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### Research Papers: Applications

J. Tribol. 2015;138(2):021101-021101-8. doi:10.1115/1.4031437.

This paper deals with the effects of grease characteristics on sound and vibration of a linear-guideway type ball bearing. First, sound, vibration, and temperature of a linear-guideway type ball bearing were measured by changing 16 types of greases. Next, in order to explain the effects of grease characteristics (absolute viscosity η0 of the base oil in the grease at atmospheric pressure, pressure–viscosity coefficient ξ of the base oil, and penetration P of grease) and linear velocity V on the sound pressure p and vibratory acceleration a of the linear bearing, dimensional analyses were carried out. The dimensional analyses derived dimensionless products Pp/(η0V) and η0ξV/P for the sound, and dimensionless products Pa/V2 and η0ξV/P for the vibration. The plot of the measured data using dimensionless products showed that both sound pressure p and vibratory acceleration a (under a certain linear velocity) decreased as the absolute viscosity η0 of the base oil in the grease at atmospheric pressure or the pressure–viscosity coefficient ξ increased. Whereas the sound pressure p decreased as the penetration P decreased, the vibratory acceleration a decreased as P increased.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021102-021102-9. doi:10.1115/1.4031583.

The new “I” type double-decker ball bearing (NITDDBB) with two inner contact ball bearings is proposed to improve the speed and load capability of the original I type double-decker ball bearing (OITDDBB). Based on the quasi-statics principle, the mechanical model of the NITDDBB is established and takes into consideration the radial load, axial load, and ball centrifugal forces, as well as the gyroscopic moments. The corresponding calculation model is established on the matlab platform. The mechanical characteristics of the NITDDBB are analyzed and compared with the OITDDBB and also with a single-decker ball bearing (SDBB). Finally, a bearing load test rig is designed and built to verify the simulation results. The results provide a theoretical and experimental basis for the application of the NITDDBB.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021103-021103-8. doi:10.1115/1.4031463.

The motion errors of hydrostatic lead screws can be smaller than the pitch errors due to the averaging effect of oil films, which is the so-called averaging effect on the pitch errors (AEPE). This paper investigates the factors influencing AEPE under low speeds and a constant external load. An equivalent model of hydrostatic lead screws with continuous helical recesses is established, and the axial motion errors of the nut are obtained by combining the dichotomy and finite difference methods. The results show that (a) the motion errors caused by the pitch errors of the lead screw have additivity, and the pitch errors of the nut have little influence on the motion errors, (b) when the lead screw only has cumulative pitch errors, the motion errors are equal to the average value of the cumulative pitch errors located on the two flank surfaces of the lead screw, and (c) when the lead screw only has periodic pitch errors, the number of nut threads with helical recesses n and the period of the pitch errors of the lead screw T are the main influencing factors on AEPE.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021104-021104-7. doi:10.1115/1.4031671.

Shallow deep drawing process of a square cup is investigated in this paper. A combined friction model is introduced which integrates the Khonsari's friction model based on asperity contact and Wilson's friction model based on lubricant flow. A film thickness ratio of 0.035 is introduced for 0.15 μm standard deviation of surface summits. Below the ratio of 0.035, Khonsari's model gives more accurate results since asperity friction is dominant. Above the ratio of 0.035, Wilson's model gives more accurate results since hydrodynamic friction is dominant.

Topics: Friction
Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021105-021105-7. doi:10.1115/1.4031792.

Movement analyses of ball bearings with regard to stable and unstable cage motion behavior are often conducted by simulations, typically by investigating the cage whirl. Some experimental studies exist in which the cage is modified in order to capture its movement with sensors. This paper presents an image-based approach for investigating the cage motion without modifications, which in turn allows a cage motion analysis of an angular contact ball bearing under operation condition. Two new image evaluation algorithms are presented in detail and their suitability is verified by experiments on a bearing test rig.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(2):021106-021106-12. doi:10.1115/1.4031989.

The damages to the structural elements, viz., inner race, outer race, rollers, cage, etc., of rolling contact bearings if not detected in time can cause tragic failures of the machineries supported by these bearings. The operating parameters like variations in the machinery speed, unbalance, operating load, etc., can cause a bearing to vibrate at higher energy levels and consequently will accelerate its wear. An attempt is made in this study, and a generalized model is developed using matrix method of dimensional analysis (MMDA) that predicted the response and correlated the dependent parameter, i.e., response with the significant independent parameters. Combined use of response surface methodology (RSM) is made to explore the dependence of various factors such as size of the defect, unbalance, speed, and their interactions on the vibration characteristics of the bearings. It is observed from the study that the model developed based on the MMDA has provided an efficient approach in recognizing the damaged bearing state, which can be easily implemented in the condition-based preventive maintenance strategies. Also, the effectiveness of MMDA as compared to the conventional Buckingham's pi theorem in the dimensional analysis (DA) practice, especially in the problems involving multiple variables, is shown in this study.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(2):021107-021107-10. doi:10.1115/1.4031990.

The possibility to reduce the amount of cutting fluids from machining processes is actively studied by the industrialists and researchers. Minimum quantity lubrication (MQL) is a solution toward cutting fluids reduction. This paper investigates the consequences on friction coefficient induced by the use of MQL. A tribometer is used in order to experimentally simulate the local tribological conditions encountered during machining. As the relative sliding speed increases, a lower amount of oil is deposited on the rough surfaces. Depending on the MQL operating conditions and sliding velocities, it is plausible to reach starvation by leaving the real rough contact partly dry. A model computing a starvation percentage by filling an estimated oil amount in a deformed topography correlates with the experimental results.

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2015;138(2):021401-021401-11. doi:10.1115/1.4031399.

In this work, an experimental and numerical study is performed to understand squeal generation and suppression of a pad-on-disk friction system. Several friction material specimens having various orientation degrees of grooves cut on their surfaces are tested. Numerical studies using the methods of complex eigenvalue analysis and dynamic transient analysis are conducted to simulate the experimental process with the finite element (FE) software abaqus. Both experimental and numerical results show that surface modifications of friction material specimens have a significant influence on the squeal instability: cutting a 45 deg or 90 deg groove on the material surface can significantly reduce squeal noise, cutting a 135 deg groove just reduces squeal noise moderately and cutting a 0 deg groove cannot reduce squeal noise. Moreover, the contact pressure distributions for the original surface and modified surfaces are studied to provide a physical explanation of the noise phenomenon. The major finding that friction-induced noise can be reduced by means of suitable structural modifications of the contact interface is expected to have important and much wider applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021402-021402-10. doi:10.1115/1.4031403.

Finite-length line contact conditions, existing in applications such as gears or roller bearings, lead to subsurface stress distribution influenced by the free boundaries. This paper presents a semi-analytical method (SAM) for the finite-length line contact problem, based on the overlapping concept and matrix formation, to consider the effect of two free-end surfaces. In order to obtain two free surfaces, three half-spaces with mirrored loads to be solved are overlapped to cancel out the stresses at expected surfaces. The error introduced by this method is analyzed and proven to be negligible. The conjugate gradient method (CGM) is used to solve the pressure distribution, and the fast Fourier transform (FFT) is used to speed up the elastic deformation and stress-related calculation. The model is verified by finite element method (FEM) and shows a high conformity and efficiency. Besides, the line contact situations are discussed to explore the effect of free surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(2):021403-021403-22. doi:10.1115/1.4032072.

The recent development of semi-analytical methods (SAM) has led to numerous improvements in their capabilities in terms of phenomena that can be accounted for and numerical efficiency. They now allow to perform fast and robust simulations of contact between inelastic—with either elastic–plastic or viscoelastic behavior—and anisotropic or heterogeneous materials. All effects may be combined, with either coating, inclusions, cavities, or fibers as inhomogeneities. The coupling between local and global scales remains numerically difficult. A framework is proposed here for contact problems considering the effect of elastic heterogeneities within an elastic–plastic matrix. The mutual interactions among heterogeneities and their surrounding plastic zone as well as the interactions between them and the contact surface through which the load is transmitted should be accounted for. These couplings are outside the validity domain of the Eshelby’s equivalent inclusion method (EIM) that assumes a uniform stress field in an infinite space far from the inhomogeneity. In the presence of heterogeneities close to the surface or located at the Hertzian depth, the yield stress can be reached locally due to the additional stress it generates, whereas the stress and strain state would remain purely elastic for a matrix without inclusion. It is well known that for rolling element bearing and gear applications, the ruin of components is often linked to cracks initiated in the vicinity of large or hard inclusions that act as stress raisers. It turned out that plastic strains tend to reduce the stress generated by the contact pressure while hard heterogeneities will increase it. As plastic strain accumulation can provide the basis for fatigue damage criteria, the second half of the paper will illustrate how the method can be used to identify and rank geometrical and material parameters that influence the location and magnitude of the maximal plastic strain.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2015;138(2):021501-021501-14. doi:10.1115/1.4031494.

A high-order polynomial gas distribution cam mechanism is investigated theoretically from the viewpoint of thermal elastohydrodynamic lubrication (EHL). First, a cam with a larger base circle radius is employed, which results in slide–roll ratio 2.0 < S < 9.0 when the two surfaces move oppositely. The pressure, film thickness, and temperature profiles at a number of angular positions of the cam are presented, together with the isothermal results. The comparison between thermal and isothermal oil characteristics is also shown. It is revealed that the isothermal analysis partly overestimates the actual film thickness and it also misses some essential local phenomena. Second, a cam with a smaller base circle radius is studied, which leads to drastic variations in the slide–roll ratio which encounters four times’ occurrences of infinity in one working period. The pressure, film thickness, and temperature profiles at some angular cam positions together with the oil characteristics are given, showing much dramatic variations. A very small film thickness is observed at the contact of the tappet with the start of the cam basic segment, which suggests a possible risk of direct contact of both surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021502-021502-11. doi:10.1115/1.4031495.

Numerical solution of mixed elastohydrodynamic lubrication (EHL) is of great importance for the study of lubrication formation and breakdown, as well as surface failures of mechanical components. However, converged and accurate numerical solutions become more difficult, and solution process with a fixed single discretization mesh for the solution domain appears to be quite slow, especially when the lubricant films and surface contacts coexist with real-machined roughness involved. Also, the effect of computational mesh density is found to be more significant if the average film thickness is small. In the present study, a set of sample cases with and without machined surface roughness are analyzed through the progressive mesh densification (PMD) method, and the obtained results are compared with those from the direct iteration method with a single fixed mesh. Besides, more numerical analyses with and without surface roughness in a wide range of operating conditions are conducted to investigate the influence of different compound modes in order to optimize the PMD procedure. In addition, different initial conditions are used to study the effect of initial value on the behaviors of this transient solution. It is observed that, no matter with or without surface roughness considered, the PMD method is stable for transient mixed EHL problems and capable of significantly accelerating the EHL solution process while ensuring numerical accuracy.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021503-021503-14. doi:10.1115/1.4031516.

Metallic microparticles of 5–100 μm in size often contaminate elastohydrodynamic (EHD) contacts and indent surfaces. The geometrical characteristics of dents by such solid particles are linked to the way surface damage may evolve and how it may affect the life of the damaged contacts. In many cases, debris dents appear with shoulders raised above the original surface. Material piling-up this way causes high-pressure spikes when dents are over-rolled by an element such as a ball in a rolling bearing. This study introduces an approximate analytical method based on the so-called expanding cavity model (ECM) to calculate pile-up geometry with simple algebraic equations in thermoviscoplastic indentation of rolling EHD contacts by ductile spherical microparticles. Based on an experimentally validated debris indentation model published by the author, the pile-up model is shown to give realistic predictions in a wide range of operating parameters. Upon experimental validation, the new model is used to study the effects of particle size and hardness, Coulomb friction coefficient (CFC), strain hardening, and rolling velocity of EHD contacts on pile-up geometrical parameters including length, height, volume, and curvature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021504-021504-10. doi:10.1115/1.4030956.

Contacts of indentors with functionally graded elastic solids may produce pressures significantly different from the results obtained for homogeneous elastic materials (Hertzian results). It is even more so for heavily loaded line elastohydrodynamically lubricated (EHL) contacts. The goal of the paper is to indicate two distinct ways the functionally graded elastic materials may alter the classic results for the heavily loaded line EHL contacts. Namely, besides pressure the other two main characteristics which are influenced by the nonuniformity of the elastic properties of the contact materials are lubrication film thickness and frictional stress/friction force produced by lubricant flow. The approach used for analyzing the influence of functionally graded elastic materials on parameters of heavily loaded line EHL contacts is based on the asymptotic methods earlier developed by authors (Kudish, 2013, Elastohydrodynamic Lubrication for Line and Point Contacts: Asymptotic and Numerical Approaches, Chapman & Hall/CRC Press, New York; Kudish and Covitch, 2010, Modeling and Analytical Methods in Tribology, Chapman & Hall/CRC Press, New York; and Aizikovich et al., 2006, Contact Problems of Elasticity for Functionally Graded Materials, Fizmatlit, Moscow, Russia). More specifically, it is based on the analysis of contact problems for dry contacts of functionally graded elastic solids and the lubrication mechanisms in the inlet and exit zones as well as in the central region of heavily lubricated contacts.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021505-021505-11. doi:10.1115/1.4030958.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021506-021506-12. doi:10.1115/1.4031752.

Combining the contact model of elastic-layered solid with the concept of asperity contact in elastohydrodynamic lubrication (EHL), a mixed-lubrication model is presented to predict friction coefficient over rough surfaces with/without an elastic-layered medium under entire lubrication regimes. Solution of contact problems for elastic-layered solids is presented based upon the classical model of Greenwood and Williamson (GW) in conjunction with Chen and Engel's analysis. The effects of the Young's modulus ratio of the layer to substrate and the thickness of the layer on the elastic real area of contact and contact load for a fixed dimensionless separation are studied using the proposed method, which is used for the asperities having contact with an elastic coating. Coefficient of friction with elastic-layered solids in boundary lubrication is calculated in terms of Rabinowicz's findings and elastic-layered solutions of Gupta and Walowit. The effect of rough contacts with an elastic layer on friction coefficient in lubrication regimes has been analyzed. Variations in plasticity index $ψ$ significantly affect friction coefficients in boundary and mixed lubrications. For a large value of $ψ$, the degree of plastic contact exhibits a stronger dependence of the mean separation or film thickness than the roughness, and for a small value of $ψ$, the opposite result is true. The effect of governing parameters, such as inlet oil viscosity at ambient pressure, pressure–viscosity coefficient, combined surface roughness, and $El/E2$ on friction coefficient, has been investigated. Simulations are shown to be in good agreement with the experimental friction data.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2015;138(2):021601-021601-6. doi:10.1115/1.4031400.

Multiphase composites are attractive for improved mechanical performance and corrosion resistance. In this research, a new composite consisting quasi-crystalline Al75Mn14Si7Fe4 alloy of icosahedral, cubic α-AlMnSiFe, monoclinic Al13Fe4 phases, and ferro-silico-aluminate geopolymer was synthesized using rapid solidification and thermal treatment methods. The concentration of icosahedral phase (i-phase) was controlled and the formation of geopolymer was obtained through heat treatment. Characterization showed that the microhardness and wear resistance were increased with the amount of i-phase. The corrosion resistance, on the other hand, was improved with the existence of the geopolymer. This research demonstrates an effective approach in processing a multiphase composite that has desired properties and performance through multiphase design and composition.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021602-021602-12. doi:10.1115/1.4031401.

Particulate aluminum matrix composites (PAMCs) with different volume percent of Al3Zr particles have been developed by direct melt reaction (DMR). Wear and friction have been studied in detail for all compositions under dry sliding conditions. Results indicate that the wear rate, normalized wear rate, and wear coefficient of PAMCs decrease continuously with increase in volume percent of Al3Zr particles, however, with applied load and sliding distance, wear continuously increases. Wear rate and wear coefficient with sliding velocity initially decrease for all compositions, attains minima, and then increase sharply. However, coefficient of friction shows increasing trend with composition and sliding velocity but with load it shows a decreasing trend and with distance slid it fluctuates within a value of ±0.025. At low load and sliding velocity three-dimensional (3D)-profilometer, scanning electron microscope (SEM), and debris studies show low Ra values and mild wear dominated by oxidative nature, whereas at high loads and sliding velocities high Ra values and wear nature change to severe wear with mixed mode (oxidative–metallic) and surface with deep grooves is observed. Further, it is also important to note from morphological studies that refinement of matrix phase takes place with in situ formation of Al3Zr particles, which helps to improve hardness and tensile properties finally contributing to low wear rate.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021603-021603-11. doi:10.1115/1.4031781.

To reduce the amount of petroleum-derived fuel used in vehicles and vessels powered by internal combustion engines (ICEs), the addition of bioderived fuel extenders is a common practice. Ethanol is perhaps the most common bioderived fuel used for blending, and butanol is being evaluated as a promising alternative. The present study determined the fuel dilution rate of three lubricating oils (pure gasoline (E0), gasoline–10% ethanol blend (E10), and gasoline–16% isobutanol blend (i-B16)) in a marine engine operating in on-water conditions with a start-and-stop cycle protocol. The level of fuel dilution increased with the number of cycles for all three fuels. The most dilution was observed with i-B16 fuel, and the least with E10 fuel. In all cases, fuel dilution substantially reduced the oil viscosity. The impacts of fuel dilution and the consequent viscosity reduction on the lubricating capability of the engine oil in terms of friction, wear, and scuffing prevention were evaluated by four different tests protocols. Although the fuel dilution of the engine oil had minimal effect on friction, because the test conditions were under the boundary lubrication regime, significant effects were observed on wear in many cases. Fuel dilution was also observed to reduce the load-carrying capacity of the engine oils in terms of scuffing load reduction.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021604-021604-10. doi:10.1115/1.4031599.

Pipelines are the most flexible, economic, and convenient way for oil and gas transportation. Material degradation by slurry erosion is a common feature in oil transmission pipeline. In the present work, slurry erosion of AISI 1018, AISI 1080, API X42, and API X70 steels is investigated in terms of slurry velocity and target material microstructure. The slurry velocity and impact angle employed were 0.2, 0.29, 0.36, and 0.43 m s−1 and 90 deg, respectively. It is found that erosion rate increases with increasing slurry velocity. Scanning electron microscopy was employed to investigate the eroded surface and subsurface of the steels. Plastic deformation, microcutting, and fracture are identified as dominant erosion mechanisms. Pearlitic microstructure exhibits superior erosion resistance compared to ferrite depending upon slurry velocity and microstructural orientation.

Commentary by Dr. Valentin Fuster

### Research Papers: Hydrodynamic Lubrication

J. Tribol. 2015;138(2):021701-021701-7. doi:10.1115/1.4031435.

The effects of microgrooves and microdimples on the load-carrying performance of mechanical gas seals are compared in this study. Numerical model based on the Reynolds equation for compressible Newtonian fluid is utilized to investigate the load-carrying performance including the hydrodynamic pressure, the load-carrying force, and gas film stiffness of the gas seals. The results indicate that both microgrooves and microdimples can improve the load-carrying performance of mechanical gas seals, particularly under a small clearance condition. Furthermore, different texture patterns achieve optimal load-carrying performance at different area density, seal clearance, and depth: microgrooves with a low area density can obtain higher load-carrying force and gas film stiffness than the dimple patterns, but with high area density, elliptical dimples yield better load-carrying performance than the groove patterns.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021702-021702-9. doi:10.1115/1.4031496.

Rotors supported by gas foil bearings (GFBs) experience stability problem caused by subsynchronous vibrations. To obtain a GFB with satisfactory damping characteristics, this study presented a novel hybrid bump-metal mesh foil bearing (HB-MMFB) that consists of a bump foil and metal mesh blocks in an underlying supporting structure, which takes advantage of both bump-type foil bearings (BFBs) and MMFBs. A test rig with a nonrotating shaft was designed to estimate structure characterization. Results from the static load tests show that the proposed HB-MFBs exhibit an excellent damping level compared with the BFBs with a similar size because of the countless microslips in the metal mesh blocks. In the dynamic load tests, the HB-MFB with a metal mesh density of 36% presents a viscous damping coefficient that is approximately twice that of the test BFB. The dynamics structural coefficients of HB-MFBs, including structural stiffness, equivalent viscous damping, and structural loss factor, are all dependent on excitation frequency and motion amplitude. Moreover, they exhibit an obvious decrease with the decline in metal mesh density.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021703-021703-11. doi:10.1115/1.4031780.

Minimization of parasitic losses in the internal combustion (IC) engine is essential for improved fuel efficiency and reduced emissions. Surface texturing has emerged as a method palliating these losses in instances where thin lubricant films lead to mixed or boundary regimes of lubrication. Such thin films are prevalent in contact of compression ring to cylinder liner at piston motion reversals because of momentary cessation of entraining motion. The paper provides combined solution of Reynolds equation, boundary interactions, and a gas flow model to predict the tribological conditions, particularly at piston reversals. This model is then validated against measurements using a floating liner for determination of in situ friction of an engine under motored condition. Very good agreement is obtained. The validated model is then used to ascertain the effect of surface texturing of the liner surface during reversals. Therefore, the paper is a combined study of numerical predictions and the effect of surface texturing. The predictions show that some marginal gains in engine performance can be expected with laser textured chevron features of shallow depth under certain operating conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021704-021704-10. doi:10.1115/1.4031782.

Machine hammer peening (MHP) is an incremental surface finishing process. It enables both surface smoothing and texturing. Compared to well-established surface texturing processes, MHP has the advantage of simultaneous induction of strain hardening and compressive residual stresses. Both texturing and surface layer modification are very beneficial in case of mixed-boundary lubrication. MHP has been only recently developed. Therefore, the influence of surface textures manufactured by MHP on tribological interactions is unknown and lacks fundamental investigations. In this work, hydrodynamics of MHP textures is investigated by means of a three-dimensional (3D) computational fluid dynamics (CFD) analysis. The analyzed MHP textures have already been experimentally used to reduce friction in strip drawing tests. Using CFD analysis, an optimal arrangement of multiple elliptically shaped surface structures for maximizing the fluid pressure and the load-bearing capacity is determined. Furthermore, a correlation between the determined process parameters and the lubrication properties is presented. Because of significantly high hydrostatic pressures, cavitation is neglected in this work. Additionally, the effect of structure pileups is neglected in this study. Within the range of parameters investigated, it was found that an arrangement of surface textures by MHP should be transversally overlapping and clearly separated longitudinally. High structure depths, lubricant viscosities, and sliding velocities further improve the load-bearing capacity as well as small fluid-film thicknesses.

Commentary by Dr. Valentin Fuster

### Research Papers: Lubricants

J. Tribol. 2015;138(2):021801-021801-5. doi:10.1115/1.4031397.

The field performance of hydraulic oils depends on factors such as the viscosity index (VI), flash point (FP), pour point (PP), and demulsibility. A hydraulic oil formulation was performed by applying a multiresponse optimization, for which the PP was minimized, the FP, demulsibility, and VI were maximized. The range of viscosities at 40 and 100 °C was selected according to the International Organization for Standardization (ISO) viscosity grade for hydraulic oil. The experiments were accomplished according to an extreme vertices mixture design using solvent neutral-100 (SN-100) and SN-500 as major components and polyisobutene (PIB) as minor component. The results shows that the optimum formulation consisted of 37.5% SN-100, 60% SN-500, and 2.5% PIB, and the values of FP, PP, VI, demulsibility at 30 mins and viscosities at 40 and 100 °C were equal to 227 °C, −6 °C, 107.5, 28/2012/40, 67.3 cS, and 9 cS, respectively.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):021802-021802-5. doi:10.1115/1.4031751.

This paper presents the Maxwell model in simple shear flow by using the complex analysis, instead of the matrix analysis which is generally used in the literature. It is found that the viscoelastic fluids will behave viscoelastically only when the elastic shear deformation is significant, say about unity. Analysis of the viscoelastic flow based on the assumption of small elastic deformation will overlook the viscoelasticity. Because the elastic deformation is always great when the viscoelasticity is observed, the constant elasticity assumption, rather than the constant viscosity assumption, will lose its effect. This paper first introduces the assumption that the shear modulus of viscoelastic fluids is linearly related to the shear strain rate, and the equivalent viscosity is compared with the experimental results for some simple hydrocarbons in the literature. The theory proposed in this paper gives predictions agreed with the experimental results as well as the Carreau model.

Commentary by Dr. Valentin Fuster

### Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2015;138(2):022201-022201-11. doi:10.1115/1.4031431.

Prediction of leakage flow and windage heating for labyrinth seals with honeycomb lands is critical in understanding gas turbine engine system performance and predicting its component life. There are several labyrinth seal configurations in use in gas turbines, and for each configuration, there are many geometric factors that can impact a seal's leakage and windage characteristics. One of the factors which has not been thoroughly investigated in previously published work is the presence of rub-grooves in the honeycomb land and its impact on seal performance. This paper describes the development of a numerical methodology aimed at studying this effect. Specifically, a three-dimensional (3D) computational fluid dynamics (CFD) model is developed utilizing commercial finite volume-based software incorporating the renormalization group (RNG) k-ε turbulence model. Using this model, a broad parametric study is conducted by varying honeycomb cell size and radial clearance for a four-tooth straight-through labyrinth seal with and without rub-grooves. The results show good agreement with available experimental data. They further indicate that presence of rub-grooves increases seal leakage and decreases windage heating. The absolute levels depend on the clearance and honeycomb cell size.

Commentary by Dr. Valentin Fuster

### Research Papers: Tribochemistry and Tribofilms

J. Tribol. 2015;138(2):022301-022301-6. doi:10.1115/1.4031402.

The ability to design, control, and synthesize a material surface with superhydrophobicity is of great interests in many engineering applications. Here, we report a cost-effective process to fabricate poly(vinylidene fluoride) (PVDF)/zirconium(IV) oxide (ZrO2) composites with superhydrophobicity. This is achieved by combining an antisolvent that induces phase separation, i.e., the precipitation of PVDF from the solution through a spray-on method on various liquids. The material surfaces possess wrinkled micron-sized beads which displayed superhydrophobicity in water without any chemical treatment. The process developed in this research presented a fast and simple approach in making hydrophobic surfaces.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Tribol. 2015;138(2):024501-024501-6. doi:10.1115/1.4031398.

A mechanized galling tester has been developed to evaluate the galling resistance of material pairs at room temperature (RT) as well as at elevated temperature condition. The test rig has a facility for online measurement of frictional torque during the test which is useful in assessing the incipient scoring. Both the test rig and the test method conform to the recent ASTM G196-08 standard. Galling resistance of two different grades of stainless steel SS 304 and 304 L has been evaluated in self-mated condition at RT and elevated temperature (300 °C). The parameter called galling50 has been reported for the materials tested. The galled surface indicated the severe plastic deformation in the direction of sliding and it is dominated by the typical adhesive wear mechanism. The recent ASTM G196-08 test method for measurement of galling resistance of material pairs appears to be superior to an older ASTM G98 because galling behavior was prevailed by the stochastic wear phenomenon.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(2):024502-024502-6. doi:10.1115/1.4031515.

In order to understand the load-carrying mechanism of thermal wedge, numerical results for a rectangular pad and a slider with parallel gaps under four types of surface boundary temperature conditions are presented. Two assumptions of rigid-solid and smooth-surface were used to exclude the effects of both thermal deformation and micro-asperity. The relation between thermal wedge and thermal boundary conditions is revealed. The load-carrying mechanism of parallel gaps is explained with the thermal wedge derived not only from the surface temperature difference (STD) as proposed by Cameron but also from the film temperature gradient (FTG) independent of STD. It is also pointed out that in numerical analysis, the very small viscosity–temperature coefficient would result in high oil temperature and therefore, the predicted loading capacity from thermal density wedge would be extremely enlarged.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(2):024503-024503-8. doi:10.1115/1.4031987.

Solid particle shape and size effects on the slurry erosion behavior of AISI 5117 carbon steels are investigated, using whirling-arm ring for two different erodent particles, namely, silica sand (SiO2) and silicon carbide (SiC). From this work, it was found that aspect ratio and circularity factor (CF) increase for silica sand and decrease for silicon carbide with increasing size. The erosion rate increased with the increase of particle size for the two types of erodent particles and its value was greater in the case of silicon carbide particles. At the same test conditions, it has been noticed that the particle size plays the major role in the slurry erosion of 5117 steels in comparison with the aspect ratio and circularity factor. Microcutting and plowing with serrated wear tracks were observed for coarse SiC particles having irregular and angular shape. But, for coarse SiO2 particles which had a rounded shape, the main mechanism was plowing with plain and smooth wear tracks for an impact angle of 30 deg. Indentations and material extrusion prevailed for the coarse size of the two erodents for an impact angle of 90 deg.

Commentary by Dr. Valentin Fuster