Most machines contain components with loaded solid surfaces that rub together. The interaction of such surfaces produces friction and results in mechanical damage. Thus, tribological phenomena play a decisive role in diverse systems. For many years, researchers have sought to alleviate these problems and to understand their origin. In this context, the reduction of friction and wear is very often the primary focus. Nevertheless, there are many applications such as brake systems and clutches in which a controlled increase in friction is desired. It is important to note that the friction coefficient, being the ratio between the friction and the applied normal force, does not represent an inherent materials property. Moreover, µ is determined by the respective tribological system which comprises for example the interacting materials, the surface roughnesses, the stress collective, the kinematics and the environmental conditions (e.g. temperature and relative humidity). The early work of Bowden and Tabor showed that one determinant aspect for friction is the interaction of asperities. Those asperities weld together and will be released after having reached a critical shear stress. According to this model, one possibility to influence frictional properties is based on tailoring the real contact area of the mating surfaces. Being able to control the frictional response of a tribological system on different scales is likely to be of
TRIBOLOGY and INDUSTRY
For lubricated conditions, many articles emphasise the importance of the dimple depth over diameter ratio h/d and the area density of the patterns. In particular, dimples with a depth in the nanoscale seem to be effective in reducing friction at low speed and high load. Wakuda et al. performed friction tests of line contacts between a steel cylinder and a Si3N4 ceramic plate. Results showed that dimples with diameters larger than the Hertzian contact width lead to lower coefficients of frictions under lubricated sliding. According to Costa and Hutchings, textures with feature sizes much larger than the elastic contact width yield film thicknesses smaller than those for non-textured specimens. Additionally, the area density of the patterns plays a decisive role in the effectiveness under hydrodynamic and mixed lubrication. Andersson et al. reported low coefficients of friction for laser textured steel surfaces under oscillating sliding conditions for low area densities (8%). Furthermore, Wang et al.
Because of very short pulse durations in the order of several nanoseconds, residual adhesive particles are removed and a micro-flattening of the mechanically prepared surface takes place. Laser surface texturing (LST) proved to be a very promising candidate because laser is a fast and environmental friendly tool offering short processing times. Moreover, by a proper variation of the laser wavelength (from UV to IR laser light), the beam polarization (linear-, circular- or elliptic-polarisation), the pulse duration (ranging from femto- to nanoseconds or even continuous mode) and finally by adjusting the energy density, different materials (e.g. metals, ceramics and polymers) can be processed. There are manifold applications of laser surface texturing under dry and lubricated sliding conditions. Since Hamilton et al. , it is well known that textured surfaces have an advantageous impact on sliding bodies under hydrodynamic lubrication. Furthermore, surface patterns are able to act as lubricant micro-reservoirs under boundary lubrication conditions and trap wear debris under dry sliding thus reducing abrasive wear and fretting fatigue. Still, the optimum design parameters for an efficient surface pattern concerning for example the pattern geometry, the lateral feature sizes or area densities are still a matter of lively discussion in the surface texturing community. Most texturing experiments are based on a simple trial and error approach.
Ultmost importance for the design of miniaturized applications e.g. micro-electromechanical systems (MEMS), positioning devices and bearings. There are many potential solutions to manipulate friction by modifying the surface materials e.g. changing the design of the rubbing elements, using hard coatings like TiC, TiB2 or (TiAl)N, multilayer coatings such as WC/DLC/WS2, lubricants e.g. oils, greases, soft metals (In, Au or Sn) or DLC films and finally by texturing the mating surfaces. In particular, the rapidly growing field of surface texturing has attracted a lot of attention in the last decades and has proven to be an effective means of improving tribological properties. Nowadays, numerous industrial texturing methods are able to modify the contacting surfaces. Basically, those methods can be separated into stochastic (e.g. shot blasting or electrical discharge texturing) and deterministic processes (e.g. electron or laser beam texturing) with regard to the resulting texture geometry. Another semi-deterministic technique is the cylinder liner honing which is well established for the enhancement of tribological properties in combustion engines. The honing process is characterised by a superposition of rotational and vertical movements of the so called honledges. Depending on the selected honing method, specific surface finishes with for example cross-hatch patterns can be generated providing an improved oil retainment capability thus leading to sufficient piston ring lubrication. A further development of the aforementioned technique is the combination with a certain laser treatment named laser honing. Usually, a nanosecond-pulsed excimer laser is applied to irradiate a honed cylinder surface.
Have provided a load-carrying capacity map for SiC thrust bearings under water lubrication. The maximum load-carrying capacity was achieved by dimples of around 350 µm in diameter, 3.2 µm in depth and with an area density of about 5%. Finally, there is a direct correlation between the tribological performance and the texture geometry as well as the orientation. Nanbu et al. performed a virtual surface generation involving a numerical texturing process in order to study the effect of texture bottom shape under hydrodynamic conditions. They could determine two hydrodynamically favourable pattern geometries enhancing lubricant film thickness, namely micro-step and wedge bearings. Moreover, Siripuram and Stephens conducted experiments with protruding and recessed asperities with different geometries e.g. circular, hexagonal and triangular shapes with regard to the influence on the friction coefficient and the leakage rate for seals. It could be shown that the friction coefficient is more or less insensitive to the geometry but sensitive to the size of the asperities. In contrast, the leakage rate is affected by both parameters leading to the best results for triangular shaped textures. As far as dry friction is concerned, much less publications exist dealing with the potential benefits of texturing a surface. Most of the work is related to the already mentioned trapping of wear debris in order to avoid third body interactions or reducing stiction in magnetic storage disks. He et al. used a micromolding technique to texture an elastomer substrate (PDMS-polydimethylsiloxane) and to study the influence of different pattern geometries on the frictional response. Moreover, Borghi et al. performed laser texturing on nitrided steel surfaces and could demonstrate a decrease in the coefficient of friction of about 10 % due to embedded wear particles in the produced laser dimples. In addition, Rapoport and co-workers showed the enhanced storing capability of laser-patterned steel surfaces for solid lubricants such as MoS2. However, most of the research is directly linked to the aforementioned effects.