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Research Papers: Applications

J. Tribol. 2017;140(3):031101-031101-12. doi:10.1115/1.4038098.

In this study, a new approach has been developed to simulate three-dimensional (3D) experimental rolling contact fatigue (RCF) spalls using a two-dimensional (2D) finite element (FE) model. The model introduces a novel concept of dividing the 3D Hertzian pressure profile into 2D sections and utilizing them in a 2D continuum damage mechanics (CDM) RCF model. The distance between the two sections was determined by the size of the grains in the material microstructure. The 2D RCF model simulates characteristics of case carburized steels by incorporating hardness gradient and residual stress (RS) distribution with depth. The model also accounts for the topological randomness in the material microstructure using Voronoi tessellation. In order to define the failure criterion for the current model, sub-surface stress analysis was conducted for the Hertzian elliptical contact. It was predicted that the high shear stress region near the end of the major axis of the contact is the cause of catastrophic damage and spall formation. This prediction was validated by analyzing the spalls observed during RCF experiments using a surface profilometer. The model was implemented to predict RCF lives for 33 random material domains for different contact geometry and maximum Hertzian pressures. The model results were then compared to the RCF experiments conducted on two different test rigs, a three-ball-on-rod and a thrust bearing test apparatus (TBTA). It was found that the RCF lives obtained from the model are in good agreement with the experimental results. The results also demonstrated that the spalls generated using the analytical results resemble the spalls observed in experiments.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2017;140(3):031501-031501-12. doi:10.1115/1.4037844.

In this study, a modified mixed lubrication model is developed with consideration of machined surface roughness, arbitrary entraining velocity angle, starvation, and cavitation. Model validation is executed by means of comparison between the obtained numerical results and the available starved elastohydrodynamic lubrication (EHL) data found from some previous studies. A comprehensive analysis for the effect of inlet oil supply condition on starvation and cavitation, mixed EHL characteristics, friction and flash temperature in elliptical contacts is conducted in a wide range of operating conditions. In addition, the influence of roughness orientation on film thickness and friction is discussed under different starved lubrication conditions. Obtained results reveal that inlet starvation leads to an obvious reduction of average film thickness and an increase in interasperity cavitation area due to surface roughness, which results in significant increment of asperity contacts, friction, and flash temperature. Besides, the effect of entrainment angle on film thickness will be weakened if the two surfaces operate under starved lubrication condition. Furthermore, the results show that the transverse roughness may yield thicker EHL films and lower friction than the isotropic and longitudinal if starvation is taken into account. Therefore, the starved mixed EHL model can be considered as a useful engineering tool for industrial applications.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2017;140(3):031601-031601-8. doi:10.1115/1.4037845.

One of the major advantages of metal matrix composites (MMCs) is that their tailorable properties meet the specific requirements of a particular application. This paper deals with the experimental investigations done on the effects of the reinforcement particulate size and content on the Al7075/SiC composite. The composites were manufactured using stir casting technique. The effect of SiC particle size (25, 50, and 75 μm) and particulate content (5, 10, and 15 wt %) on the microstructural, mechanical properties, and wear rate of the composites was studied and the results were analyzed for varied conditions of reinforcement. Scanning electron microscope (SEM) examinations were used to assess the dispersion of SiC particles reinforced into the matrix alloy and was found with reasonably uniform with minimal particle agglomerations and with good interfacial bonding between the particles and matrix material. X-ray diffraction (XRD) analysis confirmed the presence of Al and SiC with the composite. The results of mechanical tests showed that the increasing SiC particle size and content considerably enhanced the ultimate tensile strength and hardness of the composites while the ductility at this condition was decreased. The highest ultimate tensile strength of 310 MPa and hardness of 126 BHN were observed for the composites containing 15 wt %. SiC at 75 μm. Lesser the wear resistance of the reference alloy while it was enhanced up to 40% with the composites. The wear resistance was increased up to 1200 m of sliding distance for all the composites, whereas for the composite containing 75 μm SiC particles, it was extended up to 1800 m.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031602-031602-8. doi:10.1115/1.4038101.

A chromium nitride (CrN) coating was deposited on YT14 cemented carbide using a cathodic arc ion plating (CAIP). The high temperature friction and wear of the obtained coating were investigated using a wear test. The results show that the Cr and N form the atom-rich zones in the coating. The average coefficient of friction (COF) of CrN coatings at 300 °C, 400 °C, and 500 °C is 0.50, 0.62, and 0.43, respectively. The wear mechanism of CrN coating at 300 °C and 400 °C is abrasive wear, and that at 500 °C is abrasive wear, accompanied by slight oxidation wear.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031603-031603-7. doi:10.1115/1.4038102.

In this work, the TiO2 coatings were synthesized by electrophoretic deposition (EPD) of nanosized powder in order to improve the tribological properties. Several characterization methods were applied to the coated substrates. The surface topography of the EPD layers, their morphology, composition, and mechanical properties were investigated. The influence of heat treatment, which results in calcination, on the wear performance of coated films was also examined. It was noticed that the effect of the normal force and sliding velocity on the coefficients of instantaneous and stabilized friction was not the same in treated coatings and untreated ones. Moreover, the treated and uncoated films showed a close relation between the dissipated accumulated energy and the worn volume. The energetic wear coefficients of fretting wear were also studied. As expected, the treated coating reduced the energetic wear coefficient, which enhanced the resistance to fretting wear.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031604-031604-6. doi:10.1115/1.4038269.

Due to its high mechanical strength, exceptional biocompatibility, low elastic modulus, and superior corrosion resistance, Ti13Nb13Zr alloy is one of the potential candidates for implanted joints. However, the poor tribological property of Ti13Nb13Zr alloy has greatly limited its wide usage in artificial joints. The elevated temperature solid carburizing technology was used to improve tribological property of Ti13Nb13Zr alloy. It was found that the surface hardness of Ti13Nb13Zr alloy was increased to 812 HV after the carburization at 1523 K due to the formation of titanium carbide on the surface. With the increase in experimental temperature, the thickness of the carburized layer increased to 120 μm. In addition, the wear rate of Ti13Nb13Zr alloy decreased by 63.9% under serum lubrication condition after the carburization at 1473 K due to the formation of hard TiC on the surface of Ti13Nb13Zr.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031605-031605-8. doi:10.1115/1.4038355.

The present experimental study investigates the effect of constant kinetic energy on erosion wear of aluminum alloy 6063. Three different natural erodents (quartz, silicon carbide, and alumina) with different particle sizes are used to impact at 45 deg and 90 deg impact angles. For calculating the number of particles in the slurry pot, it is assumed that the solid particles are of spherical shape. The total numbers of impacting solid particles were kept constant by adjusting the solid concentration, velocity, and test duration. The scanning electron microscope (SEM) images of the three erodents show that the alumina particles have sharp edges with more angularity, and silicon carbide particles have subangular nature while quartz particles are blocky in shape. The mass loss per particle at 45 deg impact angle is observed higher than at normal impact angle. It may be due to the change in material removal mechanism with changing the impact angle. It is also found that the mass loss per particle from the target material having different particle size with constant kinetic energy remains constant for respective erodents. This indicates that the velocity exponent of impacting particles is around 2. The SEM images of eroded surfaces reveal the different mechanisms of material removal at 45 deg impact angle and at normal impact angle.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;140(3):031701-031701-10. doi:10.1115/1.4038099.

Floating ring bearings (FRBs) are widely used in automobile turbochargers. However, there is no satisfying explanation of phenomenon that ring rotation speed levels off when the shaft speed reaches a certain value under low oil-supplied pressure condition. The traditional opinion that effective viscosity decreases with increasing temperature cannot completely explain this phenomenon. In this study, the air entrainment effect is introduced and evaluated using computational fluid dynamics (CFD). CFD results considering air entrainment, viscous heating, and heat transfer are compared with experimental results to evaluate each effect. The decrease in effective viscosity as a result of air–oil–thermal coupling effect is the mechanism behind the abovementioned phenomenon. This study provides calculated data and visual results of the air entrainment in low oil-supplied pressure FRB.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031702-031702-13. doi:10.1115/1.4038104.

The journal is the part of a shaft that is inside a fluid film bearing and is usually assumed to be circumferentially isothermal. Recent work has shown that under certain vibration conditions, a significant temperature difference (ΔT) can develop around the journal circumference. The ΔT may cause the shaft to bend leading to a synchronous vibration instability problem, termed the “Morton effect” (ME). A test rig was developed to verify the asymmetric journal temperature of the ME and its prediction using a five-pad tilting pad journal bearing (TPJB) operating with an eccentric shaft to replicate a circular vibration orbit. The bearing is tested at various conditions including: supply oil temperature at 28 °C and 41 °C, bearing operating eccentricities of zero and 32%Cb, and rotor speed up to 5500 rpm. The journal temperature distribution is recorded with 20 sensors located around the journal circumference, and the measurements provide a benchmark for predictions from a time transient model with the three-dimensional (3D) fluid and solid finite element method (FEM), and with a simplified ME prediction approach using only steady-state results. The test results follow the predictions exhibiting a sinusoidal-like temperature profile around the circumference with an angular lag of the hot spot location behind the high spot location (angular position on the rotor that arrives at the minimum film thickness condition each rotation) by a speed-dependent angle. Increasing the supply oil temperature reduced the journal ΔT, while increasing the bearing operating eccentricity increased the journal ΔT. The agreement between the test and predicted results is significantly better for the 3D FEM transient model than for the steady-state-based model in terms of journal ΔT and hot spot position. An improved version of the latter approach is proposed and yields significantly better correlation with the test measurements.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):031703-031703-12. doi:10.1115/1.4038353.

The noise of a journal bearing with the compound groove textures is studied based on computation fluid dynamic (CFD) theory and broadband noise source theory. In doing so, the acoustic power levels of the noise for the journal bearing with the compound groove textures and simple ones are separately solved at different geometry sizes and positions of the groove texture, and varied lubricant parameters using CFD method. Numerical results show that the compound groove texture can more effectively lower the acoustic power level of the journal bearing, compared with the simple groove texture. This reduction depends on the groove size and its position, and density and viscosity of the lubricant.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2017;140(3):031801-031801-10. doi:10.1115/1.4038105.

A dispersant is almost an unavoidable additive in engine oils since it helps to keep the carbonaceous particles in a suspended form. Dispersants can be multifunctional and can therefore interfere with the functions of other additives either synergistically or antagonistically. The present work investigated the influence of four dispersants (with and without particles of hexagonal boron nitride (hBN) on selected lubrication-related properties of the oils using four ball tester. Particles of hBN, though known as effective anti-wear (AW) and anti-friction (AF) additives, did not prove effective in oil in the presence of dispersants. On the other hand, it proved to be a good extreme pressure (EP) additive by showing 27% improvement in weld load (WL). Worn surfaces were examined using scanning electron microscopy (SEM), energy dispersion X-ray analysis (EDAX), and Raman spectroscopy.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2017;140(3):032001-032001-10. doi:10.1115/1.4038100.

Greases are widely used in extreme conditions of load, speed, and temperature, altogether with the improvement in the service life of the machinery by reducing the noise and vibration. The present study deals with the development of nanocomposite greases and their tribodynamic evaluation under boundary lubrication (BL), antiwear, extreme pressure (EP), and vibration behavior of nonconformal metallic contacts. The recording of the vibration signals constitutes the indirect approach to evaluate the lubricity of the tribological contacts. The different nano-additives (reduced graphene oxide (rGO) nanosheets, CaCO3, and α-Al2O3 nanoparticles) are dispersed in commercial lithium grease to formulate nanocomposite greases. The microstructural studies of greases are performed on high-resolution transmission electron microscopy (HRTEM). The BL behavior is studied using four ball tester. Further, the functional groups of the greases and the chemistry of the worn surfaces are evaluated through Raman spectroscopy (RS). To explore the involvement of wear mechanism(s), the morphology of worn surfaces is evaluated using scanning electron microscopy (SEM). The results showed that doping of 0.4% rGO and 5% CaCO3 in bare lithium grease can significantly improve the antiwear, EP properties, and vibration behavior, compared to α-Al2O3 dispersed composite grease.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):032002-032002-6. doi:10.1115/1.4038407.

Lapping is still an efficient and economical way in diamond shaping process, which is important in both industrial and scientific applications. It has been known that the material removal originates from the phase change or amorphization of diamond crystal carbon atoms that are chemically activated by stress, forming a top layer of amorphous carbon atoms. In this paper, the phase change of amorphous carbon atoms undergoing the nanocutting of amorphous layer during diamond lapping process is studied by molecular dynamics (MD) simulation. Two regions, the debris layer and cutting surface underneath, are studied. In the debris layer, the change of sp2 carbon atoms is directly affected by impact, while underneath the cutting surface the changes of carbon atoms are almost not affected; the change speed of amorphous carbon atoms is higher than that of pristine crystal ones; the main phase change is transformation of sp3 into sp2; cutting depth to different extent affects the phase changes of sp3 and sp2 carbon atoms. Our study expands the understanding of diamond lapping process.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2017;140(3):032201-032201-16. doi:10.1115/1.4037846.

Numerical and experimental analyses were carried out to investigate the static characteristics of liquid annular seals with helical grooves in a seal stator. In the numerical analysis, the momentum equations with turbulent coefficients and the continuity equation, which were averaged across the film thickness, were numerically solved to obtain the leakage flow rate and the pressure distributions in the seal clearance. To accurately define the location of the step between the groove and the land regions in the calculation domain, these governing equations were expressed using an oblique coordinate system in which the directions of coordinate axes coincided with the circumferential direction and the direction along the helical grooves. The numerical analysis included the effects of both fluid inertia and energy loss due to expansion during the passage of fluid from the land region to the helical groove region and that due to contraction from the groove region to the land region. In the experimental analysis, the leakage flow rate and the fluid-film pressure distributions in the seal clearance were measured for the helically grooved seals with different helix angles of the helical groove. The numerical results of leakage flow rate and pressure distributions agree reasonably with the experimental results, which demonstrates the validity of the numerical analysis. The leakage flow rate of the helically grooved seals was influenced by two factors: fluid energy loss during passage through the step between the groove and the land, and the pumping effect by which the spinning motion of the rotor pushes the flow back upstream along the helical grooves. Under a low range of rotor spinning velocity, the leakage flow rate decreased with helix angle because the effect of fluid energy loss in the steps was significant. By contrast, under a high range of spinning velocity, the quantitative difference in the leakage flow rate due to the helix angle decreased compared to that under a low range because the reduction in the leakage flow rate due to the pumping effect was pronounced for a larger helix angle. The effects of helix angle and rotor spinning velocity on the leakage flow rate are explained qualitatively using a simplified model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(3):032202-032202-6. doi:10.1115/1.4038354.

This study reviews the work performed in the field of reciprocating shaft seals from the advent of the scientific topic in the 1940s. Concepts of leakage, film layers, friction, wear, and other concerns with shaft seals are discussed. The importance of shaft seals as it pertains to liquid springs is brought to light along with issues requiring a need for these seals to withstand high temperatures and high pressures. Issues resulting from a seal exposure to high temperatures, such as thermosetting and embrittlement, are discussed in conjunction with materials and properties that allow seals to operate in high-temperature environments. High-pressure sealing challenges are identified along with the techniques currently employed to overcome these issues, such as fiber reinforcement and backup rings. Sealing solutions have been implemented independently for both high-pressure and high-temperature applications; however, the combination of high pressures coupled with high temperatures is still a challenge today.

Commentary by Dr. Valentin Fuster

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