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Research Papers: Applications

J. Tribol. 2018;141(2):021101-021101-11. doi:10.1115/1.4041365.

In this paper, a calculation method based on matlab partial differential equations (PDE) tool is proposed to investigate the static characteristics of aerostatic spherical bearings. The Reynolds equation of aerostatic spherical bearings is transformed into a standard elliptic equation. The effects of geometric parameters and operational conditions on the film pressure, bearing film force, and stiffness are studied. The axial and radial eccentricities result in different film pressure distributions; the bearing film force and stiffness are significantly influenced by geometric parameters and operational conditions. The relative optimal parameters are confirmed based on the calculation results. A comparison between the numerical and experimental results is also presented. The highest relative error between the numerical results and the experimental data is 11.3%; the calculation results show good agreements with the experimental data, thus verifying the accuracy of the calculation method used in this paper.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2018;141(2):021301-021301-8. doi:10.1115/1.4041215.

This work investigates the effect of convexity position of ring barrel surface on the wear properties and scuffing resistance of the Cr–Al2O3 coated piston rings against with the CuNiCr cast iron cylinder liner. The scuffed surface morphology and elements distribution as well as the oil film edge were analyzed to explore the influencing mechanism of the convexity position on the scuffing resistance. The results show that the convexity offset rate on the barrel surface of the ring has no noticeable influence on both friction coefficient and wear loss near the dead points, but a suitable convexity position will result in the improved scuffing resistance. The shape of the barrel face not only affects the worn area on the ring, but also determines the oil film wedge and pressure distribution, consequently influences the scuffing resistance.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;141(2):021601-021601-12. doi:10.1115/1.4041246.

A corrosive wear model is considered at the asperity-scale of a tribocorrosive wear system as well as the traditional Archard-type mechanical wear model. The geometry of the surface asperities is modified in a contact mechanics model with respect to both corrosive and mechanical wear calculations. This model was presented and validated for prediction of the electrochemistry in the first part of this work. The material used in the experimental part of this work was CoCrMo plate working electrode (WE) and Si3N4 ball as the counter body in a reciprocating configuration. Experiments were conducted at loads of 5, 7.5, and 10 N and the contributions of total mechanical wear and corrosion were measured. The model is then tuned to predict the chemical and mechanical components of the total wear of the system. The synergistic effect of corrosion on mechanical wear and mechanical wear on corrosion are modeled numerically in this work. The values are then used to explain different components of mechanistic tribocorrosive wear models present in the literature. This deterministic model, for the first time, calculates the corrosion-enhanced wear in a tribocorrosive wear environment and proposes that changes in the topography are responsible for this synergistic effect. The results show a linear dependence of the corrosion enhanced wear, wear-enhanced corrosion, and the pure mechanical wear on the applied load. Results also suggest that the wear enhanced corrosion has a significant contribution in the overall degradation of the material.

Topics: Wear , Corrosion , Stress
Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021602-021602-12. doi:10.1115/1.4041019.

Acrylonitrile butadiene styrene (ABS) polymer is cost-effective and also possesses high toughness and resistance to corrosive chemicals. However, pure ABS does not show significant wear resistance and also it has a high friction coefficient. Incorporation of a solid lubricant and nanofiller in a polymer matrix improves its tribological properties significantly. The addition of solid lubricant makes it suitable for application where self-lubrication is desirable (sliding bearings, gears). This paper deals with the study of tribological behavior of ABS hybrid composites reinforced with nano zirconia and polytetrafluoroethylene (PTFE). ABS hybrid composites with varying proportions of nano zirconia and PTFE were prepared using melt blending. Dispersion of reinforcement in the polymer matrix has been studied with the help of transmission electron micrographs. Influence of reinforcements on the mechanical behavior is studied by tensile testing according to the ASTM standard. The tribological behavior of composites was determined in a pin-on-disk tribometer according to the ASTM G99 standard. Worn surfaces were analyzed using scanning electron microscope (SEM) in order to identify the different types of wear and various wear mechanisms. Transfer film formation was studied by analyzing the counterbody surface. A wear mechanism map has been developed, which helps in identifying various wear mechanisms involved under given loading conditions. The results reveal that the addition of PTFE reduces the wear rate and coefficient of friction (COF) significantly. Nano zirconia effectively transfers the load, thereby improving wear resistance, and the addition of PTFE results in continuous transfer film formation thereby reducing the COF. Also from the wear map, it has been identified that abrasion, adhesion, plowing, plastic deformation, melting, and delamination are the dominant wear mechanisms involved.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021603-021603-7. doi:10.1115/1.4041126.

This study aims to investigate the effect of volume fraction of commercially pure titanium (CP-Ti) on microstructural, mechanical, and tribological features of A356 aluminum matrix composites. Vacuum-assisted melt infiltration casting was performed to produce composites with 50%, 65%, 75%, and 80% CP-Ti contents. CP-Ti sawdusts were assembled under mechanical pressure in order to attain porous one-piece CP-Ti preforms which were infiltrated by A356 melt at 730 °C under 10−5 Pa vacuum atmosphere. TiAl3 layer was formed at the interface between A356 and CP-Ti phases. Owing to increased diffusion time through decreased diffusion path length, both thickness and hardness of TiAl3 phase were increased with increasing CP-Ti ratio, whereas the best wear resistance was obtained at 65% CP-Ti ratio. The main reason for decrease in wear resistance of 75% and 80% CP-Ti reinforced composites was fragmentation of TiAl3 layer during wear process due to its excessively increased brittleness. Strongly bonded TiAl3 phase at the interface provided better wear resistance, while weakly bonded ones caused to multiply wear rate.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021604-021604-14. doi:10.1115/1.4041072.

Inconel 100 (IN100) aerospace superalloy is used in manufacturing aero-engine components that operate at intermediate temperatures. It is considered to be a hard-to-cut material. Chipping of the tool edge is one of the major failure mechanisms of ceramic tools in finish cutting of superalloys, which causes a sudden breakage of the cutting edge during machining. Cutting temperature significantly depends on cutting speed. Varying the cutting speed will affect the frictional action during the machining operations. However, proper selection of the cutting variables, especially the cutting speed, can prevent chipping occurrence. In this work, the influence of controlling the cutting speed on the chipping formation in dry finish turning of IN100 aerospace superalloy using SiAlON ceramic tool has been investigated. Scanning electron microscope (SEM)/energy dispersing spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and three-dimensional wear measurements were used to make the investigations of the worn tool edges. It was found that variations of the cutting speeds in a certain range resulted in the generation of different lubricious and protective tribo-films. The presence of these tribo-films at the cutting region proved essential to prevent chipping of the cutting tool edge and to improve its wear resistance during finish turning of age-hardened IN 100 using SiAlON ceramic tools. Chip compression ratio and calculated values of the coefficient of friction at the tool–chip interface confirmed these results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021605-021605-7. doi:10.1115/1.4041257.

The in situ method of making zinc-aluminum composites wherein TiC has been introduced has been investigated in the present paper for its microstructural, physical, and dry sliding wear behavior and compared with the base alloy. In the present study, ZA-27 alloy reinforced with 5 and 10 vol % TiC was taken into consideration. The results indicate that the wear rate and coefficient of friction of composites were lower than that of base alloy. The material loss in terms of both wear volume loss and wear rate increases with increase in load and sliding distance, respectively, while coefficient of friction follows a reverse trend with increase in load. Better performance was obtained for 5% TiC reinforcement than with 10% probably due to agglomeration of particles resulting in nonuniform dispersion. Worn surfaces were analyzed by scanning electron microscopy (SEM) analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021606-021606-9. doi:10.1115/1.4041214.

The mechanical and tribological performances of 316 L stainless steel subjected to different cold rolling (CR) strains were investigated. The microhardness and strength of 316 L stainless steel were improved attributed to the formation of high-density defects, such as dislocations and parallel lamellar structures. Furthermore, the tribology tests were conducted under dry sliding at room temperature. With the increase in rolling strain, the wear rate of 316 L stainless steel gradually decreased due to the improvements in microhardness and strength. For the as-received specimen, the strong adhesive wear leads to the maximum wear rate compared with the cold rolled specimens. Under higher rolling strain conditions, the grain boundary embrittlement caused by oxygen reaction leads to the formation of oxidative abrasive under dry sliding conditions, and then the oxidative abrasive could serve as the third body at the siding interface. Consequently, there is a transition phase where the wear mechanism gradually shifts from adhesive to abrasive wear.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021607-021607-10. doi:10.1115/1.4041397.

Polycrystalline diamond compacts (PDCs) are the main cutting unit of drill bits and are a major factor in determining the drilling efficiency and service life of drill bits. Drill bit failure is caused by the severe abrasive wear it undergoes during the drilling process. The drill bit failure can prolong the drilling period, which can result in borehole instability and cause collapse in the material. A solution that can address this issue is developing an appropriate drilling method that can expel the dust in a manner that will not increase the abrasive wear on the drill bit. Here, an Amsler friction and wear-testing machines was used to investigate the friction and wear characteristics of PDC and to study the effects of the dust expelled during drilling on the wear performance of drill bits under dry air and wetting conditions. The microstructures of the worn surfaces were examined by a scanning electron microscope (SEM) and metalloscope. In addition, the chemical compositions of the PDCs' surfaces were analyzed using X-ray diffraction (XRD) after the wear and friction tests. The results demonstrate that the friction coefficients and wear rate obtained in dry air were higher than those under wetting conditions. As expected, these values are mainly ascribed to the absence of the absorber layer and lubrication under dry air. Furthermore, under wetting conditions a number of cracks were observed on the PDC surface after testing at 700 °C, which was mainly caused by two factors: The different thermal expansion coefficients between the diamond and Cobalt phase; and the residual stress generated inside the PDC under wetting conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;141(2):021701-021701-14. doi:10.1115/1.4041211.

The quality of predictions for the operating behavior of high-speed journal bearings strongly depends on realistic boundary conditions within the inlet region supplying a mixture of hot oil from the upstream pad and fresh lubricant from the inlet device to the downstream located pad. Therefore, an appropriate modeling of fundamental phenomena within the inlet region is essential for a reliable simulation of fluid and heat flow in the entire bearing. A theoretical model including hydraulic, mechanical, and energetic effects and the procedure of its numerical implementation in typical bearing codes for thermo-hydrodynamic lubrication is described and validated. Convective and conductive heat transfer as well as dissipation due to internal friction in the lubricant is considered for the space between pads or the pocket where the inlet is located. In contrast to most other models, the region between the physical inlet and the lubricant film is part of the solution domain and not only represented by boundary conditions. The model provides flow rate and temperature boundary conditions for extended Reynolds equation and a three-dimensional (3D) energy equation of film and inlet region, respectively. The impact of backflow from the inlet region to the outer supply channel possibly occurring in sealed pockets is taken into account. Moreover, the model considers the influence of turbulent flow in the inlet region.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021702-021702-10. doi:10.1115/1.4041363.

Using the experience acquired within our lab in terms of both thermohydrodynamic (THD) and thermoelastohydrodynamic (TEHD) numerical simulations, a new THD code has been improved by adding the possibility of taking into account geometrical defects, and particularly scratches, which are often discovered by turbine users during maintenance operations. To examine this issue, two numerical codes were coupled to provide the TEHD analysis presented in this work. To validate the numerical results, experimental tests were conducted using the Pprime Institute bearing test rig. The performance of the same two-lobe journal bearing (preload 0.524) as used in a previous study, lubricated with ISO VG 46 oil, was evaluated. Scratches of different depths (varying as a function of the radial clearance) were directly machined onto the shaft. TEHD solutions and experimental data were compared for various rotational speeds and applied loads. Pressure and temperature comparisons for the three scratch depths show good correlation, and give the expected results for cases with a scratch. It was also found that the asymmetry in the pressure field created by the presence of a scratch led to a slight misalignment. The comparisons were improved by taking into account this misalignment, using the balance of momentum.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2018;141(2):021801-021801-7. doi:10.1115/1.4041364.

Soft solids, such as rubbers, elastomers, and gels, are the important polymeric materials. A better understanding of their interfacial properties such as friction and adhesion is critical for variety of technological applications. Motivated by the experimental observation that interfacial properties can be modified even without changing the content of a soft solid, the effect of specimen thickness on the energy release rate (G) of a soft gelatin hydrogel is investigated in direct shear test. Slide-hold-slide (SHS) experiments have shown that shear strength decreases, while corresponding crack length increases, with increase in thickness of gel specimens. However, G at static, dynamic and residual strengths increase with specimen thickness. At the end, these observations are explained in light of mixed mode I/II fracture and shear rate effects at the sliding interface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021802-021802-7. doi:10.1115/1.4041124.

Understanding the pressure and shear dependence of viscosity is essential to an understanding of the mechanisms of film forming and friction in concentrated contacts. The blending of different molecular mass polyalphaolefins (PAOs) may permit the formulator to arrive at a desired combination of film thickness and friction. The viscosities of PAO base oils and their blends were measured versus temperature, pressure, and shear stress to 1 GPa in pressure. The Grunberg–Nissan mixing rule, with effective mole fractions, provides an excellent mixing rule for the temperature and pressure-dependent low-shear viscosity. This work provides the first look at a possible mixing rule for the non-Newtonian response of mixtures of base oils.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021803-021803-8. doi:10.1115/1.4041207.

The purpose of this work was to study the influence of soybean biodiesel addition in ultra-low sulfur diesel (ULSD) on its tribological behavior under low-amplitude reciprocating conditions, simulating the operation of a fuel injector system. The methodology was divided into three parts: the first was the fuel preparation and its physicochemical characterization, where were studied four fuels (diesel, soybean biodiesel, and mixtures of them).The following step was the evaluation of the fuel tribological properties, using the high-frequency reciprocating rig (HFRR) test. These tests were carried out by steel ball-on-disk lubricated contact, on which the friction coefficient of friction (COF), the film percentage, and the wear scar diameter (WSD) were measured, according to ASTM D6079-11. In the end, the analysis of the damages presented on the worn disk surfaces was characterized by scanning electronic microscopy (SEM) and atomic force microscopy (AFM) techniques. Results showed that the addition of biodiesel to ULSD is an excellent option to restore the lubricating ability of this fuel. The biodiesel incorporation reduces the friction coefficient and improves the film formation. Besides, the evaluation of worn disk surfaces using SEM and AFM techniques showed that biodiesel avoids damages to surface through protective film formation and reduces the superficial roughness.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021804-021804-10. doi:10.1115/1.4041304.

Performance of grease lubricated point contact under elastohydrodynamics lubrication (EHL) regime is critical in many engineering applications. The present work deals with the evaluation of rheological, film forming characteristics and elastic recovery of newly developed nanocomposite greases. The nanocomposite greases are formulated by dispersing different nano-additives to bare grease (BG). The nano-additives include reduced graphene oxide (rGO) nanosheets, CaCO3 and α-Al2O3 nanoparticles. The microstructure evaluation of the nano-additives and different greases is done using high-resolution transmission electron microscopy (HRTEM). Estimation of the rheological parameters (storage and loss moduli) is done using rotational rheometer. The film forming behavior is recorded using elastohydrodynamic (EHD) rig for range of speed at different temperatures and constant load. The results indicate that change in microstructure due to nano-additive incorporation improves the responses of different greases. Based on rheological response, CaCO3 doped grease seems better but rGO doped grease is able to bear high shear stresses. Further, based on film forming characteristics and reflow or recovery behavior, rGO doped grease is better. The rGO-based grease registers approximately 90% elastic recovery followed by 75% for CaCO3-based grease, 65% for BG and 10% for α-Al2O3-doped grease.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2018;141(2):022001-022001-9. doi:10.1115/1.4041074.

During the last decades, there has been an increased interest in the use of lignin-based composites following the ideas of developing green materials for fossil-based raw materials substitution. The biopolymer Arboform is a mixture of lignin, plant fibers, and additives, which is nowadays successfully used in many applications. As a thermoplastic, it can be molded and is therefore also called “liquid wood.” In this paper, we report a study comparing the nanomechanical and tribological properties of Arboform (AR), and Aramid-reinforced Arboform (AR-AF) composite biopolymers. The samples were produced in an industrial-scale injection molding machine. Nanoindentation experiments have revealed that, in both series of biopolymer samples, an increase in temperature or a change in the injection direction from 0 deg to 90 deg produces an increase in hardness. On the other hand, Young's modulus is slightly affected by the increase in temperature, and not affected by the injection angle. Tribological characterization has shown that all samples, except the AR-AF injected at 175 °C, present noticeable wear and have a similar friction coefficients μ ∼ 0.44–0.49 at Hertzian contact pressures p0 between 90 and 130 MPa. Interestingly, the reinforced polymer produced at 175 °C shows no wear and low friction of μ ∼ 0.19 at p0 = 90 MPa. Our results show that the reinforced Arboform biopolymers are a good candidate to replace other polymers in many mechanical and tribological applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):022002-022002-11. doi:10.1115/1.4041303.

We investigated the wear resistance properties of high-frequency induction heat (HFIH) sintered alumina (Al2O3) ceramic nanocomposites containing various multilayer graphene (MLG) concentrations. The tribology of the monolithic Al2O3 and nanocomposites samples was assessed against spherical ceramic (Si3N4) counter sliding partner at sliding loads ranging from 6 to 40 N using ball-on-disk wear test configuration. Compared with the monolithic Al2O3, the incorporation of 1.0 vol % MLG reduced the friction coefficient by 25% and the wear rate by 65% in the MLG/Al2O3 nanocomposites tested under 40 N sliding load. Based on the mechanical properties, brittle index, and microstructure, the active wear mechanisms for the nanocomposites were analyzed. The MLG contributed in the nanocomposites tribology process, indirectly, by enhancing the mechanical properties and, directly, by reducing the friction between the counter sliding partners. The synergistic role of MLG thin triboflim and twirled MLG for improving the tribological performance of the nanocomposites is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):022003-022003-9. doi:10.1115/1.4041366.

Ti/MoS2 coating was deposited by pulse laser deposition technology on Al-Si substrate. The microstructure, elemental analysis, nanotribological behavior of coating was investigated. The coating was composed of Ti, Mo, S, and O with typical diffraction peak around 2θ range from 30 deg to 70 deg. Nanoscratch with ramp loading was performed at low loads. The scratch test with ramp normal loading was analyzed for failure of coating in three ranges, viz., range A, range B, and range C. Scratch test result shows that the peeling of coating occurred at the normal load of 1327.75 μN and the lateral load of 75.96 μN. Nanowear with 2, 4, 6, 8, 10 number of cycles was performed at low load 100 μN. Nanowear results shows that wear rate decreases with increase in wear cycles, which attributed the self-lubricating property of Ti/MoS2 coating. Also, Ti/MoS2 coating display smooth wear path with no debris and cracks, which attributed plastic flow of coating around impression. Thus, mode of wear mechanism is mainly ductile and abrasive.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;141(2):022201-022201-12. doi:10.1115/1.4040574.

Numerical and experimental analyses of the static and dynamic characteristics of the liquid annular seals with axially partial helical grooves were conducted to investigate the effects of the axial length gal of a helically grooved section in a seal stator. The numerical solution and experimental procedures were applied in the same manner as in previous studies on through-helically grooved seals, wherein the grooves extend across the seal length. The numerical results qualitatively agreed with the experimental results, demonstrating the validity of the numerical analysis. The leakage flow rate Q was lower in the partially helically grooved seals than that of conventional through-helically grooved seals across a small range of rotor spinning velocities. In contrast, the reduction in Q due to the pumping effect caused by the spin of the rotor diminished with the decrease in gal. For a small concentric whirling motion of the rotor, the radial dynamic reaction force Fr and magnitude of variation in the tangential dynamic reaction force Ft with the whirling angular velocity increased with the decrease in gal, and their values approached the corresponding values for the smooth-surface seal. Under the same rotor whirling velocity, the Ft for the partially helically grooved seals became lower than that for the smooth-surface seal (similar to the case for the through-helically grooved seal), although decreasing gal tended to increase Ft. These results suggest that partially helically grooved seals can improve the efficiency and stability margin of the pumps because of the reduction in leakage flow rate and suppression of the rotor forward whirling motion (with large radial and tangential dynamic reaction forces) as compared to conventional through-helically grooved seals.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):022202-022202-6. doi:10.1115/1.4041210.

This paper deals with the reduction of sticking in a linear-guideway type recirculating ball bearing (linear bearing), which is the significant increase in the required driving force for a linear bearing in a back-and-forth short stroke operation. First, the driving force of a linear bearing with five carriage-body types (A–E, having different dimensions and shapes) under rolling moment load was measured. Simultaneously, the ball's position in the load zone was observed. The experimental results showed that regardless of the carriage-body types, the increasing rate of the driving force and the interspace (space between balls around the center of the load zone on the raised side) decreases and sticking tends to hardly occur as the maximum linear velocity and the stroke length increase. Also, the occurrence of sticking was affected by the carriage-body types. Finally, to examine the relationship of carriage-body types, carriage-body deformation, and the occurrence of sticking, the carriage-body deformation (caused by preloading and tightening torque of bolts) was calculated by finite element method (FEM). The FEM results showed that carriage-body type, which is more deformable, had a tendency to reduce sticking.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):022203-022203-8. doi:10.1115/1.4041616.

The percolation threshold strongly affects sealing performance. This paper investigates the relationship between the percolation threshold and the rough surface anisotropy, which is represented by the Peklenik number, γ. A series of anisotropic rough surfaces were generated and the conjugate gradient-fast Fourier transform (CG-FFT) method was used to determine the percolation threshold. The percolation threshold was found to be A/A00.484±0.009 (averaged over 45 surfaces) was established for an isotropic rough surface (γ=1). Furthermore, it was also found that the percolation threshold decreased from A/A00.528±0.011 to A/A00.431±0.008 as 1/γ increased from 0.6 to 2. Our results differ from the theoretical result of Persson et al., where A/A0=γ/(1+γ). Comparing our calculated results with the theoretical results established the presence of an intersection value of 1/γ that was related to the effect of elastic deformation on the percolation threshold. When 1/γ was smaller than the intersection value, our calculated results were lower than the theoretical ones; and when 1/γ was greater than the intersection value, our calculated results were higher than the theoretical ones.

Commentary by Dr. Valentin Fuster

Discussion

J. Tribol. 2018;141(2):025501-025501-1. doi:10.1115/1.4041075.

It should be noted that the literature survey in this paper [1] is greatly lacking. Moreover, the fundamentals of gas-lubricated tribo-elements are grossly overlooked or even misunderstood, pointing first to the theoretical background which in its entirety can be found elsewhere [28]. The authors do not explain how they enforce mass flow continuity across groove-land borders (see Refs. [46]). In fact, the authors use a successive over relaxation (SOR) method, i.e., using a finite difference scheme that by definition contains singularities in the derivatives of the film thickness as imbedded in Eq. (4)—the authors do not explain how that is handled. The SOR method, which was popular in the 1950s or 1960s, is a method that is clearly unfit for spiral groove seals that contain film thickness discontinuities. Moreover, with or without spiral grooves, it is also very well known that the SOR method suffers profoundly from numerical instabilities when the compressibility number is just moderately elevated, and severe numerical instabilities occur when the compressibility numbers are high (in fact, successive under relaxation is necessary, but it has very limited success in such cases). Miller and Green [46] provided numerical formulations that overcome these complications. Namely, the finite element method and the finite volume method, as detailed in the said references, handle appropriately the discontinuities in the film thickness across groove-land borders, while the upwinding mass conserving algorithms handles any value of the compressibility number. After all, if mass flow conservation is not enforced what is the meaning of using the flow Q as an objective for optimization? It is, therefore, doubtful that the results reported by the authors are meaningful, as no validation or comparison with previous work is given.

Commentary by Dr. Valentin Fuster

Closure

J. Tribol. 2018;141(2):026001-026001-1. doi:10.1115/1.4041076.

In the authors' perspective, a higher load-carrying capacity is necessary for the sealing of lubricant and to reduce their wear, and its value significantly depends on the pattern grooves and lubricant pumped in the contact interface. However, many studies contend that increase in load-carrying capacity of a spiral groove increases the leakage and also pumping of lubricants in the contact region shall result in elevated leakage. Therefore, the objective of this work [1] was to find a method to deal with these conflicting objectives.

Commentary by Dr. Valentin Fuster

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