Fabrication
Machining graphite
Graphite can be machined wet or dry with ordinary machine tools but is normally machined dry. The different steps are dust extraction, cutting, turning, grinding, bonding and polishing.
Dust extraction
Individual extraction arrangements are essential on all machines when dry machining is carried out. Centralised extraction equipment is not worth installing except in large graphite machine shops. Where workshops are predominantly engaged in metal machining it is enough to install industrial vacuum cleaners on individual machines (vacuum 300mm head of water or air velocity of about 18 m/sec.). When machining carbon and graphite it is important to prevent ingress of dust into electrical motors and control panels.
Cutting
Cutting of graphite does not necessarily require specialist tools. However optimum results may be achieved using diamond cutting wheels.
Drilling
For drilling graphite, hard metal drills are used. To avoid chipping at the drilling exit the point angle should be 70-100 and the clearance angle 10-15.
Turning
Specialist machine tools for the turning of graphite are not absolutely necessary and a wide range of machines are in use, from wood turning lathes to CNC lathes. The machines normally have to be adapted to accommodate graphite machining, in particular protection of electrical equipment from dust.
Roughening
The surface of the graphite is made rough by silicon carbide wheels with a grain size of 20-46 um and hardness F-K depending, on the hardness of the carbon material.
Milling
Specialised milling machines are not required but optimum results may be obtained using hard metal or diamond tools.
Superfinishing
Depending on the hardness of the graphite material electro-corundum wheels with grain size 120-160 um and hardness P-Z are used.
Bonding
It is possible to bond graphite; for this process all surfaces must be free of dust and grease. Bonding materials are polyesters, phenolic resins, epoxy resins and silicon resins. For high temperature use, ceramic bonds are available using aluminium oxide or zirconium oxide as a base.
Copy milling and form grinding
Preferred machining methods for the manufacture of electric Discharge Machining (EDM) electrodes are milling and high-speed milling, for-grinding and EDM-cutting. Since EDM fine-grained graphite grades still have a volume of open pores of 10 to 15 %, they can be impregnated with copper. Thus the advantages of the metal are combined with those of the graphite without impairing its excellent machinability. In spite of the large block dimensions available nowadays, a complicated graphite electrode sometimes cannot be manufactured in one piece. However, it is possible to bond several parts by means of commercially available one and two-component adhesives.
Electric Discharge Machining
Electric Discharge Machining is an electro-thermal erosion process for the machining of many electrically conductive materials, and it exploits the erosion effect of electric discharges between two electrodes. The tool is connected as an anode in an insulating liquid (dielectric liquid) and brought close to the work-piece cathode. DC-voltage induces an electric discharge. By this conversion of electrical into thermal energy, the cathode material is melted, evaporated, and eroded. The fast and efficient manufacture of even the most complicated EDM-electrodes plays the decisive role for the economy of this method. Isostatically pressed fine-graphite grades are extremely cost effective electrode materials as they have, besides their electrical and thermal conductivity as fundamental requirements, also high mechanical strength, edge stability, surface quality and good machinability. GRAPH Electric Discharge Machining.
GRAPH Electric Discharge Machining.
Finishing and Surface Treatment
Due to its manufacture, a carbon artefact contains open pores, which may make up one quarter of its volume. By filling these pores, the density, strength and conductivity of the artifacts can be increased to predetermined levels. Closing the surface pores will also reduce oxidation. Impregnating agents are usually pitch, resins and metals, which are brought into the formed bodies by a vacuum/pressure impregnating cycle. Pitch-impregnated artifacts have to be rebaked in order to carbonize the pitch, whereas resin-impregnated parts are either thermally cured and/or carbonized. The rebaking step also causes new pores to form, so that at least one more impregnating operation is necessary if a high degree of gas or liquid-tightness is requested. Usually, completely gas or liquid-tight grades are manufactured by an impregnation with furan or phenolformaldehyde resins, which are subsequently thermally cured. This resin impregnation, however, reduces the temperature stability of such grades to a maximum of only 200C or slightly above. This limit may be increased by approximately 100 degrees by means of impregnating agents that have a higher thermal resistance, e.g. polytetraflu oroethylene waxes. Wax, grease, oil, and salts play an important role as impregnating agents for special applications – in particular for carbon brushes. Not only the physical properties of the grades but also their operating behaviour in electrical service can be improved. Upon pyrolysis of gaseous hydrocarbons, so-called pyrocarbon can be deposited in the pores or on the surface of the substrates so that the density, strength and corrosion resistance of the artifacts are also considerably increased. Oxidation resistance up to elevated temperatures of approximately 800C may be reached by impregnation with borates or phosphates, whereas an efficient protection against oxidation at higher temperatures may only be achieved by coating with silicides, borides, carbides or nitrides.
High grades surface finishing
• Honing
Silicon carbide stones of grain size 69-99 um are used. In special cases for hard carbon it is also possible to use diamond coated honing strips depending on the required level of surface finish. Honing is done using the standard honing oils.
• Lapping
This process uses lapping powder of aluminium oxide and silicon carbide in grain sizez 1 2-1 6pm
• Polishing
It is essential to achieve a high degree of lapped finish before polishing is carried out. Diamond powder in grain sizes 6-10pm is used. After machining it is essential to remove the media used (honing oil, lapping and polishing powder) from the work parts.
• Ultrasonic dust removal
Fabrocation - coating
Where dust-free surfaces are required, ultrasonic dust removal can be carried out. The media used for this are water, distilled water or solvents.
• Coating
Surface properties of the material coated When using pyrolytic graphite coatings following surfaces properties may be obtained or modified: Electrical resistivity Optical reflectivity Mechanical wear Friction Hardness Adhesion Toughness Porosity surface area Pore size Pore volume Chemical diffusion Corrosion Oxidation There are several coatings for graphite parts such as there are silicon carbide (SiC), pyrolytic carbon coating (PyC). The graphite properties vary depending on the coating.
• Silicon carbide (SiC)
Silicon carbide can be used as a coating for graphite parts. SiC coatings are deposited from gaseous silicon and carbon compounds at high temperatures (CVD process). The coefficient of thermal expansion of the graphite material must be matched to that of the SiC coating. The typical thickness of the SiC coating on graphite is 50-100 um. Edges of graphite parts should be radiused before coating. The coating renders the graphite with special properties.
• Properties of SiC
- Zero porosity (graphite parts can be sealed completely by appropriate thickness of coating)
- High hardness figure
- Good thermal conductivity
- High oxidation resistance
- High purity
A pyrolytic carbon or graphite coating
A pyrolytic carbon or graphite coating on a graphite substrate is produced at high temperature and pressure in a hydrocarbon atmosphere (e.g. methane or acetylene) using the CVD process. Pyrolytic graphite production is based on a gaseous precursor. Pyrolytic graphite is an aggregate of graphite crystallites, which have dimensions that may reach several hundred nm. It has a turbostratic structure, usually with many warped basal planes, lattice defects, and crystallite imperfections. Within the aggregate, the crystallites have various degrees of orientation. When they are essentially parallel to each other, the nature and the properties of the deposit closely match that of the ideal graphite crystal. The structure of a pyrolytic graphite deposit can be either columnar, laminar, or isotropic, depending on the deposition conditions such as temperature, pressure, and composition of the input gases. It is possible to obtain the desired structure by the proper control of these deposition parameters. The properties for columnar and laminar pyrolytic graphites differ from those of isotrophic.
Properties of PyC
- High density
- No porosity
- High anisotropy
- Smooth surface
The anisotropic property comes from the growth of the pyrolytic carbon layer in parallel lattice planes. Edges of graphite parts should be radiused before coating. Parts should be purified before coating.
TABLE Properties of columnar and laminar pyrolytic graphites
Application of pyrolytic graphite
High temperature containers
Boats and crucibles for liquid-phase epitaxy
Crucibles for molecular-beam epitaxy
Reaction vessels for the gas-phase epitaxy of III-V semiconductor materials such as gallium arsenide
Trays for silicon-wafer handling
Free-standing products
Propellant rocket nozzles
Resistance heating elements
Nuclear applications
Biomedical applications
Coeatings for molded graphites
Coatings for fibres
Carbon-carbon infiltration
Chemical Vapour Deposition
CVD is a vapour phase process, which relies on the chemical reaction of a vapour near or on a heated surface to form a solid deposit and gaseous by-products. The process is very suitable to the deposition of carbon. It is used frequently in areas such as semiconductors and cutting tools. Special CVD processes are Chemical vapour infiltration, fludized bed CVD and plasma CVD.
Chemical vapour infiltration
Chemical vapour infiltration (CVI) is a special CVD process in which the gaseous reactant infiltrates a porous material such as an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fibre (or the foam), and the structure is gradually densified to form a composite. The process is used extensively in the production of carbon-carbon materials.
Fluidized bed CVD
Fluidized-bed CVD is a special technique, which is used primarily in coating particles such as nuclear fuel. A flowing gas imparts quasi-fluid properties to the particles. The fluidizing gas is usually methane, helium, or another non-reactive gas. Factors to consider to obtain proper fluidization are the density and size of the particles to be coated, and the velocity, density, and viscosity of the gases. The major applications of pyrolytic carbon deposited by fluidized bed are found in the production of biomedical components such as heart valves and in the coating of uranium carbide and thorium carbide nuclear-fuel.
Plasma CVD
The deposition of graphite can also be obtained by plasma CVD, with the following characteristics:
- Gases: propylene-argon or methane-argon.
- Plasma: radio frequency (RF) at 0.5 MHz
- Pressure: <1300 Pa
- Temperature: 300-500
In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD. This allows the coating of thermally sensitive materials. The characteristics and properties of the coating are similar to those of coatings deposited at higher temperatures (>1000C). Plasma activation is also used extensively in the deposition of polycrystalline diamond and diamond like carbon (DLC).