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MateriaIs & Design, Vol. 18, No. 1, 11-15.1997 pp. 0 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0261-3069/97 $17.00 + 0.00

Carbon-carbon composites: a summary of recent developments and applications

Torsten Windhorst, Gordon Blount*

Coventry University, School of Engineering, Priory Street, Coventry CVI 5FB, UK

Received 4 March 1997; accepted 21 April 1997

Carbon Fibre Reinforced Carbon (CFRC), or Carbon-carbon, is a unique composite material consist- ing of carbon fibres embedded in a carbonaceous matrix. Originally developed for aerospace applications, its low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for aircraft brakes, rocket nozzles and re-entry nose tips. It withstands temperatures in excess of 2000°C without major deformation. The properties are very much dependent on the manufacturing methods used for production. Although the general production technology is known, the combination of processes to achieve specially tailored properties remains the expertise of particular manufacturers. This paper reviews major developments of Carbon-carbon composites and describes actual and future applications. Improved oxidation resistance and continu- ously decreasing manufacturing costs make this family of materials more and more attractive to high performance applications as well as for general engineering design. 0 1997 Elsevier Science Ltd

Keywords: carbon matrix composites (A); fibres and filaments (B); thermal properties (E)

Introduction

Carbon-carbon composites are a family of advanced composite materials. They are the most advanced form of carbon and consist of a fibre based on carbon precursors embedded in a carbon matrix. This unique composition gives them such properties as low density, high thermal conductivity and shock resistance, low thermal expansion and high modulus. Carbon-carbon is mostly used in aerospace applications, mainly for aircraft disc brakes, rocket re-entry nose tips and for parts of rocket nozzles. The unique features and an advanced manufacturing technology which leads to a cheaper production process make this material more and more available for industrial applications.

Carbon-carbon composites were initially developed for the American defence and space industry, which was funded by the American government. They were subject on restrictions and dissemination, in particular to foreign countries. Brennan Forcht of Chance Vought Aircraft is generally credited as the discoverer of this material, which was first developed in 1958. In the mid 1960s the first rayon-based materials with acceptable tensile strength were developed. In 1969 the first high performance and lower cost polyacrylonitrile (PAN)- based fibres were commercialised and gradually re- placed rayon. The material was identified as friction material for aircraft brakes and fitted to the Concorde

*Correspondence to Prof. G. BIount. Tel.: f44 1203 838829; fax: + 44 1203 838272.

commercial supersonic passenger airliner by Dunlop in 1974l. Further research led to standard installation of carbon brakes in about 50 aircraft types. Graphitic pitch-based carbon fibres with ultra-high moduli values and very high thermal and electrical conductance val- ues became available in 1982. Today, PAN-based car- bon fibres are the dominant reinforcement used in Carbon-carbon composites.

There are about 40 companies in the world dealing with some form of carbon-carbon composites. The major market share is kept by American companies followed by French and British companies. Other coun- tries with more focus on industrial rather than aeros- pace and military applications are Germany and Japan*.

Manufacturing and machining

The fabrication of carbon-carbon composites consists mainly of producing the fibre precursor and then densi- fying this with the carbon matrix. The principles of the manufacturing process used in laboratories are well documented’-‘, but the technology used in production is normally regarded as confidential. The basic routes and precursor materials for production are the same for all carbon-carbon materials, but details of the techniques used are known only to the manufacturer.

The fibres are based on rayon, PAN or pitch, where PAN-based fibres are the most widely used. Rayon was the first precursor to be used in carbon-carbon pro-

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Carbon-carbon composites: T. Windhorsf and G. Blounf

Figure 1 Bi-directional woven fibres.

duction but the properties which can be achieved with rayon are far lower than with the other precursors. Pitch-based fibres lead to high level properties but their use is limited by the high price and the limited number of suppliers. Fibres are usually woven in ap- propriate directions to tailor specific properties. Neu- meister 3 investigated the mechanical performance of carbon-carbon composites with different fibre archi- tecture. The result of this research programme was that, although unidirectional non-woven composites had the highest tensile strength, woven material behaved more consistently and showed less variation in proper- ties. Figure I shows an example of a bi-directional woven fibre. The fibre content is in the range between 20-60% by volume depending on the properties to be achieved and the manufacturing processes used. Before impregnation, oxidized PAN- and resin-based fibres are carbonized at > 1000°C in an inert atmosphere (e.g. argon) or vacuum to convert the fibres to carbon.

Matrices are carbon-containing substances which are in the form of either liquid or gas. Liquid matrices are either resin-based or pitch-based and three processes for impregnation are used4:

1. Liquid Phase Impregnation (LPI) 2. Hot Isostatic Pressure Impregnation Carbonization

(HIP10 3. Hot Pressing.

Before liquid impregnation the fibres are pre-im- pregnated with resin or pitch and carbonized at 350-850°C. A pressure of up to 100 MPa can be applied which increases the carbon yield up to 90%. Then the liquid impregnation processes vary signifi- cantly.

In LPI a vacuum impregnation process adds more pitch or resin to the composite which increases density and interlaminar shear strength (ILSS). Intermediate graphitization at 2200-3000°C opens the closing pores again and further impregnation leads to higher density. The impregnation process is typically repeated 3-6 times.

With HIPIC the material is carbonized/impregnated at 650-1000°C at a pressure of 100 MPa. The high pressure increases carbon yield and maintains more volatile fractions of pitch in a condensed phase. The composite is then graphitized at > 2000°C without pressure. The HIPIC process produces carbon-carbon

12 Materials & Design Volume 18 Number 1 1997

composites with higher density than LPI but it is also more expensive.

Hot Pressing (also called High Temperature Consoli- dation) uses carbonization at either 650°C or lOOO”C, depending on the matrix precursor. A pressure of up to 76 MPa can be applied in an inert, reducing or vacuum atmosphere. Subsequent graphitization at 2200-3000°C without pressure can lead to graphitization of ther- mosetting resins, which are harder to graphitize than pitch. The part thickness is reduced by 50% during these processes. Figure 2 gives an overview of the manufacturing processes used.

For impregnating carbon fibres with a matrix in the form of gas Chemical Vapour Infiltration (CVI, or also called Chemical Vapour Deposition, CVD> is used. At 700-2000°C hydrocarbon gas is impregnated into the preforms, which can be prepregs (carbonized and graphitized fibres) or dry wound fibres. Three CVI methods are available:

1. Isothermal: the gas and sample kept at uniform temperature; several cycles are necessary and the surface is skinned in-between cycles

2. Temperature gradient: use of induction furnace, deposition of carbon first inside the sample; process is limited to one single sample per operation

3. Pressure gradient: a hydrocarbon gas impinges on the inside surface of the sample, the gas pressure inside the sample is higher than outside; also limited to single sample and not widely used.

The CVI process achieves a low rate of carbon deposition, but the carbon is harder than carbon from pitch or resin. Most aircraft brake materials are pro- duced by this method.

Further graphitization of the composites is possible to enhance thermal conductivity. It will result in a decrease in mechanical properties. The processes de- scribed above, especially CVI, need several weeks. The time consumption combined with the high pressures and temperatures needed make the material relatively expensive.

Gxidation of carbon-carbon composites is a real problem above 320°C. It attacks the fibre-matrix inter- face and weakens the fibre bundles. A combination of four methods can be used to counter this5.

1. Sic coating of the outer composite surface. 2. Internal protection by oxidation inhibitors intro-

duced in the matrix during lay up and densification. 3. Glassy sealant on top of the Sic coating. 4. Sic or Si,Ni, overlayer on glassy layer.

The addition of glass-forming additives like boron, silicon carbide and zirconium boride reduce the reac- tivity with air.

Machining of carbon-carbon composites is a com- plex area, especially as companies are keen to keep their knowledge under secrecy. Turning and especially thread cutting is a problem, as the fibres and fibre direction result in an uneven cutting force and high tool wear. Diamond tipped cutting tools or silicon carbide tools are commonly used. Lubrication is nor- mally not required, as the carbon is self-lubricating. Other cooling liquids than water would probably influ-

4

PrePreparation

I LPI

PRECURSORS

I Vacuum Impregnation

I HIPIC

/

Carbonization/ Impregnation

I Hot

Pressing

+

Carbonization

Carbon-Carbon Composite

Figure 2 Carbon-carbon manufacturing processes.

ence the later use of the high temperature material, as electrical conductivity are 12-15 times higher in the they contain oil. fibre direction than perpendicular to the fibres.

Properties

The properties are very much dependent on the manu- facturing processes used, on the raw materials and on additional treatments such as fibre surface modification or inclusion of oxidation protection. Considerable changes in

r roperties can be achieved by varying these

parameters 9’. Table I shows the range of properties that can be achieved and for comparison those of ferritic steel and titanium alloys. The properties for carbon-carbon are given in the fibre direction. Perpen- dicular to the fibres, the properties are much less superior.

Carbon-carbon composites have the capability of structural integrity at temperatures above 1000°C. Compared to other composite materials, carbon-carbon has a very high thermal conductivity. The thermal and

Table 1 Range of properties for Carbon-carbon composites

The mechanical properties are much superior to those of conventional graphite. In particular three-di- mensional carbon-carbon composites can be tailored to withstand damage and minimum delamination crack growth under interlaminar shearing.

The tensile strength increases above 1200°C when conventional superalloy components start to weaken; the density is only about 1.9 g cme3 compared with 8 g cme3 for superalloys and in the event of failure the material does not disintegrate catastrophically, but goes through a gradual failure that has been called graceful failure.

Heat treatment temperature has a significant effect on the mechanical properties. When carbonized at 1000°C upon subsequent graphitization at 2700°C the material shows a 54% increase in the flexural strength, a 40% decrease in the interlaminar shear strength and a 93% increase in the flexural modulus. The choice of the graphitization temperature affects the toughness of the composite. The tensile and flexural properties of

Property Unit Carbon-carbon Ferritic steel Titanium alloys

Compressive strength Density Tensile strength Thermal expansion Thermal conductivity Thermal shock resistance Young’s modulus

MPa g cme3 MPa K-1 W m-’ K-’ W mm-’ GPa

100-150 1.3-2.5 up to 900 -2-2 x 10-6 20-150 150-170 up to 300

240-400 130-1400 7.5-7.7 4.38-4.82 500-800 241-1280 12-15 x 10-6 7.9-9.8 x lo-” 23-27 4-21.9 5.5 N/A 200-205 95-125

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Carbon-carbon composites: T. Windhorst and G. Bfount

carbon-carbon composites are fibre-dominated, whereas the compression behaviour is mainly affected by density and matrix morphologies. The effect of surface treatment is significant on the mechanical properties of the resulting composites. Surface-treated fibres that have strong bonding with the polymer ex- hibit high flexural strength in polymer composites, but result in carbonized composites of poor flexural strength. For graphitized composites, the flexural strength increases monotonically with increasing treat- ment time.

Carbon-carbon composites with high thermal con- ductivity are important for first wall components for nuclear fusion reactors, hypersonic aircraft, missiles and spacecraft, thermal radiator panels and electronic heat sinks. The thermal conductivity at > 1000°C in- creases with the heat treatment temperature, particu- larly above 28OO”C, as more graphitic carbon is associ- ated with a higher thermai conductivity.

There is one major drawback, however, and that is its susceptibility to oxidation above 500°C which becomes progressively more severe as the temperature rises until, at about BOOC, the rate of oxidation is limited only by the diffusion rate of oxygen through the sur- rounding gas to the carbon surface. In certain ‘one-off applications like rocket motors this is not very impor- tant; however, in space plane and as turbine engine material the development of extended lifetimes of around 100 h is required.

Recyclability of carbon-carbon composites is an- other problem that manufacturers are dealing with. In many cases it is possible to re-use material that is wasted during the manufacturing process, but there is no method to recycle material that has been in service. Some research programmes focus on more common materials like thermoplastic matrix composites, but up to now there is no method to recycle carbon-carbon composites.

Applications

Although the specific strength and thermal properties of carbon-carbon make it the ideal material for high temperature applications, its use has been restricted by two major factors: the high costs and the susceptibility to oxidation. With more than 60% by volume, aircraft disc brakes are the main application. Compared to a steel brake, carbon-carbon has a 2.5 higher heat capac- ity, reduces the weight by 40% and doubles the service life. Other main applications are re-entry heat shields for space vehicles and missiles and rocket nozzles. Since the first use of carbon-carbon in aircraft brakes in 1974, major research programmes have led to new applications, of which some are:

1. Racing car brakes and clutches* 2. Hot glass transfer elements 3. Protective shielding 4. Vacuum/inert gas furnace insulation 5. Hot pressing moulds 6. Metal sintering trays

14 Materials & Design Volume 18 Number 1 1997

7. Electronic circuit board thermal planes 8. Semiconductor manufacturing components.

Although the load bearing ability of carbon-carbon at room temperature is not as high as that of metals, it is superior at high temperatures. This makes it the first choice for high temperature mechanical fasteners, where this material also saves weight. In braking appli- cations it is not the frictional behaviour that is of major interest but the ability to absorb and conduct large quantities of heat in a very short period of time without damaging the brake assembly.

For biomedical devices carbon-carbon is used for prosthetic implants such as hip joint replacements and it has been tested in artificial hearts for animals. It is considered to be ‘bio-active’, as it is compatible with blood, bones and soft tissue and properties can be tailored to be close to those of bones. Other future applications where this material can be considered are protective shielding against X-ray and laser, and parts in gas turbine engines such as flaps, seals, liners, vanes and tailcones. Its high purity and its resistance to heat and ionising radiation make it a possible material for the nuclear industry, it is already being used in the JET (Joint European Torus) fusion reactor.

Due to the secrecy and price of carbon-carbon com- posites, applications have been mainly restricted to military and aerospace use. This is about to change as improved manufacturing facilities and the increased amount of applications reduce the price permanently. The price varies significantly with the manufacturing methods and precursor materials and can be con- sidered currently at around &300 per kg for a medium priced material. Most manufacturers who exclusively served the military market are now expanding their knowledge into the civilian market. This offers new possibilities in major industries such as car manufactur- ing, where carbon-carbon could be used as a brake material or for engine pistons.

Conclusions

Carbon-carbon composites are an advanced composite with superior thermal properties. It is currently the ultimate material for brakes and other high perfor- mance applications. The manufacturing process is very cost intensive as high temperatures and high pressures are required. The main factors influencing the quality are:

. the quality of polymer matrix composite from which carbon-carbon is made

. the choice of pitch as it affects carbon yield l the use of resin . the choice of carbon fibre . the microstructure of mesophase (pitch-based) . the weave pattern of carbon fabric . fibre matrix bond strength . carbonization method/medium . surface treatment of carbon fibres.

The use of carbon-carbon will increase in future as costs are reduced because of larger scale production and improved manufacturing processes. Many new ap- plications can be found when this material is seriously considered as a replacement to environmental damag- ing materials such as asbestos.

Acknowledgements The authors wish to thank the staff at Coventry

University for their helpful and critical support when preparing this study. They also wish to thank Dunlop Aviation Division in Coventry for support and informa- tion provision. Other companies and institutions to include are SGL Carbon in Halifax and Advanced Furnace Technology in Cambridge as interview part- ners and Dr. Steve Appleyard from The University of Leeds for useful information.

Carbon-carbon composites: T. Windhorst and G. Blount

References

Stimson, 1. L. and Fisher, R., Design and engineering of carbon brakes. Phil. Transactions of the Royal Sociefy London A, 1980, 294,583-590. Rohini, G. and Rama Rao, K., Carbon-carbon composites - an overview. Defence Science Journal, 1993,43 (4). 369-383. Neumeister, J. et al., The effect of fiber architecture on the mechanical properties of Carbon/Carbon fiber composites. Acta Materialia, 1996, 44 (21, 573-585.

Chung, D. D. L., Carbon Fiber Composites. Buttenvorth-Heine- mann, Newton, MA, 1994. Thomas, C. R. fed), Essentials of Carbon-carbon Composites. The Royal Society of Chemistry, Cambridge, 1993. Rand, B., High Pe$orrnance Carbon Materials. HIPERMAT Con- ference on High Performance Materials, London, Sept. 1989. Sogabe, T. et al., Effect of boron-doping on structure and some properties of carbon-carbon composites. Journal of Materials Sci- ence, 1996,31, 6469-6476. Savage, G., Carbon-carbon Composifes. Chapman and Hall, Lon- don, 1993.

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