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Manufacturing Process and Material Propertiesof Carbon and Graphite Materials

Schunk Kohlenstofftechnik

Kohlenstoff-Broschuere_GB.qxd 23-11-2004 08:04 Uhr Seite 1

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Carbon and graphite materials are manufactured according to processes based on conventional ceramic technologies. Raw materialssuch as petroleum cokes, pitchcokes, carbon black or graphite

materials with a defined grain sizedistribution are mixed with a ther-moplastic binder at elevated tem-peratures. Coal tar or petroleumbased pitches as well as syntheticresins are used as binders. Mineral

additives or metal powders can be employed to achieve specialmaterial properties. As an example,copper powder is generally used forthe manufacture of carbon brushesapplied in low voltage motors.

Material Processing and Mixing

Material processing and mixing atSchunk Kohlenstofftechnik is basi-cally performed using computercontrolled continuous processes.The grain size distribution of theprocessed raw materials is controlledby laser diffraction, mostly online.

The mixing process is carried outin double screw extruders accordingto specific parameters such asthroughput, screw configurationand temperature profile.

Shaping

The ready-to-mold mixes are formedinto "green bodies" by die molding,isostatic molding or extruding. The shaping process can be carriedout at ambient or elevated tempera-tures; the pressure may vary bet-ween 2 and 400 MPa.

Baking

After the shaping process, the"green bodies" are baked. Depen-ding on the type of material, dimen-sions and the required materialcharacteristics, the baking process

is performed in continuous orbatch furnaces applying differentheating rates, maximum tempe-ratures (up to 1200 °C / 2190 °F)and furnace atmospheres.

During baking, the binder is decomposed into volatile compo-nents and carbon. This process is called pyrolysis. The resultingbinder coke ensures the integrityof the molded and baked blocks.

After baking, the blanks do not yetpossess a complete graphitic struc-ture. They are brittle and, generally,exhibit high mechanical strengthand hardness. At this stage, thematerial is called carbon/graphiteor "hard carbon". It demonstratesproperties suitable for certainmechanical applications, such assliding rings and bearings.

Graphitizing

For many applications, graphiticproperties are required which areobtained through the process ofgraphitization, a second heat treatment at temperatures up to3000 °C (5430 °F).

At Schunk Kohlenstofftechnik, graphitization is mainly performedby applying the Acheson process,

whereby the material to be graphi-tized is packed between two elec-trodes and connected as a resistancein the secondary circuit of a trans-former. Thus, the graphitizing tem-perature is reached by resistanceheating. During this process, recry-stallization occurs, yielding in largergraphitic domaines with a higherdegree of orientation. The materialproperties of the graphitized blanksare defined by the structural prop-erties of these graphitic domaines.Graphitized carbon is called elec-trographite.

Electrographite materials generallypossess excellent sliding properties,low electrical resistance, high ther-mal conductivity and an improvedcorrosion resistance. They are usedfor applications where enhancedsliding properties, high resistanceto chemical attack and temperaturecycling as well as a high purity are required as an individual char-acteristic or as a combination ofproperties.

Inductively heated graphitizing furnaces or vacuum graphitizingfurnaces are frequently used forcarbon fiber-reinforced carbon(CFRC; C/C) materials as well aswhen the highest purity is required.


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The manufacturing process of carbon and graphite materials contains the following steps:

■ Raw material processing■ Mixing■ Shaping■ Baking

■ Graphitization (only for the production of electrographite)

■ Special treatments such as impregnating, purifying, coating

■ Final machining

Binder Raw material Crushing

Spec.TreatmentsInspection

Isostatic Molding

Mixing

Die Molding

Baking

Extruding

Graphitizing

Machining

Impregnating

Milling Sieving

Inspection

Machinedcarbon and

graphite parts

Carbon graphiteblanks

HomogenizingMilling


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Impregnating

In addition to the production pro-cess for the basic material, thereare many processes to generatespecial properties for particularapplications. By impregnation with synthetic resins, the porousstructure originated by pyrolysis of the binder can be made imper-meable to gas and liquids. By impregnation with metals, an increase in hardness and strength by a factor of 2-5 can be achieved.

Resin Bonding

Special properties can also be achieved by resin bonded carbonmaterials. Materials impermeable to liquids and gases can be pro-duced without being subjected to the coking and impregnating process. Because they are not graphitized, such materials havemoderate sliding properties, whichcan be improved by using naturalgraphite or synthetic graphite asraw materials.

Resin bonded carbon materials can only be used up to the curingtemperature of the resin, generally180 °C to 280 °C (350 °F to 530 °F).The production of low electricalresistance, resin-bonded materials is not possible because of the isolating properties of the resin.

Special Treatments

Out of a variety of special treat-ments the most important onesshould be mentioned:

■ Purification of graphite parts in order to obtain products ofthe highest purity

■ Coating of the highest purity graphite with pyrolytic carbon(PyC) and/or silicon carbide (SiC)

■ High-vacuum degassing

Due to the various possibilities of modifying carbon or graphitematerials and optimizing their characteristics for special needs, the application fields for this mate-rial group reaches into all branchesof technology and is still expanding.


Characteristics depending on Structure and Bonding

Based on the special bonding char-acteristics of the carbon atoms in the graphite lattice, graphite cry-stallizes in a hexagonal layer struc-ture. The enhanced sliding proper-ties, the anisotropy of electricaland thermal conductivity as well asthe coefficient of thermal expan-sion are characteristics whichdepend on structure and bondingof the graphite.

The chemical properties of carbonmaterials are also determined bythe bonding conditions of the carbonatoms within the lattice. Due to thehigh strength of the covalent bondswithin the lattice layer, carbonmaterials exhibit a high resistanceto acids, bases, gases, melts, etc.

The resistance of carbon materials isonly limited by strongly oxidizingmedia and oxygen. In oxidizing atmospheres carbon graphite mate-rials are stable up to 350 °C (660 °F),

whereas graphitized materials startto be oxidized at 500 to 600 °C(930 to 1110 °F). Up to these tem-perature limits at least the short-term service will not be restrictedby oxygen attack.

Details on the resistance to chemicalattack encountered in practice aregiven in our brochure “ChemicalResistance” (39.12e).

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Graphite structure

A

A

Bc

a


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Process dependent Properties

In addition to the properties depen-ding on structure and bonding, car-bon materials show characteristicswhich are related to the manufactur-ing process. Carbon manufacturedaccording to the production methoddescribed above will have a poly-granular and polycrystalline micro-structure. Frequently, the microcry-stallites already present in thegrains of the solid starting materialsare randomly oriented, so thatnearly no remaining anisotropy of the crystallites is measurable.

Porosity is a property particularlyinfluenced by the manufacturingmethod and can be varied between0 and 50 %. The porosity can bedefined by the pore volume andthe pore size distribution, bothbeing characteristic for differentmaterial and production methods.In general, there are both open and closed porosity. Open porosity can be filled by impregnants, whereas closed porosity cannot.

Due to the porosity and the differ-ent graphitizability of variouscarbon materials, all industriallymanufactured polycrystalline carbons exhibit a lower bulk den-sity than that which is calculatedtheoretically based on the ideal cry-stal structure of the graphite.Depending on the productionmethod, bending strength andcompressive strength can be varied within wide limits. The bending strength may vary from10 to 150 MPa.

Methods for the Determinationof Material Properties

First, those properties should bementioned which are typical foreach grade and can easily be determined:

■ Specific electrical resistance(according to DIN 51911)

■ Hardness Rockwell(according to DIN 51917)

■ Bulk density(according to DIN EC 60413,DIN 51918)

■ Bending strength(according to DIN 51902)

■ Ash content(according to DIN 51903)

These properties allow for rapididentification of the material gradeand for quality inspection. Theyserve as a basis for delivery con-tract agreements between cus-tomer and manufacturer.

The testing methods mentionedabove are summarized with specialreference to carbon brushes in DIN EC 60413. Further standards for the investigation of carbonmaterials can be found in the DIN Standard Series 51901 through 51940.


Further Characteristic Data

In addition to the above, furthercharacteristic data can be deter-mined which require a fair amountof measurement, however, knowl-edge of which is vital for certainapplications as well as for internalinvestigation purposes. These are,for example, Young's modulus, tensile and compressive strengthsas well as thermophysical datasuch as the coefficient of thermalexpansion, thermal conductivityand specific heat capacity.

Usually, it is not sufficient to know just the porosity of a carbonor graphite material in order tocharacterize its impregnation be-haviour. It is often necessary to determine the pore size distri-bution, to examine the micro-scopic structure of the material and also, possibly, the wetting behaviour of various impregnants.

Furthermore, the influence of the impregnation process on thematerial properties (e. g. on theoxidation resistance and on thepermeability) is of interest (e. g.oxidation resistance, permeability).Not all of those properties are routinely measured, as this wouldcreate unnecessary laboratory expense. If required, special infor-mation will be gladly given onrequest.

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Micrographs of carbon structures with different porosities and grain size distributions


Application relatedExaminations

The properties of a specific prod-uct are measured with reference to special fields of application inaddition to the material propertiespreviously mentioned. Tests couldcover the determination of the:

■ Coefficient of friction againstvarious materials

■ Contact resistance of a slidingsurface

■ Wear rate under various loadconditions

■ Adsorption of gases

■ Wetting behaviour with melts

■ Radio interference behaviourwith an electrical sliding contact

■ Dependence of electrical resistance on temperature.

Polycrystalline graphite

coarse-grained FE 934

fine-grained FE219

fine-grained FE679

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finest-grained FE779


Technical Carbon

Carbon and graphite materials aregenerally manufactured in polygran-ular and polycrystalline form. Thismeans that the blanks show a ran-dom orientation of crystallites. Dueto this microcrystalline structure, a macroscopic blank does not showthe typical anisotropic properties ofa graphite single crystal. With poly-crystalline materials, the extremelyhigh anisotropy of the electricalconductivity or coefficient of ther-mal expansion is greatly reduced or nonexistent. The anisotropy ofproperties which occur in polycrys-talline carbon materials does notonly depend on the properties of the raw materials, but also on the molding method. Isostaticallymolded carbon materials, for exam-ple, show little or no anisotropy,whereas uniaxially or biaxially mold-ed parts exhibit increased aniso-tropy. The data given for propertiesparallel and perpendicular to themolding direction vary and are, therefore, indicated separately.

Another form of technical carbon or graphite is pyrolytic carbon orgraphite. This material is depositedfrom a carbon-containing vapourphase onto a heated substrate bychemical vapour deposition (CVD).The properties of these materialscorrespond much more to those ofsingle crystals.

As the manufacturing process isvery expensive, these proceduresare in general only used for surfacemodification of standard polycrys-talline materials, e. g. to achieve alow gas permeability or to producea wear resistant surface. Solid pyro-lytic graphite is used in only a fewcases, e. g. for the manufacture ofhigh capacity vacuum tube grids.

An additional class of technical carbon materials are carbon or graphite fibers. They are manu-factured by pyrolysis of polymerfibers, preferably polyacrylnitrile(PAN), or of special pitch basedfibers.

Carbon fibers are used for the rein-forcement of polymers (carbonfiber-reinforced polymers; CFRP),carbon (carbon fiber-reinforced car-bon; CFRC, C/C), ceramics (ceramicmatrix composites; CMC), andmetals.

These composites are normallyused when high stiffness andstrength together with low weightplay a decisive role. Typical appli-cations of CFRP are sporting goodsand components for aviation andspace technology which are notsubmitted to high temperatures.For high-temperature applications,e. g. in the semiconductor industryor furnace construction, C/C is theaccepted material.

Another and less expensive form ofcarbon fibers is carbon felt which isused as a thermal insulation material.

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Application Fields ofCarbon and Graphite Materials

Due to their special properties,Schunk carbon and graphite materials are used for electro-technical products, componentsin mechanical engineering, thesemiconductor industries, and for medical technology.

Details on the particular proper-ties of the products for the variousapplications are given in our spe-cial brochures.

Schunk Material Codes

The Schunk material code is basedon an alphanumerical system. Thedifferent groups of Schunk carbonmaterials are identified by one ortwo initial letters of the grade code.Materials for carbon brushes haveonly one letter to denote the gradecategory. All other materials aredenoted by two letters, in that „F“ as the first letter indicatesmainly mechanical application.

The two figures following the initalletters identify the different gradeswithin the material groups. Depen-ding on the material, a third figureindicates special manufacturingprocedures. Additional processingsteps such as infiltrations withmetals, resins or special salts areindicated by a final letter/figurecombination.

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Electrographite with SiC layer

Texture of a C/C material

Electrographite with PyC layer


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Material Code –Grade Categories

A natural graphite / copper

B, C natural graphite / copper babbitt metal alloy

E electrographite

F natural graphite, resin-bonded

H carbon graphite

K natural graphite / copper, pitch-bonded

L carbon graphite

S natural graphite / silver

U special material

BH carbon graphite, material for pantographs

WH carbon graphite, resistance material

FE electrographite

FF resin bonded materials

FH carbon graphite

FR, FP, high purity electrographite

FG highest purity electrographite

CF fiber-reinforced material

FU special material

The most ImportantSpecial Treatments

A antimony impregnation

B impregnation with lead-antimony

C copper impregnation

D lead-bronze impregnation

F, H, V impregnation to improve the operational behaviour of carbon brushes

G high vacuum degassing

M bonded laminated carbon brushes (sandwich)

Q, M, PS salt impregnation

R X-ray inspection

S pyrolyzed impregnant

T impregnation to increase the abrasion capacity of carbon brushes

U ultrasonic dedusting

X, Z, ZP resin impregnation

Y resin impregnation, pyrolyzed


Schunk Kohlenstofftechnik GmbH

Rodheimer Strasse 5935452 HeuchelheimGermany

Phone: +49 (0) 641 608 0Fax: +49 (0) 641 608 1436

www.schunk-group.comE-Mail: [email protected]

03.05e/2004


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