Fullerenes offer lubricant coatings
Composite coatings based on inorganic fullerene-like material (IFLM) nanosphere powders are being developed to reduce friction and improve wear resistance in parts where there are rolling and sliding contacts, such as ball bearings, chains, gears, pumps, screws and artificial joints.
The aim of the European FOREMOST (Fullerene-based Opportunities for Robust Engineering Making Optimised Surfaces for Tribology) programme is to save energy, extend operating times and reduce maintenance in sectors such as automotive, aerospace and medical. It comprises 31 partners, including Airbus, Renault-Nissan and Rolls-Royce Jet Engines, and a number of European universities.
The research is based on technology from ApNano Materials Inc, headquartered in New York, USA, for manufacturing nanoparticles of tungsten disulphide.
The company claims that, when incorporated into a matrix, the particles allow independent control of friction and wear, with consistent tribological performance.
'The IFLM molecules are among the strongest cage molecules known, withstanding 35GPa pressure without any damage or irreversible damage either physically or chemically,’ says Dr Niles Fleischer, Vice-President of Business and Product Development at ApNano. While diamond-like coatings, he says, are prepared in vacuum chambers, which make them more expensive, and their friction coefficient (0.1) is on the high end of IFLM. 'Our coatings are as low as 0.05.'
Talking in threes
The coatings produced are divided into three main groups – metal matrix, polymer matrix and matrix-less. Metal matrix coatings are produced using materials such as nickel and cobalt by suspending IFLM powder in a plating bathing solution. Fleischer says, ‘These offer hard coatings for high load applications. As they are based on aqueous coating techniques [an electro-less or galvanic process], they can accommodate large parts, but these components need to be able to withstand being immersed in [the] solution’.
Meanwhile, products that use polymer matrices such as polyketone, polypropylene, nylon and polyethylene are suitable for more intermediate load-bearing applications due to the softer matrix.
Fleischer adds, ‘The final type – the matrix-less coating – can be used in
situations where chemical compatibility precludes use of certain metals or polymers, where there may be issues of out-gassing from occluded items in the polymers (or the polymer itself), or from the metal composite’.
One of the main partners is thin film manufacturer Ionbond Ltd, based in Consett, UK.
Dr Peter Hatto, Director of Research at Ionbond, says, We got involved because there was potential to control the frictional properties of our coatings.’
The company manufactures thin films of typically three to four microns from nitrides of materials such as titanium, chromium and aluminium. It is researching the incorporation of IFLMs into the matrix of such coatings, as well as in situ deposition via physical vapour deposition (PVD), electroplating, sol-gels or a super-sonic cluster beam. The latter, installed on a PVD system, has so far been the most successful, says Hatto.
He adds, ‘The [challenge] is to bond the particles to the [substrate] surface while depositing the coating on top’.
The coatings arising from FOREMOST are to be marketed by the partners, with further research on finer powders, new compositions and functionalisation of the surface of the IFLM particles. The group also intends to research paints and greases, as well as exploit the strength of IFLMs in polymer and ceramic composites for helmets and bullet proof vests.