Diamond like carbon, DLC, and ta-C

Benefits in mechanical applications

If the deposition of pure DLC or ta-C commences with the impact of energetic carbon ions or atoms, then bonding of the "diamond" layer to the material being coated can be the strongest type of chemical bonding. Many materials for which carbides do not exist at STP, nevertheless have the potential for forming metastable carbides in an interfacial layer if the carbon atoms are physically constrained in the two-dimensions of the interface between substrate and pure carbon coatings. Iron and Ge are examples, and contrary to expectations, the incident carbons are not solvated into the material being coated and metastable carbides are formed in the interface. Such chemical bonds of coating to substrate are the strongest kind of attachment and do not break under mechanical stress.

Although the strudtures of pure DLC, and ta-C are like a flexible mat embedded with load-bearing "cobblestones" of diamond, the nanoscale of the structure is so fine that coatings are mirror-smooth and have a coefficient of friction against all other materials lower than Teflon (<0.1).

When chemical bonding of pure DLC and ta-C occurs by forming metastable carbides that are not stable without the 2-d interfacial constraints, it is difficult to image such chemistry because the X-ray spectrographic constants are usually not known for metastable molecules. However, in the case of bonding pure DLC to Ti the spectrographic constants for TiC are well known and the "chemistry" in the bonding interface can be imaged. Shown here is a dark field X-ray image of a (distorted) cross section made through a layer of pure DLC deposited by PVD on a Ti sample. The image is recorded at the angle corresponding to resonant scattering from the chemical bond of TiC. More red coloration shows the regions where the stoichiometry of C and Ti is more nearly that of the compound TiC. Red areas correspond to actual nanocrystals of TiC in the bonding interface.

Most process for depositing pure DLC and ta-C are "room-temperature" processes meaning that fragile materials can be coated without damage.

Pure DLC and ta-C readily bond to quartz optical windows without need for deposition of a separate interfacial layer.