In the search for achieving ultra-low friction for applications in extreme environments, we evaluate the interfacial processes of diamond/tungsten sliding contacts using an on-line macro-tribometer ...and a micro-tribometer in an ultra-high vacuum. The coefficient of friction for the tests with the on-line tribometer remained considerably low for unlubricated sliding of tungsten, which correlated well with the relatively low wear rates and low roughness on the wear track throughout the sliding. Ex situ analysis was performed by means of XPS and SEM-FIB in order to better understand the underlying mechanisms of low friction and low-wear sliding. The analysis did not reveal any evidence of tribofilm or transferfilm formation on the counterface, indicating the absence of significant bonding between the diamond and tungsten surfaces, which correlated well with the low-friction values. The minimal adhesive interaction and material transfer can possibly be explained by the low initial roughness values as well as high cohesive bonding energies of the two materials. The appearance of the wear track as well as the relatively higher roughness perpendicular to the sliding indicated that abrasion was the main wear mechanism. In order to elucidate the low friction of this tribocouple, we performed micro-tribological experiments in ultra-high vacuum conditions. The results show that the friction coefficient was reduced significantly in UHV. In addition, subsequently to baking the chamber, the coefficient of friction approached ultra-low values. Based on the results obtained in this study, the diamond/tungsten tribocouple seems promising for tribological interfaces in spacecraft systems, which can improve the durability of the components.
This work elucidates friction in Poly-Ether-Ether-Ketone (PEEK) sliding contacts through multiscale simulations. At the nanoscale, non-reactive classical molecular dynamics (MD) simulations of dry ...and water-lubricated amorphous PEEK–PEEK interfaces are performed. During a short running-in phase, we observe structural transformations at the sliding interface that result in flattening of the initial nanotopographies accompanied by strong polymer chain alignment in the shearing direction. The MD simulations also reveal a linear pressure – shear stress dependence and large adhesive friction in dry conditions. This dependence, summarized in a nanoscale friction law, is of central importance for our multiscale approach, since it forms a link between MD and elastoplastic contact mechanics calculations. An integration of the nanoscale friction law over the real area of contact yields a macroscopic friction coefficient that allows for a meaningful comparison with measurements from macroscopic tribometer experiments. Severe normal loading conditions result in significant wear and high experimental friction coefficients µ≈0.5–0.7, which are in good agreement with the calculated values from the multiscale approach in dry conditions. For milder experimental loads, our multiscale model suggests that lower friction states with µ≈0.2 originate in the presence of physisorbed molecules (e.g., water), which significantly reduce interfacial adhesion.
Graphical Abstract
In this study, we investigate the running-in behavior of brass (i.e. 95% Copper and 5% Zinc) sliding against 100Cr6 under lubricated conditions. Prior to the tribological tests, the materials are ...characterized using microindention, nanoindentation, Focused Ion Beam analysis (FIB), X-Ray Photoelectron Spectroscopy (XPS), and White Light Interferometry (WLI) in order to quantify the mechanical properties, grain sizes, chemistry, and topography. The sliding experiments are performed using “real time” tribometry with a pin on plate tribometer with varying contact pressures (1–4MPa) and sliding velocities (10–20mm/s).
We found that a significant reduction in friction during running-in occurs only at a small range of pressure (2.4–2.9MPa). We suggest that two main mechanisms are favorable for running-in. The first mechanism is the presence of ZnO in the near-surface region of the wear track, which acts as lubrication and reduces the friction. The second mechanism is the presence of C/CHX within the first 200nm of the pin which acts as a passivation layer and thus keeps the transfer film thin and stable.
•Low friction and wear obtain in a small range of velocity and contact pressures.•The good running-in behavior is attributed to different interfacial processes.•For good an poor running-in, the roughness follows the friction trend closely.•Running-in experiments reveal an accumulation of ZnO on the worn surfaces.•More carbon is observed on the counterfaces of the good running-in experiments.
Despite the great importance of the real contact area, it is a parameter which, depending on the tribological system, is difficult or impossible to obtain experimentally. In this work, a combination ...of methods was used to estimate the development of the real contact surface, and the results were compared with the friction coefficient course. The measurements were carried out with a home-built in situ tribometer, which records a 3D image of the surface after each individual friction cycle. A tungsten sample was treated by laser interference with a line-like pattern to produce a deterministic surface. This allowed for more precise tracking of the real contact area when combined with the use of an inert corundum sphere as a counter-body. The real contact area was calculated numerically from the height information obtained using a contact application. Finally, the true contact surface was compared with the parallel-recorded friction values. After a short running-in phase, the friction behavior and the real contact surface showed comparable courses. This indicates that the changes in the real contact area could explain the friction behavior of the laser-patterned sample, and the methodology was proven to be suitable for experimentally estimating the real contact area.
The reinforcement of coatings with diamond particles results in superior tribological performance in automotive applications. In addition to improving the coating's bulk properties, sliding of ...diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond-reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribochemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C–O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication, and ex situ chemical analysis by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and electron energy loss spectroscopy (EELS) provides evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations.
In order to further optimize the efficiency of today’s internal combustion engines, specific coatings are used on functional surfaces to reduce internal engine friction and wear. In the current ...research project, oxymethylene ether (OME) is discussed because it is CO2 neutral and has a strong soot-reducing effect as a fuel or fuel additive. In some operational regimes of the internal combustion engine a dilution of engine oil by fuel must be assumed. In this paper, the frictional contact between piston ring and cylinder raceway is modelled using a pin-on-disk tribometer and the friction and wear behavior between a diamond-like carbon coating (DLC) and a thermal spray coating is characterized. The wear of the spray layer could be continuously detected by radionuclide technology (RNT). With the aid of photoelectron spectroscopic measurements (XPS), the steel thermal spray coating was chemically analyzed before and after the tribometer tests and the oxidative influence of OME was investigated. In addition, confocal microscopy was used to assess the topographies of the specimens. The measurements showed that the addition of OME to the lubricant reduced the viscosity and load-bearing capacity of the lubricating film, which led to an increase in the coefficient of friction. While almost no wear on the pin could be detected at 10% OME, the first visible material removal occurs at an OME content of 20% and the layer delaminated at 30% OME. The evaluation of the RNT wear tests showed that both the tests with engine oil and with engine oil plus 20% OME achieved very low wear rates. No corrosion of the thermal spray coating could be detected by XPS. Only the proportion of engine oil additives in the friction track increased with increasing OME concentration.
We studied the formation of tribologically induced nanocrystalline layers in lubricated contacts during running-in using a pin-on-disk tribometer coupled to a radionuclide high-resolution wear ...measurement system. The experiments were stopped after different running times and the sliding surfaces were analysed by atomic force microscopy, auger electron spectroscopy and focused ion beam analysis. Initially, at very high wear rates, zinc phosphate anti-wear films are formed in the contact regions of the surfaces, which slow down the wear process. During rubbing the formed films will submerge below the surface of the materials as result of large plastic deformations. This process affects the friction coefficient and the wear rate and is considered to be a key mechanism for the running-in.
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