Surface damage of unidirectional carbon fiber reinforced epoxy composites under reciprocating sliding in ambient air

For many technical applications friction and wear are critical issues. Reinforced polymer-matrix composites are widely used under vibrating contact condition in various automotive and aerospace applications as well as in structural engineering. In this paper, the friction and wear of bulk epoxy and unidirectional carbon fiber reinforced epoxy composite have been investigated under reciprocating sliding against either alumina or stainless steel balls in ambient air. The effect of sliding direction with respect to the long and unidirectional carbon fibers has been studied. We demonstrate that the carbon fiber reinforcement greatly improves the tribological properties of the thermoset epoxy: it reduces the coefficient of friction and the debris formation. It was found that on sliding in the anti-parallel direction a more significant degradation takes place than in the parallel direction. The coefficient of friction measured on bulk epoxy sliding against either stainless steel or alumina is around 0.65, whereas the coefficient of friction measured on epoxy reinforced with carbon fibers is significantly lower, namely down to 0.11. It was found that sliding with a stainless steel ball in a direction parallel to the fiber orientation results in a lower coefficient of friction than sliding in anti-parallel direction. The reduced coefficient of friction is largely influenced by the carbon fiber reinforcement due to the auto-protecting film formed as a paste in the contact area and along the wear track edges. The relationship between friction and degradation of the composite material including surface wear and debris formation are discussed based on an in-depth analysis of the worn surfaces by optical and scanning electron microscopy, micro-Raman spectroscopy, and white light interferometry.     

Combined effects of fiber/matrix interface and water absorption on the tribological behaviors of water-lubricated polytetrafluoroethylene-based composites reinforced with carbon and basalt fibers

Tribological behaviors of two PTFE-based composites reinforced with carbon fibers and basalt fibers sliding against stainless steel under water lubrication were investigated and compared with those of pure PTFE. Results showed that carbon fibers were well bonded with PTFE matrix by dendritic PTFE nano-ribbons in a Boston ivy-like manner, but the basalt fibers were poorly bonded with the matrix. Due to the great accelerating effect of poor fiber/matrix interfacial adhesion on water absorption, BF/PTFE with the highest crystallinity unexpectedly showed the highest water absorption, resulting in serious matrix plasticization and degradation of fiber/matrix interfacial adhesion. As a result, as the reinforcement failure of basalt fibers occurred, BF/PTFE exhibited the highest wear rate. Instead, because good fiber/matrix interfacial adhesion was favor of the resistance to water intrusion, CF/PTFE composite was not dominated by remarkable matrix plasticization and fiber/matrix interface degradation, and showed the lowest wear rate.     

Effects of potassium titanate whiskers and carbon fibers on the wear behavior of polyetheretherketone composite under water lubricated condition

The tribological behaviors of polyetheretherketone (PEEK) composite reinforced by carbon fiber (CF) and potassium titanate whiskers (PTW) have been investigated using the pin-on-disk configuration at different applied loads under water lubricated condition. The effects of micrometer carbon fiber and sub-micrometer PTW on the wear properties of the hybrid composite have been discussed. It was found that the PEEK/PTW/CF composite showed excellent tribological performance in water condition. High wear resistance and low friction coefficient were achieved under a wide range of loads. It was revealed that the two fillers worked synergetically to enhance the wear resistance of the hybrid reinforced PEEK composite. The carbon fiber carried the main load between the contact surfaces and protected the matrix from further severe abrasion of the counterpart. At the same time, the exposed PTW out of the polymer matrix around the fiber inhibited the direct scraping between the fiber edge and counterpart tip in some degree, so that the fibers could be less directly impacted during the subsequent sliding process and they were protected from severe damage. In addition, the reinforcement effect of PTW on PEEK could reduce the stress concentration on the carbon fiber-matrix interface, and thereby reduce the CF failure/damage. The reinforcement effect of PTW on PEEK might also restrict the crack initiation and propagation on the surface and subsurface of the composite, and therefore to protect the matrix from fatigue failure during the sliding process.     

Evaluation of water degradation of vinylester and epoxy matrix composites by single fiber and composite tests

Degradation of the mechanical properties of vinylester and epoxy matrix composites exposed to water has been approached by monitoring the strengths of glass and carbon fibers and resins. In addition, the fiber/matrix (F/M) interface strengths and debond lengths of single-fiber composites were determined and test results were compared to test results of macroscopic composite specimens. The single-fiber tensile test results indicate a substantial loss of the tensile strength of glass fibers and the fragmentation tests reveal loss of F/M shear strength and substantial debonding for both glass and carbon fiber composites after water exposure. The transverse strengths of the composites are also degraded to large extents. The tests results identify water degradation of the F/M interface as a major strength limiting mechanism.     

Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content

Here, an anodic electrophoretic deposition was adopted to facilitate the large-scale uniform coating of nano-fillers onto carbon fibers to enhance the interfacial properties between carbon fibers and epoxy matrix. As interface–reinforcing materials, aramid nanofibers were introduced because of their superior mechanical properties and epoxy matrix-friendly functional groups. Furthermore, aramid nanofibers can be readily coated on carbon fibers via electrophoretic deposition because they are negatively-charged in solution with high electrical mobility. Finally, aramid nanofiber-coated carbon fibers showed significantly improved interfacial properties such as higher surface free energy and interfacial shear strengths (39.7% and 34.9% increases, respectively) than those of a pristine carbon fiber despite a very small amount of embedding (0.025 wt% of aramid nanofibers in a carbon fiber), and the short beam strength of the laminated composite prepared with the aramid nanofiber-coated carbon fibers was also improved by 17.0% compared to a non-modified composite.     

Tribo-mechanical characterization of reinforced epoxy resin under dry and lubricated contact conditions

The aim of the present work is to investigate the influence of the reinforcing material and architecture on the voids content, mechanical properties and tribological behavior of fiber reinforced epoxy composite laminates manufactured by VARTM under different processing conditions. Two different textile architectures, namely unidirectional non-crimp fabrics (UD) and 0/90 plain wave (PW), were considered, reinforcing an EPIKOTE RIMR 135 epoxy matrix with glass (GF) as well as carbon (CF) continuous fibers. Optical observations revealed an unexpected trend relatively to the intra- and inter-bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three points bending tests highlighted the dominant role of fiber material and architecture on mechanical properties, whereas the presence of voids played a minor role with respect to the analyzed features. Tribological outcomes evidenced a reduction of the friction coefficient (μ) when the resin is reinforced by carbon or glass fibers. The lowest values were detected when the sliding direction of the counterbody is oriented parallel to the fiber direction for UD samples. Further reduction of μ, for both UD and PW specimens, was obtained by interposing a lubricant at the interface.     

Hybrid effect of nanoparticles with carbon fibers on the mechanical and wear properties of polymer composites

Polyetheretherketone (PEEK) composites reinforced with carbon fibers (CFs) and nano-ZrO₂ particles were prepared by incorporating nanoparticles into PEEK/CF composites via twin-screw extrusion. The effects of nanoparticles on the mechanical and wear properties of the PEEK/CF composites were studied. The results showed that the incorporation of nano-ZrO₂ particles with carbon fiber could effectively enhance the tensile properties of the composites. The tensile strength and Young’s modulus of the composites increased with the increasing nano-ZrO₂ content. The enhancement effect of the particle was more significant in the hybrid reinforced composites. The compounding of the two fillers also remarkably improved the wear resistance of the composites under water condition especially under high pressures. It was revealed that the excellent wear resistance of the PEEK/CF/ZrO₂ composites was due to a synergy effect between the nano-ZrO₂ particles and CF. CF carried the majority of load during sliding process and prevented severe wear to the matrix. The incorporation of nano-ZrO₂ effectively inhibited the CF failures through reducing the stress concentration on the carbon fibers interface and the shear stress between two sliding surfaces. It was also indicated that the wear rates of the hybrid composites decreased with the increasing applied load and sliding distance under water lubrication. And low friction coefficient and low wear rate could be achieved at high sliding velocity.     

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

A Comparative Investigation of the Tribological Behavior of Short Fiber Reinforced Polyimide Composites Under Dry Sliding and Water-Lubricated Condition

The friction and wear behavior of high performance polyimide (PI) and its composites reinforced with short cut fibers such as carbon fiber, glass fiber and quartz fiber was comparatively evaluated under dry sliding and water-lubricated condition, aiming at selecting matching materials for the pumps of pure water power transmission. The wear mechanisms of the composites under the two different sliding conditions were also comparatively discussed, based on scanning electron microscopic examination of the worn composite and steel counterpart surfaces. As the results, the PI composites reinforced with carbon fiber have the best mechanical and tribological properties compared with glass fiber and quartz fiber. PI composites sliding against stainless steel register lower friction coefficients and wear rates under water-lubricated condition than under dry sliding though the transfer of PI and its composites was considerably hindered in this case. PI and its composites are characterized by plastic deformation, micro cracking, and spalling under both dry-and water-lubricated sliding. Such plastic deformation, micro cracking, and spalling is significantly abated under water-lubricated condition. The glass and quart2 fibers were easily abraded and broken when sliding against steel in water environment, the broken fibers transferred to the mating metal surface and increase the surface roughness of mating stainless steel. This is probably the cause of the increased wear rate of glass fiber and quartz fiber PI composites in this case.     

Fiber–matrix adhesion from the single-fiber composite test: nucleation of interfacial debonding

The level of fiber–matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, it was suggested that an interfacial energy parameter might constitute a valid alternative. From an overview of the literature regarding the single-fiber composite fragmentation test, it appears that energy-based approaches have already been proposed in the past, but were either not successful, or not fully developed. Our recent energy balance scheme, proposed for the analysis of the initial interface debonding which occurs at fiber breaks during a fragmentation test, is presented and expanded here. The effects of thermal residual stress in the fiber, and of friction in the debonded area, are now incorporated in the energy balance model. We use a different shear-lag parameter proposed by Nayfeh, rather than the commonly used Cox parameter. New, extensive single-fiber fragmentation data regarding the interface crack initiation regime is presented, using sized and unsized E-glass fibers embedded in UV-curable or epoxy polymers. Some data for unsized carbon in epoxy is also presented. Fiber fragmentation is forced to take place entirely in the linear elastic region of the stress–strain curve, by means of pre-stressed single fibers. The importance of this procedure is discussed. Future work will focus on the interface crack propagation regime.     

炭纤维增强聚醚醚酮复合材料在水润滑下的摩擦学行为

考察了炭纤维及PTFE增强PEEK复合材料在干摩擦和水润滑下的摩擦学性能,并研究了该复合材料在两种条件下的磨损机理.结果表明,干摩擦下复合材料的摩擦系数和磨损率随负荷的增加不断减小;水润滑下复合材料的摩擦系数随负荷的变化不大,磨损率随负荷的增加而增大.干摩擦下,复合材料的磨损以粘着磨损和磨粒磨损为主.水润滑条件下,磨损表面比较光滑,仅有微切削的痕迹,磨损方式以轻微磨粒磨损为主.干摩擦条件下,摩擦对偶表面仅有轻微的犁沟形成,表面形成一层薄而均匀且结合紧密的转移膜.水润滑下,对偶表面犁沟较深,犁削作用明显,转移膜的形成被明显抑制.水的冷却作用使得向摩擦对偶的粘着转移明显减轻,同时由于摩擦表面吸附水膜的边界润滑作用,显著改善复合材料的摩擦磨损性能.     

Interface properties of carbon fiber/epoxy resin composite improved by supercritical water and oxygen in supercritical water

The interface properties between the carbon fibers and epoxy resin were improved by the supercritical water and oxygen in supercritical water because the surface roughness and the functional groups containing oxygen were increased obviously compared with that of untreated carbon fibers. This was favorable to improvement of the adhesive strength between the carbon fibers and the resin. The maximal IFSS of the carbon fiber/epoxy resin composite treated by the supercritical water and the oxygen in supercritical water were 58.3 and 92.8 MPa, respectively, which increased by approximately 10% and 75% for 52.9 MPa of the untreated carbon fiber/epoxy resin composite.     

A methodology to measure the interface shear strength by means of the fiber push-in test

A methodology is presented to measure the fiber/matrix interface shear strength in composites. The strategy is based on performing a fiber push-in test at the central fiber of highly-packed fiber clusters with hexagonal symmetry which are often found in unidirectional composites with a high volume fraction of fibers. The mechanics of this test was analyzed in detail by means of three-dimensional finite element simulations. In particular, the influence of different parameters (interface shear strength, toughness and friction as well as fiber longitudinal elastic modulus and curing stresses) on the critical load at the onset of debonding was established. From the results of the numerical simulations, a simple relationship between the critical load and the interface shear strength is proposed. The methodology was validated in an unidirectional C/epoxy composite and the advantages and limitations of the proposed methodology are indicated.     

土木工程应用中碳纤维/环氧树脂界面在环境影响下退化的分子模拟研究进展

在土木工程领域,碳纤维增强复合材料 (CFRP) 由于有着优异的力学性能而被越来越多地用在建筑结构中。碳纤维与环氧树脂之间粘结界面的性能对于 CFRP 内部应力的有效传递极为关键,并很大程度上决定了复合材料的长期耐久性能。然而,纤维/树脂粘结界面易受到湿热、盐雾及海水等恶劣环境的侵蚀,导致界面脱粘及最终的复材破坏。为了确保复材的长期耐久性能,需要全面认识界面在环境侵蚀下的退化行为。分子动力学模拟可以“自底向上”地描述界面在环境侵蚀下的行为,有利于探究界面的退化和失效机制。本文综述了不同环境因素影响下碳纤维/环氧树脂界面退化的分子模拟研究进展,包括界面模型的建立,界面在潮湿、盐雾等环境中结构、性能的退化及其背后的机制。最后,提出了未来界面退化的研究方向,例如纤维/树脂粘结界面模型的进一步完善。      

Modeling Strength Degradation of Fiber-Reinforced Ceramic-Matrix Composites Subjected to Cyclic Loading at Elevated Temperatures in Oxidative Environments

In this paper, the strength degradation of non-oxide and oxide/oxide fiber-reinforced ceramic-matrix composites (CMCs) subjected to cyclic loading at elevated temperatures in oxidative environments has been investigated. Considering damage mechanisms of matrix cracking, interface debonding, interface wear, interface oxidation and fibers fracture, the composite residual strength model has been established by combining the micro stress field of the damaged composites, the damage models, and the fracture criterion. The relationships between the composite residual strength, fatigue peak stress, interface debonding, fibers failure and cycle number have been established. The effects of peak stress level, initial and steady-state interface shear stress, fiber Weibull modulus and fiber strength, and testing temperature on the degradation of composite strength and fibers failure have been investigated. The evolution of residual strength versus cycle number curves of non-oxide and oxide/oxide CMCs under cyclic loading at elevated temperatures in oxidative environments have been predicted.     

Anisotropy of hygrothermal damage in fiber/polymer composites: Effective elasticity measures and estimates

Measured elasticity moduli of a highly (68%) glass-fiber reinforced epoxy matrix for different amounts of fiber/matrix interface weakening and debonding, due to different hygrothermal ageing stages, are compared to estimated ones. Ultrasonic measurements provide seven of the nine elasticity moduli of the orthotropic material samples, including all the moduli significantly affected by damage. Theoretical estimates combine homogenization modeling techniques and Finite Element (FE) calculations, the latter when the effect of observed partial debonding on effective moduli is to be specified. These estimates are performed under different assumptions for the composite structure, with special attention to the existence of a fiber–matrix interphase. Analytical comparisons for the undamaged composite establish that matching US measurements with estimates cannot be obtained, regardless of the chosen model, without the assumption of an interphase layer of modified resin coating the fibers. This coating resin, when in relevant concentration with regard to literature data about fiber coating thickness, typically conserves the epoxy moduli transversally to the fiber orientation, while, in the fiber direction its moduli approach those of the fibers. The comparison of the US measurements on damaged samples to FE calculations assuming progressive one-directional debonding shows that most of the composite stiffness loss can preferentially be due to an initial interphase weakening, while the fiber/matrix debonding seems more likely delayed to long H-ageing times. This is consistent with physical interpretation of damage by water pooling through silane bridges bonding epoxy to glass. The calculations also provide the effective stiffness, at different damage stages, of the “Undamaged Equivalent Inhomogeneity” for this damaged inclusion type.     

Novel wear-resistant materials - Carbon fiber reinforced low-carbon Stellite alloy composites

This paper reports the design and development of a class of new composite materials, which are low-carbon Stellite alloy matrices reinforced with carbon fibers. The focus of the research is to compare the different effects of carbon fibers versus carbides on Stellite alloys. Stellite 25 was selected as the matrix because of its very low carbon content (0.1 wt.%), thus minimal carbide volume fraction. The composite specimens are fabricated using the hot isostatic pressing and sintering techniques. The microstructures of the specimens are examined with optical microscopy in order to identify the possible carbide formation from the carbon fibers. The material characterization of the specimens is achieved through hardness test, sliding wear test and corrosion test. These novel materials exhibit superior properties compared to existing Stellite alloys and are expected to spawn a new generation of materials used for high temperature, severe corrosion, and wear resistant applications in various industries.     

Surface Damage Characteristics and Specific Wear Rates of a New Continuous Carbon Fiber (CF) / Polyetheretherketone (PEEK) Composite under Sliding and Rolling Contact Conditions

The surface damage characteristics of a continuous carbon fiber reinforced composite, having a polyetheretherketone (PEEK) matrix, were investigated under sliding and rolling contact. The corresponding mechanisms were studied by the use of scanning electron microscopy (SEM) in three different fiber orientations, namely: Paralllel direction to the fibers, AntiParallel direction to the fibers, and Normal direction to the fibers. All wear tests were conducted against smooth steel surfaces for both contact conditions. Mechanical properties under tension, compression, and shear were investigated for the material at two different temperature levels, i.e. room temperature and 150 °C. The composite material under normal fiber orientation has the lowest specific wear rate in case of rolling wear while the parallel orientation has the lowest specific wear rate under sliding wear conditions. Both results were compared to wear data of the neat PEEK matrix.     

Using silane-functionalized graphene oxides for enhancing the interfacial bonding strength of carbon/epoxy composites

Silane-functionalized graphene oxides (sGOs) were fabricated with four different self-assembled monolayers (SAMs) to reinforce an epoxy adhesive, with the aim of improving the bonding strength of carbon/epoxy composites. The oxygen-containing groups on the surface of graphene oxide (GO) were converted by the SAMs to amine, epoxy, or alkyl groups. The successful reaction between the silane molecules of the SAMs and functional groups of GO was evidenced by the results of different characterization methods such as Fourier transform infrared spectroscopy. It was found that the average thickness of the sGO flakes was higher than that of GO flakes. The bonding strength of a carbon fiber/epoxy composite, tested with a single lap joint bonded with an epoxy adhesive, was increased by 53% after the addition of a sGO that contained amine groups. These results show that sGOs, especially those containing amine functional groups, can strengthen the interfacial bonding between the carbon fibers and epoxy adhesive.