Basics

Diamond-like carbon coatings (DLC) are generally carbon coatings with a significant fraction of diamond-like atomic carbon bonds. In addition to the diamond-like bonded carbon (sp³ hybridized bonds) the rest of the carbon atoms is mostly graphite-like bonded (sp2 hybridized). The ratio of sp³/sp² bonds and the hydrogen content in the coating are important for the properties. Some DLC coatings are produced from hydrocarbon gases and thus contain always a certain amount of hydrogen. According to Jacob/Moeller/Robertson most DLC coating types can be distinguished using a certain type of phase diagram.

Due to the nature of their synthesis process from solid graphite, ta-C coatings do not contain a significant amount of hydrogen. The sp³ fraction in ta-C coatings ranges from 50 to 90% depending on the fabrication conditions. Thus ta-C coatings are the closest of the DLC coatings that approach the properties of crystalline diamond.

The fabrication of ta-C coatings occurs under high vacuum conditions and makes use of a carbon plasma. Two critical conditions have to be fulfilled in order to achieve a high fraction of sp³-bonded carbon in the film: a high kinetic energy (some 10 eV) of all coating particles and a sufficiently low substrate temperature (< 150°C). The maximal possible sp³ content of 90% is achieved with particle energies of about 100 eV at low temperatures (i.e. room temperature).

The for ta-C synthesis required energetic carbon plasmas can be produced by vacuum arc evaporation (arc), pulsed laser ablation deposition (PLD) or high power pulse magnetron sputtering (HIPIMS) from graphite targets. So far PLD and HIPIMS are not suitable for industrial applications due to their low efficiency.

IWS engineers combined the high efficiency of the arc process with the high degree of process control of PLD processes and developed the Laser-Arc technology.

The condition for the formation of sp³ bonds is a subplantation of carbon atoms. That means that incoming carbon particles during the deposition have to penetrate several atomic layers. Neighboring atoms are displaced, which causes a brief localized high-pressure situation with simultaneous heating. Both conditions are ideal for sp³ diamond bond formation. A subsequent rapid cooling freezes this bonding state. This prevents the change back to the sp² equilibrium state. The following deposition of more and more material reduces all possibilities for further relaxation of the structure. Even heating of the final result to about 600°C will not change the bonding character of the carbon material.

Ta-C coatings are characterized by high compressive stresses (up to 10 GPa). Without further engineering these high stresses would lead to coating failure such as spalling. A suitable nanolayered film structure can prevent this. A multilayer coating with periodically varying sp³ fractions (and therefore with varying E-modulus) is naturally produced the parts rotate in front of the plasma source during the deposition. The deposition parameters are automatically periodically changed and in particular the plasma incidence angle on the surface of the coated parts. Using this effect enables the appropriate formation of a nanostructured ta-C coating.

An optimized adhesion-promoting layer serves as a transition film from substrate to coating. In combination with the nanolayered structure IWS engineers synthesize well adhering coatings of more than 10 µm in thickness on hard metals and various steel materials.