Reducing friction and wear

Tribology is an often overlooked subject area and new technologies in the area can often be over looked due to their complex and 'difficult to get right' nature.

Yet the science and technology behind interacting surfaces is vital to providing a multitude of machines continual rotational and linear motion with minimum friction build up and wear.

And there are improvements continually coming through. Some the most recent include developments in thin coating technologies that significantly reduce the dry friction coefficient of interacting materials, which also dramatically improves wear life. These can mostly laid down on a surface by using existing chemical vapour deposition and sputtering techniques.

Bearing manufacturer NSK says it has been able to improved smearing resistance by applying a proprietary diamond like carbon (DLC) coating to its bearing rolling elements. It has recently developed spherical roller bearings for papermaking machinery that eliminates micro seizing.

The phenomenon of smearing - or micro-seizing – is caused by slippage between the raceway surface of the inner and outer rings and the roller surface. It often occurs in bearings used in light load areas inside papermaking machinery and areas with poor lubrication. If not addressed, smearing can lead to flaking and fracture that result in production lines being shut down.

Its DLC coating is a hard film made mainly of carbon and it will give the bearings much more stable operation and extend maintenance intervals. The coating has an elastic modulus close to that of the base metal so any deformation is closely mimicked and followed. SKF has also improved the adhesion between the coating and base metal, making it less likely to come off, even under high surface pressure.

NSK developed its new product to improve energy savings, extend maintenance intervals, and prevent early damage to bearings with light loads, and bearings used in areas with poor lubrication conditions.

But, some of the biggest improvements have been achieved by incorporating nm sized balls of tungsten disulphide. The coatings are in most cases only about one or two microns thick, yet this is enough to drastically reduce the friction between unlubicated surfaces. This is often essential when operating under extreme conditions where lubrication maybe difficult or minimal.

These coatings also decrease wear where there is lubrication, which is useful for automotive designers working with camshafts and spherical joints, as well as various sliding and rolling element bearings.

The downside is that because stopping sliding surfaces from wearing each other out is extremely complicated, each potential use has to be examined separately in order to establish that it works effectively.

Although a coating may work in one application, it may not work in another. And some hard coatings help improve wear resistance in one kind of application but not in others. For example, chromium nitride is very good at resisting fretting but very poor when used on gears.
Professor Mark Gee, a fellow in surface engineering and tribology at NPL, says: "DLC and molybdenum disulphide (MOST) coatings that incorporate titanium metal into a molybdenum disulphide matrix are offering increasingly higher resistance to adhesive and abrasive wear. They usually have a load bearing limit in excess of 3GPa and a coefficient of friction ranging between 0.005 to 0.1 depending on load and operating conditions."

DLCs already widely used to coat rubbing parts in the motorsport community; however they can go wrong and suffer when moisture is present. However, Teer Coatings developed a patented alternative, Graphit-iC, which has a graphitic microcrystalline structure that it claims performs much better with water based lubrication.

Professor Gee also says traditional hard cutting tool materials, such as tungsten carbide in a cobalt matrix, can also be used as bearing materials. "When this is done pieces of tungsten carbide in the surface fragment are embedded in the matrix to produce a wear resistant surface," he says.

Another idea that has been around for sometime, but has never quite been developed, is to introduce very small spheres - made of a given material - into the metal matrix. But, US based ApNano are using the concept to produce commercially available lubricating coatings under the product name Nanolub.

It claims that multiple layers of tiny spherical particles lubricate surfaces as the particles roll over one another like miniature ball bearings, just on the nano scale. The coating is made up of particles of tungsten disulphide that are 20nm across. The spheres have been given the generic name of inorganic fullerenes.

NanoLub is now being added to various oils and greases, and is used to impregnate parts and as a component of polymer and metal composite films and coatings. NanoMaterials, and Israeli oil company Sonol, have got together to incorporate the technology into a top up concentrate for enhancing engine oil performance.

The particles can also be incorporated into nickel coatings, where they reduce the dry coefficient of friction to between 0.05 and 0.1, as well as cobalt, aluminium and a wide range of polymers.

The idea of using nano scale balls of material as an aid to lubrication and something that can be embedded into a metal surface has led to a European Union project called FOREMOST – Fullerene based Opportunities for Robust Engineering Making Optimised Surfaces for Tribology. The project is now nearing completion and has had high profile participation from EADS, Renault, Rolls Royce, Goodrich, Nanomaterials and IonBond.

The €18.9million project aims to substantially extend operational life, reduce maintenance requirements and reduce the environmental impact of a wide range of machines incorporating transmission trains, sliding bearings, spherical joints and roller bearings. It is hoped this project will produce a radical innovation in friction and wear protection concepts and insure that tribology continues to benefit mainstream industry and engineering.

Author
Tom Shelley

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