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Research Papers: Applications

J. Tribol. 2016;138(3):031101-031101-8. doi:10.1115/1.4032136.

Contact mechanics in wrapping a thin-shell (tape/web) around a grooved cylindrical surface (roller) under tension is investigated. The problem is analyzed along the axial direction of the roller, and the effects of wrap-angle tape/web motion are neglected. Equations of equilibrium admit analytical solutions, but the problem is nonlinear due to the unknown nature of contact area. The tape bends into the grooves and makes contact over the lands. Three distinct contact states describe the interaction of the tape/web with respect to the lands. Nondimensional analysis shows that contact state depends on the width of the groove and the land, and the nondimensional belt-wrap pressure only modulates the amplitude of the deflected profile.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031102-031102-7. doi:10.1115/1.4032489.

The cavitation erosion resistance of an X5CrNi18-10 stainless steel, solution treated at temperatures in the range of 1000–1100 °C for 5–50 mins, was investigated using a piezoceramic vibrating system. The variation of the technological parameters led to changes in the degree of the chemical homogeneity and the grain size of the austenite. Heating at 1050 °C for 25 mins, followed by water quenching, led to an increase in the cavitation erosion resistance of about 2.45 times compared to the samples heated for 50 mins. A significant improvement of the cavitation resistance was obtained for the sample maintained at 1050 °C compared to the samples annealed at 1000 and 1100 °C. It was found that the associated cavitation erosion resistance is improved for finer granulation and for higher degree of chemical homogeneity of the austenite.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031103-031103-9. doi:10.1115/1.4032525.

In this paper, an innovative system for condition-based monitoring (CBM) using model-based estimation (MBE) and artificial neural network (ANN) is proposed. Fault diagnosis of deep groove ball bearings (DGBB) is a key machine element for stability of rotating machinery. MBE model is proposed to demonstrate and estimate the vibration characteristics of bearings. It is realized that it may be worth mentioning that the vibration analysis of damaged bearings at all the positions of a structure is difficult to obtain. For this purpose, methods have been discussed to get the utmost information to notify bearing faults. The ANN approach enables us to determine the effects of various parameters of the vibrations by conducting the experiments. The results point out that defect size, speed, load, unbalance, and clearance influence the vibrations significantly. Experimental simulated data using the MBE and ANN models of rotor–bearing are used to identify the damage diagnosis at a reasonable level of accuracy. The results of the experiments consist in constantly evaluating the performance of the bearing and thereby detecting the faults and vibration characteristics successfully. The effects of faults and vibration characteristics obtained using the experimental MBE and ANN are studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031104-031104-13. doi:10.1115/1.4032526.

This work presents a numerical simulation which studies the effect of elastomeric bushing on the dynamics of a deep-groove ball bearing. To achieve the objective, a three-dimensional (3D) explicit finite element method (EFEM) was developed to model a cylindrical elastomeric bushing, which was then coupled with an existing dynamic bearing model (DBM). Constitutive relationship for the elastomer is based on the Arruda–Boyce model combined with a generalized Maxwell-element model to capture both hyperelastic and viscoelastic behaviors of the material. Comparisons between the bushing model developed for this investigation and the existing experimental elastomeric bushing study showed that the results are in good agreement. Parametric studies were conducted to show the effects of various elastomeric material properties on bushing behavior. It was also shown that a desired bushing support performance can be achieved by varying bushing geometry. Simulations using the combined EFEM bushing and DBM model demonstrated that the elastomeric bushing provides better compliance to bearing misalignment as compared to a commonly used rigid support model. As a result, less ball slip and spin are generated. Modeling with a bearing surface dent showed that vibrations due to surface abnormalities can be significantly reduced using elastomeric bushing support. It has also been shown that choosing a proper bushing is an efficient way to tuning bushing vibration frequencies.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031105-031105-9. doi:10.1115/1.4032668.

This work deals with generalized three-dimensional (3D) mathematical model to estimate the force and stiffness in axially, radially, and perpendicularly polarized passive magnetic bearings with “n” number of permanent magnet (PM) ring pairs. Coulombian model and vector approach are used to derive generalized equations for force and stiffness. Bearing characteristics (in three possible standard configurations) of permanent magnet bearings (PMBs) are evaluated using matlab codes. Further, results of the model are validated with finite element analysis (FEA) results for five ring pairs. Developed matlab codes are further utilized to determine only the axial force and axial stiffness in three stacked PMB configurations by varying the number of rings. Finally, the correlation between the bearing characteristics (PMB with only one and multiple ring pairs) is proposed and discussed in detail. The proposed mathematical model might be useful for the selection of suitable configuration of PMB as well as its optimization for geometrical parameters for high-speed applications.

Commentary by Dr. Valentin Fuster

Research Papers: Biotribology

J. Tribol. 2016;138(3):031201-031201-10. doi:10.1115/1.4031988.

The influence of the hip joint formulation on the kinematic response of the model of human gait is investigated throughout this work. To accomplish this goal, the fundamental issues of the modeling process of a planar hip joint under the framework of multibody systems are revisited. In particular, the formulations for the ideal, dry, and lubricated revolute joints are described and utilized for the interaction of femur head inside acetabulum or the hip bone. In this process, the main kinematic and dynamic aspects of hip joints are analyzed. In a simple manner, the forces that are generated during human gait, for both dry and lubricated hip joint models, are computed in terms of the system's state variables and subsequently introduced into the dynamics equations of motion of the multibody system as external generalized forces. Moreover, a human multibody model is considered, which incorporates the different approaches for the hip articulation, namely, ideal joint, dry, and lubricated models. Finally, several computational simulations based on different approaches are performed, and the main results are presented and compared to identify differences among the methodologies and procedures adopted in this work. The input conditions to the models correspond to the experimental data capture from an adult male during normal gait. In general, the obtained results in terms of positions do not differ significantly when different hip joint models are considered. In sharp contrast, the velocity and acceleration plotted vary significantly. The effect of the hip joint modeling approach is clearly measurable and visible in terms of peaks and oscillations of the velocities and accelerations. In general, with the dry hip model, intrajoint force peaks can be observed, which can be associated with the multiple impacts between the femur head and the cup. In turn, when the lubricant is present, the system's response tends to be smoother due to the damping effects of the synovial fluid.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2016;138(3):031301-031301-6. doi:10.1115/1.4031995.

In this study, the structure and tribological performance of the diamondlike carbon (DLC) films were related to deposition parameters. The feasibility of the microwave-excited plasma-enhanced chemical vapor deposition (μW-PECVD) as a process to produce good quality DLC films was the focus. The DLC films were deposited on the steel substrates with a tungsten carbide interlayer via μW-PECVD. The negative substrate bias used during the film deposition was varied. The Raman results revealed that the increased negative substrate bias increased the sp3 bonding in the DLC films as a result of the increased kinetic energy of film-forming ions during the film deposition. The tribological results clearly indicated that the friction and wear of the DLC-coated steel samples against a 100Cr6 steel ball significantly decreased with increased negative substrate bias due to the significantly improved wear resistance of the DLC films.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031302-031302-11. doi:10.1115/1.4032522.

A combination technology of the solid lubricant and the laser surface texturing (LST) can significantly improve the tribological properties of friction pairs. The plate sample was textured by fiber laser and composite lubricant of polyimide (PI) and molybdenum disulfide (MoS2) powders were filled in the microdimples. Sliding friction performances of micron-sized composite lubricant and nano-sized composite lubricant were investigated by ring-plate tribometer at temperatures ranging from room temperature (RT) to 400 °C. On the one hand, the results of the micron-sized composite lubricant show that the friction coefficient of the textured surface filled with composite lubricant (TS) exhibits the lowest level and the highest stability compared to a textured surface without solid lubrication, smooth surface without lubrication, smooth surface burnished with a layer of composite solid lubricant. The better dimple density range is 35–46%. The friction coefficients of the sample surface filled with micron-composite solid lubricant with the texture density of 35% are maintained at a low level (about 0.1) at temperatures ranging from RT to 300 °C. On the other hand, the results of the nano-sized composite lubricant show that these friction properties are better than those of MoS2-PI micron-sized composite. The friction coefficients of MoS2-PI-CNTs nano-sized composite solid lubricant are lower than those of the MoS2-PI composite lubricant at temperatures ranging from RT to 400 °C. In addition, the possible mechanisms involving the synergetic effect of the surface texture and the solid lubricant are discussed in the present work.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2016;138(3):031401-031401-7. doi:10.1115/1.4031994.

This work presents a finite element (FE) study of a perfectly elastic axisymmetric sinusoidal-shaped asperity in contact with a rigid flat for different amplitude to wavelength ratios and a wide range of material properties. This includes characterizing the pressure required to cause complete contact between the surfaces. Complete contact is defined as when there is no gap remaining between two contacting surfaces. The model is designed in such a way that its axisymmetric and interaction with the adjacent asperities are considered by the effect of geometry at the base of the asperity. The numerical results are compared to the model of curved point contact for the perfectly elastic case (known as Hertz contact) and Westergaard's solution. Once properly normalized, the nondimensional contact area does not vary with nondimensional load. The critical pressure required to cause complete contact is found. The results are also curve fitted to provide an expression for the contact area as a function of load over a wide range of cases for use in practical applications, such as to predict contact resistance. This could be a stepping stone to more complex models.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031402-031402-12. doi:10.1115/1.4032519.

Finite element (FE) simulations were performed to study yielding in single and bilayer (BL) film systems using a “yield zone map” approach. Onset of yielding was observed at the interface, substrate, surface, and film in HfB2/silicon and HfB2/stainless steel systems. The interface yield zone in HfB2/stainless steel system was found to be larger due to the dominant effect of interfacial stress gradients. Based on the FE simulations, empirical equations were derived for the maximum contact pressure required to initiate yield at the interface. For BL/substrate systems, onset of yield at the lower film/substrate interface occurred when film thickness ratio was in the range 0.5–5. The maximum contact pressure associated with the initial yielding at this interface is minimum compared to other locations. From the design point of view, for a BL system the preferable film thickness ratio was found to be 20, whereas the optimum hardness ratio ranges from 2 to 4. For these values, maximum contact pressure is very high (∼30 GPa), and thus, yield onset can be avoided at lower film/substrate interfaces. In addition, based on the obtained results, the advantages and disadvantages of using a BL film as compared to a single film and their relevance to practical applications are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031403-031403-11. doi:10.1115/1.4033132.

In order to achieve greater efficiency or to meet light weight requirements, components are downsized. This, however, increases the load, e.g., Hertzian or nominal contact pressures and stresses of tribosystems. This load is expressed as pa·v-value, the product of nominal contact pressure and sliding velocity. pa·v-values are an effective tool for design engineers for predicting low wear/high wear transitions. Therefore, in the present work, topographical analysis has been combined with the plasticity of micro-asperities and the flash temperatures to estimate the limits of pa·v diagrams. The central piece of this set of models presented here is the calculations for flash temperatures and contact mechanics of micro-asperities. This central piece is used to predict the performance of materials in high velocity (turbines, machinery) and low velocity (human joint) applications. It is shown that the model combination suggested here is a useful tool for screening and preselecting a candidate and new materials with respect to tribological requirements before engaging in expensive testing.

Topics: Wear , Temperature
Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031404-031404-13. doi:10.1115/1.4033134.

The purpose of this work is to establish an analytical model and standard way to predict the performance characteristics of a four-point contact, or gothic arch type, rolling element ball bearing. Classical rolling element bearing theory, as developed by Jones, has been extended to include the complex kinematics of the four-point contact bearing; thereby providing complete elementwise attitude and internal load distribution of the bearing under operating conditions. Standard performance parameters, such as element contact stresses, contact angles, inner ring deflections, nonlinear stiffness's, torque, and L10 life, are solved explicitly via standard Newton–Raphson techniques. Race control theory is replaced with a minimum energy state theory to allow both spin and slip to occur at the ball-to-raceway contact. The developed four-point model was programed within the orbis software program. Various test cases are analyzed and key analytical results are compared with the Jones four-point contact ball bearing analysis program, the Wind Turbine Design Guideline, DG03, and traditional two-point (angular contact) analysis codes. Model results for the internal distribution of ball loads and contact angles match the Jones program extremely well for all cases considered. Some differences are found with the DG03 analysis methods, and it is found that modeling a four-point contact bearing by overlaying two opposed angular contact bearings can result in gross errors.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2015;138(3):031501-031501-9. doi:10.1115/1.4031854.

A method of determining the response of elastohydrodynamic line contacts to low amplitude, sinusoidal variations in load is presented. It is shown that the load variations alter the Hertz width, cyclically increasing and reducing the effective entrainment velocity. This produces clearance variations in the inlet, which are transported through the conjunction altering the pressure distribution as they pass. The resulting pressure and clearance changes can be many times greater than when the load changes slowly. The results are used to determine the flexibility and damping of the conjunctions. These vary depending on the number of transported waves inside the contact. It is shown that a Maxwell model rather than the usual Voigt model is required to define the contact's behavior. While the Voigt model may be used at low frequencies, it has a damping coefficient that is not unique to the contact but depends on the total system stiffness.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031502-031502-12. doi:10.1115/1.4031991.

By coupling the equations of the modified Reynolds equation with the anisotropic slip effect, the piezoviscosity and piezodensity relations, the elasticity deformation equation, and the load equilibrium equation are solved simultaneously using the finite element method (FEM) for the elastohydrodynamic lubrication (EHL) of circular contact problems under constant load conditions. Results show that the film thickness is more sensitive to the slip length in a sliding direction (x-direction) than to the slip length in a transverse direction (y-direction). A slip in the y-direction concentrates the pressure toward the center region, and the film collects toward the central region and possesses a deeper dimple. The central pressure and coefficient of friction (COF) increase as the slip length in the y-direction increases. On the contrary, the central pressure and COF decrease as the slip length in the x-direction increases. Detailed results and animations for film thicknesses and pressure distributions are available under the “Supplemental Data” tab for this paper on the ASME Digital Collection.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031503-031503-12. doi:10.1115/1.4032137.

Surface plastic deformation due to contact (lubricated or dry) widely exists in many mechanical components, as subsurface stress caused by high-pressure concentrated in the contact zone often exceeds the material yielding limit, and the plastic strain accumulates when the load is increased and/or repeatedly applied to the surface in a rolling contact. However, previous plasto-elastohydrodynamic lubrication (PEHL) studies were mainly for the preliminary case of having a rigid ball (or roller) rotating on a stationary elastic–plastic flat with a fixed contact center, for which the numerical simulation is relatively simple. This paper presents an efficient method for simulating PEHL in a rolling contact. The von Mises yield criteria are used for determining the plastic zone, and the total computation domain is discretized into a number of cuboidal elements underneath the contacting surface, each one is considered as a cuboid with uniform plastic strain inside. The residual stress and surface plastic deformation resulted from the plastic strain can be solved as a half-space eigenstrain–eigenstress problem. A combination of three-dimensional (3D) and two-dimensional (2D) discrete convolution and fast Fourier transform (DC-FFT) techniques is used for accelerating the computation. It is observed that if a rigid ball rolls on an elastic–plastic surface, the characteristics of PEHL lubricant film thickness and pressure distribution are different from those of PEHL in the preliminary cases previously investigated. It is also found that with the increase of rolling cycles, the increment of plastic strain accumulation gradually approaches a stable value or drops down to zero, determined by the applied load and the material hardening properties, eventually causing a groove along the rolling direction. Simulation results for different material hardening properties are also compared to reveal the effect of body materials on the PEHL behaviors.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031504-031504-12. doi:10.1115/1.4032325.

Friction, wear, and vibration behaviors of oil-and grease-lubricated laser textured point contacts formed between AISI 52100 steel surfaces have been experimentally investigated under the unidirectional sliding motion using a ball-on-disk configuration. The performance behaviors of lubricated concentrated contacts formed between the lapped disk/lapped ball and textured disk/lapped ball have been compared at two Hertzian pressures (0.4 GPa and 0.7 GPa) and two sliding speeds (0.8 m/s and 2.4 m/s) using lubricating oil and grease. For the geometric configuration of point contacts adopted in the investigations, the textured point contacts lubricated with grease yielded reductions in the friction coefficient and specific wear rate (SWR) of the balls as compared to the oil-lubricated textured contacts. Moreover, reduction in the amplitudes of vibrations (at normal contact resonance frequencies) has also been observed with the grease-lubricated textured point contacts in comparison to the corresponding oil-lubricated cases.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031505-031505-9. doi:10.1115/1.4032952.

The experimental observation of the inlet dimple of the elastohydrodynamic lubrication (EHL) film was carried out in point and elliptical contacts, respectively, and then, the numerical analysis under point contacts was conducted. The inlet dimple appears and then moves upstream toward the inlet and finally disappears with the increase of the entrainment speed. In the meantime, the dimple depth increases at the beginning and then decreases. The high load leads to a wide entrainment speed range where dimple exists. The varying range of the entrainment speed corresponding to the dimple appearance is smaller at a smaller included angle between the minor axis of elliptical contact and the entrainment direction.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031506-031506-9. doi:10.1115/1.4032959.

Oil–air lubrication is widely used in the high-speed rotary machines; the oil is usually in the form of discrete droplets. The lubrication behavior of the oil droplet is rarely investigated. This paper investigates the effect of the oil droplet on lubrication performance based on the developed transient lubrication model with consideration of starvation conditions. The oil droplet is modeled as spherical segment with different heights, base radius, and positions, which will change the inlet oil supply conditions. The results show that the oil droplet with large size can generate thick oil film which is close to the one in fully flooded condition and can remain long time; the position of the oil droplet entering the contact region also has significant effect on the lubrication performance.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2016;138(3):031601-031601-8. doi:10.1115/1.4031831.

Structural parts in gas turbines undergo fretting wear, and as clearances open up, it turns into impact wear. Uncoated cobalt-based substrates that are used in such applications show poor resistance to fretting/impact damage and undergo extensive/unacceptable level of degradation. Commonly used substrate materials such as uncoated cobalt-based materials show poor resistance to fretting/impact wear. This study is focused on assessing the performance of high velocity oxyfuel (HVOF) tungsten carbide (WC) coatings and sintered tungsten carbide inserts as potential solutions for mitigating this issue. Sintered WC12Co with grain size range from 0.2 μm to 4.5 μm and HVOF coating with composition of WC–17%Co was tested. It was found that the HVOF coatings performed better than sintered material and the behavior was attributed to the hard WC particles surrounded by the higher volume fraction of cobalt binder. In the HVOF coating, the normal load was better accommodated by the decarburized WC but a fairly tough binder-surrounding matrix. An additional factor is that the sintered WC had significant volume fraction of undesirable W2C phase, which apparently underwent fracture during the test, thus showing an inferior behavior compared to the HVOF WC coating.

Topics: Wear , Coatings , Temperature
Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031602-031602-10. doi:10.1115/1.4031993.

Traditionally, iterative schemes have been used to predict evolving material profiles under abrasive wear. In this work, more efficient continuous formulations are presented for predicting the wear of tribological systems. Following previous work, the formulation is based on a two parameter elastic Pasternak foundation model. It is considered as a simplified framework to analyze the wear of multimaterial surfaces. It is shown that the evolving wear profile is also the solution of a parabolic partial differential equation (PDE). The wearing profile is proven to converge to a steady-state that propagates with constant wear rate. A relationship between this velocity and the inverse rule of mixtures or harmonic mean for composites is derived. For cases where only the final steady-state profile is of interest, it is shown that the steady-state profile can be accurately and directly determined by solving a simple elliptic differential system—thus avoiding iterative schemes altogether. Stability analysis is performed to identify conditions under which an iterative scheme can provide accurate predictions and several comparisons between iterative and the proposed formulation are made. Prospects of the new continuous wear formulation and steady-state characterization are discussed for advanced optimization, design, manufacturing, and control applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031603-031603-18. doi:10.1115/1.4032518.

Modeling of dry sliding friction and wear behavior of Ti3Al2.5V alloy sliding against EN31 steel using a multi-tribotester has been presented. Mathematical model equations in the form of natural log transformation for wear rate (WR), average coefficient of friction (μa), and a square root transformation for maximum contact temperature (Tm) considering the effect of tribological variables have been developed and validated by comparing them with the experimental results. The authors claim novelty with regard to modeling and optimization of friction and wear characteristics of Ti-3Al2.5V alloy. The results reveal that the magnitude of wear rate and maximum contact temperature increases with increase in sliding velocity and increasing normal load with few exceptions. Whereas average coefficient of friction first increases with increasing sliding velocity up to 2.51 m/s, and then decreases at highest sliding velocity. The load is found to have strongest influence on both wear rate and average coefficient of friction followed by sliding velocity, whereas sliding velocity has strongest influence on the maximum contact temperature followed by load. The perturbation plot results are also in accordance with the analysis of variance (ANOVA) analysis. The theoretical and experimental results have an average error of 5.06%, 1.78%, and 1.42%, respectively, for wear rate, average coefficient of friction, and maximum contact temperature. Optimization resulted in a maximum desirability of 0.508 at a load of 60 N and a sliding velocity of 1.5 m/s. For these values, the predicted minimum wear rate is 0.0001144 g/m, the coefficient of friction is 0.3181, and the tool-tip temperature is 59.03 °C.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031604-031604-9. doi:10.1115/1.4032523.

Frictional behavior and topographical changes of material surfaces with applications in the microelectronics industry were experimentally observed. This work was performed to unveil trends in tribological characteristics of porous polyurethane material separately against copper and silicon dioxide materials typically found in integrated circuit (IC) polishing manufacturing processes. A linear reciprocating tribometer was utilized to translate the loaded contact of the polymer and contacting materials in the presence of a colloidal silica slurry. Contact forces were monitored throughout the experiments while surface topography of contacting surfaces was quantified using profilometry. Trials of polishing experiments were performed through a range of normal pressures and velocities to identify trends of interest, which are important in polishing. Coefficients of friction (COFs) between the polymer and contacting materials showed a decreasing trend with increasing polishing time and distance traveled. The copper and polymer material contacts were found to have a lower COF than that for the silicon dioxide and polymer contacts. Surface roughness of the polymer showed a general decreasing trend with increasing polishing time. This trend indicates a potential correlation between polymer surface roughness and the COF between the polymer and contacting materials. Evolution of the surface roughness of the materials differed depending on the direction along which topography was measured. An uncertainty analysis of the quantified parameters was conducted to provide knowledge in the confidence of the experimental results. Tribological behavior of the porous polyurethane and copper and silicon dioxide contacts is gathered from this experimental work for more complete characterization of the material.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031605-031605-7. doi:10.1115/1.4032528.

Experiments were performed on a ring-on-disk tribometer under lubricated conditions. Friction force was measured throughout the friction process. The parameter predictability was used to provide a quantitative description of the intrinsic randomness of the friction force. The parameter dynamic difference was used to detect the dynamic abrupt changes. The results show that, from the perspective of dynamics, the friction process can be divided into the abrupt changing process through which the intrinsic randomness is enhanced, the dynamic stable process through which the system maintains the strong intrinsic randomness, and the abrupt changing process through which the intrinsic randomness is weakened.

Topics: Friction , Chaos , Signals
Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031606-031606-6. doi:10.1115/1.4032527.

In this work, the wear and failure mechanism of polytetrafluoroethylene (PTFE)/SiO2/epoxy composites with a high concentration of SiO2 particles under dry sliding is examined. In the composite with 12.5 wt.% PTFE, a significant rise of the coefficient of friction (COF) appears after sliding over several kilometers and under a load of 60 N against balls made out of Al2O3 and steel. It is attributed to an accumulation of back-transferred Al2O3/steel and fractured SiO2 on the worn composite surfaces as well as a reduction of PTFE on worn SiO2 surfaces. The TiC/a-C:H coated ball yields the most stable COF. It is also observed that the loading capacity of the composite decreases with increasing PTFE concentration. Massive wear of the composites is seen after few kilometers sliding when the normal load rises above the loading capacity. The increased wear is due to a high concentration of PTFE which lowers the hardness as well as the compressive strength of the composites.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031607-031607-9. doi:10.1115/1.4033133.

High temperature self-lubricating NiAl matrix composites with addition of CuO (15, 20, and 25 wt.%) were fabricated by powder metallurgy technique, and the tribological behavior from room temperature to 1000 °C was investigated. It was found that Ni–Cu and Al2O3 phases formed during the fabrication process due to reaction of NiAl and CuO. The tribological results showed that the composite with addition of 25 wt.% CuO has a favorable friction coefficient of about 0.2 and excellent wear resistance with the magnitude of 10−6 mm3 N−1 m−1 at high temperatures (800 and 1000 °C).

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2016;138(3):031701-031701-10. doi:10.1115/1.4031992.

As for the micro gas bearing operating at a high temperature and speed, one wedge-shaped microchannel is established, and the hydrodynamic properties of the wedge-shaped gas film are comprehensively investigated. The Reynolds equation, modified Reynolds equation, energy equation, and Navier–Stokes equations are employed to describe and analyze the hydrodynamics of the gas film. Furthermore, the comparisons among the hydrodynamic properties predicted by various models were performed for the different wedge factors and the different wall temperatures. The results show that coupling the simplified energy equation with the Reynolds or modified Reynolds equations has an obvious effect on the change of the friction force acting on the horizontal plate and the load capacity of the gas film at the higher wedge factor and the lower wall temperature. The velocity slip weakens the squeeze of the gas film and strengths the gas backflow. A larger wedge factor or a higher wall temperature leads to a higher gas film temperature and thus enhances the rarefaction effect. As the wall temperature is elevated, the load capacity obtained by the Reynolds equation increases, while the results by the Navier–Stokes equations coupled with the full energy equation rapidly decrease. Additionally, the vertical flow across the gas film in the Navier–Stokes equations weakens the squeeze between the gas film and the tilt plate and the gas backflow.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031702-031702-5. doi:10.1115/1.4032350.

This paper focuses on the structural stiffness of bump foils which are used for compliant foil bearings with different heat treatments. After heat treatments in vacuum environments, the mechanical properties of the foil strips were tested, and the structural stiffness was estimated from the static load versus displacement curves obtained from the experiments. High cycle dynamic load tests were also applied to the bump foil under different cycle loads, and the shape of the foil was scanned after the tests to measure the height variation of the bumps. The results show that the modulus of elasticity and strength of Inconel X-750 strip with thickness of 0.1 mm after different treatments are lower than that with the thickness of 0.18 mm at room temperature. Moreover, the sample foil strips which have been treated with a lower solution anneal temperature at 980 °C (2 hrs) and precipitation heat treatment at 732 °C (16 hrs) have the largest modulus of elasticity and strength at room temperature. Therefore, heat treatments have a great influence on the structure stiffness of the bump foil. At last, the results of the high cycle dynamic load tests show that the bump foil with suitable heat treatment will have a good load capacity and stress-relaxation property.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031703-031703-11. doi:10.1115/1.4032659.

This study deals with a comparison between new experiments on the frictional behavior of porous journal bearings and its prediction by previous numerical simulations. The tests were carried out on bearings lubricated with polyalphaolefin (PAO)-based oils of distinct viscosities. The theoretical model underlying the simulations includes the effects of cavitation by vaporization and accounts for the sinter flow by virtue of Darcy's law. The effective eccentricity ratio corresponding to the experimentally imposed load is estimated by an accurate numerical interpolation scheme. The comparison focuses on the hydrodynamic branches of the Stribeck curve by dimensional analysis (DA), where the variations of the lubricant viscosity with temperature are of main interest. The numerically calculated values of the coefficient of friction are found to reproduce the experimentally obtained ones satisfactorily well in terms of overall trends; yet, the former lie predominantly below the measured ones, which results in a low-positive correlation between the two.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031704-031704-16. doi:10.1115/1.4032951.

This paper presents direct simulation Monte Carlo (DSMC) numerical investigation of the dynamic behavior of a gas film in a microbeam. The microbeam undergoes large amplitude harmonic motion between its equilibrium position and the fixed substrate underneath. Unlike previous work in literature, the beam undergoes large displacements throughout the film gap thickness and the behavior of the gas film along with its impact on the moving microstructure (force exerted by gas on the beam's front and back faces) is discussed. Since the gas film thickness is of the order of few microns (i.e., 0.01 < Kn < 1), the rarefied gas exists in the noncontinuum regime and, as such, the DSMC method is used to simulate the fluid behavior. The impact of the squeeze film on the beam is investigated over a range of frequencies and velocity amplitudes, corresponding to ranges of dimensionless flow parameters such as the Reynolds, Strouhal, and Mach numbers on the gas film behavior. Moreover, the behavior of compressibility pressure waves as a function of these dimensionless groups is discussed for different simulation case studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031705-031705-11. doi:10.1115/1.4032961.

The Morton effect (ME) is characterized by an asymmetric journal temperature distribution, slowly varying thermal bow and intolerable synchronous vibration levels. The conventional mass imbalance model is replaced by a more accurate thermal shaft bow model. Rotor permanent bow and disk skew are synchronous excitation sources and are incorporated in the dynamic model to investigate their influence on the ME. A hybrid beam/solid element finite element shaft model is utilized to provide improved accuracy for predicting the rotor thermal bow and expansion, with practical computation time. ME is shown to be induced by initial shaft bow and disk skew. The conventional mass imbalance approach is shown to have some limitations.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2016;138(3):031801-031801-8. doi:10.1115/1.4032304.

A series of ball–disk contact friction tests were carried out using a microtribometer to study the tribological characteristics of steel/steel rubbing pairs immersed in 47 different organic compounds as lubricant base oils. The structures and their friction data were included in a back-propagation neural network (BPNN) quantitative structure tribo-ability relationship (QSTR) model. Following leave-one-out (LOO) cross-validation, the BPNN model shows good predictability and accuracy for the friction parameter (R2 = 0.994, R2(LOO) = 0.849, and q2 = 0.935). Connectivity indices (CHI) show the large positive contribution to friction, which imply that friction performance has a strong correlation with molecular structure. The BPNN–QSTR models can flexibly and easily estimate the friction properties of lubricant base oils.

Topics: Friction , Lubricants
Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031802-031802-7. doi:10.1115/1.4033191.

The structures and the wear data of 47 different organic compounds as lubricant base oils were included in a comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA)–quantitative structure tribo-ability relationship (QSTR) model. CoMFA- and CoMSIA-QSTR models illustrate good accuracy, robustness, and predictability, with the latter more accurate than the former. CoMFA-QSTR with both steric and electrostatic fields: R2= 0. 958, R2(LOO) = 0.958, and q2= 0.625; with only a steric field: R2= 0.987, R2(LOO) = 0.987, and q2= 0.692. CoMSIA-QSTR with a steric field: R2= 0.924, R2(LOO) = 0.923, and q2= 0.898, whereas CoMSIA-QSTR with a hydrophobic field gave R2= 0.985, R2(LOO) = 0.985, and q2= 0.899. QSTR with CoMFA and CoMSIA shows a strong correlation to wear scar diameter scales (WDS), and builds statistical and graphical models that relate the wear properties of molecules to their structures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):031803-031803-6. doi:10.1115/1.4033334.

The results of the studies on the formation of surface and boundary layers in commercial lithium (LT4-S3) and calcium (STP) greases near the walls of six different materials are presented. Two elastomeric materials (nitrile-butadiene rubber (NBR), silicone rubber (MVQ/VMQ)), two thermoplastic materials (polyoxymethylene (POM), polyethylene (PE)), and two metal (copper C11000 and steel 304) alloys were used in the tests. The tests were carried out using a rotational rheometer operating in the plate/plate configuration. Structural viscosity–shear rate curves were determined and dynamic oscillatory tests were carried out. The tests have shown that the metal alloys have the highest capacity to adsorb grease thickener particles on their surface. The elastomeric materials have the smallest effect on the change in structural viscosity in the vicinity of the wall, which indicates their low capacity to form a surface layer in the tested commercial greases.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2016;138(3):032001-032001-8. doi:10.1115/1.4031912.

An effort was made to study and characterize the tribological characteristics of diamond nanoparticles as compared to neat mineral oil in the presence of sliding contact typically observed in the standard ASTM D4172 four-ball test. Four-ball tests were conducted with a solution of diamond nanoparticles and mineral oil, both at varying run times and bulk oil temperatures, and a consistent reduction in wear rates was observed. Numerical simulations were performed; it was observed that by enhancing the thermal conductivity of the lubricant, the wear reduction rate was observed to match the diamond nanoparticles solution results remarkably. This effort provides evidence that this additive wear reduction is in part caused by reduced lubricant temperatures due to the enhanced conductivity of the diamond.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(3):032002-032002-8. doi:10.1115/1.4032524.

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a thermally sprayed yttria-stabilized zirconia (YSZ) ceramic coating deposited onto a hot tool steel substrate to improve the mechanical and tribological properties. The friction test results showed that the UNSM-treated coating had a smoother surface, a lower friction, and a higher resistance to wear compared to that of the as-sprayed coating. It was also demonstrated that the UNSM technique improved the adhesion behavior of the coating by about 24%. Hence, it was found that a hybrid use of thermal spray coating (TSC) and UNSM technique is a meaningful way to bring together synergy effect of two emerging surface technologies in terms of tribology.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2016;138(3):032201-032201-7. doi:10.1115/1.4032175.

Extrusion-based elastomer seals are used in many applications, such as the seal in a variable bore ram valve used in offshore oil and gas drilling. Performing full-scale closing pressure experiments of such valves to characterize the seal performance and material failure of elastomer, especially under various temperature conditions, are quite expensive and time consuming. Conversely, simple coupon tests to characterize the elastomer mechanical properties and failure do not capture the complex deformation associated with the extrusion and subsequent sealing type that these materials undergo in the valves. In view of this, a simple subscale experimental test method capable of simulating the extrusion and sealing type deformation is developed. The extrusion and sealing deformation are realized by bonding the rectangular elastomer sample to metal pieces on top and bottom surfaces, and then compressing the sample in the vertical direction, while the deformation of the three lateral surfaces is kept constrained. As a result, sample deforms and extrudes out of the front surface, eventually forming the seal against a flat rigid metal plate placed at an appropriate distance. Simple scaling rules to determine the appropriate sample size and initial sealing gap, equivalent to the full-scale valve in terms of similar strain conditions, are derived and then verified using finite element analysis (FEA). Finally, the experimental test method is demonstrated by characterizing the contact pressure of nitrile (NBR) samples under different operating temperatures, ranging from 21 °C to 160 °C using pressure-sensitive film sensor.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2015;138(3):034501-034501-4. doi:10.1115/1.4031853.

The present work includes the study of boundary lubrication properties of SAE20W40 lubricating oil added with aluminum oxide nanoparticles. Pin-on-disk tribometer is employed to study the effects of nanoparticles in different sizes (40–80 nm) and concentrations (0–1% by weight) on the friction coefficient. The experimental design consists of L18 orthogonal array involving six levels for nanoparticles concentration and three levels for nanoparticles size, sliding speed, and normal load. The presence of nanoparticles has significantly improved the lubrication properties of oil. Minimum friction coefficient is recorded at 1200 rpm rotational speed and 160 N normal load for 0.8% concentration of 60 nm sized nanoparticles. Scanning electron microscopy (SEM) and electron diffraction spectrometry (XRD) are employed to understand the friction reduction mechanism.

Commentary by Dr. Valentin Fuster
J. Tribol. 2015;138(3):034502-034502-5. doi:10.1115/1.4031753.

This note provides additional graphical representations and curve-fit expressions for characteristics of the Warner–Sommerfeld approximate solution of the Reynolds equation for cavitating journal finite bearings. In particular, curve-fit expressions for mobility components, end-leakage factor, and positive film pressure extent are provided in support of previously published graphical representations of the data.

Commentary by Dr. Valentin Fuster

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