Oil-lubricated Conditions Research Articles

Tribological Properties of 7A04 Aluminum Alloy Enhanced by Ceramic Coating

The 7A04 Al alloy is a commonly used lightweight metal material; however, its low wear resistance limits its application. In this study, the wear resistance of this alloy was improved by preparing micro-arc oxidation (MAO) coatings, MAO/MoS2 composite coatings, and hard-anodized (HA) coatings on its surface. The friction and wear behaviors of these three coatings with diamond-like coated (DLC) rings under oil lubrication conditions were investigated using a ring–block friction tester. The wear rates of the coatings on the block surfaces were determined using laser confocal microscopy, and the wear trajectories of the coatings were examined using scanning electron microscopy. The results indicated that, among the three coatings, the MAO/MoS2 coating had the lowest coefficient of friction of 0.059, whereas the HA coating had the lowest wear rate of 1.47 × 10−6 mm/Nm. The MAO/MoS2 coatings exhibited excellent antifriction properties compared to the other coatings, whereas the HA coatings exhibited excellent anti-wear properties. The porous structure of the MAO coatings stored lubricant and replenished the lubrication film under oil lubrication. Meanwhile, the introduced MoS2 enhanced the densification of the coating and functioned as a solid lubricant. The HA coating exhibited good wear resistance owing to the dense structure of the amorphous-phase aluminum oxide. The mechanisms of abrasive and adhesive wear of the coatings under oil lubrication conditions and the optimization of the tribological properties by the solid–liquid synergistic lubrication effect were investigated. This study provides an effective method for the surface modification of Al alloys with potential applications in the aerospace and automotive industries.

Influence of triangular texture composite MAO coating on the tribological properties of aluminum alloys

To compensate for the poor friction and wear performance of aluminum alloys, in this study, four triangular textures with different area occupancy rate (5.16 %, 9.17 %, 20.63 % and 28.07 %) were prepared on the surface of aluminum alloys by laser texture technology, subsequently micro-arc oxidation (MAO) coatings were prepared using MAO technique. The treated samples were characterised by X-ray diffraction, micro-morphological analysis, optical profiler and energy dispersive spectroscopy. The tribological performance under oil lubrication conditions was tested by using a pin-disc contact rotational friction wear tester. The results showed that the sharp corners at the edges of the texture caused current concentration, which resulted in more intense sparking and accelerated MAO growth, thus creating more pores and roughening the surface of the coating. In terms of tribological performance, the triangular texture with 9.17 % area occupancy rate showed the best tribological performance before MAO coating, compared with the substrate, the average friction coefficient decreased by 64.11 %; and after the MAO coating was applied, the triangular texture composite MAO coating with 9.17 % area occupancy rate still had the best tribological performance, and its average friction coefficient further decreased compared with that of the smooth MAO coating by 33.07 %. The analysis suggests that the texture can store abrasive particles to prevent the MAO coatings from being worn through, leading to coating failure, while the hard MAO coatings can resist contact stresses well enough to prevent the texture from being flattened, leading to the failure of the function of storing abrasive debris. Therefore, the integration between texture and MAO coating can greatly enhance the anti-wear properties of aluminum alloys.

Increasing the friction and wear performance of AISI 316L under dry, aqueous and oil lubricated sliding conditions with surface textures fabricated by laser-powder bed fusion (L-PBF)

In this study, it is aimed to examine the effects of surface textures on the friction and wear performance of materials under different sliding conditions (dry, aqueous and oil). AISI 316 L samples with hexagonal, circle, square and rhombus geometries were produced using Laser Powder Bed Fusion (L-PBF) and non-textured samples were produced using casting and L-PBF. L-PBF samples showed high peak intensity due to rapid heating-cooling and small shifts in Bragg angles occurred. L-PBF samples had higher hardness than casting samples since they had a finer microstructure. Textured samples exhibited better tribological performance because they trapped wear debris. A higher friction was observed in samples with sharp corners due to stress concentrations. The highest tribological performance occurred in oil-lubricated conditions.

Wear-resistant phenolic composites reinforced with attapulgite and short carbon fibers for applications subjected to both dry and oil-lubricated sliding

Attapulgite (AT) nanofibers were integrated into phenol-formaldehyde resin (PF) and PF composites reinforced with short carbon fibers (CF). Inclusion of AT significantly enhances the wear resistance of PF under both dry and oil-lubrication conditions. The hybrid nanocomposites, reinforced with both AT and CF, exhibit a high mechanical strength. Interestingly, AT and CF lower synergistically the friction coefficient of PF subjected to harsh dry sliding conditions. The hybrid nanocomposites exhibit excellent tribological performance under both dry and oil-lubrication conditions. Tribofilms’ nanostructures were comprehensively analyzed for disclosing dominant tribological mechanisms. Outcome of this work provides new ideas for developing tribo-materials of a high adaptability to various dry/oil-lubrication conditions.

Tribological Characteristics of Fibrous Polyphthalamide-Based Composites.

The aim of this study was to investigate the tribological characteristics of commercially available high-strength polyphthalamide-based composites with great contents (30-50 wt.%) of both carbon and glass fibers in point and linear contacts against metal and ceramic counterfaces under dry friction and oil-lubricated conditions at various loads and sliding speeds. The lengths of both types of fibers were varied simultaneously with their contents while samples were fabricated from granules by injection molding. When loading PPA with 30 wt.% SCFs at an aspect ratio (AR) of 200, the ultimate tensile strength and the elastic modulus increased up to 142.7 ± 12.5 MPa and 12.9 ± 0.6 GPa, respectively. In the composites with the higher contents of reinforcing fibers PPA/40CCF and AR~1000, the ultimate tensile strength and the elastic modulus were 240 ± 3 MPa and 33.7 ± 1.9 GPa, respectively. Under the applied test conditions, a composite reinforced with 40 wt.% carbon fibers up to 100 μm long at an aspect ratio of ~1000 possessed the best both mechanical properties and tribological characteristics. One of the reasons that should be considered for improving the tribological characteristics of the composite is the fatigue wear mechanism, which is facilitated by the high filling degree, the strong interfacial adhesion, and the great aspect ratio for fibers. Under the oil-lubricated conditions, both friction coefficients and wear rates decreased, so such friction units could be implemented whenever possible. The reported data can be used as practical recommendations for applying fibrous polyphthalamide-based composites as friction unit components.

The influence of pin inclination on frictional behaviour in pin-on-disc sliding and its implications for test reliability

The pin-on-disc test is a widely employed method for investigating the friction and wear performance of materials in conformal contact. In a typical pin-on-disc system, self-aligning pin holders are frequently utilized to ensure proper surface contact. This study reveals that in such a setup, pin inclination has a significant impact on test reliability, particularly under oil-lubricated conditions, which may overweight the influence of other parameters such as roughness or texture elements. Utilizing in-situ measurements, we captured the dynamic changes in pin inclination during rotational sliding. Our findings indicate that the pin inclination varies with sliding speed, showing a pitch angle difference of approximately 0.01°as the speed decreases from 2 m/s to 0.04 m/s in our test setup. Importantly, a robust correlation was identified between the friction coefficient and pin inclination, which is supported by the numerical investigation. This study underscores concerns regarding the test reliability of pin-on-disc tribometers, prompting a reconsideration of the assumptions associated with self-aligning pin holders in such experimental configurations.

Analysis of wheel-rail contact under oil lubrication conditions

Abstract This investigation centers on the characteristics of the contact between the wheel and rail when lubricated with oil, with a consideration of the impact of surface roughness. Confronting significant challenges related to low adhesion, a thorough examination is undertaken to analyze the changes in film thickness under oil lubrication, investigating trends at varying vehicle speeds and axle loads. Employing detailed modeling based on micro-elastohydrodynamic theory, the distribution of oil film thickness under diverse operational conditions is determined. The findings indicate that, under oil lubrication, a film thickness in the range of several tens of micrometers is established, effectively insulating the surfaces of the wheel and rail, thereby reducing adhesion. With increasing vehicle speed, the thickness of the oil film also rises, whereas an elevation in axle load results in a marginal decrease in film thickness. This research contributes to a comprehensive comprehension of the wheel-rail contact mechanism in oil lubrication scenarios, providing theoretical backing to address concerns related to low adhesion. The implications are substantial for improving the safety and economic efficiency of efficient railway transportation.

A Novel Unsupervised Domain Adaptation Transformation Reconstructed Gru Framework Considering Prediction Uncertainty for Machinery Prognostics Under Variable Lubrication Conditions

Abstract As critical components in industrial application scenarios, high-precision and high-confidence health assessment of rolling bearings attract more and more attention. Currently, predictive maintenance obtains outstanding achievements under the same object and working conditions. However, evaluation performances under variable working conditions and different specifications still need to be improved. This study zeroes in on the cross-domain prognostics of rotating machinery under oil and grease lubrication conditions. It proposes an unsupervised domain adaptation transform reconstruction GRU (UDATrGRU) prognostics framework, which captures the common degradation characteristics under different lubrication conditions through the designed second-order statistical quantity, facilitating the following high-precision predictions. To be specific, the vibration degradation features are first extracted through signal preprocessing and then input into UDATrGRU. The developed domain adaptation layer calculates high dimensional projections between diverse data sets, and then corresponding degradation features are statistically aligned under the pressure of the designed quantity. Subsequently, time series modeling and Bootstrap-based uncertainty estimations are carried out. Finally, life-cycle accelerated tests of the rolling bearing from PRONOSITA and ABLT-1A cross-validate the feasibility ' effectiveness of the proposed machinery prognostics framework. The results are pretty promising: compared to existing methods, our UDATrGRU framework has achieved an improvement of at least 5.65% in R2 and a reduction of at least 21.5% in RMSE.

Visualization of oil-lubrication ball bearings at high rotational speeds

Deep-groove ball bearings for the eAxles of electric vehicles must adapt to higher rotational speed conditions because the speed of eAxle motors have been increasing as the size and weight of the motors decrease. Therefore, understanding the oil-lubricated conditions inside ball bearings at high rotational speeds is essential for optimizing their design for eAxles. To clarify the oil-lubricated conditions inside ball bearings at these high speeds, a new test apparatus was developed. This apparatus is capable of simultaneously measuring the friction torque of deep-groove ball bearings, the oil-film thickness on the rolling balls, and observing the oil distributions inside the bearings at rotational speeds up to 20,000 min-1. The oil-film thickness was measured using three-wavelength optical interferometry, and the oil distribution was observed using fluorescence. It was found that the oil-film thickness became constant at rotational speed conditions exceeding approximately 7,700 min-1. Oil starvations were observed on the raceway around the rolling ball, and these regions increased with increasing rotational speeds. Additionally, in the deep-groove ball bearing with a crown-shaped cage, the oil was mainly supplied to the rolling balls from the inner ring side through the space between the cage claws that held the ball. Moreover, the amount of mixed air tended to increase as the rotational speed increased to approximately 7,700 min-1. Those oil starvations and increasing air in oils were considered to be factors that prevent the increase in oil-film thickness. The findings of the reported study will contribute to the development of multibody dynamic technology for high-speed ball bearings necessary in electric vehicles.

Tribological Performance Study of Low-Friction PEEK Composites under Different Lubrication Conditions

This study introduces a low-friction composite based on PEEK to improve its friction and wear properties. The composite incorporates PTFE as a solid lubricant and utilizes PPTA as a reinforcing material within the PEEK matrix. These components were prepared utilizing a compression molding method, followed by a series of exploratory experiments to identify the optimal preparation conditions for PEEK. This research assesses how the PTFE/PPTA/PEEK composites perform in terms of friction and wear under dry and oil-lubricated conditions. By examining wear tracks using scanning electron microscopy and white light interference microscopy, this study aims to uncover the wear mechanisms of PEEK and its composites under different lubrication scenarios. Results show that the main wear mechanisms for the PTFE/PPTA/PEEK composites and bearing steel are ploughing and adhesive wear. The presence of PPTA helps reduce wear by leveraging its strong fibers and thermal stability, while the coefficient of friction decreases as PTFE creates a smooth, solid lubricating film on the surface. Notably, PEEK composites containing 25 wt% PTFE and 6 wt% PPTA demonstrate the lowest wear rates and reduced coefficient of friction in both dry and oil-lubricated conditions.

Study on the tribological characteristics of surface triangular textured TC4 alloy prepared by SLM technology

Surface Triangular Textured TC4 Alloy specimens were prepared by 3D printing technology (SLM technology: selective laser fusion). Effects of wedge-shaped triangular textures with different texture area occupancy on hardness, friction and wear of the 3D printed TC4 alloy specimens were studied, in comparison with non- textured specimens. SFT -2 M pin-disc friction and wear testing machine was used to perform friction and wear experiments under 46 # mechanical oil lubrication conditions, and MH-500 Vickers hardness tester was used to measure hardness of the specimens. Scanning electron microscope (SEM), energy spectrometer (EDS), and electronic balance instrument were used to characterize the wear scar morphology, element content, wear amount, etc. of the specimens. Using the Fluent module in ANSYS Workbench software, we studied the mechanism for the reduction in the friction coefficient between friction pairs during the relative movement in lubricating oil. Results of the study show that the hardness of the 3D printing specimen is significantly greater than that of the non- textured one. In each group of experiments, the wedge triangular textured specimens showed better tribological properties, and the 3D printed specimen with an area occupancy of 22. 53 % exhibited the smallest average friction coefficient (0.17) and higher wear resistance than other specimens. The dynamic pressure effect due to the wedge triangle texture inside the lubricating oil film is the main reason for the reduced friction coefficient between the friction pairs.

The Tribological and Mechanical Properties of Polyimide/Polyamide Imide/Epoxy Resin, Polytetrafluoroethylene-ZrO2 Coating on an Engine Bearing

To improve the poor tribological performances of the copper alloy used in engine bearings under boundary lubrication and dry sliding wear, a new polymer coating consisting of polyimide/polyamide imide/epoxy resin (PI/PAI/EP), polytetrafluoroethylene (PTFE) and ZrO2 has been designed. The tribological and mechanical characteristics of the coatings at various thicknesses were studied by friction and wear experiments, nanoindentation experiments, simulation experiments of the stress and deformation and fluid-structure coupling simulation tests. The results showed that when the thickness of the coating was 20 μm, the deformation of the coating was the lowest and the friction coefficient of the coating was 0.167. As the thickness of the coating was increased, the support effect of the substrate on the coating decreased, and the thickness of the coating increased under positive pressure. Compared with the substrate, the friction coefficient of the coating under boundary oil lubrication conditions was reduced by 53.85%. The fluid-structure coupling calculation showed that the boundary lubrication and dry sliding wear of the copper alloy were improved by the designed coating. The fluid-structure coupling calculation showed that the peak variation rates of the coating bearing pressure was 0.02%. The coating on the surface of the bearing had little influence on the oil pressure, but at the same time, improved the tribological properties of the engine bearing.

Long-Term Stable Superlubricity Coatings Enabled by the Interaction between the Polydimethylsiloxane Brush and Silicone Oil.

Recently, materials with superlubricity captured widespread attention on account of their great potential in energy savings and environmental protection. However, certain issues still remain to be solved for the traditional materials, such as the dependence on strict conditions and an unstable superlubricity state. Herein, a long-term stable superlubricity coating was prepared using a low-cost and simple method via an epoxy-based coating with polydimethylsiloxane (PDMS) brushes under silicone oil (SO) lubrication conditions. Compared with the pure epoxy resin matrix, the friction coefficient and wear track width of the superlubricity coating with the optimal amount of 6 wt % PDMS are reduced to 0.006 and 50.9 μm (reduced by 10-fold and 5.6-fold decrease, respectively). In addition, the coating can maintain a stable superlubricity state during a 5 h tribological test. The superlubricity of the coating results from the synergistic lubrication effect of the PDMS brush and SO. First, PDMS brushes with high-stretched conformation due to the swelling effect of the SO can significantly reduce friction. Second, a stable oil film is generated between the contact surfaces, which significantly improves the frictional performance. Moreover, the PDMS incorporated into the coating matrix, along with oil-swelling PDMS brushes on the surface, is highly beneficial for enhancing corrosion resistance of the epoxy resin matrix. Such an epoxy-based coating with long-term stable superlubricity is considered as a potential lubricating and protective surface for tribological components for long-term service.

Effect of Micro-Dimple Texture on the Tribological Performance of Brass with Titanium Nitride (TiN) Coating under Oil-Lubricated Conditions

Surface texture and titanium nitride (TiN) coating have been established as effective methods for enhancing the tribological property of mechanical friction pairs. This study aims to investigate the tribological performance of dimple-textured surfaces with TiN coating under oil-lubricated conditions using a pin-on-disc wear experiment. Four types of pin samples with various end surfaces were designed, including bare rod samples, TiN-coated samples, textured samples, and TiN-coated/textured samples. The surface texture consists of a series of cylindrical micro-dimples with a diameter of 150 μm and a depth of 40 μm fabricated on the end surface of the pin. TiN coating treatment on the textured surface of the end face was performed by vacuum sputtering coating equipment. The study focuses on measuring and comparatively analyzing the friction coefficient, wear morphology, and binding force of the pin-disc friction pairs among the experiments. Compared with bare rod samples, TiN-coated/textured samples will reduce the friction coefficient (COF) of the pin-on-disc friction pair by at least 20% under oil-lubricated conditions in a 50 N normal contact load. The results indicate that the synergistic effect of dimple surface texture and TiN coating optimizes friction performance and reduces wear, highlighting the novelty of this study. Furthermore, the study identifies the hydrodynamic lubrication effect of the surface morphology formed by the dimple surface texture as a key factor in improving lubrication performance and reducing friction. Additionally, the dimple surface texture enables the mitigation of third body wear due to the wear debris storage function of the micro-dimples. This research provides valuable insights for the design and fabrication of mechanical friction pairs with high wear resistance under oil-lubricated conditions.

Effect of applied load on tribological behaviour of Ti–48Al–2Cr–2Nb alloy under oil-lubricated condition

With their low weights and excellent mechanical properties, TiAl alloys are expected to be applied in pistons to achieve energy savings and emissions reductions in internal combustion engines. The tribological behaviours of the Ti–48Al–2Cr–2Nb alloy against the QT600-3 material at different applied loads under oil-lubricated conditions were investigated in this study. The results showed that as the load increased, the friction coefficient and wear-scar volume significantly decreased. The wear surface of the TiAl alloy was analysed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The wear mechanism of the TiAl alloy under oil lubrication conditions was mainly the combined effect of adhesive wear and oxidation wear, different from the wear mechanism under dry-friction conditions. Furthermore, by observing the cross-sectional morphology of the TiAl alloy after focused ion beam, it was found that the QT600-3 wear debris had compacted and accumulated on the surface of the TiAl alloy, preventing further contact between the TiAl alloy and the QT600-3. This study has the potential to accelerate the application of the Ti–48Al–2Cr–2Nb alloy in lightweight pistons.

Effect of Bi Content on the Microstructure, Mechanical and Tribological Properties of Cu-Sn Alloy.

To reduce the use of the toxic Pb element in the Cu-Sn alloy with high friction performance, Cu-xBi-10Sn alloys with different Bi contents were prepared by gravity casting, and the effect of Bi content on the microstructure, mechanical properties and wear property of Cu-Sn alloys were studied. The results showed that the Bi element was distributed in bands or long strips on the dendritic arms and did not form compounds with other elements. With the increase in Bi content, the hardness and tensile strength of Cu-xBi-10Sn alloys present a trend of increasing first and then decreasing. When the Bi content was 7 wt.%, the maximum hardness value was obtained, and the ultimate tensile strength was close to that of Cu-10Pb-10Sn alloy. Compared with Cu-10Pb-10Sn alloy, Cu-7Bi-10Sn alloy also possessed better friction reduction and wear resistance under the oil lubrication condition.

Effect of Ultrasonic Surface Rolling on the Fretting Wear Property of 7075 Aluminum Alloy

This paper investigates the effect of ultrasonic surface rolling (USR) on the fretting wear properties of the 7075 aluminum alloy. A white light interferometer, Vickers hardness tester, and X-ray diffractometer were employed to comparatively analyze the variations in surface roughness, hardness. and grain size before and after the USR treatment. The fretting tests were carried out under oil lubricated and dry fretting conditions, using a ball-on-flat contact tangential fretting tester. The worn surface morphology, wear debris, and chemical composition were analyzed using an optical microscope (OM), a scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), etc. The results revealed that after USR treatment, the surface roughness was reduced by 90%, the hardness was increased by 13HV, and the grains were refined. Moreover, the wear was reduced under oil lubrication conditions but increased under dry fretting conditions. It can be concluded that the microstructure and mechanical properties of the 7075 aluminum alloy surface can be enhanced by the USR treatment. The improved fretting performance in oil should be attributed to the increased surface hardness, which helps reduce abrasive wear and plastic deformation. However, under dry fretting conditions, the wear was increased due to the presence of hard debris particles that peel off from the USR-treated surface, leading to aggravated abrasion.

Friction mechanism of DLC/MAO wear-resistant coatings with porous surface texture constructed in-situ by micro-arc oxidation

A porous textured DLC/MAO multilayer coating was constructed in-situ on Al alloy by depositing a diamond-like carbon (DLC) film onto a micro-arc oxidation (MAO) layer. The morphology, composition and tribological properties were investigated. The wear mechanism of the coating and the effect of load on wear were studied in detail under dry and oil-lubricated conditions. The results showed that DLC/MAO coating had excellent load-bearing capacity and wear resistance. The porous texture was generated in-situ on the aluminum alloy substrate by micro-arc oxidation technology, which played a mechanical interlocking effect in making the multi-layer coating tightly bonded. It can also capture wear debris to reduce abrasive wear under dry conditions while storing lubricating oil to supplement the lubricating film under oil-lubrication. DLC top film optimized the frictional resistance induced by the porous texture through graphitization and low-shear transfer film formation, reducing up to 70.7 % and 54.9 % in friction coefficient and wear track width under dry conditions. Under oil lubrication, the solid-liquid synergy between the graphitized layer and the liquid lubricating film can reduce the friction coefficient and wear track width by up to 20.9 % and 52.1 %. Hence, the combination of DLC top film and MAO tie layer realizes excellent anti-friction/wear and durability.

Friction reactions induced by selective hydrogenation of textured surface under lubricant conditions

The passivation of hydrogen atoms and the conformation of textured surfaces under oil-lubricated conditions are effective strategies to obtain amorphous carbon (a-C) films with extremely low friction. It is critical to understanding the influence mechanism of selective surface hydrogenation on the tribological behaviors of textured a-C film under oil-lubricated conditions. In particular, the interactions of hydrogen atoms and lubricants are confusing, which is enslaved to the in situ characterization technique. The reactive molecular dynamics (RMD) simulations were conducted to analyze the friction response of textured a-C films with selective hydrogenation surfaces under oil-lubricated conditions. The results indicate that the existence of hydrogen atoms on specific bump sites significantly decreases the friction coefficient (μ) of textured a-C film, which is highly dependent on the surface hydrogen content. The repulsion between hydrogen atoms and lubricant molecules prompts the formation of a dense lubricant film on the surface of the mating material. Interestingly, with the enhancement of the surface hydrogen content, the passivation of the friction interface and the repulsion between hydrogen atoms and lubricants play dominant roles in reducing the friction coefficient instead of hydrodynamic lubrication.

The Synergistic Lubrication Effects of h-BN and g-C3N4 Nanoparticles as Oil-Based Additives for Steel/Steel Contact.

The synergistic effect of different types of solid particles in liquid lubricants is of great interest. In this work, g-C3N4 nanosheets were initially prepared using a calcination method and then as-prepared, and h-BN were used as lubricating additives to the white oil. A comparison between the mixed additives and the single g-C3N4 or h-BN additives revealed that the base oil with the addition of g-C3N4 and h-BN showed the best lubricating properties. The results show a 12.3% reduction in friction coefficient, resulting in a 68.6% reduction in wear rate compared to the white oil when filled with 0.5 wt% g-C3N4 and h-BN (1:1 by weight). Moreover, the addition of g-C3N4 and h-BN improves the high-temperature lubrication properties of the white oil. However, the friction coefficient and wear rate increase with increasing oil temperature. The large contact area between g-C3N4 and its sliding counterpart and the strong adhesive force between h-BN and its sliding counterpart improve the film formation efficiency, leading to enhanced tribological properties under oil lubrication conditions.