High Normal Loads Research Articles

Experimental investigation of stick-slip behaviors in dry sliding friction

Friction-induced vibration, particularly the bothersome stick-slip behavior, is a significant contributor to machine damage and component failure. In this study, the vibrational displacement, sound pressure, and coefficient of friction (COF) of several metal or alloy samples in dry conditions are tested using a reciprocating friction force test apparatus. It is found that the stick-slip behavior is more likely to occur at low sliding speeds and high normal loads. Furthermore, under the same working conditions, increasing the surface roughness of the sample or enhancing the hardness difference between the sample and the counterpart can mitigate the stick-slip behavior during the friction process. By analyzing the characteristic parameters of the stick-slip behavior, it is found that the temporal period and the stick-time decrease significantly with an increase in relative sliding speed. Additionally, the amplitude and intensity of the friction-induced noise increases with the increase of the relative sliding speed and normal load. The frequencies of the sound pressure are primarily concentrated around the natural frequency of the frictional system. Finally, we extract the COF and find that the average COF as well as the maximum and minimum COF first decreases sharply, and then becomes stable with the increase of the relative sliding speed due to the transition of friction state from stick-slip behavior to smooth sliding.

Tribological properties under dry and lubricated sliding conditions of TiSiN and AlTiCrN coatings deposited on gray cast iron

In internal combustion engines, the majority of friction losses and wear occur between the cylinder liners and piston rings. Hard coatings applied to cylinder liners can increase the efficiency of internal combustion engines by improving tribological properties. The gray cast iron cylinder liner was coated with TiSiN and AlTiCrN employing the cathodic arc deposition process. The coatings were examined using nanoindentation tests, scratch tests, an optical profilometer, a scanning electron microscope, an X-ray diffractometer, and energy dispersive analysis to assess their hardness, adhesion, and structural characteristics. The reciprocating test was carried out to assess the coatings' wear and friction characteristics under lubricated and dry sliding with a ball at varying normal loads. The hardness of the TiSiN and AlTiCrN coatings was 45 GPa and 18 GPa, respectively. The cohesion strength, adhesion strength, and crack propagation resistance of AlTiCrN coating were approximately 2, 1.3 and 2.5 times higher than those of TiSiN coating, respectively. Both coatings exhibited remarkable wear performance at high normal loads. Under dry sliding conditions at 25 N normal load, the wear rate of AlTiCrN and TiSiN coatings were nearly 33 and 20 times, respectively, lower than the uncoated substrate. Under dry sliding conditions, the TiSiN coating exhibited the lowest wear rate at low normal loads (5 N and 15 N), while the AlTiCrN coating performed best at high normal loads (25 N). In lubricated sliding conditions, the AlTiCrN coating consistently showed the lowest wear rate across all normal loads.

Microstructural, mechanical, and sliding wear characterization of an Al90Cu4Fe2Cr4 spray-formed alloy

In this work, the microstructure and sliding wear behavior of a spray-formed Al90Cu4Fe2Cr4 (at%) alloy was studied. Mechanical properties under compression and Vickers microhardness were also evaluated. The microstructure of the spray-formed material consisted of an α-Al matrix reinforced with two complex intermetallic phases, λ-Al13Fe4 and Al13Cr2, which are binary based intermetallic structures, with significant amounts of the other elements of the alloy. The dry-sliding wear resistance of the composites was evaluated using a pin-on-disk test configuration, with normal loads ranging from 10 to 20 N and sliding velocities of 0.05–0.2 m.s−1. Alumina spheres were used as counter-bodies in these tests. The samples exhibited a high sliding wear resistance and showed almost no variation in the specific wear rates for all test parameters used, with values ranging from 7×10−4 to 8×10−4 mm3/N.m. The coefficient of friction also remained almost constant, around 0.4, and the main wear mechanism was the delamination of intermetallic particles. The results indicate an improved dry-sliding wear resistance at high normal loads compared to previously fabricated Al-matrix composites reinforced with quasicrystals and complex intermetallic phases. The mechanical properties of this alloy, fabricated in a single-step process, was in the same range as found for previous studies, of Al-matrix reinforced with micron-sized quasicrystals and intermetallics, prepared by more complex processing techniques. The simplicity of the one-step preparation process used in this study contributes to the development of more efficient and cost-effective tribological materials for various applications.

Frictional mechanisms of hydrated montmorillonite under normal loading

The friction behavior of clay particles plays an essential role in controlling its mechanical properties, but remains unclear in microscale. As one of the major clay particles, the nanoscale friction properties of hydrated montmorillonite have been studied using steered molecular dynamics (SMD) simulation method, considering the coupling effect of water content of 0 ∼ 30 % and normal load of 1 atm ∼ 10 GPa. The SMD friction along the y-direction has been performed to investigate the frictional mechanism of hydrated montmorillonite and its structural properties of interlayer water. The evolution of average friction load with water content, friction coefficients, and the distribution of interlayer water molecules were obtained. Furthermore, the SMD pulling along the z-direction was conducted to study the effect of interlayer water molecules on the interaction between montmorillonite layers. All simulation results showed that more energy and a higher friction load were required to slide at a higher normal load. The friction coefficient of 0.058 ∼ 0.17 and internal friction angle of 3.3°∼9.6° for hydrated montmorillonite agreed well with previous studies in microscale, showing the weak friction properties. The evolution of friction coefficient with water content was different at different normal loads, where the friction coefficient of 5 % hydrated montmorillonite was the highest at 3 ∼ 10 GPa. Moreover, the increasing normal load could change the distribution of interlayer water molecules.

Corrosion behavior, metal ions release and wear resistance of TiN coating deposited on SLM CoCrMo alloy by magnetron sputtering

In this study, the TiN coating was successfully deposited on selective laser melting (SLM) CoCrMo alloy via magnetron sputtering technique. The TiN coating microstructure and thickness were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical corrosion behavior, metal ions release and wear resistance of the SLM CoCrMo sample with and without TiN coating was compared. The results showed that there was some macroparticles and pores on the TiN coating surface and the TiN coating thickness was about 1.44 μm. The TiN coating helped the SLM CoCrMo sample to reduce the corrosion current density from 19.1 nA/cm2 to 8.12 nA/cm2 and improved the corrosion resistance but not significantly. Then, an explanation of the corrosion resistance changes of the TiN coating was given. In addition, the TiN coating could prevent the metal ions release such as Co, Cr and Mo of the SLM CoCrMo sample at short time immersion but lost the protecting effect at long time immersion due to the corrosion damage of the TiN coating. Furthermore, compared to the SLM CoCrMo sample, the TiN coating was conductive to improving the wear resistance only at lower normal load (5 N) but had similar wear resistance at higher normal load (10 N and 30 N). The reason was mainly attributed to the thinner TiN coating thickness and higher load led to the failure of the TiN coating.

An improved energy wear model of three-dimensional ball-plane contact structure and its fretting wear dynamic behaviors study

This paper proposes a method that combines the finite element method with an improved energy wear model to establish a three-dimensional (3D) ball-plane contact structures fretting wear model. The correctness of the model was verified by Hertz contact theory and fretting wear experiments. Then, the fretting wear dynamic behaviors of 3D ball-plane contact structures were investigated in detail. The results show that when the contact structure is in the partial slip regime (PSR), the cross-sectional wear scar profile changes from the “W” shape to the “U” shape along the fretting direction. When the contact structure is in the gross slip regime (GSR), the cross-sectional wear scar profile always maintains a “U” shape along the fretting direction. In addition, it was also found in the study that when the contact structure is at GSR under a low normal load or PSR under a high normal load, it will help reduce wear. This study can provide significant theoretical guidance for reducing and protecting fretting wear in contact structures.

Anomalous strain dependence of phononic friction between black phosphorus layers

This study reveals unexpected strain-dependent friction, when applied strain to one of two layers of relatively sliding black phosphorus, friction is likely to exhibit characteristic rise and subsequent fall, forming a distinct friction peak. Existence and location of such peak are closely related to magnitude of normal loads. Interfacial pinning is weak at lower normal load, matched dissipation channels are easy to be destroyed, resulting in a monotonic decrease of friction. At higher normal load, tip is also pulled apart once applying a small strain to substrate, stored deformation energy leads to greater energy dissipation than at strain of zero, causing friction to decrease non-monotonically. Moreover, ability of sliding layers to withstand strain decreases with increasing normal loads or temperatures.

Optimization of surface texture distribution on the thrust bearing using the mass-conserving cavitation boundary condition: Theory and experiments

This study developed a numerical optimization model for the texture distribution in thrust bearings, employing the flat-headed chevron and circular dimple, and utilizing a mass-conserving cavitation boundary. A hybrid of the genetic algorithm and sequential quadratic programming algorithm is utilized. The results indicate the highest load-carrying capacity can be obtained by optimizing the texture distribution based on the flat-headed chevron. The primary mechanism is that the optimal distribution can efficiently harness the collective effect of texture, thereby effectively building up pressure. The optimal distributions of the flat-headed chevron and circular dimples exhibit asymmetric chevron and herringbone shapes, respectively. Furthermore, verification experiments show that the optimal distribution can shift the transition point to a lower rotation speed and higher normal load.

Study on Shear Slip Characteristics of Sandstone Plane Joints under Normal Dynamic Load Disturbance

Abstract Rock joints are susceptible to slip instability due to dynamic load disturbances such as blasting, earthquakes, and fracturation. A series of direct shear tests under the dynamic load were conducted on sandstone plane joints using the RDS-200xl. The work investigated the effects of normal static loads and normal dynamic-load frequencies and amplitudes on plane joints. Besides, the following items were proposed, that is, the peak-to-valley response rate, shear velocity vibration dominant frequency, shear-stress reduction coefficient, and discrete element numerical simulation method for plane-joint direct shear tests. The results were as follows: (1) The normal dynamic load frequency played a role in attenuating the shear stress amplitude with a threshold value of 0.5 Hz. (2) The shear velocity of the plane joint was completely controlled by the high normal dynamic load frequency. Their vibrational dominant frequencies were identical. (3) The amplitude of shear stress increased, and the median stress decreased with the increased normal dynamic load amplitude. The reduction-coefficient equation for sandstone plane joints was proposed to evaluate the shear stress under the normal dynamic load disturbance. (4) The shear-stress hysteresis phenomenon existed in the plane joints under the normal dynamic load, which required excessive shear displacements to reach peak shear strength. The peak shear displacement increased with the increased normal static load. Numerical simulations and indoor tests showed that high- and low-shear-velocity regions were the main reason for shear-stress hysteresis. The findings are conducive to revealing the shear destabilization mechanism of rock joints under dynamic load disturbance.

The ultra-low friction achieved via proton-type ionic liquid with inorganic salts at steel/steel interfaces

Macroscale facilitated lubrication is a forward-looking strategy in modern tribology that can significantly reduce friction and wear in mechanical equipment. However, it has been studied experimentally mainly for special friction pairs or point-to-surface contact. In the present work, we have achieved the ultra-low friction for point-to-point contact at the steel/steel interfaces by using the inorganic salts (NaCl and KCl) and the proton-based ionic liquid possessing a simple synthesis step and good solubility stability in aqueous 1,3-propylene glycol (PDOaq). Besides, the addition of the proton-based ionic liquid (PIL) can enhance the maximum non-seizure load (PB value) in PDOaq to 3090 N and the addition of inorganic salts don't lower the PB value. Results show: (1) The proton ionic liquid can be adsorbed on the surface of the friction pairs through electrostatic interactions, and undergo a friction chemical reaction to form a triple film that can effectively cushion the direct contact and collision of the microgrooves. (2) The addition of inorganic salts can form hydrated ions, which can shape a hydrated molecular layer to withstand high normal loads and balance the van der Waals attraction by hydration repulsion, which is a key factor in realizing the ultra-low friction. This discovery greatly expands the application of facilitated lubrication for point-to-point contact on steel surfaces in engineering applications.

Site-controlled preparation of metallic microstructures via mechanical scratch-induced selective electrodeposition

Controlled preparation of metallic microstructures attracts wide interest in sensor and electronic fields. However, limited by fabrication technology, it faces challenges in achieving template-free and orientational formation of metallic microstructures. Herein, a site-controlled strategy via scratch-induced selective electrodeposition was proposed to prepare high-quality metallic structures on silicon (Si) surface with the assistance of scanning probe lithography (SPL). The effects of scratching and depositing conditions were systematically investigated for optimizing the preparation process. Selective electrodeposition mechanisms were addressed through topographic and conductive detections. It was found that single-cycle scratch created under higher loads was beneficial for preparing compact and continuous Cu structures. Further analysis indicated that higher normal loads in scratching can facilitate obtaining more conductive sites, promoting orientated migration of metal ions during the reaction, while the increase of reciprocating cycles can cause more amorphous layers and hinder electron transport. Moreover, post-annealing treatment can lead to the growth of Cu grain sizes, which contributes to the crystallinity. Notably, the proposed method is demonstrated with the preparation of Cu coils, which is sensitive to alternating magnetic field. These findings shed new light on the site-controlled preparation of high-quality metallic microstructures and the applications.

Numerical study on microscopic behaviour of non-crushable cemented granular media subjected to direct shear test

ABSTRACT Granular media, though discontinuous and heterogeneous, exhibit discrete behaviour at the grain scale and continuum behaviour at the macroscopic level. Understanding the correlation between micro-macro-mechanical parameters is crucial. The Discrete Element Method (DEM) effectively bridges this micro-macro-mechanical gap. This study simulates the Direct Shear Test (DST) using DEM software PFC2D on synthetic cemented granular media (CGM) under varying normal loads and strain rates. It examines micromechanical features and correlates them with macroscopic observations, focusing on confinement pressure and strain-rate dependency. Significant volumetric dilation is observed during shearing at low normal loads, which diminishes at higher normal loads. Particle-scale shear mobilization is explored through shear band formation in the sample. Grain-scale contact force distribution is highly heterogeneous throughout the sample. A probability distribution study is carried out to explore the bond breakage effect on the global response, which confirms the bond breakage to be a pressure and strain-dependent phenomenon.

Grafting of cationic molecules to hyaluronic acid improves adsorption and cartilage lubrication.

Synovial fluid lubricates articular joints by forming a hydrated layer between the cartilage surfaces. In degenerative joint diseases like osteoarthritis (OA), the synovial fluid is compromised, which leads to less effective innate lubrication and exacerbated cartilage degeneration. Studies over the years have led to the development of partially or fully synthetic biolubricants to reducethe coefficient of friction with cartilage in knee joints. Cartilage-adhering, hydrated lubricants are particularly important to provide cartilage lubrication and chondroprotection under high normal load and slow speed. Here, we report the development of a hyaluronic acid (HA)-based lubricant functionalized with cationic branched poly-L-lysine (BPL) molecules that bind to cartilageviaelectrostatic interactions. We surmised that the electrostatic interactions between the BPL-modified HA molecules (HA-BPL) and the cartilage facilitate localization of the HA molecules to the cartilage surface. The number of BPL molecules on the HA backbone was varied to determine the optimal grafting density for cartilage binding and HA localization. Collectively, our results show that our HA-BPL molecules adhered readily to cartilage and were effective as a lubricant in cartilage-on-cartilage shear measurements where the modified HA molecules significantly reduce the coefficient of friction compared to phosphate-buffered saline or HA alone. This proof-of-concept study shows how the incorporation of cartilage adhering moieties, such as cationic molecules, can be used to enhance cartilage binding and lubrication properties of HA.

Influence of ex-situ annealing on the friction and wear performance of multi-layer Ti3C2Tx coatings

2D materials have gained momentum for lubrication purposes, which holds specifically true under more severe conditions such as elevated temperatures. Among them, MXenes are a new family of 2D materials, which offer an excellent electrical conductivity, tunable surface functionality, and outstanding mechanical properties. In the past five years, MXenes have been significantly studied for tribological applications due to their ability to form easy-to-shear tribofilms with excellent wear resistance. However, little is known regarding MXenes’ tribological performance at elevated temperatures. Therefore, we systematically studied the tribological performance of spray-coated Ti3C2Tx coatings after thermal annealing at temperatures between 50 and 400 °C using linear-reciprocating ball-on-disc tribometry. Advanced materials characterization demonstrated that thermal annealing of MXenes results in the release of superficial and intercalated water, the removal of surface terminations, and the oxidation of Ti3C2Tx to TiO2. Consequently, the tribological performance of the annealed coatings deteriorated, which is particularly pronounced for thin coatings, high normal loads, and elevated annealing temperatures. Our results provide insight into the thermal stability of Ti3C2Tx coatings as well as the implications on their tribological performance.

Enhancing tribological performance of graphene sheet-embedded carbon films on steel substrates via texture engineering and chromium doping

Amorphous carbon (a-C) film is a promising solid lubricant coating due to its unique structure and excellent properties. However, its poor adhesion and long running-in period hinder the application in metal components. Graphene sheet-embedded carbon (GSEC) film has a significantly shorter running-in period but suffers from inferior tribological properties due to its rough surface and weak mechanical characteristics. In this study, we utilized an electron cyclotron resonance (ECR) plasma nano-surface manufacturing device to deposit GSEC films onto steel substrate surfaces. By manipulating the surface/interface texture and incorporating chromium (Cr) doping, the tribological properties of the GSEC films were improved both chemically and physically. The results demonstrated that the textured carbon film exhibited superior adhesion to the steel substrate and a wear life that was three orders of magnitude longer than the untextured GSEC film, under low normal load conditions while sliding against the GCr15-bearing steel ball. After doping the Cr into textured carbon films, there was a significant reduction in the friction coefficients, and the wear type of carbon film transformed from adhesive wear to abrasive wear. Moreover, the doped carbon films demonstrated prolonged wear lives under high normal loads. This improvement can be attributed to the formation of hard CrC nanocrystalline, which effectively reduced surface roughness and enhanced the mechanical properties of the carbon films. Notably, even under a contact pressure of 0.83 GPa, the doped carbon film maintained favorable tribological performances. These findings have extended the potential applications of carbon films on metal substrates.

Manufacturing of Poly-DL-Lactic Acid Nanosheets and Evaluation of Tribological Characteristics between Nanosheet Surfaces and Fingers

Attention is focused on ultra-thin polymer films (nanosheets) that have high flexibility and adhesiveness and their thickness can be controlled to several tens of nanometers. These nanosheets can be neatly attached to surfaces with complex irregularities without the use of adhesives. Therefore, the ratio of surface area to thickness is very large, and we believe that the relationship with friction is very significant in nanosheet technology for biomedical applications such as wearable devices and wound dressings. The purpose of this study is to investigate the contact mechanism of nanosheets with human fingertip skin in terms of friction coefficient by using the microgravure printing method, which enables thin film coating. From the results of film thickness measurements, it was found that nanosheets of any thickness can be fabricated by the microgravure printing method. The friction measurement results showed that the coefficient of friction of the nanosheets decreased except for vertical loads above Fz=2N. The coefficient of friction increased as the contact area increased. It was found to increase with increasing vertical load under the immersion in water conditions, and conversely, it decreased under the drying condition except for the high normal load of 2N. Furthermore, the coefficient of friction was found to increase with increasing nanosheet thickness. Observation of wear traces showed that when the vertical load was sufficiently high (Fz = 2 N), wear traces containing oily traces such as sebum and sweat appeared on the nanosheet surface. This is thought to function as a lubricant. Polymer nanosheets are a new material, and there have been few studies on friction with this material. Research on friction is very important because polymer nanosheets are expected to be applied to wound dressings and displays of electronic devices.

Laser surface texturing of dies in strip drawing of DP600 steel sheet

ABSTRACT Friction at the blank-die contact influences material flow, formability, and product surface quality in processes like deep drawing. Surface modification of the dies by laser texturing is a potential technique to alter the tribological characteristics at the interface during sliding of the sheet over the die surface in lubricated condition. In this work, the effect of laser surface texturing of dies has been investigated under high normal loads during strip drawing of dual phase (DP600) steel. The effect of texture parameters and process parameters on the interface friction between the strip and the dies has been studied. It has been found that the friction coefficient reduced, and the surface finish improved significantly with the textured dies when compared to the conventional die. This study emphasises the importance of surface modification of the dies by laser texturing for its potential application in sheet metal forming processes.

Temperature induced wear mechanism transition of Ni-Co-W alloy coatings

A temperature induced wear mechanism transition of electrodeposited Ni-Co-W alloy coatings has been revealed. At room temperature, the wear rate of the coating exhibits little variation with sliding velocity or normal load. From 100 °C to 300 °C, the wear resistance of the coating is enhanced at high sliding velocity or high normal load. With increase of ambient temperature, the wear mechanism turns from typical abrasive wear with significant parallel grooves and wear debris on the worn tracks to mild oxidative wear with shallow grooves and adhered oxide layer on the worn surfaces. The improved wear resistance at high temperature can be ascribed to the transition of wear mechanism and the formation of uniform oxides layer covered on the worn surface.

Mechanical and Tribological Properties of the CrAl/CrAlN and CrAl/CrAlN-(a-CNx) Multilayers Deposited by HIPIMS

In this study, two CrAl/CrAlN multilayer coatings of 1.5 µm and 1.2 µm thicknesses were deposited from a (50:50) CrAl sectioned target by high-power impulse magnetron sputtering (HIPIMS) onto A11 tool steel to evaluate the effect over mechanical and tribological properties by the addition of an (a-CNx) top layer. XRD analysis showed a CrAlN FCC structure (111) and a Cr phase (110). A glass-like morphology was observed in the CrAl layer and in the (a-CNx) top layer, while the CrAlN layers exhibited a columnar morphology according to the FE-SEM analysis. Raman analysis results confirmed characteristic peaks G and D for the carbon amorphous structure of the (a-CNx) top layer. Nanoindentation results showed comparable hardnesses (23.08 GPa ± 2.8 and 20.58 GPa ± 1.8) in both multilayer coatings. The CrAl/CrAlN multilayer with an (a-CNx) top layer increased the absorption energy capacity and the mechanical potential energy stored. The nano scratch test showed the strongest crack resistance and highest adhesive energy values for the CrAl/CrAlN multilayer. The addition of an (a-CNx) top layer improved the tribological behavior when higher normal loads were applied during the fretting test. Raman analysis showed an increase in the I(D)/I(G) ratio and a formation of a carbon sp2 structure (i.e., graphite-like structure) over the worn surfaces, as well as dissipated energy (J/m) during the fretting test.

Microstructure and Properties of Monolayer Ta and Multilayer Ta/Ti/Zr/Ta Coatings Deposited on Biomedical Ti-6Al-4V Alloy by Magnetron Sputtering

Two types of coatings, i.e., monolayer Ta and multilayer Ta/Ti/Zr/Ta coatings, were deposited on biomedical Ti6Al4V (TC4) alloy by magnetron sputtering to improve its performance. To evaluate the effect of the two coatings on the alloy properties, the microstructure, composition, mechanical and tribological properties, in vitro biocompatibility, and corrosion resistance were investigated. The results showed that α-Ta exists in the monolayer Ta coating, while α-Ta and β-Ta phases coexist in the multilayer Ta/Ti/Zr/Ta coating. The multilayer Ta/Ti/Zr/Ta coating possessed the highest hardness and the monolayer Ta coating had the lowest friction coefficient compared to the Ti6Al4V alloy. The friction and wear tests revealed that the anti-wear performance of the Ta coating is the best, followed by that of the Ta/Ti/Zr/Ta coating, while the anti-wear performance of TC4 alloy is relatively poor in comparison with the Ta and Ta/Ti/Zr/Ta coatings. The wear resistance of the multilayer Ta/Ti/Zr/Ta coating under low normal load is better than that under high load normal load. Finally, the in vitro and electrochemical corrosion tests showed that the Ta coating modification provides better biocompatibility and corrosion resistance than those of the uncoated Ti6Al4V alloy.