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The concept behind this Research Topic (RT) was to collect works, in which Atomic Force Microscopy (AFM) techniques are employed to study tribological phenomena and to push the resolution of measurements towards single asperity contact. Thanks to the direct determination of sample height with sub-nanometer resolution and the possibility of measuring local friction, AFM can be employed after a tribotest to detect topography and friction changes at the nanometer scale. Recently, efforts are being expended to use AFM cantilevers as tribometers, i.e., as probes altering the volume of suitable samples, thereby measuring tip and/or sample wear and friction at the nano/microscale. Thus, single asperity contact, friction, and wear can be investigated. Since friction and wear at the macroscale are the result of asperities interactions, such experiments are of great importance for better understanding of tribological processes.
Friction at the microscale during reciprocal sliding tribotests was studied for the first time with self-mated steel (100Cr6/AISI 52100) taking advantage of an atomic force microscope (AFM). To this aim, microsized steel particles were glued to the AFM-cantilever and employed as colloidal tips to perform tribotests on a steel disc. The torsion of the cantilever, which correlates with the friction force, was measured during the tests. Few tests with the same load did not yield any wear and show that the load and adhesion contributions to friction stay constant when the shape of the test particle does not change. Most of the presented tribotests engendered wear. For those tests, the increase of friction during the tribotests was attributed to the emerging plowing contribution. Furthermore, analysis of both torsion and local slope gives information on the creation of wear particles and their influence on friction.
Wear test results exhibit often large scattering. Hence, the study of spreading of experimental results requires big datasets.
In various studies, wear coefficients were found to have lognormal distributions. Therefore, it was supposed that the wear coefficient is affected by two normally distributed variables, which combine through a product.
In the present study, we demonstrate that a lognormal distribution may arise from a nonconstant wear coefficient, too, i.e., when a system does not follow Archard’s law.
In the automotive industry, hot stamping has been established as a key technology for manufacturing safety-relevant car body components with high strength-to-weight ratio. However, hot stamping tools are stressed by cyclic thermo-mechanical loads, which leads to severe wear and high friction during the forming operation. Consequently, the quality of the parts, the durability of the tools and the efficiency of the process are negatively affected. Within the scope of this work, a promising approach named laser implantation process has been investigated for improving the tribological behavior of hot stamping tools. This technique enables the fabrication of highly wear resistant, separated and elevated micro-features by embedding hard ceramic particles into the tool via pulsed laser radiation. Hence, highly stressed tool areas can be modified, which influences the thermal and tribological interactions at the blank-die interface. To clarify these cause-effect relations, numerical simulations, quenching tests as well as tribological investigations have been conducted. In this context, laser-implanted tools reveal a significantly improved tribological performance while offering the possibility to adjust the thermal properties within hot stamping. Based on these results, a tailored tool modification can be pursued in future research work, in order to enhance the effectiveness of hot stamping tooling systems.
Considerable scattering of experimental wear results affects seriously the evaluation of repeatability and reproducibility of tribological measurements and hampers detecting, studying, and verifying tribological laws. An accurate characterization of the statistics of results, the detection of the influence of operating parameters, and the verification of equations describing tribological phenomena can be achieved only through the analysis of large datasets with wide variation of parameters. Taking advantage of more than 400 experiments performed with the same material pairing under the same conditions on four different tribometers, the repeatability and reproducibility of volumetric wear measurements has been evaluated using Welch's test.
By testing Archard's law over seven orders of magnitude of sFN, it was found that this law is not universally valid and that, under certain conditions, the wear coefficient depends on sFN itself.
Contact-resonance AFM (CR-AFM) has been used in recent years for the measurement of mechanical properties. Compared with other techniques, CR-AFM has a much shorter acquisition time, compensating the incomplete theoretical understanding of the underlying physical phenomena. In the present paper, we propose a procedure, which allows to determine the elastic modulus of the sample as a parameter of the fit of the CR frequency as a function of the load. It is concluded that CR measurements are not appropriate for polymer samples. Major drawbacks are the bad resolution for moduli lower than ca. 10 GPa and the lack of a comprehensive physical model.
Pristine, reduced, and alkylated graphene oxides are applied as lubricating additives in paraffin grease. It has revealed that their crystalline structure governs the tribological properties of grease for steel tribo-pair. The microstructural analyses of grease samples showed that a loose fiber network of soap in the presence of graphene-based additive allows their facile release for efficient lubrication. The surface analyses based on the microscopic and elemental mapping show the development of a graphene-derived protective film on the worn scars, which protected the tribo-surfaces and subsided the wear. The reduced graphene oxide (rGO) with the interlamellar distance of 0.35 nm in the (002) plane provided minimum resistance to shear and exhibited maximum reduction in coefficient of friction (COF) for the paraffin grease. The presence of oxygen functionalities in the basal of pristine and alkylated graphene oxide (GO) compromised the interlamellar shearing under tribo-stress; consequently, higher COF than that of rGO.
The accurate determination of wear volumes is a prerequisite for the study of numerous tribological phenomena. Wear volumes can be measured with different techniques or else be calculated starting from some quantities measured from the wear scar. Advantages and drawbacks of the measuring techniques are shown by means of wear scars and calottes resulting from ball-on-plane tests with 100Cr6 specimens. When measuring wear volumes, white light interferometry results to be one of the most suitable techniques. When wear volumes are calculated, errors result mainly from two sources: (1) the arbitrary choice of one or few line profiles for the determination of the width and of the planimetric wear, and (2) approximations in the calculation, which are even necessary when values of the wear volumes of the single tribological partners and not only the total volume are of interest. The effect of both error sources on the accuracy in the determination of wear volumes is characterized and elucidated by examples. It is found that errors due to approximations are negligible when compared to errors due to the arbitrary choice of one line profile.
Laser material processing is an emerging technology that generates surface functionalities on the basis of optical, mechanical, or chemical properties. In the form of laser surface texturing (LST), it has attracted a remarkable amount of research to tailor surface properties towards various tribological applications. The main advantages of this single-step, laser-based technology are the contactless machining, featuring a high flexibility, efficiency, and speed, along with the excellent quality of the processed products. LST can be applied precisely localized to sub-micrometric areas, but, via laser beam scanning, it is also feasible for structuring large surface areas at the square meter size.
This Special Issue focuses on the latest developments concerning the tribological performance of laser-generated periodic surface nano- and microstructures and their applications. This includes the laser-based processing of different surface patterns, such as “self-organized” laser-induced periodic surface structures (LIPSS, ripples), grooves, micro-spikes, hierarchical hybrid nano-/micro-structures, microfeatures generated by direct laser interference patterning (DLIP), or even dimples or other topographic geometries shaped by direct laser modification or ablation. The applications of these periodically nano- and micro-patterned surfaces may improve the lubricated or non-lubricated tribological performance of surfaces in conformal and even non-conformal contact through a reduction of wear, a variation of the coefficient of friction, altered load carrying capacity, etc., resulting in energy saving, improved reliability, increased lifetimes as well as durability, leading in turn to extended maintenance intervals/reduced down-time. This can be beneficial in terms of bearings, gears, engines, seals, cutting tools, or other tribological components. Fundamental aspects addressed may involve the investigation of the relevant physical and chemical effects accompanying the laser-generated nano- and microscale topographies, such as alterations of the material structures, the hardness, superficial oxidation, the role of additives contained in lubricants, surface wettability, micro-hydrodynamic effects, etc.
For this Special Issue we aim to attract both academic and industrial researchers and would like to provide a bridge between research in the fields of tribology and laser material processing in order to foster the current knowledge and present new ideas for future applications and new technologies.
The influence of relative humidity on friction and wear is subject of several studies in the last decades. A comprehensive understanding of physical and chemical phenomena affecting the tribology is hampered by the lack of reproducible experimental results, by the large number of variables, and by several difficulties in the detection of tribochemical processes and products.
In the present work, we analyze the wear coefficient and the wear volumes of 686 unlubricated tests performed on different oscillating tribometers with 100Cr6 balls on 100Cr6 planes at different relative humidity.
Aim of this work is to assess the repeatability and reproducibility of data, to determine the dependence of the wear coefficient on the relative humidity, to understand the underlying physicochemical phenomena and to build three dimensional maps of the wear coefficient as a function of both humidity and the product of normal force and sliding distance.
Ti-Al-Nb based intermetallic layers of sufficient quality and thickness were obtained by non-vacuum electron beam cladding on the surfaces of Ti workpieces. Optical microscopy and X-ray tomography did not reveal any dramatical defects in the structure of cladded layers. X-ray diffraction as well as scanning and transmission electron microscopy were applied to thoroughly investigate the structure and phase composition of coatings. It was found that non-equilibrium cooling conditions of coatings provided by fast removal of heat to untreated Ti substrate after the electron beam cladding was terminated induced the proceeding of metastable phase transformations. For example, γ-phase formation was suppressed in these coatings. In coatings with 8 and 20 at.% Nb (46 and 43% Al respectively) along with ordered with α2, formation of disordered solution of the alloying elements in α-Ti took place. In high-Nb alloys β(B2) phase has undergone the diffusionless transformation to ω’, which is the intermediate phase in β → ω and the coating with the maximum Nb content characterized by appearance of γ1 as a main phase. ω-phase had negative influence to hardness and wear resistance of coatings, however, generally this paremeter increased in 1.3–1.75 times compared to cp-Ti. The high temperature creep and oxidation properties decreased proportionally with increasing Nb and decreasing Al content in the cladded layers.
In the present work it was shown that the tribologcal profiles of special model oxides under dry unidirectional sliding have shown, that sub-oxides have a contribution to the tribological behavior of carbides and cermets, when they are tribo-oxidatively formed, because their tribological profiles as monolithic materials are homologous in part or totally, or compete with hardmetals or cermets, depending from the operating conditions regarded.
Laser-induced periodic surface structures (LIPSS, ripples) with ~500–700 nm period were produced on titanium alloy (Ti6Al4V) surfaces upon scan processing in air by a Ti:sapphire femtosecond laser. The tribological performance of the surfaces were qualified in linear reciprocating sliding tribological tests against balls made of different materials using different oilbased lubricants. Extending our previous work, we studied the admixture of the additive 2-ethylhexyl-zinc-dithiophosphate to a base oil containing only anti-oxidants and temperature stabilizers. The presence of this additive along with the variation of the chemical composition of the counterbodies allows us to explore the synergy of the additive with the laseroxidized nanostructures.
Self-mated magnesia stabilized zirconia (Mg-PSZ) ceramic sliding couples have been investigated at 100 N load (P0max= 1324 MPa) in oscillating sliding conditions in different humidity conditions in air and in hot steam. Temperatures have been varied up to 400 °C and pressures up to 6 bars. The results show that the wear behavior of MgO-ZrO2 under high Hertzian contact pressures is strongly dependent on temperature and is similar for both dry oscillating and oscillating in hot steam. However, although the evolution in wear rates on temperature is similar and the wear rates of MgO-ZrO2 plunged above 300 °C in hot steam and air by nearly three orders of magnitude, SEM micrographs revealed in hot steam at 400 °C smooth wear tracks. In contrast, hot steam enhanced the tribochemistry of self-mated alumina couples and reduced wear rates. Hot steam decreased the coefficients of friction of MgO-ZrO2 with increasing temperature, but not the wear rates.
The tribological behavior of neat and filled PEEK and PEKK composites were compared in air and vacuum conditions. Very low friction and wear coefficient were obtained at low sliding speed while severe wear occurred at high speed. Experimental results are discussed by analysing the transfer film and wear debris.
In the present work an approach is presented which combines various aspects of the former models with probability considerations. It is used to simulate the impact of more than one billion Alumina particles onto a steel substrate. This approach permits the simulation of an entire erosion experiment on an average PC within about six hours.