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Research Papers: Coatings and Solid Lubricants

J. Tribol. 2019;141(8):081301-081301-14. doi:10.1115/1.4043714.

Cu-based brake pads applied in high-speed railway trains containing Ni-coated graphite flake and uncoated graphite flake were fabricated by powder metallurgy. The braking properties of the brake pads were investigated by a scaled down testing apparatus with the pad-on-disk configuration under various braking speeds and braking pressures. Compared with the brake pads containing uncoated graphite flake (designated GF), the brake pads containing Ni-coated graphite flake (designated NGF) exhibits a similar braking performance at lower braking speed and pressure. However, NGF shows more stable friction coefficient, lower linear wear loss, and lower maximum temperature during the braking process at worse braking conditions, e.g., 350 km/h, 1.5 MPa. The Ni-coating on the surface of Ni-coated graphite can transfer the mechanical bonding between copper and graphite to diffusion bonding so that there is a stronger interface bonding between copper and Ni-coated graphite. Further, the multiple linear regression analyses reveal that the mean friction coefficient of NGF is more sensitive to braking pressure than braking speed because of the better thermal resistance of NGF, while the mean friction coefficient of GF and the linear wear loss are mainly affected by braking speed.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2019;141(8):081501-081501-18. doi:10.1115/1.4043641.

Recent experiments have shown that the elastic deformation behaviors of a polymeric material are consistent with the Cosserat elasticity under nonuniform deformation at a millimeter scale. Thus, an elastohydrodynamic lubrication model in the framework of the Cosserat continuum theory is proposed to explore the lubrication performance that deviates from the classical elastohydrodynamic lubrication theory for the small polymer journal bearings with millimeter size. The elastic deformation of the bearing sleeve made of polymeric material and the pressure distribution in a lubricating film are obtained through an iterative solution of the equation of the Cosserat elasticity and the modified Reynolds’ equations with considering the boundary slippage. The effect of bearing size and Cosserat characteristic lengths for torsion and bending on the lubrication performance of the small polymer journal bearings is studied using the proposed Cosserat elastohydrodynamic lubrication model. It was found that the small changes in film thickness due to the Cosserat effect can result in large changes in film pressure. The Cosserat characteristic length of bending possesses a significant effect on the lubrication behaviors of the journal bearings, because the size effect is mainly caused by the increased apparent modulus due to the bending elastic deformation of the bearing sleeve. The boundary slip behaviors dependent on the Cosserat characteristic length are also studied using the Cosserat elastohydrodynamic model, and the numerical results show that the Cosserat characteristic length changes the optimal geometric parameters of the slip zone in terms of load carrying capacity for the small polymer journal bearings.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2019;141(8):081601-081601-11. doi:10.1115/1.4043779.

The microstructural morphology and wear behavior of as-cast Al–X wt% Mg2Si (X = 0.0, 5.0, 10.0, 15.0, and 20.0) composites were investigated through optical microscopy (OM), energy dispersive X-ray (EDX) spectrometry, scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM). The dry sliding wear behavior was studied against an EN 31 hardened steel disk at four different applied loads (19.6 N, 29.4 N, 39.2 N, and 49 N) with a sliding speed of 62.8 m/min for 1 h. The optical microscopy analysis exhibits that the primary Mg2Si particles average equivalent diameter and volume fraction are increased with an increase in Mg2Si (Mg and Si) concentration in the Al–Mg2Si composite. Therefore, the bulk hardness of the composites is increased, whereas the primary Mg2Si hardness decreased because the coarser primary Mg2Si particles have less compactness. The wear resistance of the commercially pure aluminum significantly improved due to Mg2Si reinforcement, and the wear resistance is increased with the increase in Mg2Si concentration up to 15.0 wt% and then decreased at 20.0 wt%. The tested composites worn surfaces and debris exhibit adhesion, delamination, microcutting-abrasion, abrasive- and oxidation-type wear mechanism.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(8):081602-081602-8. doi:10.1115/1.4043851.

In this study, friction and evolution of cohesive failures (cracks and spallations) in CoB/Co2B layers were analyzed. Thermochemical treatment was carried out at 950, 975, and 1000 °C during 6, 8, and 10 h of exposure time, respectively. The characterization techniques include scanning electron microscopy (SEM) for morphology inspection of layers and thickness measurement, X-ray diffraction for the investigation of phases formed, and instrumented indention to obtain hardness, Young’s modulus, and residual stresses. The practical adhesion of the boride layers was evaluated by a progressive load scratch test (PLST), and the critical loads of cracking, chipping, and spallation were calculated. Later, unidirectional multipass scratch tests (MPSTs) were performed by applying subcritical loads selected from the lower spallation load (54 N); these tests were conducted for a different number of scratch passes. The results show that the coefficient of friction (COF) and coating damage depend on the applied load, the number of passes, and coating thickness. In multipass scratch, the mechanical properties, residual stress state, and thickness play a significant role in the evolution of the coating damage that manifests in the form of cracks and cohesive spallations. Considering the scratch and multipass scratch results, it was found that the sample with 10 h of exposure time presents a better performance among the samples.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2019;141(8):081701-081701-12. doi:10.1115/1.4043780.

Various solutions for the design of oil-free bearings are discussed in the literature. Adding hydrodynamic preload to the foil bearings by profiling the inner bore of the bearing is one of the most frequently investigated methods for improving the bearing stability and damping character of the entire system. However, this approach leads to a reduced load capacity and thus to an increased lift-off speed of the foil bearings. Observations of this kind lead to the presentation of various solutions for active bearing contour adjustment, which benefits from different profiles of the lubricant film. Most of these concepts use piezoelectric stack actuators to generate the required alternating force, although the influence of the stiffness of adaptive elements on bearing performance is not fully discussed in the literature. The focus of this study is on the investigation of structural conformity, i.e., the harmonization of stiffness with respect to the requirements for shape control and load capacity of an adaptive air foil bearing (AAFB). The result may be a basis for the consideration of additional degrees of freedom in any concept with shape control as the main design framework in interaction between the lubricant and compliant structure in an air foil bearing from both static and dynamic points of view.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2019;141(8):082002-082002-5. doi:10.1115/1.4043853.

Frictional properties of a self-lubricating nanocomposite material with an A–OOO polyimide matrix reinforced by gas-phase synthesized tungsten diselenide (WSe2) nanoparticles are studied with a homemade low sliding speed tribometer. Tungsten diselenide is often used as a solid lubricant due to its layered structure yielding to anisotropy, which enhances lubrication properties. To facilitate molecular adhesion friction mechanism, friction against a very smooth steel surface (a Johansson gauge block) was used. It is shown that the composite material reinforced with WSe2 nanoparticles has enhanced frictional performance including lower friction and adhesion.

Commentary by Dr. Valentin Fuster

Errata

J. Tribol. 2019;141(8):087001-087001-1. doi:10.1115/1.4043908.

Unfortunately, the original publication of the article contained duplicate images in Fig. 9. The images have been corrected as follows:

Commentary by Dr. Valentin Fuster

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