Using a coupled FEM-DEM method to study the nonlinear phenomena of third-body behavior

Third-body lubrication is widely applied in the design of mechanical frictional pair, but the relation between the lubrication performance and third-body rheology is hardly focused on. In this paper, the behavior of two plates shearing granular third-body is modeled by a coupled FEM-DEM method to study the nonlinear phenomena during this process. The upper plate is modeled by FEM, while the third-body and lower plate are modeled by DEM. During the simulation, the upper plate compresses the granular third-body at a certain pressure. Then, the lower plate shears at a constant speed. The fluctuations of friction force and dilatancy phenomenon of clearance between two plates, which show strong nonlinearity over time, are studied over different external loads, shearing speeds, and elastic moduli. The nonlinear distributions of averaged stress and velocity across the height of third-body are also studied to further reveal the relation between the nonlinear phenomena and the discreteness of third-body. The perspective of the paper is that the coupled FEM-DEM method can elucidate the nonlinear nature of third-body by efficiently processing both the third-body rheology and first-body deformation.

Effects of magnetic ionic liquid as a lubricant on the friction and wear behavior of a steel-steel sliding contact under elevated temperatures

A magnetic ionic liquid (abridged as MIL) [C6mim]5[Dy(SCN)8] was prepared and used as the magnetic lubricant of a steel-steel sliding pair. The tribological properties of the as-prepared MIL were evaluated with a commercially obtained magnetic fluid lubricant (abridged as MF; the mixture of dioctyl sebacate and Fe3O4, denoted as DIOS-Fe3O4) as a control. The lubrication mechanisms of the two types of magnetic lubricants were discussed in relation to worn surface analyses by SEM-EDS, XPS, and profilometry, as well as measurement of the electric contact resistance of the rubbed steel surfaces. The results revealed that the MIL exhibits better friction-reducing and antiwear performances than the as-received MF under varying test temperatures and loads. This is because the MIL participates in tribochemical reactions during the sliding process, and forms a boundary lubrication film composed of Dy2O3, FeS, FeSO4, nitrogen-containing organics, and thioether on the rubbed disk surface, thereby reducing the friction and wear of the frictional pair. However, the MF is unable to form a lubricating film on the surface of the rubbed steel at 25 °C, though it can form a boundary film consisting of Fe3O4 and a small amount of organics under high temperature. Furthermore, the excessive Fe3O4 particulates that accumulate in the sliding zone may lead to enhanced abrasive wear of the sliding pair.

EVALUATION OF WEAR AND CORROSION RESISTANCE OF TITANIUM AND STAINLESS STEEL JOINTS Made OF Al, Cu AND Ni

In the present study, commercial pure titanium (Grade 2) was joined to the stainless steel (X5CrNi18-10) by diffusion bonding using aluminium, copper, and nickel as interlayers (100 μm). The investigation focuses on comparing the wear and corrosion resistance of the obtained diffusion joints. The microstructure of the joints was investigated using scanning electron microscopy equipped with an energy dispersive X-ray system (EDS) to determine the chemical composition of joint. The value of friction force and the wear resistance of diffusion bonded joints were carried out by block-on-ring frictional pair, preformed on the tribological tester T-05. The study was carried out under conditions of technically dry friction for the concentrated sliding contact loaded with 300 N. The friction distance for each test was 400 m. The results show that the maximum values of the friction coefficient and mass loss were obtained for joints with a nickel interlayer. The galvanic corrosion tests were carried out in 0.5 M Na2SO4 solution acidified to pH 1 with a sulphuric acid solution. The potentiodynamic polarization curves show that the lowest corrosion current was registered for the joints performed by copper.

Role of Magnesium Perrhenate in an Oil/Solid Mixed System for Tribological Application at Various Temperatures

Magnesium perrhenate used as a lubricating additive was prepared by an aqueous solution method in this paper, and was suspended in a base oil poly alpha olefin (PAO6) with the aid of surface active agents (SA). The thermal stability of the mixed oil with/without magnesium perrhenate and surface active agents was investigated by thermogravimetry testing. The influences of magnesium perrhenate as solid lubricating additive on the extreme pressure performance and the friction-reducing properties over a wide temperature range of the mixed lubricants were determined by four-ball tests and ball-on-disc frictional tests for the commercially available silicon nitride ball and a Ni-base superalloy frictional pair. The results revealed that the added magnesium perrhenate did not obviously affect the thermostability and oxidation resistance of the base oil. Meanwhile, it minimized the coefficients of friction and wear scar diameter to a certain extent in the four-ball experimental conditions. Ball-on-disc rubbing tests suggested the mixed oil had a similar lubricating performance to the base oil below the decomposition temperature point. The most significant advancement was the impressive antifriction improvement at the high temperature range, while the friction coefficients of the oil containing magnesium perrhenate compound were obviously below that of the base stock. This better tribological performance of the mixed lubricant was attributed to the native shear susceptible property and chemical stability of magnesium perrhenate under high temperature conditions, which could form an effective durable and stable antifriction layer with the oxides from the superalloy matrix, thereby decreasing the friction in the high-temperature environment.

Research on the lubrication performance of engine piston skirt–cylinder liner frictional pair considering lubricating oil transport

In current lubrication analysis of piston skirt, the flooded status is generally considered in the piston skirt–cylinder liner frictional pair in all strokes of an engine operating cycle. However, the quantity of lubricating oil at the entrance of piston skirt cannot always ensure the sufficient lubrication status of piston skirt–cylinder liner frictional pair when the piston moves from the bottom dead center to the top dead center in actual engine. In this article, based on the model of piston secondary motion, fluid lubrication, and lubricating oil flow, the lubrication performance of piston skirt–cylinder liner frictional pair is analyzed, in which the quantity of lubricating oil detained on the surface of cylinder liner after the piston skirt moves from the top dead center to the bottom dead center and is considered as the quantity of lubricating oil at entrance of piston skirt when the piston moves from the bottom dead center to the top dead center. The results show that compared with current analysis, in which the sufficient lubrication of piston skirt–cylinder liner frictional pair is assumed in all strokes of engine, there are remarkable changes for the lubrication performance of piston skirt–cylinder liner frictional pair and the piston secondary motion when the lubrication status of the frictional pair in the upstroke of piston is determined by considering actual lubricating oil transport in the lubrication analysis of piston skirt.

Scaling effects of measuring disc brake airborne particulate matter emissions – A comparison of a pin-on-disc tribometer and an inertia dynamometer bench under dragging conditions

An important contributor to non-exhaust emissions in urban areas is airborne particulate matter originating from brake systems. A well-established way to test such systems in industry is to use inertia dynamometer benches; although they are quite expensive to run. Pin-on-disc tribometers, on the other hand, are relatively cheap to run, but simplify the real system. The literature indicates promising correlations between these two test stands with regard to measured airborne number distribution. Recent studies also show a strong dependency between the airborne number concentration and the disc temperature. However, a direct comparison that also takes into account temperature effects is missing. The aim of this paper is, therefore, to investigate how the transition temperature is affected by the different test scales, under dragging conditions, and the effects on total concentration and size distribution. New and used low-steel pins/pads were tested against cast iron discs/rotors on both the aforementioned test stands, appositely designed for particulate emission studies. A constant normal load and constant rotational velocity were imposed in both test stands. Results show that a transition temperature can always be identified. However, it is influenced by the test scale and the frictional pair status. Nevertheless, emissions are assessed similarly when an equivalent frictional pair status is analysed (e.g. run-in). Further investigations for fully run-in samples on the pin-on-disc should be performed in order to finally assess the possibility of using the tribometers for the initial assessment of different friction materials.