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### Research Papers: Applications

J. Tribol. 2017;139(6):061101-061101-8. doi:10.1115/1.4036320.

Movement analyses of bearings focusing on cage motion behavior are often conducted by simulations, typically by investigating the cage whirl. Some experimental studies exist in which a metal cage or a modified one is analyzed with sensors. This paper presents an image-based approach for investigating the cage motion of an injection molded, window-type cage under operation condition. Besides analyses at constant rotational speed, the cage center movement behavior for different accelerations is investigated.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061102-061102-10. doi:10.1115/1.4036378.

In this paper, a model was developed to study the effects of rotor and support flexibilities on the performance of rotor–bearing–housing system. The system is composed of a flexible rotor and two supporting deep-groove ball bearings mounted in flexible bearing housings. The dynamics of the ball bearings were simulated using an existing dynamic bearing model, which was developed using the discrete element method (DEM). The explicit finite element method (EFEM) was used to model the flexibilities of the rotor and bearing support. In order to combine the dynamic bearing model with finite element rotor and support system, new contact algorithms were developed for the interactions between the various components in the system. The total Lagrangian formulation approach was applied to decrease the computational effort needed for modeling the rotor–bearing–housing system. The combined model was then used to investigate the effects of bearing clearances and housing clearances. And it was found that, as the rotor is deformed due to external loading, the clearances have a significant impact on the bearing varying compliance motion and reaction moments. Results also show that deformation of the flexible housing depends on the total force and moment generated within the bearing due to rotor deformation. The first critical speed of rotor was simulated to investigate the unbalance response of the rotor–bearing system. It was demonstrated that rotor critical speed has a significant effect on inner race displacement and reaction moment generated at bearing location.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061103-061103-10. doi:10.1115/1.4036630.

This paper presents mathematical expressions to identify the existence of localized surface defects on the raceways of the deep groove ball bearings. For the formulation of the mathematical expressions, matrix method of dimensional analysis based on force, length, time, and temperature (FLTϴ) system of unis is used. The model is based on the complete set of physical dimensions and operating parameters of the deep groove ball bearing in that the spall size is directly allied with vibration responses. The formulated governing model equations are solved numerically by applying a scheme of empirical modeling through multiple factorial regression analysis. Experiments are performed on the laboratory test rig to verify the results obtained from the developed model equations. For the experiments, deep groove ball bearings designated as SKF 6307 are used. These bearings are having artificially induced square-shaped surface defects of different sizes on the outer and inner races and are analyzed for different operating speeds. A good similarity between the predicted numerical values and the experimental results is noticed. This study showed that the proposed methodology can be successfully used for the characterization of the localized surface defects on the raceways of the deep groove ball bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061104-061104-7. doi:10.1115/1.4036447.

In order to use electric motors which run up to 40,000 rpm in future automotive applications, this study aims to experimentally investigate if splash lubrication technique is worth considering for high-speed gears, i.e., for tangential gear speed up to 60 m/s. To this end, a specific test rig has been used to operate a single spur or helical gear in various operating conditions (lubricant, oil sump volume, temperature, etc.). Churning loss is measured and, as the fraction of air in the lubricant (namely, oil aeration) can be influential on this source of dissipation, a specific sensor is also employed for online monitoring of oil sump aeration. By inserting some moveable walls in the gearbox, it is demonstrated that churning losses and oil aeration can be significantly reduced by mounting these flanges at an appropriate distance to the gear lateral faces. Based on dimensional analysis, an engineering criterion is proposed to properly choose the clearance between the tested gear and the flanges.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061105-061105-9. doi:10.1115/1.4036493.

The wear behavior of grease lubricated gears is significantly affected by a number of factors besides the composition of the grease that is used for lubrication. A main influence factor on the wear behavior of grease lubricated gears is the lubrication supply mechanism (“circulating” or “channeling”), which is, among other things, strongly affected by the rotational speed of the gears. Especially at higher rotational speed, the dominant grease lubrication supply mechanism for grease lubricated gears tends to be channeling, which generally promotes increased wear on account of limited lubricant availability in the mesh as well as limited heat dissipation from the mesh. Experimental investigations conducted herein have shown that the gear wear behavior, especially at higher rotational speed, can be influenced by the internal geometry of the gear casing as well as by the direction of rotation of the gears. Additional investigations that were conducted focus on the effect of the material pairing and heat treatment on the wear behavior of grease lubricated gears in comparison to a case-carburized reference gear set. Furthermore, the effect of the gear size on the lubrication supply mechanism and thus on the wear behavior was investigated. For both gear sizes investigated (mn = 1.0 mm and mn = 0.6 mm), circulating was observed at low rotational speeds. At high rotational speeds, channeling effects were dominant.

Topics: Wear , Gears
Commentary by Dr. Valentin Fuster

### Research Papers: Coatings and Solid Lubricants

J. Tribol. 2017;139(6):061301-061301-9. doi:10.1115/1.4036175.

W, Mo, and W–Mo alloy films are prepared on 316L stainless steel by means of ion beam assisted deposition (IBAD). Tribological behaviors of the treated surfaces are investigated under lubrication with molybdenum dialkyldithiocarbamate (MoDTC) on a MS-T3000 friction and wear tester. The micromorphologies and chemical characteristics of the tribofilms generated on the worn surfaces are analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), respectively. The results showed that the obvious synergistic effects of better friction reduction and wear resistance between W–Mo film and MoDTC are attributed to the hard surface and the high decomposition rate of additives on the worn W–Mo surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061302-061302-9. doi:10.1115/1.4036450.

Uniform nanoparticles of cobalt oxide precursors were prepared by the chemical precipitation in which the headspace vapors of ammonium hydroxide solution of known concentration were allowed to bubble through the aqueous solutions of cobalt sulfate, containing appropriate amount of the nonionic surfactant, octylphenoxy poly ethoxy ethanol. Scanning electron microscope (SEM) images showed that uniformity in particle size was dependent upon the applied precipitation conditions. Extensive optimization was therefore performed for the attainment of uniformity in particle size and shape. The amorphous precursor was transformed into crystalline Co3O4 as confirmed by X-ray diffractometry. These particles, with isoelectric point (IEP) at pH ∼ 8.4, were then employed as reinforcement additive for strengthening the electrodeposited nickel matrix. Effect of various parameters, i.e., stirring rate, applied current density, and temperature, was studied on the amount of the codeposited Co3O4 particles in the nanocomposite coatings (Ni–Co3O4) during the electrodeposition process. pH of the coating mixtures was kept below IEP value of Co3O4 so that the latter particles carried net positive surface charge. The coated surfaces were subjected to various tests, i.e., microhardness, wear/friction, and corrosion. Results revealed that irrespective of the amount of the embedded Co3O4 particles, nanocomposite coatings demonstrated superior performance as compared to pure nickel coatings.

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2017;139(6):061401-061401-10. doi:10.1115/1.4036181.

In this work, the influence of different crack arrangements in the stress distribution of hard chromium (HC) coatings was determined. Three parameters for position and length of the cracks for two different types of coatings were probabilistically modeled based on measured scanning electron microscopy (SEM) images. Probability density functions (PDF) for those parameters were obtained to characterize each kind of coating. A two-dimensional finite element (FE) model of the coating in contact with a rigid disk was developed, modeling cracks with elliptical shapes. A Monte Carlo method was used to simulate different crack distributions for each kind of coating, and values of stress and strains in the domain were obtained. Both the J-integral and the stress intensity factors (SIFs) were taken as comparative parameters of the results. Coatings which statistically present larger quantities of shorter cracks have lower values of J-integral and SIFs, and, therefore, distribute stresses better than those with low density of longer cracks.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061402-061402-7. doi:10.1115/1.4036183.

Interfacial damping in assembled structures is difficult to predict and control since it depends on numerous system parameters such as elastic mismatch, roughness, contact geometry, and loading profiles. Most recently, phase difference between normal and tangential force oscillations has been shown to have a significant effect on interfacial damping. In this study, we conduct microscale (asperity-scale) experiments to investigate the influence of magnitude and phase difference of normal and tangential force oscillations on the energy dissipation in presliding spherical contacts. Our results show that energy dissipation increases with increasing normal preload fluctuations and phase difference. This increase is more prominent for higher tangential force fluctuations, thanks to larger frictional slip along the contact interface. We also show that the energy dissipation and tangential fluctuations are related through a power law. The power exponents we identify from the experiments reveal that contacts deliver a nonlinear damping for all normal preload fluctuation amplitudes and phase differences investigated. This is in line with the damping uncertainties and nonlinearities observed in structural dynamics community.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2017;139(6):061501-061501-7. doi:10.1115/1.4036171.

The first calculations of film thickness for an oil/refrigerant system using quantitative elastohydrodynamics are reported in this work. It is demonstrated that primary measurements of the properties of the oil/refrigerant system can be employed to accurately predict film thickness in concentrated contacts. An unusual response to lubricant inlet temperature is revealed, wherein the film thickness may increase with temperature as a result of decreasing refrigerant solubility in oil when the inlet pressure is high. There is competition between the reduction in viscosity of the oil and the reduction of refrigerant concentration with increased temperature. For high inlet pressure, the dilution effect is dominant, whereas for low inlet pressure, the temperature dependence of the viscosity of the solution dominates over the range of inlet temperatures considered. It seems that only central film thicknesses have been experimentally measured for oil/refrigerant systems leaving these calculations as the only means of assessing the minimum.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061502-061502-11. doi:10.1115/1.4036274.

In this paper, the formulas of elastohydrodynamic traction coefficients of four Chinese aviation lubricating oils, namely, 4109, 4106, 4050, and 4010, were obtained by a great number of elastohydrodynamic traction tests. The nonlinear dynamics differential equations of high-speed cylindrical roller bearing were built on the basis of dynamic theory of rolling bearings and solved by Hilber–Hughes–Taylor (HHT) integer algorithm with variable step. The influence of lubricant traction coefficient on cage's nonlinear dynamic behavior was investigated, and Poincaré map was used to analyze the influence of four types of aviation lubricating oils on the nonlinear dynamic response of cage's mass center. The period of nonlinear dynamic response of cage's mass center was used to assess cage's stability. The results of this paper provide the theoretical basis for selection of aviation lubricating oil.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2017;139(6):061601-061601-9. doi:10.1115/1.4035779.

The influence of load applied on wear depth of stir cast hybrid Gr/SiC/Al 6082 composites in a two-body abrasion was investigated in as cast (AC) and T6 heat-treated condition (T6). The obtained results were compared with its unreinforced alloy and SiC/Al 6082 composites. The parameters of the applied load (5–15 N), grit size (100 μm and 200 μm), and sliding distance of 75 m were used in this study. At 200-μm grit size, the wear depth of hybrid composites with respect to unreinforced matrix alloy was reduced by 38.1% (at 5 N load) and 16.2% (at 15 N load) in AC condition; 25.1% (at 5 N load), and 27% (at 15 N load) in T6 condition. The wear mechanisms were demonstrated through the analysis of wear surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061602-061602-8. doi:10.1115/1.4035844.

In the present study, wear resistance composite cladding of Ni-based + 20% WC8Co (wt. %) was developed on SS-304 substrate using domestic microwave oven at 2.45 GHz and 900 W. The clad was developed within 300 s of microwave exposure using microwave hybrid heating (MHH) technique. The clad was characterized through scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Vicker’s microhardness, and dry sliding wear test. Microstructure study revealed that the clad of approximately 1.25 mm thickness was developed by partial mutual diffusion with substrate. It was observed that the developed clad was free from visible interfacial cracks with significantly less porosity (∼1.2%). XRD patterns of the clad confirmed the presence of Cr23C6, NiSi, and NiCr phases that eventually contributed to the enhancement in clad microhardness. Vicker’s microhardness of the processed clad surface was found to be 840 $±$ 20 HV, which was four times that of SS-304 substrate. In case of clad surface, wear mainly occurs due to debonding of carbide particles from the matrix, while plastic deformation and strong abrasion are responsible for the removal of material from SS-304 substrate.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061603-061603-9. doi:10.1115/1.4036185.

The modern technology developments have seeded for the necessity of composite materials that are incorporated with high hardness, high tensile strength, and better wear properties. Cu–Sn–Ni alloy as well as the composites of varying weight percentage of Si3N4 (5, 10, and 15) are fabricated by liquid metallurgy technique. The alloy and composites are tested for their tensile strength and hardness on Universal Testing Machine and Vickers microhardness tester, respectively. Based on the tests, Cu–Sn–Ni/10 wt. % of Si3N4 is found to have optimum mechanical properties. The scuff type adhesive wear behavior is studied through pin-on-disk tribometer under dry sliding conditions for Cu–Sn–Ni/10 wt. % of Si3N4 composite. Taguchi's design of experiments technique based on L27 orthogonal array model is used for analyses of process parameters in three levels such as applied load (10, 20, and 30 N), sliding distance (500, 1000, and 1500 m), and sliding velocity (1, 2, and 3 m/s). The parameters are ranked based on the signal-to-noise ratio and the analysis of variance approach. Based on wear results, applied load is found to have highest stature on influencing wear rate followed by sliding distance and sliding velocity. A generalized wear rate equation is obtained based on the linear regression model and its feasibility is checked. Scanning electron microscope (SEM) analyses revealed severe delamination occurred on maximum load condition. The development of this copper composite can have the possibility of replacing aluminum bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061604-061604-12. doi:10.1115/1.4036184.

Inverse problem of manufacturing is studied under a framework of high performance manufacturing of components with functional surface layer, where controllable generation of surface integrity is emphasized due to its pivotal role determining final performance. Surface modification techniques capable of controlling surface integrity are utilized to verify such a framework of manufacturing, by which the surface integrity desired for a high performance can be more effectively achieved as reducing the material and geometry constraints of manufacturing otherwise unobtainable during conventional machining processes. Here, thermal spraying of WC–Ni coatings is employed to coat stainless steel components for water-lubricated wear applications, on which a strategy for direct problem from process to performance is implemented with surface integrity adjustable through spray angle and inert N2 shielding. Subsequently, multiple surface integrity parameters can be evaluated to identify the major ones responsible for wear performance by elucidating the wear mechanism, involving surface features (coating porosity and WC phase retention) and surface characteristics (microhardness, elastic modulus, and toughness). The surface features predominantly determine tribological behaviors of coatings in combination with the surface characteristics that are intrinsically associated with the surface features. Consequently, the spray process with improved N2 shielding is designed according to the desired surface integrity parameters for higher wear resistance. It is demonstrated that the correlations from processes to performance could be fully understood and established via controllable surface integrity, facilitating solution to inverse problem of manufacturing, i.e., realization of a material and geometry integrated manufacturing.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061605-061605-20. doi:10.1115/1.4036273.

Friction and wear studies of Nimonic 80A and 21-4N valve materials against GGG-40 under dry sliding conditions, at temperatures ranging from 50 °C to 500 °C, are presented in this paper. Friction coefficient was found to be continuously decreased with time for all tests with prominent running-in behavior seen in the 50 °C and 500 °C tests. Higher friction coefficient and wear were observed at 300 °C as compared to those at 50 °C and 500 °C. Formation of oxide Fe3O4, at 300 °C, was confirmed by Raman spectroscopy, which resulted in a higher friction coefficient and wear. Raman spectroscopy further revealed the presence of α-Fe2O3, hematite, in most cases, with the presence of oxides of Ni–Cr and Ni–Fe as well. Energy dispersive spectroscopy (EDS) results on the samples confirmed the same. Wear at 500 °C was found to be the least for both the valve materials with scanning electron microscopy (SEM) confirming the formation of well-developed glaze layers.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061606-061606-5. doi:10.1115/1.4036448.

Circular translation pin-on-disk (CTPOD) tests were performed for ultrahigh molecular weight polyethylene (UHMWPE) with a view to reproducing wear mechanisms that prevail in total hip prostheses. The contact surface diameter varied from 3.0 mm to 30 mm, while the slide track diameter was fixed, 10 mm. The counterface was polished CoCr, and the lubricant was diluted alpha calf serum. Either the nominal contact pressure (1.1 MPa) or the load (126 N) was kept constant. With a constant contact pressure, the wear factor decreased steeply when the contact diameter exceeded the slide track diameter, apparently because the wear debris was not readily conveyed away from the contact. With constant load, both the wear factor and the coefficient of friction increased linearly with increasing contact area. This trend was in agreement with clinical observations that the wear rate of UHMWPE acetabular cups increased with increasing femoral head size. With nominal contact pressures approaching 10 MPa however, the bearing surface topography markedly differed from clinical observations. This was probably due to overheating of the contact and plastic deformation that resulted in the formation of protuberances not seen clinically. The present study emphasized the importance of test parameters in the pin-on-disk wear screening of prosthetic joint materials. It appeared that the contact surface diameter of the flat-on-flat test should be below the slide track diameter, and that the nominal contact pressure should be of the order of 1 MPa.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061607-061607-7. doi:10.1115/1.4036449.

In this research, rheological and tribological performance of additive V2O5 nanoparticles in a light mineral oil has been investigated. For rheological performance, the addition of 0.2 wt. % V2O5 could reduce the viscosity of the base oil for 6%. Considering the overall friction reduction in boundary, mixed, and hydrodynamic lubrication regimes, that with 0.1 wt. % V2O5 exhibited the best effect. Friction coefficient of base oil could be reduced by 33%. In terms of wear, the addition of 0.2 wt. % V2O5 showed the lowest wear rate, which is 44% reduction compared to base oil. Through Raman spectrum and energy dispersive spectroscopy (EDS) analysis, it was found that V2O5 involved tribochemical reaction during rubbing. Vanadium intermetallic alloy (V–Fe–Cr) was found to enhance the antiwear performance. This research revealed that V2O5 nanoparticles could be an effective additive to improve tribological performance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061608-061608-8. doi:10.1115/1.4036321.

The potential of applying thermodynamics to study the tribological response of a tribological system is addressed in this paper. In order to do so, a model was developed to obtain the entropy flow generated by three different dissipative processes present in dry sliding, namely, thermal gradient, heat conduction, and abrasion. The flash and bulk temperatures at the contact interface were obtained with the aid of the finite element method (FEM), and pin-on-disk tests were performed by using titanium alloy (Ti6Al4V) disks and tungsten carbide (WC/10Co) pins. Then, the wear rate obtained from the tribological tests was correlated with the calculated entropy flow, and a degradation coefficient was associated to the sliding process. A linear dependence of the wear rate and the degradation coefficient was observed regardless of the variation of the points of operation of the system, so it is proposed that the coefficient of degradation used is inherent to the tribological system.

Commentary by Dr. Valentin Fuster

### Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;139(6):061701-061701-11. doi:10.1115/1.4036629.

This paper mainly reports stability investigations of rotors supported on fluid film journal bearings possessing multilocational slip-no-slip zones at the bush–film interface. The coupled solution of governing equations (Reynolds equation, energy equation, heat diffusion equation, lubricant rheological relation, and thermal boundary conditions) has been used to find pressure distributions in the lubricating film followed by evaluation of bearing coefficients. These coefficients have been used to determine stability limit speed (SLS) of the system and its robustness for both short (nearly inflexible) and long (flexible) rotors. Numerical simulations show that the pattern of pressure distribution with multiple slip-no-slip zones is similar to that obtained for multilobe bearings, resulting in substantial improvement of rotor–bearing stability irrespective of eccentricity ratio. A reduction in friction force (up to Sommerfeld number 1.8) and an increase in SLS and robustness compared to conventional bearings are observed when used with short rotors. Typically, up to six pairs of slip-no-slip zones improve SLS of the rotor–shaft system and robustness for short rotors, although more pairs deteriorate both. However, for long rotors, where dynamic rotor forces also act, these bearings provide marginal improvement in stability and robustness only for a small range of slip length.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061702-061702-10. doi:10.1115/1.4036631.

A detailed analysis of the effective thermal resistance for the bump foil of air foil bearings (AFBs) is performed. The presented model puts emphasis on the thermal contact resistances between the bump foil and the top foil as well as between the bump foil and the base plate. It is demonstrated that most of the dissipated heat in the lubricating air film of an air foil bearing is not conducted by microcontacts in the contact regions. Instead, the air gaps close to the contact area are found to be thin enough in order to effectively conduct the heat from the top foil into the bump foil. On the basis of these findings, an analytical formula is developed for the effective thermal resistance of a half bump arc. The formula accounts for the geometry of the bump foil as well as for the surface roughness of the top foil, the bump foil, and the base plate. The predictions of the presented model are shown to be in good agreement with measurements from the literature. In particular, the model predicts the effective thermal resistance to be almost independent of the applied pressure. This is a major characteristic property that has been found by measurements but could not be reproduced by previously published models. The presented formula contributes to an accurate thermohydrodynamic (THD) modeling of AFBs.

Commentary by Dr. Valentin Fuster

### Research Papers: Lubricants

J. Tribol. 2017;139(6):061801-061801-7. doi:10.1115/1.4036271.

Polyaniline (PANI) was doped as lubricating additive to afford grease. The effect of PANI on the physicochemical characteristics, corrosion resistance, and tribological performances of lubricating grease was investigated in details, and the tribological action mechanisms of lubricating grease were analyzed in relation to worn surface analyses by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscope (EDS). Results indicate that the PANI-doped grease has superior conductive and thermal properties. And PANI-doped grease has an excellent corrosion resistance, which is attributed to the isolation effect and the compact passivated film generated by reaction of PANI and metal. In the meantime, the PANI-doped grease performs superior friction reduction and wear resistance under different applied loads and frequencies. It is mainly ascribed that the PANI can perform like spacers to avoid direct contact between the contact interfaces, and the protective tribofilm is generated by physical adsorption and chemical reaction.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):061802-061802-13. doi:10.1115/1.4036379.

The present work investigates the tribological properties of castor oil with various carbonaceous friction modifiers (nano and microsize additives) assessed using four-ball tester as per ASTM D 4172 and ASTM D 2783. Castor oil has been chosen because of its high viscosity and ease of availability. Graphite, multiwalled carbon nanotube (MWCNT), and multilayered graphene are used as friction modifiers (FMs) in castor oil on weight percentage basis. Significant enhancements of tribological properties with a certain level of concentration of friction modifiers have been observed. The surface features of the tested balls were analyzed using a three-dimensional noncontact type profilometer, scanning electron microscope (SEM), and energy dispersive system (EDS). Decrease in surface roughness indicated better antiwear properties in case of nanofriction modifiers-based castor oil as compared to micrographite-based and neat castor oil (NCO). In order to assess the suitability of castor oil as a replacement for mineral oil, the results of castor oil samples are also compared with commercially available mineral oil. The tribological properties of castor oil are found to be competitive and generally superior to the mineral gear oil. The data generated are used to develop a neural network model to map the input–output correlation.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro-Nano Tribology

J. Tribol. 2017;139(6):062001-062001-13. doi:10.1115/1.4036174.

The vibration characteristics of a thermal fly-height control (TFC) head slider in the proximity and asperity contact regimes attract much attention, because the head–disk spacing (HDS) must be less than 1 nm in order to increase the recording density in hard disk drives. This paper presents a numerical analysis of the microwaviness (MW)-excited vibrations in the flying head slider during the touchdown (TD) process. We first formulate the total force applied to the TFC head slider as a function of the HDS, based on rough-surface adhesion contact models and an air-bearing force model. Then, the MW-excited vibrations of a single-degree-of-freedom (DOF) slider model at TD are simulated by the Runge–Kutta method. It is found that, when the MW amplitude is less than the spacing range of static instability in the total force, the slider jumps to a contact state from a near-contact or mobile-lubricant-contact state. It then jumps to a flying state even when the head surface is protruded further by increasing the TFC power. When the MW amplitude is relatively large, a drastically large spacing variation that contains a wide range of frequency components below 100 kHz appears in the static unstable region. These calculated results can clarify the mechanisms behind a few peculiar experimental phenomena reported in the past.

Commentary by Dr. Valentin Fuster

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

J. Tribol. 2017;139(6):062201-062201-9. doi:10.1115/1.4036380.

Mechanical face seals are constitutive components of much larger turbomachines and require consideration of the system dynamics for successful design. The dynamic interplay between the seal and rotor is intensified by recent trends toward reduced clearances, higher speeds, and more flexible rotors. Here, the “rotor” consists of the flexible shaft and the rotating seal seat. The objective here is to, for the first time, determine how the rotor affects the seal performance and vice versa. Thresholds can then be established beyond which the rotor influences the seal but not vice versa (i.e., the rotordynamics can be sent to the seal analysis as an exogenous input). To this end, a model of a flexibly mounted stator face seal is provided including the coupled dynamics of the flexible rotor. The model accounts for axial and angular deflections of the rotor and seal. Coupled rotordynamics are modeled using a lumped-parameter approach including static and dynamic rotor angular misalignments. For expediency, linearized expressions for fluid forces are used, and the resulting steady-state equations of motion are solved analytically to investigate how rotor inertia, speed, and angular misalignment influence the coupled seal dynamics. Importantly, results from the study reveal that in some operating regimes, neglecting the rotordynamics implies healthy seal operation when instead intermittent rub exists between the faces. This work also shows that when the rotor inertia is much larger than the seal inertia, the rotordynamics can be solved separately and used in the seal model as an external input.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(6):062202-062202-11. doi:10.1115/1.4036272.

The segmented carbon seal is regularly used for sealing bearing chambers of aeronautical turboengines or as part of a buffer seal in space turbopumps. The seal operates with contaminated air or with an inert gas and is made of many identical carbon segments (generally three or six) with reciprocally overlapping ends. The segments are pressed against the rotor by the pressure difference between the upstream and the downstream chambers and by a circumferential (garter) spring. The pressure difference and an axial spring press the segments also against the stator. The inner cylindrical surface of each segment is provided with pads that create an aerodynamic lift proportional to the rotor speed. Following this lift force, the segments of the seal are pushed away from the rotor and the seal opens. The contact between the rotor and the segments is lost, and an axial leakage path is thus created. Although it was developed since long, a model for calculating the characteristics of the segmented seal is completely absent from the scientific literature. The goal of the present work is to fill this gap at least for the static characteristics (leakage and torque). The analysis is carried out for a single segment of the seal by supposing that all the segments have the same characteristics. Each segment has a planar motion (i.e., three degrees-of-freedom (3DOF)), and therefore the film thickness under each pad is not uniform. Given the stationary operating conditions (pressure difference and rotation speed), the present model calculates the equilibrium position of each segment on the bases of the lift and the friction force acting on the pads, the friction forces acting on the nose of the seal, and the radial and axial springs. Once found the static equilibrium position, the leakage and the torque of the seal are calculated. A parametric study enlightens the importance of the pad waviness, the pocket depth, and the spring forces on the characteristics of the segmented seal.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Tribol. 2017;139(6):064501-064501-5. doi:10.1115/1.4035869.

This technical brief studied the cavitation erosion behavior of the silicified graphite. The phase constituents, surface microstructure, and chemical compositions of silicified graphite were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), respectively. Cavitation experiments were carried out by using an ultrasonic vibration test system. The experimental results show that the silicified graphite exhibits an excellent cavitation erosion resistance; this can be attributed to the fact that the silicified graphite has the characteristics of both the silicon carbide and the graphite. The SEM morphology studies of the erosion surfaces indicated that the inherent brittleness of SiC ceramic material results in the formation of erosion pits on the surface of silicified graphite.

Commentary by Dr. Valentin Fuster

### Technical Brief: Technical Brief

J. Tribol. 2017;139(6):064502-064502-5. doi:10.1115/1.4035780.

Electrical analogy has been used extensively in modeling various mechanical systems such as thermal, hydraulic, and other dynamic systems. However, wear modeling of a tribosystem using electrical analogy has not been reported so far. In this paper, an equivalent electrical analogous system is proposed to represent the wear process. An analogous circuit is developed by mapping the wear process parameters to that of the electrical parameters. The circuit, thus, developed is solved by conventional electrical circuit theory. The material properties and operating conditions are taken into account by model parameters. Accordingly, a model equation in terms of model parameters is developed to represent the wear rate. It is also demonstrated how this methodology can be used to take various system parameters into account by incorporating the equivalent resistance of the parameters. The nonlinear model parameters are evaluated by Gauss–Newton (GN) algorithm. The proposed model is validated by using experimental data. A comparison of the proposed model with the experimental results, based on statistical methods: coefficient of determination (R2), mean-square-error (MSE) and mean absolute percentage error (MAPE), indicates that the model is competent to predict the wear with a high degree of accuracy.

Commentary by Dr. Valentin Fuster