Polar molecules are known to affect the friction and wear of steel contacts via adsorption onto the surface, which represents one of the fundamental boundary-lubrication mechanisms. Since the basic chemical and physical effects of polar molecules on diamond-like carbon (DLC) coatings have been investigated only very rarely, it is important to find out whether such molecules have a similar effect on DLC coatings as they do on steel. In our study the adsorption of hexadecanol in various concentrations (220 mmol/l) on DLC was studied under static conditions using an atomic force microscope (AFM). The amount of surface coverage, the size and the density of the adsorbed islands of alcohol molecules were analyzed. Tribological tests were also performed to correlate the wear and friction behaviours with the adsorption of molecules on the surface. In this case, steel surfaces served as a reference. The AFM was successfully used to analyze the adsorption ability of polar molecules onto the DLC surfaces and a good correlation between the AFM results and the tribological behaviour of the DLC and the steel was found. We confirmed that alcohols can adsorb physically and chemically onto the DLC surfaces and are, therefore, potential boundary-lubrication agents for the DLC coatings. The adsorption of alcohol onto the DLC surfaces reduces the wear of the coatings, but it is less effective in reducing the friction because of the already inherently low-friction properties of DLC. Tentative adsorption mechanisms that include the environmental species effect, the temperature effect and the tribological rubbing effect are proposed for DLC and steel surfaces.
COBISS.SI-ID: 12744987
Fatty acids are known to affect the friction and wear of steel contacts via adsorption onto the surface, which is one of the fundamental boundary-lubrication mechanisms. The understanding of the lubrication mechanisms of polar molecules on diamond-like carbon (DLC) is, however, still insufficient. In this work we aimed to find out whether such molecules have a similar effect on DLC coatings as they do on steel. The adsorption of hexadecanoic acid in various concentrations (220 mmol/l) on DLC was studied under static conditions using an atomic force microscope (AFM). The amount of surface coverage of the adsorbed fatty-acid molecules was analysed. In addition, tribological tests were performed to correlate the wear and friction behaviours in tribological contacts with the adsorption of molecules on the surface under static conditions. A good correlation between the AFM results and the tribological behaviour was observed. We confirmed that fatty acids can adsorb onto the DLC surfaces and are, therefore, potential boundary-lubrication agents for DLC coatings. The adsorption of the fatty acid onto the DLC surfaces reduces the wear of the coatings, but it is less effective in reducing the friction. Tentative adsorption mechanisms that include an environmental species effect, a temperature effect and a tribochemical effect are proposed for DLC and steel surfaces based on our results and few potential mechanisms found in literature.
COBISS.SI-ID: 13053979
In this study we investigate the correlation between the wetting, the contact angle, the spreading, the surface energy and the surface tension for five types of DLC coatings (doped, non-doped, hydrogenated and non-hydrogenated) and steel, wetted with synthetic, non-polar, polyalphaolefin (PAO) oils with two distinctly different viscosities. We show how these parameters correlate with each other and how and why they change when different DLC or steel surfaces come into contact with the oils. Several previously reported inconsistencies in the wetting and the surface-energy properties of DLC coatings are discussed. The spreading parameter (SP) presented in this work provides the necessary information to reveal the actual wetting properties of the DLC coatings and the steel with oils, instead of the contact angle, which is the parameter that is normally considered, but was found here to be inappropriate. A correlation between the polar surface energy and the DLC's hydrogen content is clearly observed. The results further indicate that the polar surface energy is a key parameter for tailoring the DLC wetting and the related surface tribochemical characteristics. In addition, the influence of various models for the surface-energy calculation is described.
COBISS.SI-ID: 12982299
Despite several studies that have confirmed the beneficial effect of MoS2 and WS2 nanoparticle-assisted lubrication, an understanding of how the nanoparticles behave in different, even very common contact conditions, such as roughness, is still missing. As a result we have focused on a comparison of the lubrication behaviour of MoS2 nanotubes mixed with PAO oil using steel surfaces with different roughnesses. Moreover, we have investigated the MoS2-nanotubes-assisted lubrication of steel/steel contacts in all lubrication regimes and also the effect of the running-in of these contacts. It was realized that the friction with the nanotubes-containing oil was 40-65% lower compared to the base oil, depending on the different contact conditions used. Furthermore, we showed that by using MoS2 nanotubes in the oil the friction is the same for rough and smooth steel surfaces, meaning that the nanotubes completely govern the lubrication behaviour in self-mated steel contacts in the boundary- and mixed-lubrication regimes, irrespective of the surface roughness or the running-in.
COBISS.SI-ID: 12730139
We report on the successful realization of a contactless, non-perturbing, displacement-measuring system for characterizing the surface roughness of polymer materials used in tribological applications. A single, time-dependent, scalar value, dubbed the collective micro-asperity deformation, is extracted from the normal-displacement measurements of normally loaded polymer samples. The displacement measurements with a sub-nanometer resolution are obtained with a homodyne quadrature laser interferometer. The measured collective micro-asperity deformation is critical for a determination of the real contact area and thus for the realistic contact conditions in tribological applications. The designed measuring system senses both the bulk creep as well as the micro-asperity creep occurring at the roughness peaks. The final results of our experimental measurements are three time-dependent values of the collective micro-asperity deformation for the three selected surface roughnesses. These values can be directly compared to theoretical deformation curves, which can be derived using existing real-contact-area models.
COBISS.SI-ID: 12590619