Some experiments directed toward the development of improved hard, lubricious coating materials have involved direct fluorination of proprietary diamondlike coats on various substrates. The idea is to retain the hardness of the diamondlike material while adding surface molecular structures that impart the chemical inertness (especially resistance to erosion by atomic oxygen) and lubricity of fluorinated polymers.
The proprietary diamondlike coating material is Amorphic Diamond™, which comprises nodules of carbon that have the molecular/crystalline structure of diamond, with sizes of 100 to 200 nm, densely and uniformly packed in a net of amorphous carbon polytypes. This material has a low coefficient of friction and is almost as hard as diamond is. However, it is not brittle: it can withstand flexure and shock without cracking. It can be applied to plastic, polymer, metal, and ceramic substrates.
In the experiments reported thus far, material samples coated with the diamondlike material were exposed to fluorine to create a surface layer of highly inert carbon/fluorine molecular bonds. Fluorination did not degrade the lubricity and hardness of the diamondlike coats. Among the samples were sheets of Kapton™ polyimide 5 mils (about 0.1 mm) thick. The sheets remained flexible after fluorination of the diamondlike coats. The coating layers were not compromised, even when the sheets were creased.
At the time of submission of information for this article, the ability of the fluorinated coats to resist erosion by atomic oxygen remained untested. Also untested at that time was the combined effect of ultraviolet radiation and erosion by atomic oxygen (this is of concern because the rates of erosion of fluoropolymers by atomic oxygen have been observed to increase in the presence of vacuum ultraviolet radiation.
This work was done by Mark S. Hammond and A. Wesley Moorehead of SI Diamond Technology, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.