DLC and Carbon Based Tribological Coatings

Aside from liquid lubrication, surface engineering of tribological contacts is a way of controlling friction and wear. With carbon based thin films surface properties can be modified within a wide range from soft graphite to hard diamond. Modification takes place in the near surface region within thickness from nanometres to micrometres.

Carbon

Carbon is a real shapeshifter, from graphite and diamond to carbon nanotubes and football like fullerenes. With shape shifts its properties. Carbon atom has two energy levels of electrons. When it wants to form four single bonds with neighbouring atoms, electrons form four identical sp3 orbitals and material is called diamond.  On the other side of the scale exist graphite with flat sp2 bonded sheets with some extra electrons that can conduct electrical current.

Graphite

Graphite is 100% sp2 bonded stable form of carbon in normal atmosphere. It is so called solid dry lubricant, in which internal, layered sliding planes can easily move relative to each other.

Diamond

Pure diamond would be 100% sp3 chemically bonded with diamond crystallographic structure. Diamond has the highest known thermal conductivity of any solid material at room temperature, 5 times higher than that of copper. Diamonds are not forever though, if we would have enough time, we would observe diamond turning onto a more stable graphite.

DLC: The Best of Both Worlds

In between graphite and diamond lies a vast family of "Diamond-Like Carbons" (DLC). These films are typically amorphous carbon networks (a-C) containing a mixture of sp2 and sp^3 bonds. By "doping" these networks with hydrogen (a-C:H) or other elements, we can tune the coating to be as flexible as a polymer or nearly as hard as a natural diamond

a-C:H

When we use hydrocarbons like ethylene in PE-CVD, some hydrogen becomes part of the film and amorphous diamond like carbon films commonly contains hydrogen. Hydrogen content in a film has a most significant influence on its hardness, modulus, elasticity, density and sp3/sp2 ratio. In general, higher hydrogen content downgrade properties of diamond but provides higher flexibility and lower synthesis cost.

Hard = tough ?

Understanding of tribomechanisms that leads to friction and wear can be a complex in action by adhesion, ploughing, wear by adhesion, abrasion and fatigue combined with material fracture. One can specify the level of friction, hardness and wear for a given application but because tribological behavior is complex, selecting the right film requires expertise beyond simple hardness scales. That’s why engineers have developed number of carbon-based films, to meet the varying requirements of applications and to provide energy savings, economic and environmental benefits.

Ultimately, a surface is where your product meets the world. Whether you need the extreme hardness of a diamond-like structure or the self-lubricating properties of a hydrogenated film, let’s work together to reduce friction.