Average Contact Research Articles

Test and DEM study on cyclic hysteresis characteristics of stereoscopic geogrid–soil interface

The bearing resistance provided by the geogrid's transverse ribs is a non-negligible aspect of the strength mechanism in mobilizing the geogrid–soil interface. Therefore, studying its influence on the response mechanism of geosynthetic-reinforced soil structures under cyclic loading is crucial. The stereoscopic geogrids were manufactured using 3D printing technology by quantitatively thickening the transverse ribs of planar geogrids. To investigate the cyclic hysteresis relationship and stress–dilatancy phase-transformation characteristics of the stereoscopic geogrid–coarse particle interface, cyclic direct shear tests were conducted. Additionally, a discrete element method (DEM) was employed to study the evolution of shear bands and fabric anisotropy at the interface under cyclic loading. The results of the study indicate that the stress–displacement phase angle of the stereoscopic geogrid in the horizontal direction of cyclic shear is smaller compared to the planar geogrid. Furthermore, thickening the transverse ribs decreases the stress–dilatancy phase-transformation angle of the interface. The thickness of the interface shear band in the stereoscopic geogrid is greater than that of the planar geogrid. Moreover, as the transverse-rib thickness increases, the principal direction of the average normal contact force and average tangential contact force under cyclic loading also increases.

Dynamic impact of sleeper unsupported defects on the heavy haul locomotive-ballasted track coupling system

In this study, a dynamics analysis framework combining the multi-body dynamics method (MBD) and the discrete element method (DEM) capable of accounting for sleeper unsupported defects is proposed. Different sleeper hanging defects (e.g., number of unsupported sleepers and the hanging height) is an input for an analysis of the heavy haul locomotive dynamic behavior and related ballasted bed degradation in terms of ballast particle wear. Simulated results indicate that the sleeper hanging defects dramatically increase the average contact stress underneath the well-supported sleeper adjoining the sleeper hanging area and the corresponding ballasted bed stress, indicating that the unsupported height should be suppressed within 5 mm even if only there exist single or two consecutive unsupported sleepers. When occurring minor sleeper unsupported defects and not being maintained in time, the vibration acceleration and dissipation energy of the ballasted bed will further increase sharply, which will aggravate the wear of the ballast particles and may change the particle size distribution of the ballasted bed or even pollute it.

Model of shear strength of ultra-deep fractured sandstone considering fracture morphology

Ultra-Deep fractured tight sandstone gas reservoir, the hotpot unconventional petroleum resources, has been the subject of investigation recently. Due to the presence of natural fractures, drilling in ultra-deep fractured tight sandstone may cause complicated wellbore instabilities, which resulted in a substantial challenge to the efficient exploitation and production of tight gas resources. In order to understand the rock mechanical properties of the ultra-deep fractured tight sandstone under shear stress, tight sandstone core samples were artificially shear-fractured, the 3D characteristics of the fractures were reconstructed by CT scanning, and the fracture aperture and roughness characteristics were systematically evaluated. Simultaneously, triaxial shear strength tests were conducted to evaluate the mechanical properties of fractured rock subjected to varying confining pressures. Considering the contact relationship of the fracture surface during shearing, two indexes of average effective shear angle and contact area ratio were proposed to correct the fracture roughness, and a new fracture shear strength model was constructed, which served as the failure criterion for the fractured formation. The results show that the intact rock has a high compressive strength and can only collapse under conditions of substantial differential stress. Consequently, shear failure of fracture ought to be the primary source of wellbore instability. The fracture aperture's spatial distribution is irregular and conforms to a Gaussian distribution. A clear anisotropy in the fracture roughness is present, and there is less roughness along the initial shear failure path and more roughness perpendicularly. With an increase in normal stress, the fracture's peak shear strength rises, while the friction angle steadily falls. The innovative model's accuracy in predicting shear strength is higher than that of the weak-surface model and the JRC-JCS model.

The effects of composition and microstructure on compressive strength of Ag-Au nanoparticles

Hemispherical nanoparticles of Ag-33 at.% Au and Ag-60 at.% Au alloys were fabricated by solid state dewetting of Ag-Au bilayers deposited on a sapphire substrate. The in-situ microcompression tests of the particles in the scanning electron microscope have revealed the plastic deformation behavior typical of plasticity which is controlled by the nucleation of new dislocations: high elastic strain reaching about 5% of the particle diameter, followed by a catastrophic particle collapse. The modulus-normalized average contact pressure at the onset of plasticity calculated employing the isotropic Hertz model was concentration-independent for pure Ag and two studied alloys. The atomistic molecular dynamics simulations of shear and compressive deformation of bulk Ag-Au alloys revealed a weak solid solution hardening effect for dislocation nucleation stress and its absence for the nucleation strain. The particles strength and critical strain obtained in simulations were in excellent quantitative agreement with the experimental results. The lack of solid solution hardening or weakening effects in the Ag-Au has been discussed in terms of small atomic size mismatch between Ag and Au. Finally, the effect of the twin boundaries observed in the Ag-Au particles on their strength was discussed.

Indentation and Detachment in Adhesive Contacts between Soft Elastomer and Rigid Indenter at Simultaneous Motion in Normal and Tangential Direction: Experiments and Simulations.

In reported experiments, a steel indenter was pressed into a soft elastomer layer under varying inclination angles and subsequently was detached under various inclination angles too. The processes of indentation and detachment were recorded with a video camera, and the time dependences of the normal and tangential components of the contact force and the contact area, as well as the average contact pressure and average tangential stresses, were measured as functions of the inclination angle. Based on experimental results, a simple theoretical model of the indentation process is proposed, in which tangential and normal contacts are considered independently. Both experimental and theoretical results show that at small indentation angles (when the direction of motion is close to tangential), a mode with elastomer slippage relative to the indenter is observed, which leads to complex dynamic processes-the rearrangement of the contact boundary and the propagation of elastic waves (similar to Schallamach waves). If the angle is close to the normal angle, there is no slipping in the contact plane during the entire indentation (detachment) phase.

Characterisation of the Contact between Cross-Country Skis and Snow: On the Multi-Scale Interaction between Ski Geometry and Ski-Base Texture

In elite endurance sports, marginal differences in finishing times drive ongoing equipment improvement to enhance athlete performance. In cross-country skiing, researchers, since the 1930s, have faced the challenge of minimising the resistance caused by friction in the contact between skis and snow. This study was designed to evaluate the multi-scale interaction between the macro-scale ski-camber profile and the micro-scale ski-base texture. Considerations included real contact area, average interfacial separation, and total reciprocal interfacial separation between the ski and snow, which are properties that are intimately coupled to ski–snow friction. We found that both the profile of the ski camber and the texture of the ski base play decisive roles in determining viscous friction. At the same time, the texture of the ski base exerts a greater impact on the average real contact pressure, real contact area, and minimal average interfacial separation between the ski and snow than the ski-camber profile.

Small-scale Experimental Research on the Key Parameters Affecting Metal-to-metal Sealability

The harsh service environment of high pressure and high temperature (HPHT) gas well have posed great challenge to the seal integrity of the casing string thread connections. All the time, due to the complexity of metal-to-metal sealing mechanism, there is no uniform reliable sealability criterion in the Finite Element Analysis process of the casing premium thread, so a systematic research on the sealing mechanism of the casing premium thread metal-to-metal seals. In this paper, a series small-scale metal-to-metal seal tests considering different influence factors were conducted, the results showed that the average contact stress at the leakage threshold increases with gas pressure, thread compound and metal roughness has significant influence on the metal sealing property, tests with fine surface roughness (<0.05um) and without thread compound show the lowest critical average contact stress value. Based on the test results, the key parameters for calculating the sealability criterion model were calibrated, and the sealability criterion is recommended for use in high-pressure applications.

Analysis of freezing of a sessile water droplet on surfaces over a range of wettability

HypothesisNonwetting surfaces, by virtue of their water-repelling trait, offer desirable anti-icing characteristics. Surface roughness, type and wettability are important interfacial characteristics that affect the icing dynamics that can be tailored to achieve desired anti-icing designs. Experiments and SimulationsThe present study systematically explores the effect of surface roughness on the freezing behaviour of water droplets on surfaces ranging in their wettability. Surfaces with tailored textures and wettability were fabricated using chemical etching and electrodeposition by varying the voltage. The surfaces studied include bare copper, five different dry nonwetting copper surfaces, and five different lubricant-infused copper surfaces that ranged in surface texture fractal dimension from nearly 1.0 to 1.92 and wettability measures of average water contact angle from 91° to 162° and sliding angle from less than 3° to greater than 50°. A computational model is developed to simulate the freezing dynamics on the surfaces studied. FindingsWith increasing roughness features, the freezing time increased due to the dual effects of increased contact angle and poor interfacial conductance caused by trapped air or infused liquid within the asperity textures. In general, the nonwetting surfaces increased the freezing time by a factor of at least 1.33 and up to about 3.2 compared to freezing on bare copper surfaces. The computational model shows close agreement with experimental measurements on the freeze front progression as well as freeze time. Design guidelines on the suitability of the different nonwetting surfaces for anti-icing purposes are derived from the systematic study, with the overall design recommendation favoring lubricant infused surfaces.

Investigation of ultrasonic mechanism and development of tool wear model in ultrasonic elliptic vibration assisted cutting of stainless steel

By analyzing the kinematics and dynamics of ultrasonic vibration under various conditions, the tool wear mechanism in Ultrasonic Elliptic Vibration-Assisted Cutting (UEVAC) were investigated. Two distinct mechanisms for mitigating tool wear emerge: In non-separated UEVAC, the pivotal factor is the reduction of material yield stress through the acoustic softening effect. Conversely, in separated UEVAC, tool wear reduction is attributed to the decrease in average contact stress via stress superposition and the hammer effect. These findings highlight the effectiveness of aligning ultrasonic energy field action times and intensity to reduce stainless steel tool wear rates in non-separated UEVAC. Furthermore, the proposed tool wear model demonstrates robust reliability for separated UEVAC processes.

Design of Superhydrophobic Shape Memory Composites with Kirigami Structures and Uniform Wetting Property.

With the continuous increase in human demand to improve aircraft performance, intelligent aircraft technologies have become a popular research field in recent years. Among them, the deformable skin structure has become one of the key technologies to achieve excellent and reliable performance. However, during the service, deformable skin structures may encounter problems such as surface impact and adhesion of droplets in rainy weather or surface icing in low-temperature environments, which can seriously affect the flight safety of the aircraft. One way to overcome these issues is to use superhydrophobic shape memory materials in the structure. In this regard, first, shape memory composites were prepared with shape memory epoxy resin as the matrix and carbon fiber orthogonal woven fabric as the reinforcement material. Superhydrophobic shape memory composites (SSMCs) were then obtained by casting the kirigami composite with superhydrophobic carbon nanotube-polydimethylsiloxane (CNT@PDMS) mixture, and the surface was processed by laser micromachining. Shape memory performance and surface wetting performance were determined by material testing methods. The results showed that the shape memory recovery rate can reach 85.11%, the surface is superhydrophobic, the average water contact angle is 156.9 ± 4.4°, and the average rolling angle is 3 ± 0.5°. The three-point bending test of the specimens with different kirigami cell configurations showed that the shape memory composite based on the rectangular structure has the best deformability with an aspect ratio of 0.4. From the droplet impact test, it was found that the impact speed of water droplets and the curvature of the surface can greatly affect the dynamic performance of water. This work is expected to be of significant research value and importance for developing functional deformable skin materials.

SARS-CoV-2 risk in household contacts of healthcare workers: a prospective cohort study

BackgroundFew studies have assessed whether the increased SARS-CoV-2 risk of healthcare workers (HCW) is carried on to their household contacts. Within a prospective HCW cohort, we assessed the SARS-CoV-2 risk of household contacts of HCW depending on the HCWs cumulative exposure to COVID-19 patients and identified factors influencing this association.MethodsHCW aged ≥ 16 years from nine Swiss healthcare networks participated. HCW without any household contacts were excluded. For HCW, cumulative patient exposure (number of COVID-19 patient contacts times average contact duration during a 12-month follow-up) was calculated. During follow-up, HCW reported SARS-CoV-2 nasopharyngeal swab results and positive swab results of their household contacts. We used multivariable logistic regression to identify variables associated with SARS-CoV-2 household positivity.ResultsOf 2406 HCW, 466 (19%) reported ≥ 1 SARS-CoV-2 positive household. In multivariable analysis, patient exposure of HCW (adjusted OR [aOR] 1.08 per category, 95% CI 1.04–1.12), household size (aOR 1.53 per household member, 95% CI 1.35–1.73) and having children (aOR 0.70, 95% CI 0.53–0.94) remained associated with household positivity. Vaccinated HCW had a lower risk (aOR 0.54, 95% CI 0.38–0.77) of reporting a positive contact, as were those using respirator masks in contact with COVID-19 patients (aOR 0.65, 95% CI 0.49–0.86). Among vaccinated HCW, delayed first vaccination was associated with increased household SARS-CoV-2 positivity (aOR 1.14 per month, 95% CI 1.08–1.21).ConclusionsSARS-CoV-2 positivity in household contacts of HCW increases with higher cumulative COVID-19 patient exposure of HCWs. Measures reducing the SARS-CoV-2 risk in HCW might indirectly reduce the infection risk of their households.

Screen printable fire through nickel contacts for silicon solar cells

Metallization of crystalline silicon (Si) solar cells is indispensable for reducing the cost while increasing the overall efficiency. Developing alternative materials to the most commonly used screen printed silver (Ag) contacts is a critical factor. Here, in this study, a nickel (Ni) metal paste consisting of Ni metal particles, glass frit and an organic vehicle is fabricated for screen printed Ni contacts. To prevent possible Schottky barrier formation, a graphene layer is placed on the front surface of the Si solar cell between Ni and Si forming a metal-2D-semiconductor structure ensuring the ohmic (or ohmic-like) contacts. It is demonstrated here that the graphene is transferred successfully onto the textured front surface of the cells and G/2D peaks are observed clearly, indicating the good quality of the graphene layer. Constituents of Ni metal paste, glass frit and organic vehicle, are fabricated in concordance with nickel metal powder and mixed to achieve optimal electrical output parameters. The findings suggest that the transition temperature of the glass frit is between 270 °C and 300 °C resulting in around 475 °C softening temperature, which gives excellent etching behavior to the paste. The average contact resistance of the screen printed Ni contacts governed by etching process is measured around 6.9 mΩ cm2. The SEM images of the contacts show a uniform distribution and sintering behavior similar to that of silver counterparts. The light current-voltage measurements read the open circuit voltage of 660 mV, the short circuit density of 39.11 mA/cm2 and the fill factor of 81.4% resulting in around 21% efficiency.

A Preliminary Study of New Experimental Low-Cost Fe–P-Based and Mn–Fe–P-Based Brazing Filler Metals for Brazing of Non-Alloy and Low-Alloy Steels

Seventeen new experimental filler metals from eight different alloy systems based on Fe–P–X and Mn–Fe–P–X (X = B, C, Si in various combinations) were created and experimented with. DSC analyses were performed to determine the solidus and liquidus temperatures and the melting ranges. Hardness measurements of the alloys were performed in the as-cast state. The alloys contain primary and eutectic intermetallic compounds that make them very hard with average hardness values ranging from 590 HV10 to 876 HV10. The wettability was determined at 1000 °C, 1040 °C and 1080 °C on C22 non-alloy steel and 15CrNiS6 low-alloy steel in Ar 4.6 and 78 vol% H2-22 vol% N2 atmospheres. The results show good wettability at T = 1080 °C in both atmospheres, as the contact angles were mostly ≤30°. Thirteen alloys exhibit very good wettability with average contact angles of ≤15.5°. Nine alloys exhibit excellent wettability with their average contact angles being ≤10°. Wettability improves at higher temperatures. The liquid alloys are reactive to solid steels and form a diffusion joint. Diffusion of P, B, C, and Si from the filler metal into the base material dealloys the composition of the melt near the joint interface. For the same reason, a continuous layer of solid solution forms on the joint interface. When brazing with filler metals rich in carbon, strong carburisation of steels can be observed near the joint.

Examining layer‐by‐layer assembly of polyelectrolyte‐drug interactions using Quartz Crystal Microbalance with Dissipation

AbstractThe incorporation of drug molecules into layer‐by‐layer (LbL) assemblies attracts great interest in biomedical applications. This work describes the preparation and characterization of LbL multilayers including drugs, namely ibuprofen (IBF‐Na) and Diclofenac sodium salt (DCF‐Na) as anionic molecules. The main aim is to enhance the drug loading capacity within the LbL assembly. The effects of drug‐LbL multilayered films obtained from poly(diallyl dimethylammonium chloride) (PDADMAC)/IBF‐Na and PDADMAC/DCF‐Na interactions on multilayer growth and loading of model molecules are examined using UV‐Visible and Quartz‐Crystal Microbalance with Dissipation (QCM‐D). The wet mass of PDADMAC/IBF‐Na is found to be 67.2 ng/cm2, while PDADMAC/DCF‐Na film exhibits 201.6 ng/cm2 mass adsorption. Furthermore, the multilayer stabilities are assessed against neutral, acidic, and basic solutions. The results demonstrate that the LbL films have robustness, without any signs of multilayer degradation. By the way, the incorporation of IBF‐Na into the feed solution leads to the formation of a rigid structure, whereas the inclusion of DCF‐Na results in a comparatively flexible structure. The use of a higher molecular weight and more polar drug in the feed causes a noticeable rise in surface hydrophilicity and surface roughness. PDADMAC/IBF‐Na and PDADMAC/DCF‐Na display average contact angles of 46 ± 1.3° and 76 ± 2° while they have roughness values of 5.9 and 8.0 nm, respectively. LbL multilayers fabricated from a polycation and anionic drug can be considered as a promising material platform for controlled drug loading due to their tunable physicochemical and morphological properties as well as surface wettability.

Surface treatment of large-area epoxy resin by water-perforated metal plate electrodes dielectric barrier discharge: Hydrophobic modification and uniformity improvement

Wind power plays a vital role in the field of renewable energy. However, a significant challenge arises when operating wind turbines with epoxy resin blades in temperatures below 0℃, as they tend to ice up, causing unexpected shutdowns. Low-temperature plasma technology emerges as a promising solution that can produce hydrophobic thin films with excellent mechanical properties in an eco-friendly way, but its application is currently limited to small objects. This paper presents a novel design for a large-scale DBD (Dielectric Barrier Discharge) water-perforated metal electrode plasma treatment device that was optimized through simulations to achieve uniform gas discharge at low flow rates. The device successfully deposited a hydrophobic thin film uniformly on the surface of an epoxy resin plate with an average water contact angle of 139.5 ± 2.5°, which is an 87% improvement compared to the untreated surface. In addition, the device reduced the required working gas flow rate by 40–60% compared to existing large-area processing devices, and improved the effectiveness by 16%. Physical and chemical property tests demonstrated that the device’s design increased surface roughness and generated a dense film containing hydrophobic silicon groups (such as Si-O-Si and Si-(CH3)x), contributing to the improvement in hydrophobicity. Overall, the presented device offers an efficient, eco-friendly, and effective method for depositing hydrophobic thin films on large-scale materials, which can enhance wind turbine performance by preventing ice formation on epoxy resin blades.

Characterizing force-chain network in aggregate blend using discrete element method and complex network theory

The force chain network in aggregate blend indicates load transfer path and is the main carrier to resist external loads. The objective of this study is to analyze the force chain network characteristics of aggregate blend on the basis of complex network theory. The virtual aggregate blend models with three stone matrix asphalt (SMA) gradations and three dense asphalt concrete (AC) gradations were constructed using discrete element method (DEM). The force chain network topology structure of each aggregate blend was established according to the aggregate contact parameters extracted from DEM software and the introduced complex network theory. The characteristics of each force chain network were analyzed by the various network evaluation indicators. The results showed that there are two or more contacts between most particle pairs. The average coordination number and the average contact number of aggregate blend are not equal. The degree distribution of each gradation shows a good power-law distribution. The force chain network of aggregate blend is a scale-free network. Big particles have greater average strength and smaller average weight distribution difference compared with small particles. The reference lower limit has better clustering performance for SMA-16, and the reference upper limit has better clustering performance for AC-25. The average path length of SMA-16 is longer than that of AC-25, indicating that SMA-16 has better load transfer ability and skeleton properties.

Evaluating the performance of wearable devices for contact tracing in care home environments

COVID-19 has had a devastating impact worldwide, including in care homes where there have been substantial numbers of cases among a very vulnerable population. A key mechanism for managing exposure to the virus and targeting interventions is contact tracing. Unfortunately, environments such as care homes that were most catastrophically impacted by COVID-19 are also those least amenable to traditional contact tracing. A promising alternative to recall and smartphone-based contact tracing approaches is the use of discrete wearable devices that exploit Bluetooth Low Energy (BLE) and Long-Range Wide Area Network (LoRaWAN) technologies. However, the real-world performance of these devices in the context of contact tracing is uncertain. A series of experiments were conducted to evaluate the performance of a wearables system that is based on BLE and LoRaWAN technologies. In each experiment, the number of successful contacts was recorded and the physical distance between two contacts was compared to a calculated distance using the Received Signal Strength Indication (RSSI) to determine the precision, error rate, and duration of proximity. The overall average system contact detection success rate was measured as 75.5%; when wearables were used as per the manufacturer’s guidelines the contact detection success rate increased to 81.5%, but when obstructed by everyday objects such as clothing or inside a bag the contact detection success rate was only 64.2%. The calculated distance using RSSI was close to the physical distance in the absence of obstacles. However, in the presence of typical obstacles found in care home settings, the reliability of detection decreased, and the calculated distance usually appeared far from the actual contact point. The results suggest that under real-world conditions there may be a large proportion of contacts that are underestimated or undetected.

Photodegradation of dyes using electrospun polymeric membranes containing titanium oxide and iron

This study aimed to analyze the photocatalytic degradation of Reactive Red 195, a textile dye belonging to the class of azo dyes. For this, a polymer membrane of Ecovio® (polymeric blend formed by PBAT/PLA) was obtained by the electrospinning technique, with the incorporation of semiconductors titanium dioxide (TiO2-R) and magnetite iron oxide (Fe3O4-M). The hydrophobic nature of the membrane required the treatment with sodium hydroxide (NaOH), in this way, the membranes were characterized with and without the semiconductors, as well as, with or without the alkali treatment. The techniques used for characterization were: contact angle analysis, infrared spectroscopy with Fourier transform (FT-IR), thermogravimetrics (TGA), X-ray diffraction (XRD), and differential exploratory calorimetry (DSC). The contact angle analysis of the membrane surface showed that it was hydrophobic before treatment with NaOH, with an average contact angle of 121.45 ± 1.44° and consequently low humidity. After treatment, there was a change in the surface with a change of the average contact angle value to 80.68 ± 1.52° and an increase in wetness. The FT-IR spectroscopy identified bands characteristic of the Ecovio® functional groups in the polymer blend, and discreet changes occurring after alkaline treatment. The TGA analysis showed that the incorporation of two semiconductors and the alkaline treatment resulted in a decrease in thermal stability for both the PLA polymer and the PBAT due to the possible interaction of the TiO2-R and Fe3O4-M semi-conductors with the polymers that we can promote a lower arrangement of fibers. The DSC analysis indicated possible changes in the polymer's structure due to the presence of semiconductors and alkaline treatment. A decrease in melting calories of the treated membranes compared to the untreated membrane indicates possible hydrolysis of the polymers. In the DRX analysis, an increase in the intensity of the peaks of the polymer membrane containing the semiconductors and subjected to alkaline treatment was observed. However, comparing membranes containing the semiconductors with pure membranes (without the semiconductors) subjected or not to alkaline treatment, possibly an alteration in crystallinity had a greater relationship with the incorporation of the two semiconductors in the pure membrane subjected to alkaline treatment continued to have predominantly amorphic characteristics. After obtaining and characterizing the membrane, studies were conducted to evaluate its potential in the photodegradation of the Reactive Red 195, using an Ecovio® polymer membrane incorporated with TiO2-R and Fe3O4-M, and treated with an alkaline solution. The results showed degradation above 80% after 24 h, in addition, the potential for reuse of this membrane was analyzed in 4 cycles of photocatalytic degradation, which showed that in all the cycles using the same membrane, the degradation remained above 80%, but at the end of the fourth cycle, a membrane still intact but visually more fragile was noted. Kinetic studies have shown that degradation follows the pseudo-first-order kinetic model. After the degradation cycles, the Fourier transform infrared spectroscopy (FT-IR) showed no new bands. Thus, the polymer membrane proved promising for the photodegradation of Reactive Red 195 given its efficiency, ease of separation from the reaction medium, and the possibility of reuse for several cycles.

The lubrication regimes and transition laws of gallium liquid-metal

Gallium-based liquid metal is a novel lubricant for high-end equipment due to its low friction coefficient and excellent load-carrying capacity. In this work, the stribeck curve, static friction and a series of fixed-speed friction tests for steel self-mated pairs under gallium lubrication are conducted to reveal the lubrication regimes and transition laws. In the sliding speed ranging from 10−8 to 1.4 m/s and the normal load ranging from 2 to 20 N, gallium can only provide boundary and mixed lubrication. Uniformly distributed or island-like distributed gallium-rich films are formed under boundary or mixed lubrication regimes, respectively. The different formation mechanisms are revealed. The failure of the gallium-rich film is mainly related to the average contact stress of the frictional interfaces.

Identifying differences in the tribological performance of GUR 1020 and GUR 1050 UHMWPE resins associated to pressure × velocity conditions in linear reciprocating sliding tests

In this work, two commercial UHMWPE (ultra-high molecular weight polyethylene) resins used in orthopedics, GUR 1050 and GUR 1020, were evaluated through linear reciprocating dry friction tests. Average contact pressures (P) of 34 MPa and 50 MPa and sliding velocities (V) of 0.02 m/s and 0.10 m/s were selected to perform tests in four PV conditions. The friction coefficient (COF) with both resins was around 0.18 in average, without significant distinctions by PV; however, a distinction was seen in COF dispersion; it was in the range of 5%–19%, in dependence of the PV condition and resin type. COF with GUR 1020 was more disperse, and it was related to the vulnerability of the resin to undergoing dynamic changes in the intensity of adhesive (higher COF) or abrasive (lower COF) wear mechanisms. Both wear mechanisms are displayed simultaneously, but random changes in intensity may occur during the friction process. Such randomness was associated to the susceptibility to have the structure modified by friction, higher in GUR 1020 than GUR 1050. Concerning wear amount, contact pressure was the most influencing parameter on it. GUR 1020 performed more than 30% inferior than GUR 1050 under contact pressure higher than the yield strength of the material. Under pressures near the material strength, the wear level was in the range of surface roughness and both resins performed equal in average; however, in this case, the dispersion was systematically lower for GUR 1050, evidencing its better tribological stability. It was concluded that analyses on the dispersion of the tribological responses disclosed relevant information on stability related performance. Also, when procedural dependent properties, as such friction and wear, are considered as evaluation parameters, care must be taken to compare results from different tribosystems.