Autonomic healing of carbon fiber/epoxy interfaces

Self-Healing Composites: A Path to Redefining Material Resilience-A Comprehensive Recent Review

Polymeric composites are prone to undergoing damage, such as microcracks, during their operation, which can ultimately lead to catastrophic failure. To contradict such a problem, efforts have been carried out, by the scientific community, towards developing self-healing composites that, by mimicking biological systems, can autonomously and prematurely repair flaws, extending the durability and improving the security of materials. The present review explores the progress made in this area, focusing on extrinsic self-healing methods, as these can be employed to a variety of materials. Reservoir-based techniques, which resort to capsules, hollow fibers or microvascular networks, and thermoplastic-based ones are overviewed, prioritizing innovative approaches made in recent years. At last, promising practical applications for self-healing composites are highlighted and future challenges and opportunities are pointed out.

Enhanced Mechanical and Self-Healing Properties of Carbon Fiber-Reinforced Epoxy Laminates Using In Situ-Grown ZnO Nanorods and Thermo-Reversible Bonds

Advanced hierarchical carbon fiber epoxy laminates with an engineered interface using in situ-grown ZnO nanorods on carbon fiber resulted in strong mechanical interlocking with the matrix. To further strengthen the interface, "site-specific" modification was realized by modifying the ZnO nanorods with bismaleimide (BMI), which facilitates "thermo-reversible" bonds with graphene oxide (GO) present in the matrix. The resulting laminates exhibited an improvement in flexural strength by 20% and in interlaminar shear strength (ILSS) by 28%. In order to gain a mechanistic insight, few laminates were prepared by "nonselectively" modifying the ZnO-grown carbon fiber (CF) with BMI. The "nonselectively" modified laminates showed flexural strength and ILSS improvement by 43 and 39%, respectively. The "nonselective" modification resulted in a strong improvement in mechanical properties; however, the "site-specific" modification yielded a higher self-healing efficiency (81%). Raman spectroscopy, scanning electron microscopy (SEM) micrographs, atomic force microscope (AFM) analysis, and contact angle analysis indicated a strong interaction of the modified CFs with the resin. Enhanced surface area and energy, along with a decrease in segmental molecular mobility observed from dynamic mechanical analysis, confirmed the mechanism for a better performance. Microscopic images revealed an improved interfacial behavior of the fractured samples, indicating a higher interfacial adhesion in the modified laminates. Besides mechanical properties, these laminates also showed excellent electromagnetic interference (EMI) shielding performance. The laminates with only ZnO-modified CF showed a high shielding effectiveness of -47 dB.

Structural integrity and healing efficiency study of micro-capsule based composite materials via 1H NMR relaxometry

In this work we present a novel approach utilizing nuclear magnetic resonance (NMR) relaxometry to assess the structural stability of microcapsules employed as self-healing agents in advanced aerospace composites both in ambient and harsh environmental conditions. We successfully correlate the amount of the encapsulated self-healing agent with the signal intensity and confirm non-destructively the quantity of the encapsulated self-healing agent mass for the first time in the literature using ¹H NMR spin-spin relaxation techniques on urea-formaldehyde (UF) microcapsules of different diameters containing an epoxy healing agent. The amount of self-healing agent is shown to increase by reducing the capsule diameter; however, the reduced shell mass renders the capsules more fragile and prone to failure. Most notably, via NMR experiments conducted during thermal cycling simulating flight conditions, we demonstrate that the microcapsule integrity under thermal fatigue varies according to their size. Especially we experimentally verify that the microcapsules with the most sensitive shells are the 147 nm and 133 nm diameter microcapsules, which are the most commonly used in self-healing systems. Finally, we were able to retrieve the same results using a portable NMR spectrometer developed in-house for in situ microcapsule testing, thus demonstrating the potential of NMR relaxometry as a powerful non-destructive evaluation tool for the microcapsule production line.

Self-Healing Coatings Containing Core-Shell Nanofibers with pH-Responsive Performance

The micro-nanofibers prepared by the electrospinning technique can be used as a good container for loading healing agents. The core-shell electrospun nanofibers with polyacrylonitrile as the outer shell and tannic acid (TA) and tung oil as the core healing agents were synthesized by a coaxial electrospinning method and exhibited pH-sensitive ability. The nanofibers as additives were added to an epoxy resin coating as a self-healing coating. The morphological stability of the electrospun nanofibers were observed by a scanning electron microscope and a transmission electron microscope. Fourier transform infrared spectroscopy and fluorescence microscopy reveal that the successful synthesis and uniform distribution of core-shell fibers. The mechanical properties test revealed that the tensile properties of the coating could be improved by adding nanofibers. The infrared mapping test, energy-dispersive spectrometry, and X-ray photoelectron spectroscopy, which were carried out on the scratched part of the coating, proved the release of the healing agent in the damaged part. TA forms a protective film on the exposed metal surface through molecular adsorption under acidic conditions. Meanwhile, the curing of tung oil can effectively compensate into the microcracks to form a TA protective film, which could improve the self-healing performance. As the tung oil dries and solidifies in the alkaline solution, the cross-linking effect of the molecules is combined to form a tight film and strength the self-healing ability. TA as an acidic healing agent and tung oil as an alkaline healing agent played the role of pH-sensitive products in healing the cracked coating. The self-healing rates of coating immersing in 3.5 wt % acidic NaCl solution and alkaline solution were 81.6 and 71.2%, respectively. The composite coating shows a great pH-sensitive self-healing ability to heal the cracked coating.

Facile immobilization of graphene nanosheets onto PBO fibers via MOF-mediated coagulation strategy: Multifunctional interface with self-healing and ultraviolet-resistance performance

The surface of poly (p-phenylene benzobisoxazole) (PBO) fibers with self-healing and ultraviolet (UV)-resistance performance play the key role in prolonging their service lifespan. Although great advances have been made in the single aspect of above two properties, integration of self-healing and anti-UV performance into the surface of PBO fiber is still a challenge. In this study, the coagulation strategy mediated by metal-organic framework (MOF) is proposed to construct the multifunctional surface of PBO fibers. The spindle-like iron (III)-based MOF (MIL-88B-NH₂) nanocrystals are firstly immobilized onto the surface of PBO-COOH through hydrothermal reaction, then serving as the medium layer to further immobilize sufficient graphene oxide (GO) nanosheets. Benefitting from the favorable near-infrared (NIR, 808 nm) photothermal conversion performance of GO nanolayers, the monofilament composite-PBO@Fe-MIL-88B-NH₂-GO-TPU (thermoplastic polyurethane) exhibited a stable and high self-healing efficiency (approximately 80%) within five cycle times. Meanwhile, the cooperative adsorption and shielding weaken effects of MOF-GO nanolayers enabled PBO fibers with excellent anti-UV properties that are superior to much reported literatures after 96 h aging time and eventually increased by 75% compared with untreated PBO fiber. In view of the varieties and multifunctionalities of MOFs and carbon nanomaterials, MOF-mediated coagulation strategy would provide guidance for preparing multifunctional composite materials.

The preparation and characterization of a carbon fiber-reinforced epoxy resin and EPDM composite using the co-curing method

Due to the development of the aerospace technology, the requirements for composite materials have become stricter. Thus, in this work, a completely novel technology, which has not been reported elsewhere, was used to prepare a composite of a carbon fiber-reinforced epoxy resin (CFRP) and ethylene-propylene-diene rubber (EPDM), which was denoted as CFRP/EPDM; CFRP and EPDM are commonly used as a shell and heat insulation layer, respectively, in the solid rocket industry. The composite system had good adhesive ability, as confirmed by the 90° peel strength test, even though the EPDM rubber is non-polar in nature. Additionally, the adhesive mechanism between CFRP and EPDM was determined using scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) indicated that the T_d10% value of the CFRP/EDPM composite was slightly higher than that of CFRP. According to the nuclear magnetic resonance (NMR) spectroscopy results of the EPDM rubber and the interlaminar shear strength (ILSS) of CFRP, we can conclude that the co-curing method will not damage the properties of CFRP and EPDM.

The CFRP/EPDM composite was prepared by a one-step co-curing method, which was very convenient and energy effective.

Cyclodextrin as sizing for carbon fibers: new bonding mechanism improves adhesion in carbon fiber reinforced epoxy resin

In this study, β-cyclodextrin was covalently bonded on carbon fibers and its influence through host-guest complex formation was analyzed. Since cyclodextrins act as host molecules for aromatic compounds, complex formations can be performed in carbon fiber reinforced epoxy resins, between the carbon fiber surface and the aromatic compounds of the surrounding plastic. This interface design leads to improved adhesion between fiber and plastic. An increase of the interfacial shear strength from 12 MPa to 38 MPa was detected. It was possible to increase the interfacial shear strength even further, to 41 MPa, through a prior complex formation with one of the plastics components. In addition to the micromechanical analysis, energy-dispersive x-ray spectroscopy and contact angle measurements were performed to confirm the covalent coating of cyclodextrin on the carbon fiber surface.

Effect of the Chain Structure of Self-Emulsifying Polyester Sizing Agent on ILSS of Carbon Fiber/Unsaturated Polyester Resin Composites

Herein, we report self-emulsifying anionic unsaturated polyester emulsions with different chain segments as novel sizing agents. The epoxy modified unsaturated polyester emulsions were synthesized via a self-emulsifying technique with no organic solvents. Emulsions were characterized by dynamic light scattering (DLS), Zeta potential, centrifuge, and cryo-scanning electron microscopy (Cryo-SEM). The results showed that the emulsions obtained were mono-dispersed nanospheres that had adequate colloidal stability. The maximum Zeta potential of the sizing agent is −52.88 mV. In addition, these emulsions were investigated as the sizing agents in order to improve interfacial adhesion between carbon fibers (CFs) and unsaturated polyester resin (UPR). Compared with the CFs sized with the commercial epoxy sizing agent, the interlaminar shear strength (ILSS) of CF/UPR composites from the CFs sized by these emulsion sizing agents with three different chain structures were enhanced by 25%, 29%, and 42%, respectively. The emulsion sizing agent composed of a flexible segment synthesized from adipic acid, neopentyl glycol, and maleic anhydride is most effective and can achieve the highest enhancement of the ILSS of CF/UPR composites.

Multifunctional Thermal Barrier Application Composite with SiC Nanowires Enhanced Structural Health Monitoring Sensitivity and Interface Performance

Carbon fiber (CF)-reinforced ceramic composites show the attractive potential for next generation thermal protection materials because of their outstanding reliability and excellent high-temperature resistance but are facing great challenges in the combination of the engineering practicality and versatility. Herein, it is demonstrated that silicon carbide nanowires can be grown on the surface of CF to create a multifunctional thermal barrier application composite. The embedding of the silicon carbide nanowires in the interface of CF and ceramic matrix significantly increased the structural health monitoring sensitivity and interface strength of the composites. Compared to the conventional CF/ZrC composites, the structural health monitoring sensitivity of the composites with SiC nanowires is greatly elevated with a 14-fold improvement. Additional investigations revealed that the multifunctional SiC_nws-CF/ZrC nanocomposites enjoyed a low thermal conductivity of 0.49 W/(m·K), a light weight (0.76-1.85 g/cm³), and a relative high compressive strength of 23.64 MPa, which is favorite in applying as a thermal barrier material. Furthermore, the interface design strategy could be extended as a universal method in fabricating various fiber-reinforced composites for a wide range of other applications.

Effect of Graphene Nanowall Size on the Interfacial Strength of Carbon Fiber Reinforced Composites

Graphene nanowalls (GNWs) with different sizes (i.e., length and height) were grown directly on the surface of individual carbon fibers (CFs) using a radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique. The size was controlled by varying the deposition time. The GNW-modified CFs were embedded into epoxy resin matrix to prepare a series of carbon-fiber-reinforced composites (CFRCs). The results indicated that GNWs were remarkably effective in improving the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of the carbon-fiber-reinforced composites. The enhancement effect on the strength strongly depended on the size of GNWs. It increased with the increase in the GNWs’ size and reached the maximum upon the incorporation of GNWs that were grown for 45 min. Noticeable increases of 222.8% and 41.1% were observed in IFSS and ILSS, respectively. The enhancement mechanism was revealed by means of scanning electron microscope (SEM) fractography analysis. However, further increase of GNW size led to no more improvement in the shear strength. It could result from the increased defect concentration and wrinkle size in the GNWs, which deteriorated the strength.

Carbon Nanofiber Arrays Grown on Three-Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating

Carbon nanofibers (CNFs) were grown around the carbon fiber architecture through a plasma enhanced chemical vapor deposition method to enhance the interface performance between CF architecture substrate and ZrC preceramic matrix. The synthesized 3D CF hierarchical architectures (CNFs-CF) are coated with zirconium carbide (ZrC) ceramic to enhance their antioxidant property and high temperature resistance. The composition and the crystalline phase structure of the composite were detected with the X-ray photoelectron spectroscopy and X-ray diffraction. The results of scanning electron microscopy show that, the as-prepared CNFs and consistent ZrC ceramic coating are uniformly covered on the surface of carbon fiber architecture substrate. The ZrC ceramic products with excellent crystallinity were got from the pyrolysis of preceramic polymer at 1600 °C in inert atmosphere. Comparing with the untreated CF, the loading of ZrC ceramics around the CNFs-CF architecture surface are significantly increased. The thermal stability and mechanical property of CNFs-CF/ZrC nanocomposites have been promoted obviously compared with the CF/ZrC ceramic nanocomposite. The prepared CNFs-CF/ZrC ceramic nanocomposite is one of the potential candidate materials for the thermal protection application.

Design Rules for Enhanced Interfacial Shear Response in Functionalized Carbon Fiber Epoxy Composites

Carbon-fiber reinforced composites are ideal light-weighting candidates to replace traditional engineering materials. The mechanical performance of these composites results from a complex interplay of influences operating over several length and time scales. The mechanical performance may therefore be limited by many factors, one of which being the modest interfacial adhesion between the carbon fiber and the polymer. Chemical modification of the fiber, via surface grafting of molecules, is one possible strategy to enhance interactions across the fiber-polymer interface. To achieve systematic improvements in these modified materials, the ability to manipulate and monitor the molecular structure of the polymer interphase and the surface grafted molecules in the composite is essential, but challenging to accomplish from a purely experimental perspective. Alternatively, molecular simulations can bridge this knowledge gap by providing molecular-scale insights into the optimal design of these surface-grafted molecules to deliver superior mechanical properties. Here we use molecular dynamics simulations to predict the interfacial shear response of a typical epoxy/carbon-fiber composite for both pristine fiber and a range of surface graftings. We allow for the dynamic curing of the epoxy in the presence of the functionalized surface, including cross-link formation between the grafted molecules and the polymer matrix. Our predictions agree with recently reported experimental data for these systems and reveal the molecular-scale origins of the enhanced interfacial shear response arising from functionalization. In addition to the presence of interfacial covalent bonds, we find that the interfacial structural complexity, resulting from the presence of the grafted molecules, and a concomitant spatial homogeneity of the interphase polymer density are beneficial factors in conferring high interfacial shear stress. Our approach paves the way for computational screening processes to design, test, and rapidly identify viable surface modifications in silico, which would enable rapid systematic progress in optimizing the match between the carbon fiber treatment and the desired thermoset polymer matrix.

Sustainable self-healing at ultra-low temperatures in structural composites incorporating hollow vessels and heating elements

Self-healing composites are able to restore their properties automatically. Impressive healing efficiencies can be achieved when conditions are favourable. On the other hand, healing might not be possible under adverse circumstances such as very low ambient temperature. Here, we report a structural composite able to maintain its temperature to provide a sustainable self-healing capability-similar to that in the natural world where some animals keep a constant body temperature to allow enzymes to stay active. The composite embeds three-dimensional hollow vessels with the purpose of delivering and releasing healing agents, and a porous conductive element to provide heat internally to defrost and promote healing reactions. A healing efficiency over 100% at around -60°C was obtained. The effects of the sheets on the interlaminar and tensile properties have been investigated experimentally. The proposed technique can be implemented in a majority of extrinsic self-healing composites to enable automatic recovery at ultra-low temperatures.

Biomimetic Self-Healing

Self-healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness-enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self-healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.

A shape-recovery polymer coating for the corrosion protection of metallic surfaces

Self-healing polymer coatings are a type of smart material aimed for advanced corrosion protection of metals. This paper presents the synthesis and characterization of two new UV-cure self-healing coatings based on acrylated polycaprolactone polyurethanes. On a macroscopic scale, the cured films all show outstanding mechanical properties, combining relatively high Young's modulus of up to 270 MPa with a strain at break above 350%. After thermal activation the strained films recover up to 97% of their original length. Optical and electron microscopy reveals the self-healing properties of these coatings on hot dip galvanized steel with scratches and microindentations. The temperature-induced closing of such defects restores the corrosion protection and barrier properties of the coating as shown by electrochemical impedance spectroscopy and scanning vibrating electrode technique. Therefore, such coatings are a complementary option for encapsulation-based autonomous corrosion protection systems.

Self-healable castor oil based tough smart hyperbranched polyurethane nanocomposite with antimicrobial attributes

Smart, tough hyperbranched polyurethane nanocomposite with excellent rapid and repeatable self-healing and antimicrobial attributes is fabricated.

Photothermal healing of a glass fiber reinforced composite interface by gold nanoparticles

Scheme of interfacial healing process.

Interfacial healing of carbon fiber composites in the presence of gold nanoparticles as localized “nano-heaters”

In this paper, interfacial healing was achieved in carbon fiber composites via local heating generated by photothermal effect of gold nanoparticles (Au NPs).