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Technical Brief

J. Tribol. 2019;141(10):. doi:10.1115/1.4044299.

Finite element analysis is conducted on an elastic-plastic full stick contact between a rigid flat and a deformable coated sphere with a soft coating. A combined normal and tangential load is applied to the rigid flat, and the sliding inception is assumed as a result of the decreasing tangential stiffness of the contact junction. As the coating becomes thicker, the static friction coefficient first decreases reaching a minimum and then increases. This demonstrates a friction reduction effect of soft coatings that is opposite to hard coatings. The static friction coefficient decreases with increasing normal load as the contact junction contains more plasticity and becomes more compliant.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2019;141(10):. doi:10.1115/1.4044293.

The dry sliding tribological behavior of a columnar-grained Fe2B intermetallic compound under different normal loads was evaluated by scanning electron microscopy (SEM), XPS, and 3D laser scanning microscope. The results indicated that under a load of 12 N, after a 35 min break-in period, the dynamic friction coefficient decreased from 0.78 to about 0.6 and this low value was maintained until the end of test. When the normal load increased from 4 N to 20 N, both the average friction coefficient and wear rate values initially decreased and then increased. The lowest values of the average friction coefficient and wear rate were obtained under a load of 12 N. As the load increased from 4 N to 12 N, a complete film formed on worn surface. Nevertheless, when the load increased to 16 N, severe self-induced vibration occurred and a corrugated ribbon appeared on the surface. Furthermore, severe damage on the worn surface was caused by cycling vibration under the 20 N load. Under 4 N and 12 N, the main wear mechanism was abrasive wear, while under a load of 20 N, fracture wear and abrasive wear were the mian wear mechanisms. The friction products were composed of B2O3, H3BO3, SiO2, and FexOy. More specifically, Fe2O3 was generated under 4 N load, Fe2O3 and Fe3O4 were produced under 12 N load, and the mixture of FeO, Fe2O3, and Fe3O4 appeared under 20 N load.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(10):. doi:10.1115/1.4044298.

Solid particle erosion (SPE) as a common damage mechanism in industrial applications can reduce the effective operation of components or contribute to failure. However, it has beneficial usages in manufacturing processes, especially in abrasive sandblasting and waterjet cutting. The aim of this paper is an investigation of erosive behavior of Ti-6Al-4V alloy through numerical and experimental approaches. A three-dimensional finite element (FE) model is developed using the representative volume element (RVE) to simulate multiple particles impact on Ti-6Al-4V target. Failure and plastic behavior of the target surface due to particles impact is described by Johnson-Cook constitutive equations. Furthermore, erosive behavior of the alloy is experimentally researched by multiple SPE tests. Verification of the implemented approach is studied by comparing the results of the FE model and the SPE experiments. Effects of particles impact angle considering Johnson-Cook coefficient values and particles velocity on erosive behavior of Ti-6Al-4V are also studied. Both numerical and experimental results show a maximum erosion rate of the alloy at an impact angle of 45 deg for spherical sand particles with a diameter of 100 µm. According to the scanning electron microscopy (SEM) images, the erosion process involves both ductile and brittle mechanisms at this angle.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(10):. doi:10.1115/1.4044295.

The goal of this work is to investigate the dynamic responses of the parallel mechanism with irregular clearances caused by wear and to further reveal the influences of multiple clearance interaction on wear. The motion model and the force model of spherical clearance joint based on a continuous contact force model and a static friction model are established. The dynamic equation of the spatial parallel mechanism considering two spherical clearance joints is derived. A general wear analysis strategy to establish spherical clearance joint with sustainable updation of the surface profile is presented, and the dynamic responses of parallel mechanism after wear are studied. The interaction between two wear joints with different initial clearance values is further investigated. The results show that it is necessary to consider the factor of irregular clearances caused by wear in the analysis of dynamics behavior for precision mechanisms. Proper distribution of clearance values can reduce wear of clearance joint and improve the useful life of mechanism to a certain extent. This work provides a foundation for life prediction and reliability analysis of parallel mechanisms.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(10):101601-101601-8. doi:10.1115/1.4044081.

Super duplex stainless steel (SDSS) has excellent mechanical properties and corrosion resistance. However, currently, there are few researches conducted on its fretting wear performance. This paper studies the influence of different heat treatment temperatures and medium environment on the fretting wear performance of SAF 2507 SDSS. Results show that the combined effect of the sigma phase and seawater lubrication can significantly improve the wear resistance of SAF 2507 SDSS. After treated with different heat treatment temperatures, different contents of sigma phases are precipitated out of SAF 2507 SDSS, which improves the wear resistance of the material to different degrees. In addition, the fretting wear performance of SAF 2507 SDSS also relates to the lubrication medium. In air, the friction and wear performance of SAF 2507 SDSS is poor, while in seawater, solution and corrosion products that acted as a lubricant dramatically improve the wear resistance of the material. Under the combined action of heat treatment and seawater lubrication medium, the friction coefficient and wear reduce by 70% and 91%, respectively.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2019;141(10):. doi:10.1115/1.4044294.

In this paper, the antiwear and antifriction performance of MoS2 nanoparticle in castor oil was studied. The ball-on-disc tests were performed for different concentrations of MoS2 nanoparticle. Coefficient of friction, wear loss, and worn surface morphology were investigated. The results show that MoS2 nanoparticle could reduce the possibility of asperities direct contact, resulting in the reduction of the coefficient of friction and adhesive wear. However, MoS2 nanoparticle in excessive concentration could agglomerate into large particles, playing the role of an abrasive particle, which reduces the beneficial effects of MoS2 nanoparticle.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2019;141(10):101301-101301-8. doi:10.1115/1.4044178.

Hydrophobic and self-cleaning photocatalytic ceramics and concrete with potential for the superhydrophobicity are promising novel materials for civil engineering applications including buildings, bridges, road pavements, and airport runways. Due to embedded liquid-repellent properties, such materials have low water and salt absorption and, therefore, enhanced durability. However, in applications requiring high traction (e.g., tire and pavement), there is a concern that reduced adhesion may compromise the friction. This paper reports on wetting, dry friction, and roughness properties of TiO2 coated (hydrophilic) and polymethyl hydrogen siloxane (PMHS) coated (hydrophobic) self-cleaning ceramic tiles. The coefficient of friction values of the tile–rubber interface do not change significantly with the applications of the coatings up to 0.67 for hydrophilic TiO2 based and up to 0.46 for hydrophobic TiO2 + PMHS coatings versus 0.45 for uncoated reference. Friction has adhesion and roughness-related components and this response can be attributed to the roughness component of friction due to TiO2 coating. The challenges related to hydrophobic coatings, including the durability and future research, are also discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2019;141(10):101401-101401-12. doi:10.1115/1.4044131.

Three types of surfaces in the Schroeder–Webster (SW) theory, i.e., sliding, mixed, and sticking surfaces, have been verified via finite element analysis of an axisymmetric compression test for a metallic specimen. Judging from (i) the radial profile of the pressure at the top elements and (ii) the radial displacement at the top nodes, the three types of SW surfaces are not manifested in the numerical simulation. However, the SW friction compensation model developed for the SW-sliding surface is remarkably reliable in predicting the measured stress–strain curve of the barreled specimen down to the height-to-diameter ratio of 0.1. The origin of this reliability is discussed along with recommendations for using the SW friction compensation model for the SW-sliding surface.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2019;141(10):101701-101701-9. doi:10.1115/1.4044083.

Effects of shaft shape errors are studied on dynamic characteristics of a rotor-bearing system. Stability characteristics of the cylindrical journal bearing are studied. It is shown that the rotating speed at which the oil whip occurs increases when the shape errors exit. And, there is a threshold speed of the bearing with shaft shape errors; before the speed is increased to the threshold, orbits of the center of the journal decrease, and when the speed exceeds the threshold, the orbits increase dramatically and oil whip appears. Furthermore, the quantitative relationship between shaft shape errors and bearing reaction forces of the rotor-bearing system is obtained, which is verified by experiments using rotors with different machining precisions. In order to reduce computing time, variational principle is applied when solving Reynolds’ equation.

Commentary by Dr. Valentin Fuster

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