Carbon, Vol. 33, No. 4, pp. 427-433, 1995 Pergamon Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223195 $9.50 + .OO 0008-6223(94)00167-7 OXIDATION PROTECTION OF CARBON FIBERS BY COATINGS Yu-QING WANG, BEN-LIAN ZHOU, and ZUO-MING WANG Institute of Metal Research, Academia Sinica, Shenyang 110015, China (Received 28 August 1994; accepted in revised form 10 November 1994) Abstract-Sic coatings prepared by chemical vapor deposition and from polycarbosilane (PCS) solution; also, SiO, and Al,O, coatings by Sol-Gel method were investigated in detail, to improve the oxidation re- sistance of carbon fibers as important reinforcement in advanced composites. A P-Sic coating was ob- tained on carbon fibers by pyrolysis of CH,SiCI, and Hz at 1373-1573 K. The coating thickness could be controlled according to the requirements in practice. A uniform and fine Sic coating was also obtained by immersion and pyrolysis of PCS solutions with PCS contents of 5-15 wt%, and was composed of a mixture of Sic, SiO,, and free carbon after pyrolysis in argon. Fine and dense SiO, and A1,03 coatings could be obtained from silica and alumina sols, respectively, under exact control of process parameters. All coatings had good effects on the oxidation resistance of carbon fibers in the sequential order of CVD Sic, PCS-Sic, SiO,, and Al,O,. A composite coating of CVD Sic and a mixing coating of SiOz and AI,O, was suggested to decrease defects like pores and microcracks in individual coatings to obtain the best oxidation protection for carbon fibers. Key Words-Carbon fibers, oxidation resistance, coatings. 1. INTRODUCTION gas was used as carrier gas and diluent, and argon gas as the sealing gas for the tubular ends. Part of the HzCarbon fibers are the most important reinforcement gas passed through an evaporator containing liquidused in advanced composites because of their relatively MMT kept constantly at about 298 K, and carried somelow cost and excellent properties, such as high specific vaporized MMT into the reactor. The hot-wall arrange-strength, modulus, low expansion coefficient, and rel- ment of CVD was used for the continuous Sic coat-ative flexibility. However, they have low oxidation resis- ing on carbon fibers in the device with a wheel fortance in an oxidative environment (carbon fiber begins driving the carbon fiber yarn, schematically shown into be oxidized at 750 K in air) and chemical reactivity, Fig. 1. Sizing agent on the surface of carbon fibers wasand are therefore not widely used either in metal-matrix removed by heating at 673 K in the first furnace; thenand ceramic-matrix composites or in high-temperature carbon fibers entered the deposited reactor, which waspolymer-matrix composites. To widen the application a resistance-heated alumina tube of 10 mm innerof carbon fibers, it is necessary to improve their oxi- diameter.dation resistance. The parameters used for deposition of Sic coating In fact, much attention has been paid to oxidation were as follows:protection of carbon fibers; for example, coating met-als like Al, Ni, Ta, or Ti on carbon fibers directly by Deposition temperature: 1370 K-1523 Kchemical vapor deposition (CVD), physical vapor de- Molar ratio of H2/MMT: <20position (PVD), plating (electric or electroless), or Hz gas-flow rate: 2-540 ml/minplasma sputtering; coating nonmetals like B, Si, or P Ar gas-flow rate: 2 x 0.1 M’/hby CVD; coating ceramic compounds like carbides, Wheeling speed: lo-60 cm/minnitrides, or borides by CVD or other modern tech- 2.2 Structure of Sic coatingniques[l-31, and oxides like Si02, or Al,O, by solu- Using the parameters given above, carbon fiberstion methods, etc.[4,5]. In general, the results show were well coated with a uniform and continuous Sicthat the application of ceramic compound coatings is coating by the process, as presented in Fig. 2. Thea successful approach for improving the oxidation re- thickness of the coating could be adjusted by changingsistance of carbon fibers. the wheeling speed of the carbon fibers, as shown in This paper mainly reports the results of a study of Fig. 3. Corresponding to increment of coating thick- oxidation resistance of coated carbon fibers with Sic ness, the color of the Sic-coated carbon fibers changedby CVD, and with Sic, SiOZ, and A1203 by solution in the order of brown, dark blue, blue, clear blue,method carried out at our Laboratory. gold, dark red, and red-green. Of course, these were closely related to the types of carbon fibers used. If 2. Sic COATING BY CHEMICAL the coating thickness was over 0.5 pm, a bridging phe- VAPOR DEPOSITION nomenon was observed across fibers, and the coated2.1 Coating process fibers became quite brittle. This must be avoided. CH,SiCl, (MMT) was used as the precursor for In most cases of the process, cubic P-Sic with a dis-the deposition of Sic. A metered flow of purified Hz ordered phase was obtained. The other phases were 427
428 U.-Q. WAN0et al 4 t t Cool in:: water Cool in:: water Fig. 1. Schematic drawing of SIC deposition process on carbon fibers by CVD: 1. supply wheel; 2. fibers; 3. furnace; 4. depositing furnace; 5. take-up wheel.also found to be in the form of free carbon or/and 2.3 Oxidation resistance of thefree silicon under different parameters given above. Sic-coated carbon fibersThe typical X-ray diffraction patterns of Sic coatings The thermal weight loss test of the fibers was car-are shown in Fig. 4. As seen from the figure, the com- ried out with thermal balance model Cahn-2000. Thepositions and structure of the SIC coating are affected results of the measurement showed that the Sic-coatedby the molar ratio of H,/MMT. With the mechanism fibers had higher oxidation resistance than the as-of SIC deposition from MMT, these are under- received carbon fibers. In the case of step-heatingstandable. When the ratio is large, the thermal de- mode, as-received carbon fibers began oxidation atcomposition of the CH;. group is retarded and the 753 K, and then lost its weight at a rate of 9.9%/minproduction of HCI is promoted by more H2 gas in the over 773 K until they were oxidized completely. Bydeposition process. These lead the production of an comparison, the weight loss of the Sic-coated carbonSic coating with high silicon content (free silicon). fibers was only l/50-1/30 of that of the as-receivedWhen the molar ratio is small and/or the deposition fibers, as shown in Fig. 5.temperature is very high, carbon deposition is pro- Oxidation resistance of the coated carbon fibers ismoted and SIC coating with high carbon content (free closely related to the coating thickness. The relation-carbon) is obtained. Only when the molar ratio of ship between the weight loss and the coating thicknessH,/MMT is appropriate (in our experiments, the mo- on carbon fibers is presented in Fig. 6 (of course, thelar ratio was about S-7), the stoichiometric SIC struc- oxidation included the contribution of two end cross-ture and composition can be deposited. The apparent sections because they were not covered by SIC coat-activity energy of the whole process was estimated to ing). The oxidation rate decreased with increase inbe about 167 KJ/Mol. The existence of free carbon coating thickness when it was less than 0.1 pm, theand silicon in SIC coating on carbon fibers certainly weight loss becoming constant when the thickness wasaffects the properties of coated fibers. over 0.1 km. This showed that the coated carbon fi- bers were oxidized by diffusion of oxygen through the coating and reaction with carbon. Hollow tubes were 5 10 15 20 Take-up speed(mn/Sec) Fig. 2. Surface of Sic-coated carbon fibers. Pig. 3. The coating thickness vs wheeling speed
Oxidation protection of carbon fibers by coatings 429 Ternperature(‘C) i l 700 0 500 l e-a--* A 500 (h) Siiiron rich Tenpera~ure, 1371°K 0 0.1 0.2 Sic coating thickness(rrm) Fig. 6. Weight loss vs the coating thickness. existence of free carbon in the Sic coating was also a kind of channel of oxygen atoms. These caused con- stant oxidation at a low rate, although carbon fibers (L) SroiLhiastric SIC were coated by SIC. Temperature, 142"K 1 Considering the rapid diffusion of oxygen from the outer surface into inner interface between carbon fiber and the coating through microcracks and other defects, the reaction between carbon and oxygen occurred: c+ 02 = COZT (1) and/or2u 25 311 35 40 45 sn 55 sn b5 70 75 20 ._ 2c + 02 = zcot. (2)Fig. 4. X-ray spectrum of deposited SIC on carbon fibers. The reaction would result in an accumulation of CO + CO2 at the surface between carbon fiber and the coat- ing. The advance of further reaction requires that new oxygen atoms arrive rapidly at the reaction front andobtained from complete oxidation of the Sic-coated the products diffuse out through the coating. The dif-carbon fibers at 973 K for a long time, as shown in fusion rate of oxygen and products is smaller than theFig. 7. The coating thickness could be determined to reaction rate of carbon with oxygen. So the controlbe about 0.15 Km. The microcracks and other defects, factor of the oxidation process was the diffusion rateas Medford proposed[7,8], were also observed. As of the gaseous species.the coating was thickened, the defects in the coating If the Arrhenius equation,seemed to increase. These defects were the channel ofoxygen atom transportation. On the other hand, the K = A exp( -E/RTJ, (3) 80 0 50 I?0 ISQ Time(min)Fig. 5. Weight loss of carbon fibers coated by different coat- ings with step-heating mode (lO”C/min). Fig. 7. Hallow tubes of Sic-coating after complete oxidation.
430 Y.-Q. WANG et a[.is used, the reaction rate of the oxidation reaction (u) 3. CERAMIC COATING BY SOLUTION METHODcan be described as 3.1 Sic coating by solution method u = XA expl -E/RT), (4) A process of making inorganic Sic fibers by con- trolling pyrolysis of silicon-polymer precursor was de-where K presents the constant of reaction rate in oxi- veloped by Professor Yajima and his colleagues indation; A and X are constants, respectively; R is the 1975[1 I]. This process suggested a cheap, simple waygas parameter; E is the apparent activation energy; for coating carbon fibers with Sic from the pyrolysisand T is temperature. Therefore, the apparent activa- of polymer containing silicon and carbon.tion energy of the reaction can be estimated from the 3.1.1 Experimental procedure. Polycarbo-oxidation rate. That is, silane (PCS) with a mean molecular weight of 1300 was used as the Sic precursor, and was dissolved in di- Ln u = -E/RT + Ln(Ah). (5) methylbenzene solution at different concentrations. The coating process is schematically presented in Fig. 9. In light of the experimental results, the apparent The fibers were passed sequentially through a furnaceactivation energy of the reaction process can be cal- to remove the sizing agent at 730 K, a vessel contain-culated to be about 91 KJ/Mol. ing the PCS solution kept at 303 K, a chamber to sta- bilize the PCS molecules at 473 K in air to avoid its2.4 Effect of the composite coating melting in the high temperature, and finally a drying SIC coating on carbon fibers seriously degrades the furnace to pyrolyze PCS in argon gas at 1473 K to ob-strength of carbon fibers, although it improves the tain the Sic coating. The take-up speed was 40-60oxidation resistance. In order to maintain the high cm/min.strength of carbon fibers, it is necessary to coat a soft 3.1.2 Structure of PCS-Sic coating. Thelayer like carbon between carbon fiber and Sic coating coated carbon fibers were brown or blue in color, de-to obtain a duplex coating. The effects of pyrolytic pending on the gaseous environment in the final py-carbon (PC) on the strength of coated fibers are shown rolysis. In some cases, filaments of carbon fibersin Fig. 8. The duplex coating had a good contribution adhered together, but not strongly. The adhesion wasto the oxidation resistance, compared with the single related to the concentration of PCS in the solution.coating; however, it was not so obvious. According to In general, the higher the concentration, the strongerthe biomimetic design of interface in composites[ lo], the adhesion of carbon fibers. Other factors were alsothe multiple-gradient coating of PC//SiC//Si (the important, such as the take-up speed of the fibers andouter surface was silicon) on carbon fibers was coated what types of fibers were used (e.g., high modulus orand had best oxidation resistance at high tempera- high strength), etc. The adhesion among filaments oftures, as shown in Fig. 5. At the preliminary period carbon fibers was easily removed by the optimizationof oxidation, silicon on the surface of composite coat- of parameters in the process, and a uniform and con-ings was oxidized to form Si-0 compounds, result- tinuous coating was obtained on carbon fibers, asing in increases in weight, which balanced the weight shown in Fig. 10.loss caused by oxidation of carbon fibers; the weight The PCS with the molecular weight of 1300 wouldremained unchanged. Finally, silicon was completely melt at high temperature, so that stabilization must beoxidized to form Si02, and oxidation process of the carried out before pyrolysis. The PCS was stabilizedcoated fibers was controlled by gaseous species diffu- by heating in air at 463-493 K, whereby the ctoss-sion as discussed above, so that the oxidation rate linking of short chains like Si-H, C-H, or Si-Sistayed in the low level. groups in the PCS molecules took place to form Si-O-Si or Si-O-C bonds and also polymeriza- tion into large molecules. During pyrolysis at high temperatures, the cross-linking of the polymer chains occurred again, and side chains containing hydrogen, methyl group, and siliceous group were decomposed. Therefore, the composition and structure of the resul- tant coating were directly related to the environmen- I 0.05 0 IJ 0.15 0.20 Sic coating thic!messCw) Fig. 9. Schematic drawing of coating process by solution: 1. supply wheel; 2. furnace for removing sizing agent; 3. con.Fig. 8. Strength of the SiC-coated carbon fibers vs the coat- tainer; 4. drying furnace; 5. pyrolytic furnace; 6. take-up ing thickness. wheel.
Oxidation protection of carbon fibers by coatings 431 (a) (b) Fig. 10. Surface of PCS-SC-coated carbon fibers with PCS content of (a) 5 wt% and (b) 15 wt%.tal gas. In argon gas, the coating contained Sic, nally pyrolyzed to obtain the oxide[12,13]. This pro-SiOZ, free carbon and free silicon; in vacuum, it con- cess is called Sol-Gel method. The method has beentained SIC, SiOZ, and free carbon. Their molecular widely studied to fabricate ceramic powder, glass fi-ratio was about SiC:Si02:C = 1:0.35:0.58. Oxygen ber, surface coating, etc.[l4].in the coating came from the cross-link reaction dur- 3.2.1 Experimental procedure. The hydrolysising the stabilization of PCS. X-ray diffraction showed of Si(OC,H,), was performed by introducing the alk-the structure of the coating was in the form of oxide and water into C2H50H at 298-308 K. The fol-microcrystallites. lowing reactions occurred during the process: 3.1.3 Oxidation resistance of the coated car-bon fibers. The results of weight-loss measurements Si(OC,H,) + nH,O = (HO),Si(OC,H,),_, + nClH,OHon the PCS-Sic-coated carbon fibers on heating in airat 773 K are presented in Fig. Il. It remained un- (hydrolysis) (6)changed for 60 min, but the as-received carbon fibers andlost 40% of their weight in this period. When heatedfor 140 min at 773 K, the weight loss of the coated fi-bers was about 3%; in comparison, the as-received fi- )Si-OCZHI + OH-Sic = GSi-0-Si c + C2H,0Hbers were oxidized completely. These numbers showthat the PCS coating improved the oxidation resis- (polymerization) (7)tance of carbon fibers efficiently. or3.2 SiO, coating by Sol-Gel method Metal alkoxide (or salt) were hydrolyzed at room +Si-OC2HS + HO-Sic = $Si-0-Si $ + HZ0temperature or at higher temperature to obtain a sol,which was then gelled to a network structure, and fi- (polymerization). (8) In this process, the catalyst, such as HCI acid, and strong stirring were necessary to obtain a clear sol. Of 70 course, the amount of added water, catalyst, and so- 60 lution temperature must be controlled exactly. The coating process of carbon fibers was similar to coat- 50.c: ing Sic by the solution method mentioned above. The3 O pyrolysis temperature was 873 K. The take-up speedT 30 of fibers was 40-60 cm/min.6 20 3.2.2 Structure of the coating. A transparent3 10 SiOZ coating was coated uniformly on carbon fibers under the optimization of process parameters. Some 0 particles were observed on the surface, and were iden- tified to be SiOz when the coating thickness was over 100 150 200 0.15 Frn. Sometimes cracks appeared on the coating Time(mln) when the thickness was over 0.15 pm and the take-upFig. 11. Weight loss of the coated carbon fibers with a thick- speed was higher. The coating was in the form of ness of 0.1 pm. amorphous SiO,.
432 U.-Q. WANG et al 3.2.3 Oxidation of SO,-coated carbon fi- (440) (311) (400) hbers. The SiOZ coating improved efficiently the oxi- (2221dation resistance of carbon fibers, as shown in Fig. 11.However, during heating at 873 K for 0.5 h, the coatedfibers had a high weight loss of 50 wt%. This was ALx.zzz b. 873K Ihrcaused by micropores in the coating. Experimental re-sults showed that the oxidation resistance of the coatedfibers was better if the coated fibers were heated at1073 K for 1 h in argon gas to self-heal the micropores.3.3 AlJO coating by Sol-Gel method a. 393K 4Hr On the basis of the principle mentioned above, anAl,O, coating was deposited by Sol-Gel process. 3.3.1 Experimental procedure. Aluminum iso- 20 40 60 80propoxide, A1(OC3H7)3, was used as precursor. The 20hydrolytic reactions, Fig. 12.X-ray diffractogram of Al?O,solduring heating process. AI(OC,H,)3 H,O =(OC;H,)2AIOH + CqH,OH, (9)and polymerization, 3.3,3 Oxidation resistance of AlJO,-coated carbon fibers. The oxidation-resistance of carbon fi- :AI-OC,H,+~H-AI: = ~AI-o-A~/-+c,H,~H, bers at high temperatures was improved by the A1203 (10) coating, as shown in Fig. 11. Heating at 773 K for 140or min, the as-received carbon fibers were oxidized com- pletely, whereas the coated fibers lost less than 10% of their weight. The content of A&O, in the sol and +AI-OH +oH-Ali= ~AI-o-AI~ +~,0, the coating quality of course affected the results. (11) Many microcracks also existed in the coating, which resulted in oxidation of the coated fibers.were performed by introducing the alkoxide into ex-cess water and alcohol under vigorous stirring. The 4. DISCUSSIONmolar ratio of water to alkoxide was over 100. It mustbe noted that it was difficult to change the slurry of Carbon fibers have higher surface areas because ofwater and alkoxide to a clear so1. The peptization their small diameter, so that surface coating is aprocess should be performed to produce sol with a net- method of protecting them from oxidation. SIC is verywork structure; the amount and kind of acid added stable in air below 1273 K, so that it is used for oxi-should be controlled; and temperature of water was dation protection of carbon fibers in most cases. Un-also controlled exactly at 348 K during mixing and at fortunately, Sic coated by CVD or other methods368 K during peptizing. The coating process was sim- have many defects, such as pores, incompleteness ofilar to the procedure for coating Sir by the solution structure, and nonstoichiometric structure accessiblemethod mentioned above. Here the last pyrolysis tem- to transport oxygen atoms. Furthermore, the thickerperature stayed at 1123 K. The moving speed of car- the coating is, the more defects there are in the coat-bon fibers was 40-80 cm/min. ing. As seen from our results, the composite coating 3.3.2 Structure of the coating. After optimiz- is a satisfactory way to protect carbon fibers from ox-ing parameters of the process, a uniform and trans- idation. The silicon in the coating is initially oxidizedparent coating was obtained on the carbon fiber at high temperature, and its volume increases to formsurface. Some particles on the coating were also ob- a finely dispersed and dense SiO, film, which fillsserved by SEM, which were identified to be Al,O, by some pores that are the possible channels for the trans-EDAX. Some amount of the A1203 sol was heated in port of oxygen atoms. However, it is difficult andthe same way as in the coating process to investigate costly to obtain this composite coating.the changes of the sol and structure of resultant coating. In the coating obtained from solution, more poresThe pattern of X-ray diffraction for samples is pre- existed. So the oxidation resistance is lower than thesented in Fig. 12. The structure of the gel became very CVD coating. Some measurements indicated that thecomplex after heat treating at 393 K. It was mainly porosity was 63 k 1% in the A&O, coating. Elec-composed of aluminum hydroxide and hydroxide with tron micrographs showed two types of pores present.small amounts of chlorine and alcohol. When heat The smaller pores having a radius below 500 nm aretreating at 873 K, the water and/or alcohol was de- barely visible. The larger river-like pores have a 1000sorbed from the gel, which was transformed to to 1500 nm radius, and are separated by distances ofxpAl,O,. After heating at 1023 K, the coating be- about 1 pm. These larger pores appear to contributecame to a-A&O,. somewhat less than one-tenth of the total porosity.
Oxidation protection of carbon fibers by coatings 433These pores are possible channels accessible to trans- of oxidation can be considered as the transportationport oxygen atoms, causing the oxidation of carbon of oxygen atoms from outer surface of the coating tofibers. The additional heat treatment at high temper- the interface between the carbon fiber and the coat-ature above 973 K caused a porosity decrease because ing to cause continuous oxidation of fibers. The rateof self-healing at high temperature with the SiOZ of the oxidation process was limited by diffusion ofcoating, but in the case of the AlzO, coating, the pore the gaseous species. In order to improve the oxidationvolume showed relatively small changes with the ad- resistance and strength of the coated fibers, it was sug-ditional heat treatment. That is, the oxidation resis- gested that a multiple-gradient coating be applied,tance of the SiO,-coated carbon fibers could be such as PC//SiC//Si, deposited on carbon fibers byimproved by the additional heat treatment above the CVD process. For the solution process, the pyro-973 K, whereas the A&O3 coating could not. In order lytic gas environment must be controlled for coatingto improve the Al*O, coating, it is suggested that a PCS-SIC, and the addition of some silica sol into alu-suitable silicon oxide sol be added to the alumina sol minum oxide (3-50 wt%) sol decreased pores andto obtain a mixing sol for coating, the resultant coating cracks m A&O, coating.containing AlzO, and some amount of SiOZ (3-50 wt%, Comparing coatings prepared by different meth-preferably), being dense and having higher oxidation ods, the CVD coating showed the higher oxidation re-resistance. This needs further research in the future. sistance; however, it degraded the strength of carbonThe PCS Sic coating had a relatively lower porosity fibers. The coating deposited by the solution methodand a relatively higher oxidation resistance. On the had a relatively low oxidation resistance, but its pro-other hand, the porosity in coating was closely related cess was simple and of low cost, so that it could beto the coating process parameters. Hence, the process widely applied in the fabrication of composites.must be optimized and exactly controlled at every step.The key problem is to get a real sol containing the Acknowledgements-The research work was supported fi-coating materials. nancially by the National Natural Science Foundation of Comparing coatings from CVD with that from Sol- China and the Corrosion Science Laboratory, Shenyang, Ac-Gel method, the oxidation resistance of the CVD coat- ademia Sinica. The authors wish to express their gratitude to Prof. Zhu-Hong Shen and Prof. Jun-Ying Yang for theiring was higher than that of the Sol-Gel coating. suggestive and beneficial discussions, to Jiu-Hong Zheng,However, the strength of the coated fibers by the so- Guo-Bin Zheng, Min Liu, and Rei Wen for their contribu-lution method remained generally unchanged[l7]. tion to this work, and also to Xue-Ying Wang for her help in preparing this manuscript. 5. CONCLUSION Utilizing CVD technique and CH$iCI, as a pre- REFERENCEScursor, a uniform P-Sic coating was obtained contin-uously on carbon fibers at 1370-1523 K and a suitable 1. M. F. Amateau, J. Composife Muter. 10, 279 (1976). 2. A. A. Baker, A. Matin, and R. J. Bathe, Composites9,H,/MMT molar ratio. The coating thickness was ad- 154 (1971).justed by the take-up speed of carbon fibers. The Sic 3. K. Honjo and A. Shindo, Yogyo Kyokai-Shi 99, 172coating improved the oxidation resistance of carbon (1986).fibers. 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