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### Research Papers: Applications

J. Tribol. 2017;139(4):041101-041101-10. doi:10.1115/1.4034955.

Ball piston is an essential design element in eccentric ball piston pump. The objective of this paper is to investigate the dynamic force and lubrication characteristics of the ball piston. In this study, a tribodynamic model of the ball piston is proposed. The model couples the elastohydrodynamic lubrication model of ball–ring pair and the elastohydrodynamic model of ball–cylinder pair with the dynamic model of the ball piston. The interaction between tribological behavior and dynamic performance and the effects of centrifugation and structural deformation of the ball piston are considered. The pressure distributions and film profiles of the ball–ring pair and ball–cylinder pair are calculated. The dynamic normal contact forces and friction forces are analyzed. The friction torque loss and mechanical efficiency of the whole pump are obtained by combing the tribodynamic model of the ball piston with a simplified friction torque model of cylinder-valve shaft pair. A test bench is established for validating the simulation results. The results show that the normal contact force of the ball–ring pair waves in each stroke, and in discharge stroke, the contact force is much bigger than that in suction stoke because of loading force, while the friction force changes very little for the ball sliding in the outer ring. Comparing with the average friction force of the ball–ring pair, the average friction force of the ball–cylinder pair is small, which reflects that the ball–ring pair is more prone to wear. For the ball–ring friction pair, when the load increases, the secondary contact pressure peak appears, the profile of film presents a horseshoe shape, and the classical shrinkage appears at the outlet of contact region. For the ball–cylinder pair, the film pits and the contact pressure jumps at the entrance of contact region for the heavy load and the high shear stain of film. In addition, the friction torque presents a nonlinear growth trend with the increasing working pressure

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041102-041102-7. doi:10.1115/1.4035346.

High chromium white irons (HCCIs) are used extensively throughout the mineral processing industry to handle erosive and corrosive slurries. This study is an investigation of the effect of impact angle and velocity on slurry erosion of HCCI. The tests were carried out using a rotating whirling-arm rig with particle concentration of 1 wt. %. Silica sand which has a nominal size range of 500–710 μm was used as an erodent. The results were obtained for angles of 30 deg, 45 deg, 60 deg, and 90 deg to the exposed surface and velocities of 5, 10, and 15 m/s. The highest erosion resistance of HCCI was at normal impact and the lowest at an angle of 30 deg, irrespective of velocity. The low erosion resistance at an oblique angle is due to large material removal by microcutting from ductile matrix and gross removal of carbides. The effect of velocity, over the studied range from 5 m/s to 15 m/s, on the increase in the erosion rate was minor. The change of impact velocity resulted in changing the slurry erosion mechanisms. At normal incidence, plastic indentation with extruded material of the ductile matrix was the dominant erosion mechanism at low impact velocity (5 m/s). With increasing impact velocity, the material was removed by the indentation of the ductile matrix and to smaller extent of carbide fracture. However, at high impact velocity (15 m/s), gross fracture and cracking of the carbides besides plastic indentation of the ductile matrix were the dominant erosion mechanisms.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041103-041103-7. doi:10.1115/1.4035339.

This paper investigates the nonlinear characteristics of varying compliance contact resonance in a rotor–bearing system and takes into consideration the Hertzian contact deformation and internal radial clearance. We created an experimental rig of a rigid rotor supported by rolling element bearings. In the course of the rotational speed run up and down, the frequency–amplitude curves of the varying compliance vibrations were observed during experiments using different radial loads and compared with the results of our numerical simulations. The experimental and numerical results indicate that the varying compliance contact resonance in the vertical direction presents the soft spring characteristic, while the soft and hard spring characteristics coexist for the horizontal resonance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041104-041104-9. doi:10.1115/1.4035338.

Solid processing storage and conveying units (e.g., hoppers, silos, tumblers, etc.) often involve the collision of granular media with relatively thin walls. Therefore, the impact of a sphere with a thin plate is a problem with both fundamental and practical importance. In the present work, the normal elastoplastic impact between a sphere and a thin plate is analyzed using an explicit finite element method (FEM). The impact involves plastic deformation and flexural vibrations, which when combined results in significant energy dissipation. One way to quantify the energy dissipation is to employ the coefficient of restitution (COR), which is also a key input parameter needed in various granular flow models. The results were validated against the available experimental data. It is observed that, in addition to material properties and impact parameters, the energy dissipation is strongly dependent on the ratio of plate thickness to sphere diameter. A comprehensive parametric study is conducted to evaluate the effect of material properties, geometry, and impact parameters on the energy dissipation. For the impact velocities commonly observed in granular systems (V = 5 m/s or less), it was determined that the energy lost to flexural vibrations can be neglected if the plate thickness is more than twice the sphere diameter, i.e., tcr > 2d. In this scenario, the mode of energy dissipated is primarily due to the plasticity effects.

Commentary by Dr. Valentin Fuster

### Research Papers: Coatings and Solid Lubricants

J. Tribol. 2017;139(4):041301-041301-6. doi:10.1115/1.4034766.

This paper compares the tribological properties of transmission oil dispersed with molybdenum disulfide (MoS2) and tungsten disulfide (WS2) nanoparticles. Lubricant samples are prepared by dispersing MoS2 and WS2 nanoparticles in 0.5 wt.% in transmission oil. The nanoparticles are stabilized in the lubricant by surface modification with surfactant SPAN 80. The stability of the lubricant in terms of size variation of dispersed nanoparticles is evaluated using particle size analyzer. The antiwear, antifriction, and extreme pressure (EP) properties are tested on a four-ball wear tester and a comparison is made to assess the relative performance of MoS2 and WS2 nanoparticles. The friction and wear characteristics of lubricant dispersed with nanoparticles are strongly dependent upon the load taken into consideration. The lubricant dispersed with WS2 nanoparticles gave higher weld load and load wear index (LWI) than that of lubricant dispersed with MoS2 nanoparticles. The metallographic studies show that under high load conditions, the WS2 nanoparticles deposit more than MoS2 nanoparticles, thereby giving better performance at higher load conditions.

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2017;139(4):041401-041401-10. doi:10.1115/1.4034767.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041402-041402-9. doi:10.1115/1.4034960.

Engineered surfaces (ground and similarly structured rough surfaces) show anisotropic characteristics and their topography parameters are direction dependent. Statistical characterization of these surfaces is still complex because of directional nature of surfaces. In this technical brief, an attempt is made to simulate anisotropic surfaces through use of topography parameters (three-dimensional (3D) surface parameters). First, 3D anisotropic random Gaussian rough surface is generated numerically with fast Fourier transform (FFT). Numerically generated anisotropic random Gaussian rough surface shows statistical properties (texture direction, texture ratio) similar to ground and similarly directional anisotropic rough surfaces. For numerically generated anisotropic Gaussian rough surface, important 3D roughness parameters are determined. Sayles and Thomas' (1976, “Thermal Conductance of Rough Elastic Contact,” Appl. Energy, 2(4), pp. 249–267.) theoretical model for directional anisotropic rough surface is adopted here for calculating the summit parameters, i.e., equivalent bandwidth parameter, mean summit curvature, skewness of summit height, standard deviation of summit height, and equivalent spectral moments. This work demonstrates the variation of spectral moments in both across and parallel to the lay directions with pattern ratio ($γ=βx/βy$). Correlation length (βx) is fixed $10μm$ and correlation length (βy) is varied from 100 to 10 $μm$. Variation of summit parameters with pattern ratio is also discussed in detail. Results shows that mean summit curvature and skewness of summit heights increase with increase in pattern ratio, whereas standard deviation of summit heights and equivalent bandwidth parameter (αe) decreases with pattern ratio. A significant difference is found in “Abbott-Firestone” parameters when calculated in both perpendicular and parallel to lay directions. Effect of these parameters on wear process is discussed in brief.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2017;139(4):041501-041501-12. doi:10.1115/1.4034956.

This paper investigates coupling strategies for finite element modeling (FEM) of thermal elastohydrodynamic lubrication (TEHL) problems. The TEHL problem involves a strong coupling between several physics: solid mechanics, fluid mechanics, and heat transfer. Customarily, this problem is split into two parts (elastohydrodynamic (EHD) and thermal) and the two problems are solved separately while an iterative procedure is established between their respective solutions. This weak coupling strategy involves a loss of information, as each problem is not made intimately aware of the evolution of the other problem's solution during the resolution procedure. This typically leads to slow convergence rates. The current work offers a full coupling strategy for the TEHL problem, i.e., both the EHD and thermal parts are solved simultaneously in a monolithic system. The system of equations is generated from a finite element discretization of the governing field variables: hydrodynamic pressure, solids elastic deformation, and temperature. The full coupling strategy prevents any loss of information during the resolution procedure leading to very fast convergence rates (solution is attained within a few iterations only). The performance of the full coupling strategy is compared to that of different weak coupling strategies. Out of simplicity, only steady-state line contacts are considered in this work. Nevertheless, the proposed methodology, results, and findings are of a general nature and may be extrapolated to circular or elliptical contacts under steady-state or transient conditions.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2017;139(4):041601-041601-5. doi:10.1115/1.4035014.

The effect of graphite (Gr) content on tribological performance of copper-matrix composites against H13 steel was investigated using a pin-on-disk test in the range of 3.14–47.1 m/s. The composites with different weight fractions of Gr (up to 18%) were fabricated by powder metallurgy technique. The results showed that the friction coefficient and wear rate generally decreased with the increase in Gr content. However, the friction coefficient and wear rate differ at various speeds. At 200 and 500 r/min, the friction coefficient and wear rate kept lower with the increase in Gr content, because the third body of Cu–Al–3%Gr specimen had strong fluidity and plasticity. By contrast, the particle third body of Cu–Al–12%Gr specimen, which contained higher content of Gr, could roll easily. Increased Gr feeding to the third body was reasonable for the decreasing of friction coefficient and wear with the increasing of the amount of Gr content at the speed in the range of 1000–2000 r/min. Under the high-speed, the friction coefficient showed slight change because the friction temperature induced all the third bodies to extend and flow effortlessly without componential influence. However, wear decreased significantly because the third body possessed more metal, which favored attachment to the counter disk.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041602-041602-10. doi:10.1115/1.4035155.

LM13/AlN (10 wt. %) metal matrix composites (MMC) and unreinforced aluminum alloy were produced under stir casting route. Microstructural characteristics were examined on the developed composite using optical microscope. The hardness and tensile test were carried out on both unreinforced aluminum alloy and composite using Vickers hardness tester and universal testing machine (UTM), respectively. Dry sliding wear behavior of the composite and unreinforced aluminum alloy was evaluated using pin-on-disk tribometer based on the design of experiments approach. Experimental parameters such as applied load (10, 20, and 30 N), velocity (1, 2, and 3 m/s), and sliding distance (500, 1000, and 1500 m) were varied for three levels. Signal-to-noise (S/N) ratio analysis, analysis of variance, and regression analysis were also performed. The characterization results showed that reinforcement particles were uniformly distributed in the composite. The hardness and tensile test revealed greater improvement of property in composite compared to that of unreinforced alloy. Wear plot showed that wear was increased with increase in load and decreased with increase in velocity and sliding distance. S/N ratio analysis and analysis of variance (ANOVA) indicated that load has greater significance over the wear rate followed by velocity and sliding distance. Regression analysis revealed greater adequacy with the constructed model in predicting the wear behavior of composite and unreinforced aluminum alloy. Scanning electron microscopy (SEM) analysis is evident that the transition of wear from mild to severe occurred on increase of the load in the composite.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041603-041603-9. doi:10.1115/1.4035147.

In the present work, a wear-resistant composite cladding of Ni-based+10% SiC was developed on martensitic stainless steel (SS-420) through a recently developed process microwave hybrid heating (MHH) technique. In the current investigation, domestic microwave oven of frequency 2.45 GHz and 900 W power was used for the development of clads. The metallurgical and mechanical characterizations of developed clads were carried through scanning electron microscope (SEM), X-ray diffraction (XRD), and Vicker's microhardness. The developed clad is uniformly developed and it is metallurgically bonded with the substrate. The average Vicker's microhardness of the clad was 652 ± 90 HV. The tribological behavior of cladding has been investigated through pin-on-disk sliding method against an EN-31 (HRC-62). The clad surface showed good resistance to the sliding wear. It is observed that in case of the clad samples, wear occurs due to dislocation of particles, smearing off of tribofilm, and craters due to pullout of carbides from the matrix.

Commentary by Dr. Valentin Fuster

### Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;139(4):041701-041701-15. doi:10.1115/1.4035343.

Active magnetic bearings (AMBs) have been utilized widely to support high-speed rotors. However, in the case of AMB failure, emergencies, or overload conditions, the auxiliary bearing is chosen as the backup protector to provide mechanical supports and displacement constraints for the rotor. With lack of support, the auxiliary bearing will catch the dropping rotor. Accordingly, high contact forces and corresponding thermal generation due to mechanical rub are applied on the dynamic contact area. Rapid deterioration may be brought about by excessive dynamic and thermal shocks. Therefore, the auxiliary bearing must be sufficiently robust to guarantee the safety of the AMB system. Many approaches have been put forward in the literature to estimate the rotor dynamic motion, nonetheless most of them focus on the horizontal rotor drop and few consider the inclination around the horizontal plane for the vertical rotor. The main purpose of this paper is to predict the rotor dynamic behavior accurately for the vertical rotor drop case. A detailed model for the vertical rotor drop process with consideration of the rotating inclination around x- and y-axes is proposed in this paper. Additionally, rolling and sliding friction are distinguished in the simulation scenario. This model has been applied to estimate the rotor drop process in a helium circulator system equipped with AMBs for the 10 MW high-temperature gas-cooled reactor (HTR-10). The HTR-10 has been designed and researched by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University. The auxiliary bearing is utilized to support the rotor in the helium circulator. The validity of this model is verified by the results obtained in this paper as well. This paper also provides suggestions for the further improvement of auxiliary bearing design and engineering application.

Topics: Bearings , Rotors
Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041702-041702-12. doi:10.1115/1.4035157.

A method to design hybrid hydrostatic/hydrodynamic journal bearings, with the criterion of optimized self-compensation under misaligning loads, is presented. An analysis considering laminar and turbulent flow of a Newtonian incompressible lubricant between the bearing and a misaligned shaft, with restricted lubricant supply to each recess, is discussed. The mathematical model considers the modified steady-state Reynolds lubrication equation, an exact function for the local bearing radial clearance with a misaligned shaft, the continuity integral–differential equations at the recess limits, and boundary conditions at the cavitation zone and outer limits. The finite-difference method was used, and a modular computer program was developed. The procedure follows a univariate search to determine the optimum size and position of recesses and therefore obtain the design with the maximum reactive moment under misaligning loads. A validation of the model was obtained comparing the results with experimental and calculated data from the literature. Results for a 4 + 4 LBP hybrid bearing design are presented.

Commentary by Dr. Valentin Fuster

### Research Papers: Lubricants

J. Tribol. 2017;139(4):041801-041801-7. doi:10.1115/1.4035345.

The goal of this paper is to investigate tribological characteristics of nanographene platelets and hybridized nanocomposite of multiwalled carbon nanotubes (MWCNTs)/multilayer nanographene platelets (MLNGPs)/lithium based-grease. Characterization is done through high resolution transmission electron microscopy (TEM) and X-ray diffraction. While grease properties were tested using Falex four-ball testing machine. Scanning electron microscopy (SEM) and energy dispersive X-ray diffraction (EDX) were utilized to characterize the lubrication mechanism and the worn surface. The results showed that 1% of MLNGPs is the optimum concentration. Wear scar diameter (WSD) was reduced by 66%, friction coefficient was reduced by 91%, while maximum nonseizer load was increased by 90 kg over ordinary lithium grease. Hybrid MWCNTs\MLNGPs were studied, and the optimum ratio of MLNGPs to MWCNTs was found to be 4:1.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(4):041802-041802-8. doi:10.1115/1.4035342.

Recently, ionic liquids (ILs) have received an increasing attention as lubricants owing to their intriguing properties such as tunable viscosity, high thermal stability, low emissions, nonflammability, and corrosion resistance. In this work, we investigate how the incorporation of octadecyltrichlorosilane (OTS) functionalized silica nanoparticles (NPs) in 1-butyl-3-methylimidazolium (trifluoromethysulfony)imide influences the tribological properties and rheological properties of IL under boundary lubrication and elastohydrodynamic conditions, respectively. It was found that the coefficient of friction was depended on the concentration of NPs in IL with a concave upward functional trend with a minimum at 0.05 wt.% for bare silica NPs and at 0.10 wt.% for OTS-functionalized silica NPs. For steel–steel sliding contact, the presence of functionalized NPs in IL at the optimum concentration decreased the coefficient of friction by 37% compared to IL and 17% compared to IL with bare silica NPs. While IL with bare NPs demonstrated a shear thinning behavior for all concentrations, IL with functionalized NPs showed a Newtonian behavior at low concentrations and shear thinning behavior at high concentrations. Overall, this study provides new insights into the antifriction and antiwear additives for lubrication systems involving ILs.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Tribol. 2017;139(4):044501-044501-9. doi:10.1115/1.4035153.

Fluid film bearings (FFBs) provide economic wear-free performance when operating in hydrodynamic lubrication regime. In all other operating conditions, except hydrostatic regime, these bearings are subjected to wear. To get wear-free performance even in those conditions, a hybrid (hydrodynamic + rotation magnetized direction (RMD) configured magnetic) bearing has been proposed. The hybrid bearing consists of square magnets to repel the shaft away from the bearing bore. Load-carrying capacities of four configurations of hybrid bearings were determined. The results are presented in this paper. The best configuration of hybrid bearing was developed. A test setup was developed to perform the experiments on the fluid film and hybrid bearings. The wear results of both the bearings under same operating conditions are presented.

Commentary by Dr. Valentin Fuster

### Discussion

J. Tribol. 2017;139(4):045501-045501-1. doi:10.1115/1.4034824.

The work is an addition to the numerous investigations that establish temperature rise in rubbing as a powerful, yet practical, diagnostic parameter for tribosystems. For effective condition monitoring, it is imperative to progress from diagnostics to prognostics. The discusser argues that a pathway toward prognosis of wear catastrophes is to consider the physical interpretation of the temperature rise rather than its numerical value. To that end, the thermodynamic definition of temperature being an indicator of the intensity of thermal energy present within a material volume of interest is beneficial. Accordingly, the temperature to be utilized in expressing material properties is indicative of the level of thermal energy necessary to initiate tribofailure (i.e., expressing tribofailure thresholds in terms of a “critical energy density”).

Commentary by Dr. Valentin Fuster

### Closure

J. Tribol. 2017;139(4):046001-046001-1. doi:10.1115/1.4035845.

We thank Dr. Abdel-Aal for his comments and discussion. Our intention was the description of temperature, topography, and the wear behavior in quite different tribological systems by known models. Such models provide the opportunity for the construction of pa·v diagrams as a helpful tool to determine transition lines which mark the change of wear mechanisms or the beginning of tribofailure. In order to establish meaningful pa·v diagrams, the determination of the temperature in the region of contact is essential. For the determination of this temperature, the dependency of the respective material properties on the temperature was explicitly considered. Therefore, thermal properties and their variation were implicitly linked to wear damage and the strength of the respective material [1].

Commentary by Dr. Valentin Fuster