Research Papers: Applications

J. Tribol. 2016;139(1):011101-011101-15. doi:10.1115/1.4033054.

Large bearings employed in wind turbine applications have half-contact widths that are usually greater than 1 mm. Previous numerical models developed to investigate rolling contact fatigue (RCF) require significant computational effort to study large rolling contacts. This work presents a new computationally efficient approach to investigate RCF life scatter and spall formation in large bearings. The modeling approach incorporates damage mechanics constitutive relations in the finite element (FE) model to capture fatigue damage. It utilizes Voronoi tessellation to account for variability occurring due to the randomness in the material microstructure. However, to make the model computationally efficient, a Delaunay triangle mesh was used in the FE model to compute stresses during a rolling contact pass. The stresses were then mapped onto the Voronoi domain to evaluate the fatigue damage that leads to the formation of surface spall. The Delaunay triangle mesh was dynamically refined around the damaged elements to capture the stress concentration accurately. The new approach was validated against previous numerical model for small rolling contacts. The scatter in the RCF lives and the progression of fatigue spalling for large bearings obtained from the model show good agreement with experimental results available in the open literature. The ratio of L10 lives for different sized bearings computed from the model correlates well with the formula derived from the basic life rating for radial roller bearing as per ISO 281. The model was then extended to study the effect of initial internal voids on RCF life. It was found that for the same initial void density, the L10 life decreases with the increase in the bearing size.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011102-011102-9. doi:10.1115/1.4033051.

The objective of this study was to investigate the influence of contaminants on the tribological behavior of wheel–rail contact. Sand, phosphate, sulfur, and cement were the studied contaminants identified after a Tunisian railway expertise. All friction tests under different contaminants were conducted using pin-on-disk machine, maintaining the same sliding velocity and Hertzian pressure, respectively, at 0.1 m/s and 1000 MPa. All results were compared with reference of two configuration contacts: wheel tread-rail head (clean dry condition) and wheel flange-rail gauge (clean lubricated condition). The main findings of this study could be listed as follows. First, with reference to clean and dry condition tests, sand and cement showed a higher adhesion than phosphate and sulfur. Second, all contaminants increased the adhesion coefficient with reference to clean and lubricated conditions. Third, sulfur generated the lowest energy-wear coefficient yielding a mild wear. Fourth, sand, cement, and phosphate generated a higher energy-wear coefficient yielding an abrasive wear. Finally, the highest energy-wear coefficient was obtained with sand.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011103-011103-8. doi:10.1115/1.4033136.

This paper deals with the load–displacement relationship of a ball bearing with axial, radial, and angular displacements for both the inner and outer rings. First, the expressions for the load–displacement relationship of ball bearings with any number of balls under the combined axial, radial, and moment loads were presented by using a system in which both the inner and outer rings are allowed to move in the axial, radial, and angular directions. Second, the presented expressions were compared with Jones' expressions (which are typical conventional expressions for the load–displacement relationship), then the range of application of Jones's expressions were elucidated. Third, the relative axial displacement, the relative radial displacement, and the relative angular displacement of a miniature ball bearing type 692 under the combined axial, radial, and moment loads were calculated. Finally, it was shown that the relative angular displacement in the case with no inner ring angular displacement is Ri/Ro times the relative angular displacement in the case with no outer ring angular displacement, in which Ri and Ro are the radii of the inner and outer race curvature center loci.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011104-011104-7. doi:10.1115/1.4033189.

In the present study, the composition of ion nitriding layer of forged CoCrMo alloy was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The tribocorrosion was investigated in 25% calf serum solution. The results showed that CrN and Cr2N phases formed in ion nitriding layer. The content of CrN phase was about 70.2% and Cr2N is only about 29.8%. The corrosion potential (Ecorr) of ion nitriding sample was about −310 mV and the untreated sample was about −820 mV. On condition of tribocorrosion, the friction coefficient of untreated sample was less than those of nitriding sample under different applied loads. At the same applied load, the current density of ion nitriding sample was higher than that of the untreated one. Ktot and Kw of ion nitriding sample were less than the untreated one, which showed the better tribocorrosion resistance. The ratio of Kc/Kw for ion nitriding sample lied in the range of 0.1 < Kc/Kw < 1 under three tested loads, showing that the mechanism was controlled by a wear dominated corrosion.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011105-011105-7. doi:10.1115/1.4033336.

A slurry impingement rig containing 6 wt.% SiO2 particles was used to investigate synergistic erosion–corrosion behavior of X-65 carbon steel at various impingement angles. Maximum erosion–corrosion and erosion rates occurred at impingement angles of about 25 deg and 40–55 deg, respectively. The synergy value highly depended on the impingement angle. The formation of patches of porous corrosion product followed by the formation of corrosion pits led to a positive synergy under impingement angle of 25 deg. At higher impingement angles, the absence of pits probably due to the formation of a more durable tribocorrosion layer resulted in a negative synergy.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2016;139(1):011301-011301-7. doi:10.1115/1.4033130.

Wear characteristics were influenced by the parameters of wear-testing apparatus including configuration of contact surface and form of the relative motion. The tribological behavior of polytetrafluoroethylene (PTFE) disk against AISI1045 steel pin under unidirectionally rotating, linearly reciprocating, and torsional motion was studied. The friction coefficients under unidirectional rotating, linearly reciprocating and torsional motion were 0.1, 0.118 and 0.12, respectively. The highest wear mass loss of PTFE was obtained under linearly reciprocating. The wear mass loss under torsional motion was lowest. The wear mechanism of PTFE under unidirectional rotating, linearly reciprocating, and torsional motion was slight plowing, serious abrasive wear, and adhesive wear, respectively. Through finite element analysis, a higher normal stress induced by the edge effect of steel pin promoted a higher shear stress in PTFE disk. The plastic ratcheting mechanism occurred on the contact edge when the steel pin entered and exited the contact zone, as led to higher wear mass loss under linearly reciprocating and unidirectional rotation. The plastic ratcheting mechanism did not occur under torsional motion. Different transfer films with various topographies were formed on the steel pins under the three motions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011302-011302-9. doi:10.1115/1.4033190.

These days wear-resistant coatings including Fe–TiC composites because of their properties such as high melting point, hardness, and wear resistance are used in different fields such as aerospace, transport, cutting, and abrasive. In situ synthesis of Fe–TiC nanocomposite as a wear-resistant coating by the plasma-spray process is the purpose of this study. Ilmenite concentrate and carbon black were used as raw materials. Three kinds of powders with different conditions were prepared and sprayed on CK45 steel substrates in constant conditions. Microstructure, phase identification, wear resistance, and hardness of coated samples were determined. The results showed that activated sample was synthesized during the plasma spray, but in situ synthesize did not happen for inactive sample which was sprayed by plasma spray. Also, wear resistance and hardness tests showed by synthesis of Fe–TiC composite in coated samples, wear resistance, and hardness were increased.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2016;139(1):011401-011401-8. doi:10.1115/1.4033100.

With a subscale rolling-sliding apparatus, the objective of this study is to explore the adhesion and rolling contact fatigue characteristics of wheel/rail rollers with sanding under water condition. Sanding improves adhesion coefficient but aggravates the surface damage of wheel and rail materials. With the particle diameter and feed rate increasing, the adhesion coefficient is further improved. However, the surface damage (spalling and pits) becomes severer as well as the surface roughness. Note that pitting is a special damage type when sanding is used to improve the adhesion. Big pits and fatigue cracks appear on subsurface under larger particle diameter and feed rate conditions. Severe cracks initiate from big pits and develop into material to a depth, which results in bulk material breaking.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011402-011402-9. doi:10.1115/1.4033052.

A statistical model for torsional friction of plate-on-plate contact is constructed. The torsional responses including T–θ curves, proportion of slip asperities, and the radius of gross slip can be obtained from the model. The torsional friction response of monomer cast (MC) nylon against 316L stainless steel was calculated with this model and a torsional friction experiment of MC nylon against 316L stainless steel was performed to verify the model. The calculated T–θ curves exhibit different shapes under different torsional angular displacements. The calculations demonstrated that the torsional regime determined only through T–θ curves was inaccurate. The statistical results of asperities located at the torsional interface more directly reflected the torsional regime. The T–θ curves obtained from theoretical calculation and experiments are consistent in shapes, whereas the torque magnitude from the theoretical calculation is larger than that from experiments. When gross slip is indicated by the maximum torque on the T–θ curves, about 93% of the contact asperities were in a slip status rather than 100% and the gross slip radius in the whole torsional contact interface was about 3 mm.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2016;139(1):011501-011501-12. doi:10.1115/1.4032963.

Elastohydrodynamic lubrication (EHL) is a common mode of fluid-film lubrication in which many machine elements operate. Its thermal behavior is an important concern especially for components working under extreme conditions such as high speeds, heavy loads, and surfaces with significant roughness. Previous thermal EHL (TEHL) studies focused only on the cases with smooth surfaces under the full-film lubrication condition. The present study intends to develop a more realistic unified TEHL model for point contact problems that is capable of simulating the entire transition of lubrication status from the full-film and mixed lubrication all the way down to boundary lubrication with real machined roughness. The model consists of the generalized Reynolds equation, elasticity equation, film thickness equation, and those for lubricant rheology in combination with the energy equation for the lubricant film and the surface temperature equations. The solution algorithms based on the improved semi-system approach have demonstrated a good ability to achieve stable solutions with fast convergence under severe operating conditions. Lubricant film thickness variation and temperature rises in the lubricant film and on the surfaces during the entire transition have been investigated. It appears that this model can be used to predict mixed TEHL characteristics in a wide range of operating conditions with or without three-dimensional (3D) surface roughness involved. Therefore, it can be employed as a useful tool in engineering analyses.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011502-011502-9. doi:10.1115/1.4032968.

Ball bearings have been used for a long time. Nevertheless, the description of their behavior remains incomplete in spite of the large number of surveys dedicated to ball bearings. Particularly, the exact balls kinematics has still to be addressed in depth. This paper proposes a new way to calculate the balls kinematics by using a simplified quasi-static approach for dry lubricated and axially loaded ball bearings. This method does not use the classical restrictive race control assumptions. More specifically, the role played by the balls kinematics is emphasized by means of the power dissipated within contacts between balls and races. The need for a correct evaluation of the balls behavior is illustrated by using an example, viz., a ball bearing of cryogenic engine turbopump. Indeed, the dissipated power is one of the main concerns in this particular case.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011503-011503-17. doi:10.1115/1.4033048.

A theoretical and computational framework for the analysis of fully transient, thermomechanically coupled, frictional rolling contact based on an arbitrary Lagrangian–Eulerian (ALE) kinematical description is presented. In particular, a computationally efficient methodology for mixed control between the ALE referential velocities and their corresponding driving forces is developed and discussed in depth. Numerical examples involving two-dimensional (2D) cylinder–plate rolling contact are presented, covering a range of transient, thermomechanically coupled rolling contact phenomena, taking place on a broad range of time scales. Here, particular points of emphasis include dynamical effects in the vicinity of the contact region and the time scales on which mechanical and thermal mechanisms operate.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2016;139(1):011601-011601-12. doi:10.1115/1.4033097.

AA5052/ZrB2 particulate aluminum matrix composites (PAMCs) have been produced by in situ reaction of K2ZrF6 and KBF4 compounds with molten alloy at about 860 °C. Dry sliding wear and friction of composites have been investigated for a particular sliding velocity and sliding distance at different loads from ambient temperature to 200 °C. It is revealed that for a particular load and temperature, wear rate and normalized wear rate decrease with increase in the volume percentage of ZrB2 particles whereas coefficient of friction (COF) shows a reverse trend. Wear rate and COF also increase with increase in temperature for a constant load and composition. Whereas with load for a particular temperature, wear rate and wear rate per unit vol. % ZrB2 increase while COF decreases. Worn surface and wear debris morphology examined under scanning electron microscopy (SEM) and profilometer to understand the wear mechanism revealed that wear mode transition takes place from mild-oxidative to severe-metallic at 100 °C for unreinforced alloy, whereas a shifting is observed in transition temperature from 100 to 150 °C for composite with 9 vol. % ZrB2 particles. Energy dispersive spectroscopy (EDS) analysis of worn surface confirms the oxidative wear mode. Profilometry results indicate that wear surface has higher surface roughness at higher values of load and temperatures. Prior to wear and friction studies, composites were also characterized by X-ray diffraction (XRD) and SEM for morphology and microstructural characteristics to correlate with wear results. The findings are very helpful to make the AA5052/ZrB2 composites suitable for the applications, where high-temperature wear is a limiting factor.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2016;139(1):011701-011701-9. doi:10.1115/1.4032971.

An analytical numerical model to optimize the shape of concave surface texture for the achievement of low friction in reciprocating sliding motion has been developed. The model uses: (i) average Reynolds equation to evaluate friction coefficient and (ii) genetic algorithm (GA) to optimize and obtain several preferable texture shapes. Analysis of distribution contour maps of hydrodynamic pressure gives the possible mechanisms involved. Moreover, experimental comparisons of tribological performances between the optimized and the circular textures were conducted to verify the simulation results. It is shown that surface textures of the elliptical and fusiform shapes can effectively enhance the load bearing capacity and reduce the friction coefficient compared with circular textures. The increase in hydrodynamic pressure for these optimized texture shapes is considered to be the major mechanism responsible for improving their tribological performance. Experimental results confirm that the elliptical-shaped textures have preferable tribological behaviors of low friction coefficient under the operating condition of light load.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011702-011702-11. doi:10.1115/1.4033135.

In this paper, the start-up process of the ring/liner system with surface texturing is studied. By employing a thermal-mixed lubrication model considering the oil supply, the tribological behavior of the textured surface under the cold and hot start-up conditions is investigated. It is found that the friction coefficient curve under the cold start-up condition is different from the hot start-up result. The textured surface is easier to form the hydrodynamic lubrication than the smooth surface, which is helpful to separate the mixed lubricated contact surfaces. With the textured features on the ring face, the less friction heat is generated at the start-up phase. These effects could prove beneficial in applications with the frequent start and stop conditions. Besides, the inlet wedge of ring can also influence the start-up performance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011703-011703-9. doi:10.1115/1.4033361.

Hydrostatic lead screws possess superior motion characteristics and often run under high speeds and variable loads. This paper presents research on the transient motion of capillary compensated hydrostatic lead screws with continuous helical recesses under high speeds and variable loads. The model is based on the Reynolds equation, the equation of motion and the Euler equation, and takes pitch errors into account. The simulation results show that: (a) the displacement curves of the nut thread calculated under lower static loads can be obtained approximately by translating the displacement curve calculated under no load, (b) under high speeds and higher static loads, the displacement curves of the nut thread present differences, (c) under step loads, the hydrostatic nut has a smooth transient process, and (d) under sinusoidal loads, the amplitude of transient response path is smaller than the displacement of the nut thread under a static load with the same magnitude, and decreases with increase in the frequency of the sinusoidal load.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011704-011704-11. doi:10.1115/1.4033128.

This study investigates a mechanism of textured features taking into account the balance of moment termed “balancing wedge action.” The principle of the suggested mechanism is that a change in moment applied to the lubricated area by incorporating textured features promotes the entire wedge action over the lubricated area. In the current study, multiple dimples are created on the stationary surface of an infinite pad bearing. A one-dimensional incompressible Reynolds equation is solved numerically to determine the load-carrying capacity of infinite pad bearings with a centrally located pivot. Numerical results show the importance of the balancing wedge action. When multiple dimples are created at the inlet side or outlet side of the lubricated area, positive load-carrying capacity is realized. When multiple dimples are located around the central area, no balance solution is obtained for the pad. The dimple depth, width, and distribution are varied to investigate the behavior of the load-carrying capacity realized by the action of the balancing wedge.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011705-011705-11. doi:10.1115/1.4033888.

The Morton effect (ME) is a thermally induced instability problem that most commonly appears in rotating shafts with large overhung masses, outboard of the bearing span. The time-varying thermal bow due to the asymmetric journal temperature distribution may cause intolerable synchronous vibrations that exhibit a hysteresis behavior with respect to rotor speed. The fully nonlinear transient method designed for the ME prediction, in general, overhung rotors is proposed with the capability to perform the thermoelastohydrodynamic analysis for all the bearings and model the rotor thermal bow at both overhung ends with equivalent distributed unbalances. The more accurate nonlinear, coupled, double overhung approach is shown to provide significantly different response prediction relative to the more approximate linear method based using bearing coefficients and the single-overhung method, which assumes that the ME on both rotor ends can be decoupled. The flexibility of the bearing pad and pivot is investigated to demonstrate that the pivot flexibility can significantly affect the rotordynamics and ME, while the rigid pad model is generally a good approximation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011706-011706-10. doi:10.1115/1.4033892.

This paper performs the parametric studies corresponding with the theoretical Morton effect (ME) model explained in Part I of this paper, where the fully nonlinear transient analysis based on the finite element method is introduced. Operating parameters, such as oil supply temperature, bearing clearance, oil viscosity, etc., are perturbed from the testing conditions to investigate the shifting of critical speeds and ME instability onset speed (IOS). The ME is significantly affected by the rotor bending mode with large overhung deflections, and operating parameters should be adjusted to increase the separation margin between the operating speed and the corresponding critical speed for ME mitigation. Reducing the carryover flow ratio and using the asymmetric bearing pivot offset are capable to suppress the ME by reducing both the average and differential journal temperature. The heat barrier sleeve with air or ceramic isolation is designed to prevent the heat flux into the journal and can successfully mitigate the ME based on the simulations.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011707-011707-12. doi:10.1115/1.4033715.

The viscous dissipation of micropolar lubricant results in temperature increase of hole-entry hybrid journal bearing. Thermohydrostatic (THS) performance characteristics are computed by the concurrent solution of micropolar Reynolds, micropolar energy, and conduction equations. The results obtained numerically indicate that bearing is significantly affected by increase in temperature. Hence, it is essential to consider the thermal effects for bearing operating with micropolar lubricant to produce realistic bearing characteristic data.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):011708-011708-11. doi:10.1115/1.4033887.

Surface topography of sealing interface is a key factor affecting sealing performance. In industry, it has always been desirable to optimize the performance of static seals by optimizing the surface topography. The evolution of leak channels and the quantitative effects of surface topography on leak rates are expected to be clarified. This paper proposes a novel approach to calculate leak channels and leak rates between sealing surfaces for specific surface topographies, based on three-dimensional (3D) finite-element contact analysis. First, a macromechanical analysis of the entire sealing structure is conducted to calculate the interfacial pressure. Second, the surface topography data are measured and processed. Third, the interfacial pressure is used as the boundary condition applied on the microscale 3D finite-element contact model, which is built based on the specific surface topography. Fourth, the geometrical models of leak channels are extracted from the finite-element contact model, and the leak rates are calculated using the computational fluid dynamics (CFD) method. The proposed approach was applied to a hollow bolt-sealing structure. Finally, experimental results verified the accuracy and effectiveness of the proposed approach, which can provide valuable information for optimizing surface processing techniques, surface topography, and static seal performance.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2016;139(1):011801-011801-8. doi:10.1115/1.4033050.

Measurements are reported for dipentaerythritol hexaisononanoate (DiPEiC9) of pressure–volume–temperature (pVT) response to pressures to 400 MPa and temperatures to 100 °C, and of viscosity at pressures to 700 MPa and temperatures to 90 °C and shear stress to 18 MPa. These data complement the low-shear viscosities published by Harris to pressures to 200 MPa and the compressions by Fandiño et al. to 70 MPa. The improved Yasutomi correlation reproduces all viscosity measurements with accuracy better than the Doolittle free volume and the Bair and Casalini thermodynamic scaling models which require an equation of state (EoS). The interaction parameter for thermodynamic scaling, γ = 3.6, is less than that reported by Harris (γ = 4.2) and the difference is primarily in the choice of EoS. The shear stress at the Newtonian limit, about 6 MPa, is exceptionally large given the high molecular weight of DiPEiC9. The large Newtonian limit is also seen in the oscillatory shear response.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2016;139(1):012001-012001-6. doi:10.1115/1.4033188.

The current study is aimed to investigate the tribological properties of ultrahigh molecular weight polyethylene (UHMWPE) reinforced with organoclay Cloisite (C15A). Nanocomposites are prepared using a high energy ball milling process followed by hot pressing. Three different loadings of 0.5 wt.%, 1.5 wt.%, and 3 wt.% of C15A, respectively, are used as reinforcement. Results from the ball-on-disk wear tests showed that nanocomposites reinforced with 1.5 wt.% of C15A exhibited best wear resistance and lower coefficient of friction (COF), with C15A reducing the wear rate by 41% and the COF by 38%, when compared to the pristine UHMWPE. These improvements are attributed to the uniform dispersion of the nanosized clay platelets preventing large-scale material removal and formation of a thin tenacious, continuous transfer film on the counterface for C15A organoclay composites. X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical profilometry are used to characterize the morphology of the nanocomposites and the wear tracks. SEM images of worn surfaces indicated more abrasive wear for the case of pristine UHMWPE as compared to organoclay composites.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2016;139(1):012201-012201-8. doi:10.1115/1.4033101.

The objectives of this investigation were to develop a coupled dynamic model for turbocharger ball bearing rotor systems, correlate the simulated shaft motion with experimental results, and analyze the corresponding bearing dynamics. A high-speed turbocharger test rig was designed and developed in order to measure the dynamic response of a rotor under various operating conditions. Displacement sensors were used to record shaft motion over a range of operating speeds. To achieve the objectives of the analytical investigation, a discrete element angular contact ball bearing cartridge model was coupled with an explicit finite element shaft to simulate the dynamics of the turbocharger test rig. The bearing cartridge consists of a common outer ring, a pair of split inner races, and a row of balls on each end of the cartridge. The dynamic cartridge model utilizes the discrete element method in which each of the bearing components (i.e., races, balls, and cages) has six degrees-of-freedom. The rotor is modeled using the explicit finite element method. The cartridge and rotor models are coupled such that the motion of the flexible rotor is transmitted to the inner races of the cartridge with the corresponding reaction forces and moments from the bearings being applied to the rotor. The coupled rotor–cartridge model was used to investigate the shaft motion and bearing dynamics as the system traverses critical speeds. A comparison of the analytical and experimental shaft motion results resulted in minimal correlation but showed similarity through the critical speeds. The cartridge model allowed for thorough investigation of bearing component dynamics. Effects of ball material properties were found to have a significant impact on turbocharger rotor and bearing dynamics.

Commentary by Dr. Valentin Fuster
J. Tribol. 2016;139(1):012202-012202-10. doi:10.1115/1.4033131.

In this research work, pulverized biochar obtained by the pyrolysis of rice husk is used as particulate reinforcement in unsaturated polyester matrix. The effects of the particle loading and particle size on tribological properties of the particulate composites were investigated. The average size of biochar particles obtained through pulverizing using ball-mill varied from 510 nm to 45 nm while milling for a duration ranging from 6 hrs to 30 hrs. The particle loading in the composite was varied from 0.5 wt.% to 2.5 wt.%. It was observed that the particle size and particle content played a vital role in the tribological properties of the composites. The specific wear rate of the specimen having particle loading of 2.5 wt.% with 45 nm particle size exhibited a decrease of 56.36% upon comparing with the specific wear rate of cured pure resin. The coefficient of friction of the same sample decreased by 6.42% when compared to that of a cured pure resin. The biochar particles were subjected to X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and atomic force microscope analysis for characterization. Morphological studies were performed on the worn surfaces by scanning electron microscope (SEM) and optical microscopy.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2016;139(1):014501-014501-5. doi:10.1115/1.4033049.

In this technical brief, a Cr3Si nanocrystalline film was deposited on 304 stainless steel (SS) substrate using a double glow discharge plasma technique. The film was characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, nanohardness tester, and scratch tester. The as-deposited film with a thickness of 5 μm consisted of A15 structured Cr3Si phase with an average grain size of 8 nm. The hardness values of the film were determined to be 26 GPa, which was ten times greater than 304 SS. A self-designed ultrasonic vibration cavitation erosion apparatus was employed to evaluate the cavitation erosion resistance of the Cr3Si film. The results showed that after cavitation tests of 30 hrs, the erosion mass loss of the film was only 60% of that for 304 SS substrate. SEM observation of the erosion surfaces indicated that the surface damage degree of the Cr3Si film is significantly less than that of 304 SS.

Commentary by Dr. Valentin Fuster

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