Similar names, different materials
Diamond-like carbon, often shortened to DLC, is a family of thin carbon coatings that share some traits of diamond: they are hard, smooth, wear resistant and often very slippery. Despite the name, they are not diamond. True diamond is a crystal; DLC is amorphous, meaning its atoms have no long-range repeating order.
This distinction is easy to miss because coating suppliers sometimes use “diamond” loosely. The useful question is always what the structure is and what performance it delivers on a given part.
The bonding difference
Carbon can bond in a diamond-type way, with four neighbors in a tetrahedral pattern, or in a graphite-type way, with three neighbors in a flat sheet. Diamond is entirely the first type, arranged in a perfect lattice.
DLC is a disordered mix of both. Its hardness comes from the diamond-type bonds, and the proportion of them varies between coatings. Many DLC films also contain hydrogen, and some contain added elements such as silicon, nitrogen or metals to adjust their behavior. Types are usually described as hydrogenated or hydrogen-free, with the hydrogen-free, high diamond-type content coatings being the hardest.
True diamond films have the crystalline structure described in the overview of diamond crystal structure, with grains that may be microscopic or nanoscopic. The discussion of nanocrystalline diamond covers one version of those films.
How they are made
DLC is generally deposited at much lower temperatures than diamond. Typical routes include physical vapor deposition, such as sputtering or arc evaporation, and plasma-assisted chemical vapor deposition using hydrocarbon gases. In these processes, energetic carbon ions or fragments land on the part and build up a dense film.
Crystalline diamond films usually need conditions that favor diamond growth over graphite, such as hot-filament or microwave-plasma reactors and substrate temperatures often several hundred degrees Celsius or higher. Hydrogen-rich gas is typically used. That requirement limits which materials can be coated.
Side-by-side comparison
| Aspect | Diamond-like carbon | Crystalline diamond film |
|---|---|---|
| Structure | Amorphous, mixed bonding | Crystalline, diamond bonding |
| Hardness | Hard, varies widely by type | Typically harder, near bulk diamond |
| Friction | Often low, especially against steel | Can be low, depends on surface finish |
| Deposition temperature | Usually low to moderate | Typically high |
| Thickness | Usually thin, often a few micrometers | Can be thicker |
| Surface | Smooth as deposited | Often rough unless polished |
| Thermal conductivity | Low | Very high |
Treat these as general tendencies, since both families have wide ranges.
Where DLC does well
- Low friction and wear on moving parts. Engine components, gears and pump parts can benefit from a smooth, hard, slippery surface, which affects the friction coefficient.
- Coating steel and other substrates. Because deposition temperatures are modest, many hardened steels can be coated without losing their temper.
- Cutting tools for non-ferrous materials. DLC is often used on tools for aluminum and similar metals, where chip sticking is a problem.
- Medical and consumer items. Surgical tools, implants and razor blades have used DLC, and its biocompatibility is a subject of ongoing study.
- Magnetic storage and other precision components. Very thin protective layers are used on some of these parts.
Where true diamond films do better
Crystalline diamond films are chosen when the job demands the extreme hardness or other bulk diamond traits that DLC cannot match: severe abrasion, high heat flow, or operation at elevated temperature. This is typical of machining abrasive composites and graphite with diamond-coated tools. Details on this use are in the discussion of diamond films.
Limitations of DLC
- Temperature. Many DLC coatings begin to change, as the structure shifts toward graphite-type bonding, at temperatures far lower than where diamond stays stable. Hydrogen-containing types are especially limited, and the exact limit depends on the coating.
- Internal stress. Dense DLC often carries high compressive residual stress, which can lead to peeling on thick films.
- Adhesion. Good coating adhesion generally requires clean surfaces and an interlayer, often a metal or metal-carbide layer.
- Ferrous machining. Like diamond, carbon-based coatings can react with iron at cutting temperatures, so they are usually not the first choice for machining steel.
- Thin layer. Because the film is only a few micrometers, it will not rescue a part if the substrate underneath is too soft to support it.
Choosing sensibly
The two coating types address different problems. If the need is low friction at moderate temperature on a hardened substrate, DLC is often the practical option. If the need is maximum abrasion resistance in a demanding cutting or wear role, a crystalline film or a diamond-based material may be justified, at a higher cost and with more substrate constraints.
For how these coatings fit with graphite, graphene and the rest of the family, see the overview of carbon allotropes.