0

### Research Papers: Applications

J. Tribol. 2019;141(4):041101-041101-9. doi:10.1115/1.4042199.

To describe the dynamic evolutionary law and tribological behavior of the tribopair AISI 52100-AISI 1045, rotational experiments were conducted by sliding a disk against a static pin. The multidimensional phase spaces were reconstructed based on the scalar time-series by the time-delay embedding technique, and the multivariate graph-based method was used to visualize the overall picture of the phase space. The evolution of radar plots and the corresponding multivariate graph centrobaric trajectory (MGCT) is consistent with the description of “running-in, steady-state and increasing friction stages,” and can serve as effective indicators for the friction state transitions. Results show that the radar plot can inform quantitative interpretations of friction process identification. Therefore, the multivariate graph-based method is a useful approach to characterize the nonlinear dynamics of tribological behaviors.

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2019;141(4):041401-041401-9. doi:10.1115/1.4042099.

A numerical method for modifying cylindrical roller profile was proposed to smooth axial pressure distributions of finite line contacts under the mixed lubrication regime. The mixed lubrication model, in which the Reynolds equation modified by Patir and Cheng has been solved with implementing the rough surface contact model of Kogut and Etsion for the stochastic solution of hydrodynamic pressure and asperity-contact pressure, was established and it is validated by the comparison between simulation results and experiments. Some common roller profiles were carried into the mixed lubrication model and obvious increment of pressure appears near the roller ends or at the central contact area. A numerical running-in method was developed to smooth pressure shapes and the crown drop of roller profile was modified gradually implementing Archard's wear law, where a higher asperity-contact pressure leads to a larger crown drop on a roller profile. The results of the numerical running-in method indicated that pressure distributions of finite line contacts are uniform if the optimized roller profile is employed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041402-041402-11. doi:10.1115/1.4042270.

A coupled multibody elastic–plastic finite element (FE) model was developed to investigate the effects of surface defects, such as dents on rolling contact fatigue (RCF). The coupled Voronoi FE model was used to determine the contact pressure acting over the surface defect, internal stresses, damage, etc. In order to determine the shape of a dent and material pile up during the over rolling process, a rigid indenter was pressed against an elastic plastic semi-infinite domain. Continuum damage mechanics (CDM) was used to account for material degradation during RCF. Using CDM, spall initiation and propagation in a line contact was modeled and investigated. A parametric study using the model was performed to examine the effects of dent sharpness, pile up ratio, and applied load on the spall formation and fatigue life. The spall patterns were found to be consistent with experimental observations from the open literature. Moreover, the results demonstrated that the dent shape and sharpness had a significant effect on pressure and thus fatigue life. Higher dent sharpness ratios significantly reduced the fatigue life.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041403-041403-10. doi:10.1115/1.4042268.

To quantify the friction mechanism of the interface of the brake disk-pad pair, an analytical model of coefficient of friction (COF) is established from the perspective of contact mechanics. The model takes into account the surface topography of the disk, mechanical properties of brake pair, and the ingredients of the brake pad. As the reinforcing fillers, the effect of particle size and amount on the COF are analyzed, and the simulation results are consistent with the experimental data. The model and results presented here offer some insight into real brake pair design.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2019;141(4):041501-041501-14. doi:10.1115/1.4042503.

This paper presents a thermal elastohydrodynamic lubrication (EHL) model for analyzing crowned roller lubrication performances under the influence of frictional heating. In this thermal EHL model, the Reynolds equation is solved to obtain the film thickness and pressure results while the energy equation and temperature integration equation are evaluated for the temperature rise in the lubricant and at the surfaces. The discrete convolution fast Fourier transform (DC-FFT) method is utilized to calculate the influence coefficients for both the elastic deformation and the temperature integration equations. The influences of the slide-to-roll ratio (SRR), load, crowning radius, and roller length on the roller lubrication and temperature rise are investigated. The results indicate that the thermal effect becomes significant for the cases with high SRRs or heavy loads. The proposed thermal EHL model is used to study the thermal-tribology behavior of an apex seal–housing interface in a rotary engine, and to assist the design of the apex seal crown geometry. A simplified crown design equation is obtained from the analysis results, validated through comparison with the optimal results calculated using the current crowned-roller thermo-EHL (TEHL) model.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2019;141(4):041601-041601-10. doi:10.1115/1.4042039.

Friction stir processing is performed on RZ 5 Mg alloy to produce surface metal matrix composites reinforced with hard reinforcement particles. Boron carbide, multiwalled carbon nanotubes, and a mixture of zirconia and alumina particle reinforcements were introduced. The developed surface composites (SCs) exhibited lower wear rates at various normal loads than the base RZ 5 Mg alloy owing to their improved microhardness. The wear resistance of the composites was 1.2–1.9 times greater than the base alloy, and hence, the wear rates were 18–50% lower than the base alloy. Maximum reduction in wear rate is observed in B4C-reinforced SC. Abrasion, adhesion, and oxidative wear mechanisms are operational during the wear test performed at loads ranging between 10 N and 75 N.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041602-041602-22. doi:10.1115/1.4042198.

Al–Zn–Mg–Cu matrix composites reinforced with (0–20 wt %) Al2O3 particles have been manufactured by enhanced stir casting technique. Microstructural characterization of cast composites by optical, field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDS) and X-ray diffraction (XRD) reveals homogeneous distribution of reinforcements in Al-alloy matrix with MgZn2 plus Al2CuMg intermetallics. With increasing particle content, hardness of composite rises considerably in spite of marginal rise in porosity. Tribological performance under two-body abrasion has been studied considering central composite design (CCD) apart from identification of mechanisms of wear via characterizations of abraded surfaces and debris. Composites exhibit significantly reduced wear rate and coefficient of friction (COF) irrespective of test conditions, since mechanisms of abrasion are observed to change from microplowing and microcutting in unreinforced alloy to mainly delamination with limited microplowing in composites. Effects of four independent factors (reinforcement content, load, abrasive grit size, and sliding distance) on wear behavior have been evaluated using response surface-based analysis of variance (ANOVA) technique. Dominant factors on both wear rate and COF are identified as reinforcement content followed by grit size and load. Combined optimization of wear rate and COF employing multiresponse optimization technique with desirability approach as well as regression models of individual responses have been developed, and their adequacies are validated by confirmatory tests. The developed mathematical models provide further insight on the complex interactions among wear performances of the selected materials and variables of abrasive system. The optimum amount of reinforcement is identified at around 15 wt % for achieving the lowest values of both wear rate and COF.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041603-041603-8. doi:10.1115/1.4042038.

Frictional and fretting wear behaviors of Inconel X-750 alloy against GCr15 steel ball were investigated in dry contact condition with ∼60% air humidity. Fretting tests were run at the high frequency tribosystem SRV 4 in room temperature and ball-on-flat contact configuration were adopted with the relative oscillatory motion of small displacement amplitude (40 μm). Sliding regimes, wear volumes, frictional properties, and material damage mechanisms were studied with regard to different normal loading and test durations. After the tests, the worn surface morphologies were analyzed by three-dimensional (3D) optical surface profiler, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to distinguish fretting running conditions and material responses for different test cases. It was found that the material removals by abrasive and adhesive wear, debris formation and oxidization, and wear delamination were the main damage mechanisms under the lower normal load where the full slide or gross slip regime (GSR) was dominant between the contact surfaces. On the other hand, fretting regime was found to be a stick-slip or a partial slip at greater loads where damage mechanisms were correlated with deformed asperities, fatigue cracks, and thick layer removal due to highly concentrated cyclic stresses. Time dependence was crucial during GSR where the wear volume increased substantially; however, the wear volumes and scars sizes were consistent over time because of stick-slip effects under the higher normal load.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041604-041604-10. doi:10.1115/1.4042098.

While commercial biomedical titanium alloys present excellent biocompatibility and corrosion resistance, their poor wear resistance remains a major limitation. In this study, alloying with aluminum was used to improve the tribological performance of an experimental Ti−Si−Zr alloy. The effect of Al content on the alloy's microstructure and mechanical properties was evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers hardness measurements. Sliding wear testing was performed in a ball-on-disk setup, using stainless steel and silicon nitride counterparts and serum solution lubrication. Microstructural examinations showed that an increase in Al content induced a change from eutectic cell microstructure to regular near-equiaxed particles and produced a solid solution strengthening, increasing alloy's hardness. The adhesive tendencies of the α-Ti matrix to the counterpart dominated the frictional response, and a lower friction coefficient was found against silicon nitride compared to stainless steel. In wear tests against stainless steel counterparts, the alloys showed significantly higher wear rates than the CoCr and Ti−6Al−4V references due to severe abrasive wear, induced by the adhesion of titanium matrix to the counterpart. The Al addition had a positive effect on the wear resistance against silicon nitride due to the solid solution strengthening and the change in microstructure, which reduced the risk of brittle delamination. However, while this gave a trend for a lower wear rate against silicon nitride than the Ti−6Al−4V alloy, the wear rate was still approximately three times higher than that of CoCr.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041605-041605-9. doi:10.1115/1.4042273.

Metal matrix composite (MMC) claddings can be effectively used in severe wear working conditions. The present work successfully develops the MMC clad of Ni + 10% WC8Co + Cr3C2-based material on SS-316 L substrate using cost-effective microwave hybrid heating (MHH) technique. The developed composite clads showed the refined microstructure with the existence of randomly dispersed reinforcement particles inside the Ni based matrix. The phase analysis study results of the clad region revealed the formation of various hard phases of Co3W3C4, Cr7Ni3, NiC, NiW, W2C, Fe6W6C, Fe7C3, FeNi3 during microwave heating. The hard phases present in the clad region contributed to the enhancement in the microhardness. The mean microhardness of the clad region was observed as 550 ± 40 HV. Further, composite clad exhibited excellent wear resistance than SS-316 L substrate under different tribological conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041606-041606-12. doi:10.1115/1.4042274.

Superalloys are metallic systems commonly used in components for aerospace and energy generation applications. In this paper, results of an investigation developed to analyze the effect of heat treatment on the wear behavior of a Haynes 282® superalloy under sliding, nonlubricated conditions are presented. Room temperature pin-on-roll wear tests were undertaken at a constant load and for a fixed sliding distance of 7.5 km. It was found that the wear rate of the alloys was greater for the heat treated specimens compared to the specimens that were tested in a cast and forged condition. Inspection of the alloys in both metallurgical conditions suggests that the wear phenomenon was characterized mostly by severe plastic deformation of the alloy matrix at both surface and subsurface regions by the well-known mechanism of plowing. The test specimens also experienced the formation of a tribofilm whose characteristics were different for each test condition. The formation of tribofilms also had a considerable influence on the wear behavior of the systems studied because they were also present on the surface of the counter rolls with this phenomenon being an additional wear mechanism experienced by the tribosystems studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041607-041607-9. doi:10.1115/1.4042391.

This study presents the prehot corrosion effect on erosion behavior of AISI 446 SS in simulated heat exchanger environment at elevated temperature. Samples were spray deposited using two salt mixture (Na2SO4/NaCl). Subsequently, low-temperature hot corrosion tests were carried out at 550, 650, and 750 °C for 20 h. Chlorination followed by sulfidation was mainly responsible for the passive layer formation during the process of hot corrosion. The prehot corroded samples were subjected to air-jet erosion test using alumina as the erodent, at impact velocity of 100 m/s and flux rate of 4.2 g/min, with variable impingement angles of 30 deg, 60 deg, and 90 deg. The passive layer formed during corrosion underwent detachment of metallic flakes through cracking during the impact of erodent, and was responsible for a significant change in erosion rate. Cutting, plowing, lip formation, and particle embedment were identified as the operative mechanisms during erosion.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):041608-041608-11. doi:10.1115/1.4042506.

The present investigation explores the collective outcome of hard particle reinforcement with deep cryogenic treatment (DCT) on wear responses of magnesium metal matrix nanocomposites (MMNC). A multilevel factorial design of experiments with control factors of applied load (20 and 40 N), sliding speed (1.3, 1.7, 2.2, and 3.3 m/s), reinforcement % (0% and 1.5%), and cryogenic treatment (cryogenic-treated and nontreated) was deployed. Around 1.5 wt % WC-reinforced MMNC were fabricated using stir-casting process. DCT was performed at −190 °C with soaking time of 24 h. The dry sliding wear trials were done on pin-on-disk tribometer with MMNC pin and EN8 steel disk for a constant sliding distance of 2 km. The WC reinforcement contributed toward the improvement in wear rate of MMNC appreciably by absorbing the load and frictional heat at all loads and speeds. During DCT of AZ91, the secondary ß-phase (Mg17Al12) was precipitated that enriched the wear resistance, only for the higher load of 40 N. Scanning electron microscope analyses of the cryogenic-treated MMNC ensured the existence of both ß-phase precipitates and WC in the contact area. As a result, the adhesiveness of this pin was lesser, which attributed to the improved wear resistance (approximately 33%) as compared to base alloy. The coefficient of friction was also less for cryogenic-treated MMNC. A regression analysis was made to correlate the control elements and the responses.

Commentary by Dr. Valentin Fuster

### Research Papers: Mixed and Boundary Lubrication

J. Tribol. 2019;141(4):042101-042101-10. doi:10.1115/1.4042074.

This study presents a generalized model of mixed elastohydrodynamic lubrication, in which the dimensional Reynolds equation is discretized according to a modified differential scheme based on the full analysis of the pressure balance within the lubrication region. The model is capable of a wide range of lubrication regimes from fully hydrodynamic down to boundary lubrication, and both the steady-state and the time-dependent conditions can be considered. A simplified computational procedure is proposed for elliptical contacts without the ellipticity parameters specified. The evolution of lubrication behavior at startup and shutdown conditions is investigated and the transient effect of surface waviness is discussed. The model application is then extended to contacts of multilayered materials, and the effects of the layer stiffness and the fabrication methods on the stress fields and lubrication performance are analyzed. The conclusions may potentially provide some insightful information for the design and analysis of functional materials and their engineering structures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):042102-042102-8. doi:10.1115/1.4042392.

It has been proved that surface nanocrstallization pretreatment is beneficial to plasma nitriding of steel by enhancing nitrogen diffusion, while the tribological properties of the nitrided nanostructured steel under boundary lubrication are not clear. In this work, AISI 316 L stainless steel with and without ultrasonic cold forging technology (UCFT) pretreatment was plasma nitrided at 500 °C for 4 h. The effects of UCFT pretreatment on the microstructure and properties of the nitrided layer and the tribochemical interactions between the nitrided layer and friction modifier molybdenum dithiocarbamate (MoDTC) and antiwear additive zinc dialkyldithio-phosphate (ZDDP) were investigated using SRV tribometer, scanning electron microscopy (SEM), vickers hardness tester, optical microscope, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Surface analyses confirm the formation of a 20 μm thick nitrided layer on the UCFT-pretreated sample and it had higher hardness than that on the unpretreated sample. Furthermore, the nitrided UCFT-pretreated sample presented better synergetic effect with MoDTC and ZDDP on tribological behaviors than the nitrided unpretreated sample. This is attributed to the higher contents of Mo, S, Zn, P, and MoS2/MoO3 ratio in the tribofilms on the nitrided UCFT-pretreated sample.

Commentary by Dr. Valentin Fuster

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

J. Tribol. 2019;141(4):042201-042201-9. doi:10.1115/1.4042272.

This paper experimentally studies the leakage and rotordynamic performance of a long-smooth seal with air–oil mixtures. Tests are performed with inlet gas-volume-fraction gas volume fraction (GVF) = 0%, 2%, 4%, 6%, and 10%, rotor speed ω = 5, 7.5, 10, and 15 krpm, inlet temperature Ti = 39.4 °C, exit pressure Pe = 6.9 bars, and pressure drop (PD) = 31, 37.9, and 48.3 bars. Test results show that adding air into the oil flow does not change the seal's mass flow leakage $m˙$ discernibly but significantly impacts the seal's rotordynamic characteristics. For all PDs and speeds, K increases as inlet GVF increases from zero to 10% except for 6% ≤ inlet GVF ≤ 10% when PD = 48.3 bars, where K decreases as inlet GVF increases. The K increment will increase a pump rotor's natural frequency and critical speed. Increasing the rotor's natural frequency would also increase the onset speed of instability (OSI) and improve the stability of the rotor. Adding air into the oil flow has little impact on cross-coupled stiffness k, direct damping C, and effective damping Ceff. Ceff = C − k/ω + mqω, where mq is the cross-coupled virtual-mass. Test results are compared to predictions from San Andrés's (San Andrés, 2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503.) bulk-flow model, which assumes that the liquid–gas mixture is isothermal and homogenous. The model reasonably predicts $m˙$, C, and Ceff. All predicted K values are positive, while measured K values are negative for some test cases. Predicted k values are close to measurements when ω = 5 krpm and are larger than measurements when 7.5 ≤ ω ≤ 15 krpm.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Tribol. 2019;141(4):044501-044501-10. doi:10.1115/1.4042390.

According to that the fuel pump and injectors of the diesel engines are lubricated by the fuel itself, so the lubrication property of the fuels is an important issue in internal combustion engines. Biodiesel is one of the most famous biofuels that can be used in diesel engines. In this research, wear characteristics of biodiesel derived from sunflower and soybean oil blends were investigated. The five fuel blends were tested under steady-state conditions (with durations of 1500 and 3600 s) at four different rotational speeds of 600, 900, 1200, and 1500 rpm. An optical microscope was also applied to check out the worn surfaces of the balls. The results indicated that wear and friction as tribological properties were reduced with the increase in the rotating speed under the steady-state condition. It was found that with an increase in the biodiesel concentration, the friction coefficient was reduced at lower rotating speeds due to free fatty acids, monoglycerides, and diglycerides as the components of biodiesel which help improve the lubrication properties of biodiesel and reduce the friction more than that of other blends. However, in higher rotational speeds, friction and wear of fuel blends included biodiesel increased due to reduced viscosity as the causes of oxidation which helps in the exposure of biodiesel to air at higher temperature. So, B100 has better lubricity properties compared to other fuel blends at lower rotational speeds, and better performance belongs to B20 at higher rotational speeds.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):044502-044502-7. doi:10.1115/1.4042271.

In the present study, the tribological behavior of polyaryletherketones (PAEKs) and their composites was investigated in air and vacuum environment. Polymer matrices were filled with either glass or carbon fibers and compared with a standard bearing material containing 10% carbon fiber (CF), 10% graphite, and 10% polytetrafluoroethylene (PTFE). The samples were tested in a pin-on-disk configuration under continuous sliding against a rotating steel disk (AISI 52100) at different sliding speeds. The results indicated that the tribological performance of these materials in vacuum depends on both compositions and test conditions. At low sliding speed, a very low friction and wear coefficients were obtained while at higher speed, severe wear occurred. In particular, CF filled composites showed excessive wear that led to the ignition after opening the vacuum chamber. Experimental results are discussed by analyzing the transfer film and wear debris.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):044503-044503-6. doi:10.1115/1.4042504.

This work demonstrates that granular flows (i.e., macroscale, noncohesive spheres) entrained into an eccentrically converging gap can indeed actually exhibit lubrication behavior as prior models postulated. The physics of hydrodynamic lubrication is quite well understood and liquid lubricants perform well for conventional applications. Unfortunately, in certain cases such as high-speed and high-temperature environments, liquid lubricants break down making it impossible to establish a stable liquid film. Therefore, it has been previously proposed that granular media in sliding convergent interfaces can generate load carrying capacity, and thus, granular flow lubrication. It is a possible alternative lubrication mechanism that researchers have been exploring for extreme environments, or wheel-regolith traction, or for elucidating the spreadability of additive manufacturing materials. While the load carrying capacity of granular flows has been previously demonstrated, this work attempts to more directly uncover the hydrodynamic-like granular flow behavior in an experimental journal bearing configuration. An enlarged granular lubricated journal bearing (GLJB) setup has been developed and demonstrated. The setup was made transparent in order to visualize and video capture the granular collision activity at high resolution. In addition, a computational image processing program has been developed to process the resulting images and to noninvasively track the “lift” generated by granular flow during the journal bearing operation. The results of the lift caused by granular flow as a function of journal rotation rate are presented as well.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(4):044504-044504-5. doi:10.1115/1.4042505.

A deterministic adhesive model for the contact between an elastic layered medium with surface roughness and a smooth elastic microsphere was developed on the basis of the Lennard–Jones surface force law. Through numerical simulations, the adhesive contact behavior of the layered medium with the measured three-dimensional (3D) surface topography was comparatively analyzed with that of the homogeneous medium. Furthermore, the contact characteristics of the layered medium with pre-assigned roughness parameters were investigated with the aid of a computer-generated technique for simulating surface roughness. Results showed that the pull-off force for the contact problem involving rough surfaces was influenced by the contact location, and the average value for the contact between an alumina (SiO2) microsphere and a diamond-like carbon/silicon (DLC/Si)-layered medium was smaller than that for the contact between a SiO2 microsphere and a Si homogeneous half-space. In addition, the effect of the diamond-like carbon (DLC) layer on reducing adhesion was smaller than that of the surface roughness. Finally, the average pull-off force for a DLC/Si-layered medium with computer-generated surface roughness rapidly decreased; however, it eventually became almost unchangeable with the increase in the root-mean-square (RMS) deviation.

Commentary by Dr. Valentin Fuster