ICMCTF2013 Session E1-3: Friction, Wear, and Lubrication; Effects & Modeling

Solid lubricants are unusual in their ability to provide low friction and wear in otherwise unlubricated sliding conditions. In almost every case, thin (10-100nm) and well-adhered transfer films accompany low friction and wear during dry sliding and there is increasing evidence that the transfer films are responsible for the friction and wear reductions. Further elucidating the role of these transfer films in tribology requires direct studies of their tribological properties. Unfortunately, films of this scale are extremely challenging to probe with forces and contact areas that are sufficiently small to isolate their properties from those of the bulk. While the atomic force microscope is uniquely able to probe tribological surfaces in a wear-free single-asperity contact, instrument calibration challenges have limited the usefulness of this technique for quantitative tribological studies. A number of AFM calibration techniques have been proposed and used, but none has gained universal acceptance due to limitations or significant potential error sources. This paper describes a simple ‘in-situ method’ of calibrating AFM friction coefficients which: (1) allows simultaneous calibration and measurement for a given configuration of the AFM system, thus eliminating tip damage and confounding effects of instrument setup adjustments; (2) is insensitive to adhesion, PSD cross talk, transducer/piezo-tube axis misalignment, and shear-center offset; (3) is applicable to integrated tips and colloidal probes since ‘calibration’ is performed on the very substrate for which the friction coefficient is determined; (4) is applicable to any reciprocating friction coefficient measurement.

Friction and related tribological phenomena play a decisive role in technological systems. For many years, a lot of research groups have sought to understand the origin of friction and to enhance the tribological performance of rubbing surfaces. The ability to modify frictional forces on different scales is of utmost importance. There are several techniques that offer the possibility to tailor the contact area thus leading to an improvement in the tribological behaviour. The laser interference metallurgy (LIMET), which is one possible approach of laser surface texturing, is used to produce a well defined surface topography with line-like pattern on both contacting surfaces. Commercial stainless steel and titanium samples were irradiated with a high power pulsed solid state laser (pulse duration of 10 ns and wavelength of 355 nm). The laser experiments were performed using a two beam interference configuration resulting in line-like patterns with three different structural wavelengths. Furthermore, a detailed study of the chemical and microstructural state before and after laser texturing was conducted. The tribological testing was done using a ball on disk configuration (ball material: 100Cr6 steel) in linear oscillating test conditions. In order to control the involved contact geometries and the frictional response, both contacting bodies were structured with the same structural wavelength.

The tribological tests demonstrate that the LIMET is a powerful tool to create a well defined surface topography and to design the contact area of rubbing surfaces. Furthermore, it can be stated that depending on the relative alignment and the structural periodicity, geometrical interlocking between the contacting bodies is possible thus leading to modified frictional properties and enhanced run-in behaviour. It could be shown that dry friction between two laser-patterned solids depends not only on the wavelength of the structuring but also on the relative orientation between the patterns.

Numerous electrical contacts worldwide are exposed to sliding motion. Commonly conductor materials are gold and copper. Very often, electrical contacts are gold plated.

A basic understanding of the fundamental aspects during film growth and methods for a subsequent optimization or tailoring of the film microstructure and surface topography is absolutely essential for improving the physical properties of the as-deposited films. Independent of the deposition technique used (e.g. sputtering or electron beam evaporation), metallic thin films exhibit an amorphous or a polycrystalline microstructure when deposited under typical conditions, such as no or only a moderate substrate heating. In the latter case, films are mainly characterized by a logarithmic normal grain size distribution with randomly oriented grains. As far as technical applications are concerned, microstructural randomness leads to highly inhomogeneous and non-optimized device characteristics. Consequently, circumventing the random grain alignment by a laser-interference induced recrystallization for example with specified threshold energies results in a control of nucleation sites and grain orientations and thus superior properties.

In this context, gold thin films with a nominal thickness ranging between 300 and 700 nm were deposited on Si-substrates by electron beam evaporation. Subsequently, the as-deposited films were laser-patterned by a novel interference technique allowing for long-range ordered and periodic grain architectures and topographies (e.g. line-, dot- and cross-like patterns) on the micron-scale. Depending on the used laser energy density and the number of interfering laser beams, novel engineered microstructures and surfaces appear with beneficial properties.

The resulting tailored gold films were analyzed by high resolution techniques such as electron backscatter diffraction and transmission electron microscopy concerning the microstructure and correlated with thermal simulations. Moreover, detailed studies of the achieved topography after the laser treatment were performed by white light interferometry.

Finally, the results of the sliding tests under dry friction conditions will be presented showing a 40 % reduction of the friction coefficient and an enhanced wear resistance compared to the pristine sample state. Additionally, a Greenwood-Williamson approach is used to explain the tribological findings.

Molybdenum disulfide is regarded as one of the most widely used so lid lubricants. One of the application methods of MoS₂ on substrates is the electrodeposition in metal matrix. Ni–MoS₂ composite coatings were developed on AISI 304 stainless steel substrates by electroplating from Watts bath containing suspended MoS₂ particles. The effects of three types of surfactants; depramin C, ammoniumlignosulfonate, sodiumlignosulfonate and the MoS₂ concentration on the tribological behavior and particle distribution of electrocodeposited Ni-MoS₂ composite coatings were studied by pin on disc tribometer and a scanning electron microscope. Addition of 10 g/l MoS₂ (1.2 micron average particle size) into the Watts bath, decreased the friction coefficient of nickel coatings from 0.75 to 0.45. The use of sodiumlignosulfonate and ammoniumlignosulfonate were more effective in homogenously distributing MoS₂ particles in nickel matrix and reducing the friction as compared to Depramin C. Moreover, increasing the MoS₂ content to higher levels caused further reductions in the friction coefficient.

In this study, three composites, titanium grade 2 based with different particle rates, were tested under fretting wear conditions. The fretting wear analysis was performed under gross slip regime, for several pressures and sliding conditions. Both friction and wear rates responses have been investigated. Like for aluminium composite, it is shown the TiC particle concentration highly influences of the wear resistance. Various hypothesis including oxidation mechanisms and micro-cracking of TiC particules are discussed.

[1] R.N. Rao, S. Das; Materials and Design 31 (2010); 1200–1207

[2] D.B. Miracle, Compos. Sci. Technol. 65 (2005) 2526–2540