室温和高温下三维针刺炭/炭复合材料的弯曲性能及破坏机理

2016-10-31李典森姚倩倩

新型炭材料 2016年4期

李典森，　姚倩倩，　姜　楠，　江　雷

(1.北京航空航天大学化学与环境学院，仿生智能界面科学与技术教育部重点实验室，仿生能源材料与器件北京市重点实验室，北京100191；2.东华大学纤维材料改性国家重点实验室，上海201620；3.北京农学院国际学院，北京102206)

李典森1,2，姚倩倩1，姜楠3，江雷1

本文制备了高密度三维针刺炭/炭复合材料，研究了该材料在室温和高温下的弯曲性能，并从宏、细观角度研究了材料的变形与失效机理。结果表明，三维针刺炭/炭复合材料具有好的抗弯曲性能，400 ℃以下的载荷-挠度曲线呈线弹性和脆性破坏特征；而更高温度下的曲线表现出明显的韧性和塑性失效。由于氧化作用的加重，材料的弯曲性能随着温度的升高而显著减小。材料呈现锯齿状断裂特征，在500 ℃以下，主要的损伤形式表现为基体开裂，90°纤维/基体脱粘，0°纤维的局部扭曲和断裂；而在更高温度下，复合材料的氧化特征更加明显，纤维和基体间的界面粘结性能显著下降。

三维针刺炭/炭复合材料；高温；弯曲性能；失效机理

1　Introduction

Recently, three-dimensional (3D) composites have been developed largely by the advanced textile techniques of weaving, braiding, knitting and stitching to overcome the flaws of the conventional 2D laminates. In particular, 3D needle-punched composite is produced from woven fabrics and non-woven webs by through-thickness needling technique, which has an advantageous combination of high interlaminar properties and low cost processing. In recent years, the application of 3D needle-punched carbon/carbon (C/C) composites is growing rapidly in the fields of aerospace, marine and automobile industries[1-3].

Much effort has been spent on performance characterization and failure mechanism of the needle-punched C/C composites. For example, Li et al.[4]showed that the C/C composite produced from chopped fibers/resin carbon possessed a low tensile strength due to its poor microstructure. Yasuo et al.[5]examined the strength-controlling factors in the C/C composites and showed that tensile strength was sensitive to interfacial bonding strength. Hiroshi et al.[6]found that the C/C composite exhibited a high elastic modulus and a brittle fractural mode under tensile loading, but a low modulus and a large fracture strain under shear loading. Zhang et al.[7]studied static compressive behavior of the 3D needle-punched C/C composites and showed that transverse and longitudinal compressive strength with dual matrix were bigger than those of the composites with single matrix. Failure modes of the composites under transverse and longitudinal compressive loading were shear and extension, respectively. Yuan et al.[8]investigated the dynamic compressive fracture behavior of the composites. They found that the compressive strength and stiffness increased with the strain rate and the fiber failure was characterized by a multiple splitting without extensive debonding. Cai et al.[9]studied the bending properties of the 3D needled C/C composites and showed that flexure strength was 98 MPa. Luo et al.[10]investigated the effect of fiber architecture on flexural properties of the C/C composites. Their results showed that the composites with a plain cloth structure possessed the highest flexural strength and a pseudo-plastic fracture behavior. Chen et al.[11]studied the influence of the needle-punched felt structure on the flexural properties of the C/C composites. They found that flexural strength and modulus increased when the mass ratio of non-woven cloth to short-cut fiber web changed from 7∶3 to 6∶4. Hu et al.[12]presented the relationship between preform density and flexure properties of the C/C composites. In addition, Farhan et al.[13]examined the ablation behavior of the C/C composites with different fiber orientations and densities in the preform. While Mouritz et al.[14-18]thoroughly investigated the properties of a 3D z-pinned composite by modelling of internal geometry and characterization of mechanical properties to reveal the damage development and failure mechanisms during static and dynamic testing. However, little work has been done on the high temperature properties and the failure mechanism of the 3D needle-punched C/C composites. Recently, Li et al.[19]reported the high temperature compression properties and failure mechanism of these composites. Their results showed that compression properties decreased significantly with temperature due to material oxidation. Moreover, the composite exhibited a 45° shear fracture, and the local and plastic failure became more obvious after 600 ℃。

It is valuable to study the high temperature properties and failure mechanism of the 3D needle-punched C/C composites because its wide potential applications in engineering, especially in aerospace are at high temperature environments. In this paper, the 3D needle-punched C/C composites with a high density have been fabricated successfully. The high temperature bending properties of the composites were investigated experimentally. The damage and fracture morphology are observed from macroscopic and microscopic views and the failure mechanism is demonstrated. Furthermore, the influences of temperature on the properties and failure mechanism are also analyzed. The aim of our study is to assist in establishing the database and provide an experimental basis for the potential applications of the 3D needle-punched C/C composites in high temperature.

2　Experimental

2.1Materials and samples

A 3D needle-punched preform was prepared using T700, 12K carbon fiber non-woven cloths (450 g/m2) and fiber web (50 g/m2) by needle-punching in the through-thickness direction. Fig.1 shows the 3D needle-punching process. The density of as-fabricated preform in this study is about 1.15 g/cm3.

Fig. 1　The schematic of 3D needling process.

The 3D needle-punched preform was firstly heat-treated at 2 000 ℃ for 2 h, and then densified by chemical vapor deposition (CVD) at 1 300 ℃ under 1 kPa. Natural gas was used as the pyrocarbon precursor and H2as the carrier and diluent gas. The density of as-obtained 3D needle-punched C/C composite is 1.72 g/cm3. The composite was then cut into rectangles for bending tests at different temperatures. Fig. 2 shows that the photograph of the tested samples and their detail specifications are summarized in Table 1.

Fig. 2　Photograph of the bending sample.

2.2Characterization

The three-point bending tests at five temperatures (25, 300, 400, 500, 600 ℃) were conducted according to ASTM C1341 and GB/T l449-2005 standards on a high-temperature electronic testing machine (WDW-100). The dimensions of the samples are 38.0 mm (length) × 4.0 mm (width) × 3.0 mm (thickness) with a span length of 20 mm. At high temperatures, a box high temperature furnace (GWX-1200)and a guarded hot fixture have been developed. The furnace was controlled by the GWX-1200B control cabinet and heated by resistance wire to keep the high temperature during all experiments. The temperature was raised at a rate of 3 ℃/min. When the temperature reached the preset value, it was kept for 30 min and then the high-temperature bending properties of composites were measured. The deformation was measured by an extensometer directly fixed on the top of the connecting rod of the fixture. Through the furnace window, the damage evolution and failure of the composites could be observed in the process of the experiment. The crosshead speed was set at 0.5 mm/min. The load directions were perpendicular to the surface of the carbon felt or cloth. The applied load was released to zero when the test force reached up to 40 kN or the sample was fractured totally. Five samples were tested at each temperature and the average values of experimental results were obtained. The experimental procedure is shown schematically in Fig. 3.

Table 1　Details of the bending samples.

Fig. 3　The experimental procedure.

3　Results and discussion

3.1Bending curves at different temperatures

The typical load-deflection curves tested at different temperatures are illustrated in Fig. 4. It can be observed that the temperature has a significant impact on the load-deflection response. The curves go down gradually with increasing the temperature. All the curves increase linearly at the initial stage, but with different slopes. At room temperature (25 ℃), the curves show an obvious linear elastic feature up to failure. The material shows a brittle fracture characteristic. This is because the interface between matrix and the 0° fiber bundles and 90° fiber acts as the main load-bearing function, which can bear compression, tension and shear stress up to fracture. At 300 ℃, the slope of the curves reduces significantly due to the performance degradation of carbon fibers and matrix carbon. When the curve attains the peak stress, it declines rapidly and the material also shows a clear brittle feature. At 400℃, the curves keep the same trend as those in 25 ℃ and 300 ℃, and no clear yield phenomenon can be observed in the whole loading process. This is because the composites can bear loading as a whole due to the needle-punched structure. However, the curve climbs slower due to the further degradation of constituents’ properties. At 500 ℃, as the material oxidation reactions begin[20], the slope of curves reduces further. The curve increases linearly to the maximum load and drops sharply, it rises again after a short falling with a small slope. However, the second load has decreased about 40% of the maximum load. This indicates the material has been damaged, and it cannot withstand load effectively. At 600 ℃, the curve rises more slowly and shows a non-linear trend, the material exhibits an obvious plastic plateau and toughness fracture. This is mainly because the composite undergoes a severe oxidation, making the stiffness decrease significantly. Moreover, as the surface and internal oxidation are different, 0° fibers are fractured and 90° fiber/matrix interface is debonded gradually from the surface to interior under the coupling of compressive, shear and tensile stress, which causes a gradual damage of the material.

Fig. 4　Load vs. deflection curves at different temperatures.

3.2Bending strength and bending modulus

The bending strength and modulus of the composite tested at different temperatures are calculated according to the following formulate:

(1)

(2)

wherePis maximal bending load,Δpis loading increment,Δfis deflection increment,lis span length,bandhare sample width and thickness, respectively.

Fig. 5 shows the comparison of bending properties at different temperatures. It can be seen that both bending strength and modulus decrease with the temperature. The strengths are 200.58, 150.28, 137.42, 111.02 and 53.82 MPa, respectively, at 25, 300, 400, 500 and 600 ℃, and the decrements are 25.08%, 31.49%, 44.65%, and 73.17% with increasing temperature compared with that at 25 ℃. On the other hand, the decline for modulus with temperature is smaller than that of strength. The modulus tested at 25, 300, 400, 500 and 600 ℃ are 11.23, 9.14, 8.71, 8.26 and 6.72 GPa, respectively, and it decreases 18.61%, 22.44%, 26.45% and 40.16% with increasing temperature compared with that at 25 ℃. From Fig. 5, it can also be found that 500 ℃ is the key point, the decreasing of strength and modulus with the temperature is almost linear below 500 ℃ the decrement is more prominent above 500 ℃. So, the temperature is an important factor affecting the bending properties of the 3D needle-punched C/C composite, and the bending properties decrease significantly with the temperatures. The reason is that at higher temperature, the performance of carbon fiber and matrix carbon may be degraded due to the oxidation, especially when the temperature is above 500 ℃, the materials are oxidized more severely[21]. Moreover, at higher temperature, the interfacial bond strength between fiber and matrix is weakened, causing a decrease in load-carrying capacity. On one hand, 90° fiber/matrix interface debonding occurs more easily due to a coupling between compression and shear stress. At the same time, the 0° fibers are more prone to be broken due to a coupling between tension and shear stress. On the other hand, as the matrix generates shear failure more easily, the fiber layer delamination is more serious. Therefore, the bending properties of the are detoriateds significantly with increasing temperature.

Fig. 5　Comparison of the bending strength and modulus at different temperatures.

3.3Failure mechanism at high temperatures

The fracture morphology is observed by photos and with Quanta 250 FEG Environmental Scanning Electron Microscope (ESEM) to investigate the damage and failure mechanisms. Fig. 6 shows the fracture photographs of the composite at different temperatures. Fig. 7 shows SEM photographs at different temperatures. It is found that the damage and failure morphology are different at different temperatures.

It is known that under bending load, the upper surface of the composite bears compressive stress, which is defined as compression surface, the bottom surface of the composite bears tensile stress, which is defined as tension surface. From Fig. 6, it can be found that all the samples are broken into two segments. The fracture surface is irregular, and shows an obvious zigzag fracture, which indicates that the material is subjected to a coupling effect from tensile, compression, bending and shear stress. Through lateral observation, no obvious interlayer delaminating can be found. Through-thickness needle-punched yarns enhance the delamination resistance of the composite significantly. At 25 ℃, the compression and tension surface is smooth and samples exhibit local damage and a typical brittle fracture(Fig. 6(a)). At 300 ℃, the roughness of the fracture surface increases and some fragments can be found. Moreover, the fracture is uneven. This is because the matrix cracks due to thermal stress and the micro-cracks expand along weak fiber/matrix interface where the energy spreads easily (Fig. 6(b)). At 400 ℃, the damage areas on the fracture surface are clearly visible due to the oxidation and a local shear fracture is distinguished (Fig. 6(c)). With the temperature increases further (500 ℃), from Fig. 6(d), it can be found the surface of the composite becomes brown due to an severe oxidation. Moreover, shear fracture occurs on compression surface while the fracture on tension surface is flush. At 600 ℃, the composite is oxidized to red. Lots of ablated holes occur on the surface and the fibers are pulled out (Fig. 6(e)). These indicate that the oxidation is dominant above 500 ℃, resulting in the loss of mechanical properties.

Fig. 6　Fracture photographs of the bending samples at different temperatures:

By further SEM observations, it is found that the bonding of carbon fiber/matrix carbon interfaces is strong at room temperature, and a number of fibers are fractured in brittle and pulled out (Fig. 7(a,b)). This is because the stress at the interface is not relaxed, when the crack propagates to this area, the stress concentration makes it penetrate through the fibers directly. At 300 ℃, the fiber/matrix interface shows damage which decreases the mechanical properties of the composite. At the same time, carbon fibers are fractured, which is bonded closely with matrix carbon (Fig. 7(c,d)). At 400 ℃, the fiber breakage is very smooth and also characterized as a brittle fracture as shown in Fig. 7(e,f). However, the interface debonding can be observed due to the oxidation of carbon fibers and matrix carbon. Meanwhile, the fibers exhibit a shear fracture and the matrix between the fibers also generates a significant shear deformation due to shear stress. At 500 ℃, the microstructure changes more obviously, a serious oxidation and cracking is visible and the interface debonding is serious. Moreover, carbon fiber has experienced an oxidation, leaving obvious oxidation ablation holes (Fig. 7(g,h)). Therefore, at this temperature, the cracks propagate through matrix cracking, fiber/matrix debonding, fiber breakage and pulling-out. And the energy consumption in this process prevents the brittle fracture and the composite shows certain toughness. At 600 ℃, more severe oxidation occurs on the fibers and matrix and a large amounts of ablated pores are distributed in the matrix, which exhibits a mesh structure (Fig. 7(i)). Fiber/matrix interface debonding is more severe, and carbon fibers become thinner and their diameter is reduced (Fig. 7(j)). These also verify that the oxidation is dominant at this temperature, which reveals a significant plastic failure behavior.

Fig. 7　SEM photographs of bending fracture at different temperatures:

Therefore, when the bending load is applied on the composite to a certain extent, the damage of compression surface occurs accompanied with local cracking of matrix carbon, fiber/matrix interface debonding for 90° fiber layers, as well as bending deformation of 0° fibers. As the load increases, on tension surface, matrix is also damaged, 90° fiber/matrix interface is debonded and 0° fibers are fractured in a brittle mode due to the tensile stress. Cracks in the thickness direction expand along the easiest path of energy diffusion and the propagation is a gradual process from both the external layers on compression and tension surface to interior layers. At room temperature, the fiber/matrix interface is well bonded, which effectively hinders the crack propagation. When the cracks penetrate through the thickness, the composite is fractured in brittle completely. In this case, the fibers are the main load-bearing object which provides a high mechanical strength. The main damage of the composite is in the form of matrix cracking, 90° fiber/matrix debonding, local twisting and breakage of 0° fibers. As the temperature increases, the properties of fibers and matrix are degraded and micro-cracks can take place easily due to mismatching of thermal-stress between fibers and matrix. At the same time, the interface becomes weakened due to material oxidation, especially above 500 ℃. The weakened interface makes fibers debonding easily from the matrix, which is mainly manifested by debonding between 90° fiber layers, easy fracture of 0° fibers and delaminating between fiber layers. All of these result in the significant losses of properties at high temperatures. The main damage is in the form of oxidation for fibers and matrix, fiber/matrix interface debonding, delaminating and fiber fracture.

4　Conclusions

A 3D needle-punched C/C composite with a high density has been fabricated successfully. The bending properties of the composite are studied at room and elevated temperatures. The results show the load-deflection curves below 400 ℃ have an obvious linear elasticity and brittle fracture feature, while the curves at high temperature show an obvious toughness and plasticity failure. Moreover, the temperature is an important factor that affects the bending properties that decrease with the temperature. Especially, above 500 ℃, the properties are decreased significantly due to material oxidation.

The macro- and micro-fracture morphology examinations indicate that the damage and failure patterns vary with temperature. The composite shows an obvious zigzag fracture and the crack propagation is a gradual process from both external layers on compression and tension surface to the interior layers. Below 500 ℃, the main damage is in the form of matrix cracking, 90° fiber/matrix debonding, local twisting and breakage of 0° fibers. At the temperatures above 500 ℃, the oxidation is dominated and the interfacial adhesion between fibers and matrix is decreased significantly, which results in low mechanical properties. The main damage is in the form of oxidation of fibers and matrix, fiber/matrix interface debonding, delaminating and fiber fracture.

[1]Rao M V, Mahajan P, Mittal R K. Effect of architecture on mechanical properties of carbon/carbon composites[J]. Compos Struct, 2008, 83(2): 131-142.

[2]Beier U, Fischer F, Sandler J K W, et al. Mechanical performance of carbon fibre-reinforced composites based on stitched preforms[J]. Compos Part A, 2007, 38(7): 1655-1663.

[3]Ansar M, Wang X W, Zhou C W. Modeling strategies of 3D woven composites: A review[J]. Compos Struct, 2011, 93(8): 1947-1963.

[4]Li J S, Luo RY, Li Q. Study of the infiltration and mechanical behavior of C/C composites prepared by ECV I[J]. Mater Sci Eng A, 2008, 480(1-2): 278-282.

[5]Kogo Y, Sumiya R, Hatta H, et al. Examination of strength-controlling factors in C/C composites using bundle composites[J]. Adv Compos Mater, 2003,12(2-3): 139-154.

[6]Hiroshi H, Keisuke T, Yasuo K. Compressive strength of three-dimensionally reinforced carbon/carbon composite[J]. Carbon, 2005, 43(2): 351-358.

[7]Zhang J C, Luo R Y, Xiang Q. Compressive fracture behavior of 3D needle-punched carbon/carbon composites[J]. Mater Sci Eng A, 2011, 528(15): 5002-5006.

[8]Yuan Q L, Li Y L, Li H J. Quasi-static and dynamic compressive fracture behavior of carbon/carbon composites[J]. Carbon, 2008, 46(4): 699-703.

[9]Cai Y, Fan S, Yin X. Microstructures and mechanical properties of three-dimensional ceramic filler modified carbon/carbon composites[J]. Ceram Int, 2014, 40(1): 399-408.

[10]Luo R, Yang C, Cheng J. Effect of preform architecture on the mechanical properties of 2D C/C composites prepared using rapid directional diffused CVI processes[J]. Carbon, 2002, 40(12): 2221-2228.

[11]Chen T, Liao J, Liu G. Effects of needle-punched felt structure on the mechanical properties of carbon/carbon composites[J]. Carbon, 2003, 41(5): 993-999.

[12]Hu Y J, Luo R Y and Zhang Y F. Effect of preform density on densification rate and mechanical properties of carbon/carbon composites[J]. Mater Sci Eng A, 2010, 527(3): 797-801.

[13]Shameel F, Li K Z, Guo L J, et al. Effect of density and fibre orientation on the ablation behaviour of carbon-carbon composites[J]. New Carbon Materials, 2010, 25(3): 161-167.

(Shameel F, Li K Z, Guo L J, 等. 密度和纤维取向对炭/炭复合材料烧蚀性能的影响[J]. 新型炭材料, 2010, 25(3): 161-167. )

[14]Mouritz A P. Compression properties of z-pinned composite laminates[J]. Compos Sci Technol, 2007, 67(15): 3110-3120.

[15]Chang P, Mouritz A P, Cox B N. Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension[J]. Compos Part A, 2006, 37(10): 1501-1513.

[16]Mouritz A P, Cox B N. A mechanistic interpretation of the comparative in-plane mechanical properties of 3D woven, stitched and pinned composites[J]. Compos Part A, 2010, 41(6): 709-728.

[17]Isa M D, Feih S,Mouritz A P. Compression fatigue properties of z-pinned quasi-isotropic carbon/epoxy laminate with barely visible impact damage[J].Compos Struct, 2011, 93(9): 2269-2276.

[18]Nanayakkara A, Feih S, Mouritz A P. Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites[J]. Compos Part A, 2011, 42(11): 1673-1680.

[19]Li D S, Luo G,Yao Q Q, et al. High temperature compression properties and failure mechanism of 3D needle-punched carbon/carbon composites[J]. Mater Sci Eng A, 2015, 621(3): 105-110.

[20]Zhang C, Yan K, Qiao S, et al. Effect of oxidation on fracture toughness of a carbon/carbon composite[J]. Journal of Wuhan University of Technology-Mater Sci Ed, 2012, 27(5): 944-947.

[21]Sauder C, Lamon J, Pailler R. Thermomechanical properties of carbon fibres at high temperatures (up to 2000 ℃) [J]. Compos Sci Technol, 2002, 62(4): 499-504.

Bend properties and failure mechanism of a carbon/carbon composite with a 3D needle-punched preform at room and high temperatures

LI Dian-sen1,2,YAO Qian-qian1,JIANG Nan3,JIANG Lei1

(1.KeyLaboratoryofBio-InspiredSmartInterfacialScienceandTechnologyofMinistryofEducation,BeijingKeyLaboratoryofBio-InspiredEnergyMaterialsandDevices,SchoolofChemistryandEnvironment,BeijingUniversityofAeronauticsandAstronautics,Beijing100191,China;2.StateKeyLaboratoryforModificationofChemicalFibersandPolymerMaterials，DonghuaUniversity，Shanghai201620，China;3.InternationalCollege,BeijingUniversityofAgriculture,Beijing102206,China)

A 3D needle-punched C/C composite with a high density was fabricated and its bend properties were investigated at room and high temperatures. Macro-fracture and SEM micrographs were examined to understand the deformation and failure mechanism. Results show that the load-deflection curves below 400 ℃ exhibit a linear elastic and brittle fracture failure, while the curves at temperatures above 500 ℃ show an obvious tough and plastic failure. The bend strength and modulus decrease significantly with increasing temperature due to severe carbon oxidation. Below 500 ℃, the main damage to the composite is in the form of matrix cracking, 90° fiber/matrix debonding, local twisting and fracture of the 0° fibers. Above 500 ℃, the oxidation of the composite is significant and the interfacial adhesion between fibers and matrix is decreased significantly.

3D needle-punched C/C composites; High temperature; Bending properties; Failure mechanism

National Natural Science Foundation of China (11272001，11522216); Foundation of State Key Laboratory of Explosion Science and Technology (KFJJ15-21M); Foundation of State Key Laboratory of Structural Analysis for Industrial Equipment (GZ15213); Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF14B14); Foundation of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK0904); Foundation of State KeyLaboratory of High Performance Ceramics and Superfine Micro-structure (SKL201304SIC); Fundamental Research Funds for the Central Universities (YWF-15-HHXY-004).

LI Dian-sen. E-mail: lidiansen@buaa.edu.cn; JIANG Nan. E-mail: jiangnan@bua.edu.cn

date: 2016-06-02;Reviseddate: 2016-07-29

10.1016/S1872-5805(16)60023-9

1007-8827(2016)04-0437-08

TQ342+.74

国家自然科学基金(11272001，11522216)；爆炸科学与技术国家重点实验室基金(KFJJ15-21M)；工业装备结构分析国家重点实验室基金(GZ15213)；摩擦学国家重点实验室基金(SKLTKF14B14)；纤维材料改性国家重点实验室基金(LK0904)；高性能陶瓷和超微结构国家重点实验室基金(SKL201304SIC)；中央高校基本科研业务费专项基金(YWF-15-HHXY-004).

李典森. E-mail: lidiansen@buaa.edu.cn; 姜楠. E-mail: jiangnan@bua.edu.cn

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).