Succinimide additives play an important role in combating engine deposits and are therefore commonly blended in fuels. As many of the methods currently used to quantify them in fuel rely on time-consuming techniques and the use of expensive laboratory equipment, a more practical approach was explored. For this purpose, an existing method for aqueous samples involving a colour reaction with Rose Bengal dye and spectrophotometric detection in the UV/Vis range was modified for usage in the nonpolar fuel matrix and tested for applicability. The result was an accessible method for determining the succinimide additive content of diesel fuel—including biodiesel—that is easy to implement in the laboratory routine.

Oxidation limits the performance and lifetime of lubricants, and phenolic antioxidants are commonly used to slow this process by scavenging hydrocarbon peroxyl radicals. The performance of phenolic antioxidants is largely determined by the stability of the antioxidant radical that remains after hydrogen donation. To explore the relationship between antioxidant chemical structure and radical stability, we used REACTER-based reactive molecular dynamics simulations to model the reverse hydrogen transfer reaction from polyalphaolefin hydroperoxides to phenoxyl radicals. Simulations were run for 718 distinct single-ring phenoxyl radicals with varied substituent types and positions in a polyalphaolefin hydroperoxide environment. Reaction rates were obtained from the time evolution of hydrogen transfer events, where lower reaction rates correspond to higher radical stability and better antioxidant performance. Analysis of diffusivity, hydrogen bonding, and steric hindrance showed that strong hydrogen bonding and high steric hindrance around the phenoxyl oxygen atom decreased the reaction rate, while faster diffusion increased it. A multivariate linear model confirmed that hydrogen bonding was the dominant contributor to radical stability in the low reaction rate region. These results highlight the molecular features that influence antioxidant behavior and demonstrate that reactive simulations offer an efficient route for screening and designing antioxidant molecules.

IntroductionGrease oxidation in rolling bearings causes a loss of grease in the track and hence a loss of lubricity in the bearing. This work addresses loss of grease via evaporation and oxidation due to thermal-oxidative stress applied to two lubricating greases.MethodsFor this purpose, the oxidation stability of two greases was determined using a modified micro-coking test over a wide temperature range from 100°C to 190°C. Grease analyses by means of antioxidant content, oxidation and weight loss were used to develop a lifetime model to describe the change in mass as a function of temperature, time, and grease type due to oxidation based on the Arrhenius equation.Results and discussionThe developed model describes the correlation between the findings from grease analyses and shows increased oxidation and evaporation after the induction period when antioxidants are almost consumed.

This study aims to investigate if sensors used for condition monitoring of lubricated systems can provide high-level robustness against environmental factors, such as temperature, humidity, vibrational load and mechanical shock, thereby ensuring long-term reliable operation. After successful laboratory tests of the Humidity Sensor in Axle Bearings (HSAB) system using accelerated aging for robustness evaluation, a field demonstration was performed to assess its functionality for monitoring water in grease-lubricated axlebox bearings of rail wheelsets. For that purpose, a humidity sensor was integrated inside the bearing cover to measure the relative humidity of the air surrounding the grease. Despite harsh environmental conditions, the HSAB system provided reliable output signals under varying environmental conditions. In this study, the field evaluation of a unique approach for the detection of water in lubricated wagon components is presented. The key element of this system is a robust humidity sensor located in the immediate atmosphere of the investigated grease-lubricated rail component. The HSAB system proved to have satisfactory robustness for both the sensor system and the developed algorithm to calculate the grease–water content for the intended application in axle bearings. Furthermore, the grease–water content of the investigated lubrication grease showed a good correlation with the prevailing weather conditions. The proposed method can significantly enhance the reliability and reduce the maintenance costs and downtime of railway wagons. The presented approach paves the way for an online monitoring tool to predict the water content of grease-lubricated rail parts.

Accelerated Life Testing (ALT) is used to detect and understand failure mechanisms, as well as to calculate and evaluate the robustness and resulting reliability of electronic components such as sensors subjected to various influencing factors in different applications. Based on the successful lab and field robustness validation of the customized Humidity Sensor in Axle Bearings (HSAB) system utilized for the condition monitoring of lubricated rail components, an ALT methodology was designed for a micromachined humidity sensor, and respective results are presented. ALT was aimed at quantifying the robustness of the selected sensor under higher-than-normal use environmental loads (temperature and humidity), with a focus on its sensor element. Thus, ALTs under constant high temperatures combined with low-and high-humidity conditions were executed until all the tested sensor elements failed. Thereafter, the resulting failures were investigated using various methods. The solder joint failure of the sensor element was determined as a central failure mode. Based on the data obtained for the time to failure of the sensor elements, a two-parameter Weibull distribution function was fitted, in agreement with comparable scientific works on solder joint failures. As the aged sensor elements themselves did not seem to be significantly influenced or even damaged by the executed ALTs, their existing functionality was proven afterwards. For this purpose, they were first resoldered and then tested using a developed step-validation test program for temperature and humidity. After a statistical evaluation of the sensor signal deviations relative to a calibrated reference sensor, the amount of still operational sensor elements was assessed. As a result, it was determined that sensor elements aged at high temperatures in a high-humidity atmosphere failed to a significantly greater extent owing to the damaging effect of water. This indicates that water significantly affected not only the solder joints of the sensor but also the sensor element itself under the investigated test conditions.

The growing demand for sustainable lubricant solutions is driving the exploration of bio-based materials that deliver comparable performance to conventional, primarily fossil-based lubricant chemistries. This study focuses on squalane as a sustainable base oil, which can be derived from different renewable sources. A total of two squalane products were evaluated for thermal-oxidative stability and benchmarked against a polyalphaolefin, PAO 4, of the same total carbon number. Oils artificially altered in a closed reactor were sampled and subjected to conventional lubricant analyses, including infrared spectroscopy, to determine the changes due to autoxidation over time. For in-depth information, direct-infusion high-resolution mass spectrometry and gas chromatography coupled with triple quadrupole mass spectrometry were employed to identify degradation products from thermo-oxidative stress. The results revealed substantial variability in the stability of squalane products, suggesting that differences in raw materials and production processes have a major impact on their performance, including rheological properties. The degradation products of polyalphaolefin and squalane, identified through detailed mass spectrometry, were analyzed to understand their impact on conventional physicochemical properties. While polyalphaolefin predominantly generated carboxylic acids with short to medium chain lengths as degradation products, squalane oxidation produced carboxylic acids with medium to long chain lengths as well as several alcohols and ketones. Despite these differences, squalane demonstrates its potential as a non-fossil hydrocarbon base oil, as squalane products matched and even exceeded PAO 4 stability.

The condition monitoring of the health status of lubricating greases used in axle box bearings can be realized by applying well-established electrical or optical measurement principles. Furthermore, some novel methods have been reported that make use of humidity sensors or of dielectric thermoscopy. One of the most important grease condition parameters is the water content of the lubricating grease, as water can degrade grease to the point that it is no longer able to provide suitable lubrication and can also damage the bearing due to corrosion and cavitation. In this study, a new approach for water detection in lubricated wagon components is presented that is based on commercially available humidity sensors. The core element of this sensor system is a robust humidity sensor mounted in the immediate atmosphere of the grease-lubricated wagon axle bearing. In the case of water intake, the humidity of the gaseous atmosphere above the grease increases and can be detected by the customized sensor concept Humidity Sensor in Axle Bearings (HSAB). As this sensor system has to be sufficiently robust, it must be able to withstand environmental impact factors. The most important of these factors are temperature, relative humidity, and mechanical load, like vibrations and shocks, depending on the relevant railway application. To mimic these field effects under controlled laboratory conditions, the “lab-to-field” approach was set up and employed. Of the utmost importance was the installation of a development environment for the sensors that enabled the transfer of laboratory results to the respective rail field application. As a result, the HSAB system shows promise with respect to enhancing the reliability of railway wagons and decreasing maintenance costs, thereby reducing the downtime of railway wagons significantly.

The Austrian rail network is very diverse: The lines in the alpine areas have very tight radii while the lines in the flat areas (east and north of Austria) are mainly straight high-speed lines. This leads to different levels of wheel and rail wear. Accordingly, the objective of this study was to experimentally model a laboratory-scale cyclic wheel-rail contact to investigate wear behavior and rolling contact fatigue at different radii (RCF).

Digitalization in mobility is considered the key to success to increase efficiency, reliability and safety in rail transport, both track and rolling stock. “Big data” analytics are therefore implemented. In the European Joint Undertaking “Shift2Rail”, tremendous efforts are dedicated to condition-based maintenance (CBM). The task of AC2T research GmbH is to enable data collection, processing and evaluation by appropriate sensor systems for online health status monitoring of lubricated components in locomotives and wagons to increase safety and availability while reducing of maintenance costs and unplanned downtime. Special attention is paid to robustness of both sensor and algorithm to meet the high demands for reliability in rail transport. Suitable sensor systems were compiled for three use cases at locomotives and wagons by the field-to-lab approach and validated for use in CBM.

For the understanding of oil degradation in a combustion engine, in particular zinc dialkyl dithiophosphate (ZDDP) deterioration and its impact on wear expressed as iron content, a field test with a passenger car was carried out, which covered a distance of 19,800 km and represented an entire oil change interval. Condition monitoring of the SAE 5W-30 engine oil usedin theturbocharged petrol engine combined the use of conventional and advanced analytical methods. The conventional data collected from the used oils revealed the progress of additives (antioxidants, base reserve, ZDDP), oil degradation products (oxidation, nitration, sulfation, acids), and contaminations (water, soot, wear, fuel dilution). High-resolution mass spectrometry was included to identify ZDDP additive compounds and their fate during the field test as well as their correlation with wear formation. Dialkyl dithiophosphates as the main ZDDP compounds were rapidly degraded and no longer detected after 6000 km. Dialkyl thiophosphate as intermediate ZDDP degradation product was formed and largely depleted within the first 6000 km. Dialkyl phosphates, phosphoricacid, and sulfuric acid as organic and inorganic ZDDP degradation products were generated early and reached high levels at the end of the field test. The presence of intact ZDDP and its degradation products, notably phosphoric and sulfuric acid, correlated with the oil's iron content. Wear largely remained at low level as long as intact ZDDP was available for tribofilm formation. The lack of ZDDP along with the formation of inorganic acids from ZDDP resulted in an increase in the wear rate by a factor of four.

The 23rd International Colloquium Tribology in January 2022 was held for the first time as an online conference only, with about 300 participants. The organising team, speakers and participants therefore made the best of this traditional event in Germany in view of the Covid-19 pandemic, even though the “online conference is not a substitute for a conference on site”, as one participant stated aptly. In almost 140 talks, 8 of which were plenary talks, new findings were reported on the topics of lubricants and additives, measuring techniques, digitalisation, coatings and surfaces, transport and industry, and sustainability. Several presentations were dedicated to the EU project “i-Tribomat”, which will soon be launched as “The European Tribology Centre” that offers services for the tribological characterisation of materials and lubricants.

Lubrication of an internal combustion engine is critical for energy and material losses. Engine lubricants contain a number of functional additives including zinc dialkyldithiophosphate, which is a commonly used antiwear additive that forms by in situ decomposition a protective interface at the metal surface. Here, we present a detailed nanoscale investigation of carbonaceous soot nanoparticles generated from real engine conditions. By combining macroscale XPS with high-resolution STEM-EELS-EDX, we reveal that such a soot nanoparticle matrix contains also 3-5 nm ZnO-based nanoparticles with additions of phosphorus and sulfur, originating from the organometallic antiwear additive. Under the consideration of the obtained chemical information on the carbonaceous matrix and (ZnO:P,S) nanoparticles and the generally known suggestion of potential toxicity for soot nanoparticles, our method allows us to predict nanoparticle-based hazards from mechanochemical applications and also their formation mechanism. These are critical information and also the basis of toxicity assessment, both for theoretical predictions and experimental testing for the estimation of overall life-cycle analysis, including the environmental impact. Our results unravel the tribofilm decomposition under real field conditions and hint toward potentially unidentified toxicological nanoparticle hazards with respect to organophosphate-containing lubricants.

In this chapter, synergistic effects between graphene/graphite and glycerol are investigated and proved to be effective to achieve superlubricity for steel, carbon coating, and ceramics counterfaces. The sources of carbon could be graphite-containing materials, graphite from penciling, or tribo-induced graphene-like structures from glycerol. Furthermore, perfectly layered or ordered graphite (such as highly ordered pyrolytic graphite) is not necessary for friction reduction. Higher content of graphitic clusters in tetrahedral amorphous carbon shows shorter running-in period and lower friction than poorer one. The benefit of cooperating graphite/graphene and glycerol is to provide aromatic carbon sources which could serve as precursors to form oxygen- or nitrogen-containing lamellar graphitic structures in contacts.

Lubricant degradation by additive depletion, contamination and build-up of degradation products deteriorates lubrication performance. It can be expected that engine oil degradation also has an impact on performance parameters. Engine development tends to higher power density, lower engine oil viscosity to optimize fuel efficiency, which heavily stress engine oils and thus demand increased oil performance. Correlations between engine oil degradation, friction and wear during an engine oil’s lifetime are a valuable asset in engine development towards performance, longevity and cost reduction. Here, physical-chemical and tribological properties of engine oils collected from passenger cars were determined. The results revealed the deterioration of friction (loss of fuel efficiency) and wear (increasing loss of material) depending on mileage. Defined used engine oils were produced by artificial alteration in a large-scale device and showed a very good correlation with the field samples.

Lubrication of an internal combustion engine is critical for unwanted energy and material losses. Zinc dialkyldithiophosphate (ZDDP) is a commonly used anti-wear additive that forms by in situ decomposition a protecting interface between sliding surfaces. The interface consists of the tribofilm on both surfaces and oil in the contact. Soot particles from a petrol engine and gas engine were analyzed using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques: electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). These techniques revealed that the end-products in soot contain 3-5 nm ZnO-based particles with additions of phosphorus and sulfur, originating from the ZDDP anti-wear additive. Our results unravel the tribofilm decomposition under real field conditions and hint toward potentially unidentified hazards with respect to ZDDP-containing lubricants.

In this study, interaction of ionic liquid (IL) and borate esters (SBs) as antiwear (AW) additives with steel surfaces in tribological contacts was examined using blends which contained no prior AW additives but all the other ingredients present in a fully formulated engine oil. In detail, low phosphorus oil blends were prepared by adding trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (P_DEHP) at 700ppm phosphorus and 2-methoxy-4,4,6-trimethyl-1,3,2-dioxaborinane or trimethoxyboroxine at 200ppm boron treat rate to a partially formulated oil. The tribological properties of these novel ionic liquid (IL) additive and IL+SB additive mixtures were compared with those of zinc dialkyldithiophosphate (ZDDP) at equal phosphorus levels in the oil blends. Tribological experiments with a reciprocating cylinder on flat contact revealed that both P_DEHP and binary mixtures of P_DEHP+SB offer superior wear protection than ZDDP and the partially formulated oil without AW additives, expressed by a wear reduction of minimum 50%. X-ray absorption near edge structure spectroscopy (XANES) analysis revealed that tribologically formed films are primarily composed of calcium phosphate for oils with AW additives. The interaction of P_DEHP with SB results in additional boron oxide/boric acid and to some extent boron phosphate domains incorporated into the tribofilms.

This work aimed to bridge the gap between engine oil degradation, in particular zinc dialkyl dithiophosphate (ZDDP) deterioration, its effect on tribofilm formation, and its eventual impact on friction and wear. Artificial oil alteration of a commercial engine oil SAE 0W-20 was used to produce a series of altered oils that were subjected to high resolution mass spectrometry in order to identify ZDDP degradation products and their abundances over time. It was found that ZDDP was depleted at an early stage of alteration by the replacement of its sulfur atoms by oxygen atoms and the release of the alkyl side chains. As final products of ZDDP deterioration, sulfuric and phosphoric acid were identified. Selected oils were subjected to tribometrical experiments consisting of an oscillating ball-on-disc contact. ZDDP concentration as well as the type and abundance of ZDDP degradation products in the oils directly influenced tribofilm formation and consequently friction and wear. However, the superior tribological performance of the fresh engine oil could not be regained with any of the artificially altered oils. X-ray photoelectron spectroscopy revealed that sulfur occurred mainly in sulfide state in the tribofilm formed from fresh oil. With increasing degree of oil degradation, the sulfate state became more dominant. By trend, the amounts of sulfur, phosphorus, zinc and calcium declined with increasing degree of oil degradation, indicating that tribofilm formation had become increasingly difficult.

An experimental approach using the stage-gate process is presented to efficiently select and assess ionic liquids (ILs) for their usability in space mechanisms, thus, for their potential to replace commonly used liquid lubricants based on perfluorinated polyethers and multiply alkylated cyclopentanes. This methodology was based on a five-stage selection procedure comprising the determination of rheological properties, outgassing properties, corrosion-inhibiting capabilities, screening of friction and wear performance in vacuum, which was completed by tribometrical lifetime assessments. Five ILs were benchmarked against Fomblin® Z25 as reference at the end of each stage and selected for the next stage depending on the performance. One IL of the type pyrrolidinium bis(trifluoromethylsulfonyl)amide outperformed Fomblin® Z25 in all stages except pour point. Thus, only in the case of fluidity at very low temperature showed Fomblin® Z25 a better performance. Additives slightly improved corrosion inhibition of this IL but showed adverse effects on friction and wear in comparison to the neat IL. In lifetime experiments, the IL resulted in a lifetime extension of at least factor 23 and 31 compared to the reference. Even with the use of additives in this IL, the lifetime extension was still by a factor of 6 to 15 compared to Fomblin® Z25.

Modern space exploration missions, such as planetary exploration of Mars, have significantly different tribological concerns compared to conditions faced by mechanical devices in satellites. Space ...