R. Simič

Friction and Lifetime of Laser Surface–Textured and MoS2-Coated Ti6Al4V Under Dry Reciprocating Sliding

The use of lasers for creating defined textured patterns on surfaces has steadily gained attention during the past decade. These textures can contribute to friction reduction by acting as a reservoir for lubricant, hydrodynamic bearing and trap for wear debris. In the present work, titanium alloy surfaces were textured using a nanosecond pulsed laser and subsequently coated with MoS2. The samples were tested under dry reciprocating sliding conditions using two different oscillation amplitudes. During the test, the evolution of the coefficient of friction as a function of the number of cycles was measured until coating failure. The influence of the dimple distance on friction and lifetime was evaluated and verified by post-mortem analyses of the samples using optical, confocal and SEM microscopy as well as EDS and XPS analyses. The results show that under certain conditions, surface texturing can reduce friction, extend the lifetime of the coating and provide a progressive coating degradation until failure.

Fatty Acid Adsorption on Several DLC Coatings Studied by Neutron Reflectometry

The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out.

ATR-IR Investigation of Solvent Interactions with Surface-Bound Polymers

Solvent interactions with bulk and surface-bound polymer brushes are crucial for functionalities such as controlled friction and thermoresponsive adhesion. To study such interactions, the temperature-induced solvent-quality changes and the effect of surface tethering on the mechanical and tribological properties of poly(dodecyl methacrylate) (P12MA) brushes have been investigated by means of attenuated total reflection infrared spectroscopy (ATR-IR), as well as atomic force microscopy (AFM) and lateral force microscopy (LFM). These results have been compared with temperature-dependent UV–visible spectrophotometry (UV–vis) data for the corresponding bulk polymer solutions. The ATR-IR results clearly show that increasing temperature enhances ethanol uptake in P12MA, which results in film swelling. This is accompanied by a marked increase in both adhesion and friction. We have also shown that a combination of solvents, such as toluene and ethanol, can lead to a temperature-dependent solvent partitioning within the polymer brush. To our knowledge this is the first time preferential solvent uptake in a grafted-from brush has been monitored via in situ ATR-IR. Moreover, we have observed remarkably different behavior for polymer chains in solution compared to the behavior of similar chains bound to a surface. The presented findings on the temperature-dependent solvent interactions of surface-grafted P12MA reveal previously unknown solvation phenomena and open up a range of possible applications in the area of stimuli-responsive materials.