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The amazing tribology of hydrogen-free amorphous carbon coatings

Tuesday (21.02.2017)
11:15 - 12:20 Rohrersaal

11:15 - 12:20

Diamond-like carbon coatings (DLC) are known for their

outstanding tribological properties and are therefore industrially used as

protective coatings especially on engine parts and various kinds of tools. Depending

from the deposition technique, DLC coatings can contain hydrogen and metal and

differ in their amount of diamond bonds (sp3) versus graphite bonds (sp2). The

most “diamond like” DLC coating is the hydrogen-free, pure carbon ta-C with

sp3-contents up to 85%. Despite the extraordinary properties the industrial use

of ta-C started very late compared to other DLC coating types because of the limited

availability of industrial deposition technique. A few years ago ta-C reached

the level of industrial production.


In tribological investigation of ta-C some fascinating,

but also curious phenomena have been observed. Under ambient unlubricated

conditions in air, very low friction and wear coefficients can be measured.

Altering the humidity, strong effects on friction and wear will occur. Superiour

tribological properties completely vanish under very dry air conditions or in

vacuum, revealing that chemical surface effects determine the tribology of ta-C

rather to mechanical coatings properties. The nature of chemical interaction

can also be seen under lubricated conditions using different kinds of lubricant

types. Friction coefficients range from “normal” values with formulated and

non-formulated engine oils to ultra-low or even super-low values with several

types of polyalcohols, fatty acids or various ester types. Astonishingly,

extraordinarily high coating wear is found with some lubricants even under

ultra-low friction conditions, while almost zero wear is observed with other

lubricants on the same friction level. This contradictory ta-C behaviour is not

yet fully understood, but there are some mechanisms proposed to account for the

observed phenomenons.


Apart from tribological behaviour with organic

lubricants friction and wear behaviour of ta-C under engine oil lubrication

with selected additives is also of great interest. In tribological studies, the

wear behaviour of ta-C was compared to hydrogenated DLC coatings. It could be

shown, that ta-C in contrast to hydrogenated DLC does not suffer from high wear

even with extensively Mo additivated oils. 







Dr. Volker Weihnacht
Fraunhofer Institute for Material and Beam Technology IWS Dresden
Additional Authors:
  • Stefan Makowski
    Fraunhofer-Institut für Werkstoff- und Strahltechnik IWS
  • Dr. Andreas Leson
    Fraunhofer-Institut für Werkstoff- und Strahltechnik
  • Gregor Englberger
    Fraunhofer-Institut für Werkstoff- und Strahltechnik - IWS
  • F. Schaller
    Fraunhofer-Institut für Werkstoff- und Strahltechnik - IWS