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The performance of titanium alloy (Ti6Al4V) surfaces was investigated in lubricated reciprocating sliding tribological tests (RSTT). Special emphasis was laid on the effect of surface nanostructures in area of contact on the respective friction and wear behaviour. These so-called laser-induced periodic surface structures (LIPSS, ripples) were produced on the titanium alloy surface upon scan processing in air by an ultrashort pulsed femtosecond (fs) laser. As lubricant served two types of base oils, a pure polyalcylene-glycol, and an SAE 0W30 oil containing only antioxidants and temperature stabilizers. Tribological tests were carried out on polished as well as LIPSS covered areas using both types of base oil. A test metrics was established, combining the additive 2-ethylhexyl-zincdithiophosphate (ZDDP) or the ionic liquid [P6,6,6,14] [DEHP] (98% purity) with the respective base oils. The test metrics also considered the orientation of motion with respect to the orientation of the structures formed on the surface. Results are presented which show that the interplay between LIPSS and the local chemistry formed by the respective additives is beneficial for the tribological behaviour of the titanium alloy. Certain combinations of base oil, additive and LIPSS reduced friction and wear significantly in the tribological contact.
Commercial grade-1 titanium samples (Ti, 99.5% purity) were treated using three alternative methods, i.e., fs-laser processing in air, thermal heat treatment in an oven, or anodization in an electrochemical bath, all resulting in the formation of differently conditioned superficial oxide layers. The laser processing was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of several square-millimetres large surface areas covered homogeneously by these nanostructures. The thermal processing in the oven was done at two different temperatures, while the electrochemical anodization was performed at room temperature, aiming to generate different polymorphs of titanium oxide at similar oxide layer thickness. The irradiated surface regions were characterized by optical and scanning electron microscopy, and micro Raman spectroscopy. The tribological performance of the differently treated titanium surfaces was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in un-additivated paraffin oil and fully formulated engine oil as lubricants. The specific tribological performance of the differently treated surfaces is discussed on the basis of possible physical and chemical mechanisms.
Laser-induced periodic surface structures (LIPSS, ripples) were generated on steel and titanium surfaces upon irradiation with multiple linear polarized femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas covered homogeneously by the nanostructures. The irradiated surface regions were subjected to optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM) revealing sub-wavelength spatial periods. The nanostructured surfaces were tribologically tested under reciprocal sliding conditions against a sphere of hardened 100Cr6 steel at 1 Hz using paraffin oil and engine oil as lubricants. After 1000 sliding cycles at a load of 1.0 N, the corresponding wear tracks were characterized by OM and SEM. For specific conditions the laser-generated nanostructures endured the tribological treatment. Simultaneously, a significant reduction of the friction coefficient was observed in the laser-irradiated (LIPSS-covered) areas when compared to the non-irradiated surface, indicating the potential benefit of laser surface structuring for tribological applications.
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.
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.
In order to meet the goal of reducing CO2 emissions, automotive industry places significant importance on downsizing components to achieve greater efficiency through lower weight and reduced friction. As friction reductions are associated with energy efficiency and wear protection with resource conservation, ever greater attention has been given to adamantine carbon-
based coatings and high-alloyed steels. Such applications are, however, associated with high production costs and energy expenditures, as well as many technical difficulties. Therefore a key issue in meeting the goals of friction reduction, wear protection and development of comprehensive lightweight strategies is whether or not the functional profiles of state-of-the art alloys can be enhanced by affordable solutions.
The running-in phase of mechanical systems is inevitable and, from a tribological standpoint, critical for the lifetime of such systems, though receives little attention and is poorly understood.
The growth of micro-cracks accelerates premature 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. Two mechanisms were
investigated:
a. Cold work hardening and
b. Chemical tribofilm formation.
Steels that showed a tendency toward work hardening and tribofilm formation in previous testing were chosen for this investigation. Thorough characterization of the chosen Steels was carried out before any pre-conditioning techniques were applied. The widely used 20MnCr5, seen as a reference gear steel, underwent case-hardening and subsequent deep freezing treatments in an attempt to yield discrete sample groups with respect to residual austenite.
This allowed for a more thorough investigation into the effects of residual austenite on the properties of this material. The high-performance alternative steels, 36NiCrMoV1-5-7 (hot working steel) and 45SiCrMo6 (spring steel), were heat treated as recommended by their respective manufacturers, and were not case-hardened. The selection of materials with and materials without case-hardening allows for an investigation into whether or not case-hardening is even necessary to deliver acceptable friction behaviour and wear performance. Elemental
analyses were conducted by multiple methods to ensure accurate results. Residual Austenite contents of the steels and the depth profiles of residual stresses were determined by X-Ray diffraction (XRD), for 20MnCr5 ranging from approximately 6 – 14 vol.%, and under 2 vol.% for
the alternative alloys. Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or
100Cr6/102Cr6, than of a non-case-hardened 20MnCr5. Residual stresses from machining and case-hardening were measured directly at the sample surface. The high-performance Steels fulfilled manufacturer expectations in terms of elemental content, with hardness values between
50 – 55 HRC and strongly martensitic microstructure character. With characterization of the chosen materials complete, the materials could then be subjected to pre-conditioning.
The first pre-conditioning method involved targeted generation of cold work hardening as induced boundary layers to protect the contact zone against wear. Work hardening was identified both by variations in residual stress profiles, i.e. the introduction of beneficial compressive residual stresses, and hardness increases in the contact zone, providing enhanced wear resistance. Parameters for work hardening were further optimized to reduce damage to the surface substrates of the treated materials. The second pre-conditioning method involved
the targeted generation of chemically reactive tribolayers (tribofilms) on twin disk testing rigs.
The lubrication strategies were based on:
a. CaCO3, which is predominant in engine oils, and
b. MoDTC, which is commonly used in engine and gear oils.
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 and lasting 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.
Finally, slip-rolling endurance testing (T = +120 °C, 107 cycles, approximately 19 days in
a factory fill engine oil) was carried out on all materials. It was shown that both pre-conditioning methods could achieve significant reductions in friction and wear during testing at up to and including P0Mean =1.94 GPa (P0Max = 2.91 GPa, FN = 2,000 N). Ultimately, this research showed that:
1. non-case-hardened high-performance steels offer competitive wear performance and better friction behaviour than the case-hardened 20MnCr5.
2. pre-conditioning led to COF reductions to under 7/10 and wear coefficient reductions to an astonishing 1/10 of the original values for the untreated steels under mixed/boundary lubrication.
3. the observed improvements to friction behaviour and wear performance are indicative of
a technically simple, cost- and energy-efficient pre-conditioning strategy that may prove
This chapter deals with the development of polyetheretherketone (PEEK) and polytetrafluoroethylene (PTFE)-based composites, optimized for low friction and low wear performance under extreme environments. It is demonstrated that the incorporation of a harder polymer component into PTFE (such as PEEK particles), a short fiber reinforcement (e.g. carbon fibers (CFs)) and internal lubricants (e.g. PTFE particles) helps to reduce the friction and to improve the wear resistance over a very wide temperature range.
Microstructure and wear behavior of improved NbC cermets for (BS-1452) grey cast iron machining
(2017)
Tungsten carbide (WC) and cobalt (Co) based cermets are the most successful commercial cermets beacuse of the good combination of physical, mechanical and behavioral properties. Niobium carbide (NbC) has good mechanical and physical properties, such as high hardness, very high melting point and low density. SPS refined the NbC grains and increased the hardness, while substitution of Co with Ni improved the fracture toughness.