1、文章编号: 1007- 8827( 2014) 06- 0444- 10纤维种类对炭/炭复合材料微观结构和力学性能的影响郝名扬， 罗瑞盈， 向巧， 侯振华， 杨威， 商海东( 北京航空航天大学 物理科学与核能工程学院，北京 100191)摘要: 采用 6K 的预氧丝和炭纤维制备预制体， 通过化学气相渗积制备炭/炭复合材料。通过偏光显微镜、 拉曼光谱、 纳米硬度和三点弯曲等手段研究其微观结构和力学性能。结果表明， 预氧丝复合材料的基体为暗层和粗糙层炭， 厚度分别为 1 4-2 6m和 10 2- 11 6m; 而炭纤维复合材料的基体为光滑层和粗糙层炭， 厚度分别为 8m 和 4 4m; 预氧丝纤
2、维的模量和硬度明显小于炭纤维， 同时基体的模量和硬度随消光角的增加而降低; 低模量的基体和纤维导致预氧丝复合材料的拉伸强度、拉伸模量、 弯曲强度和模量分别降低了 145%- 242%、 9 7%- 19 8%、 7 3%- 15 4%和 15 1%- 18 6%; 但其韧性指数却提高了224%- 235%， 这是高含量的粗糙层炭和纤维的石墨化收缩所致; 同时提出了一个三单元复合模型用来模拟复合材料的拉伸模量， 模拟误差小于 9 9%。关键词: 炭/炭复合材料; 微观结构; 力学性能; 化学气相渗积中图分类号:TQ342+ 76文献标识码:A收稿日期: 2014- 05- 23; 修回日期: 2
3、014- 12- 08基金项目: 国家自然科学基金( 21071011) 通讯作者: 罗瑞盈， 教授 E- mail:ryluo buaa edu cn作者简介: 郝名扬， 博士研究生 E- mail:haomingyangniat163 comEffects of fiber- type on the microstructure andmechanical properties of carbon/carbon compositesHAO Ming- yang， LUO ui- ying， XIANG Qiao，HOU Zhen- hua， YANG Wei， SHANG Hai- don
4、g( School of physics and Nuclear Energy Engineering，Beijing University of Aeronautics and Astronautics，Beijing100191，China)Abstract:Two carbonized oxidized polyacrylonitrile fiber ( OPF)felts and one polyacrylonitrile- based carbon fiber ( CF)feltwere used as preforms to prepare two kinds of carbon/
5、carbon composites by chemical vapor infiltration， and the effect of fiber typeon the microstructure and mechanical properties of the composites were investigated The microstructure was characterized bypolar-ized light microscopy and aman spectroscopy and the mechanical properties were characterized
6、by nanoindentation and three- pointbend tests The two carbonized OPFs are surrounded by a darklaminar layer about 1 4- 2 6 m thick followed by a rough laminarlayer of about 10 2- 116 m， while the CFs are surrounded by a smooth laminar layer about 8 8 m thick and arough laminar layerof about 4 4 m Na
7、noindentation indicates that the modulus and hardness of the carbonized OPFsare obviously lower than those ofthe CFs，and the modulus and hardness of the matrix decrease with increasing extinction angle The low modulus of the matrix andthe OPFsresult in a decrease of the tensile and flexural strength
8、 by about 14 5%- 24 2% and 7 3%- 15 4% and a decrease of thetensile and flexural modulus by about 9 7%- 19 8% and 15 1%- 18 6%，respectively，for the OPF- derived composites comparedwith the CF- derived composites However，for the OPF- derived composites the ductility factor increases by about 224%- 23
9、5% be-cause of the high content of rough laminarcarbon and the obvious shrinkage of the OPFs after graphitization Meanwhile，a modelin-volving the three components in the composites is proposed to predict their tensile modulus，which shows deviations between experi-mental and predicted results below 9
10、9%Keywords: Carbon/Carbon composites;Microstructure;Mechanical properties;Chemical vapor infiltrationFoundation item:National Natural Science Foundation of China ( 21071011)Corresponding author:LUO ui- ying，Professor E- mail:ryluo buaa edu cnAuthor introduction:HAO Ming- yang，Ph D E- mail:haomingyan
11、gniat163 comEnglish edition available online ScienceDirect (http: www sciencedirect comsciencejournal18725805 ) DOI: 10 1016/S1872- 5805( 14) 60149- 9第 29 卷第 6 期2014 年 12 月新型炭材料NEW CABON MATEIALSVol 29No 6Dec 20141IntroductionCarbon/carbon ( C /C ) composites are widelyused for structural and fricti
12、onal applications in aero-nautic and space industries，as well as brake materialfor high speed vehicles owing to their high specificstrength，stiffness and toughness，self- lubricating ca-pability，low thermal expansion coefficient and out-standing refractory properties1- 3 Chemical vapor in-filtration
13、( CVI)of carbon fiber ( CF)preforms is theaccepted process for mass production of the C /C com-posites used in the aircraft brake industry4 But thehigh cost largely limits their civil applications such asbuilding materials，sport accessories and biologicalprostheses because of the expensive high modu
14、lus CFsand the long densification time5 A simple and ef-fective way to cut the cost is to substitute the highmodulus CFs by the cheap oxidized polyacrylonitrilefibers ( OPFs)obtained by oxidation of polyacryloni-trile fibers in a temperature range of 200- 300 inair6 Moreover，Jia et al7 reported that
15、 the elonga-tion to break of the OPFs is almost four times as highas that of the CFs，which is advantageous for wea-ving Manocha et al8 and Ko et al9 have used OPFco- carbonization with resins to make the C /C com-posites with acceptable mechanical properties Chenet al10 have studied the mechanical p
16、roperties of theC /C composites from the carbonized OPF preformswith CVI/resin carbon hybrid matrix and found thatthe flexural strength ( 100- 115 MPa)was low More-over，Su et al11 have used the CF cloth and the OPFfelt alternately needled as preforms to prepare the C /Ccomposites with remarkable abl
17、ative behaviors How-ever，the fiber- types，which are important to deter-mine the microstructure and mechanical properties ofcomposites，have hardly been taken into account informer studies， and the essential information related totheir effects on the microstructure and mechanicalproperties is relative
18、ly rareThe current work is to compare the microstructureand mechanical properties of the C/C composites pre-pared from carbonized OPF felts with those from CFfelts Because the carbonization of the OPFs has beenhighlighted by previous researchers 12 ，this process isnot discussed and taken into accoun
19、t here So，theproperties of the OPFs after carbonization are only mo-nitored and used directly to compare with that of CFs2Experimental2 1Material preparationNeedle- punched integrated felts were used as pre-forms and the mass ratio of non- woven cloth to short-cut fiber web was 7 3 The fiber types o
20、f the feltswere OPFs ( Jilin carbon plant，China) ，OPFs ( TohoTenax，Japan)and CFs ( Jilin carbon plant，China)named as Nos 1，2 and 3，respectively Moreover，the obtained C /C composites were denoted using thesame names as their preforms The CF felt was usedfor comparison purpose and the two OPF felts we
21、reused to confirm the conclusion The thickness of OPFand CF felts was about 25 and 20 mm，respectivelyThe density of OPF and CF felts was 0 60 and0 45 g /cm3，respectively It is necessary for the OPFfelts to be carbonized before densification because thetwo OPF felts exhibit high shrinkages in cross-
22、sectionand length when used for CVI directly10， 13 To char-acterize the performance of the OPFs after carboniza-tion，6 K OPFs the same as the OPFs for the feltswere selected for this work The carbonization was asfollows:room temperature 200 for 5 h;200-550 for 20 h;550- 700 for 10 h;700- 900 for10h;
23、 900- 1000 for 10 h; 1000 for 2 h Both theOPFs and their felts were carbonized in an N2( purity，99 99%)environment in carbonization furnace with-out stretching The density of the OPF felts after car-bonization was about 0 45 g /cm3 The felts were di-rectly used as preforms after carbonization withou
24、thigh- temperature treatmentThe CF preforms wereheat- treated at 2300 for 2 h After all of the pre-forms were machined to the size of 200 20 mm，they were densified by isothermal CVI，using naturalgas as pyrocarbon precursor，hydrogen as dilute andcarrier gas at a temperature of 1050- 1080 under apress
25、ure of 1- 5 kPa The volume ratio of natural gas/hydrogen was 2/1 The three kinds of preforms wereinfiltrated about for 400 h and the crust was removedby machining every 50 h in the last 200 h The finalbulk density of the three preforms was 1 722，1 718and 1 715 g /cm3for Nos 1，2 and 3，respectively，an
26、d then they were graphitized at 2100 for 2h2 2Characterization of the fibersThe density of the fibers was obtained at 25 by the density gradient column method The crosssection of the fibers was observed by an optical micro-scope JXA- 840 and the average diameter was calculat-ed by the statistical so
27、ftware nano measurer Thetensile strength of the fibers was measured on an In-stron 5565- 5KN tester using a fiber filament with agauge length of 150 mm and a loading speed of1 mm /min The properties of the fibers are listed inTable 1544第 6 期HAO Ming- yang et al:Effects of fiber- type on the microstr
28、ucture and mechanical properties Table 1Properties of the fibersFiberOPFsNo 1No 2BeforecarbonizationAftercarbonizationBeforecarbonizationAftercarbonizationCFsNo 3Density ( g/cm3)1 361731421751 76Fiber diameter ( m)15 3710671356795700Carbon content ( %)62 539046604789839250Tensile strength ( GPa)0 23
29、187033203353Tensile modulus ( GPa)5 35163008962120023000Elongation ( %)14 8013815201451 512 3Characterization of the compositesThe microstructure of the composites perpendicu-lar to needle punched surface was determined on pol-ished cross- sections under a polarized light microscope( PLM，Neophot21)
30、The distinction of the differentpyrocarbon layer was made by measuring the extinc-tion angles The optical textures are determined bytheir relationship to extinction angle ( Ae) The clas-sification of the texture of pyrocarbon is made as sug-gested by Duppel et al14 and is given in Table 2The thickne
31、sses of the pyrocarbon layers were meas-ured on polished samples The reported figures aremean values of at least 15 measurements The pol-ished surfaces of the C /C composites were analyzedby aman spectrometry ( Avaaman- 532 ) in therange from 1000 to 2000 cm1 The laser power wasapproximately 10 mW a
32、t a wavelength of 514 nmTable 2The textures of the pyrocarbon and thecorresponding extinction angles14 TexturesExtinction angle ( Ae)ough laminarAe18Smooth laminar12Ae18Dark laminar4Ae12IsotropicAe 4The elastic modulus and hardness of the fibersand pyrocarbon matrix were measured by the nanoin-denta
33、tion measurements using Nanoindenter XP sys-tem with a Berkovich- type- pyramid shape diamond in-denter The displacement resolution for the nanoin-denter was 0 01 nm，and the load resolution was50 nN Before beginning the nanoindentation tests，the system was calibrated using the fused silica The2 000
34、nm depth indentation was conducted while theindentation load，F，and displacement，h，were con-tinuously recorded during one complete cycle of load-ing and unloading The hardness of nanoindentationwas determined from the load- displacement ( F- h)curve ( Fig 1) The hardness value，H，is definedaccording t
35、o the following Eq ( 1) :H =FmaxA( 1)Where Fmaxis the maximum load， and A is the pro-jected area of contact that is deduced from an empirical-ly determined function described as the Eq ( 2) :A = ( hmax FmaxS)2( 2)Fig 1Schematic representation of indentation load vsdisplacement curve Fmax，the maximum
36、 load;hmax，the maximum penetration depth;hf，residual penetration depth;Smax，the slope of the unloading curve at h = hmaxWhere and is the apical angle of indenter， andthey are 2644 and 075 for the Berkovich indenter，re-spectively S is the initial unloading stiff， S =dF/dhThe elastic modulus，E，for the
37、 material is deter-mined by evaluating the effective modulus ( Eeff)ofthe contact revealed by the Eq ( 3)and the effectivemodulus suggested by the Eq ( 4) :1Eeff=1 v2E+1 v2iEi( 3)Where the poissom ratio for the indenter viis0 07 and the indenter modulus Eiis 1 140 GPa v isassumed to be 0 28 for the
38、materials suggested byMarx15 The effective modulus has been suggestedby the following Eq ( 4) :Eeff=S2槡A( 4)By combining Eq ( 3)and Eq ( 4) ，the modulusof the indented materials can be determinedThe tensile and three- point bending tests werecarried out on a universal testing machine MTS880- 50644新型
39、炭材料第 29 卷KN to determine the mechanical properties of thecomposites The size of the specimens for tensile andflexural strength was 100 10 7 mm3and 70 10 6 mm3，respectively The gauge length of tensile andthe span of bending tests were 60 mm The tensileloading and the flexural loading direction was pa
40、rallelto and perpendicular to the cloth layer direction，re-spectively The load- displacement curves were recor-ded with a loading speed of 1 mm /min The averagestrength，modulus and strain to failure were calculatedfrom five test samplesToughness of the samples( FD)was determined by a ductility facto
41、r suggestedby Guellali et al16 and was used to compare the qua-si- ductile facture behavior of the C /C composites Itcan be determined with Eq ( 5) :FD=1 Es/Ee( 5)Where Esand Eeare the secant modulus and theelastic modulus in the typical stress- strain curves( Fig 2) ，respectively After flexural tes
42、ting，frac-ture surfaces were examined by a scanning electronmicroscope ( SEM，JSM- 6700F)without depositingconductive layers3esults and discussion3 1Effect of fiber- types on the microstructure ofthe C/C compositesPLM images of the three composites are shownin Fig 3 They exhibit various types of matr
43、ix micro-structure resulting from the different fibers As shownin Fig 3a- c，all the fibers are surrounded by two la-mellae with different degrees of optical anisotropyThe first pyrocarbon layer in composites No 1 and 2exhibits extinction angle， Ae = 6 and 5， respective-ly It is a dark laminar pyroca
44、rbon layer，which is a-bout 2 6 and 1 4 m in the thickness;the second py-rocarbon layer gives higher extinction angle ( Ae =22 and 23) ，which is a characteristic of a roughlaminar pyrocarbon layer about 10 2 and 11 6 m inthe thickness In contrast，the extinction angle of theinner and outer layer in co
45、mposites No 3 are 15 and21，corresponding a smooth laminar and rough lami-nar layer，respectively;their mean thickness are 8 2and 4 4 mFig 2Description of the ductility factor FD16 Fig 3PLM images of the three composites:( a)composite No 1;( b)composite No 2;( c)composite No 3The above identification
46、of microstructures canbe further confirmed by the results given in Fig 4，which are the aman spectra of the three compositesfor fibers and matrixes after graphitization The ex-plored sites in the polished surface of the three com-posites are illustrated in Fig 4a，where F1，F2and F3represent fiber Nos
47、1， 2 and 3 M1，M3and M5re-present the first pyrocarbon layer of composites No1， 2 and 3，respectively M2，M4and M6representthe second pyrocarbon layer of composite No 1，2and 3，respectivelyIt can be observed that eachspectrum shows two broad peaks， the typical addition-al bands of disordered carbons ( D
48、)at around 1 340-1 350 and intrinsic graphite peak ( G ) bands at1 580 cm1( Fig 4b- d) The average microcrystal-line in- plane size and the ability of graphitization areinversely proportional to the intensity ratios = ID/IG The high values of F1( 3 742) ，F2( 3 845)and F3( 3 665) ，indicates that all
49、the fibers are poor-ly graphitizable They also indicate that the micro-crystalline in- plane size of the fibers is in the sequenceof F3 F1 F2 The values of the first pyrocarbonlayer M1( 2 132) ，M3( 2 405)and M5( 1 652) ，in-dicate that the microcrystalline in- plane size is in the744第 6 期HAO Ming- ya
50、ng et al:Effects of fiber- type on the microstructure and mechanical properties sequence of M5 M1 M3 In contrast，the val-ues of the second pyrocarbon layer M2( 1 461) ，M4( 1 421)and M6( 1 537) ，indicates that M4of com-posite No 2 has the highest lattice perfectionFig 4aman spectra of the fibers and
