ICMCTF2015 Session B3: Deposition Technologies and Applications for Diamond-like Coatings

Amorphous hydrogenated carbon (a-C:H) films have been grown using different hydrocarbon precursors in order to find the best set of mechanical and tribological proprieties. The addition of noble gases to the hydrocarbon precursor atmosphere is expected to increase the ratio of ion to neutral radicals on the surface of the growing film without changing the H/C ratio of the gas mixture. This is in fact a powerful way to investigate the effect of ion bombardment on the structural arrangement and properties of a-C:H films.

In this work, acetylene (C₂H₂) was be studied using argon as an inert additional gas, in order to determine the mechanical and tribological properties and microstructure of a-C:H films. The films were deposited employing an asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition (PECVD) system and an active screen that worked as an additional cathode.

The a-C:H films were analyzed according to their microstructure, mechanical, and tribological properties as a function of the amount of argon diluted in the acetylene. The film’s microstructure and the hydrogen contents were probed by means of Raman spectroscopy. The internal stress was determined through measurement of the change in the substrate curvature by means of a profilometer, while nanoindentation experiments allowed to determinate the hardness and the elastic modulus of the film. The friction coefficient and wear resistance of the films were determined using a tribometer, while the adhesion of the films was evaluated via the scratch test. In order to improve the a-C:H films’ adhesion to steel substrates, a thin amorphous silicon interlayer was used.

The results showed that the atmosphere of argon diluted in acetylene induced modifications in the properties of the a-C:H films. Hard, adherent, low-stress, and high wear resistant a-C:H films were deposited on steel substrates using a combination of a modified and asymmetrical bipolar pulsed-DC PECVD system, an active screen as additional cathode, and acetylene-argon atmospheres. The use of an amorphous silicon interlayer improved the a-C:H films’ deposition onto steel substrates. These results suggest that the used methodology represented a step forward for thin film growth by using lower pressure and higher plasma density than the conventional PECVD system and may represent a new and useful alternative for mechanical and tribological applications.

The hardness and density of the coatings was measured by instrumented indentation tests and X-ray reflectivity, respectively. The wettability was acquired by water contact angle measurements. The electrical conductivity was recorded by a four-point-probe. The hydrogen content was analyzed by carrier gas hot extraction method. The hardness was found to decrease with increasing p(C₂H₂) and at higher substrate temperature and is correlated to the density of the a-C:H coatings. The reason for the reduced hardness with increasing p(C₂H₂) is the decrease in ion energy. Therefore the generation of sp³ bonded carbon is reduced (less kinetic energy for subplantation) and the number of sp² bonded carbon is increased. The change of the hydrogen content and the water contact angle is correlated and discussed with respect to the changes of the a-C:H film properties.

Diamond-like carbon has excellent tribological properties such as high wear resistance and low friction. At present, DLC films are of significant interest for automobile parts, because they possess the potential to reduce the friction coefficient under various sliding conditions. We focused on the silicon-containing DLC (DLC-Si) films produced by the DC-PACVD, and its method features a considerably higher deposition rate, higher throwing power, and lower equipment cost and treatment cost compared to conventional methods such as RF-PACVD and sputtering. Therefore, it has a significant potential for wide applications in various industries including the automobile one.

Furthermore, the DLC-Si films shows tribological properties different from those of the conventional DLC film: i.e., low friction in a poorly lubricated atmosphere like dry, water, fuel, etc. The influence of the silicon content in the DLC-Si films on the friction property of the films was evaluated by a tribological test. In addition, the low friction mechanism of the films was examined by analysis of the wear surface. From the obtained surface analytical results, it was suggested that the DLC-Si films exhibited a low friction property caused by the adsorbed water on Si-OH under the dry sliding condition.

However, DLC-Si films have not been able to be used in many applications due to their weak adhesion to a steel substrate. Thus, we developed the pre-activation surface process prior to the DLC-Si films, which have a strong adhesion to steel substrates. The developed coating process has a significant potential for application in various machine components. This technology can be applied to automobile parts such an engine components and drive parts, as well as machine tool parts, dies and jigs. In this session, I will review the DLC-Si coating technology for an automobile application, and share the latest data about their friction performance in tribological tests.

High-rate synthesis of dense and hard amorphous carbon (a-C) thin films using conventional magnetron sputtering based methods such as direct current magnetron sputtering is challenging. This is due to low sputtering yield of C as well as difficulty in generating highly ionized C fluxes which are essential for synthesizing dense and hard a-C structures such as diamond-like carbon (DLC). In this contribution, we present sputtering-based routes for high-rate synthesis of dense and hard a-C thin films using high power impulse magnetron sputtering (HiPIMS). We compile and implement a strategy for generating highly ionized carbon fluxes using Ne-based HiPIMS discharge that entails energetic electrons as compared to Ar-based HiPIMS discharge facilitating the generation of highly ionized C fluxes as well as a-C thin films with mass densities in the order of 2.8 g/cm³ and film hardness exceeding 40 GPa.

Hydrogenated amorphous carbon (a-C:H) thin films are among the best coating solutions for the reduction of automotive fuel consumption because of its very good tribological properties. Friction losses are indeed one of the main sources of energetic consumption in an automotive engine. 80 % of friction occurring in the valve train system is only due to the cam-follower contact, representing 20 % of the total losses in the engine. That is why the reduction of friction coefficient in dry and lubricated conditions is still in demand. Quite high values for hardness are also required to withstand the severity of this contact. Nowadays, DLC coatings are a standard and well recognized solution but research development is still necessary to improve the performance’s baseline of standard DLC regarding friction performance without impact on wear resistance.

Many metallic elements have been introduced in the DLC (Diamond-Like Carbon) matrix to modify their tribological properties. In particular, elements which are not able to form carbide phases like Al enable to benefit from doping without altering the matrix properties. Al-DLC films are produced in an industrial coater by a hybrid PECVD - magnetron sputtering process using different power on the Al target to explore various Al/C ratios. X-ray photoelectron spectroscopy is performed to determine the chemical composition of the films and the structure is observed by Raman spectroscopy. Then hardness is measured by nanoindentation. Finally, tribological tests are performed with an alternative ball-on-disc tribometer to measure wear resistance and friction coefficient in dry and lubricated conditions.

For a static configuration, a-C:H coatings are doped from 0.5 to 11.5 at. % of Al and compared to a standard pure DLC. Tribological tests in dry conditions show a friction reduction from 0.21 to 0.04 by increasing the Al content to 4.7 at. %. In lubricated conditions, Al containing DLC with more than 2 at. % of Al showed a too low wear resistance to be able to evaluate the friction reduction. In order to overcome this issue and to combine wear resistance with friction decrease, multilayer coatings with non-linear Al content have been designed and evaluated.

Its industrial implementation took a while, but the wide family of Diamond Like Carbon coatings (“DLC”) has found its way in many different markets and applications. The combination of achievable coating properties through different deposition techniques and process settings have led to a wide range of engineered materials. These coatings are typically deposited at very mild processing temperatures, making it applicable to a wide range of substrate materials. Today’s industrial coating machines make the deposition of such DLC materials affordable for many industries and applications. One of the ongoing challenges is to process it cheaper and faster to even further increase the penetration in the market place.

In general the family of amorphous DLC materials is well known for a high hardness, relatively low Young’s modulus, low coefficient of friction, chemical inertness, biocompatibility, semi-conductor properties, IR transparency, wettability behavior, and distinct black and grey colors. The only weak property is the in general low temperature stability. Addition of elements like Si help to increase it, but it is still less stable than mostly known for ceramic hard films.

DLC coatings are rarely deposited as a single layer material. In most cases multi-layer and nano-layer configurations are formed in order to gain extra advantages out of the DLC layers. A common multi-layer approach is to start with a metallic layer in order to achieve sufficient adhesion between the desired DLC coating and the substrate material of choice. Pre- and/or post-processing steps are common in order to get maximum benefits out of the applied DLC materials.

The key feature to the tool industry is the chemical inertness in combination with high hardness. In forming or machining of aluminum based alloys there is much less sticking of the aluminum to either dies or cutting edges. The high intrinsic hardness increases the abrasive wear resistance of Si fillers in the aluminum alloys. Anti-sticking behavior improvements are also seen in processing materials like rubbers and food.

The depositions were carried out at an industrial scale unbalanced magnetron sputtering equipment with 4 sputtering cathodes, 6-axes planetary rotary substrate table and with effective loading space of 450mm in diameter and 600mm in height. After etching and interlayer formation by sputtering, depositions of 2-17micron thick DLC coatings were carried out under 3Pa of acetylene and by applying MF-AC power of 3kW range. The deposition rate for the substrates on 2-fold rotation fixtures under full load conditions was 5-6microns/hr and 2-17microns thick DLC coatings were obtained by adjusting deposition time. The hardness of DLC coatings was almost constant around 20GPa whereas the thickness was varied from 2 to 17microns.

Carbon films include plasma polymer films, amorphous carbon films (diamond-like carbon, DLC), CVD diamond films as well as graphite films [1, 2]. DLC is an amorphous network solid, containing a high fraction of carbon sp³ sites, but also sp² sites and hydrogen. DLC films have attractive properties, such as high mechanical hardness, wear resistance, optical transparency and chemical inertness and hence have widespread applications as protective coatings in several areas such as car parts, micro-electromechanical systems (MEMS) and as magnetic storage disks. Site-selective coating of the carbon protective layer on trenches is one of the concerns to realize coatings on submicron wide trench substrates. So far, we have succeeded in Site-selective coating of Cu films on trench, and have realized sub-conformal, conformal and anisotropic deposition, for which Cu is filled without being deposited on sidewall of trenches, using a H-assisted plasma CVD method [3-6]. We have applied the method to carbon protective layer on trenched substrates in order to control deposition profile [7, 8]. Here we have demonstrated three kinds of deposition profiles of carbon films on substrates with submicron wide trenches using H-assisted plasma CVD of Ar + H₂ + C₇H₈. The three deposition profiles are sub-conformal, conformal and anisotropic deposition, for which carbon is deposited on top and bottom of trenches without being deposited on sidewall of trenches. Experimental deposition profiles are determined by the balance between deposition of carbon containing radicals and etching by H atoms. Irradiation of ions hardens films and hence decreases the etching rate. When the etching rate surpasses the deposition rate of carbon containing radicals, no deposition takes place there. Eventually we have realized site-selective coating, coating on top surface, on bottom surface, on sidewall surface, or combination of them on trenches.

Acknowledgements: Work supported by MEXT, JSPS, and JST.