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Eingeladener Vortrag
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Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys in the Hertzian contact pressure range up to P0max of 4 GPa.
Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys such as e.g. CSS-42L™, ASP2012, BIMAX42+, in the Hertzian contact pressure range up to P0max of 4 GPa. Through-hardened 100Cr6H (AISI 52100), case-hardened 20MnCr5 (AISI 5120H) and nitrogen alloyed Croni-dur30 (AMS 5898) still compete in terms of slip-rolling and wear resistance and load carrying capacity, whereas Maraging- and PM-type steel alloys offer superior strength and toughness properties.
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.
Substituting tungsten carbide (WC) as cutting tools and for wear protection by niobium carbide (NbC)
(2016)
As a refractory carbide, niobium carbide (NbC) is today still a forgotten carbide with hidden properties, such as wear resistance, thus qualifying NbC for the group of tribological materials with enhanced wear resistance. Several phases, like Nb₄C₃, Nb₆C₅ and other short and long range ordered phases occur In the region of homogeneity of NbCx (0,75≤ x ≤1.0) in the binary phase diagram, which enable the tailoring of properties and offers a much wider process window than known from WC. The type binder (Co, Ni, NiMo) and its concentration as well as the sintering process (SPS, liquid phase sintering) determine the box “hardness-toughness”, which will be illuminated based on more than 100 grades.
The impact of these tailored properties on sliding wear (T= 22/400°C; v= 0,1-10 m/s) and cutting performance will be illuminated by tribological and machining results. Cutting test results under emulsion and coolant-free operation of cobalt and Fe3Al bonded, straight NbC versus WC based inserts against different alloys (C60, 100Cr6, 42CrMo4, X90CrMoV18, 300WA, GG35) will be highlighted.
Several phases, like Nb₄C₃, Nb₆C₅ and other short and long range ordered phases occur in the region of homogeneity of NbCₓ (0,75≤ x ≤1.0) in the binary phase diagram. Properties, like micro-hardness, hot hardness, sliding wear resistance, elastic modulus and toughness can be tailored by the C/Nb ratio, the addition of secondary carbides and the type of binder. Supporting results from different grades with varying C/Nb ratio or binder types were illuminated. Thus, the NbC system offers to producers a wider process window, than WC. The impact of These tailored properties on sliding wear and cutting performance was illuminated by tribological and machining results. Niobium is today largely available. NbC can be synthesized by carbothermal conversion of Nb₂O₅ or be metallurgically grown and leached out, provides comparably low friction in many relevant tribo-contacts and displays low wear. NbC and Nb₂O₅ have so far no REACH classification related to human toxicology and are not listed as substances of very high concern contrary to WO₃ and Co₃O₄.
In the state-of-the-art technology, the impact of additives on friction and wear is evaluated in recipes with fixed concentrations. For the development of formulations it is desirable to determine the lowest effective concentration or the concentration at which failure was initiated or excessive wear began. Through a new development of the SRV® machine it is now possible to change the lubricant composition of a test specimen in a reproducible manner, in sub-pro Mill steps during the SRV® testing. This technology is used in the present article to define the optimum additive concentration with regard to wear and friction in a system of base oil and additive. Exemplary selected base oil and additive variants are tested here. The load parameters during testing are selected in such a way that they correspond to later application. The new data acquisition capabilities enable new possibilities for optimizing lubricant formulations in practice-oriented model-tribometer tests. “Cliff“ testing aims to identify in engine or gear tests the induction time or off-set point (“cliff“) after which wear and friction increased of failure occurred. Explanations for friction and wear increased as well as failures, which occurred during engine tests, can be derived from SRV® testing of oil samples taken or collected at different engine test times and correlating these with their friction, wear and EP data in respect to depleting curves for specific additives or other oil properties. A “cause-root” analysis is done by plotting SRV® data versus functional properties. The collected oil samples must be fully SRV® tested and chemically analyzed.
This review article comprises of three parts. Firstly, reports of brake manufacturers on the beneficial impact of solid lubricants for pad formulations are surveyed. Secondly, since tribofilms were identified to play a crucial role in friction stabilization and wear reduction, the knowledge about tribofilm structures formed during automotive braking was reviewed comprehensively. Finally, a model for simulating the sliding behavior of tribofilms is suggested and a review on modelling efforts with different model structures related to real tribofilms will be presented. Although the variety of friction composites involved in commercial brake systems is very broad, striking similarities were observed in respect to tribofilm nanostructures. Thus a generalization of the tribofilm nanostructure is suggested and prerequisites for smooth sliding performance and minimal wear rates have been identified. A minimum of 13 vol.% of soft inclusions embedded in an iron oxide based tribofilm is crucial for obtaining the desired properties. As long as the solid lubricants or their reaction products are softer than magnetite, the main constituent of the tribofilm, the model predicts smooth sliding and a minimum of wear.
In this study the so-called laser-induced periodic surface structures (LIPSS, ripples) were produced on titanium alloy (Ti6Al4V) surfaces upon scan processing in air by a Ti:sapphire femtosecond (fs) laser. The tribological performance of the resulting surfaces was qualified in linear reciprocating sliding tribological tests (RSTT) against balls made of different materials (100Cr6 steel/Al2O3/Si3N4) using different oil-based lubricants. The admixture of the additive 2-ethylhexylzinc-dithiophosphate (ZDDP) to a base oil containing only anti-oxidants and temperature stabilizers disclosed the synergy of the additive with the laser-oxidized nanostructures. This interplay between the laser-textured sample topography and the local chemistry in the tribological contact area reduces friction and wear.
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.
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.
Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) 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 setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.
Hydraulic oils are well established formulations developed around the pump test. Requirements for energy efficiency and also environmentally acceptable properties increased the demand for development. In the same time, system pressure and the used of servo-hydraulic vanes increased bringing specific formulations and materials to their limit.
The vane pump is a key component in hydraulic systems and many other tribosystems in hydraulic circuits operate under different tribological operating conditions. The concept for ranking of hydraulic oils consists in using existing SRV-based ASTM test methods for evaluation of friction, wear and extreme pressure properties: D6425, friction and wear of oils (homologue to DIN 51834-2), D7421, extreme pressure properties of oils and D7755, wear volumes of ball and disks (homologue to DIN 51834-3. The tribological properties are assessed under the regime of mixed/boundary lubrication The SRV-based concept of tribological profile generates the wear volumes on two triboelements (specimen) and the extreme pressure properties as well as the evolution of friction in the wear and extreme pressure load step test. Thus, the benchmark and validation of hydraulic oils bear on a wider range of tribological properties. Round robin test results using the aforementioned testing concept will be shown. Another outcome is tribological set limits for inclusion in hydrau¬lic oil speci¬fications.
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.
In a first test series, the friction and wear properties of 6 polymeric candidate materials for application in LNG tribosystems were tested. The most favorable properties showed PTFE composite, filled with carbon fibers and PEEK, which showed low and stable friction and a wear coefficient in the order of 10-7 mm3N-1m-1. PTFE containing PEEK composites seem to be also appropriate for LNG applications. Admixtures of graphite as solid lubricant result in lower wear. Contrary, a lubricating mechanism of MoS2 was not observed. The PI-materials showed comparatively high friction and medium wear.
In a first test series, the friction and wear properties of 6 polymeric candidate materials for application in LNG tribosystems were tested. The most favorable properties showed PTFE composite, filled with carbon fibers and PEEK, which showed low and stable friction and a wear coefficient in the order of 10-7 mm3N-1m-1. PTFE containing PEEK composites seem to be also appropriate for LNG applications. Admixtures of graphite as solid lubricant result in lower wear. Contrary, a lubricating mechanism of MoS2 was not observed. The PI-materials showed comparatively high friction and medium wear.
The talk gives a historical introduction and introduces some of the biggest and most important research institutes in Germany dealing with tribology at Universities, scientific societies and companies. Some examples for research projects and focus areas are given. As an example for a research institute with a long history in tribological research the activities of BAM in Berlin are presented.
The friction and wear properties of polyetherimide composites under dry oscillating sliding condition at room temperature (RT) as well as at elevated temperature (120 °C) was investigated. The polymer specimens were made to oscillate against steel cylinder as a counterpart. The friction and wear properties of PEI and composites were strongly influenced by the temperature. In case of carbon fiber composite abrasive action of carbon fibers has severely damaged the counterpart and resulted in accelerated wear of the composite at RT. Solid lubricants filled (PTFE, MoS2, graphite) along with glass fiber is beneficial in improving the friction and wear performance of the PEI composite at RT, whereas at elevated temperature wear performance was deteriorated.
Tribology in hot steam
(2017)
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2, alumina and antimony impregnated carbon.
The wear rates of self-amted alumina couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. The presence of water and ist amount available in the surrounding system either in liquid or in gaseous from plays a key role for friction and wear behavior and cabe benefical for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2, alumina and antimony impregnated carbon.
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2, alumina and antimony impregnated carbon.
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above-mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2 alumina and antimony impregnated carbon.
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.
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.