Diamond-like carbon (DLC) is a class of amorphous carbon material that exhibits some of the basic properties of diamond. It is often applied as a coating on virtually any material that is compatible with a vacuum environment. When DLC is applied in pure form, it is rock hard like natural diamond. These diamond coatings provide excellent protection against wear and attack from chemical vapors and atmospheric moisture.
Different Forms of Diamond-like Carbon
DLC occurs in seven different forms, which contain considerable amounts of sp3 hybridized carbon atoms. This is because diamond has two different crystalline polytypes. Common diamond found in jewelry has carbon atoms that are organized in 3-D cubic lattices, while the rare form called lonsdaleite has a hexagonal lattice, similar to cells in a beehive.
Although DLC is smooth when observed with visible light, it in fact has the form of a cobblestone street. The cobbles are amorphous and not crystalline because they are made from haphazard alternations between hexagonal and cubic lattices. The cobbles do not have long-range order and hence have no fracture planes along which to break. This results in an exceptionally strong material that is also flexible and conformal to the underlying shape being coated. When more and more layers of these nodules are stacked, DLC coatings can be developed into flexible, amorphous, and yet purely sp3 bonded diamonds (Figure 1).
Figure 1. Nanostructure of pure DLC showing the nodules (cobblestones) in which carbon atoms are arranged in 3-D diamond bonding. The scale marker is about 1% of the thickness of a human hair. Image made with electron microscopy. Image Credit: Wikimedia Commons/ Drac2000
Tetrahedral Amorphous Carbon
Tetrahedral amorphous carbon , also known as ta-C, is the hardest, slickest and strongest mixture. For instance, a coating of just 2µm thickness can increase the resistance of a common stainless steel against abrasive wear and thus increase its lifetime from one week to 85 years. Ta-C is believed be the pure form of DLC given that it includes only sp3 bonded carbon atoms. In order to reduce manufacturing costs or to impart other preferred properties, graphitic sp2 carbon, hydrogen, and metals are often utilized in six other known forms of impure DLC. Such economies can significantly reduce the service lifetimes of the articles being coated.
Applications of Diamond-like-Carbon Coatings
Diamond-like-carbon coatings are utilized in a wide range of applications. They are used in non-ferrous, automotive ferrous material machining and industrial engineering settings where surfaces of components are exposed to intermittent lubrication, high contact pressure, or dry sliding and rolling environments. Harsh adhesive and abrasive wear conditions are quite common in such environments.
DLC coatings are also used on a wide range of dental products, medical devices and implants to give a black, bio-compatible and anti-reflective surface with improved tribological protection properties.
Amorphous DLC has a unique combination of mechanical, physical, chemical and optical properties. It is often deposited at a substrate temperature of less than 250°C. The percentage of metal content, bonded sp2 and sp3 carbon atoms, and hydrogen content of the films can be manipulated by both the methodology and deposition chemistry such as evaporation, sputter and PACVD.