ICMCTF2010 Session E1-2: Friction and Wear of Coatings: Lubrication, Surface Effects and Modeling

This paper is related to the development of CrN/AlN thin films on H13 tool steel produced with the physical vapor deposition (PVD) process, where the objective was to determine the friction and wear of the layers on this tool steel used for hot work. The films were produced by reactive magnetron sputtering of aluminum under s everal modes of injection of nitrogen to find the optimum deposition conditions to form a satisfactory structure such that it provided a good adhesion to the substrate. The injection methods used were: convectional injection of Ar + N₂ mixture away from the substrate varying the amount of nitrogen, Ar + N₂ mixture away of the substrate with extra nitrogen near the substrate and direct injection of N₂ near the substrate. The injection method that results in a richer nitrogen atmosphere seems to produce better outcomes. The resulting coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive X-ray micro analysis (SEM-EDS) and their tribological characterization was carried out by using a pin-on disk tribometer.

Nature lubricates with polymer monolayers: The substrates are soft, the base lubricant is water, and the polymers are glycoproteins. Glycoproteins consist of a proteinaceous backbone with oligosaccharide side chains, and it is these latter structures that appear to lead to the lubricious behavior of the glycoprotein-covered surface.

Lubricating man-made devices with glycoproteins is not very practicable. They are expensive to obtain in quantity, and susceptible to bacterial attack. They are also not suited to the lubrication of many surfaces, especially those of hard materials. Nevertheless, some of the natural principles can be utilized for the design of lubricating polymer systems that can work in a variety of environments on a variety of substrates.

The majority of studies carried out in our laboratory have concerned aqueous lubricant systems. Water is rarely used at present as a lubricant in an industrial environment, but the combination of novel polymer additives with corrosion resistant sliding surfaces, such as engineering polymers or ceramics (e.g. Si₃N₄), could lead to the introduction of a new generation of oil-free bearing systems. The common factor in the polymer monolayers appled to date is that they have been surface tethered, like the oligosaccharides in nature, often leading to the formation of brush-like structures upon exposure to a "good" solvent. Polymer brushes have a variety of interesting applications in many different technologies, and their fabrication is either carried out by attaching ready-made polymers to the surface by one end ("grafting to"), or by immobilizing initiators on the substrate and then growing polymer chains out of the surface ("grafting from"). Both approaches have been utilized in our laboratories and have been shown to lead to substantial friction reduction under aqueous lubrication.

Water has a major drawback as a lubricant, in that its viscosity is essentially pressure independent. This results in poor performance under classical EHL conditions, as encountered in traditional ball bearings, for example. Two approaches can mitigate this situation: the use of elastomeric contacting surfaces, such that the soft EHL regime is encountered, and the addition of a viscosity-enhancing fluid, such as glycerol, to the water. The latter approach has the advantage that the transition to mixed and hydrodynamic lubrication happens at a lower speed, leading to a lower friction coefficient over a wider range of conditions. The pressure coefficient of viscosity of glycerol, being greater than that of water, also brings with it additional benefits.

A Calotest micro-abrasion machine (which is used widely to determine the thickness of hard coatings) was used to

perform abrasive wear tests on AISI 1018 steel substrates after a variety of surface treatments such as

carburizing, nitro-carburizing, electrochemical boriding and pack boriding. Using a variety of abrasive slurries

(containing alumina, SiC, and diamond particles), the effects of test duration on the wear resistance of untreated

and surface treated substrates were evaluated and the results were compared in terms of wear crater depths and

the weight losses. Using the present technique, wear resistance of single phase boride layers (Fe₂B) for pack

boriding, electrochemical boriding, and double phase boride layers (FeB + Fe₂B) formed during electrochemical boriding have been calculated. We found that borided surfaces are much more resistant to abrasive wear than the surfaces that were treated by other methods.

In this study, the effects of rhenium on the tribological properties of Al-Ti-N and Cr-N coatings were investigated. The coatings were produced by cathodic arc + magnetron sputtering hybrid physical vapor deposition technique. Al-Ti-N and Cr-N coatings were deposited by using cathodic arc. The arc current and N₂ pressure were 80 Amps and 1 Pa, respectively. In order to incorporate rhenium into Al-Ti-N and Cr-N coatings, a rhenium target was simultaneously sputtered with 200 W. The rhenium content of Al-Ti-Re-N and Cr-Re-N were 2.52 at.% and 1.58 at.%, respectively. In order to characterize the coatings, Calotest, X-Ray Diffraction, Scanning Electron Microscopy, EDS and micro hardness techniques were used.

The tribological properties of the coatings were investigated by using ball-on-disc and reciprocating tests against Al₂O₃ balls at different temperatures starting from room temperature up to 200 °C. The wear of the coatings and the counter body surfaces were observed with 3D optical profilometer and SEM. The wear debris in the wear tracks were analyzed with Micro Raman Spectroscopy. The wear behavior of the coatings were correlated with the chemistry of the compounds formed in the wear track. The interdependent role of temperature and rhenium content on the tribological behavior of the coatings were presented.