Charmed by Plasma

Creating a Hydrogen-free DLC Using Vacuum Arc Deposition

Diamond-like carbon (DLC) is a hard, amorphous carbon with desirable properties such as low friction and low wear. It is now gaining ground as a coating film for cutting tools and molds in place of hard nitride films.

DLCs can be divided into two categories – those which are purely composed of carbon and those which contain additional elements. The properties of each type of DLC depend on its composition and the way it was synthesized. DLC development researcher Professor Hirofumi Takikawa outlined the reasons for this as follows:

"What’s really important is whether a DLC contains hydrogen. Any DLC that doesn’t contain hydrogen will display superior hardness, density, heat resistance and refractive index. However, current popular methods of synthesizing DLC films, such as plasma CVD and ion vapor deposition use hydrocarbon gas or hydrocarbon vapor, which means that the resulting DLC will always contain hydrogen.

In order to develop a hydrogen-free DLC film, researchers have developed methods that use solid carbon (graphite), which does not contain hydrogen, as a raw material. There are four synthesizing methods that use graphite as a vapor source: vacuum deposition, electron beam deposition, sputtering deposition and vacuum arc deposition. Ions are required to make DLC films, which limits us to sputtering deposition or vacuum arc deposition, although sputtering deposition has low ion energy which results in a softer DLC. This is why I use vacuum arc deposition for synthesizing hydrogen-free DLC films."

Vacuum arc deposition is a method that uses arc discharge, that is, plasma (ionized gas), in a vacuum. In this method, a cathode spot is formed on a cathode solid surface, from which the cathode material (graphite) rapidly evaporates. At the same time, a large number of thermions are emitted and the evaporated graphite is ionized. When this occurs, a high ion energy is obtained and a super DLC (tetrahedral amorphous carbon (ta-C)), which has properties like a diamond, can be synthesized.

Professor Takikawa says that, "A cutting tool coated with a super DLC that has been synthesized using this method does not chemically react with metal or glass during cutting because it does not contain hydrogen. This means that it can make clean cuts without leaving a burr. Also, it won’t cause an unnecessary reaction and stick to materials when used in molds, which makes it suited for manufacturing high-precision camera lenses that need to be pressed at a high temperature, for instance.

Using a pure diamond as a cutting tool would obviously result in clean cuts, but diamonds are too expensive to be used for such applications. Diamonds are also crystals, which means that it’s impossible to form an even thin film. The properties of an amorphous (non-crystalline) DLC makes it able to produce an even and smooth film."

T-shaped Device to Prevent Adherence of Microparticles

Vacuum arc deposition is required to make hydrogen-free DLC, but this method has its drawbacks. When microparticles called droplets are emitted from the cathode spot, these droplets adhere to the film and cause the film to lose uniformity or peel off. Films in such conditions cannot be used in cutting tools for micromachining and molds for camera lenses that require precision. Torus-shaped (that is, donut-like 3D-shaped) filters have been developed to prevent droplets from adhering to the film, but these filters cannot remove all droplets and require much effort to maintain.

"Facing this problem, we developed a device that has a T-shaped filter. The main advantage of this device is in transferring vacuum arc plasma generated from the cathode to a film-forming chamber with a magnetic field generated using an electromagnet and an electric field applied to the filter duct. Because the filter has a T-shape, the plasma bends 90° while the droplets proceed in a straight line. In other words, the plasma and the droplets can be separated from each other, which results in a clean film. The duct also has a simple straight shape, making it easy to maintain."

Using carbon which is a burnable substance enables the film that covers the surface of the tool or mold to be separated and removed in the gaseous phase of oxygen plasma, making maintenance simple. That is, the DLC film can be easily removed, meaning that expensive tools and molds can be reused even if the thin film coating them is damaged.

Expanding Scope of Research to Meet Corporate and Social Needs

Development of this device first began decades ago as a joint research project with Itoh Optical Industrial Co., Ltd., a local manufacturer of lenses.

Professor Takikawa recalls, "Itoh Optical knew that using ions to form a film with vapor deposition resulted in a harder film. They came to us for technical consultancy on what was needed in order to effectively apply this knowledge. After some time, we developed a process of making high-precision lenses in a mold instead of polishing the lenses by hand, as was previously done. That was the first application of the results of our joint research."

Following this, Professor Takikawa and his team developed the device in collaboration with a manufacturer, was granted a patent and made many improvements to the device and manufacturing process. Currently, Professor Takikawa is looking into how to create laminated films by adding a sputtering vapor source to the device.

"By laminating many films together, we can add the functions of wear-resistance and toughness. We are now applying this technology to not just cutting tools and molds but also many other products in response to requests from various companies."

Professor Takikawa has also been working on a range of other projects. For instance, he has been developing self-supporting DLC films with no base, which are expected to be used in filters and laser-driven heavy ion targets for treating cancer. He has also developed a nano-powder called a "carbon nano-balloon," which is a type of nanocarbon used for electrode materials in electric vehicle capacitors and which increases electric capacitance. For all such endeavors, Professor Takikawa has searched for practical ways of applying the results of his research through joint research with companies.

While seemingly unrelated to the developments described above, Professor Takikawa has also branched out into research on spectroscopic pyranometers for agricultural use and has developed a method of more accurately obtaining information on solar irradiance. This method involves producing sensors at low cost with a simple system, using many of those sensors to detect abnormal values and cancelling out the influence of shadows. Professor Takikawa’s pyranometer is already being sold by a manufacturer under a licensing agreement. The prolific inventor is also working on a device that can disperse light every 100 nm and aims to further develop his products in order to assess harvesting times in greenhouses and to achieve more efficient operation at plant factories.

"Technology is made to be used, so I want to keep monitoring the market and applying my findings in the fields that need them. Pyranometers and carbon film research may seem completely unrelated to each other, but heat and light energy emitted by the sun, which we need to live, are the result of a nuclear fusion reaction of hydrogen plasma. In other words, plasma is the source of all living things and materials. That is why our research lab’s name bears the words 'Plasma Energy System.' We will continue to research plasma in ways that benefit society."