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### Research Papers: Applications

J. Tribol. 2017;139(5):051101-051101-5. doi:10.1115/1.4036173.

Magnetorheological elastomers (MREs) are smart materials that have been studied widely for their material properties. The elasticity modulus or hardness of an MRE can be changed when an external magnetic field is applied. In this study, a study of MREs applied to rolling friction control under various external magnetic strengths is conducted. To accomplish this, the rolling friction property of an elastomer on a rigid plate is analyzed. Then, MREs are prepared, and a rolling friction tester is designed to evaluate the changes in the rolling friction coefficient. The results show that the rolling friction coefficient can be changed with different magnetic field strengths. The rolling friction coefficient of the MRE can be controlled by the applied magnetic field, which can be applied to control the slip rate and be adapted to achieve the optimal friction effect in the future.

Commentary by Dr. Valentin Fuster

### Research Papers: Coatings and Solid Lubricants

J. Tribol. 2017;139(5):051301-051301-11. doi:10.1115/1.4035383.

In the present study, the effects of TiC content on the microstructure, hardness, and wear property are to be investigated. Magnesium matrix hybrid composites reinforced with varying wt.% of TiC (0, 5, 10, 15, and 20) and a fixed wt.% of MoS2 (7.5) were produced by powder metallurgy. The microstructure of the hybrid composite samples was analyzed using optical microscopy. Elemental composition of sintered specimens was determined by energy dispersive X-ray spectroscopy (EDS) analysis. The Vicker's hardness test was performed in different locations on the sintered specimen surface with a load of 5 g and 15 s dwell time. The dry sliding wear test was carried out in a pin-on-disk wear testing machine at various load (5–30 N), velocity (0.5–3 m/s), and sliding distance (500–3000 m). Tribological investigation was statistically analyzed using Taguchi L27 orthogonal array with four factors at three levels. A graphical and numerical optimization technique was used to find the optimum value of TiC content using the predicted value of the responses. The tribological properties of the fabricated composites improved significantly compared to that of the magnesium matrix due to the combined effect obtainable by both reinforcements.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051302-051302-9. doi:10.1115/1.4036169.

Ni–B coatings have been deposited directly on commercial purity magnesium and evaluated by means of sliding wear and friction testing. The nickel and boron are distributed throughout the whole thickness of the coating. Parametric optimization has been carried out using the design of experiment based on Taguchi analysis. The friction coefficients of the coatings vary between 0.01 and 0.3, depending on the evaluated conditions. The lowest wear rate of the coating was 0.19 × 10−5 mm3 N−1 m−1. Wear and friction coefficient maps of Ni–B electroless coatings formed on magnesium surfaces are reported. Nanoparticles present in the lubricating fluid act as a third wear body increasing the wear of the electroless coating

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2017;139(5):051401-051401-8. doi:10.1115/1.4035776.

A new tangential contact model between a rough surface and a smooth rigid flat is proposed in this paper. The model considers the contribution of both elastically deformed asperities and plastically deformed asperities to the total tangential load of rough surface. The method combining the Mindlin partial slip solution with the Hertz solution is used to model the contact formulation of elastically deformed asperities, and for the plastically deformed asperities, the solution combining the fully plastic theory of normal contact with the bilinear relation between the tangential load and deformation developed by Fujimoto is implemented. The total tangential contact load is obtained by Greenwood and Williamson statistical analysis procedure. The proposed model is first compared to the model considering only elastically deformed asperities, and the effect of mean separation and plasticity index on the relationship between the tangential load and deformation is also investigated. It is shown that the present model can be used to describe the stick–slip behavior of the rough surface, and it is a more realistic-based model for the tangential rough contact. A comparison with published experimental results is also made. The proposed model agrees very well with the experimental results when the normal load is small, and shows an error when the normal load is large.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2017;139(5):051501-051501-19. doi:10.1115/1.4035154.

This work presents an exact and general model order reduction (MOR) technique for a fast finite element resolution of elastohydrodynamic lubrication (EHL) problems. The reduction technique is based on the static condensation principle. As such, it is exact and it preserves the generality of the solution scheme while reducing the size of its corresponding model and, consequently, the associated computational overhead. The technique is complemented with a splitting algorithm to alleviate the hurdle of solving an arising semidense matrix system. The proposed reduced model offers computational time speed-ups compared to the full model ranging between a factor of at least three and at best 15 depending on operating conditions. The results also reveal the robustness of the proposed methodology which allows the resolution of very highly loaded contacts with Hertzian pressures reaching several GPa. Such cases are known to be a numerical challenge in the EHL literature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051502-051502-5. doi:10.1115/1.4036167.

Transient thermal elastohydrodynamic lubrication (EHL) line contact simulations are carried out to study the traction behavior during accelerated motion considering realistic shear-thinning behavior. Using three lubricants with different inlet viscosity and shear-thinning parameters, the application of present analysis for lubricant selection is demonstrated. Owing to squeeze film action, the film evolution is delayed, and EHL traction during acceleration is found to increase much above the designed value. This effect decreases with increasing starting speed. The most shear-thinning test oil considered here yields the lowest traction coefficient with minimum variation in its value desirable for smooth and vibration-free operation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051503-051503-6. doi:10.1115/1.4035777.

The paper deals with an experimental study of an elastohydrodynamic contact under insufficient lubricant supply. Theoretical studies published in this research area focus mainly on the development of theoretical models, and there is a lack of experimental validation of the theoretical models. This paper presents original experimental results and aims to describe the starvation severity level as a function of the inlet film thickness and contact geometry. Experimental data are compared with an analytical model for point contacts published by Chevalier. The study was also extended to elliptical contacts to achieve a comparison with the different parameters of the side-flow resistance used by Damiens. Both models agree well with the experiments.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2017;139(5):051601-051601-13. doi:10.1115/1.4035344.

The nonlubricated sliding wear of SiC–B4C–Si cermets against a diamond indenter was studied. The cermets containing 2, 5, 10, and 20 wt.% of Si were fabricated by both conventional sintering and spark plasma sintering (SPS) techniques. It has been observed that wear depth, volume of the wear debris, and wear rate increases with increasing applied load for both cases. Minimum wear depth and lowest wear rate was obtained for the cermet containing 10 wt.% Si. Three-body abrasion is the main wear mechanism which results in surface delamination, and formation of grooves and cavities on the damaged surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051602-051602-10. doi:10.1115/1.4035340.

The effect of bifilm oxides on the dry sliding wear behavior of Fe-rich (1.5 wt.%) F332 Al–Si alloy under as-cast and T6 heat-treated conditions was investigated. Toward this end, the surface oxides were intentionally incorporated into the molten alloy by surface agitation. The results showed that, after sliding under the applied load of 75 N, due to the presence of bifilms, the wear rate of base (0.2 wt.% Fe) and 1.5 wt.% Fe-containing alloys increased by almost 22% and 14%, respectively. The results also indicated that, despite the positive effect on the hardness, T6 heat treatment adversely affected the wear resistance of alloys made under surface turbulence condition. This negative effect can be attributed to the expansion of bifilms which, during heat treatment, are converted to the potential sites for initiation and propagation of subsurface microcracks. However, the strengthening effect exerted by the thermally modified β-Al5FeSi platelets showed that it can compensate the negative effects of bifilm oxides because it improves the wear rate of 1.5 wt.% Fe-containing F332-T6 alloy by about 5% under the applied load of 75 N.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051603-051603-5. doi:10.1115/1.4035341.

The static and dynamic friction properties of a steel pin on polyoxymethelyne homopolymer disk were studied at temperatures ranging from 22 to 160 °C. Samples were tested at externally applied normal loads ranging from 20 to 80 N. Under this range of temperatures, the friction coefficients displayed a linearly increasing dependence on the load. The load dependence is attributed to an enhanced contribution of the plowing friction mechanism at higher loads. As load increases, the pin asperities penetrate into the hard, injection mold-induced skin layer, causing an increase in the frictional plowing. The coefficient of friction was observed to decrease from 0.08 at 22 °C to 0.05 at 50 °C, and subsequently rise to 0.07 at 160 °C. The initial drop was caused by a decrease in the modulus of elasticity attributed to the rise in molecular mobility with increased available thermal energy. As the temperature increased to 160 °C, however, the further decrease in modulus allowed the penetration of the pin asperities to increase significantly, requiring increased material displacement to initiate frictional motion.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051604-051604-6. doi:10.1115/1.4035309.

The tribosystem stability in different wear stages was analyzed by the friction vibration energy feature, which was extracted from the phase-space-matrix of friction vibration attractor with singular value decomposition (SVD). An energy feature parameter K of friction vibration was defined as the norm of the singular value feature vector, and the variation of K in different wear stages was also investigated in this paper. Results show that K becomes larger in the running-in wear stage, friction vibration energy becomes higher and tribosystem stability gets worse; K fluctuates smoothly and steadily in the stable wear stage, friction vibration energy is stable and the tribosystem is dynamically stable; and K increases sharply in the severe wear stage, the friction vibration energy increases dramatically and the tribosystem stability decreases greatly. Therefore, the friction vibration energy can reflect the tribosystem stability in different wear stages with the energy feature parameter K.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051605-051605-9. doi:10.1115/1.4035477.

Very smooth surface topography of CFs (CFs) and its chemical inertness have been a concern while selecting them for reinforcement in polymer composites. Such fibers do not show adequate fiber–matrix adhesion and hence do not contribute to enhance the property to the extent possible. Chemical treatment or sizing of fibers using proper coupling agents appears to be the only choice to deal with this problem. In the case of friction materials (FMs), hardly anything is reported on exploring the potential of few surface treatments of CFs, though CF is used as very efficient ingredient of FM. In this paper, two methods (one most primitive, i.e., nitric acid and second, most recent, i.e., nano-YbF3 particles) were selected to treat short CF and to develop FMs based on treated and untreated fibers for comparison. The studies revealed that the nanoparticles treatment proved most effective and enhanced almost all the performance properties of FMs.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051606-051606-14. doi:10.1115/1.4035478.

Effects of wear and member flexibility on the dynamic performance of a planar five-bar mechanism with joint-clearance are investigated. The equation of motion of the mechanism is derived based on the absolute nodal coordinate formulation (ANCF). In order to enhance the accuracy of the contact force, the slope of the load–displacement curve of the cylindrical joint with clearance is used. The contact deformation couples the joint wear to the contact state. The contact force model of Flores and coworkers is improved, by the introduction of the stiffness coefficient. The wear depth is predicted by using the Archard's wear model. Simulations show that the multiclearance joints can generate stronger contact forces relative to single clearance joint case. This leads to more severe wear in the joint. However, the mechanism with multiple flexible links can absorb more of the energy arising from the clearance joint, and this improves the wear phenomenon.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051607-051607-8. doi:10.1115/1.4036168.

Recently, one of the present authors proposed a new model to explain the generation mechanism of brake squeal based on the time delay between a varying applied normal force and the resulting friction force. The present work conducts a series of experimental tests examining the behavior of this time delay using a special test apparatus. The test apparatus suppresses the effect of interference in the time delay owing to the excitation normal force to the greatest extent possible. Several calibration tests of the test apparatus are conducted to ensure the validity of the normal force and friction force measurements. The varying friction force is extracted from the overall friction force signal without phase distortion using a zero-phase filter. The test results demonstrate a time delay between the varying normal force and the resulting friction force under various testing parameters. The time delay is found to increase with increasing excitation frequency. The generation mechanism of the time delay is also discussed.

Commentary by Dr. Valentin Fuster

### Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;139(5):051701-051701-11. doi:10.1115/1.4035773.

Magnetorheological fluids (MRFs) are applicable for achieving semi-active control in smart bearings. For hydrodynamic bearings lubricated with MRF, changes of the viscosity induced by magnetic field lead to changes of the dynamic characteristics such as stiffness and damping properties, providing the controllability to the bearings in rotor applications. Two main defects of the MRF, however, may potentially limit the use of this kind of bearings. One is that the magnetic field-induced viscosity alteration capability decreases as the shear rate increases; the other is the extra friction introduced by iron particles in the MRF in external magnetic field. In this study, the floating ring bearing (FRB) and semi-floating ring bearing (sFRB) are introduced to replace common journal bearing for MRF-lubricated smart bearings. Performance enhancement is achieved using FRB and sFRB. The lubrication behavior of MRF is studied using the Herschel–Bulkley (HB) model that incorporates the yield stress and the shear-thinning effect, which are the two main features of the MRF under shearing. A kind of MRF is developed for lubrication application, and a test rig is setup to measure its shear rate–stress relationship and then to identify its HB model parameters. With the identified HB model, stiffness and damping characteristics of the MRF-lubricated FRB and sFRB are studied. Results show that, compared to MRF-lubricated common journal bearings, the MRF-lubricated FRB and sFRB both achieve better performances in damping enhancement, while limiting the journal friction to a relatively lower degree.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051702-051702-10. doi:10.1115/1.4035868.

This work relates to previous studies concerning the asymptotic behavior of Stokes flow in a narrow gap between two surfaces in relative motion. It is assumed that one of the surfaces is rough, with small roughness wavelength μ, so that the film thickness h becomes rapidly oscillating. Depending on the limit of the ratio h/μ, denoted as λ, three different lubrication regimes exist: Reynolds roughness (λ = 0), Stokes roughness (0 < λ < ∞), and high-frequency roughness (λ = ∞). In each regime, the pressure field is governed by a generalized Reynolds equation, whose coefficients (so-called flow factors) depend on λ. To investigate the accuracy and applicability of the limit regimes, we compute the Stokes flow factors for various roughness patterns by varying the parameter λ. The results show that there are realistic surface textures for which the Reynolds roughness is not accurate and the Stokes roughness must be used instead.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051703-051703-11. doi:10.1115/1.4035778.

Surface texturing is a technique for improving frictional and hydrodynamic performances of journal bearings because microtextures can serve as reservoirs for oil or traps for debris and may also generate hydrodynamic pressure. Over the past two decades, many researchers have experimentally demonstrated that texturing of various tribological elements can reduce friction force and wear, contributing to improvement of lubrication performance. Some numerical studies have examined the hydrodynamic lubrication conditions and reported that surface texturing affects the static characteristics of journal bearings, such as their load carrying capacity and friction torque. However, the validity of these numerical models has not been confirmed because of a lack of experimental studies. This study proposes a numerical model that includes both inertial effects and energy loss at the edges of dimples on the surface of a journal bearing in order to investigate the bearing's static characteristics. Experimental verification of journal bearings is also conducted with a uniform square-dimple pattern on their full-bearing surface. The results obtained by the model agree well with those of experiment, confirming the model's validity. These results show that under the same operating conditions, textured bearings yield a higher eccentricity ratio and lower attitude angle than the conventional ones with a smooth surface. This tendency becomes more marked for high Reynolds number operating conditions and for textured bearings with a large number of dimples.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051704-051704-13. doi:10.1115/1.4036275.

This paper presents a new approach for modeling flexure-pivot journal bearings (FPJB) employing a three-dimensional (3D) elasto-hydro-dynamic (EHD) lubrication model. The finite element (FE) method is adopted for an analysis of the (1) pad-pivot dynamic behavior and the (2) fluid force. The isoviscosity Reynolds equation is utilized to calculate the fluid force acting on a flexure-pivot pad bearing and spinning journal. Computational efficiency is achieved utilizing modal coordinate transformation for the flexible pad-pivot dynamic analysis. Fluid film thickness plays a critical role in the solution of Reynolds equation and is evaluated on a node-by-node basis accounting for the pad and web deflections. The increased fidelity of the novel modeling approach provides rotating machinery designers with a more effective tool to analyze and predict rotor–bearing dynamic behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051705-051705-9. doi:10.1115/1.4034765.

A dynamic model of a rotating shaft on two textured hydrodynamic journal bearings is presented. The hydrodynamic mean pressure is computed using multiscale periodic homogenization and is projected on a flexible shaft with internal damping. Harmonic balance method (HBM) is used to study the limit cycles of unbalance response of the coupled system discretized by finite element method (FEM). Stability is analyzed with Floquet multipliers computation. An example of an isotropic texturing pattern representing laser dimples on a lightweight rotor is analyzed. Vibration amplitude and stability zone are compared with plain bearing lubrication. It is shown in an example that full surface texturing leads to relatively higher vibration amplitude compared to plain bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051706-051706-6. doi:10.1115/1.4035774.

This paper presents an approximate solution of Muijderman's model for compressible spiral grooved gas film. The approximate solution is derived from Muijderman's equations by Adomian decomposition method. The obtained approximate solution expresses the gas film pressure as a function of the gas film radius. The traditional Runge–Kutta method is avoided. The accuracy of the approximate solution is acceptable, and it brings convenience for performance calculation of spiral grooved gas seal. A complete Adomian decomposition procedure of Muijderman's equations is presented. The approximate solution is validated with published results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051707-051707-10. doi:10.1115/1.4036170.

Operation parameter influences on the behavior of the gaseous bubble in the lubricant for a groove textured journal bearing are studied under the consideration of the thermal effect of the bearing–shaft system. The influence is analyzed by simultaneously solving Rayleigh–Plesset (RP), energy, and Reynolds equations. The computer code for the analyzing the bubble behavior is validated. Numerical results show that appropriately increasing the width–diameter ratio of the bearing and rotational speed of the shaft, or decreasing the applied load and inlet temperature of the lubricant, can decrease the maximum radius, collapse pressure, and temperature of the bubble.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051708-051708-20. doi:10.1115/1.4035812.

In this paper, a mixed lubrication model is presented to analyze the tribological behavior of the textured journal bearings operating from mixed to hydrodynamic lubrication regimes. In particular, the effects of fluid piezoviscosity and the non-Newtonian fluid behavior are also considered. The presented model solves the hydrodynamic lubrication problem by a mass-conserving formation of the Reynolds equation, whereas the metal–metal contact is considered by using the Greenwood and Tripp (GT) contact model which is linked with the hydrodynamic model based on the concept of Johnson's load sharing. As a result, the performance of the textured journal bearing system under different lubrication regimes, including boundary lubrication regime, mixed hydrodynamic lubrication regime, and hydrodynamic lubrication regime, can be evaluated. Using the journal bearing systems operated under the start-up condition as examples, prediction demonstrates the influences of texture distributions on friction and wear. It is found that the friction reducing effect induced by texturing is influenced by the distribution of the texturing zones. In particular, the hydrodynamic friction can be reduced when the eccentricity ratio is changed from high to low. Moreover, it appears that the shear-thinning effect of lubricant cannot be neglected in the transient analysis of journal bearing system.

Commentary by Dr. Valentin Fuster

### Research Papers: Lubricants

J. Tribol. 2017;139(5):051801-051801-5. doi:10.1115/1.4036172.

Antimony nanoparticles, whose surfaces were modified by alkyl phenol polyoxyethylene ether (OP-10), were used as one of the types of lubricating additives in liquid paraffin (LP). The tribological properties of antimony nanoparticles as lubricating additives were evaluated and compared with those of pure LP on a four-ball test machine. The morphology and chemical composition of the worn surface were investigated and analyzed by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the additives can obviously improve the anti-wear and friction reducing properties of LP, which are better under high friction load. The double-layer crystal structure of antimony can be separated and glided along the cleavage plane by a friction-shear force and a normal load, respectively. The separating and gliding of antimony can form a physical adsorption film, which can separate the friction surface to avoid direct contact of the friction surface and play an important role in improving the anti-wear and friction reducing properties.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro-Nano Tribology

J. Tribol. 2017;139(5):052001-052001-9. doi:10.1115/1.4035775.

Greases are widely used for variety of applications at extreme pressures, temperatures, and speeds with obligation of high bearing and shaft life with low noise. The present study deals with the development of nanocomposite greases and records their flow and frictional characteristics. The commercial grease is modified, by dispersing nanoparticles, to get the nanocomposite greases. Reduced graphene oxide (rGO), calcium carbonate (CaCO3), and alpha-alumina (α-Al2O3) are used as nano-additives. The microstructure of newly developed greases is examined using high-resolution transmission electron microscopy (HRTEM), and the presence of different chemical functional groups is explored using Fourier transform infrared spectroscopy (FTIR). Further, the new greases are investigated for rheological, consistency, and tribological behavior using Visco Tester, penetrometer, and elastohydrodynamic (EHD) rig, respectively. The flow properties reveal the shear-thinning, yielding, and thixotropic nature of lubricating greases. The study shows that there is an optimality in concentration of different nano-additives above which grease's flow and tribological performance degrades. Up to 35%, 27%, and 10% reduction in coefficient of friction is recorded for optimum concentration of rGO nanosheets, CaCO3, and α-Al2O3 nanoparticles, respectively. The difference in the performance of nanocomposite greases can be attributed to the appearance of different friction mechanisms for different nano-additives.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Tribol. 2017;139(5):054501-054501-5. doi:10.1115/1.4035156.

This study investigated the use of pressure sensitive paint (PSP) as new measuring technique for measuring the pressure distribution of a gas bearing. An externally pressurized circular thrust gas bearing with single gas supply hole was used as the test bearing to investigate the suitability of this technique. The test bearing was 30 mm in diameter, with a gas supply hole of diameter 0.7 mm. A coat of PtTFPP, the substance used as the PSP, was applied to the bearing surface using an air-assisted spray. The PSP luminescence characteristics were calibrated before the tests because of their dependency on temperature and pressure. The pressure distribution was obtained by averaging 50 images captured by a 12-bit complementary metal-oxide semiconductor (CMOS) camera. These experimental results were compared with the results of a numerical analysis based on the divergence formulation method. There was good agreement between the experimental and analytical results, thus demonstrating the effectiveness of using PSP for pressure distribution measurements.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):054502-054502-8. doi:10.1115/1.4035506.

The aim of this technical brief is to provide a numerical approach to investigate the lubricity enhancement effect of microgrooves texture on tools' rake face. The key parameters related to cutting condition and grooves morphology were considered in the analytical model of tool–chip friction pair. The fully textured surfaces with the periodic microgrooves were systematically studied by solving the nondimensional Reynolds equation with the multigrid method. The results indicated that the microgrooves texture generates extra carrying capacity comparing to the flat tool and the optimum grooves direction is vertical to the chip sliding. Higher area density and optimum grooves width can further promote hydrodynamic lubrication. By modifying the tool rake face geometry to restrict the tool–chip slope angle, efficiency of surface texture could be greatly extended. In addition, the film's average pressure was nearly proportional to the chip velocity. Hence, the textured tool is more effective in high-speed cutting.

Commentary by Dr. Valentin Fuster

J. Tribol. 2017;139(5):055001-055001-11. doi:10.1115/1.4035347.

Short-fiber (e.g., steel wool, rock fibers, and kevlar) reinforced flyash cenosphere–molybdenum disulfide combination-filled phenolic composite materials for friction braking applications were developed and evaluated for their physical (density, void fraction, acetone extraction, and ash content), mechanical (hardness, compressibility, shear strength), and thermal characteristics (thermogravimetric analysis (thermo gravimetric analysis (TGA)/differential thermo gravimetry). Performance correlations between thermomechanical characteristics and friction braking data obtained from Krauss friction testing machine is attempted. The frictional fade (μF) and recovery (μR) behavior vis-a-vis amplitude of friction fluctuations (μmaxμmin) performance were observed to be enhanced due to the incorporation of “cenosphere–molybdenum disulfide” combination. The enhanced wear resistance was attributed to the lower temperature sensitivity of posttemperature degradation kinetics. Analysis of friction and wear data indicated μF and intrinsic material coefficients to be major determinants for μP and wear, respectively. Our study demonstrates the performance synergism due to the incorporation of cenosphere–molybdenum disulfide as the hybrid “filler–lubricant” combination in composite materials for friction braking application.

Commentary by Dr. Valentin Fuster

### Errata

J. Tribol. 2017;139(5):057001-057001-1. doi:10.1115/1.4036176.

The correct version of Table 4 is:

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):057002-057002-1. doi:10.1115/1.4036178.

The correct version of the equation is Display Formula

(3)$cos β0=2Bd−e2Bd$

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):057003-057003-1. doi:10.1115/1.4035888.

Display Formula

(6a)$n˙=1e[r˙1+R˙1sb11−r˙6−R˙6sA16]=1e[r˙1+R1(ω¯1×sb11)−r˙6−R6(ω¯6×sA16)]$
Display Formula
(28)$Fx=FN sin θz sin θy+Ft cos θz cos θy$
Display Formula
(42)$ω=ar cos lzl6zlz⋅l6z,l6z=R6[00z]T$

Commentary by Dr. Valentin Fuster