ICMCTF2010 Session G1-1: Innovations in Surface Coatings and Treatments

Coating deposition using cathodic arc technology has become widely commercialized during the last 3 decades, e.g. for hard coatings on tools and mechanical components and for durable decorative coatings. In such applications the advantages of an ionized and energetic coating stream for promoting adhesion and controlling coating structure have been found to outweigh the disadvantages of the splattered droplets (“macroparticles”) generated by the arc and incorporated into the coating. Although “filtering” of the coating stream, using magnetic and electric fields to divert the plasma while mechanically blocking the macroparticles, has been known since the late 1970s, the inefficiency and complexity of available equipment has hindered commercialization of this technology. Improvements in filtered source design in recent years by several groups, notably in increasing the ion current output, have made it practical to use filtered arc coatings in a number of products now being manufactured on an industrial scale. This presentation will review some current applications of filtered arc deposition, and examine factors influencing industrial adoption, including source and equipment design and cost, materials capabilities, and the emergence of competing methods providing ionized coating atoms.

Hollow cathode arc discharges with hot emitters have been studied for some decades in basic research and different fields of practical application. Especially in physical vapor deposition (PVD) they are often used for metal evaporation and the generation of plasma. Shortcomings preventing a broader application were found in the high throughput of working gas necessary for a stable operation and in the very inhomogeneous, locally concentrated ionization efficiency.

A promising way to overcome these limitations was found by arranging the hollow cathode in a strong axial magnetic field, allowing for a drastic reduction of working gas flow without loss of discharge stability. Moreover, with the reduction of gas flow not only a strong increase of the discharge impedance was observed, but also a multiplication of ion current density together with a highly expanded volume of plasma plume.

By means of spatially resolved Langmuir probe measurements, combined with the usage of an energy-resolved mass-spectrometer, plasma density profiles and energy distribution functions of electrons and ions have been measured. Generally, with an increase of the magnetic field and with the reduction of the working gas flow those energy distribution functions shift from mean values of a few eV to 10 eV and more, while charge carrier densities increase from 10¹⁶ m^-3 to more than 10¹⁹ m^-3. A systematic overview of the measurements will be presented and conclusions for the application of the plasma source in different fields of surface treatment technologies will be derived.

In the second part of the presentation two promising applications related to the coating of cutting tools in industrial batch devices will be discussed in detail: the sputter etching with argon ions and the reactive pulse magnetron sputter deposition of wear resistant nitride layers. Whereas the first mentioned process provides pre-heating and etching rates higher than all actually used in tool coating industry, the second one offers great advance for film growth kinetics leading to significant improvements in structure, surface morphology, elastic modulus and hardness of the deposited layers.

Recently, internal plasma processing of halls and tubes such as DLC (Diamond-Like Carbon) coating of molds and mechanical parts is strongly desired. Uniform processing of such a 3-dimensional metal surface is typically obtained by RF and DC plasmas where negative voltage is applied to a metal substrate against a grounded chamber at low gas pressures. However, the plasma electron density, n_e of RF and DCplasmas is typically no more than 10⁹−10¹⁰ cm⁻³, by which high-speed processing can not be expected. Furthermore, such low-density plasma can not steadily exist inside small halls and tubes less than 1 cm in width under typical processing conditions, In order to solve the latter problem for iinternal plasma processing of narrower metal halls and tubes, the use of higher-density (n_e>10¹¹ cm⁻³) plasma is essential. Therefore, we proposed a new method which can steadily generate such high-density plasma along 3-dimensional metal surfaces by using microwave propagation along plasma-sheath interfaces bounded by metal surfaces.

In our previous works, it was demonstrated by using the new method that plasma column is steadily sustained inside a narrow metal tube whose inner diameter is in the range of millimeters and the length is more than 10 times of it. Furthermore, in this work, we developed a new plasma CVD apparatus by using the plasma column, and prepared DLC film on the inner surface of a stainless-steel tube (SUS304, JIS) 4.4 mm in inner diameter and 50 mm in length. Here, the flow rate ratio of source gases, methane and tetramethylsilane (TMS), were controlled to be 5:1, at a total gas pressure of 27 Pa, where a negative voltage of −180 V was applied to the pipe with 2.45-GHz microwaves injected at a forward power of 120 W.

In order to investigate the tribological properties of the DLC film deposited, rotational friction test of the film a was conducted against cast iron shaft (FC250, JIS) 4.2 mm in outer diameter and 10 mm in length; as a result, friction coefficient was 0.17 at the position about 40 mm from the bottom end of the pipe where microwaves were injected during deposition. However, the friction coefficient was 0.4 at the rest positions (10 and 25 mm from the bottom end), where the films were peeled off the substrate.

This work was partly supported by a Grant-in-Aid for Young Scientists (B), No. 19740343 (2007-2008), from the Japan Society for the Promotion for Science, and Tokai Region Nanotechnology Manufacturing Cluster, Knowledge Cluster Initiative (The Second Stage, 2008-).

Plasma ion implantation has proven to be an effective tool to enhance surface properties. Many attempts have been made to implant internal wall of cylindrical bore using this technique. However the difficulties have been well demonstrated due to severe non-uniformity of incident dose stemming from external plasma source. A novel technique has been proposed in our lab based on internal inductively-coupled radio-frequency discharge. The RF coil acts as both plasma source and grounded electrode to eliminate the overlapping effect of plasma sheath in tubes. In order to comprehend the implantation dynamics, numerical simulation based on particle-in-cell/Monte Carlo method has been performed. The uniformity of injected ions, the influence of ion-neutral collisions on the impact energy, incident dose and impact angle have been investigated at different gas pressure. The shadow effect of RF coil appears in the implanted wall and minimum incident dose happens at the center of shadow regions. Incident ions fly towards the internal wall at about ±30°off normal with higher impact energies. As the gas pressure increases, the uniformity of incident dose is improved slightly however the impact energy decreases significantly. A higher gas pressure leads to wider distribution of impact energy and angle of incident ions. The numerical results have demonstrated that this novel technique is capable for implanting internal wall of cylindrical bore and a manipulation system has to be utilized to achieve uniform implantation.

In a typical micro-fabrication process, micro scale pattern transfer is achieved by using photolithography. In this process, each substrate is covered by a light sensitive resist. The resist covered substrate is then exposed to light through a patterned mask – this either develops or destroys the resist in the areas exposed to light. Thereafter, materials can be plated on or etched off from the exposed areas. This platform technology, therefore, allows micro sized patterns of materials to be transferred on to a substrate.

We have developed a process to transfer micro scale patterns on a fully exposed substrate. The method uses electrochemical means and a specialised electrochemical reactor for pattern transfer. This process uses a metallic material with a resist pattern, which serves as an electrochemical tool. The substrate, which is fully exposed, is placed facing the tool, within close proximity. The tool and the substrate are electrically connected so that the tool is the cathode and the substrate is the anode. Electrolyte is pumped through the system to deliver fresh solution to the anode and cathode as well as remove reaction by products.

Our experiments, involving copper as the tool as well as substrate material, showed that micro scale patterns by electrochemical etching and plating could be transferred with good reproducibility. In our reactor, they were placed within a distance of 500 microns. We have successfully transferred micro patterns which are significantly smaller than the electrode gap, namely, 50, 100 and 200 microns. The steep walls and cubic shape demonstrate the feasibility of the process. The process has also been applied to metal deposition and lines to 100 microns width have been reproduced. Copper deposits up to 2.5 microns thick has been plated using the technique.

We have developed a model which describes the underlying physical phenomemenon of the micropattern transfer process. The model is a reaction distribution model (which, in electrochemical engineering is called a potential or current distribution) model. We used modelling software, ElSyCA-2D, to understand the critical processes enable micropattern transfer. The model is tested against the experimental data obtained in the experiments.

Since a single tool can be used to transfer a pattern numerous times, this opens the the possibility of greatly reducing the use of photolithography for pattern transfer on to metallic substrates. This means that chemical solvents used for photolithography can be minimised. In addition clean room use can also be greatly reduced. Finally, the technique requires very dilute solutions, which means that the process is non-hazardous.