Effect of needle-punched preform parameters on the mechanical properties of carbon/carbon composites

ZHENG Jin-huang,LI He-jun,CUI Hong,,ZHANG Xiao-hu,DENG Hai-liang

(1.State Key Laboratory of Solidification Processing,Northwestern Polytechnical University,Xi'an 710072,China;2.Xi'an Aerospace Composite Research Institute, Xi'an 710025, China.)

Effect of needle-punched preform parameters on the mechanical properties of carbon/carbon composites

ZHENG Jin-huang1,LI He-jun1,CUI Hong1,2,ZHANG Xiao-hu2,DENG Hai-liang2

(1.State Key Laboratory of Solidification Processing,Northwestern Polytechnical University,Xi'an 710072,China;2.Xi'an Aerospace Composite Research Institute, Xi'an 710025, China.)

Carbon fiber preforms were prepared and then densified to produce carbon/carbon composites using needle-punching and chemical vapor infiltration technique,respectively.The effects of the preform manufacturing parameters,including the needle-punched density and its depth,and the mass-to-area (M/A) ratio of short-cut fiber felt,on the mechanical properties of the composites were investigated.The relation between the mechanical properties of the composites and the bulk density of the preforms was also discussed.Results show that the tensile strength of the composites increase and thereafter decrease as the needle-punched density increased from 20 to 50 pin/cm2,while the interlaminar shear strength of the composites tends to increase.The tensile and interlaminar shear strengths exhibit an increasing tendency with the increase of needle-punched depth from 10 to 16 mm,whereas they reduce as theM/Aratio of short-cut fiber felt is increased from 100 to 300 g/m2.Both the tensile and interlaminar shear strengths are dominated by the preform density when only one parameter of the needle-punched density and its depth,and theM/Aratio was varied.The preform density could be employed as a key macro-parameter to predict the mechanical properties of the composites.

carbon/carbon composites;preform;needle-punched parameters;mechanical properties

0 Introduction

Carbon/carbon (C/C) composites have been developed to adapt the high performance requirements for the rocket engines of strategic missiles[1-4].In addition,the composites become very well suited for aerospace applications to provide some advantages in performance,efficiency,and affordability.

Needle-punched carbon fiber preform is the most promising reinforcement of C/C composites for the aerospace applications,due to the high designable character,good structural integrity,and high interlaminar shear strength[5-6].Moreover,the low density,high thermal diffusivity and small coefficient of thermal expansion combined with the high temperature capability providing advantages to C/C composites used as the nozzle exit cone or extension of rocket engines[7-10].

The manufacturing parameters, such as the needle-punched density and its depth,and the mass-to-area (M/A) ratio of short-cut fiber felt,are dominant factors for the mechanical properties of the preforms and C/C composites[11-13].In this work,the effects of these manufacturing parameters on the mechanical properties of C/C composites are investigated.The relation between the mechanical properties of the composites and the preform density is also analyzed.

1 Experimental

1.1 Preparation of preform and C/C composites

Carbon fiber preforms were prepared by alternate lamination of polyacrylonitrile based carbon fiber cloth and short-cut fiber felts with a stepwise needle-punching.The schematic of this needle-punching process is shown in Fig.1.Short-cut fiber felt is set on carbon fiber cloth to form dual-layered structure.Hook needles are used to punch the short-cut fibers through the fabric layers when the dual-layered structures are laminated.Thus a three-dimensional preform is formed.In this process,the needle-punched density and depth is varied in the ranging 20～50 pin/cm2and 10～16 mm respectively.TheM/Aratio of short-cut fiber felt are increased from 100 to 400 g/m2.The preforms are heat-treated at 1 800 ℃ for 2 h and thereafter densified using isothermal chemical vapor infiltration (ICVI) at the deposition temperature of 940 ℃ under a pressure 4 kPa.Propylene (C3H6) gas is used as the carbon matrix precursor and N2is adopted as a carrier gas.The final bulk density of C/C composites is 1.60～1.65 g/cm3.

1.2 Mechanical test of C/C composite

The samples were machined from C/C plate reinforced by needle preform made from carbon fiber cloth with 2×2 twill.Tensile and interlaminar shear tests were performed in an INSTRON universal testing machine to evaluate the mechanical properties of the composites.The sample dimensions for the tensile and interlaminar shear modes areφ6 mm × 52 mm and 15 mm × 15 mm × 20 mm,respectively.More than five samples of the composites prepared from different preforms were tested in these two modes.The morphology of the preforms was observed using a scanning electron microscope (SEM).

2 Results and discussion

2.1 Structure of needle-punched preform

Integral structure of needle-punched perform is formed by needling with fiber cloth and fiber felt layer alternately.By Hook needles,the fiber in plane of fiber felt are transformed toZ-direction bundles,whoseZ-direction fibers with the certain length is linked with carbon cloth layer and felt layer tightly by twisting each other betweenZ-direction fiber bundles and short-cut fiber felts.Carbon cloth and short-cut fiber felts are restricted each other to form a three dimensional net structure like “pin” structure,which increases the integrity of preform.The SEM micrographs of the needle-punched preform is shown in Fig.2.The carbon fiber cloth and short-cut fiber felts are linked byZ-direction fiber bundles which can provide an excellent interlaminar strength on the preforms.However,many long fibers are inevitably damnified in the interbundle and intrabundle positions of the carbon cloth during the needle-punching (Fig.3).

These fractured long fibers unfavor the plane strength of the preforms. Hence,the balance between the interlaminar strength and the fiber damnification in the plane direction is very important for optimizing the mechanical properties of the preforms and the composites.

2.2 Mechanical properties of C/C composites

2.2.1 Effect of the needle-punched density

The relation between the preform needle-punched density and the mechanical properties of the composites is shown in Fig.4.The tensile strength increases from 51.33 to 99.77 MPa as the needle-punched density rises from 20 to 40 pin/cm2,and then decreases with a further increase to 50 pin/cm2.The interlaminar shear strength of the composites is increased to 24.94 MPa at the needle-punched density of 50 pin/cm2,which is improved by 191.02 % compared with the interlaminar shear strength of 8.57 MPa at 20 pin/cm2.

The fiber volume fractions under different preform manufacturing parameters are shown in Fig.5.It can be seen that the change of fiber volume fraction with needle-punched density is similar to that of the tensile strength.The higher fiber volume fraction is obtained at the needle-punched density 40 pin/cm2,which is advantageous to improve the tensile strength of the composites.However,the number of the pinholes in the preform increase at the higher needle-punched density of 50 pin/cm2, leading to a decrease of the fiber volume fraction.Moreover,the number of fractured fibers in the plane direction of the preforms increases during the needle-punching.Therefore the tensile strength of the composites reduces with a further increase of the needle-punched density to 50 pin/cm2.It is well known that the number of theZ-direction fibers increases as the needle-punched density rises.The spacing between the carbon fiber cloth and the short-cut fiber felts is compacted at a higher needle-punched density.The higher needle-punched density could also intensity the interlaced degree of carbon fibers.Consequently,the interlaminar shear strength is improved with the increasing needle-punched density.

2.2.2 Effect of the needle-punched depth

The mechanical properties of the composites densified from the preforms with different needle-punched depths are shown in Fig.6.The relation between the mechanical strength and the needle-punched depth are approximatively linear.The tensile and interlaminar shear strengths reach higher values of 118.25 MPa and 20.43 MPa at the needle-punched depth about 16 mm,respectively.These results suggest that the needle-punched depth is an effective parameter for adjusting the mechanical properties of the composites compared with the needle-punched density.

From Fig.5(b),the fiber volume fraction of the preform increases with the increasing needle-punched depth, which improves the mechanical properties of the composites.Moreover,the effect of needle-punched depth on the plane fiber damnification is mainly dominated by the configuration of needle barb.

At a certain value of the needle-punching depth,its influence on the damnification of long fibers is weakened.Therefore, the tensile strength of the composites is enhanced with the increasing needle-punched depth from 10 to 16 mm.The lengths ofZ-direction fiber bundles induced by the needle barbs (the barb positions shown in Fig.1)is shown in Fig.7.The length of short-cut fibers is about 70 mm which is larger than the needle-punched depths.Thus,the length and number of theZ-direction fiber bundles are obviously increased by the increasing needle-punched depth. Additionally,the interlaced degree of carbon fibers between the fiber cloth and short-cut fiber felts is enhanced.Consequently,the interlaminar shear strength of the composites is improved by the increasing needle-punched depth.

2.2.3 Effect of the mass-to-area ratio of short-cut fiber felt

The mechanical properties of the composites prepared from the preforms with differentM/Aratios of short-cut fiber felts is shown in Fig.8.The tensile strength decreases from 119.0 MPa at theM/Aratio of 100 g/m2to 52.4 MPa at 300 g/m2,and then increases to 57.5 MPa as theM/Aratio increases to 400 g/m2.The variation of interlaminar shear strength is similar to that of the tensile strength. The higher and lower values are 18.2 and 13.0 MPa which are obtained at theM/Aratio of 100 and 300 g/m2, respectively.

The fiber volume fraction of the preforms decreases with the increase of theM/Aratio (Fig.5(c)).The volume fraction of long fiber cloth with high tensile strength decreases at a higherM/Aratio under the same thickness of the preforms.Therefore,the tensile strength tends to decline with the increasingM/Aratio of the short-cut fiber felt. However,the tensile strength of the fiber felts is enhanced by the increasingM/Aratio,which restricts a further decline of the mechanical properties.This is an important reason that the tensile strength of the composites at 400 g/m2is slightly higher than that at 300 g/m2.

For the lowerM/Aratio at 100 g/m2,the thickness of the dual-layered structure composed of fiber cloth and felt is small.In this case,the dual-layered structure has high punching probability under the same preform thickness and needle-punched depth. For the higherM/Aratio of 400 g/m2,more fibers can be introduced intoZdirection of the preform for every punching.Therefore the volume fraction ofZ-direction fiber bundles is high,leading to the higher interlaminar shear strength of the composites.

2.3 Relation between the preform density and the mechanical properties of the composites

Bulk density of the preforms is readily varied by the manufacturing parameters including the needle-punched density,the needle-punched depth,and theM/Aratio of fiber felts.Hence,the preform density can be used as a macro-parameter to predict the mechanical properties of the composites. In order to study the relations between the preform density and the mechanical properties of the composites,Vary parameters are designed.when only needle-punched density is varied from 20 to 50 pin/cm2,the needle-punched depth is 12 mm and theM/Aratios of felt is 200 g/m2.when only needle-punched depth is varied from 10 to 16 mm,the needle-punched density is 30 pin/cm2and theM/Aratios of felt is 200 g/m2.when only theM/Aratios of felt is varied from 100 to 400 g/m2,the needle-punched density is 30 pin/cm2and the needle-punched depth is 12 mm.

The mechanical properties of the composites as functions of the preform density at different preform manufacturing parameters are shown in Fig.9.The preform density impacts remarkably on the tensile and interlaminar shear strengths of the composites.Based on the variations of the mechanical properties,the relation between the strength and the preform density is fitted using origin software.The mechanical properties of the composites have parabolic shapes with the preform density,except that the interlaminar shear strength at different needle-punched depths and the tensile strength at different needle-punched densities exhibit a nearly linear increase.

These fitted curves under different preform manufacturing parameters follow Equ.(1).Obviously,the preform density can be applied as a key macro-parameter to predict approximatively the mechanical properties of the composites when only one of these manufacturing parameters is varied.

Y=A+BX+CX2

(1)

whereYis the mechanical strength of the composites,Xis the preform density,A,BandCare constants which are listed in Table 1.

3 Conclusions

The tensile strength of the composites increases and thereafter decreases as the needle-punched density increases from 20 to 50 pin/cm2, while the interlaminar shear strength tends to increase with the needle-punched density.Both the tensile and interlaminar shear strengths increase with the increasing needle-punched depth from 10 to 16 mm,whereas they tend to decrease as theM/Aratio increases to 300 g/m2.The variations of the mechanical strengths with preform parameters are attributed to the fiber volume fraction and the fiber damnification by needle-punching.The tensile and interlaminar shear strengths are mainly dominated by the bulk density of the preforms in the case that only one of the manufacturing parameters is varied.The preform density can be employed as a key macro-parameter for predicting the mechanical properties of the composites.

ManufacturingparameterStrength/MPaY=A+BX+CX2ABCNeedle⁃puncheddensity/(pin·cm-2)Needle⁃puncheddepth/mmM/Aratiooffelt/(g·m-2)Tensile712．60-2659．612681．11Shear-35．8488．930Tensile-152．16468．39-80．59Shear-1570．936029．45-5702．58Tensile905．23-3530．533648．01Shear161．13-572．89552．14

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针刺预制体参数对C/C复合材料力学性能的影响

郑金煌1，李贺军1，崔 红2，张晓虎2，邓海亮2

(1.西北工业大学 凝固技术国家重点实验室，西安 710072；2.西安航天复合材料研究所，西安 710025)

通过针刺与化学气相沉积分别制备碳纤维预制体与碳基体，获得针刺C/C复合材料。研究了针刺密度、针刺深度、网胎面密度等预制体成型工艺参数对C/C复合材料力学性能的影响，并探讨了预制体体积密度与C/C复合材料力学性能关联关系。结果表明，针刺密度在20～50针/cm2之间时，C/C复合材料拉伸强度先增后减，而层间剪切强度一直上升；针刺深度在10～16 mm之间时，拉伸强度和层间剪切强度随针刺深度的提高而增加；网胎面密度在100～300 g/m2之间时，拉伸强度和层间剪切强度随网胎面密度的提高而降低；当只改变针刺密度、针刺深度、网胎面密度其中一个成型参数时，拉伸强度和层间剪切强度受预制体密度影响显著，预制体密度可作为预测C/C复合材料力学性能的一个宏观成型参数。

C/C复合材料；针刺参数；针刺预制体；力学性能

V258 Document Code：A Article ID：1006-2793(2017)02-0221-07

10.7673/j.issn.1006-2793.2017.02.016

Received date：2016-05-02；Revised date：2017-01-19。

Foundation：National Natural Science Foundation of China(51202233);Equipment Advanced Research Foundation of China(9140A12060514HT43190)

Biography：ZHENG Jin-huang(1973—),male,doctor,Speciality:research on properties of C/C,C/C-SiC composites,refractory metal modified high temperature composites.E-mail：zjh114@sohu.com