curing agent from fiber-to-resin (CFL→H). This reveals that pre-heating is more effective for fiber that has weaker interactions with sizing agent. This work is believed to be a preliminary but crucial trial toward obtaining high-performance fiber composites by simply applying it in the already existed fiber production line.

Interfacial reinforcement of carbon fiber composites through a chlorinated aramid nanofiber interphase

Carbon fiber-reinforced polymers (CFRPs) rely on a strong interfacial bond between the reinforcing fibers and polymeric matrix to yield the high strength and toughness expected by a composite material. Poor interfacial strength leads to sub-optimal load transfer and introduces stress concentrations, which can reduce overall performance and result in catastrophic failure. Aramid nanofibers (ANFs) have shown significant promise for interfacial reinforcement in polymeric composite systems due to their high tensile strength, large specific surface area, and abundant polar functional groups. However, due to the chemically inert nature of carbon fibers, ANFs do not readily bond to their surface – thus limiting their application to CFRPs. In this work, we demonstrate that chlorination of ANFs and oxygen plasma treatment of carbon fibers enables the formation of a chlorinated ANF (Cl-ANF) interphase through chemical and physical adsorption using a simple dip-coating process, while fully preserving the tensile strength of the carbon fibers. The Cl-ANF interphase yielded a 79.8 % increase in interfacial shear strength and a 33.7 % increase in short beam strength. By enhancing the interfacial bond between fiber and matrix without degradation of the fiber's tensile strength, this method provides a rapid and reliable process to improve the mechanical properties of CFRP composites.

Enhancing the interfacial strength of jute fiber/polylactic acid composites via surface carboxymethylation pretreatment and in situ growth of ZnO nanorods

ABSTRACT Jute fibers (JFs) are considered an excellent reinforcement due to their abundant resources, environmental friendliness, low cost, lightweight, high specific strength and high specific modulus. However, they still suffer from surface defects that result in poor interfacial compatibility with polymeric matrices. This paper proposes a novel strategy that combines carboxymethylation pretreatment of JFs with in situ growth of ZnO nanorods (NRs) to enhance the mechanical properties of JFs and interfacial properties of JFs/polylactic acid (PLA) composites. The results indicate that carboxymethylation is a more effective method for removing pectin from the surface of JFs compared to conventional acid/alkali treatment. Subsequently, ZnO NRs are deposited in situ on the surface of carboxymethylation-treated JFs (c-JFs) through a seed-growth process, resulting in ZnO NRs@c-JFs. The impact of diverse process parameters, namely reaction time (t), reaction temperature (T) and concentration of zinc source (C), on the morphology and size of ZnO NRs was thoroughly investigated. Optimal process conditions were determined to be t = 6 h, T = 95°C, and C = 37.5 mmol·L−1, resulting in well-aligned ZnO NRs that completely filled up the grooves on JFs’ surface. Compared to untreated JFs, the tensile strength and tensile modulus of ZnO NRs@c-JFs increased by 30.4% and 81.6%, respectively, while exhibiting lower hygroscopicity and higher thermal stability. Furthermore, JFs-reinforced PLA composites were fabricated via hot pressing and their interfacial strength was evaluated using a microdroplet debonding test. Compared to untreated JFs/PLA, the combination of carboxymethylation and ZnO growth in ZnO NRs@c-JFs/PLA resulted in a significant 334% increase in interfacial shear strength (IFSS), indicating highly improved interface bonding between JFs and PLA resin, which was primarily attributed to the formation of a ‘zipper-like’ mechanical interlocking structure between ZnO NRs and PLA. This study provides valuable guidance for enhancing the interface of natural fiber/polymer composites and highlights their potential applications.

Using Nitroxides to Enhance Carbon Fibre Interfacial Adhesion and As An Anchor for "Graft to" Surface Modification Strategies.

Nitroxide groups covalently grafted to carbon fibres were used as anchoring sites for TEMPO-terminated polymers (poly-n-butylacrylate and polystyrene) in a "graft to" surface modification strategy. All surface modified fibres were evaluated for their physical properties, showing that several treatments had enhanced the tensile strength and Young's modulus compared to the control fibres. Up to an 18% increase in tensile strength and 12% in Young's modulus were observed. Similarly, evaluation of interfacial shear strength in an epoxy polymer, showed improvements of up to 144% relative to the control sample. Interestingly, the polymer grafted surfaces showed smaller increases in interfacial shear strength compared to surfaces modified with a small molecule only. This counterintuitive result was attributed to incompatibility, both chemical and physical, of the grafted polymers to the surrounding epoxy matrix. Molecular dynamics simulations of the interface suggest that the diminished increase in mechanical shear strength observed for the polymer grafted surfaces may be due to the lack of exposed chain ends, whereas the small molecule grafted interface exclusively presents chain ends to the resin interface, resulting in good improvements in mechanical properties. This article is protected by copyright. All rights reserved.

Mechanical strength enhancement of natural fibre composites via localized hybridization with stitch reinforcement

Abstract Stress concentration is an unavoidable phenomenon during the fabrication of composite structures. This research focuses on reducing stress concentration. Circular holes made on the composite structural member induce stress-concentrated regions. These stress-concentrated regions around the circular hole (considered as a weak section) were strengthened by adding synthetic fibre (local hybridization) in the form of fibre stitches (locked loop stitch). Through conducting a single fibre pull-out test, the present study analysed the impact of incorporating synthetic fibres in stitch form and the enhancement of Interfacial Shear Strength (IFSS). The load acting on the broken warp fibres is distributed to the broken weft fibres when the number of concentric stitches increases. The increment of concentric stitches resulted in the increase of IFSS due to localized hybridization. The tensile test results also show a significant improvement with 110 Nmm axial fastening force, from 6.749 MPa (for the natural fibre composite) to 76.91 MPa (for locally hybridized with eight concentric stitches). A simple bolted lap joint with local hybridization around the hole has been evaluated for different clamp-up forces. The best combinations of the clamp-up force and the extent of hybridization have been identified. SEM images illustrate the reduced voids and the reduced fibre pull-out due to the local hybridization around delamination-prone areas.

Boosting interfacial adhesion in PBO fiber/epoxy composite by employing finely granular-structured polyaniline as an interfacial intensifier

Poor interfacial adhesion of composites deriving from the surface smoothness and inertness of poly (p-phenylene benzobisoxazole) (PBO) fibers severely constrains the extensive application of the fillers. Constructing the functional coating is generally considered a non-destructive and scalable strategy to simultaneously improve the asperity and activity of PBO fiber surfaces. Herein, we report a facile method to boost the interfacial adhesion in PBO fiber/epoxy composite by employing finely granular-structured polyaniline (PANI) as an interfacial intensifier. Via a mild and efficient in situ polymerization technique, the positively charged micron-scale PANI particles are uniformly anchored onto the negatively charged PBO fibers to form delicate leather-like surface layer containing substantial amino groups. Meanwhile, the resulting PBO fibers maintain excellent mechanical properties. Therefore, the PANI-coated PBO fiber/epoxy composite (PBO@PANI/EP) shows the significant increase in interfacial shear strength (41.0 MPa) and interlaminar shear strength (44.9 MPa), soaring by 60.16 % and 54.83 %, respectively. The debonding behavior and fracture surfaces confirm that the interfacial failure mechanism of PBO@PANI/EP converts from adhesive failure to both cohesive and substrate failure. The strategy in this work to modify PBO fiber surfaces for increasing the interfacial combination of composite is cost-effective and feasible to scale up.

Designing Carbon Fiber Composite Interfaces: Reactive Sizing Derived from Terpenes

Terpenes, typically obtained from conifer plants, were chemically modified to include functional groups that could react with both the surface of carbon fiber and the surrounding epoxy resin. This strategy was coupled with the installation of reactive amines (NH2) and thiols (SH) on the fiber surface to specifically react with these sizings. Increases in interfacial shear strength (IFSS) up to 195% were observed for fibers coated in these reactive sizings in combination with the tailored surface chemistry. Improvements in IFSS were also observed for unmodified fibers, suggesting passive functionalization. The presence of persistent carbon-centered radicals was proposed as a way that the fiber surface could react with the sizing agents, stabilized by the inherent high degree of conjugation present within the fiber structure. Interrogating commercially available carbon fiber via electron paramagnetic resonance (EPR) spectroscopy confirmed the presence of radicals within or on the carbon fiber surface (g-value = 1.9984).

Surface modification of carbon fibres with ammonium cerium nitrate for interfacial shear strength enhancement

An ongoing research focus in the carbon fibre (CF) field has been to identify a practical and more effective surface modification method for improving CF's interfacial adhesion and bonding capacity, which is vital for achieving high performance CF composites. In this research, carbon fibres were modified by using a new and facile treatment process in an aqueous solution of ammonium cerium nitrate (ACN) at a low temperature (20 °C–50 °C). The surface analysis results revealed that the ACN treatment imparted a combined effect of CF surface oxidation and cerium (IV)/(III) immobilisation on the oxidised CF surface through coordination bonds, leading to a significantly increased specific surface area, surface energy and surface reactivity. These new surface characteristics directly contributed to an improved CF interfacial adhesion and load transfer between the modified CFs and an epoxy matrix. The study demonstrated that under specified conditions, the new treatment method resulted in 304% increase in interfacial shear strength (IFSS) and 130% increase in interlaminar shear strength (ILSS) without any significant reduction in CF's mechanical properties. In comparison, the ACN modification yielded CFs with a much higher ILSS than the current industrial processes using electrochemical oxidation and sizing.

Enhancing mechanical strength of carbon fiber-epoxy interface through electrowetting of fiber surface

The interfacial shear strength between carbon fiber and polymer matrices plays a critical role in bulk mechanical performances of carbon fiber-reinforced polymer composites. However, finding simple and effective methods for modulating the interfacial adhesion remains challenging even after over a decade of research and development. Herein, a novel method based on electrically-assisted wetting technique was developed to strengthen the interfacial adhesion between carbon fiber and polymer matrix. The results indicated the polymer completely infiltrated the small grooves on the fiber surface under applied external electric field during the curing process, leading to the formation of conformal contact interface. The interfacial properties were characterized by utilizing the microdroplet debonding and single fiber fragmentation tests. It is found that the as-prepared composites exhibit a maximum increased interfacial shear strength (IFSS) reaching up to 119.69 MPa, a value 81.02% higher than that of the original fiber-based composites. The reinforcement mechanism was ascribed to the improved wettability and mechanical interlock effect induced by the applied electrical stimulus which was supported by dynamic simulation of two-phase flow. In sum, the suggested simple, facile, and environmentally friendly strategy can effectively regulate the interfacial load transfer capacity, thereby suitable for the optimal design of fiber-reinforced composites. • A green, and economical electrically-assisted wetting method is proposed to improve the interfacial properties. • Electrically-assisted wetting promotes preferable resin wetting and formation of mechanical interlocking. • The two-phase flow dynamic simulation reveals the rheological behaviors of the resin and the reinforcement mechanism. • The proposed method will not cause damage to the fiber and is compatible with other modification methods.

Enhanced interfacial shear strength in ultra-high molecular weight polyethylene epoxy composites through a zinc oxide nanowire interphase

The mechanical properties of fiber-reinforced composites are largely dictated by fiber-matrix interactions, where a well-adhered interface is desired. Ultra-high molecular weight polyethylene (UHMWPE) fibers suffer from poor interfacial properties due to their smooth and inert surface. Therefore, enhancing the interfacial properties of such composites will lead to an increased use in structural applications where high modulus, tenacity, low density, high impact and abrasion resistance properties are needed. In this work, an interfacial modification process consisting of an oxygen plasma surface functionalization followed by the grafting of zinc oxide nanowires (ZnO NWs) onto the UHMWPE fiber surface is investigated to improve the fiber-matrix interaction in UHMWPE composites. The UHMWPE fiber surface is subjected to varying oxygen plasma treatment durations such that the surface oxygen content can be increased to improve ZnO NW adhesion. Through single fiber pullout testing, the interfacial shear strength (IFSS) of oxygen plasma functionalized and ZnO NW coated UHMWPE fibers was shown a maximum increase of 135% with a 30 s plasma treatment prior to ZnO NWs. This increase in IFSS is attributed to an increased surface area interaction and mechanical interlocking between the fiber and matrix. The results detailed in this work demonstrate a benign, simple, and effective way to significantly improve the interfacial properties of UHMWPE fiber reinforced composites.

Pulsed Laser Deposition of SWCNTs on Carbon Fibres: Effect of Deposition Temperature.

Single wall carbon nanotubes (SWCNTs) were grown on either sized or desized carbon fabric in a self-designed reactor by Pulsed Laser Deposition (PLD). The uniqueness of the PLD system lies, among other things, in the ability to keep the substrate at a low temperature, compared to the 1100 °C needed for the SWCNTs synthesis, thus, rendering it undamaged. Samples were placed at different positions on a cold finger (CF), where a temperature gradient develops, in the range 25–565 °C. The chemical composition and morphology of desized and surface treatments, as well as SWCNTs grown on carbon fibres, were verified by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-Ray Spectroscopy (EDX), while the quality of SWCNTs was proven by confocal micro-Raman Spectroscopy and High-Resolution Scanning Transmission Electron Microscopy (HR-STEM). Fibres covered with SWCNTs by PLD were characterized using contact angle and the surface free energy was calculated. A micro-droplet pull-out test was used to evaluate the effect of SWCNTs over interfacial properties of a carbon-epoxy composite. A 20% increase in interfacial shear strength (IFSS) was observed by deposition at 290 °C, compared to the commercial carbon fibre sizing. The carbon fibres kept their tensile properties due to the low deposition temperatures.

Surface modification of carbon fibres using ring-opening metathesis polymerization

A series of alkene bearing small molecules were electrografted to the surface of carbon fibers using the in situ generation of aryldiazonium salts. These molecules were used as an anchoring point for the use of Ring Opening Metathesis Polymerization (ROMP) in a ‘graft from’ approach to carbon fiber surface modification. After modification, evaluation of these modified fibers in an epoxy resin, using the single fiber fragmentation test, showed significant increases in interfacial shear strength (IFSS) of up to 189%, relative to untreated pristine fibers. Dilution of strained norbornene alkenes at 2:1 (alkane:alkene) had minimal effect on IFSS, while dilution at 10:1 (alkane:alkene) still resulted in improved, albeit suppressed, IFSS gains of 70% relative to pristine fibers. This study shows the potential of using ROMP as a means to tailor the surface chemistry of carbon fibers for use in advanced composites.

Improving the effects of plasma polymerization on carbon fiber using a surface modification pretreatment

Plasma and electrochemical treatments of carbon fibers for enhanced properties are often presented in opposition to each other. This work demonstrates the combination of these methodologies through the electrochemical attachment of nitroaryl moieties to the surface of the carbon fiber, prior to the deposition of plasma polymerized acrylic acid to the surface. Notably, the tensile strength of fibers having undergone both surface modification and plasma polymerization showed a significant increase (3.76 ± 0.08 GPa), relative to control fibers (3.31 ± 0.11 GPa), while plasma polymerization alone showed no change (3.39 ± 0.09 GPa). Additional benefits resulting from both treatments were observed when determining the fiber-to-matrix adhesion. Plasma polymerization of acrylic acid alone returned a 49% increase in interfacial shear strength (IFSS) compared to control (28.3 ± 1.2 MPa vs 18.9 ± 1.2 MPa, respectively). While the presence of nitrophenyl groups on the fiber prior to polymerization conferred an additional 24% improvement over plasma polymerization alone and a 73% improvement relative to control fibers (32.7 ± 0.5 MPa vs 18.9 ± 1.2 MPa, respectively). Finally, we present the first comparison of scanning electron microscopy (SEM) and helium ion microscopy (HIM) to visualize polymers on the carbon fiber surface. HIM shows a clear advantage over conventional SEM in visualizing non-conductive coatings on carbon fibers. Analysis of the samples by X-ray photoelectron spectroscopy (XPS) confirmed the desired chemistry had been imparted onto the surface, consistent with the plasma-polymerized acrylic acid coating and presence of nitro-aryl moieties.

Effects of chain length of polyether amine on interfacial adhesion of carbon fiber/epoxy composite in the absence or presence of polydopamine bridging platform

To understand the complementary mechanisms between molecular interlocking and covalent interconnection that affects interfacial shear strength (IFSS), the adherence of polyether amine with varying chain length onto the surface of carbon fiber in the absence or presence of bio-derived adhesive polydopamine (PDA) as a bridge was subtly designed and studied. Without PDA, up to 25.6% and 34.7% increases in IFSS relative to untreated CF was achieved respectively for shorter and longer molecular chain of polyether amine. The longer chain length encourages molecular interlocking with epoxy molecules and produces a higher penetration depth into epoxy matrix, thus giving a larger increase in IFSS. In the presence of PDA, 39–42% IFSS increase by analogy to CF is observed due to the synergistic effects of hybrid interfacial interactions, the increasing rigidity of interphase and the improved wetting behavior. While, in this case, impact of chain length becomes less noticeable, which is mainly attributed to the trade-off between the enhanced chemical cross-linking points and the less molecular entanglement with resin.

Laccase-Enzyme Treated Flax Fibre for Use in Natural Fibre Epoxy Composites

Natural fibres have a high potential as reinforcement of polymer matrices, as they combine a high specific strength and modulus with sustainable production and reasonable prices. Modifying the fibre surface is a common method to increase the adhesion and thereby enhance the mechanical properties of composites. In this study, a novel sustainable surface treatment is presented: the fungal enzyme laccase was utilised with the aim of covalently binding the coupling agent dopamine to flax fibre surfaces. The goal is to improve the interfacial strength towards an epoxy matrix. SEM and AFM micrographs showed that the modification changes the surface morphology, indicating a deposition of dopamine on the surface. Fibre tensile tests, which were performed to check whether the fibre structure was damaged during the treatment, showed that no decrease in tensile strength or modulus occurred. Single fibre pullout tests showed a 30% increase in interfacial shear strength (IFSS) due to the laccase-mediated bonding of the coupling agent dopamine. These results demonstrate that a laccase + dopamine treatment modifies flax fibres sustainably and increases the interfacial strength towards epoxy.