Diamond-like carbon (DLC) coatings have attracted significant attention due to their low friction, high hardness, high elastic modulus, chemical inertness, biocompatibility, and high wear resistance. These coatings have been grown using different deposition methods and hydrocarbon precursors in order to find the best set of mechanical and tribological proprieties. The major disadvantage of hard DLC coatings deposition and, therefore, their technical applications is that there is often a relatively low adhesion on metallic substrates caused by very high total compressive stress on these coatings.

In this work, deposition of hard and adherent DLC coatings employing an asymmetrical bipolar pulsed-DC PECVD system and an active screen that worked as an additional cathode is presented. This system represented a step forward for thin coating growth by using very lower pressure (about 0.1 Pa) in collision less regime and higher plasma density than the conventional PECVD system. Acetylene gas was used as a precursor. In order to overcome the high residual stress and low adherence of DLC coatings on steel substrates, a thin amorphous silicon interlayer was deposited as an interface. The interlayer was synthesized using silane as precursor gas and varying the applied self-bias voltage during deposition.

The DLC coatings were analyzed according to their microstructural, mechanical, and tribological properties as a function of the substrate bias voltage. The film's atomic arrangements and the hydrogen content of the films were estimated by means of Raman spectroscopy. The total stress was evaluated through the measurement of the substrate curvature, using a profilometer, while nanoindentation experiments helped determine the films' hardness and elastic modulus. The friction coefficient and the wear rates of the films were determined using a tribometer in unlubricated sliding friction experiments, while the critical load of failure was determined by a classical scratch test.

The results showed that the use of the modified experimental setup and an amorphous silicon interlayer improved the DLC films' deposition onto steel substrates, producing good adhesion, low compressive stress, and a high hardness. The composition, microstructure, and mechanical and tribological properties of the films were dependent on the applied bias voltage. These results suggest that a combination of a modified pulsed-DC PECVD system, the use of an active screen as an additional cathode, and acetylene as a precursor gas for growing DLC films may represent a new and useful alternative for mechanical and tribological applications.

Diamond-like carbon (DLC) coatings are used in numerous tribological applications, for example components of the automotive powertrain. The hardness of these coatings contributes to the reduction of the component wear and correlates with the sp³/sp² bond ratio between the carbon atoms, where sp³ bonds are similar to the diamond structure. Physical vapor deposition (PVD) processes like pulsed laser deposition (PLD) or filtered cathodic arc (FCA) have been used for the deposition of DLC coatings with high sp³/sp² ratios, so far. However, coatings deposited by PLD or FCA exhibit high defect densities and surface roughnesses. Therefore, research is upon the high power pulsed/impulse magnetron sputtering (HPPMS/HiPIMS), which in contrast to mentioned processes provides smooth coatings and therefore less post processing. Synthesizing DLC coatings with a high sp³/sp² ratio requires high energetic carbon ions, whose generations strongly depend on the HPPMS process parameters. The synthesis of hard DLC coatings by means of HPPMS, especially with different process gases has been reported in few works, yet. Plasma diagnostics have shown that the energy of the carbon ions in the plasma strongly depends on the process gas mixture, the process gas pressure and the bias voltage. An increased fraction of high energetic carbon ions in the HPPMS plasma has been reported for process gases with a higher ionization energy as well as for higher bias voltage and lower process gas pressure. Therefore, the aim of this work is to investigate the correlation of the plasma parameters with the properties of DLC coatings. Ar, Ne and He were used as process gases. Process gas pressure and bias voltage were varied between p = 0.5 – 2.0 Pa and U_B = -300 – 0 V, respectively. The coatings were deposited in a high volume semi-industrial coating unit. The coating properties were analyzed by means of nanoindentation, Raman spectroscopy, scanning electron microscopy and confocal laserscanning microscopy. It was observed that the coating properties strongly correlate with the results from the plasma diagnostics. Dependencies of the sp³/sp² ratios and hardness could be found for increasing ionization energy of the process gas. Furthermore, an influence of the process gas pressure and bias voltages could be observed. Hardness values up to HU = 45 GPa have been achieved. In conclusion it can be claimed that synthesis of DLC coatings with high hardness is possible by HPPMS processes using process gas mixtures of Ar, Ne and He.

The fullerene-like nanostructure hydrogenated carbon films exhibited high hardness and ultra-high elasticity though having high sp2 hybrid carbon bonds, which implies that the hardness of carbon films is not only related to the content of sp3 hybrid carbon bonds, but also contributed by the microstructure. The reason is due to the curvature structure of fullerene like structure, which extends the strength of graphite plane hexagon into three dimension space network, in turn, increasing the hardness and elasticity of carbon films. More importantly, the films demonstrated super low friction in air, meaning low friction solid lubricant with practical application value. With five- and seven-member ring as the representative of fullerene like structure, the fullerene like structure of the carbon films could be adjusted via the hydrogen content in the deposition gas sources. It was found that there is direct relationship between the fullerene like structure and the friction coefficient, the more fullerene like structure, the lower friction coefficient of the carbon film, indicating that the fullerene like structure could govern the friction behavior of the carbon film. During the sliding process, the friction induced more fullerene like structure generated at the frictional interface, accounting for ultra low friction. Some applications of the fullerene like structure carbon films with super low friction are carried out to engine parts such as high pressure common rail fuel system to save energy and reduce emission. The results will expand and ensure the application of carbon films to most frictional parts of engine, and thus to save fuel and reduce emission much more.

[1] Qi Wang, Chengbing Wang, Zhou Wang, Junyan Zhang*, Deyan He, Applied Physics Letters, 2007, 91(14), pp141902,

[2] Yongfu Wang, Junmeng Guo, Kaixiong Gao, Bin Zhang, Aimin Liang, Junyan Zhang*, Carbon, 2014,77, pp518