Tribology of Diamond and Nanocomposite Coatings

The development of low-friction and wear-resistant materials was traditionally driven by highly specific industrial applications, such as machining, stamping and forming tools [1]. These coatings are also now making an impact in a wider range of products including razor blades, magnetic hard discs, critical engine parts, mechanical face seals, scratch-resistant glasses, invasive and implantable medical devices, and microelectromechanical systems (MEMS) [2].

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Low-friction, low-wear and anti-adhesion properties are the highly desirable characteristics of these coatings, which include diamond-like carbon (DLC) coatings as well as nanostructured metal nitrides, such as molybdenum nitride and titanium nitride [2][3]. Nanocomposite coatings are now becoming commonplace for the protection of the contact surfaces of mechanical systems as they allow higher speeds, increase lifetime, and improve corrosion resistance [1][2].

Diamond-Like Carbon Coatings (DLC)

Many DLC films are extremely hard and provide hardness data approaching 90 gigapascals (GPa). Super-hard materials are defined as having a hardness of above 40 GPa [4]. At the same time, from a tribological point of view they provide some of the lowest known friction and wear coefficients.

Outstanding optical and electrical properties and a chemical inertness to corrosive and oxidative attacks in both acidic and saline media make DLCs unique. Understanding of the manner in which dynamic lubrication mechanisms operate at DLC surfaces is still in its infancy and studies have been assumed to examine how chemical interactions between DLC and lubricants affect the tribology [3][5]. Some researchers suggest that hydrogen passivates ‘dangling bonds’ on DLC surfaces to lower the coefficient of friction. While other studies have looked at the interaction of DLC with water and have suggested that hydrophilic hydroxyl groups play a role in reducing friction at DLC surfaces. It is concluded that the presence of unpassivated sigma and pi bonds on DLC surfaces is a primary cause of increased friction [3].

Alcohol and Fatty Acid Interaction

A 2013 study examined the interaction of hexadecanol with DLC surfaces using atomic force microscopy (AFM) and tribological testing. AFM indicated the amount of surface coverage, namely the size and the density of the adsorbed islands of alcohol molecules. Tribological tests were then performed to correlate wear and friction behaviour with the adsorption of molecules on the surface. The study showed that alcohols adsorb both physically and chemically onto the DLC surfaces and are likely to act as boundary-lubricating compounds for DLC coatings. Alcohol adsorption onto the DLC surfaces reduces the wear of the coating but it is less effective in reducing friction of DLC further. The proposed adsorption mechanism implicates a temperature effect and tribological rubbing effect and suggests a bond between oxides on the DLC surface and the hydroxyl group hydrogen so that the long alkoxy (hexyl) chain is outward facing and able to take part in the lubrication processes.

Alcohols are well known for their lubricative properties [3]. A study [6] comparing the adsorption properties for hexadecanoic acid and hexanol on DLCs showed that both alcohol and fatty acid could provide wear protection to the DLC surface, especially at temperatures above 80°C. Again there was no tenable reduction in friction because of the inherent low friction characteristics of DLC coatings. However, at low concentrations (2 to 5 mmol/l) of fatty acids provided lower wear compared to alcohols proving the enhanced adsorption ability of the acid. Adsorption at the DLC surface was also thought to be due not only to the formation of passivating bonds (ester in the case of acids and ether in the case of alcohols) but also to the development of a triboplasma from free ions and electrons promoting favorable tribochemical processes [6].

Metal Nitride Coatings

The application of nitride-based hard coatings on steel is increasing because of their high hardness, low friction factor, excellent adhesion to different steels, and superior thermal and chemical stability.

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The thermal stability of single-layer coatings of TiN and MoN is not very high. At temperatures of 550–600°C, these coatings begin to oxidize, and consequently theirhardness is severely decreased [7, 8]. Therefore, multilayer nanostructured composites of metal nitrides showing improved physical, chemical, and tribological properties are being used for a range of applications.  Such metal nitride multilayer nanocomposite materials were recently examined in Poland [7] using a universal mechanical testing jig for a ball-on-plate sliding mode. The study showed that the wear, friction, and load-carrying properties of multi-layered composites of TiN and MoN varied with their grain size.

It was concluded that decreasing monolayer thickness resulted in an increased hardness and smaller grain structure. An additional study on the effect of nano-layer thickness [8] revealed that nanostructured multilayer coatings, obtained by ARC/PVD methods, with layer thickness between 10 and 20 nm produced a friction coefficient in the range of 0.09 to 0.12. A 2015 study [9] reported that the use of cathode-arc evaporation (vacuum-arc method) for producing TiN/MoN alternating layers with superior tribological and physico-mechanical properties. The elemental and phase compositions, tribological properties, hardness and elastic modulus of such coatings show promises for their use as protective coatings for cutting tools, turbine blades, walls of chemical and nuclear reactors [9]. The deposition methods were followed by an annealing process that reduced the grain size of the coatings to impart such superior tribological and mechanical properties.

Universal Mechanical Testing

Mechanical testers are the key means of acquiring good data about the mechanical and tribological properties of both DLC coatings and nanocomposites. Development of stronger, harder, low friction, and inert materials leads to the development of technology that provides lighter, stronger, and less expensive tools, construction materials and consumer products. Bruker’s UMT TriboLab (Figure 1) is a state-of-the-art universal mechanical tester that can provide a full range of tribological and mechanical tests [10] for evaluation of such coatings. The UMT TriboLab test platform provides:

• Easy transformation from rotary to reciprocating motion
• Sub-newton to kilo newton force measurement
• Temperature from ambient up to 1000°C for environmental testing
• A motor that accommodates the full range of speeds and torques
• Four interchangeable mechanical drives
• Wide selection of configurations including rotary, reciprocating, block-on-ring, linear tribology and scratch-test

Figure 1. Bruker’s UMT TriboLab

References

1. S. Veprek and M. J.G. Veprek-Heijman, Industrial applications of super hard nanocomposite coatings, Surface & Coatings Technology 202 (2008) 5063–5073
2. A. Erdemir and C. Donnet, Tribology of diamond-like carbon films: recent progress and future prospects, J. Phys. D: Appl. Phys. 39 (2006) R311–R327
3. M. Kalin and R. Simiˇ Atomic force microscopy and tribology study of the adsorption of alcohols on diamond-like carbon coatings and steel, Applied Surface Science 271 (2013) 317– 328
4. S. Stupp, Annual review of materials research, Volume 31, Annual Reviews, 1 Sep 2001, Science, p2
5. Erdemir A and Donnet C 2000 Modern Tribology Handbook, ed B Bhushan (Boca Raton, FL: CRC Press) pp 871–908
6. R. Simič and M. Kalin, Comparison of Alcohol and Fatty Acid Adsorption on Hydrogenated DLC Coatings Studied by AFM  and Tribological Tests, Journal of Mechanical Engineering 59(2013)12, 707-718
7. A Pogrebnjak et al, Structure and properties of multilayer nanostructured coatings TiN/MoN depend on deposition conditions, Acta Physica Polonica A, 2014, vol 125, no6, p1280-1283
8. B. O. Postolnyi and A Pogrebnjak et al, The effect of nanolayer thickness on the structure and properties of multilayer TiN/MoN coatings, Technical Physics Letters; Mar2014, Vol. 40, Issue 3, p215
9. O. V. Bondar B. A. Postol’nyi. M. Beresnev , Composition, structure and tribotechnical properties of TiN, MoN single-layer and TiN/MoN multilayer coatings, Jan 2015 Journal of Superhard Materials
10. Bruker - Tribological and Mechanical Testers

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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