Surface energetics of reinforcement is one of the most important properties in case of adhesion of reinforcement in composites . For this study, PAN fibres were stabilized isothermally with three different timings (viz. 0, 1, 2 h) and eventually prepared fibres having different surface energetics were evaluated by DCA 322. Composites

were made by using these stabilized fibres (designated as OP, 1 P, and 2P) with coal tar pitch as matrix precursor using match mould die technique. Green composites

were carbonized upto 1000°C, then impregnated and heat treated to 1500, 2000 and 2600°C. Green composites as well as heat treated composites were studied for their

mechanical properties . Microstructure as well as interfacial studies were carried out using optical microscope.

     Optical microscopic examination of composite samples show that Ill treated fibres offer much better adhesion with matrix precursor and the matrix also exhibits

an obvious increase in the anisotropic domain size in case of 2P composites . Density of stabilized fibres and also of green composites has been found to increase with

stabilization time. Flexural strength of green composites prepared with 0 and 2h treated fibres decreases ( 123 MPa to 60 MPa). However, as a result of better bonding between

fibre and matrix , in case of IP composites , strength is always high except in green stage . An attempt has been made to correlate the surface energetics of fibre with

mechanical properties as well as matrix microstructure of carbon /carbon composites

Bahl, O P

Dwivedi, Hariom

Mathur, R B

English

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137CORRELATION BETWEEN THE SURFACE ENERGETICS OFREINFORCEMENT AND MECHANICAL PROPERTIES OFCARBONICARBON COMPOSITESO.P. BAHL, R.B_ MATHUP AND HARIOM DWIVEDICarbon Technology Unit, National Physical Laboratory, New Delhi -110 012.AbstractSurface energetics of reinforcement is one of the most important properties incase of adhesion of reinforcement in composites . For this study, PAN fibres werestabilized isothermally with three different timings (viz. 0, 1, 2 h) and eventually pre-pared fibres having different surface energetics were evaluated by DCA 322. Com-posites were made by using these stabilized fibres (designated as OP , 1 P, and 2P) withcoal tar pitch as matrix precursor using match mould die technique. Green compositeswere carbonized upto 1000°C, then impregnated and heat treated to 1500, 2000 and2600°C. Green composites as well as heat treated composites were studied for theirmechanical properties . Microstructure as well as interfacial studies were carried outusing optical microscope.Optical microscopic examination of composite samples show that Ill treatedfibres offer much better adhesion with matrix precursor and the matrix also exhibitsan obvious increase in the anisotropic domain size in case of 2P composites . Densityof stabilized fibres and also of green composites has been found to increase withstabilization time. Flexural strength of green composites prepared with 0 and 2h treatedfibres decreases ( 123 MPa to 60 MPa). However, as a result of better bonding be-tween fibre and matrix , in case of IP composites , strength is always high except ingreen stage . An attempt has been made to correlate the surface energetics of fibre withmechanical properties as well as matrix microstructure of carbon /carbon composites.1.0 IntroductionCarbon/carbon (C/C) composites are the ceramic materials used for high tem-perature applications because of their far superior thermo-mechanical properties com-pared to the conventional materials[ 1]. These are good reasons to make them practi-cally the ideal material for use in aerospace applications, such as rocket nozzles, ex-haust cones, heat shields etc. [2]. However, processing of C/C composites is a com-plex process influenced by surface chemistry and topography of reinforcement, charor coke yield of the matrix resin and the fibre/matrix interactions at the interface[3 `6].138All these parameters are known to influence the performance of the composites. Theinteractions at the interface between the fibres and matrix are crucial in determiningthe mechanical properties of the composites. Coal-tar pitch is unique among carbonmatrix precursors because of its low price and its flexibility to produce carbons withvery different microstructures. A compromise between strong and weak bonding atthe interface is necessary to optimise the efficiency with which fibre properties areutilized and the fracture toughness of the composites increased[7-8]. A strong inter-face is known to lead to a brittle fracture behaviour, whereas a weak interface permitsfibre/matrix debonding and sliding and no proper transfer of load to the reinforce-ment.One of the main disadvantages with C/C composites is their high cost. With theincreased emphasis on lower cost of production of C/C composites, more attention isbeing directed toward lower cost fibres and the use of very high char yield resins andpitches as impregnants. This paper deals with the former aspect. Since PANEX fibresundergo chemical and physical transformations during pyrolysis along with the ma-trix, the microstructure of the matrix will be different than the carbon fibre basedcomposites.During pyrolysis of preforms, especially upto 1000°C there is a large relativeshrinkage of the matrix precursor compared to carbon fibres, which have already seena temperature of 1000°C and above. As such there is hardly any shrinkage in thefibres and the shrinkage of the matrix causes stress concentrations at the fibre/matrixinterface. One can therefore observe matrix cracks, fibre damage and therefore lowerstrength of the resulting composites. The advantage in using oxidised PAN fibres.`PANEX', thermally stabilized upto 250°C only, besides their low cost is that theyalso undergo considerable shrinkage along with the matrix pitch and chances of stressrelated cracks are rare. However, the shrinkage will be influenced by the surfaceenergetics of the fibre, which may inhibit the free shrinkage of the matrix. This willfurther influence the mechanical properties of the composites.Very few studies have been reported on the oxidised PAN fibres based carbon/carbon composites using either phenolic resin or pitch as matrix precursors [5,9-11 ] .A systematic approach is therefore required to understand the role of surface energeticsof the precursor PANEX fibres and the subsequent development of microstructure inthe matrix and its overall influence on the mechanical properties of the resulting C/Ccomposites. Present study is one such attempt in this direction.2.0 Experimental2.1 Preparation of PANEX FibresPolyacrylonitrile (PAN) fibres tow containing 6000 monofilaments of 1.2 d'texCARBON/CARBON COMPOSITES - PROPERTY CORRELATION 139each and diameter about 12.5 µm obtained from M/s Courtauld's, U.K. were used inthe present study. These were first converted into PANEX fibres on a continuous scaleby oxidising PAN fibres under different stabilization conditions viz: oxidation tem-perature 250°C and stabilization time 0, 1 and 2h respectively. About 500meters ofPANEX fibre was prepared for each batch. Table- l shows various physical charac-teristics of the stabilized fibres, while Table-2 shows the surface energetics for eachbatch. One can clearly notice that the three samples exhibit completely different char-acteristics and therefore lead to different behaviour of the resulting C/C composites.2.2 Preparation of composites using PANEX fibres :Unidirectional composites of size 130 x 5 x 4 mm3 (approx.) were made byreinforcing separate batches of PANEX fibres with coal tar pitch as matrix. The speci-fications of the pitch are given in Table-3. A standard match mould die technique wasused to prepare several batches of green composites A,B and C, corresponding to 0, 1and 2h PANEX fibres as reinforcement. These composites were then subjected tovarious heat treatment temperatures viz. 600, 1000, 1500, 2000 and 2600°C, in ultrahigh purity nitrogen atmosphere to observe the transformation in the composite micro-structure at each stage.2.3 Characterzation of the composites :Mechanical properties of the fibres as well as that of the composites were meas-ured on an 1NSTRON 4411 model. Density of the fibres was measured by sink floatmethod while that of the composites was determined using Archimedes principle. In-situ dilatometry of the PANEX fibres as such and that of the green composites wascarried out on Mettler TA-4000 thermal analyser upto 1000°C. Surface energy of thePANEX fibres was determined by measuring dynamic contact angle using CAHNDCA 322 system. Polarised light microscopy of the cross-section of the polishedsamples of the composites was carried out on an Olympus rotating stage optical mi-croscope, to understand the microstructure of the C/C composites at all the heat treat-ment temperatures.3.0 Results and Discussion3.1 Characteristics of the PANEX fibres :Table-1 shows the characteristics of the three PANEX fibre samples identifiedfor reinforcement. Column 2 of the table shows that the aromatization index increaseswith oxidation time showing that 2h sample is the best stabilized one. This is alsoreflected in the DSC results of the sample which show the value of AH to be only 82kJ/mol for 2h as compared to 973 kJ/mol for 0 h sample suggesting it to be least140stabilized of the three, Likewise other properties like tensile strength and the densityalso show linear trend indicating gradual structural changes in the three samples.Similarly the surface energy of the fibres shown in table-2, especially the polar com-ponents of the surface groups which are responsible for interaction, shows a variationfrom 3.8 dyne/cm to 10.24 dyne/cm for 2h sample and does not follow the trend as inTable-1.The difference in the characteristics of the fibres should form different bondswith the matrix coal tar pitch in the green stage and the effect should very much bereflected in the shrinkage behaviour of the composites when subjected to pyrolysis,especially upto 1000°C. In order to observe the effect minutely; in situ dilatometry ofthe green composites was planned. Shrinkage of the PANEX fibres was followedalong the length whereas in case of the composites it was measured in both the direc-tions i.e. parallel as well as perpendicular to the fibre tows. This could give an ideaabout the total volume shrinkage of the composites.3.2 Dilatometry of the compositesThe green composites prepared from all the three PANEX samples were sub-jected to thenno-mechanical analysis on Mettler TA-4000 thermal analyser, to under-stand the in situ shrinkage behaviour during pyrolysis. The dilatometry was carriedout in notrogen atmosphere and the heating rate was kept at 5°C/min.Figure 1 (a) shows the shrinkage behaviour of the PANEX fibres alone whileFigs .l (b) and (c) show the shrinkage behaviour of green composites parallel andperpendicular direction of the reinforcement. As expected, the fibres stabilized for0 h show maximum shrinkage while the 2h stabilized sample shows minimum shrink-age. Interestingly, similar trend is not followed if one looks at the curves obtained forthe composites {Fig. I (b) and (c)]. In order to correlate these curves with the behav-iour of the interactions at the fibre matrix interface, total shrinkage, following thesecurves, in different temperature ranges are reproduced in Table-4.As shown in Table-4, one finds that in the longitudinal i.e. parallel to fibredirection there is maximum shrinkage i.e. about 9% in case of composites A and C inthe temperature range 0 to 400°C. On the other hand the shrinkage in the case of fibresitself is only -3% and + 0.5 % respectively. That is there is additional shrinkage of 6%and 10% in the case of composites, suggesting strong fibre matrix interactions. This isalso in tune with the larger values of surface energies for these fibres (Table-2). Incase of composite B the additional shrinkage is only 2.5%. It can therefore be inferredthat in case of composite B, the fibre matrix interactions are different. In the tempera-ture range 400-1000°C there is almost no significant difference in the numerical val-ues as well as the general trend of the shrinkages in the parallel direction.CARBON/CARBON COMPOSITES - PROPERTY CORRELATION 141Similarly in case of shrinkage in the perpendicular direction, there is a largedifference in the values between the composites ranging between 9 to 37%. The differ-ence is almost 4 times between A and B and 3 times between B and C, and here too thedifference is more pronounced upto 400"C alone. If one looks at the combined effectof the longitudinal and the perpendicular shrinkage it is clear that there is large vol-ume shrinkage in case of composites A and C and very little in case of composite B.The density of composite B should therefore be minimum. However, as shown in Fig.2, the density of composite B is maximum at 1000°C. It can therefore be concludedthat the interactions in the fibre and matrix in composites B are such that there ismuch less evolution of volatiles comparatively. Density for composite B is thereforemaximum. The char yield or the weight loss during co-carbonization of the PANEXand the pitch matrix is in the order 43%, 39% and 45% respectively which confirmsthis view point. Since the evolution of the volatiles in the composite C is maximum, itsdensity is minimum at 1000°C.3.3 Mechanical Properties of the composites :Figure-3 shows the comparative bar chart of the flexural strength and flexuralmodulus of the composites A, B and C after heat treatment at different temperatures.As discussed above because of the better bonding between the fibre and the matrix incase of composites B, it shows highest values for strength at each stage of heat treat-ment. The influence is more pronounced in flexural modulus of composite B and issignificantly higher comparatively.3.4 Optical microscopy of the green as well as carbonized composites :Fig.4 (a-c) shows the optical micrographs of the composites A,B and C treatedat 600°C. Composite A shows the development of fine mosaic texture of the matrix.Composites B and C show essentially medium coarse to coarse mosaic texture of thematrix. There are areas where some flow type behaviour of the matrix pitch is alsoobserved in composite C. One important point of notice is the difference in the diam-eters of the fibres in the three samples. Although the difference in the diameters of thePANEX fibres is not significant (Table-1), in the composites the diameter of the fibrein composite A, as measured from image analyser, is 9.5 µm as compared to 7.4 gm incomposite B. Fig. 5 shows plots of diameter of the fibres in the composites subjectedto different heat treatment . The higher diameter of the PANEX fibres in composite Bcould be attributed to strong fibre matrix interactions which do not allow the fibre toshrink along its cross section . The same behaviour was also observed by the authorsearlier [6]. However, between 1000°C and 1500°C the matrix microstructure gener-ally turns into a coarse mosaic type with flow type microtexture of the matrix in thepockets which are not influenced by the fibre energetics i.e. areas which are compara-tively farther from the fibre surface. The diameters of the fibres, as expected, is also142reduced considerably.Composite B, which shows maximum strength shows only a coarse grain typematrix microstructure for all the heat treatment temperatures upto 2600°C (Fig.6-8).More importantly one does not observe matrix microcracks, as has generally been thecase for Carbon fibre based C/C composites. This confirms the advantage with PANEXfibres which undergo co-carbonization along with the pitch and therefore stresses dueto mismatch of fibre matrix shrinkage during pyrolysis are much less in these com-posites.Beyond 1500°C it is quite evident that the fibres start developing anisotropyand the sample treated to 2600°C clearly shows the development of radial type ofmicrotexture in the fibre cross section. This is quite unusual of PAN based carbonfibres, which generally show onion type microtexture. Radial type microtexture of thefibres as reinforcement is more preferable for high performance composites.4.0 ConclusionThe study clearly reveals the importance of having optimum surface energeticsof the reinforcement which will lead to strong C/C composites. Experiments are inprogress to densify the composites to at least 1.8g/cc for the evaluation of ultimatemechanical properties of PANEX based C/C composites.5.0 AcknowledgementThe authors are grateful to Prof. A.K. Raychaudhuri for his keen interest in thework and his permission to publish the results. We are also thankful to the Indo-French centre for the promotion of Advanced studies for sponsoring the projectNo.1408-1, under which the studies have been carried out. Hariom Dwivedi is thank-ful to the centre for providing the grant for a research fellow.References1. "Carbon-carbon composites" (Ed. G. Savage) Pub: Chapman and Hall,London, 1993.2. Thomas, C.R. - Overview-What are carbon-carbon composites andwhat do they offer ?, Chapter 1 in Essentials of carbon-carboncomposites, 1994.3. L.M. Manocha and O.P. Bahl, Carbon, 26 (1988) 13.4. L.M. Manocha, E. Yasuda, Y. Tanabe and S. Kimura, Carbon,26 (1988) 333.CARBON/CARBON COMPOSITES - PROPERTY CORRELATION 1435. E. Fitzer, K.H. Geigle and W. Huttener, Carbon, 18 (1980) 265,6. T.L. Dhami, O.P. Bahl and P.K. Jain, Carbon, 33 (1995) 517.7. Fitzer, E., Carbon, 25 (1987) 25,8. Kowbel, W. and Shan, C.H., Carbon, 28 (1990) 287.9. V. Markovic and S. Marinkovic, Carbon, 26 (1988) 329.10. L.M. Manocha and Ketan G. Patel, Proc. 23rd Biennial Conference ofCarbon Pennsylvania, vol. II (1997) p. 418.11. V. Markovic and et.al., Proc. 15th Biennial Conf. on Carbon,Pennsylvania, 1980, p.272.Table 1: PANEX Fibres characteristicsFibre Type A.I.WeightgDiameterP-MEnthalpykl/molT.S.MPaY.bl.MPaDensityg/w0 h 51 0 .7524 11.81 973 . 9 401 1405.4 1.301 h 60 0 . 7690 11 .67 475. 3 249 739.3 1.432 h 75 0 .7626 11 .44 82 . 2 206 390.6 1.49Table 2: Surface energy of PANEX fibresCosO(ndvancing) Geometric Mean Method (dyne/cm)Water ml Disp. Polar Total0 h 0.231+0.018 0.693+0.011 31.58 6.55 38.131 h 0.136+0.006 0.704+0.016 33.52 3.80 37.322 h 0.331+0.021 0.678+0.011 29.38 10.24 39.62144Table 3: Properties of pitchesCharacteristics Preforming pitch Impregnating pitchSoftening point (IC) 98 85Quinoline insoluble (%) 9.9 0.6Toluene insoluble (%) 29.1 21.0Coking value (%) 57.4 51.0Table 4Percent change in length in PANEX fibres and the green composites (from Fig. 1)Temperature 0-400°C 400-600•C 600-1000°C 0-10000CPANEX fibres Parallel (longitudinal)PANEX 1 -3.0 -2.5 -4.0 -9.5PANEX 2 -1.5 -2.3 -3.8 -7.6PANEX 3 -0.5 -2.5 -4.0 -6.0Green composites ParallelComposite A -9.0 -3.5 -3.3 -15.8Composite B 4.0 -3.0 -3.3 -10.3Composite C -9.8 -2.7 -2.7 -15.2Green Composites PerpendicularComposite A -37.0 -7.5 -3.0 -47.5Composite B -9.5 -7.0 -5.0 -21.5Composite C -27.0 -3.0 -2.0 -32.0CARBON/CARBON COMPOSITES - PROPERTY CORRELATION 145(a)FIBRESEI i i200 400 • 600 , 800To mpavrin t(b)COMPOSITES95654 60o 600T.mperotwq,CCOMPOSITES200 400 sop GooTsmp.rotw. CFig. 1 - Dilatometry of panex fibres and the green composites146DENSITY OF COMPOSITESFig. 2 - Variation of composites density with temperature.I I I \ I It \I ' I ItI 'c, I I I14012010040200GREEN 1000 °C 1500 °C 2000 °C 2600°CI I I \I 1, \I \I, -101 t IGREEN 1000 °C 1300 °C 2000 °C 2600°CFig. 3 - Mechanical properties of composites after heat treatmentat different temperature.CARBON/CARBON COMPOSITES - PROPERTY CORRELATION 147Fig. 4 - Optical Micrographs of composites HTT 600° C148TemperatureFig. 5 - Changes in the diameters of the PANEX fibres i n the compositesat different HTTCARBONYCARBON COMPOSITES - PROPERTY CORRELATION 149Fig. 6 =• Optical Micrographs of composites HTT 1 000° C150Fig. 7 - Optical Micrographs of composites HTT 15000 C

Correlation between the Surface Energetics of Reinforcement and Mechanical Properties of Carbon/Carbon Composites

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Correlation between the Surface Energetics of Reinforcement and Mechanical Properties of Carbon/Carbon Composites

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