Basics

The basics of ta-C coating fabrication

Einteilung der DLC-Schichttypen nach Jacob/Möller/Robertson
© Photo Fraunhofer IWS Dresden

Einteilung der DLC-Schichttypen nach Jacob/Möller/Robertson

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.

Conditions for producing ta-C coatings

20 µm dicke Diamor®-Schicht auf Stahl als Bruch-Querschnitt im Rasterelektronenmikroskop
© Photo Fraunhofer IWS Dresden

20 µm dicke Diamor®-Schicht auf Stahl als Bruch-Querschnitt im Rasterelektronenmikroskop

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.

ta-C coating formation

Atomistische Simulation der ta-C-Struktur mit sp3- (dunkel) und sp2- (hell) hybridisierten C-Atomen
© Photo Fraunhofer IWS Dresden

Atomistische Simulation der ta-C-Struktur mit sp3- (dunkel) und sp2- (hell) hybridisierten C-Atomen

Diamor®-Schichtquerschnitt mit nanolagiger Struktur im Transmissionselektronen-mikroskop
© Photo Fraunhofer IWS Dresden

Diamor®-Schichtquerschnitt mit nanolagiger Struktur im Transmissionselektronen-mikroskop

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.