Tribological Layer Research Articles

The Role of Triboloading Conditions in Tribolayer Formation and Wear Resistance of PES-Based Composites Reinforced with Carbon Fibers.

In this paper, the tribological characteristics of polyethersulfone-based composites reinforced with short carbon fibers (SCFs) at aspect ratios of 14-250 and contents of 10-30 wt.% are reported for linear metal-polymer and ceramic-polymer tribological contacts. The results showed that the wear resistance could be greatly improved through tribological layer formation. Loading PES with 30 wt.% SCFs (2 mm) provided a minimum WR value of 0.77 × 10-6 mm3/N m. The tribological layer thicknesses were estimated to be equal to 2-7 µm. Several conditions were proposed, which contributed to the formation of a tribological layer from debris, including the three-stage pattern of the changing kinetics of the time dependence of the friction coefficient. The kinetics had to sharply increase up to ~0.4-0.5 in the first (running-in) stage and gradually decrease down to ~0.1-0.2 in the second stage. Then, if these levels did not change, it could be argued that any tribological layer had formed, become fixed and fulfilled its functional role. The PES-based composites loaded with SCFs 2 mm long were characterized by possessing the minimum CoF levels, for which their three-stage changing pattern corresponded to one of the conditions for tribological layer formation. This work provides valuable insight for studying the process parameters of tribological layer formation for SCF-reinforced thermoplastic PES composites and revealing their impact on tribological properties.

Lubrication mechanism analysis of nickel-carbon-based tribological layer on sliding interfaces in automotive engines

Automotive engine oils demand superior tribological properties to prolong the lifespan of mechanical components. This study contributes to that objective by investigating the impact of nickel-coated multi-walled carbon nanotubes (Ni-MWCNTs) as additives to engine oil on tribological properties. Tribological tests were conducted utilizing a reciprocating motion tribometer to simulate the tribo-system of the piston ring and cylinder liner in automobile engines. Additionally, advanced characterization techniques were employed to study wear and lubrication mechanisms. Ni-MWCNTs nano-additives diminished the friction coefficient and wear rate by 15–23% and 68–87%, respectively, compared to baseline oil. The lubrication process is ascribed to the generation of a tribofilm brought about by thermophysical and tribochemical mechanisms. Ultimately, this study offers new insights into improving lubrication performance in automobile engines to enhance engine durability and fuel economy.

Application of Raman Spectroscopy to Evaluate the Structure Changes of Lubricating Grease Modified with Montmorillonite after Tribological Tests

This study shows the experimental data obtained by Raman spectroscopy to evaluate the structural changes of vegetable lubricants modified with montmorillonite after tribological tests. The analysis of the friction factor and limiting load of wear in the test for the examined grease compound shows a substantial effect on this parameter for grease. A change in the evaluated tribo-parameter results in a modification of the structure of the tested lubricant and changes in the protection efficiency of the tribological system. The amount of thickener, oil base and additive in the grease structure has an influence on the value of anti-wear properties, as shown by the data obtained in the tribological test described in this paper. The Raman spectroscopy tests showed that, in the tribological processes, some of the ingredients undergo an oxidative reaction, which leads to the formation of oxygenated organic substances that form an organic layer on the metal surface and counteract the wear of the lubricated contact surfaces. Other compounds come into close contact with the tribological layer, which increases its ability to resist wear and shear. The efficiency of the used additive is based on the formation (during friction) of a low-shear and high-plastic-strength thin film which is chemically highly bonded to the material and has a high level of durability against wear processes. As a consequence of the thermal decomposition of the additive, chemical interactions occur among the ingredients of the material of the substrate and the lubricants.

Comparison of Various Conversion Layers for Improved Friction Performance of Railway Wheel-End Bearings

With a growing global railway market which needs to reduce its energy consumption and emissions, railway wheel-end bearing units are being optimized to further reduce power losses with no compromise on reliability. One of the different solutions being evaluated is the selection of the optimum surface engineering. Inner and outer rings of railway bearing units are currently coated with a zinc-calcium phosphate conversion coating designed for anti-corrosion, anti-fretting, and mounting properties. In this study, different conversion layers, like zinc-calcium phosphate, manganese-phosphate and tribological black oxide, have been compared in terms of friction performance using a single-contact tribometer and a grease-lubricated bearing friction test rig. Results demonstrate that an optimum tribological black oxide conversion layer can reduce the bearing torque by up to 30% in both low and intermediate speeds relevant to intercity trains.

Structure, mechanical and tribological properties of Ta-xN coatings deposited by reactive HiTUS

This work investigates the structure, mechanical, and tribological properties of Ta-xN coatings deposited using reactive High Target Utilization Sputtering (HiTUS) with a focus on the role of tribological layers formed in the sliding contacts. The scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy observations revealed that the gradual increase of nitrogen flow results in the transition from textured fcc Ta structure (without nitrogen) toward mixed sub-stoichiometric Ta-Ny/Ta2N/amorphous structures up to near-stoichiometric fcc TaN coatings. The highest hardness HIT ∼37 GPa and indentation modulus EIT ∼375 GPa as well as the lowest coefficients of friction (COF) in the range 0.16–0.36 and wear rates around 3 × 10−7 mm2/N·m were obtained in the layer with Ta2N structure. The unique combination of low COF and high wear resistance combined with high hardness and stiffness were attributed to the shear sliding between Ta2O5-based transfer film in the wear track and pure alumina ball.

Tribological behavior of a novel Si- and WC- co-reinforced a-C multilayer coating at 25– 500 °C

To improve the applicability of a-C coatings in a wide temperature range, a novel Si- and WC- co-reinforced amorphous carbon coating (SiC-nWCC), consisting of Si-doped a-C nanolayers and WC/a-C nano-multilayers, is proposed in this paper. A series of SiC-nWCC coatings with different modulation ratios (0.49–1.98) were fabricated by a closed field unbalanced magnetron sputtering system. The influences of modulation ratio on the microstructure, mechanical properties together with thermal stability of SiC-nWCC coatings were explored. The tribological behaviors of SiC-nWCC coatings against M50 steel balls were investigated at room temperature to 500 °C in ambient air. The results show that the SiC-nWCC coating with the modulation ratio of 1.98 achieved superior wide-temperature applicability, featuring the average friction coefficient lower than 0.17 and the wear rate less than 9.84 × 10−6 mm3/N·m in the whole temperature range of 25–500 °C. The excellent wide-temperature tribological performance of the SiC-nWCC coating are mainly attributed to the good thermal stability and the formation of temperature adaptive tribological layers. At 25 °C, sp2-rich carbon is the main lubrication phase of tribological layers, and at 200– 400 °C, the lubrication performance of coating relies on both sp2-rich carbon and Si-rich oxides, and at 500 °C, the self-lubrication phases of Si-rich oxides and WO3 are account for the outstanding lubricating performance of coating. This work provides a profound insight for developing the wide-temperature applicable a-C based coatings.

Effect of Tribological Layer Formation on Wear Resistance of PI- and PEI-Based Nanocomposites in Point and Line Contacts

The tribological performance of both PI- and PEI-based nanocomposites, reinforced with chopped carbon fibers (CCF) and additionally loaded with halloysite nanotubes (HNTs) as well as carbon nanotubes (CNT), was investigated. Metal (GCr15 steel) counterparts were utilized in the point (“ball-on-disk”) and linear (“block-on-ring”) tribological contacts. In the point contact, the PEI/10CCF/1HNT nanocomposite was characterized by the maximum wear resistance and the absence of microabrasive damage of the steel counterpart (Ra = 0.02 µm). The effect of tribological layer formation through creep and mixing mechanisms was proposed to make it possible to protect (shield) the contacting surfaces. In the linear contact at the higher Ra counterpart roughness of 0.2 µm, the tribological layer was formed on both PI- and PEI-based nanocomposites. This was governed by the development of both creep and mixing processes under the cyclic action of the tangential load transmitted from the sliding counterpart and being localized on the wear track. Due to the combination of both higher manufacturability and lower cost, the PEI-based nanocomposite loaded with CCFs and HNTs is a promising inexpensive material for fabricating components of metal–polymer friction units.

Influence of Tribolayer on Rolling Bearing Fatigue Performed on an FE8 Test Rig—A Follow-up

The tribological contact between raceways and rolling elements is essential for rolling bearing performance and lifetime. The geometrical description of these contacts is well known and can be used in several mechanical simulation tools. The material description, especially of the near-surface volume after interaction with lubricants, is not as simple. In particular, the Schaeffler FE8-25 test with cylindrical roller thrust bearings exhibits different failure modes depending on the lubricant chemistry. The main failure mechanisms of this test are sub-surface fatigue damage due to WECs (White Etching Cracks) and/or surface-induced fatigue damage (SIF). The harsh test conditions with mixed friction at high speeds and multiple slip conditions over the raceway width additionally provides different tribological conditions on a small area. This leads finally to the formation of certain tribological layers on the raceway because of the interaction of the surface with the lubricant chemistry under local frictional energies, which are worth investigating. The characterization of the layers was performed by the two less time-consuming, spatially resolved analysis methods of µXRF and ATR FTIR microscopy adapted by Schaeffler. This paper shows the results of this research and offers new approaches to optimizing rolling bearing testing and predicting the risk of early failures.

Volatile Lubricants Injected Through Laser Drilled Micro Holes Enable Efficiently Hydrocarbon-Free Lubrication for Deep Drawing Processes

In order to reduce the use of classic lubricants such as synthetic or mineral oils, emulsions or waxes in the deep drawing process, a new tribological system based on volatile lubricants was investigated. In this system, a volatile medium is injected under high pressure through laser drilled micro holes directly into the contact zone between the tool and the sheet metal and serves as a temporary lubricant. In order to investigate this tribological system under realistic conditions, strip drawing experiments with different volatile lubricants (air, nitrogen, carbon dioxide and argon) were performed on galvanized sheets. Therefore, a new generation of strip drawing tools was designed and numerically calculated for low elastic deformations to ensure a uniform contact pressure distribution over the entire friction contact area. To obtain a homogeneous distribution of the volatile lubricants, a number of micro holes with a depth of several millimeters were drilled into the hardened strip drawing jaws using ultrashort pulsed laser radiation. Taking into account the capabilities of this laser drilling technique in terms of size and shape of the micro holes, computational fluid dynamics simulations were performed to predict the flow behavior of the lubricant within the micro hole as well as the contact zone and were compared with observable effects in outflow tests. The chemical composition of the acting tribological layers was characterized by means of X-ray photoelectron spectroscopy and their changes during the deep drawing process were correlated with the lubricants used as well as the measured wear and friction values.

Effect of silicon-doping on the wide-temperature tribological behavior and lubrication mechanism of WC/a-C film

To expand the lubricating temperature range of a-C based film in air atmosphere, a novel Si and WC co-doping Si/WC/a-C film was proposed. The Si/WC/a-C film with different Si contents were prepared by an unbalanced magnetron sputtering system. The effects of Si on the microstructure, thermal stability and mechanical properties of WC/a-C films were explored, and its wide-temperature tribological behaviors against M50 steel were researched at temperatures ranging from 25 °C to 500 °C in air atmosphere. The Si-doping inhibited the crystallization of WC in films, leading to the microstructural transformation from nanocrystalline-amorphous to amorphous. The introduction of Si significantly improved the nanohardness, thermal stability and wide-temperature lubrication properties of WC/a-C film. Therein, the Si/WC/a-C film with 18.85 at.% Si exhibited the best tribological performance featuring the average friction coefficients less than 0.25 at 25–500 °C. Its prominent wide-temperature lubrication performance can be attributed to the successive generation and their combined effect of the self-lubricating phases of sp2-rich carbon, Si–O–C and WO3 in different temperature ranges. Remarkably, appropriate Si-doping can effectively inhibit the oxidation of sp2-rich carbon in tribological layers, thus it is the dominant lubrication phase throughout the wide temperature range of 25–500 °C.

Role of Testing Conditions in Formation of Tribological Layers at Line Contacts of Antifriction CF-Reinforced PI- and PEI-Based Composites.

High-strength PI and PEI polymers differ by chemical structure and flexibility of the polymer chains that ensure lower cost and higher manufacturability of the latter. The choice of a particular polymer matrix is of actuality at design of antifriction composites on their basis. In this study, a comparative analysis of tribological behavior of PI and PEI- based composites was carried out with linear contact rubbing. The neat materials, as well as the two- and three-component composites reinforced with chopped carbon fibers, were investigated. The third components were typically used, but were different in nature (polymeric and crystalline) being solid lubricant fillers (PTFE, graphite and MoS2) with characteristic dimensions of several microns. The variable parameters were both load and sliding speed, as well as the counterface material. It was shown that an improvement of the tribological properties could be achieved by the tribological layer formation, which protected their wear track surfaces from the cutting and plowing effects of asperities on the surfaces of the metal and ceramic counterparts. The tribological layers were not formed in both neat polymers, while disperse hardening by fractured CF was responsible for the tribological layer formation in both two- and three component PI- and PEI-based composites. The effect of polymer matrix in tribological behavior was mostly evident in two-component composites (PI/CF, PEI/CF) over the entire P⋅V product range, while extra loading with Gr and MoS2 leveled the regularities of tribological layer formation, as well as the time variation in friction coefficients.

Elastoplastic properties of tribological layers of WC – (Fe – Mn – C) composites formed after high-speed sliding on steel

In this work, the authors studied the elastoplastic properties of the formed tribological layers of WC – (Fe – Mn – C) composites with matrices consisting of γ-iron (containing 4 % Mn (WC – 80G20)) and γ + α′ (containing 20 % Mn (WC – 80G4)) after friction on a high-speed steel disk at contact pressure of 5 MPa and sliding speeds in the range from 10 to 37 m/s. It was established that the main factor determining the morphology of the worn surface is sliding speed. At sliding speeds of 10 and 20 m/s, finely dispersed mechanically mixed tribolayers 3 – 4 µm thick are formed. As the sliding speed increases to 30–37 m/s, the thickness of the tribolayers reaches 10 – 15 µm, and the structure consists of oxidized fragments of WC – (Fe – Mn – C) composites and FeWO4 complex oxide and does not have a sharp boundary, like the tribolayers formed at lower sliding speeds. The highest values of nanohardness (~33 GPa) and effective Young’s modulus (~523 GPa) were achieved in the WC – 80G4 tribolayer after friction at 10 m/s when the nanoindenter was embedded into agglomerates of fragmented WC grains. This contrasted with the properties of the tribolayers formed at sliding speeds above 20 m/s. The results of nanoindentation showed an obvious effect of tribochemically induced softening in the emerging tribolayer after high-speed sliding at a speed of 37 m/s. Such a layer had a composite microstructure consisting of fragmented composite components cemented in-situ by tribochemically formed FeWO4 and, in addition to antifriction properties, had an increased indentation fracture resistance.

Investigation into the evolution of tribological white etching layers

When wheels and rails are in operation, their complicated and changeable contact conditions contribute to the formation of white etching layers on their contact surface. Martensite white etching layers (M-WELs) can induce the initiation of rolling contact fatigue (RCF) cracks, accelerating wheel/rail failure; Tribological white etching layers (T-WELs) can improve wear resistance. Therefore, to investigate the formation of T-WELs is to provide theoretical bases for developing surface strengthening technology. In order to delve into the mechanism for the formation of T-WELs, this paper conducted dry friction sliding wear experiment on ingot iron (equiaxed ferrite), D1 wheel steel (lamellar eutectoid ferrite) and FCB wheel steel (lath-like bainite-ferrite), and generalized the impacts of ferrite with different morphology and the second phase on the evolution of T-WELs. The result showed that T-WELs formed on the worn surfaces of all of these materials under the condition of sliding wear, and the evolution of T-WELs displayed similarities. Under the impact of dislocation slip mechanism, size reduction and dimension reduction were realized and finally ultrafine ferrite grains formed. But the second phase would increase the critical shear stress value of the transformation of ultrafine ferrite grains, hindering the formation of T-WELs and eventually affecting the size of ultrafine ferrite grains inside T-WELs.

Elastic-Plastic Properties of Tribological Layers of WC–(Fe–Mn–C) Composites Formed after High-Speed Sliding Against a Steel

Elastic-Plastic Properties of Tribological Layers of WC–(Fe–Mn–C) Composites Formed after High-Speed Sliding Against a Steel

Self-Lubricating Effect of FeWO4 Tribologically Synthesized from WC-(Fe-Mn-C) Composite during High-Speed Sliding against a HSS Disk

WC-(Fe-Mn-C) composites with γ-iron and γ + α’ matrices were sintered and then tested at sliding speeds in the range 7–37 m/s. The coefficient of friction was exponentially reduced as a function of sliding speed reaching its minimum at 37 m/s. This behavior was provided by the mechanochemical formation of iron tungstate FeWO4 on the worn surfaces of composite samples. The lubricating effect of iron tungstate did not, however, allow for a reduction in wear. The worn surface was represented by a 3–10 μm-thickness tribological layer composed of fine WC and iron particles cemented by FeWO4. This layer provided the self-lubricating effect in high-speed (high-temperature) sliding because of its easy shear and quasi-viscous behavior. The underlying 25–65 μm of thickness layer was composed of only heat-affected WC and recrystallized iron grains.

Mechanical properties and wear behavior of Al5083 matrix composites reinforced with high amounts of SiC particles fabricated by combined stir casting and squeeze casting; A comparative study

Combined stir casting and squeeze casting processes were used to fabricate Al5083 matrix composites reinforced with 20, 25, and 30 wt% SiC p . The microstructure, mechanical properties and wear behavior of the composites fabricated by combined stir casting and squeeze casting were compared with those fabricated by stir casting. The results revealed that the combined casting method improved the distribution of SiC particles through the reduction of the agglomeration of SiC particle and reduced the porosities of the samples from 2.32% to 1.29% in the sample containing 30 wt% SiC. These modifications led to the enhancement of mechanical properties i.e. increased the hardness to 85 BHN and the compressive strength to 350 MPa for the sample containing 30 wt% SiC fabricated by the combined casting method. In addition, the wear resistance of the samples fabricated by the combined casting method improved because of the reduced size of the wear debris as well as the smaller worn region. The dominant wear mechanism of all the composite samples fabricated by both methods was the delamination of the tribological layer while adhesion wear was dominant in the monolithic Al alloy.

Influence of the kinematic roughness resulting from facing of AMC specimens on preconditioning of friction surfaces

Aluminium matrix composites (AMCs) consist of a comparatively soft aluminium alloy and hard ceramic particles. Due to elevated properties, these materials are used for high-performance lightweight components. Their application in tribologically loaded brake systems requires a defined friction behaviour, which can be gained by appropriate geometrical surface properties as a prerequisite for a specific tribolayer formation. The aim of this research is to enhance the generation of nearly non-wearing tribolayers on AMC specimens reinforced with 35% volume proportion of silicon carbide particles. For that purpose, the influence of the corner radius and the feed on the surface properties is analysed in face turning using CVD diamond tipped indexable inserts. On the machined AMC specimens, tribolayers are generated in a conditioned pin-on-disc arrangement applying pins made of automotive brake lining material. The focus lies on the tribolayer build-up phase as the stage in which the protective tribolayer begins to form. Surface analyses show that a high kinematic roughness resulting from facing leads to a more intensive build-up of the tribolayers, which is further promoted by voids in the AMC surface. This study provides a first overview of the facing conditions that can be applied to support the subsequent formation of the tribological layer.

Metal inclusions in the seams and fracture of the tool during friction stir welding of aluminium alloys

ABSTRACT It is shown that the appearance of foreign metal inclusions in the seams of aluminium alloys created by friction stir welding is a consequence of the interaction of the working tool with the metal being welded, as well as the tool wears itself. Working tool fracture occurs when a tribological layer is formed on the tool surface as a result of the welded metal diffusion interaction and the tool material. The formation of the tribological layer is accompanied by the formation of aluminium penetration tongues along the grain boundaries into the steel from which the tool is made. Foreign metal inclusions are an intermetallic iron-aluminium particles of the FeAl3 type, which are detached from the tool surface and get mixed in the weld metal. It was found that tool wears increases with an increase in the temporary resistance of the welded aluminium alloy and with the presence of reinforcing ceramic particles SiC in its structure.

Relationship Between the Microstructural Evolution and Wear Behavior of U71Mn Rail Steel

During train operation in a wheel-rail system, the service lives of the wheel and rail materials are heavily influenced by microstructural evolution. This study investigated the relationship between microstructural evolution and the wear properties of U71Mn rail steel under conditions of dry sliding wear using an MRH-5A wear tester. The results demonstrated that tribological white etching layers (T-WELs), which comprise nanocrystalline α-Fe and fine carbide, are formed after a certain number of cycles. The T-WEL thickness increased with the number of cycles; however, more cycles caused T-WEL flaking, which resulted in a reduced T-WEL length. The high microhardness of T-WELs (approximately 950 HV) is mainly attributed to ferrite grain refinement and cementite dissolution. During initial wear, the existence of grinding marks on the surfaces of the samples indicated an obvious increase in the wear loss of the steel. The high microhardness of the T-WELs can enhance wear resistance; thus, as the number of cycles was increased, the wear loss of the steel was reduced. At this point, an adhesive wear mechanism acted on the rail steel. The formation of fatigue cracks at the interface between the T-WEL and the plastic deformation layer induces T-WEL flaking, which increases wear loss. Consequently, after 6000 cycles, wear loss increased as the wear mechanism changed to fatigue wear. It is confirmed that T-WEL flaking is the primary cause of wear failure in U7Mn rail steel.

Charge trapping with α-Fe2O3 nanoparticles accompanied by human hair towards an enriched triboelectric series and a sustainable circular bioeconomy.

This work reports a new approach to amending polydimethylsiloxane (PDMS) by supporting α-Fe2O3 nanoparticles (NPs), thereby generating a material suitable for use as a negative triboelectric material. Additionally, human hair exhibits a profound triboelectrification effect and is a natural regenerative substance, and it was processed into a film to be used as a positive triboelectric material. Spatial distribution of α-Fe2O3 NPs, the special surface morphologies of a negative tribological layer containing nano-clefts with controlled sizes and a valley featuring a positive tribolayer based on human hair made it possible to demonstrate facile and scalable fabrication of a triboelectric nanogenerator (TENG) presenting enhanced performance; this nanogenerator produced a mean peak-to-peak voltage of 370.8 V and a mean output power density of 247.2 μW cm-2 in the vertical contact-separation mode. This study elucidates the fundamental charge transfer mechanism governing the triboelectrification efficiency and its use in harvesting electricity for the further development of powerful TENGs suitable for integration into wearable electronics and self-charging power cells, and the work also illustrates a recycling bioeconomy featuring systematic utilization of human hair waste as a regenerative resource for nature and society.