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Editorial

Research Papers: Applications

J. Tribol. 2016;138(4):041101-041101-8. doi:10.1115/1.4032787.

Rotating machinery is inherently susceptible to costly and dangerous faults. One such commonly encountered fault is undesirable dynamic contact between the rotor and stator (i.e., rotor–stator rub). The forces generated during rotor–stator rub are fundamentally tribological, as they are generated by contact and friction and result in wear. These forces are typically found by assuming linear elastic contact and dry Coulomb friction at the rotor–stator interface, where the normal force is a linear function of the interference. For the first time, this work incorporates viscoelasticity into the stator support and investigates its influence on the global dynamics of rotor–stator rub. The viscoelastic stator supports are modeled using fractional calculus, an approach which adeptly and robustly characterizes the viscoelasticity. Specifically, a fractional derivative order of one-half is employed to generate an analytic time-domain form of viscoelastic impedance. This approach directly assimilates viscoelasticity into the system dynamics, since the rotor equations of motion are integrated numerically in the time-domain. The coupled rotor–stator dynamic model incorporating viscoelastic supports is solved numerically to explore the influence of viscoelasticity. This model provides a framework for analysis of dynamic systems where viscoelasticity is included.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041102-041102-8. doi:10.1115/1.4032913.

Lab-scale electromagnetic launcher (EML) tests for the baseline case of an aluminum armature spanning a pair of copper rails reproduced excessive aluminum melt wear depth leading to loss of conductive contact and resultant electrical transition before completion of launch. A simple thermal model partitioning interfacial Joule heat input between conduction into the rails and melting of the armature provided compact expressions describing armature wear behavior. A quantity from this expression, termed the armature melt resistance (AMR), predicts a decrease of armature wear and the likelihood for resultant electrical transition with increasing rail thermal conductivity and heat capacity, as well as armature heat capacity, latent heat, and melt point. With this AMR metric to guide materials' selection, in subsequent tests, a slight increase in wear indeed occurred upon substitution of stainless steel rails, with more dramatic order of magnitude decreases in wear and avoidance of electrical transition instead realized upon substitution of solid molybdenum armatures.

Topics: Wear , Rails , Aluminum , Copper
Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2016;138(4):041401-041401-7. doi:10.1115/1.4032786.

Rotating machines and associated triboelements are ubiquitous in industrial society, playing a central role in power generation, transportation, and manufacturing. Unfortunately, these systems are susceptible to undesirable contact (i.e., rub) between the rotor and stator, which is both costly and dangerous. These adverse effects can be alleviated by properly applying accurate real-time diagnostics. The first step toward accurate diagnostics is developing rotor–stator rub models which appropriately emulate reality. Previous rotor–stator rub models disavow the contact physics by reducing the problem to a single esoteric linear contact stiffness occurring only at the point of maximum rotor radial deflection. Further, the contact stiffness is typically chosen arbitrarily, and as such provides no additional insight into the contacting surfaces. Here, a novel rotor–stator rub model is developed by treating the strongly conformal curved surfaces according to their actual nature: a collection of stochastically distributed asperities. Such an approach is advantageous in that it relies on real surface measurements to quantify the contact force rather than a heuristic choice of linear contact stiffness. Specifically, the elastoplastic Jackson–Green (JG) rough surface contact model is used to obtain the quasistatic contact force versus rotor radial deflection; differences and similarities in contact force between the linear elastic contact model (LECM) and JG model are discussed. Furthermore, the linear elastic model's point contact assumption is assessed and found to be inaccurate for systems with small clearances. Finally, to aid in computational efficiency in future rotordynamic simulation, a simple exponential curve fit is proposed to approximate the JG force–displacement relationship.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041402-041402-13. doi:10.1115/1.4032888.

Trilayer materials consisting of a functional outer layer on a substrate containing one intermediate layer are widely used in data-processing devices, biomedical components, and mechanical elements. The recent analytical frequency response functions (FRFs) derived by the authors' group for the contact of multilayer materials lead to the novel deterministic modeling of frictionless and frictional contact involving a trilayer material system designed with various thickness and elastic property combinations. Displacements and stresses for point contacts are calculated effectively by employing the discrete-convolution and fast Fourier transform (FFT) method based on the influence coefficients obtained from the analytical FRFs. The maximum von Mises stress and its location, which are critical information for understanding the material contact status, are thoroughly investigated for a wide range of trilayer materials. The results provide an informative guideline for the design of bilayer coatings without contact failure.

Topics: Stress , Coatings , Friction
Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041403-041403-8. doi:10.1115/1.4032820.

Nanoindentation experiments with a Berkovich indenter and a spherical indenter were performed to study the effects of annealing at temperatures below the glass transition temperature and room temperature ion irradiation on the near surface mechanical response of Ti40Cu32Pd14Zr10Sn2Si2 metallic glass (MG) ribbons. The specimens were isothermally annealed in vacuum at 573 K and 673 K for 4 hrs. Annealing was seen to increase the hardness of the specimens and decrease their ductility. The annealed specimens were subsequently irradiated by 3.5 MeV Cu2+ ions at room temperature using a fluence of 1 × 1012 ions/cm2 or 1 × 1016 ions/cm2. Nanoindentation experiments on the annealed and irradiated specimens showed a reduction in hardness and an increase in ductility for the specimens irradiated at a fluence of 1 × 1012 ions/cm2. Although the values of the mean contact pressure and critical shear stress under the spherical indenter showed an easier formation of shear bands after irradiation, increasing the irradiation fluence to 1 × 1016 ions/cm2 was seen to increase the hardness value and decrease the ductility of the specimens.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041404-041404-6. doi:10.1115/1.4032822.

Soft matter mechanics are characterized by high strains and time-dependent elastic properties, which complicate contact mechanics for emerging applications in biomedical surfaces and flexible electronics. In addition, hydrated soft matter precludes using interferometry to observe real areas of contact. In this work, we present a method for measuring the real area of contact in a soft, hydrated, and transparent interface by excluding colloidal particles from the contact region. We confirm the technique by presenting a Hertz-like quasi-static indentation (loading time > 1.4 hrs) by a polyacrylamide probe into a stiff flat surface in a submerged environment. The real contact area and width were calculated from in situ images of the interface processed to reduce image noise and thresholded to define the perimeter of contact. This simple technique of in situ particle exclusion microscopy (PEM) may be widely applicable for determining real areas of contact of soft, transparent interfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041405-041405-7. doi:10.1115/1.4032917.

This paper describes a new method, based on a recent analytical model (Hertzian biphasic theory (HBT)), to simultaneously quantify cartilage contact modulus, tension modulus, and permeability. Standard Hertzian creep measurements were performed on 13 osteochondral samples from three mature bovine stifles. Each creep dataset was fit for material properties using HBT. A subset of the dataset (N = 4) was also fit using Oyen's method and FEBio, an open-source finite element package designed for soft tissue mechanics. The HBT method demonstrated statistically significant sensitivity to differences between cartilage from the tibial plateau and cartilage from the femoral condyle. Based on the four samples used for comparison, no statistically significant differences were detected between properties from the HBT and FEBio methods. While the finite element method is considered the gold standard for analyzing this type of contact, the expertise and time required to setup and solve can be prohibitive, especially for large datasets. The HBT method agreed quantitatively with FEBio but also offers ease of use by nonexperts, rapid solutions, and exceptional fit quality (R2 = 0.999 ± 0.001, N = 13).

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2016;138(4):041501-041501-7. doi:10.1115/1.4032790.

This work investigates the sensitivity of the surface roughness lubricant life adjustment factor for rolling-element bearings to variations in measured roughness. Roughness measurement results using different values of stylus tip dimension and short wavelength cutoff filter were obtained for several bearing raceway surfaces and used as the inputs for a lubricant life adjustment factor based on the lambda ratio—the ratio of elastohydrodynamic lubrication (EHL) film thickness to the composite surface roughness. The effect on predicted bearing life is shown to be significant. As one means of verification of surface roughness measurements, an ISO type D random/repeating precision roughness specimen was also investigated and shown to have excellent sensitivity to stylus tip dimension, demonstrating its effectiveness as a tool for assessment of stylus radius and condition.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2016;138(4):041601-041601-8. doi:10.1115/1.4032842.

Entropic and energy-based approaches are employed for prediction of wear in dry sliding contact between crossed cylinders. The methodology requires measurement or estimation of the temperature rise in the sliding system. The results of experimental tests reported in literature in conjunction with measured degradation coefficients are used to examine the validity of the proposed methodology. The approach presented is shown to be capable of predicting the wear rate for different tribopairs and under different loading conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041602-041602-7. doi:10.1115/1.4032819.

Frictional heating occurring during pin-on-flat tribotesting of ultrahigh molecular weight polyethylene (UHMWPE) pins was measured and modeled. A full factorial experiment was conducted to determine if testing parameters can produce sufficient frictional heat to alter tribological properties of the bovine serum used as lubricant in the system. Temperature of the surrounding bovine serum was monitored during tribotests using varying pin sizes and sliding speeds to determine typical temperature rises due to frictional heating. This work examined two sliding speeds (40 mm/s and 80 mm/s) and two pin diameters (6.35 mm and 9.5 mm) at a single static load. Gravimetric analysis for wear determination and coefficient of friction measurement were performed for each test. Results showed that frictional heating increased the bulk temperature of the surrounding serum and correlated to sliding speed and average coefficient of friction. No correlation was seen at this temperature range between serum temperature rise and wear rate, providing evidence that the tested parameters are acceptable for tribotesting of UHMWPE. A computational model was developed to predict bulk serum temperature increase. This model closely predicted the temperature increase to within 2 °C, which is sufficient accuracy for identifying if bovine serum protein precipitation is likely during tribotesting. This work serves as an initial estimate and prediction for appropriate testing parameters based on lubricant responses to frictional heating.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041603-041603-8. doi:10.1115/1.4032821.

Constitutive and dynamic relations for friction coefficient are presented. A first thrust combines the laws of thermodynamics to relate heat, energy, matter, entropy, and work of forces. The equation sums multiple terms—each with a differential of a variable multiplied by a coefficient—to zero. Thermodynamic considerations suggest that two variables, internal energy and entropy production, must depend on the others. Linear independence of differentials renders equations that yield thermodynamic quantities, properties, and forces as functions of internal energy and entropy production. When applied to a tribocontrol volume, constitutive laws for normal and friction forces, and coefficient of friction are derived and specialized for static and kinetic coefficients of friction. A second thrust formulates dynamics of sliding, with friction coefficient and slip velocity as state variables. Differential equations derived via Newton's laws for velocity and the degradation entropy generation (DEG) theorem for friction coefficient model changes to the sliding interface induced by friction dissipation. The solution suggests that the transition from static to kinetic coefficient of friction with respect to slip velocity for lubricant starved sliding is a property of the motion dynamics of sliding interacting with the dynamics of change of the surface morphology. Finally, sliding with stick-slip was simulated to compare this model to others.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041604-041604-8. doi:10.1115/1.4032843.

During the running-in process, a friction pair experiences drastic evolution in many of its tribological parameters, such as surface roughness, wear rate, and coefficient of friction until steady-state is attained. In this paper, we present a model for predicting the behavior of the running-in process. Specifically, we determine a general relationship between the wear loss and surface roughness during the running-in stage and test the validity of its prediction of wear rate by comparing to available experimental results. We show, by using a dimensional analysis and applying the Buckingham Pi theorem, that there exists a linear relationship between the transient dimensionless wear, the dimensionless initial surface roughness, and dimensionless running-in time.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041605-041605-7. doi:10.1115/1.4032823.

Iterative numerical wear models provide valuable insight into evolving material surfaces under abrasive wear. In this paper, a holistic numerical scheme for predicting the wear of rubbing elements in tribological systems is presented. In order to capture the wear behavior of a multimaterial surface, a finite difference model is developed. The model determines pressure and height loss along a composite surface as it slides against an abrasive compliant countersurface. Using Archard's wear law, the corresponding nodal height loss is found using the appropriate material wear rate, applied pressure, and the incremental sliding distance. This process is iterated until the surface profile reaches a steady-state profile. The steady-state is characterized by the incremental height loss at each node being nearly equivalent to the previous loss in height. Several composite topologies are investigated in order to identify key trends in geometry and material properties on wear performance.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2016;138(4):041701-041701-8. doi:10.1115/1.4032824.

The thrust bearing duty in a pump-turbine generator can be quite arduous, since the pad support system must be symmetrical about the center of the pad, yet the oil-film must converge adequately for either direction of rotation. Special care must be taken with large machines since the thermal and elastic deformation of the pads will increase nonlinearly with size B of the pad, for example, as B2 when thermal deformation is considered. However from first principles, the thickness of the oil film will increase with only the square root of size B½. Poorly shaped films can develop when a design standard is scaled-up to larger sizes. Three options for the thrust bearing design of a particular pump-turbine were considered: (a) “semihard” supports for the pads such as a spring-disk insert, (b) “piston-type” supports in the back of the pads, which are machined to form shallow pistons that fit into recesses, allowing the pads to be supported hydrostatically, and (c) a symmetric arrangement of coil springs. In this instance, an upper limit of thrust bearing temperature was specified. Penalties would incur if this were exceeded. It is shown using a design code (GENMAT) that the best performance is achieved with a spring support (option c), arranged to give a convex film shape in the direction of sliding, and a slightly concave film in the radial direction. This is achieved by limiting the extent of the spring pack in the circumferential direction so that there are unsupported “overhangs” at the lead and trail edges. The radial concavity is arranged by having the spring pack extend edge-to-edge in the radial direction. The bearing has performed very well since commissioning. The original machining patterns are untouched after thousands of reversals under load. The pads appear as new.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041702-041702-9. doi:10.1115/1.4033417.

Aerodynamic slider bearings are currently used in various types of turbomachinery. Many such systems perform at increasingly faster speeds and may operate in the supersonic regime. Although there is extensive research on compressible lubrication extrapolated to high-speeds, very little of it addresses the potential supersonic nature of the flow. It is well known in compressible flow that many of the tendencies of subsonic flow actually reverse themselves as the singularity at Mach one is traversed. Thus, examination of this high-speed regime may yield some unanticipated results. The behavior of a thin film of air in the supersonic regime is studied in the two-dimensional flow case with rigid sliding surfaces. The one-dimensional bearing studied has a dual profile consisting of an inlet region converging wedge of constant slope and an exit region of constant gap. Two approaches are compared: the solution of a modified Reynolds equation, and the solution to a version of Navier–Stokes equations adapted to thin films. The results show that the modified Reynolds equation approach, which is useful to describe the behavior of lubricating fluids at high subsonic speeds may be inadequate in the supersonic regime. The present studies show the absence of shock and expansion wave phenomena for cases in which the film thickness ratio does not exceed 0.01.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041703-041703-14. doi:10.1115/1.4032911.

In this paper, a hypothesis for the operating tribological mechanisms and phenomena occurring in compliant surface gas foil bearings subjected to low ambient pressure conditions, such as occur at high altitude or in soft vacuum, will be presented and discussed. Both theoretical and experimental evidence supporting the proposed hypothesis will be presented to show that, under low ambient pressure conditions (i.e., something akin to starved fluid film lubrication), the shaft is supported by a combination of hydrodynamic and morphological elements. The theoretical treatment of the compressible fluid film in a simple gas bearing is highly nonlinear in-and-of-itself, and especially more so when combined with a compliant surface supported on a frictional-elastic foil foundation. Adding a “molecularly starved gas film” to this highly nonlinear system, one encounters a very interesting and complex system that, heretofore, has not been considered. When operating compliant foil gas bearings in a near or soft vacuum, the term hydrodynamic may be considered oxymoronic in that there is little or no apparent fluid/gas to provide “a full hydrodynamic” action. However, theoretical and experimental evidence of compliant surface foil gas bearings operating at low ambient pressures show that they do continue to work and, in fact, can do so quite well given the appropriate compliancy and other factors, as yet to be discussed. In this paper, the situation will be addressed based upon the experimental evidence that resulted in the essential hypothesis that there are elements at work that go above and beyond purely hydrodynamic phenomenon or so-called solid lubrication. These elements include both tribological and morphological interactions, which are at work at all times and it is the respective ratios of hydrodynamic and morphological elements that characterize operation. Evidence is presented to the effect that, even when hydrodynamic effects dominate, morphological interactions contribute to bearing performance and load-carrying capacity and that, when morphological effects dominate, third body and surface elements impart to the interface many of the characteristics and effects of a hydrodynamic film. Thus, by combining classical Reynolds equation modified for compressible media with the quasi-hydrodynamic/continuum equations and the appropriate rheological and morphological parameters, meaningful solutions for foil bearing operating with extreme low-pressure boundary conditions are possible, and which result in increased load-carrying capacity contrary to classical hydrodynamic theory.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):041704-041704-10. doi:10.1115/1.4032912.

Olsson's interphase condition (OIC) is carefully examined and scrutinized with respect to both physical and mathematical implications. It is a genuine initial value problem statement so that its full compliance is mandatory for analyzing time-dependent cavitation problems in the journal bearing. Implementation of OIC must include: (1) an unambiguous description of the initial state of the entire fluid film of the bearing, (2) a realistic description of fluid supply configuration, (3) accurate determination of the locations of the void boundaries together with the corresponding pressure gradients, and (4) a suitable morphology model for the cavitated fluid. A new computation algorithm is proposed for cavitation studies that are governed by cross-boundary interface continuity (CBIC), which is a modified statement of OIC.

Commentary by Dr. Valentin Fuster

Research Papers: Magnetic Storage

J. Tribol. 2016;138(4):041901-041901-12. doi:10.1115/1.4032797.

Fretting wear at the dimple/gimbal interface of a hard disk drive suspension was investigated for stainless steel dimples in contact with stainless steel gimbals coated with diamondlike carbon (DLC) of different thicknesses and different elastic moduli. Scanning electron microscopy (SEM) was used to evaluate the size and characteristics of the wear scar of both the dimple and the gimbal. Fretting wear and fatigue-type cracks were found predominantly on the dimple. For different dimple/gimbal combinations tested in this study, the least amount of wear was obtained for the case of a 690 nm thick DLC overcoat. Numerical simulations were performed to calculate the maximum principal stress in the dimple and the gimbal with the goal of correlating wear and the maximum principal stress. The maximum principal stress in both the dimple and the gimbal was found to increase with an increase of the elastic modulus of the DLC overcoat on the gimbal. On comparing the experimental and simulation results, we conclude that wear and fatigue crack formation can be explained by the different level of the maximum principal stress in both the dimple and the gimbal.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2016;138(4):042001-042001-8. doi:10.1115/1.4032818.

Wear rates of polytetrafluoroethylene (PTFE) filled with micrometer- and nanometer-sized particles of copper, silicon nitride, and γ-phase alumina were measured under dry sliding conditions using a pin-on-plate tribometer. In their ability to limit the wear rate, micrometer-sized copper particles were found to be better than their nanometer-sized counterparts, though by only small margins, with a 20 wt.% loading of the micrometer-sized copper particles resulting in a tenfold reduction in the wear rate over that of unfilled PTFE. With 10 wt.% loading of micrometer-sized particles of silicon nitride and γ-phase alumina, very low wear rates of ∼5 × 10−7 mm3/N·m and ∼2.5 × 10−7 mm3/N·m, respectively, were measured. Wear rate of unfilled PTFE under the same testing conditions, also measured here, was found to be about 3.6 × 10−4 mm3/N·m. In all the three cases (copper, silicon nitride, and γ-phase alumina), wear resistance was either lost fractionally or completely when the size of the filler particles was reduced from the microscale to a few tens of nanometers, with nanoscale silicon nitride filler resulting in even slightly higher wear rates and larger platelike wear debris than unfilled PTFE. Micrographs of the wear tracks and the generated wear debris seem to indicate that all three filler materials in the form of more effective larger microparticles reduce wear by a common mechanism of interrupting wear debris production and limiting wear debris size, further supporting Tanaka and Kawakami's 1982 proposal of a broad general mechanism of PTFE wear reduction by filler particles having at least a requisite microscale size. Recent reports of extreme PTFE wear resistance imparted by few limited nanofiller particles appear to be reflective of an additional wear reduction mechanism they may specifically possess, rather than a contradiction of previously proposed microparticle wear reduction mechanisms.

Commentary by Dr. Valentin Fuster

Research Papers: Mixed and Boundary Lubrication

J. Tribol. 2016;138(4):042101-042101-9. doi:10.1115/1.4032841.

The ability to predict contact surface temperatures in rolling/sliding contacting bodies is important if failure of tribological components is to be avoided. Many works on surface temperature analysis and prediction have been published over the past 75 years or so, but most of the analytical solutions that are readily available do not include such important factors as finite body geometry, surface roughness, or convective cooling. Approaches for addressing these deficiencies are presented in this paper. This paper builds on previous analytical work by the authors and others, and presents models that are based on experimental observations of contact temperatures and factors that affect them. It is shown that the total surface temperature rise above ambient temperature is the sum of nominal temperature rise and flash temperature rise. Models are developed for calculating nominal surface temperature rise for sliding bodies of finite size, including effects of both convection and conduction. Flash temperature models are developed for both single and multiple contacts, as would be found with rough surfaces. Methods are presented that are valid for a variety of geometries and kinematic operating conditions, and techniques are also presented for partitioning the frictional heat between the two contacting surfaces. Examples of the use of the methodology are presented, along with experimental verification of the predictions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):042102-042102-4. doi:10.1115/1.4032889.

Structural hydrogel materials are being considered and investigated for a wide variety of biotribological applications. Unfortunately, most of the mechanical strength and rigidity of these materials comes from high polymer concentrations and correspondingly low polymer mesh size, which results in high friction coefficients in aqueous environments. Recent measurements have revealed that soft, flexible, and large mesh size hydrogels can provide ultra low friction, but this comes at the expense of mechanical strength. In this paper, we have prepared a low friction structural hydrogel sample of polyhydroxyethylmethacrylate (pHEMA) by polymerizing an entangled polymer network on the surface through a solution polymerization route. The entangled polymer network was made entirely from uncrosslinked polyacrylamide (pAAm) that was polymerized from an aqueous solution and had integral entanglement with the pHEMA surface. Measurements revealed that these entangled polymer networks could extend up to ∼200 μm from the surface, and these entangled polymer networks can provide reductions in friction coefficient of almost two orders of magnitude (μ > 0.7 to μ < 0.01).

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;138(4):042103-042103-3. doi:10.1115/1.4032890.

Gemini hydrogels have repeatedly produced low friction under conditions generally not thought to be favorable to superlubricity: low sliding speeds, low contact pressures, macroscopic contact areas, and room temperature aqueous environments. A proposed explanation for this unique behavior is that thermal fluctuations at the interface are sufficient to separate the surfaces, with solvent (water) shearing in this region being the main source of dissipation. In this paper, we demonstrate that very soft and correspondingly large mesh size Gemini hydrogels show superlubricity with the lowest measured friction coefficient being μ = 0.0013 ± 0.0006.

Topics: Friction , Hydrogels
Commentary by Dr. Valentin Fuster

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