A layered solid with unusual versatility
Graphite is built from flat honeycomb sheets of carbon stacked on top of each other. Within a sheet the atoms are held by strong bonds, but between sheets the attraction is weak. Combined with some free electrons that move along the sheets, that structure explains most of what graphite does in industry: it slides, it conducts, and it tolerates heat.
Those three behaviors correspond to the three roles in the title.
Graphite as a lubricant
Because the layers shear easily, graphite works as a solid, or dry, lubricant. It can be used as a powder, mixed into grease or oil, or applied as a bonded coating or paste. It is helpful in places where liquid lubricants run into trouble: very high temperatures, dusty environments, or sliding parts that are hard to relubricate.
An important and often misunderstood point is that graphite’s slipperiness depends partly on adsorbed vapors, such as water, between its layers. In very dry or vacuum conditions, ordinary graphite can lose much of its lubricating ability and wear faster. That is one reason some other solid lubricants are favored in space and vacuum equipment. Graphite also oxidizes in air at high temperature, so its useful limit depends on the atmosphere. The general concept is described under friction coefficient.
Typical uses include bearings and bushings with graphite in the material, lock and hinge lubricants, hot forging and wire-drawing compounds, and brushes and contacts that must slide while carrying current.
Graphite as an electrode
Graphite conducts electricity well, resists many chemicals and can handle intense heat, which makes it a standard material for electrodes.
- Electric arc furnaces. Large graphite electrodes carry current into the furnace to melt scrap steel. The electrodes are consumed gradually.
- Electrochemical processes. Graphite anodes and electrodes appear in chlorine production and other industrial electrolysis, depending on the process.
- Electrical discharge machining. Shaped graphite electrodes erode a conductive workpiece through controlled sparks, producing intricate cavities in hard materials.
- Batteries. Graphite is the usual anode material in lithium-ion batteries, a major growth area and a part of the story of energy storage.
- Brushes. Carbon-graphite brushes pass current to rotating parts in motors and generators.
Graphite as a refractory
A refractory is a material that keeps its shape and strength at very high temperatures. Graphite does not melt at ordinary pressure and keeps its strength to high temperatures, even gaining some strength as it heats, as long as oxygen is excluded. It also has a low thermal expansion for a solid, and good thermal conductivity, which together give it strong resistance to thermal shock.
It is used in:
- crucibles and molds for casting and melting some metals,
- furnace linings, heating elements and fixtures for vacuum and inert-gas furnaces,
- carbon-bonded refractory bricks, which combine graphite flakes with oxide grains such as alumina or magnesia to improve resistance to thermal shock and slag attack,
- continuous casting components, and
- sintering trays, dies and molds for ceramic and powder metallurgy parts, relevant to the sintering process.
The major weakness is oxidation in air above several hundred degrees Celsius, which burns graphite away. Coatings or protective atmospheres are used, and this is a key point in the wider subject of refractory materials. Graphite is also mechanically weak compared with ceramics, and it is not wetted by many molten metals, which can be an advantage.
Natural versus synthetic
- Natural graphite is mined and sold as flake, amorphous or vein material. Flake graphite is valued for refractories, lubricants and battery use after purification and shaping.
- Synthetic graphite is made by heating carbon sources such as petroleum coke and pitch to very high temperatures, so the carbon rearranges into ordered layers. It can be made very pure and in large shapes, which suits electrodes and machined parts. Production is energy intensive.
Synthetic graphite blocks are often machined into shapes, and fine-grained grades are used where precision matters.
Other related roles
- Seals and gaskets. Flexible graphite sheet, made from expanded graphite, is used in seals and gaskets that must tolerate heat and chemicals, including some pump seals.
- Thermal management. Sheets of graphite spread heat in electronics.
- Nuclear applications. Graphite has long been used as a moderator in certain reactor designs.
- Pencils. Graphite mixed with clay for writing is the best-known use, though a small part of industrial volume.
Putting it in context
Graphite lacks the hardness of diamond, yet it excels in places diamond cannot: electrical conduction, lubrication and use as a machinable high-temperature solid. The two are chemically identical in composition and wildly different in behavior, a contrast explored in the overview of carbon allotropes. Graphene, a single sheet of graphite, is discussed separately, and carbon-carbon composites use graphite-like carbon as both fiber and matrix.