Stable Friction Coefficient Research Articles

Enhanced tribological performance of PLA/CNC composites: A comparison with phenolic resin and nylon

PLA has been developed to replace plastic because it is degradable. For more advanced applications, more research is needed on PLA. This study investigates the tribological properties of phenolic resin, nylon, and PLA/CNC composites under varying sliding distances and loads. Both phenolic resin and nylon demonstrate exceptional wear resistance and stable friction coefficients. PLA/CNC composites exhibit improved wear resistance, showing a 17% reduction in friction coefficient at a 3 wt.% CNC content. While the wear volume of PLA/CNC composites increases with sliding distance, the addition of CNC enhances PLA’s self-lubricating properties and overall wear resistance. The correlation between dissipated energy and wear volume confirms that higher CNC content significantly improves the durability of PLA. These findings suggest that CNC has considerable potential as an additive to enhance the tribological performance of PLA composites, making it a valuable material for various applications requiring superior wear resistance.

Wear-resistant and stable low-friction nanodiamond composite superhard coatings against Al2O3 counter-body in dry condition

Conventional machining lubricants pose environmental hazards and increase production costs. This study addresses this challenge by investigating nanodiamond composite (NDC) coatings deposited on WC–Co substrates as a lubricant-free alternative for sustainable machining. The NDC films show superior hardness (65 GPa) compared to the substrate (22 GPa) and enhanced adhesion (HF2). The NDC's unique self-lubrication results in a low and stable friction coefficient (COF ≤ 0.095) and exceptional wear resistance (7.45 × 10−8 mm3/N·m) in dry tesing against Al2O3 counter-body. Compared to CVD diamond, NDC coatings show a 47.8 % reduction in COF and a 31.65 % enhancement in wear resistance, promoting environmentally friendly machining practices.

Study on Friction and Wear Performance of Sliding Metal Seal Materials Under Reciprocating Motion.

During petroleum drilling, the reciprocating motion in the seal device leads to piston and sleeve wear, which may cause leakage of the sealing medium. Selecting appropriate materials for the piston and sleeve, along with surface modifications, can effectively prolong the seal service life of the seal. The friction and wear properties of piston and sleeve pairs of different materials in a metal sealing device were simulated by the laboratory "pin-on-block" reciprocating friction test. Pins made of 45# steel, 35CrMo, and 20Cr13 were used to simulate piston bulges, while 35CrMo samples were used to simulate sleeves. Additionally, the influence of DLC (diamond-like carbon) coating and QPQ (Quench-Polish-Quench) nitriding on the wear resistance of the materials was studied. Based on this, the friction and wear properties, along with the wear mechanism of different material pairs, were analyzed. The results show that the friction coefficient curves of the three piston base materials and the 35CrMo sleeve are similar, and the friction coefficient of 45# steel is lower than that of 35CrMo and 20Cr13 at the initial stage. The DLC surface coating exhibited the best anti-wear performance, with the lowest friction coefficient, minimal wear, and the most stable friction coefficient. Surface QPQ nitriding treatment can also improve the wear resistance of the base material. However, due to the oxide formed during nitriding being prone to flaking, the friction coefficient fluctuates significantly at the initial stage of testing, and its anti-wear performance was inferior to that of the DLC coating. This study on material pairing and surface modification provides theoretical support for material selection and surface modification design of pistons and sleeves in oil drilling sealing devices.

Nb-doped hydrogenated diamond-like carbon coated biodiesel injectors material: Synthesis, structure and properties

Despite the economic and environmental benefits of biodiesel, it poses significant challenges, including coking on metal surface and increased wear on diesel engine injector nozzles. Therefore, this study investigates the synthesis of niobium-doped diamond-like carbon (Nb-DLC) coatings on two types of injector nozzle materials (viz. H13 steel, AISI 420 stainless steel) using the Closed-Field Unbalanced Magnetron Sputtering (CFUBMS) technique. Comprehensive characterizations, including microstructural analysis, crystal structure evaluation, hardness assessment, and tribological property assessment, were conducted. The Nb-DLC coatings demonstrated a stable friction coefficient, with values four times lower than that of the substrate materials. The present findings demonstrate significant enhancements in terms of structural composition, crystalline phase, hardness, friction coefficients, and adhesion properties due to the Nb-DLC coatings. Therefore, Nb-DLC coated materials could be the promising candidate material for biodiesel engine injector nozzle applications. This emphasizes their potential to contribute significantly to the sustainable consolidation of biodiesel within automotive engine.

RESISTANCE OF HEAT-RESISTANT YTTRIUM-CONTAINING SEALING COATINGS TO MECHANICAL FRACTURE WHEN FORMING CUTTING PATHS

According to the results of tribotechnical tests of coatings made of KNA-82 alloy ligatures with the addition of yttrium of 0.1%, 0.3%, 0.5%, data were obtained that allowed us to establish the nature of changes in the dynamic coefficient of friction over the test period and numerous values of the energy intensity of material wear. The evaluation of coatings formed by the gas-flame and ion-plasma method was based on the following premise. The maximum resistance to mechanical fracture is determined by the manifestation of a constant minimum value of the dynamic coefficient of friction. This serves as an indicator of reduced friction force before reaching the fatigue limit. Another key factor is the number of separated particles produced per unit of integral work during the friction process. These evaluation parameters are lined up in a row by the number of points from 1 to 4. The maximum score corresponds to the maximum resistance, i.e., a lower value of the energy intensity of material wear and the minimum value of the stable friction coefficient. It has been determined that the same coincidence of these parameters according to the scores is almost at all stages of testing (I-III) was the coating formed by the gas-flame method with a yttrium concentration of 0.3%-0.5%. The exception was the coating formed by the ion-plasma method with a yttrium concentration of 0.1% at the fourth stage of testing.

Macroscopic superlubricity achieved by hydrolysis reaction of ethyl lactate on silicon-doped diamond-like carbon film

Superlubricity between steel/diamond-like carbon (DLC) film could be achieved at vacuum or nitrogen condition, but it would be failed at ambient conditions. In this work, the macroscale superlubricity was achieved at ambient conditions by introducing ethyl lactate into ethylene glycol as lubricant additive for the friction pairs of silicon-doped diamond-like carbon (Si-DLC)/steel. Stable friction coefficient (μ = 0.002) and wear rate of friction pairs with the introduction of ethyl lactate could be respectively reduced by 99 % and 35 %. The characterization tests and density functional theory (DFT) calculation both demonstrated that the partial ethyl lactate was hydrolyzed into lactic acid due to the catalysis effect of steel surfaces. The molecular dynamics (MD) simulation result showed that the lactic acid molecules could be chemically adsorbed on the surfaces of friction pairs, forming a tribofilm through hydrogen bond with ethylene glycol molecules, which led to a significant reduction in the friction coefficient. This work presents a novel approach to achieve superlubricity on Si-DLC film with liquid, providing a great support for industrial application of superlubricity on DLC film.

Experimental study of wear and rolling contact fatigue in railway wheel steels coupled with various brake block materials: Insights from innovative small-scale testing

This study presents a comprehensive analysis using an innovative testing method of two wheel steels paired with cast iron and organic composite brake block materials. By conducting tests under consistent conditions and varying in duration, the study examines temperature profiles, friction coefficients, surface characteristics, weight loss, and microstructural changes in wheel samples, emphasizing the distinct behaviour of these materials in braking applications and the damage evolution over time. The results demonstrate that organic composite brake samples outperform those in cast iron, showcasing smoother wheel sample surfaces and stable friction coefficients. Weight loss analysis reveals the environmental benefits of organic composite brakes, emitting fewer particulates than cast iron counterparts. Microstructural examinations uncover the formation of a Thermal White Etching Layer (T-WEL) on wheel samples tested with cast iron samples, leading to cracks and material detachment. Conversely, extended use of organic composite samples led to a "thermal fuse effect", impacting their efficiency and suggesting the need of careful temperature management in sustained braking scenarios. Despite significant differences in wheel steels, the study underscores the critical role of brake material in braking improvements. The findings not only enhance the scientific understanding of brake material behaviour but also introduce an innovative, cost-effective, and fast 4-contact machine testing method.

Influence of Ag Doping on Wide-Emperature Tribological Properties of γ-Fe2O3@SiO2 Nanocomposite Coatings on Steel

γ-Fe2O3@SiO2-Ag nanocomposite coatings were prepared to investigate the lubrication performances of the nanocomposite coatings under a wide range of temperatures. The effect of Ag doping on the tribological properties of γ-Fe2O3@SiO2-Ag nanocomposite coatings was studied from room temperature to 600 °C, and the synergistic effect of Ag and oxides in the nanocomposite coatings was investigated. The coefficient of friction and the wear rate of γ-Fe2O3@SiO2-Ag nanocomposite coatings decrease with an increase in Ag content. The tribological properties of 24 wt.%Ag of the nanocomposite coatings are excellent. The stable coefficient of friction is 0.25 at 100 °C and the coefficient of friction is reduced to 0.05 at 500 °C. It was found that the synergistic effect of γ-Fe2O3 and Ag is helpful in improving the tribological properties of γ-Fe2O3@SiO2-Ag nanocomposite coatings over a wide temperature range. Ag plays a lubricating role at low and medium temperatures and oxides play a role in lubrication at high temperatures.

Tribological properties of ductile cast iron with in-situ textures created through abrasive grinding and laser surface ablation

This paper discusses the proposed abrasive grinding (AG) and laser surface ablation (LSA) method for creating in-situ textures on ductile cast iron (DCI) surfaces. LSA method removes spherical graphite of DCI’s surface, forming martensite and high residual stress on the matrix, thus increasing hardness. AG method uses Al2O3 abrasives to disrupt spherical graphite and form grinding-layer. Compared to LSA, AG method avoids residual stress and burrs, simplifies the processing technology. Tribological tests show that LSA-treated samples have a lower coefficient of friction (COF) and specific wear rate, especially under dry friction conditions, due to increased surface hardness. AG-treated samples have a more stable COF because the oxide layer reduces adhesive wear, grinding-layer provides solid lubrication and reduces localized contact stress.

Mechanically stable superhydrophobic surfaces constructed by laser surface texturing and micro-arc oxidation of TC4 alloy prepared based on SLM

In this paper, a superhydrophobic surface was prepared on the surface of Ti–6Al–4V (TC4) alloy prepared by selective laser melting (SLM), and the water contact angle (WCA) was 156.8°. The laser surface texturing (LST) and chemical modification treatments were combined with a micro-arc oxidation (MAO) treatment to improve the mechanical durability of the superhydrophobic surface. The surface morphology, wettability, wear resistance, and abrasion resistance of the composite coatings were analyzed, and the mechanism of abrasion resistance on superhydrophobic surfaces was discussed. The superhydrophobic surface that was prepared withstood abrasion under 600 # sandpaper at a pressure of 2.9 kPa and a movement distance of 1020 cm. In addition, the prepared superhydrophobic surface showed better wear resistance and friction reduction in friction experiment, exhibiting more stable coefficient of friction (COF) curve, lower COF value, and narrower abrasion marks. The results show that the prepared composite coatings not only exhibit superhydrophobicity, but also good mechanical stability, which effectively improves the mechanical property of superhydrophobic sample and significantly prolongs its service life in harsh environments. This study provided a new idea and preparation method for the preparation of mechanically stabilized superhydrophobic surface.

Preparation and tribological properties of WS2 solid lubricating coating with dense structure using HiPIMS

In this study, a dense WS2 coating was deposited via high-power impulse magnetron sputtering (HiPIMS) after solving the glow discharge problem, and it was compared with WS2 coating deposited via radio frequency (RF) sputtering. The latter showed a columnar growth, with a loose and porous microstructure. The columnar crystal growth of the former was inhibited, resulting in a more compact coating structure and a more uniform, flatter and smoother surface. Therefore, the structure and tribological properties of WS2 coating prepared via HiPIMS were investigated under different loads and frictional frequencies. Under low loads (up to 15 N), the coefficient of friction (COF) decreased with polishing wear mechanism. Under a high load of 20 N, the COF rapidly increased, the wear rate was the highest, and the wear mechanism changed from polishing to abrasive wear. At low frequencies (5 and 10 Hz), the coating had a low and stable COF, good wear resistance and the wear mechanism was abrasive wear. At high frequencies (≥15 Hz), the COF and wear rates rapidly increased and the wear mechanism changed from abrasive to adhesive wear and plastic fracture. However, the friction and wear rates were similar across different wear mechanisms.

Effects of Al2O3 powder characteristics on microstructure, mechanical and tribological properties of Inconel 625-Al2O3 non-skid coatings prepared by plasma enhanced high-velocity arc spraying

The effects of the Al2O3 powder characteristics on the microstructure and properties of Inconel 625-Al2O3 non-skid coatings were investigated. The friction coefficient of the coatings increased with the size of the Al2O3 powder, which is beneficial for non-skid performance. However, the porosity and microdefects increased accordingly, degrading the mechanical properties and wear resistance. Among them, 25 wt% nano/micro Al2O3 powder is a better solution. m-Al2O3 with large size provided a high and stable friction coefficient, and the diffuse n-Al2O3 could repair and optimise the structural homogeneity. Meanwhile, n-Al2O3 enrichment areas on the wear scar result in excellent wear resistance. The in-flight sprayed particle size distribution could be adjusted by changing the Al2O3 powder particle size, enabling effective friction coefficient control.

Facile fabrication of Co-containing coating to enhance the wear resistance of 24CrNiMo steel at elevated temperature

High-speed train brake discs, relying on surface friction for braking, can reach up to 600 °C, significantly increasing wear and adversely affecting performance and lifespan. Here, we propose a facile approach employing ultrasonic shot peening with the addition of cobalt (Co) particles (termed UEG) to creates a dense Co-containing coating on the 24CrNiMo steel. Friction tests show that UEG samples exhibit more stable friction coefficients and lower wear rates at temperatures ranging from 25 to 600 °C. Experimental and analytical results reveal a higher dislocation density in the UEG-treated samples surface and the formation of a dense Co-enriched tribofilm, effectively enhancing wear resistance. The findings indicate that UEG has considerable promise for enhancing the wear resistance of brake discs in high-temperature environments.

Microstructure, mechanical properties and wear behavior of Mg matrix composites reinforced with Ti and nano SiC particles

This study examines the mechanical properties and wear mechanisms of magnesium (Mg) metal matrix composites reinforced with titanium (Ti) and silicon carbide (SiC) particles. Three different composite formulations were investigated: Mg-30 wt% Ti (A), Mg-25 wt% Ti-5 wt.% SiC (NB), Mg-20 wt% Ti-10 wt% SiC (NC), and Mg-15 wt% Ti-15 wt% SiC (ND). These composites were fabricated through ball milling and spark plasma sintering (SPS). The incorporation of SiC particles significantly enhanced grain refinement and phase formation within the composites. Density analysis revealed that the actual densities of the composites were lower than the theoretical values, with Composite A exhibiting the highest actual density of 2.15 g/cm³ and the lowest porosity of 16.31%. The introduction of SiC particles increased porosity, with Composite NB displaying the highest porosity at 30.58%. Hardness testing indicated that Composite NC, containing 10 wt% SiC, achieved the highest hardness of 137 HV. In contrast, Composite ND, with 15 wt% SiC, showed a reduced hardness of 115 HV, attributed to increased porosity and potential SiC particle agglomeration. Wear behavior was evaluated using a pin-on-disc tribometer. Weight loss measurements indicated that Composite A had the lowest weight loss (1.1–2.1 mg), while Composite NB experienced the highest weight loss (2.8–8.3 mg) due to increased porosity. Composite NC demonstrated a balance with moderate weight loss (2.0–3.8 mg). The coefficient of friction (COF) varied with SiC content and applied loads (2, 4, and 8 N), with Composite A demonstrating the lowest COF values (2.3–2.8) and stable performance across different loads. Composite NB exhibited higher COF values (3.5–4) and significant fluctuations due to elevated porosity and the presence of SiC particles. Composite NC showed better wear resistance and more stable COF values (2.5–2.9) compared to NB and ND.

Fabrication of gradient structure for enhancing wear resistance of GCr15 bearing steel friction shim

The wear resistance of metallic shims under heavy load is crucial for the friction behavior of friction damper in civil engineering. This study verifies the potential of the strengthening grinding process (SGP) to enhance the wear resistance of metallic materials under heavy load. A multidisciplinary effort is shown in this paper to investigate the wear mechanism of SGP-treated GCr15 bearing steel in terms of the application scenario of civil engineering. The SGP treatment uses a multiphase abrasive comprising steel balls, corundum powder, and strengthening liquid to impact the surface of the GCr15 bearing steel. Then, a dry friction sliding test was performed on the polished, sandblasted, and SGP-treated GCr15 bearing steel. The microtopography and microstructure of these three friction shims with different treatments before and after the wear test were compared and analysed. The test results indicated the feasibility of SGP in enhancing the wear resistance of GCr15 bearing steel friction shims under heavy loads in terms of the application scenario of civil engineering. The SGP-treated sample had a more stable friction coefficient (0.475) and slighter wear compared to the other two samples due to its enhanced layer with a gradient structure and a thickness of approximately 23 μm.

Technical overview of the main types, designs, and materials of brake pads for mobile agricultural machinery

Brake pads are a critical element of any machine, as they directly affect the safety of its use. Accordingly, the quality of brake pads, their resistance, and durability are key aspects that must be considered when developing braking systems for mobile agricultural machinery. The purpose of this study was to review scientific sources related to the study of the tribological properties of brake pads, their operating modes, and friction materials included in brake linings. The main parameters affecting the efficiency of brake pads were analysed and the main criteria for selecting materials for brake pads of mobile agricultural machinery were defined, namely, wear resistance, temperature resistance, and corrosion resistance. Accordingly, the materials used in the production of brake pads for such equipment must be capable of operating under any conditions, have high thermal conductivity, help reduce the wear rate, have a stable friction coefficient, and be environmentally friendly. The study focused on an overview of the types and design of brake pads, their systematisation by various features (by purpose; by design features; by friction material composition; by the presence of wear sensors) and composition (semi-metallic, non-asbestos organic, and ceramic). The study described modern components of friction materials for brake linings and found that they are usually composites formed by hot pressing coarse powders, which include many different components: a binder (thermosetting phenolic resins, often with rubber added), structural materials (metal, carbon, glass, and/or Kevlar fibres), fillers (mica and vermiculite), and friction additives (graphite and various metal sulphides). The study also assessed the main characteristics of friction material components used in the manufacture of brake linings. The findings of this study can provide researchers and scientists with useful information on the types and design of brake pads and the main materials used in the manufacture of brake linings and be useful for further practical development of braking mechanisms

Variable cycle fretting conditions meditated wear behavior and mechanism in a self-lubricating composite coating on TC21 titanium alloy substrate

To simulate the variational working conditions of the titanium fastener in the aero-engine, variable cycle fretting wear tests were carried out for normal loading (80/40 N), frequency (100/30 Hz), displacement amplitude (80/40 µm) and operating temperature (500/350 °C) in a self-lubricating composite coating on TC21 titanium alloy substrate, and the fretting wear behavior and wear mechanism were thoroughly investigated. The results shows that the coefficient of friction (COF) and the wear volume present a significant difference under variable cycle fretting conditions. The specimen exhibits a relatively stable COF under variable operating temperature. Whereas for the specimen under variable displacement amplitude presents a lowest wear volume. The adhesive wear is mainly found in the specimens under variable normal loading and frequency. Whereas for the specimens under variable displacement amplitude and operating temperature, abrasive wear and oxidation wear is mainly dominated, respectively. The dissipated energy calculation results demonstrate that the variable displacement amplitude and operating temperature have greater impact on the fretting wear behavior. The microstructure analysis in the cross-section of specimens reveals that the sub-surface is composed of amorphous-nano crystalline layer, plastic deformation layer and the origin coating. Additionally, the wear mechanism of the specimens under variable fretting conditions were deeply revealed. This work can provide essential information on fretting behavior and mechanism under the variational working conditions.

Anisotropic tribology and electrification properties of sliding-mode triboelectric nanogenerator with groove textures

Sliding-mode triboelectric nanogenerator (S-TENG) is based on the coupling of triboelectrification and electrostatic induction, converting electrical energy from sliding motion. Introducing micro-textures into the sliding surface, and adjusting the angle between the texture and sliding direction (direction angle) may achieve performance anisotropy, which provides novel ideas for optimizing the tribology and electrification performance of S-TENG. To guide the performance optimization based on the anisotropy, in this paper, groove micro-textures were fabricated on the surface of S-TENG, and anisotropic tribology and electrification performance were obtained through changing the direction angle. Based on the surface analysis and after-cleaning tests, the mechanism of the anisotropy was explained. It is shown that the anisotropy of friction coefficient can be attributed to the changes of texture edge induced resistance and groove captured wear debris, while the voltage anisotropy is due to the variations of debris accumulated on the sliding interface and the resulting charge neutralization. Among the selected 0°–90° direction angles, S-TENG at angle of 90° exhibits relatively small stable friction coefficient and high open-circuit voltage, and thus it is recommended for the performance optimization. The open-circuit voltage is not directly associated with the friction coefficient, but closely related to the wear debris accumulated on the sliding interface. This study presents a simple and convenient method to optimize the performance of S-TENG, and help understand the correlation between its tribology and electrical performance.

Effects of in situ formed nanostructures on the structure and properties of C/SiOC composites

AbstractIn this paper, Ni was introduced into the matrix of carbon fiber reinforced SiOC (C/SiOC) composites by the precursor infiltration and pyrolysis method to catalyze the generation of in situ formed nanostructures including carbon nanotubes (CNTs), turbostratic carbon, and SiC. The effect of in situ formed nanostructures catalyzed by Ni on the properties of the C/SiOC composites was investigated. The results demonstrated that the presence of in situ formed nanostructures in the matrix improved the densification and mechanical properties of C/SiOC composites. In addition, the incorporation of in situ formed nanostructures facilitated the formation of thermal conductivity pathways within the composites, thereby enhancing the thermal conductivity of C/SiOC composites. Furthermore, in situ formed nanostructures‐containing C/SiOC composites developed a carbon friction layer comprising CNTs and turbostratic carbon, which significantly enhanced the abrasion resistance, endowing C/SiOC composites stable friction coefficients, and lower levels of wear.

Excellent lubricating performance of a novel polysaccharide lubricant on reducing friction-induced deformation

Minimizing the deformation caused by friction on surfaces enabled to maintain a stable friction force and coefficient of friction (COF), and valuable insights could be gained from studying bio-based lubrication systems. A natural polysaccharide was derived from fresh okras in order to enhance the lubricating efficiency of aqueous system. The lubricating performance and mechanism of okra polysaccharide were unveiled through the implementation of multiple wear tests between the steel and PA66. Particular attention was paid to the relationship between the deformation, friction force and COF. The results indicated that the surface fluctuated deformations caused comparable fluctuations in both the friction force and the COF. However, the inclusion of okra polysaccharide at a concentration of 2 wt% significantly diminished the amplitudes of friction force by decreasing surface deformations. As a result, almost 50% decrease in both COF and wear loss were observed. The lubricating mechanism of okra-polysaccharide lubricant could be described as the interaction between the hydrophilic groups and water molecules, which forms a hydration layer on the surfaces in contact, thus reducing shear force. The acquired knowledge in this study was used to develop a green lubricant that addresses the issue of instability in frictional systems caused by friction-induced deformation.