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This paper shows how Angle-Resolved Scattering can reveal wear on engineered surfaces. The samples studied, three discs made of steel 100Cr6H used in gear wheels in the automotive industry, were assessed after they had undergone ball-on-disc tests. Scattering maps recorded for spatial frequencies from 0.1 µm-1 to 2.7 µm-1 show a contrast inversion around 0.36 µm-1 revealing the wear trace. Besides measurements of rms roughness, where mean values are 20 nm and 24 nm depending on the locations considered, stationarity and isotropy, we demonstrate the ability of the technique to reveal localized wear on this type of surface in a rapid, robust and convenient way. We show that the tool trace is influenced by the load magnitude rather than by the number of oscillation cycles and highlight the key role played by surface roughness in how the sample responds to wear tests.
Sub-100-nm laser-induced periodic surface structures (LIPSS) were processed on bulk titanium (Ti) surfaces by femtosecond laser pulse irradiation in air (30 fs pulse duration, 790 nm wavelength). The laser peak fluence, the spatial spot overlap, and the number of overscans were optimized in a sample-scanning geometry in order to obtain large surface areas (5 mm × 5 mm) covered homogeneously by the LIPSS. The laser-processed regions were characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). The friction coefficient of the nanostructured surfaces was tested during 1000 cycles under reciprocal sliding conditions (1 Hz, 1.0 N normal load) against a 10-mm diameter ball of hardened 100Cr6 steel, both in paraffin oil and in engine oil used as lubricants. Subsequently, the corresponding wear tracks were qualified by OM, SEM, and energy dispersive X-ray analyses (EDX). The results of the tribological tests are discussed and compared to that obtained for near wavelength-sized fs-LIPSS, processed under somewhat different irradiation conditions. Some constraints for a beneficial effect of LIPSS on the tribological performance are provided.
Titanium nitride (TiN) was coated on different substrate materials, namely pure titanium (Ti), titanium alloy (Ti6Al4V) and steel (100Cr6), generating 2.5 μm thick TiN layers. Using femtosecond laser pulses (30 fs, 790 nm, 1 kHz pulse repetition rate), large surface areas (5 mm × 5 mm) of laser-induced periodic surface structures (LIPSS) with sub-wavelength periods ranging between 470 nm and 600 nm were generated and characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). In tribological tests, coefficients of friction (COF) of the nanostructured surfaces were determined under reciprocating sliding conditions (1 Hz, 1.0 N normal load) against a 10-mm diameter ball of hardened 100Cr6 steel during 1000 cycles using two different lubricants, namely paraffin oil and engine oil. It turned out that the substrate material, the laser fluence and the lubricant are crucial for the tribological performance. However, friction and wear could not be significantly reduced by LIPSS on TiN layers in comparison to unstructured TiN surfaces. Finally, the resulting wear tracks on the nanostructured surfaces were investigated with respect to their morphology (OM, SEM), depth (WLIM) and chemical composition by energy dispersive X-ray spectroscopy (EDX) and, on one hand, compared with each other, on the other hand, with non-structured TiN surfaces.
Laser texturing is an emerging technology for generating surface functionalities on basis of optical, mechanical, or chemical properties. Taking benefit of laser sources with ultrashort (fs) pulse durations features outstanding precision of machining and negligible rims or burrs surrounding the laser-irradiation zone. Consequently, additional mechanical or chemical post-processing steps are usually not required for fs-laser surface texturing (fs-LST). This work aimed to provide a bridge between research in the field of tribology and laser materials processing. The paper reviews the current state-of-the-art in fs-LST, with a focus on the tribological performance (friction and wear) of specific self-organized surface structures (so-called ripples, grooves, and spikes) on steel and titanium alloys. On the titanium alloy, specific sickle-shaped hybrid micro-nanostructures were also observed and tribologically tested. Care is taken to identify accompanying effects affecting the materials hardness, superficial oxidation, nano- and microscale topographies, and the role of additives contained in lubricants, such as commercial engine oil.
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 running-in phase of mechanical systems is critical from a tribological standpoint, though poorly understood. Microcracks accelerate material failure and wear during this phase of heightened friction. With this in mind, the ultimate goal of this current work is to transfer the running-in phase into the final step of the mechanical finishing process through the targeted pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments and compare these to conventional, case-hardened steels. This pre-conditioning involved the targeted implementation of two specific lubricant packages, the first with CaCO3 as the active ingredient and the second with MoDTC as the active ingredient, to generate chemically reactive tribolayers (tribofilms) on twin disk testing rigs. Pre-conditioning was carried out up to 104 load cycles (approximately 25 min). The films generated in pre-conditioning were analyzed by SEM-EDX with Element-Mapping, Raman spectroscopy, and XPS to elucidate their molecular composition and concentration on the sample surfaces. The combination of these methods of analysis gave a clear indication that 104 cycles were sufficient to generate stable chemical tribofilms. CaO and CaCO3 were the main components of the tribofilm from the first lubricant package, while MoS2, MoO2 and MoO3 were the main components from the second lubricant package. Tribofilm-protected samples were then subjected to slip-rolling endurance testing (T=+120 °C, 10,000,000 cycles, approximately 19 days in a factory fill engine oil) to determine any changes in friction behavior or wear performance. Some significant reductions in coefficients of friction at the end of endurance testing were observed, though in certain cases, no definitive improvement was observed. In contrast, very strong reductions in wear were observed across the entire spectrum of materials and testing loads. In some cases, sample surface wear reduction from pre-conditioning via tribofilms reached over 90%. The observed improvements to friction behavior and wear performance are indicative of a technically simple, cost- and energy-efficient pre-conditioning method that may prove to be competitive with existing thermochemical treatments for steel alloys.
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.
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.
Different fretting tests of thin steel wires under different conditions (loads, strokes, number of cycles and mean pressures) were performed. The wires used had 0.45 mm in diameter and the material was a cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength over 2800 MPa. The tests were carried out with 90° "crossed cylinders" configuration varying the stroke, normal load and testing time (number of cycles). Afterwards the volumetric wear was obtained using surface profilometry with diamond stylus.
With a classical study of the coefficient of wear with respect to the sliding distance, the results show that the wear coefficient increases with normal load and stroke, consequently, for the same sliding distance, the wear produced was higher when the load and/or stroke is increased. In this work, the coefficient of wear is decreasing continuously as the test is running. In order to analyze better the wear behaviour, another method, in which the volumetric wear is compared with respect to the Archard's loading factor, is proposed. As result of this method, two distinctive behaviours were detected; one corresponds to the running-in period with a more aggressive wear behaviour and the second one to the stable steady state period. In all the tests the running-in period was similar with regard to the wear rates but in the stable period the wear rates were increasing with higher loads or strokes.