Showing papers in "Journal of Tribology-transactions of The Asme in 2020"

The Effect of Boron Nitride on Tribological Behavior of Mg Matrix Composite at Room and Elevated Temperatures

Abstract: The goal of the study is to examine the dry sliding wear behavior of pure Mg and Mg/nano-boron nitride (BN) composite at elevated temperatures. The wear behavior of the samples was evaluated under loads of 5, 10, and 20 N, at sliding speed of 80, 130, and 180 mm s−1 and at temperatures of 25, 100, and 175 °C. The examination of worn surface, counterface, and wear debris was performed. The results showed that nano-BN particles lead to substantial enhancement of wear resistance for both room and elevated temperatures. Mg/0.25 BN has lower coefficient of friction values due to the presence of BN which act as solid lubricant. The wear mechanisms are thermal softening, melting, oxidation, abrasion, and delamination.

A Reconstruction and Contact Analysis Method of Three-Dimensional Rough Surface Based on Ellipsoidal Asperity

Abstract: The 3D rough surface modeling and contact analysis is a difficult problem in the study of rough surface contact. In this paper, a new method for reconstruction and contact analysis of asperities on 3D rough surfaces is proposed based on real rough surfaces. Watershed algorithm is used to segment and determine the area of asperities on the rough surface. According to the principle of minimum mean square error, ellipsoid fitting is carried out on asperities. Based on the elastic-plastic contact model of a single ellipsoidal asperity, a stable and efficient method for 3D rough surface contact analysis and calculation is proposed. Compared with existing calculating methods, the present method has the following characteristics: (1) the constructed surface asperity is closer to the real asperity in contact, and the calculation of asperity parameters has better stability under different sampling intervals and (2) the contact pressure, contact area, and other contact parameters of the 3D rough surface are calculated with high accuracy and efficiency, and the calculation convergence is desirable. The reconstruction and contact analysis method of the 3D rough surface asperity proposed in this paper provides a more accurate reconstruction and calculation method for the study of contact fatigue life and wear failure of rough surfaces.

Experimental Investigations on the Tribological Performance of Electric Discharge Alloyed Ti–6Al–4V at 200–600 °C

Abstract: Nowadays, the titanium alloys observe broad applicability in aerospace, marine, automobile, and bio-medical industries due to their lightweight, bio-compatibility, good fatigue strength, and corrosion resistance. However, it possesses poor tribological behavior characterized by a high coefficient of friction (CoF) and the specific wear rate (SWR). In this paper, the impact of the electrical discharge machining on the tribological properties of Ti6Al4V (Ti64) was compared before and after electrical discharge machining (EDM) using a pin-on-disk tribometer under un-lubricated and ambient temperature. Besides, ex-situ analysis was performed on the distinct pin surfaces using (i) scanning electron microscopy (SEM), (ii) energy-dispersive X-ray spectroscopy (EDX), and (iii) X-ray diffraction spectroscopy (XRD) techniques to elucidate the associated wear mechanisms. The mechanical properties such as nano-hardness and elastic modulus of the test surfaces were also determined using a nano-indenter. Significant improvement in SWR (65.44% reduction) with a passable compromise for CoF (22.5% increment) occurred during experimentation (before and after EDM) at 100 N. Besides, the wear behavior of the electrical discharge alloyed Ti64 (ETi64) was evaluated at 200 °C, 400 °C, and 600 °C under the applied loads of 50–150 N. At 400 °C and 600 °C, the SWR initially decreased and then increased with variation in load from 50–100 N and 100–150 N, respectively. The enhanced tribo-behavior at 100 N was due to the protective influence of tribo-oxides, viz., TiO2 and Ti8O15 assisted by the hard Ti24C15 carbides in the recast layer (RL).

Friction and Wear Characterization of Spark Plasma Sintered Hybrid Aluminum Composite Under Different Sliding Conditions

Abstract: Friction and wear behaviors of hybrid aluminum matrix self-lubricating nanocomposite, containing varying amount of nano-aluminum oxide (Al2O3) and graphene nanoplatelet (GNP) particles were investigated under dry and boundary lubrication sliding conditions. All the specimens were prepared by ultrasonic probe mixing followed by high-energy mechanical ball milling and spark plasma sintering fabrication route. The objective of this present study is to determine the tribological behavior of GNP as nanoreinforcement under dry and wet lubricating conditions. The wear test was carried out at room temperature on a ball-on-disc reciprocating tribometer with a contact load of 5 N, a frequency of 30 Hz, a stroke of 2 mm, and a sliding distance of 120 m. The chromium-plated chrome steel ball is used as the counter body for tribo-testing. From the results, it has been observed that GNP, as the solid lubricant reinforcement, reduces the friction by 25.33% and 68.14% under dry and wet lubricating conditions, respectively, for the hybrid composite. Reduction in wear volume by 91.15% and 90.50% is observed under dry and wet lubricating conditions, respectively, for the hybrid composite in comparison to the base eutectic Al–Si alloy. The reduction in the coefficient of friction and wear volume for the hybrid composite is attributed to the multilayer two-dimensional GNP nanoparticles that cause easy shearing by forming a conformal protective film layer on the sliding interface. Ultra-mild to severe wear regime was obtained for different composition samples including base composition to hybrid composition.

An Experimental Investigation on the Wear of Lubricated Steel Against PEEK Gears

Abstract: The application of polymer gears in power transmission has been considered recently due to their advantages over metal gears, e.g., lighter weights and lower costs. Much further research work needs to be carried out to understand their wear mechanisms and to establish their design methods. This paper aims to investigate the wear and failure behavior of the poly-ether-ether-ketone (PEEK) gear against the steel gear under oil jet lubrication through the durability test with the standard FZG test rig. The service life and wear loss of PEEK gears were experimentally recorded under different moderate loading conditions ranging from the output torque of 10–20 Nm with a fixed input rotational speed of 1000 rpm. Moreover, the tooth surface morphologies and the wear loss of PEEK gears under different running stages were investigated. The experimental measurements reveal that there is a critical load magnitude for the transition of wear performance. With a load above 15 Nm, the PEEK gear wear rate increases rapidly, leading to a quick breakage failure, whereas the gear wear rate with a torque below 15 Nm is much lower. All tested PEEK gears finally failed with a pitting-induced tooth breakage mode under moderate loading conditions.

Effect of Laser Shock Peening on the Wear–Corrosion Synergistic Behavior of an AZ31B Magnesium Alloy

Abstract: Laser shock peening (LSP) is one of the widely used surface processing techniques for tailoring functional behavior of surfaces. LSP has been used to enhance friction, wear, and mechanical properties. However, understanding of LSP-treated surfaces involving tribological contacts in electrochemically active environments is limited because the mechanism of wear–corrosion interactions (tribocorrosion) for such surfaces is still unclear. In the present study, the effect of LSP on the wear–corrosion behavior of an AZ31B Mg alloy is investigated. A zero-resistance ammeter (ZRA) method is utilized to examine the evolution of open circuit potential (OCP) during wear–corrosion analysis. The study finds that the LSP processing can decrease the corrosion potential difference between worn and unworn regions of the surface, thereby mitigating the effect of wear-accelerated corrosion during sliding. The effect of wear-accelerated corrosion is evident from the change in average surface roughness (Sa) of the unworn areas. It is found that understanding the change in surface roughness due to wear–corrosion interactions is necessary to investigate the onset and propagation of galvanic corrosion. Based on these results, the study details the mechanism of wear–corrosion interactions during sliding.

Tribological Property Investigation of Self-Lubricating Molybdenum-Based Zirconia Ceramic Composite Operational at Elevated Temperature

Abstract: Three mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with 0.5 wt% of magnesium oxide (MgO) and 6 wt% of molybdenum (Mo) were prepared by the pressureless sintering process, and the friction and wear behavior of the ceramic composite were studied against the alumina disc. Tribological tests were carried out both at room temperature as well as at an elevated temperature (500 °C). The result revealed that a substantial reduction of ∼50% in the friction coefficient and ∼31% reduction in the wear rate were achieved while 6 wt% Mo was added into the 3Y-TZP matrix operational at 500 °C. No significant tribological influence was observed with the addition of Mo at the normal operating temperature. The minimum coefficient of friction and low specific wear rate were achieved because of the formation of MoO3 in between the mating surfaces at elevated temperature. The worn surfaces were characterized by means of field emission scanning electron microscopy (FESEM). The formation of MoO3 phases was identified by wear debris analysis which was performed with the help of X-ray photoelectron spectroscopy (XPS).

Synergistic Effect Between Phosphonium-Based Ionic Liquid and Three Oxide Nanoparticles as Hybrid Lubricant Additives

Abstract: The tribological properties of ionic liquid (IL) trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate along with Al2O3, CuO, and SiO2 nanoparticles (NPs) have been investigated as a lubricant additive in a group 1 mineral base oil. About 0.5 wt% concentration of additives were added in base oil, and tribological tests were conducted at mild (stipulated) and severe (ASTM D 4172D) working conditions to assess the synergy between IL and NPs. This study shows the excellent synergy between IL, Al2O3, and CuO NPs in improving tribological and extreme pressure (EP) properties. Al2O3 and CuO hybrid nanolubricants decreased friction by 19% and 24%, whereas wear by 32% and 36%, respectively, at ASTM test conditions. IL displayed very good EP properties with a total improvement of 19%, and the highest load-bearing capacity was observed for Al2O3 and CuO hybrid nanolubricants with an improvement of 30% and 34%, respectively. No conclusive evidence of synergy has been observed between IL and SiO2 NPs. Surface characterization techniques, such as scanning electron microscope, energy dispersive X-ray spectrometer, and Raman spectra, demonstrated the formation of a tribofilm rich in phosphate and tribosintered NPs on the worn surface responsible for improved triboperformances.

Exact Spectral Moments and Differentiability of the Weierstrass-Mandelbrot Fractal Function

Abstract: Fractal mathematics using the Weierstrass-Mandelbrot (WM) function has spread to many fields of science and engineering. One of these is the fractal characterization of rough surfaces, which has gained ample acceptance in the area of contact mechanics. That is, a single mathematical expression (the WM function) contains characteristics that mimic the appearance of roughness. Moreover, the “roughness” is “similar” across large dimension scales ranging from macro to nano. The field of contact mechanics is largely divided into two schools of thought: (1) the roughness of real surfaces is essentially random, for which stochastic treatment is appropriate, and (2) surface roughness can be reduced to fractal mathematics using fractal parameters. Under certain mathematical constraints, the WM function is either stochastic or deterministic. The latter has the appeal that it contains no randomness, so fractal mathematics may offer closed-form solutions. Spectral moments of rough surfaces still apply to both approaches, as these represent physical metrology properties of the surface standard deviation, slope, and curvature. In essence, spectral moments provide a means of data reduction so that other physical processes can subsequently be applied. It is well known, for example, that the contact model of rough surfaces, by Greenwood and Williamson (GW), depends on parameters that are direct outcomes of these moments. Despite the vast amount of publications on the WM function dedicated to surfaces, two papers stand out as originators, where the others mostly rework their results. These two papers, however, contain some omissions and approximations that may lead to gross errors in the estimation of the spectral moments. The current work revisits these papers and adds information, but departs in the mathematical treatment to derive exact expressions for the said moments. Moreover, it is said that the WM function is nondifferential. That is also revisited herein, as another approach to derive the spectral moments depends on such derivatives. First, the complete mathematical treatment of the WM function is made, then the spectral moments are derived to yield exact forms, and finally, examples are given where the physical meanings of the approximate and exact moments are discussed and their values are compared. Numerical procedures will be introduced for both, and the effectiveness of the computational effort is discussed. One numerical procedure is particularly effective for any digitized signal, whether that originates from analytical functions (e.g., WM) or real surface measurements. [DOI: 10.1115/1.4045452]

Numerical Simulation of Ultrasonic-Assisted Grinding Surfaces With Fast Fourier Transform

Abstract: Numerical simulation of three-dimensional rough surfaces based on fast Fourier transform (FFT) is revisited. A more systematic approach, which is an extension of the current FFT-based simulation models, is proposed to approach surface reconstruction. Moreover, the simulation of the surfaces with machining signature by prescribing the parameters, take ultrasonic-assisted grinding as an example, has been taken as the research focus for the first time. The effectiveness is tested by three cases of simulation examples. Excepting the surface with exponential autocorrelation function, the simulation of surfaces with grinding machining marks is considered both by prescribing the theory autocorrelation function and by measuring a small area as a sample. The results show that the proposed method has great potential in engineering applications.

Scratch and Sliding Wear Testing of Electroless Ni–B–W Coating

Abstract: Electroless Ni–B–W coating is deposited on low carbon steel in an alkaline sodium borohydride-reduced electroless bath. The mechanical and tribological properties of such coatings are much necessary to be assessed to carry out application-based studies. The present work focuses mainly on the evaluation of hardness and fracture toughness of electroless Ni–B–W coatings using a scratch tester. Coating's response toward scratching is also studied thoroughly. The characteristic short-range order present in its lattice structure causes the generation of a specific behavioral pattern. Furthermore, a linear sliding wear test is carried out on coatings' surface to analyze the wear behavior at different loading conditions. The specific wear rate is observed to be minimum at a normal load of 22.5 N against Si3N4 counterbody. The patterns of tribological behavior of the coating at different load values are examined from the worn surface morphologies. But before embarking on the scratch and sliding wear tests, the synthesized coatings are characterized under field emission scanning electron microscope and X-ray diffraction in an exhaustive manner. The growth rates with respect to time and the changes in morphological aspects of the coating are also evaluated. The present study establishes electroless Ni–B–W deposits as a suitable option for protecting mechanical components against wear.

A Dynamic Wear Model for Micro-Grooved Water-Lubricated Bearings Under Transient Mixed Lubrication Condition

Abstract: The study presents a dynamic wear model for micro-grooved water-lubricated bearings considering the transient mixed elastohydrodynamic lubrication (mixed-EHL) condition. In the established model, the modified Archard wear model and the mixed-EHL model are bridged to study the transient interdependent relationship between the sliding wear behavior and the mixed-EHL performance. In order to consider the effect of the transient mixed-EHL performance on the sliding wear, the Archard model is extended to include the time-varying wear coefficient based on the fatigue concept. To verify the presented model, the comparisons with the experimental results available in the literatures have been conducted. In this study, the evolution of the wear and mixed-EHL performance distribution over time is predicted, and the impact of the radial clearance, boundary friction coefficient, and surface parameters on the numerical predictions is evaluated. The simulation results reveal that the worn region moves toward the rotational direction slowly. The simulation results also reveal that the wear rate and the wear coefficient first decrease considerably, and then decrease gently, and the sliding wear geometry promotes the hydrodynamic effects and reduces the asperity contact during the operation. Furthermore, the parametric study demonstrates that dynamic wear and mixed-EHL performance is sensitive to the radial clearance, boundary friction coefficient, and surface parameters.

Investigation on Friction, Anti-wear, and Extreme Pressure Properties of Different Grades of Polyalphaolefins With Functionalized Multi-walled Carbon Nanotubes as an Additive

Abstract: In the present investigation, the COOH-functionalized multi-walled carbon nanotubes (MWCNTs) having an outer diameter of 20–30 nm and length 1−2 μm were dispersed in four different grades of polyalphaolefins (PAOs; i.e., PAO 4, PAO 6, PAO 40, and PAO 100) at various concentrations (0.025, 0.05, 0.075, 0.10, and 0.15 wt%) to evaluate friction, anti-wear, and extreme pressure properties. The tribological test was conducted as per ASTM standard using four-ball tester. The test results showed that with the addition of MWCNTs, the friction and wear properties of PAOs had been improved significantly as compared to the base oils. It was also observed that MWCNTs exhibited excellent anti-wear properties than friction properties. The possible reasons for the improvement in friction and wear properties are discussed with the aid of various analytical tools.

Abrasive Wear Mechanism of Microwave-Assisted Compression Molded Kenaf/HDPE Composite

Abstract: The present work aims to study the abrasive wear of kenaf/high-density polyethylene (HDPE) composites with 20% weight fraction reinforcement of the kenaf fiber. A unique technique of the microwave-assisted compression molding (MACM) was used to fabricate the composites. The pin-on-disc setup was used for two-body abrasive wear, in which the kenaf/HDPE composite acts as a pin and the abrasive paper (P100) acts as a counter surface. Two-body abrasive wear tests were conducted for HDPE and kenaf/HDPE composites at normal loads of 10 N, 20 N, and 30 N and the sliding speed of 1 m/s, 2 m/s, and 3 m/s within 100 m of sliding distance. Tribofilm formation was observed at higher values of load and speed, which helps in reducing the wear-rate of the composites. Wear mechanism of the kenaf/HDPE composite is discussed in detail and supported with scanning electron microscope (SEM) fractography.

Modeling a Hydrodynamic Bearing With Provision for Misalignments and Textures

Abstract: Surface texturing is one effective means to improve bearing performance. Available studies are mainly focused on the perfectly aligned journal bearing with textures. The studies involving the effect of surface texturing on the misaligned bearing are still limited. In the present study, a transient mixed lubrication model that allows predicting evolution of friction characteristics with high efficiency was proposed. The misalignment of journal bearing under the time-varying applied load was considered using two deflection angles. The performance of journal bearing with provision for misalignments and textures was studied using the proposed model. The results involving the orbits of the journal center and the energy loss of different misaligned journal bearings were obtained. The influences of misalignment degree and surface texturing on these parameters were analyzed. The results show that along with the increased deflection angles, the effect of texturing on the tribological performance of journal bearing would turn beneficial into harmful.

Dry Sliding Wear Performance of Ni–SiC Composites Developed Through an In Situ Microwave Casting Process

Abstract: Metal matrix composites of nickel-based powder reinforced with silicon carbide are processed through a domestic microwave applicator. In situ melting and casting of composites were carried out using microwave energy with average processing time of 25 min. Phase analysis of processed composites revealed the formation of some hard-intermetallic compounds such as nickel silicides (NiSi, Ni2Si, and Ni3Si2) and carbides (Cr3C2 and Cr7C3). Microstructure analysis confirms the favorable growth of equiaxed grains with uniform dispersion of reinforcement and low porosity defects (1.5–1.8%). The formation of hard-intermetallic compounds and the presence of SiC reinforcement led to the increased microhardness of composites. Sliding wear tests under dry sliding conditions with varying load and velocity conditions revealed the formation of stable oxide tribolayers at 1.0 m/s of sliding velocity and 15 N load. Fractography of worn-out samples revealed abrasion of surfaces at the lower load (10 N) condition. However, on increasing the load, the shearing of surfaces due to adhesion, plastic deformations, and surface pullout was observed. At higher loads (20 N) and higher sliding velocities (1.5 m/s), particle pullout and three-body abrasive wear mechanisms were observed. The overall weight loss study revealed that the addition of 5% and 10% volume fraction of SiC reinforcement reduced the wear-rate by 58.9% and 80.6% in comparison to the pure nickel casting at the sliding velocity of 1 m/s and under 15 N load.

Slurry Erosion Performance of Ni + B4C Microwave Composite Clads

Abstract: A composite clad of Ni-based alloy and B4C was developed on austenitic steel substrate through microwave hybrid heating. The B4C was added with Ni-based alloy powder in various weight proportions (0%, 5%, 10%, and 20%). The clads were investigated for the microstructural details, nanohardness and slurry erosion performance. Silt collected from river Uhl was used as the erodent for the slurry erosion test. Slurry erosion performance of the clads was evaluated at an impact angle of 90 deg and jet velocity of 40 m/s. Microstructural characterization confirms the uniform distribution of hard (boride and carbide) phases in the Ni-based matrix of the microwave composite clads. The presence of hard phases in the Ni-based matrix enhanced the slurry erosion resistance performance.

An Optimization Design of Contact Interface Material Stiffness for Improving the Uniformity of the Contact Pressure

Abstract: Material stiffness, a significant parameter of a contact interface, is investigated to improve the uniformity of the contact pressure. A contact interface material stiffness optimization design algorithm is developed based on the modified solid isotropic material with the penalization (SIMP) method. The uniformity of the contact pressure field is represented by its variance and is defined as the optimization objective. A node-to-node frictionless elastic contact theory is adopted to perform the contact analysis. The effectiveness of the interface material stiffness design for improving the uniformity of the contact surface is verified based on two contact cases. Because the relationship between the material stiffness and the hard-and-soft degree of a contact interface is always a positive correlation, the results in this paper could be extended so that the design of the contact interfaces’ hard-and-soft degree will improve the distributing uniformity of the contact surface.

Maximizing the Lubricant Film Thickness Between a Rigid Microtextured and a Smooth Deformable Surface in Relative Motion, Using a Soft Elasto-Hydrodynamic Lubrication Model

Abstract: We design a pattern of microtexture features to increase hydrodynamic pressure and lubricant film thickness in a hard-on-soft bearing. We use a soft elastohydrodynamic lubrication model to evaluate the effect of microtexture design parameters and bearing operating conditions on the resulting lubricant film thickness and find that the maximum lubricant film thickness occurs with a texture density between 10% and 40% and texture aspect ratio between 1% and 14%, depending on the bearing load and operating conditions. We show that these results are similar to those of hydrodynamic textured bearing problems because the lubricant film thickness is almost independent of the stiffness of the bearing surfaces in full-film lubrication.

A Study of Hole-Entry Grooved Surface Hybrid Spherical Journal Bearing Operating With Electrorheological Lubricant

Abstract: The performances of the tribo-pairs are greatly influenced by introducing the grooved surfaces. Developments of the newer type of lubricants have made a great impact on the performance of fluid film bearings. This article investigates the non-Newtonian behavior of electrorheological lubricant on the performance of grooved hybrid spherical journal bearing. The effect of different arrangements of grooves, i.e., partially grooved or fully grooved on the bearing surface, has been studied. The finite element method is used to numerically simulate the results. Furthermore, a parametric study is performed for optimizing the groove attributes. The present work demonstrates that the different grooved arrangements have a substantial influence on the bearing performance. It is revealed that the provision of grooves on the bearing surface decreases frictional losses and enhances the stiffness coefficients of the bearing. Furthermore, numerically simulated results indicate that the electrorheological lubricant enhances the value of minimum fluid film thickness and the stiffness coefficients (S¯xxandS¯yy) of spherical hybrid journal bearing. Improved bearing performance can be achieved by using the optimized grooved attributes together with the electrorheological lubricant.

Calculation of Gear Meshing Stiffness Considering Lubrication

Abstract: Gear meshing stiffness is the key parameter to study the gear dynamic performance. However, the study on the calculation of gear meshing stiffness considering lubrication, especially mixed lubrication, is still insufficient. Based on the three-dimensional linear contact mixed elastohydrodynamic lubrication model and the contact stiffness calculation method of rough surface, a method for calculating the gear meshing stiffness under mixed lubrication is proposed in this paper. According to the proposed calculation method, the effects of speed, external load, and roughness amplitude on gear meshing stiffness are further explored. The method can take into account the real rough surface topography and lubrication in the meshing process, so it may be more advantageous than the conventional method to some extent.

Design of Novel Gas Foil Thrust Bearings and Test Validation in a High-Speed Test Rig

Abstract: Small gas foil bearings (FBs) with shaft diameter below 25 mm can find many applications in air compressors for fuel cells, electrical turbo chargers, small unmanned air vehicles, turbo alternators, etc. These small machines are characterized by very light load to the radial FBs, and thus rotordynamics stability is more challenging than load capacity. However, a main challenge of gas foil thrust bearings (GFTBs) is how to increase the load capacity, and the challenge remains the same regardless of the size. In previous publications on experimental studies on GFTBs, the measured load capacity is well below the prediction due to challenges in testing as well as manufacturing of GFTBs. Difficulty in achieving the design load capacity often leads to increasing the bearing size in actual applications with penalty of higher power loss. This paper presents design feature of a novel GFTB with outer diameter of 38 mm and static performance up to 155 krpm under external load of 75 N using a high-speed test rig. The 38 mm GFTB presented in this paper is a three-layered structure for easy design and manufacturing, and the unique design feature allows easy scale down and scale up to different sizes. Reynolds equations for compressible gas and the two-dimensional thin plate model were adopted for fluid–structure interaction simulation to predict load capacity and power loss of the GFTB. The predicted power loss and load capacity agree well with the measurements.

Study of Oil Starvation in Journal Bearing Using Acoustic Emission and Vibration Measurement Techniques

Abstract: This present article evaluates the state of starvation in a journal bearing using acoustic emission (AE) and vibration measurement techniques. A journal bearing requires a constant supply of oil in an adequate amount to develop a hydrodynamic film, thick enough to separate the surfaces and avoid asperity contacts. On a microscopic level, the surface interaction under starved lubrication results in deformation and fracture of asperities. This causes a proportionate increase in AE and vibration. The AE activities resulting from asperities interaction have significant energy in the frequency range of 100–400 kHz with peak frequencies in the range of 224–283 kHz. Further, the peak frequency shifts from the higher to lower side as the asperity interaction transits from the elastic to plastic contact. This information derived from the spectral analysis of AE signals can be used to develop condition monitoring parameters to proactively control the lubrication and prevent bearing failure.

Experimental Investigation on Two-Body Abrasion of Cast Aluminum–Alumina Composites: Influence of Abrasive Size and Reinforcement Content

Abstract: The occurrence of abrasion is inevitable in most engineering systems. Abrasive wear specifically two-body causes higher material and dimensional loss than other modes of wear. Two-body abrasion is yet to be fully comprehended as it is governed by several intrinsic and extrinsic variables. In this article, tribo-performances of Al-composites were experimentally studied with specific emphasis on the role of abrasive size and amount of reinforcement. AA7075 alloy matrix composites with different amounts of alumina particles were fabricated by the advanced stir-casting method. Besides measurements of density, porosity, and Vickers hardness, in-depth characterizations of microstructures were performed. Specific wear-rate (SWR), coefficient of friction (COF), and abraded surface roughness (SR) of developed materials were measured under two-body abrasion over a vast range of distance, load, velocity, and abrasive size. Under all abrasion conditions, composites exhibited higher SR but lower SWR and COF over alloy; the differences increased with reinforcement quantity. SWR, COF, and SR rose with an increase in abrasive size; however, only SR varied with sliding distance for any material. The effects of different variables on the recorded tribo-performances were explained through identification of various micro-mechanisms of abrasion via extensive post wear characterizations and microstructural features. Finally, the criteria for the occurrence of three-body abrasion even in two-body test configuration were highlighted. The wear coefficient value of 10 × 10−3 was identified as the demarcation between two-body and two-body plus three-body abrasion for Al-matrix composites.

On the Negative Influence of Roller-End Axial Profiling on Friction in Thermal Elastohydrodynamic Lubricated Finite Line Contacts

Abstract: This paper presents a finite element model for the solution of thermal elastohydrodynamic lubrication in finite line contacts, including edge effects. The model is used to investigate the influence of roller-end axial profiling on the frictional behavior of such contacts. Roller-end profiling in finite line contacts has always been used to enhance fatigue life by increasing lubricant minimum film thickness and reducing stress concentration at roller ends. The influence on friction on the other hand has often been overlooked in the literature. The current work reveals that roller-end profiling has a negative effect on friction. In fact, it turns out that the improvement in fatigue life comes at the expense of increased friction.

Enhancing Wear Resistance of Selective Laser Melted Parts: Influence of Energy Density

Abstract: Selective laser melting (SLM) is a rapidly developing metal additive manufacturing technology. SLM process parameters have a direct impact on the microstructure of parts, which further affect wear behaviors. Increasing the wear resistance by tailoring process parameters, instead of postprocessing, is crucial for enhancing surface properties of the SLM-fabricated parts with complicated structures. In this study, 316L stainless steel samples were fabricated using different energy densities by varying hatch spacing and scanning speed. The relative density and hardness were measured, and the microstructures were examined. The wear resistance was evaluated by performing scratch tests. Results show that high hardness was found in the bottom region of the samples by small hatch spacings and the highest hardness of 302.8 ± 4.3 HV was measured in the sample by a hatch spacing of 10 μm. With the increase of energy density from 178 to 533 J/mm3 by reducing hatch spacing, the fraction of cellular structures decreases and columnar structures are more likely to be aligned in a relatively constant tilted angle from the build direction, which significantly improve the ability to resist slipping and deformation, indicated by 90.1%, 45.0%, and 15.7% reductions in wear rates under 1, 3, and 5 N, respectively. With the increase of energy density from 182 to 545 J/mm3 by reducing the scanning speed, the number of cellular structures increases but pores also form, which negatively affects wear resistance.

Tribological Performance of Steel With Multi-Layer Graphene Grown by Low-Pressure Chemical Vapor Deposition

Abstract: To explore the potential of directly grown multi-layer graphene as an agent in reducing friction and wear of steel on steel tribo-pair, multi-layer graphene films were synthesized on GCr15 steel in a low-pressure chemical vapor deposition (LPCVD) setup using a gaseous mixture of acetylene and hydrogen onto a bearing steel substrate. An interlayer of electroplated nickel was deposited on steel to assist and accelerate the graphene deposition. The tribological performance was evaluated using a ball-on-disc tribometer with an average Hertzian pressure of 0.2, 0.28, 0.34, and 0.42 GPa over a stroke length of 5 mm against GCr15 steel ball and compared with bare steel and nickel-plated steel. The results indicate that the friction coefficient is dependent on the applied load and decrease with increasing load, and the minimum friction coefficient of ∼0.13 was obtained for an applied normal load of 1 N; however, the coating failed after 250 cycles. The decrease in friction coefficient has been attributed to the homogenization of the deposited multi-layer graphene along the sliding direction and transfer of graphene to counter-face ball leading to inhibition of metal-metal contact. The investigation suggests that this kind of coating has the potential of improving the tribological performance of metal-metal tribo-pairs.

Analysis of Spiral-Grooved Gas Journal Bearings by the Narrow-Groove Theory and the Finite Element Method At Large Eccentricities

Abstract: A finite groove approach (FGA), based on the finite element method (FEM), is used for analyzing the static and dynamic behavior of spiral-grooved aerodynamic journal bearings at different eccentricities, number of grooves and compressibility numbers. The results of the FGA are compared with the narrow-groove theory (NGT) solutions. For the rotating-groove case a novel time-periodic solution method is presented for computing the quasi-steady state and dynamic pressure profiles. The new method offers the advantage of avoiding time-consuming transient integration, while resolving a finite number of grooves. The static and dynamic solutions of the NGT and FGA approach are compared and they show good agreement, even at large eccentricities (ε = 0.8) and high compressibility numbers ∗Corresponding author 1 Copyright c © by ASME (Λ = 70). Stability maps at different eccentricities are presented. At certain operation points, a stability decrease towards larger eccentricities is observed. The largest stability deviations of the NGT from the FGA solutions occur at large groove angle, low number of grooves and large compressibility numbers.

Elevated-Temperature Wear Study of HVOF spray Cr3C2–NiCr-Coated Die Steels

Abstract: Wear and surface damage of tools (die materials) in the hot metal forming industry is a critical problem observed and is producing an adverse effect on the process economy. The problem occurs when tool and workpiece interact at higher temperatures. However, the research related to the wear and friction of dies is still lacking. In the present investigation, tribological (wear and friction) studies were conducted with an aim to explore the potential of Cr3C2–NiCr surface coating. The high-velocity oxy-fuel (HVOF) spray technique was used to formulate the coatings. The microhardness, surface roughness, bond strength, and porosity of the coated specimens were found and analyzed. Subsequently, elevated-temperature wear and friction study of the uncoated and coated specimens were done in the laboratory. The study was conducted at 25 N and 50 N loads. The coated specimens showed an increase in wear resistance at all test parameters. The lowest value of coefficient of friction (COF) and the specific wear-rate for the coated specimens were observed at 400 °C. Wear mechanisms were studied by the scanning electron microscopy (SEM) technique. The wear mechanisms were observed to be adhesive in nature at room temperatures and the combination of abrasive/oxidative/adhesive in nature at higher temperatures for the Cr3C2–NiCr-coated specimens.

An Investigation of the Tribological Behavior of Hf-Based Bulk Metallic Glass and Crystalline Alloys

Abstract: The use of bulk metallic glasses (BMGs) as advanced materials for many applications is attractive, due to their improved mechanical performance over their crystalline counterparts: typically providing higher strength and hardness. Hafnium-based alloys of two similar compositions were prepared by arc melting and suction casting to produce 6-mm-diameter cast rods of an amorphous alloy and a crystalline one. The selected compositions were Hf48Cu29Ni10Al13 (amorphous) and Hf58Cu20Ni16Ti6 (crystalline), as confirmed by X-ray diffraction. The hardness of the amorphous alloy was higher than that of the crystalline one. A detailed study of their tribological behavior was carried out, using a pin-on-disc wear tester, with tungsten carbide counterface balls. There were no statistically significant differences in friction coefficient or wear-rates between the two materials tested. Adherence of material from the tested alloy to the ball, and vice versa, were detected. No changes to the crystallinity of the bulk samples were induced by the wear process, as determined by X-ray diffraction. However, amorphous debris were obtained from both samples, indicating the possibility of local vitrification of particles detached from the crystalline alloy and confirming thermal stability of the amorphous alloy.