Diamond like Carbon

Figure 1- showing the Ternary phase diagram for the C, H systems.^[1]

Dimond like carbons (DLC) are a distinct set of amorphous carbon materials which share structural similarity to diamond, due to them both possessing sp³ hybridised carbon atom. The irregularity in its arrangement comes from the presence of filler atoms such as sp² hybridised carbons, metals and even C-H bonds. The presence and arrangement of these filler atoms in the sp³ system, causes the material to exhibit unique and/or improved properties not observed in a simple. In fact there are 7 classified type of DLC commonly used (demonstrated in figure 1), the most abundant of which being tetrahedral amorphous carbon (ta-c), consisting of an even blend of sp³ and sp² carbons, making the material stronger, smoother, have a higher gas barrier performance as well as a better biocompatibility compared to diamond, some research suggesting that it is even possible to scratch diamond.

Why and What are they used for?

The unique properties of DLC materials have made them one of the most sought after and researched materials within a large range of fields. The amazing this about DLCs is that in most applications they are only needed as coating materials, which are most formed as thin films (about 5 m thick), this means that not a lot of material is needed and therefore cuts cost for manufactures. These coats can be used on several materials to improve their hardness, they are so successful at improving toughness, that when steal is coated with DLC and exposed to rough wear, its lifetime was improved from a few weeks to 85 years^[1]. Because of this DLCs have been found to have a great importance in the durability of materials ranging from scratch resistant car windows to coating space shuttles to prevent wear during launch, due to their high environmental temperatures. DLCs can be made to have an incredible smooth surface allowing them to have an extremely low friction. This technology has found its way to the locomotive industry, where coated gears and equipment make a perfect replacement to lubricants, as well as increase the longevity of the vehicle.

For a long time now DLCs have been researched and developed for biomedical use due to their superior mechanical properties and biocompatibility, they have already been used in the field of medicine biomedical applications. Implants made from DLCs have shown great success, since tissue can easily adhere to the surface as well as when blood is present a layer of protein is formed around the surface making the body less prone to blood clots and less likely to reject the implant. Because of this, DLC coating has been of great important to the development stents, a device which is able to expand veins and arteries. DLCs, much like diamonds, are very inert, research shows that they are very resistant to acidic substances, making them ideal for storage of highly corrosive and dangerous chemical which would otherwise seep through when using uncoated glass, as well as to protect sensitive equipment^[1].

The structure of diamond is well known for being extremely electrically insulating, whilst the its graphite allotrope is very conducting along its planes, since DLCs have an internal structure consisting of a  mixture of diamond and graphite carbons, they have been observed to have conducting properties. The extent of which is directly proportional to the amount of conducting sp² carbons and doping which the material has, this conductivity is achieved via quantum mechanical tunnelling between sp² sites (pockets of electron site). Because of this DLCs can be easily manufactured to have a range of different conductivities from super conducting to insulating, this also means that right at a key sp² percentage semiconducting properties are observed. Moreover, the ease that which DLC’s properties can be modified, means that they can be fine tunned to have a desired band gap for a particular job, making them a very useful and prominent technology in the semiconducting industry^[2]. Unfortunately, the market is currently controlled by silicon semiconductors, due to them being cheaper and having more investment, meaning that current DLCs are used to coat and improve on the properties of already developed silicon based semiconductors. Due to the incredible conductivity features that DLCs can manufactured to have, means that they are very regularly used in the electronic community both for passive and active materials.

How are they manufactured?

Figure 2- showing 5 types of ion beam deposition methods ^[2]

Since the invention of DLCs in the early 70s, there have been a multitude of ways to develop them, all of which based on deposition methods to grow thin films. The first DLCs where developed via ion beam deposition, which have expanded into several beam type depositions as shown in figure 2, all of which sharing the general features, carbons ions are created by a plasma sputtering to a graphite surface. Sputtering is the process where an accelerated ion is targeted on to the surface of a material (in this case graphite) to remove particles of the material. The ejected carbon ions can them be guided using a forward bias to a substrate target where the thin films can grow from. Unfortunately, this process does require immense temperatures and a high vacuum environment which reduces the number of materials that it can grow from as many materials might decompose in the process. The other major deposition technique is called chemical vapour decomposition (CVD) where a solid material is vaporised via a chemical reaction and then deposited to the surface of a substrate. This technique is widely used in the formation of thin films for semiconductor materials. ^[2]

Their future?

As mentioned before the properties of DLCs can be highly tuned by controlling their manufacturing process, because of this most current research usually targets the production process of this material. Recent improvements in the field of DCLs focuses on developing deposition methods which do not require a high vacuum and temperature, Keio university in Japan have been studying these novice deposition growths, able to develop thin films at atmospheric temperature and observing any changes in the structure and properties.^[3]

References:

[1] Rajak, D., Kumar, A., Behera, A. and Menezes, P., 2021. Diamond-Like Carbon (DLC) Coatings: Classification, Properties, and Applications. Applied Sciences, 11(10), p.4445.

[2] American Elements. 2022. Robertson, J., 2002. Diamond-like amorphous carbon. Materials Science and Engineering: R: Reports, 37(4-6), pp.129-281.

[3] Hasebe, T., Ishimaru, T., Kamijo, A., Yoshimoto, Y., Yoshimura, T., Yohena, S., Kodama, H., Hotta, A., Takahashi, K. and Suzuki, T., 2007. Effects of surface roughness on anti-thrombogenicity of diamond-like carbon films. Diamond and Related Materials, 16(4-7), pp.1343-1348.