Ceramics International 23(1997)209-213 Printed in Great Britain. All rights reserved PII:S02728842(96)000272 0272.8842/9717.00+00 The Oxidation Behaviour of carbon Reinforced Glass Matrix Composites K. L Page D.J Duquette Rensselaer Polytechnic Institute, Troy, New York, 12180-3590, USA (Received 28 September 1995: accepted 7 January 1996) Abstract: Oxidation experiments have been performed on carbon fibre reinforced glass matrix composites as a function of glass composition. The oxidation resis- tance of the carbon fibres has been shown to be related to the microstructure of the glass and the adherence of the glass to the carbon fibres. Coating of the fibres with a Sic Cvd precursor has been shown to improve the oxidation resistance of the fibres. However, post-exposure mechanical characterization has shown an inverse relationship between oxidation resistance and the ability of the composites to exhibit "graceful"failures. C 1997 Elsevier Science Limited and Techna S r 1. All 1 INTRODUCTION poration of graphite fibres in glass serves to protect the relatively stiff and high strength fibres from The demand for stiffer, stronger and lighter mate- oxidation while lending strength and stiffness to rials has resulted in the development of a large the glass matrix. Early work that examincd this number of aligned fibre composites. The matrices concept showed that the incorporation of graphite for these materials can be polymeric, metallic, fibres in Pyrex glass resulted in a significant ceramic, or, as in the material described here, glass. increase in the work of fracture. -However, these The reinforcing phases are generally selected for same studies indicated that the use of glass/ high stiffnesses and, accordingly, are often ceramic graphite composites was oxidation limited, oxygen materials, although there are a few composites that attacking the fibres through cracks in the glass utilize metals, and a number of polymer based matrix, or at fibre ends. 2. 9 The cracks in the gla composites utilize graphite as the reinforcing matrix generally form during fabrication because phase. Graphite is seldom used with metal matrix of the coefficient of thermal expansion(cte)dif- composites because it reacts with most metals. For ference between the glass and the graphite. Typical composites that are intended for use at elevated glasses have Cte values of 1.7-3.2 ppm/K, while temperatures, graphite has the added disadvantage PAN-based carbon fibres have Cte values of of extraordinary oxidation rates at temperatures 5.5-8.4 ppm/K in the transverse direction and greater than a few hundred degrees Celsius. The -0.5 to-1 3 ppm/K in the longitudinal direction. carbon fibres can be, and are, incorporated into The objective of this work was to attempt to ceramic matrices, but the fabricability of such understand the oxidation behaviour of carbon materials is difficult. An alternative to ceramic fibres in glass matrices as a function of glass com- matrices is glass. Glass has the advantage of being position, and to attempt to develop protective relatively easy to fabricate and to form into struc- coatings for the fibres tural shapes. It is also generally chemically inert has a low density and is relatively inexpensive. The disadvantages of glass are its inherent low tough- 2 EXPERIMENTAL ness at low and intermediate temperatures and its low viscosity as temperature is increased, Incor- The carbon fibres used in this investigation were PAN-based 5C fibres provided by Fiberfil Fibers To whom correspondence should be addressed Inc, a subsidiary of AKZo International. The
Ceramics International 23 (1997) 205-2 13 Q 1997 Elsevier Science Limited and Techna S.r.1. PII: SO272-8842(96)00027-2 Printed in Great Britain. All rights reserved 0272-S&42/97 $17.00+ .OO The Oxidation Behaviour of Carbon Reinforced Glass Matrix Composites K. L. Page & D. J Duquette” Rensselaer Polytechnic Institute, Troy, New York, 12180-3590, USA (Received 28 September 1995; accepted 7 January 1996) Abstract: Oxidation experiments have been performed on carbon fibre reinforced glass matrix composites as a function of glass composition. The oxidation resistance of the carbon fibres has been shown to be related to the microstructure of the glass and the adherence of the glass to the carbon fibres. Coating of the fibres with a Sic CVD precursor has been shown to improve the oxidation resistance of the fibres. However, post-exposure mechanical characterization has shown an inverse relationship between oxidation resistance and the ability of the composites to exhibit “graceful” failures. 0 1997 Elsevier Science Limited and Techna S.r.1. All rights reserved 1 INTRODUCTION The demand for stiffer, stronger and lighter materials has resulted in the development of a large number of aligned fibre composites. The matrices for these materials can be polymeric, metallic, ceramic, or, as in the material described here, glass. The reinforcing phases are generally selected for high stiffnesses and, accordingly, are often ceramic materials, although there are a few composites that utilize metals, and a number of polymer based composites utilize graphite as the reinforcing phase. Graphite is seldom used with metal matrix composites because it reacts with most metals. For composites that are intended for use at elevated temperatures, graphite has the added disadvantage of extraordinary oxidation rates at temperatures greater than a few hundred degrees Celsius. The carbon fibres can be, and are, incorporated into ceramic matrices, but the fabricability of such materials is difficult. An alternative to ceramic matrices is glass. Glass has the advantage of being relatively easy to fabricate and to form into structural shapes. It is also generally chemically inert, has a low density and is relatively inexpensive. The disadvantages of glass are its inherent low toughness at low and intermediate temperatures and its low viscosity as temperature is increased. Incor- *To whom correspondence should be addressed. 209 poration of graphite fibres in glass serves to protect the relatively stiff and high strength fibres from oxidation while lending strength and stiffness to the glass matrix. Early work that examined this concept showed that the incorporation of graphite fibres in Pyrex @ glass resulted in a significant increase in the work of fracture.‘-* However, these same studies indicated that the use of glass/ graphite composites was oxidation limited, oxygen attacking the fibres through cracks in the glass matrix, or at fibre ends.2$g The cracks in the glass matrix generally form during fabrication because of the coefficient of thermal expansion (CTE) difference between the glass and the graphite. Typical glasses have CTE values of 1.7-3.2 ppm/K, while PAN-based carbon fibres have CTE values of 5.5-8.4 ppm/K in the transverse direction and -0.5 to - 1.3 ppm/K in the longitudinal direction.‘O The objective of this work was to attempt to understand the oxidation behaviour of carbon fibres in glass matrices as a function of glass composition, and to attempt to develop protective coatings for the fibres. 2 EXPERIMENTAL The carbon fibres used in this investigation were PAN-based 5C fibres provided by Fiberfil Fibers, Inc., a subsidiary of AKZO International. The
K L Page, D. Duquette Table 1. Properties of glass matrices The composites were fabricated by aligning the Glass Tg( C) Density CTE E(GPa) fibres in (g/cm3)(10-6/K with a slurry of glass powder, an organic surfac- 7070 2.13 3.2 tant, an organic binder and distilled water. After B10 590 infiltration the water was driven off with a heat B15 510 60 lamp, resulting in a single composite ply. The plies were cut to size(5 mmx5 mm)and heated in air at 325C for 3 h to remove the surfactant and binder Seven plies were stacked together and hot pressed 1210 in an argon atmosphere at 1200C for 7070, 1.0103 l400° C for B10and1350° C for e15. A pressure of 7 MPa was applied throughout the heating and 80104 cooling cycle of approximately 120 min. This fabrication process resulted in fibre reinforced glass matrix composites approximately 3 mm thick ∑4.010 containing approximately 30-40% fibres by volume depending f the glass and of the 2010 fibres(coated vs uncoated ). The actual volume fractions were determined utilizing an image 0 uTTTTTTTTTTTTT processing program and an optical microscope 050100150200250300350 Oxidation studies were performed on 45 mIn Time (hours) square samples cut from the hot pressed composite Fig. 1. Oxidation behaviour of a 7070 glass/carbon fibre Samples, in dried laboratory air, at 450 and 550C composite at450°C Oxidation rates were determined from thermo gravimetric measurements. Physical observations indicted that all of the oxidation damage occurre fibres were received as"tapes"consisting of 4 fibre on the fibres and the oxidation rates reported here tows with 50,000 fibres per tow. The tensile are normalized for the fbre volume fractions in the strength of the fibres was 2760 MPa and the tensile composites modulus was 345 GPa, according to the manufac The effects of oxidation on the mechanica turer's specifications. The fibres are circular in properties of carbon fibre reinforced glass compo cross-section,with a diameter of 6.7=0.5 um. The sites were also examined. For these tests composite fibres exhibit an"onion skin"structure, with the samples were tested in three-point bending at room outer skin consisting of graphene with a(0002) temperature. The samples tested were 32 mm x texture, and the core consisting of amorphous 3 mm x 2 mm rectangular samples. Unoxidized carbon. Some of the fibres were coated either by samples were compared with samples that had hemical vapour deposition (CVD) or simply been oxidized for 300 h at either 450C or at by dipping the coatings in a CVD precursor. The 550C. The mechanical properties were determined precursor used for CVd was 1, 3-dimethyl-3-methyl- from the standard elastic bent beam formula deposits amorphous Sic at temperatures less thao a= bd, where a is the flexural strength, P is the 1000.CI1 The dip coating was a highly branched dimensions of the sample hydridopolycarbosilane developed at Rensselaer Polytechnic Institute. 12 Three glass compositions were used as matrix 3 RESULTS AND DISCUSSION materials: Corning Glass Works 7070, a sodium borosilicate glass containing 1%Al 2O3, 26% B2O3, Figure 1 shows the results of oxidation of a 7070 0.5%0 Li2O and 1% K2O; b10 glass containing glass/uncoated carbon fibre composite at 450C 10% B2O3; and bl5 glass containing 15% B2O3. plotted as the square of the weight loss vs time, The glasses containing B2O3 as the sole additive indicating that the oxidation rate is parabolic were selected because NayO is believed to be Figure 2 compares the data at 450C with data at catalyst for the oxidation of carbon. The 7070 glass 550C. Note that the oxidation rate at the higher was obtained commercially in powder form, while temperature is linear, indicating a change in the the B10 and b15 glasses were fabricated from a rate determining mechanism sol-gel process. The properties of the glasses are Figure 3 shows the weight loss vs time plots for shown in table 1 the bl5 glass at 450 and 550C and demonstrates
210 K. L. Page, D. J Duquette Table 1. Properties of glass matrices The composites were fabricated by aligning the fibres in a metal frame and infiltrating the fibres with a slurry of glass powder, an organic surfactant, an organic binder and distilled water. After infiltration the water was driven off with a heat lamp, resulting in a single composite ply. The plies were cut to size (5 mmx 5 mm) and heated in air at 325 “C for 3 h to remove the surfactant and binder. Seven plies were stacked together and hot pressed in an argon atmosphere at 1200°C for 7070, 1400°C for BlO and 1350°C for B15. A pressure of 7 MPa was applied throughout the heating and cooling cycle of approximately 120 min. This fabrication process resulted in fibre reinforced glass matrix composites approximately 3 mm thick, containing approximately 3@-40% fibres by volume, depending on the nature of the glass and of the fibres (coated vs uncoated). The actual volume fractions were determined utilizing an image processing program and an optical microscope. Glass Tg (“(3 Density (s/cm3) CTE E (GPa) (1 0-6/K) 7070 495 2.13 3.2 51 BIO 590 2.10 1.7 60 815 510 2.10 2.2 60 -_ ; 6.0 r” 4.0 + 2.0 z z? - 0 50 100 150 200 250 300 350 Time (hours) Fig. 1. Oxidation behaviour of a 7070 glass/carbon fibre composite at 450 “C. fibres were received as “tapes” consisting of 4 fibre tows with 50,000 fibres per tow. The tensile strength of the fibres was 2760 MPa and the tensile modulus was 345 GPa, according to the manufacturer’s specifications. The fibres are circular in cross-section, with a diameter of 6.7 f 0.5 pm. The fibres exhibit an “onion skin” structure, with the outer skin consisting of graphene with a (0002) texture, and the core consisting of amorphous carbon. Some of the fibres were coated either by chemical vapour deposition (CVD) or simply by dipping the coatings in a CVD precursor. The precursor used for CVD was 1,3-dimethyl-3-methylsilamethylene- 1,3-disilacyclobutane. This precursor deposits amorphous Sic at temperatures less than 1000°C.” The dip coating was a highly branched hydridopolycarbosilane developed at Rensselaer Polytechnic Institute.12 Three glass compositions were used as matrix materials: Corning Glass Works 7070, a sodium borosilicate glass containing 1% A1203, 26% B203, 0.5% Liz0 and 1% K,O; BlO glass containing 10% B203; and B15 glass containing 15% B203. The glasses containing BzOs as the sole additive were selected because NazO is believed to be a catalyst for the oxidation of carbon. The 7070 glass was obtained commercially in powder form, while the BlO and B15 glasses were fabricated from a sol-gel process. l3 The properties of the glasses are shown in Table 1. Oxidation studies were performed on 45 mm square samples cut from the hot pressed composite samples, in dried laboratory air, at 450 and 550 “C. Oxidation rates were determined from thermogravimetric measurements. Physical observations indicted that all of the oxidation damage occurred on the fibres and the oxidation rates reported here are normalized for the fibre volume fractions in the composites. The effects of oxidation on the mechanical properties of carbon fibre reinforced glass composites were also examined. For these tests composite samples were tested in three-point bending at room temperature. The samples tested were 32 mm x 3 mm x 2 mm rectangular samples. Unoxidized samples were compared with samples that had been oxidized for 300 h at either 450°C or at 550 “C. The mechanical properties were determined from the standard elastic bent beam formula: 0 = $, where o is the flexural strength, P is the applied load and Z, b and d are the physical dimensions of the sample. 3 RESULTS AND DISCUSSION Figure 1 shows the results ‘of oxidation of a 7070 glass/uncoated carbon fibre composite at 45O”C, plotted as the square of the weight loss vs time, indicating that the oxidation rate is parabolic. Figure 2 compares the data at 450°C with data at 550°C. Note that the oxidation rate at the higher temperature is linear, indicating a change in the rate determining mechanism. Figure 3 shows the weight loss vs time plots for the B15 glass at 450 and 550°C and demonstrates
Carbon reinforced glass matrix composites 211 0.50 60 0.40 450°C 0.50 550°C 0.40 7070 0.30 98 a0.20 E0.20 0.10 010 00 9050100150200 3003500004== 1952635 Time (hours) Time (hours) Fig. 2. A comparison of the oxidation behaviour of a 7070 Fig 4. Comparative oxidation behaviour for 7070, B10 and glass carbon fibre composite at 450 and 550C. blS glass composites at 450C 4.0 7070/UCF 3.0 0 450°C 59z 15 7070/cvD 0.10 550° 0.05 1.0 000 异0.0 050100150200250300350 050100150200250300350 Time (hours) Time (hours) Fig. 3. Oxidation behaviour of B15 glass composite at Fig. 5. Oxidation behaviour of 7070 glass compo CvD coated fibres at 450.C that the behaviour of the B15 composite is similar to least oxidation resistance the b15 glass is inter- that of the 7070 glass composite, exhibiting parabolic mediate, and the 7070 glass, with the largest amount behaviour at 450C and linear behaviour at 550 C. of B O3, exhibits the best oxidation resistance Figure 4 compares the oxidative weight losses of the Since the 7070 glass matrix composite exhibited carbon in the three glass matrices at 450 C the best oxidation behaviour of the three glass n contrast to the assumptions that Na20 would matrices examined, composites with either CVD or atalyse the carbon oxidation reaction, it appears dip coated fibres were fabricated. CVD coating that B2O3 inhibits oxidation. This behaviour of the fibres increases the oxidation rates of the can be understood from examinations of the composites, as shown in Fig. 5, for a 7070 glass microstructures of the composites. The B10 and matrix composite. Dip coating of the fibres, on the B15 glasses, which exhibit a higher glass transition other hand, results in significant decrease in the temperature, are less viscous during fabrication by oxidative weight loss of the 7070 glass matrix pressing (although different pressing temperatures composites, Figure 6 shows the oxidative weight gere utilized for each glass to account for the loss behaviour of the 7070 glass matrix composite differences in viscosity ). Accordingly, the glasses and shows that at 550 C the oxidation behaviour that contain less B2O3 as a vitrifier tend to be more of the composite containing coated fibres changes orous and show poorer adherence to the carbon to a parabolic rate in contrast to the linear oxida fibres. Both phenomena lead to short circuit diffu tion rate exhibited by the composite containing sion paths for oxygen, as well as for the gaseous uncoated fibres. Some benefit of dip coating the oxidation products. Thus the B10 glass shows the fibres is also observed at 450C
Carbon reinforced glass matrix composites rA 0 50 100 150 200 250 300 Y, Time (hours) Fig. 2. A comparison of the oxidation behaviour of a 7070 glass/carbon fibre composite at 450 and 550 “C. l 2 1.0 Y( .W & z a 0.0 I 0 50 100 150 200 250 300 350 Time (hours) Fig. 3. Oxidation behaviour of B15 glass composite at 450 and 550°C. that the behaviour of the B 15 composite is similar to that of the 7070 glass composite, exhibiting parabolic behaviour at 450°C and linear behaviour at 550°C. Figure 4 compares the oxidative weight losses of the carbon in the three glass matrices at 450 “C. In contrast to the assumptions that Na20 would catalyse the carbon oxidation reaction, it appears that B203 inhibits oxidation. This behaviour can be understood from examinations of the microstructures of the composites. The BlO and B15 glasses, which exhibit a higher glass transition temperature, are less viscous during fabrication by pressing (although different pressing temperatures were utilized for each glass to account for the differences in viscosity). Accordingly, the glasses that contain less B203 as a vitrifier tend to be more porous and show poorer adherence to the carbon fibres. Both phenomena lead to short circuit diffusion paths for oxygen, as well as for the gaseous oxidation products. Thus the BlO glass shows the Time (hours) Fig. 4. Comparative oxidation behaviour for 7070, BlO and B15 glass composites at 450 “C. u 0.05 2 E 3 0.00 I”“I’-l~“‘l~~~‘I~~~‘l’~~~ 0 50 100 150 200 250 300 3 Time (hours) i0 Fig. 5. Oxidation behaviour of 7070 glass composite containing CM) coated fibres at 450 “C. least oxidation resistance, the B15 glass is intermediate, and the 7070 glass, with the largest amount of Bz03, exhibits the best oxidation resistance. Since the 7070 glass matrix composite exhibited the best oxidation behaviour of the three glass matrices examined, composites with either CVD or dip coated fibres were fabricated. CVD coating of the fibres increases the oxidation rates of the composites, as shown in Fig. 5, for a 7070 glass matrix composite. Dip coating of the fibres, on the other hand, results in significant decrease in the oxidative weight loss of the 7070 glass matrix composites. Figure 6 shows the oxidative weight loss behaviour of the 7070 glass matrix composite, and shows that at 550°C the oxidation behaviour of the composite containing coated fibres changes to a parabolic rate in contrast to the linear oxidation rate exhibited by the composite containing uncoated fibres. Some benefit of dip coating the fibres is also observed at 450°C
212 K.L. Page, D 060 7070/DCF5509 b0.50 (450 -uNoxidized 200」 -Oxidized 450.C 0.40 o-700/UcF(550°C) 030 E0. 0. 0.004午千 050100150200250300350 0010020.03004005 Time (hours) Strai Fig. 6. Oxidation behaviour of 7070 glass composite containing Fig 8. Mechanical properties of B10 glass composite before dip coated fibres(dCf)and uncoated fibres(UCF and after oxidation at 450.C 250 Unoxidized Oxidized 450%C 3001 Oxidized550℃ 一烟 150 50 0002 0.003 010020030.04 Strain Strain Fig. 7. Mechanical properties of 7070 glass composite before Fig. 9. Mechanical properties of bl5 glass composite before and after oxidation at 450 and 550 e. The increased oxidation rate of the CVD coated matrix composite is degraded by oxidation;the fibres can be directly correlated to the structure and degree of degradation being controlled by the properties of the coating. The CVD coatings tended amount of oxidation to crack and pull away from the fibres during The mechanical properties of the B10 and B15 fabrication. This resulted in short circuit paths for glass composites are shown in Figs 8 and 9. res- oxygen, and the conversion of the oxidation rate pectively. Both composites exhibit lower flexural from diffusion control (parabolic kinetics)to reac- strengths than the 7070 glass composites.However tion rate control(linear kinetics). The dip coatings, both composites exhibit"graceful"failure, with on the other hand, were adherent to the carbon, strains to failure in the 4% range(in contrast to the provided a diffusion barrier to gas transport, and 7070 glass composite which exhibited a maximum retained diffusion controlled kinetics to 550oC strain of 0. 25%). Also, the 7070 glass compositc Figure 7 compares the flexural strength of the exhibited a completely brittle failure while the b10 7070 glass oxidized at 450 and 550C for 300 h In and B15 glass composites did not exhibit a clean each case the mechanical response of the compo- break. Finally, although the flexural strength of the sites indicates a strong bond between the fibres and B10 and b15 glass composites was markedly the matrix. The flexural strength of the 7070 glass reduced, the strains to failure for these composites composite is also the highest of the three glasses were virtually unaffected by prior oxidation Oxida also indicative of a strong bond between the fibres tion attacks the fibres closest to the surfaces of the ind the matrix. In each case, however, the flexural composite samples, reducing their flexural strength strength and the strain to failure of the 7070 glass Fibres removed from the surface, however, provide
K. L. Page, D. J Duquette + 707O/DCF(%O”C) + 7070/UCF(450°C) 0.20 0.10 0.00 0 50 100 150 200 250 300 350 Time (hours) Fig. 6. Oxidation behaviour of 7070 glass composite containing Fig. 8. Mechanical properties of BlO glass composite before dip coated fibres (DCF) and uncoated fibres (UCF). and after oxidation at 450 “C. 0.001 0.002 0. Strain Fig. 7. Mechanical properties of 7070 glass composite before and after oxidation at 450 and 550°C for 300 h. Fig. 9. Mechanical properties of B15 glass composite before and after oxidation at 450 and 550 “C. The increased oxidation rate of the CVD coated fibres can be directly correlated to the structure and properties of the coating. The CVD coatings tended to crack and pull away from the fibres during fabrication. This resulted in short circuit paths for oxygen, and the conversion of the oxidation rate from diffusion control (parabolic kinetics) to reaction rate control (linear kinetics). The dip coatings, on the other hand, were adherent to the carbon, provided a diffusion barrier to gas transport, and retained diffusion controlled kinetics to 550 “C. Figure 7 compares the flexural strength of the 7070 glass oxidized at 450 and 550°C for 300 h. In each case the mechanical response of the composites indicates a strong bond between the fibres and the matrix. The flexural strength of the 7070 glass composite is also the highest of the three glasses, also indicative of a strong bond between the fibres and the matrix. In each case, however, the flexural strength and the strain to failure of the 7070 glass Strain .$150 J UJ 100 f ; 50 E = 0 0 0.01 0.02 0.03 0 Strain 14 matrix composite is degraded by oxidation; the degree of degradation being controlled by the amount of oxidation. The mechanical properties of the BlO and B15 glass composites are shown in Figs 8 and 9, respectively. Both composites exhibit lower flexural strengths than the 7070 glass composites. However, both composites exhibit “graceful” failure, with strains to failure in the 4% range (in contrast to the 7070 glass composite which exhibited a maximum strain of 0.25%). Also, the 7070 glass composite exhibited a completely brittle failure, while the BlO and B15 glass composites did not exhibit a clean break. Finally, although the flexural strength of the B 10 and B 15 glass composites was markedly reduced, the strains to failure for these composites were virtually unaffected by prior oxidation. Oxidation attacks the fibres closest to the surfaces of the composite samples, reducing their flexural strength. Fibres removed from the surface, however, provide
Carbon reinforced glass matrix composites 213 the crack bridging mechanism required to provide oxidation resistance of the fibres in a 7070 toughness and"graceful "failures. The results of glass matrix compositc he mechanical property studies, combined with 5. Composites fabricated with 7070 glass show the oxidation results, confirm the microstructural high flexural strength but poor toughness bservations that the 7070 glasses exhibit a strong Pre-oxidation of the composites reduces the bond with the carbon fibres while the b10 and b15 flexural strength and the strain to failur glasses do not. The result of the nature of the glass nt of B2o to fibre bonding is that the 7070 glasses exhibit matrix of the composites results in a reduction better intrinsic oxidation resistance. However the in flexural strength, but the reduction in weaker bonding in the b10 and b15 glasses results interfacial adhesion provides for"graceful perior composite properties, although the failures. Pre-oxidation of these flexural strengths are lower and the reductions in also reduces the fexural strength, but the flexural strength as a result of oxidation of the failure mode remains"graceful carbon fibres are greater ACKNOWLEDGEMENTS 4 CONCLUSIONS The authors would like to acknowledge the generous The following conclusions can be drawn financial assistance of AKZO International and the 1. The oxidation kinetics of glass matrix/carbon Gallo encouragement of Dr Francis Via and Dr Thomas fibre composites with 40% carbon fibres is parabolic at 450C, but becomes linear at REFERENCES 2. Reducing the amount of B2O3 in matrix of the composite, which incr the 1. PREWO, K.w., Am. Ceram. Bull., 68(2)(1989)395 glass transition temperature, increases the oxi 2. SAMBELL, R. A J, BOWEN, D. H.& PHILLIPs, D dation rate of the carbon fibres The increase in C.,J. Mater.Sci.,7(1972)663 3. PREWOK. M. brennan. J laYdeN. G. K oxidation rate is associated with greater poros Ceran.bll,5(2)(1986)305 ity in the glass and poorer adhesion of the glass 4. CRANMER, D. C, Ceram. Bull., 68(2)(1989)415 to the carbon fibres. Both phenomena provide 5. PREWO, K. M.& brennaN, J J,, J Mater. Sci., 15 (1980)463. short circuit gas diffusion paths 6. PREWO, K.M.,J. Mater. Sci, 23(1988)2745 3. CVd coating of the carbon fibres with SiC 7. PREWO, K. M.,J. Mater. Sci, 17(1982)3549 prior to fabrication results in increases of oxi 8. PREwo, K M.& Batt, J. A.,J. Mater. Sci., 23(1988) dation rates for 7070 glass matrix composites. 9. IACCOCA, R.G,Ph D.Dissertation,Rensselaer The increase in oxidation rates is associated Polytechnic Institute Troy New York, 1991 with cracking and debonding of the coating 10. CHAWLA, K.K., Composite Materials. Springer Verlag Berlin, 1987. during fabrication, providing short circuit 11. LARKIN.D J. INTERRANTE. L v. huDSoN.JB diffusion paths between the coating and the Han, B, Mater. Res. Soc. Symp. Proc., 204(1991)141 carbon fibres 12. WHITMARSH. C. INTERRANTE. L. V 4. Dip coating of carbon fibres with a CVD Organometallics, 10(1991)1336 13. JABRA, R, PhaLIPPOU, J. ZaRzYCKI, J, J. precursor prior to fabrication increases the on-Cryst. Solids, 42(1980)489
Carbon reinforced glass matrix composites the crack bridging mechanism required to provide toughness and “graceful” failures. The results of the mechanical property studies, combined with the oxidation results, confirm the microstructural observations that the 7070 glasses exhibit a strong bond with the carbon fibres, while the BlO and B 15 glasses do not. The result of the nature of the glass to fibre bonding is that the 7070 glasses exhibit better intrinsic oxidation resistance. However, the weaker bonding in the BlO and B15 glasses results in superior composite properties, although the flexural strengths are lower and the reductions in flexural strength as a result of oxidation of the carbon fibres are greater. 213 oxidation resistance of the fibres in a 7070 glass matrix composite. 5. Composites fabricated with 7070 glass show high flexural strength but poor toughness. Pre-oxidation of the composites reduces the flexural strength and the strain to failure. 6. Reducing the amount of B203 in the glass matrix of the composites results in a reduction in flexural strength, but the reduction in interfacial adhesion provides for “graceful” failures. Pre-oxidation of these composites also reduces the flexural strength, but the failure mode remains “graceful”. ACKNOWLEDGEMENTS 4 CONCLUSIONS The 1. 2. 3. 4. following conclusions can be drawn: The oxidation kinetics of glass matrix/carbon fibre composites with -40% carbon fibres is parabolic at 45O”C, but becomes linear at 550 “C. Reducing the amount of B203 in the glass matrix of the composite, which increases the glass transition temperature, increases the oxidation rate of the carbon fibres. The increase in oxidation rate is associated with greater porosity in the glass and poorer adhesion of the glass to the carbon fibres. Both phenomena provide short circuit gas diffusion paths. CVD coating of the carbon fibres with SIC prior to fabrication results in increases of oxidation rates for 7070 glass matrix composites. The increase in oxidation rates is associated with cracking and debonding of the coating during fabrication, providing short circuit diffusion paths between the coating and the carbon fibres. Dip coating of carbon fibres with a CVD precursor prior to fabrication increases the The authors would like to acknowledge the generous financial assistance of AKZO International and the encouragement of Dr Francis Via and Dr Thomas Gallo. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. PREWO, K. W., Am. Ceram. Bull., 68(2) (1989) 395. SAMBELL, R. A. J., BOWEN, D. H. & PHILLIPS, D. C., J. Mater. Sci., 7 (1972) 663. PREWO, K. M., BRENNAN, J. J. & LAYDEN, G. K., Ceram. Bull., 65(2) (1986) 305. CRANMER, D. C., Ceram. Bull., 68(2) (1989) 41.5. PREWO, K. M. & BRENNAN, J. J., J. Mater. Sci., 15 (1980) 463. PREWO, K. M., J. Mater. Sci., 23 (1988) 2745. PREWO, K. M., J. Mater. Sci., 17 (1982) 3549. PREWO, K. M. & BATT, J. A., J. Mater. Sci., 23 (1988) 523. IACCOCA, R. G., Ph.D. Dissertation, Rensselaer Polytechnic Institute, Troy, New York, 1991. CHAWLA, K. K., Composite Materials. Springer Verlag, Berlin, 1987. LARKIN, D. J., INTERRANTE, L. V., HUDSON, J. B. & HAN, B., Mater. Res. Sot. Symp. Proc., 204 (1991) 141. WHITMARSH, C. & INTERRANTE, L. V., Organometaliics, 10 (1991) 1336. JABRA, R., PHALIPPOU, J. & ZARZYCKI, J., J. Non-Cryst. Solids, 42 (1980) 489