ICMCTF2013 Session B3-1: Deposition Technologies for Diamond Like Coatings

Electron-beam physical vapor deposition (EB-PVD) is a widely used process to obtain metallic or ceramic coatings for many applications including space, turbine, optical and biomedical industry. The principle is the evaporation of a solid ingot by a focused electron beam at low pressure. If a discharge is created between substrate holder and chamber wall, a part of vapor is ionized and coating microstructure can be modified. When ionic deposition is associated with reactive gas such as N₂, O₂ there is reaction between metallic vapor and fragments produced in the plasma to form nitrides or oxides of the metal evaporated. The control of chemical composition or stoechiometry in the deposit can be achieved by changing reactive gas partial pressure.

Carbide films can be obtained with the introduction of gazeous hydrocarbon in the EB-PVD reactor. But one has to note that even without any evaporation a coating of amorphous hydrogenated carbon (a-C:H) is formed on the cathode by plasma enhanced chemical vapor deposition (PECVD). So, in association with evaporation, there is two sources of “vapor” (including ions, atoms, radicals,…) incoming at the cathode, leading to an hybrid PECVD / EB-PVD process. To our knowledge, this technique has never been reported in the literature to synthesize doped a-C:H thin films. For example, the introduction of small amount of Si can improve the tribological properties of the coating. It is thus proposed to demonstrate in this paper the feasibility of doping a-C:H by silicon with the evaporation of Si ingot in hydrocarbon plasma discharge.

Actually, the difficulty is to obtain a carbon rich film with small and controlled silicon content. To keep unchanged the a-C:H matrix, the control of the chemical composition cannot be realized by changing hydrocarbon partial pressure or the cathodic bias. To reduce the silicon evaporation rate, it would be natural to consider the decrease of electron-beam power or the modification of beam sweeping. But, when the ingot surface is not homogeneously melt, the evaporation rate is unstable and this leads to very poor reproducibility. The proposed solution is a partial mechanical filtering of evaporated vapor using baffles with different opened surfaces placed just above the Si ingot. Measurements of instantaneous deposition rate with quartz crystal microbalance are reported showing that vapor flux is in relation with the opened surface of the filter. Chemical composition analysis by ERDA-RBS and X-ray photoelectron spectroscopy (XPS) of a-C:H:Si films are also presented and discussed. Silicon content in the a-C:H film could be controlled (between 1 and 5 at.%) by changing filter.

Due to their outstanding properties DLC (diamond-like carbon) coatings are applicable in the prevention of friction and wear in tribological systems.

The properties of various DLC coating systems have been further improved in recent years. Many commercial applications would simply not be possible were it not for the effectiveness of these coatings. DLC coatings on the needles of fuel injection pumps used in combustion motors is only one example of many applications. Problematic however, is the ability to achieve extremely planar surfaces when coatings are thick and component geometries and sizes are large. A prime example of such an application requiring an extremely planar surface and specific coating thickness is sliding and counter rings for use in gas pipeline gas seals. In order to make acceleration and deceleration of these in contact sliding and counter rings as risk-free as possible, a coating thickness of 6 micrometers is desirable. Even with sealing rings often having a circumference greater than 500 mm, deviations from the desired coating thickness commonly only in the nanometer range in the circular and radial directions are allowed so as not to influence the sealing ring’s aerodynamics and thus, its ability to function correctly.

A pre-condition for the necessary deposition parameters is a new RF-PECVD device design, developed at IWM. Unlike commercial RF-PECVD devices, there is a flexible reactor design with a usable volume which can be easily extended. This novel technique operates completely clean (without deposits outside of the substrate electrode) and has the capability of being scaled up.

Neon, argon, and krypton were used to explore the role of inert gases for the deposition process of carbon-nitride (CN_x) thin films in reactive high power pulsed magnetron sputtering (HiPIMS) and direct-current magnetron sputtering (DCMS) modes. The thin film synthesis and the plasma characterization took place in an industrial deposition chamber, where a pure graphite target was sputtered in Ne, Ar or Kr / N₂ atmosphere. The N₂-to-inert gas flow ratio was varied between 0 % and 100 % at a constant deposition pressure of 400 mPa. For both deposition modes, the applied average target power was similar. The carbon discharges were investigated using mass spectrometry measurements performed at the substrate position. Here, the ion flux was analyzed with regards to composition and ion energy. The ion energy distributions (IEDs) were measured for inert and reactive gas ions, C⁺, and C_xN_y⁺ (x, y < 2) ions. These results are related to the corresponding thin films with regards to their chemical bonding and microstructure obtained by X-ray photoelectron spectroscopy and transmission electron microscopy, respectively.

IED functions in HiPIMS mode exhibited generally a broader distribution, thus an increased mean energy, compared to DCMS. This was most pronounced for C⁺ and reactive gas ions, whereas only minor differences were found for the inert gas ions. HiPIMS and DCMS processes involving Ne and small contents of N₂ yielded the highest particle energies. This was mirrored in the microstructure of the thin films as an ordering towards fullerene-like was obtained when Ar and Kr were used as inert gas, whereas CN_x films deposited in Ne atmosphere were fully amorphous. HiPIMS processes yielded the most distinct fullerene-like structure compared to films synthesized by DCMS due to pulse assisted chemical desorption processes. Moreover, the efficiency to dissociate and ionize N₂ increases with decreasing inert gas mass, which is reflected by increased N⁺/N₂⁺ ratios in the plasma and elevated N contents in thin films deposited in Ne containing atmosphere.

A very recent paper [1] describes precisely an analysis of the energy consumption due to friction in passenger cars and its consequences. As an example, one-third of the fuel energy is used to overcome friction in the engine. Introduction of low friction coating technology such as diamond-like carbon (DLC) coatings on engines parts has improved engines efficiency by reducing this energy consumption.

This work brings the main results obtained by using low frequency plasma enhanced chemical vapour deposition (LF PECVD) for depositing advanced DLC type coatings. Thus, amorphous hydrogenated carbon (a-C:H), Si-containing a-C:H (Si-C:H) and a-C:H/Si-C:H multilayered films have been deposited by low frequency plasma enhanced chemical vapour deposition (LF PECVD) from cyclohexane-hydrogen and tetramethylsilane-argon mixtures for the a-C:H and Si-C:H layers respectively.

Structural and mechanical properties of single layers have been first studied. Then, previous results have been exploited to develop a-C:H/Si-C:H multilayered coatings. Some results show the possibility to obtain thick multilayered coatings (~10µm), with period thickness down to 25 nm. Surface texturing was also investigated in order to improve the tribological properties of a DLC/steel lubricated sliding contact. A direct coating texturing process based on a laser lithography technique has been developed and used for patterning hydrogenated amorphous carbon (a-C:H) layers. The effects of cavity dimensions on friction and wear behaviors were highlighted. It has been shown that creation of small and shallow cavities on a DLC layer allowed a significant reduction of friction coefficient of a DLC/steel contact comparing to a system with a non-textured DLC film.

[1] Kenneth Holmberg, Peter Andersson, Ali Erdemir

Tribology International 47 (2012) 221–234

DLC coatings of the a-C:H type were first described by Schmellenmeier in 1953. First patents for deposition methods for industrial applications were filed in the 70’s, e.g. Weißmantel et.al. .Ta-C coatings deposited by the filtered arc were published by. Aksenov et. al. in the 70’s. In the 80’s, the a-C:H:Me coatings were developed by Dimigen et. al. . Another big step was the introduction of non metal doped a-C:H:X coatings, e.g. X: Si,O, named diamond like nanocomposites (DLN) beginning of the 90’s by Dorfmann et. al., now applied as Dylyn®. Other X elements were introduced later on: N, F, B. DLC coatings are deposited by a variety of techniques. PVD techniques, PA-CVD techniques, or a hybrid process of PVD plus PA-CVD are among the most common methods for the DLC coating types. The PVD deposition technique for the adhesion promoting metallic interlayers and hard supporting under layers is magnetron sputtering however electron beam evaporation is also used (specifically for Cavidur®). Magnetron sputtering and arc evaporation (direct, filtered arc) of carbon targets are used to deposit hydrogen free DLC (a-C, ta-C) coatings. Special solutions for a-C:H coatings have been developed which combine carbon sputtering using argon as the sputtering gas and adding carbon containing gases. The PA-CVD techniques are based on glow discharges, mainly using either pulsed DC or RF. Typically, an additional activation source (e.g. electron injection) is utilized to stimulate the carbon gas decomposition and the ionization. Ion source processes, MW discharges and PLD are additional deposition methods. DLC coatings were commercialized initially in the two application areas: I) coating of hard discs in the nm-scale in the mid 80’s. II) coating of automotive parts for racing in the early 90’s (Cavidur®). The breakthrough for automotive applications was in the late 90’s, when VW introduced the unit pump system. The ta-C coating was introduced to the automotive industry for tappets by Nissan in 2007. The demand to further reduce the CO₂ emission is the main driver for the application of the DLC coatings on automotive parts. Large scale automotive applications demand coating systems especially designed for mass production. Also in demand, are coating systems designed to coat large and heavy parts like gears of wind power transmissions. Today, the application of industrial DLC coatings is wide spread and covers tribological applications for combustion engines, transmission systems, medical applications, semiconductor equipment, plastic manufacturing, cutting tools, forming tool, various general engineering parts, decorative applications and more.

Diamond-like carbon (DLC) based coatings are ideal for low friction and wear resistant applications. Those may expose the coatings to high temperature environments or to localized elevated temperature induced by. Therefore, the thermal stability of DLC based films is a key property for their long-term performance. In this investigation, DLC-MoS₂ composite thin film was synthesized using biased target ion beam deposition (BTIBD) technique in which MoS₂ was produced by sputtering a MoS₂ target using Ar ion beams while DLC was deposited by an ion source with CH₄ gas as carbon source. A pure DLC film deposited under similar conditions without sputtering was used as reference sample. After the deposition, DLC and DLC-MoS₂ thin films were heat-treated in ambient air, N₂ and vacuum environments at different temperatures ranged from 100 to 600 º C for 2 h, respectively. The effect of annealing on the structure, mechanical and tribological properties of the resulting films were studied by means of Raman spectroscopy, scanning electron microscopy, X-ray diffraction, nanoindentation, and ball-on-disc testing. DLC-MoS₂ thin films showed a slower rate of graphitization and higher structure stability throughout the range of annealing temperatures, indicating a relatively higher thermal stability.

To solve this problem the PEVCD chamber has been equipped with several additional self-constructed and calibrated RF-sensors at numerous positions in the matching network to measure voltage, current and phase angle. The plasma is monitored by non-invasive diagnostic methods. The behavior of the resonance frequency, the plasma resistivity and, indirectly, the collision rate are monitored in-situ by Nonlinear-Extended-Electron-Dynamics. The plasma chemistry is analyzed by Optical Emission Spectroscopy. By the analysis of the emission band shape of diatomic molecules the gas temperature is determined. Based on the actinometrical approach, it is possible to calculate the electron temperature and the particle densities of emitting species.

To obtain a time-resolved overview of all of these parameters a monitoring system, which connects all of these parameters, was developed. Furthermore, we studied the behavior of the RF-parameters related to the plasma state and the resulting layer properties according to the load condition of the chamber at different generator powers and gas fluxes. The aim of the investigations was to define the influence of the RF-parameters and the plasma state on layer properties. Two main results were achieved in this work. Firstly, it is possible to transfer established layer properties directly to different substrate sizes and to another devices. This is possible by characterizing the process by means of some of these outlined parameters. Secondly, a new process control technique for exactly this procedure was developed.