Diamond-like carbon

By mixing these polytypes at the nanoscale, DLC coatings can be made that at the same time are amorphous, flexible, and yet purely sp3 bonded "diamond".

Fillers such as hydrogen, graphitic sp2 carbon, and metals are used in the other 6 forms to reduce production expenses or to impart other desirable properties.

[8] Naturally occurring diamond is almost always found in the crystalline form with a purely cubic orientation of sp3 bonded carbon atoms.

The internal energy of the cubic polytype is slightly lower than that of the hexagonal form and growth rates from molten material in both natural and bulk synthetic diamond production methods are slow enough that the lattice structure has time to grow in the lowest energy (cubic) form that is possible for sp3 bonding of carbon atoms.

Because they occur independently at many places across the surface of a growing film or coating, they tend to produce an analog of a cobblestone street with the cobbles being nodules or clusters of sp3 bonded carbon.

As a result, ta-C may have the structure of a cobblestone street, or the nodules may "melt together" to make something more like a sponge or the cobbles may be so small as to be nearly invisible to imaging.

As implied by the name, diamond-like carbon (DLC), the value of such coatings accrues from their ability to provide some of the properties of diamond to surfaces of almost any material.

The primary desirable qualities are hardness, wear resistance, and slickness (DLC film friction coefficient against polished steel ranges from 0.05 to 0.20 [10]).

The highest quality of diamond-like properties was affirmed to be correlated with the proximity of the map point plotting the (X,Y) coordinates of a particular material to the upper left corner at (0,1), namely 0% hydrogen and 100% sp3 bonding.

Hardness is often measured by nanoindentation methods in which a finely pointed stylus of natural diamond is forced into the surface of a specimen.

Conversely, if the probing stylus enters a film thick enough to have several layers of nodules so it cannot be spread laterally, or if it enters on top of a cobblestone in a single layer, then it will measure not only the real hardness of the diamond bonding, but an apparent hardness even greater because the internal compressive stress in those nodules would provide further resistance to penetration of the material by the stylus.

The tool used a comparison of force applied to indentation depth, similar to the Rockwell Scale hardness measurement method.

[19] The same internal stress that benefits the hardness of DLC materials makes it difficult to bond such coatings to the substrates to be protected.

The most simple is to exploit the natural chemical bonding that happens when carbon ions impact the surface of the item to be coated at high energies.

If that item is made of a carbide-forming substance such as Ti or Fe in steel a layer of carbide will be formed that is later bonded to the DLC grown on top of it.

DLC is often used to prevent wear on razor blades and metal cutting tools, including lathe inserts and milling cutters.

[22] If a DLC material is close enough to ta-C on plots of bonding ratios and hydrogen content it can be an insulator with a high value of resistivity.

Perhaps more interesting is that if prepared in the "medium" cobblestone version such as shown in the above figure, electricity is passed through it by a mechanism of hopping conductivity.

Such high values allow for electrons to be emitted from ta-C coated electrodes into vacuum or into other solids with application of modest levels of applied voltage.

Virtually all of the multi-bladed razors used for wet shaving have the edges coated with hydrogen-free DLC to reduce friction, preventing abrasion of sensitive skin.

DLC coats the cutting edges of tools for the high-speed, dry shaping of difficult exposed surfaces of wood and aluminium, for example on automobile dashboards.

This has enabled many medical procedures, such as Percutaneous coronary intervention employing brachytherapy to benefit from the unique electrical properties of DLC.

DLC has proved to be an excellent coating to prolong the life of and reduce complications with replacement hip joints and artificial knees.

The implantable human heart pump can be considered the ultimate biomedical application where DLC coating is used on blood contacting surfaces of the key components of the device.

These are measured values; though in the case of the 2 μm coating the lifetime was extrapolated from the last time the sample was evaluated until the testing apparatus itself wore out.

[26] Currently there are about 100 outsource vendors of DLC coatings that are loaded with amounts of graphite and hydrogen and so give much lower g-numbers than 66 on the same substrates.

Dome coated with DLC for optical and tribological purposes.
A ta-C thin film on silicon (15 mm diameter) exhibiting regions of 40 nm and 80 nm thickness.
A Co-alloy valve part from a producing oil well (30 mm diameter), coated on the right side with ta-C , in order to test for added resistance to chemical and abrasive degradation in the working environment.
SEM image of a gold-coated replica of a ta-C "diamond-like" coating. Structural elements are not crystallites but are nodules of sp 3 -bonded carbon atoms. The grains are so small that the surface appears mirror smooth to the eye.
STM image of surfaces at the edge of a 1 μm thick layer of ta-C "diamond-like" coating on 304 stainless steel after various durations of tumbling in a slurry of 240 mesh SiC abrasive. The first 100 min shows a burnishing away from the coating of an overburden of soft carbons than had been deposited after the last cycle of impacts converted bonds to sp 3 . On the uncoated part of the sample, about 5 μm of steel were removed during subsequent tumbling while the coating completely protected the part of the sample it covered.