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Research Papers: Coatings and Solid Lubricants

J. Tribol. 2018;140(6):061301-061301-11. doi:10.1115/1.4039930.

This study sought to investigate the tribological properties of nitrile rubber (NBR)/ultrahigh molecular weight polyethylene (UHMWPE)/nano-molybdenum disulfide (nano-MoS2) nanocomposites containing various quantities of nano-MoS2. The apparatus used for these tests was a marine stern tube bearing testing apparatus SSB-100V that was water-lubricated and was run at low speed under heavy duty conditions. The coefficient of friction coefficient (COF), wear rate, and surface abrasion of the composite were obtained to determine the effect of the addition nano-MoS2 and to obtain the optimum nano-MoS2 content. The mechanical and physical properties of the rubber-plastic material met the requirements of the Chinese Ship standard CB/T769-2008 and U.S. military standard MIL-DTL-17901C(SH). The experimental results showed that the nanocomposites that contained 9 phr nano-MoS2 (parts by weight per hundred parts of rubber materials) exhibited good comprehensive friction and wear properties. It is believed that the experience achieved from this study can form a theoretical foundation for the improving the properties of the subject rubber-plastic material.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2018;140(6):061401-061401-10. doi:10.1115/1.4040052.

Carburized gears are applied extensively in large-scale heavy duty machines such as wind turbines. The carburizing and quenching processes not only introduce variations of hardness from the case to the core but also generate a residual stress distribution, both of which affect the rolling contact fatigue (RCF) during repeated gear meshing. The influence of residual stress distribution on the RCF risk of a carburized wind turbine gear is investigated in the present work. The concept of RCF failure risk is defined by combining the local material strength and the multi-axial stress condition resulting from the contact. The Dang Van multi-axial fatigue criterion is applied. The applied stress field is calculated through an elastic-plastic contact finite element model. Residual stress distribution and the hardness profile are measured and compared with existed empirical formula. Based upon the Pavlina–Tyne relationship between the hardness and the yield strength, the gradient of the local material strength is considered in the calculation of the RCF failure risk. Effects of the initial residual stress peak value and its corresponding depth position are studied. Numerical results reveal that compressive residual stress (CRS) is beneficial to RCF fatigue life while tensile residual stress (TRS) increases the RCF failure risk. Under heavy load conditions where plasticity occurs, the accumulation of the plastic strain within the substrate is significantly affected by the initial residual stress distribution.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061402-061402-9. doi:10.1115/1.4040051.

Rolling contact fatigue (RCF) induces a complex subsurface stress state, which produces significant microstructural alterations within bearing steels. A novel modeling approach is presented in this paper, which investigates the effects of microstructural deterioration, phase transformations, and residual stress (RS) formation occurring within bearing steels subject to RCF. The continuum damage mechanics approach was implemented to capture microstructural decay. State and dissipation functions corresponding to the damage mechanics process were used via an energy criterion to predict the phase transformations of retained austenite (RA). Experimental measurements for RA decomposition and corresponding RS were combined to produce a function providing RS formation as a function of RA decomposition and stress history within the material. Microstructural decay, phase transformations, and internal stresses were implemented within a two-dimensional (2D) finite element analysis (FEA) line contact model to investigate variation in microstructural alterations due to RSs present within the material. In order to verify the model developed for this investigation, initial simulations were performed implementing conditions of previously published experimental work and directly comparing to observed RA decomposition and RS formation in 52100 steel deep groove ball bearings. The finite element model developed was then used to implement various RS profiles commonly observed due to manufacturing processes such as laser-shot peening and carburizing. It was found that some RS profiles are beneficial in altering RA decomposition patterns and increasing life while others proved less advantageous.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;140(6):061501-061501-13. doi:10.1115/1.4039867.

A stochastic model for predicting the evolutions of wear profile and surface height probability density function (PDF) of initial line contacts during running-in under mixed lubrication condition is presented. A numerical approach was developed on the basis of stochastic solution of mixed lubrication, which combined the Patir and Cheng's average flow model for calculation of the hydrodynamic pressure and the Kogut and Etsion's (KE) rough surface contact model for calculation of the asperity contact pressure. The total friction force was assumed to be the sum of the boundary friction at the contact asperities and the integration of viscous shear stress in the hydrodynamic region. The wear depth on the contact region was estimated according to the modified Archard's wear model using the asperity contact pressure. Sugimura's wear model was modified and used to link the wear particle size distribution and the variation of surface height PDF during wear. In the wear process, the variations of profile and surface height PDF of initial line contacts were calculated step by step in time, and the pressure distribution, friction coefficient, and wear rate were updated consequently. The effect of size distribution of wear particles on the wear process was numerically investigated, and the simulation results showed that the lubrication condition in which small wear particles are generated from the asperity contact region is beneficial to reduce friction coefficient and wear rate, and leads to a better steady mixed lubrication condition.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;140(6):061601-061601-8. doi:10.1115/1.4039133.

The effect of the gas-nitriding thermochemical treatment on the cavitation erosion resistance of a Cr-Ni-Mo alloy is analyzed using a piezoceramic vibrating equipment and following the ASTM G32-2010 standard. The evaluation of the cavitation erosion behavior was made based on the analysis of the mean depth of erosion (MDE) and mean depth of erosion rate (MDER), for samples subjected to the cavitation erosion for different times. The surface topography and the structural changes in the marginal layer were analyzed through optical and scanning electron microscopy. Following nitriding the cavitation erosion resistance was about 9.6 times higher compared to the annealed state and about 8.2 times higher compared to the hardened and tempered state.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061602-061602-12. doi:10.1115/1.4040160.

The objective of this research work is to synthesize functionally graded Cu-11Ni-4Si/10 wt % SiC, Cu-11Ni-4Si/10 wt % Si3N4 composite using horizontal centrifugal casting method and to analyze its mechanical and adhesive wear behavior. The cast samples with dimension of Øout100 × Øin70 × 100 mm were synthesized and variation in volume of SiC and Si3N4 particles on inner (1 mm), middle (8 mm), and outer surfaces (15 mm) along radial direction of the composites was analyzed. Microstructural images revealed that inner zone of the both composites had highest distribution of reinforcement particles. Tensile tests on inner (1–7 mm) and outer (8–15 mm) zones of composites revealed that the inner zones had highest tensile and yield strength. Fractography test was conducted for both composites at inner and outer zones to observe the mode of failure. Hardness tests taken along radial direction of the composites revealed that, the inner surface had better hardness and it reduced toward outer periphery. The outer and inner surfaces of Cu/SiC were compared with Cu/Si3N4 composites and results revealed that inner surface of Cu/SiC had highest wear resistance among all surfaces of composites. It was also observed that, while increasing load, wear rate increased with it for all composites. Wear rate of composites majorly decreased while increasing the sliding velocity due to formation of tribolayer. Scanning electron microscopy (SEM) analysis carried out on worn surfaces of Cu/SiC and Cu/Si3N4 composite revealed that, plastic deformation, and plowing were the dominant wear mechanism for varied parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061603-061603-8. doi:10.1115/1.4039996.

In the present experimental study, the application of microwave heating is used to develop the composite clads of Ni-based metallic powder (matrix) and Al2O3 powder (reinforcement) on the surface of AISI 304 stainless steel substrate. A domestic microwave oven working at 2.45 GHz frequency and 900 W was used to conduct the experimental trials. The Ni + 10% Al2O3 composite clads were characterized through X-ray diffractometer (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microhardness tests. The pin-on-disk type tribometer was used for analyzing the sliding wear behavior of Ni + 10% Al2O3 clads. The microstructural results revealed the presence of randomly dispersed Al2O3 particles inside Ni matrix. The average microhardness (Vicker's) of composite clad was enhanced by 3.5 times that of the substrate. The clad exhibited 156 times more wear resistance than AISI 304 substrate. Craters and groove formation were responsible for wear loss in the clad material while plastic deformation caused the failure of AISI 304 substrate.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061604-061604-12. doi:10.1115/1.4040071.

The purpose of this study is to explore the effect of water on the contact and friction properties of a friction lining. The results show that the water absorption capacity and the sensitivity to the water molecules of the friction lining determine the load-carrying capacity. The change of the actual contact area is related to the load-carrying capacity under dripping water condition. The presence of water played a role in lubricating the surface, which resulted in a reduction of the friction coefficient. In addition, water absorbed onto the surface of the lining to produce an absorbent layer, and the load-carrying capacity of the absorbent layer exerted a more intuitive effect on the friction coefficient.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061605-061605-7. doi:10.1115/1.4040078.

In this work, two polymer materials have been tested in a lubricated reciprocating pin-on-plate contact geometry using water-based hydraulic fluids to simulate sliding conditions of seal materials used in offshore equipment. The effect of load, speed, water content of the lubricant, and soaking of the ultra-high molecular weight polyethylene (UHMWPE) and a polyketone (PK) sliding against a super-duplex stainless steel (SDSS) was studied. The results showed that for UHMWPE, an increase in normal force leads to a decrease in coefficient of friction for all velocities. While under the same sliding conditions, no relevant influence of load on friction coefficient was found for PK. On the other hand, an increase in sliding speed decreased the coefficient of friction for both materials. The effect of the water content of the hydraulic fluid on the tribological performance was also studied. In UHMWPE-SDSS system, increasing water content in the hydraulic fluid resulted in steady growth of the transfer film. One reason for this might be the decreasing lubricant viscosity, which moves the system toward the boundary lubrication regime. In addition, it was found that the incubation of both UHMWPE and PK in water-based lubricants showed a beneficial effect on friction and wear, which was explained by the change in polymer visco-elastic behavior.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;140(6):061701-061701-9. doi:10.1115/1.4039957.

In this paper, a profile design of surface texture was applied for improving the tribological performance of piston ring–cylinder pair. The design of texture pattern was implemented by numerous textures of different depths in sliding speed direction to imitate the outline of barrel-shaped ring. The thickness and pressure distribution of oil film were obtained through a joint solution of modified Reynolds equation and other governing equations for textured surface. The results indicate that the novel texture pattern has the best overall performance in oil film thickness and friction force compared with normal surface textures, and the average friction power is 10.04% and 16.85% less than normal surface textures and barrel-shaped ring in the whole working cycle. Extra microhydrodynamic lubrication can be observed in the middle region of textured piston ring through the pressure distribution of oil film. At last, the experiment was conducted in a motored test rig and exhibited up to 3% reduction in friction power.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061702-061702-8. doi:10.1115/1.4039958.

This paper aims to improve the tribological performance of journal bearings by optimizing the coverage area of circular microtextures in divergent region of the sleeve. A numerical model is proposed to calculate the friction coefficient and bearing load of textured journal bearings. The surface of the sleeve is divided into rectangular squares. Textures that located at the center of rectangular grids are assumed to be present or absent, marked as 1 and 0, respectively. Afterward, different texture coverage area arrangements are evolved and selected based on the genetic algorithm (GA). The area of semi-elliptical shape is obtained as the novel and preferable textured coverage area design for journal bearings. Influences of width and eccentricity ratio are discussed, which confirm the semimajor and semiminor axes of the semi-elliptical shape of texture coverage area equal to one-third of the circumferential length and half of the width of the journal bearing, respectively.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061703-061703-9. doi:10.1115/1.4040114.

The following paper focuses on an experimental and analytical study aimed at identifying the dynamic force coefficients of hydrostatic gas films for recessed flat plates. The motivation for the effort was brought upon by the necessity of generating more accurate models for hydrostatic gas films found in hybrid gas bearings. Pressurized air at room temperature up to 120 psi was used to test different recess geometries on a flat plate test rig, capable of characterizing the stiffness and damping force coefficients for varying supply pressures, gas film thickness values, excitation frequencies, and vibration amplitudes. The test rig design and operation is described. Experimental results include frequency-dependent stiffness and damping coefficients, and leakage. The test results show that using external pressurization can generate large stiffness values while exhibiting small leakage. However, the results also show that the majority of the test configurations portray high negative damping values. An analytical model is presented and numerical predictions are compared to experimental results. Example damping trends as a function of frequency, pressure, and film thickness are presented in addition to force coefficient plots as functions of pressure ratio.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):061704-061704-8. doi:10.1115/1.4040149.

Methods for scalable surface texturing continue to receive significant attention due to the importance of microtextured surfaces toward improving friction, wear, and lubrication ability of mechanical devices. Controlled textures on surfaces act as fluid reservoirs and receptacles for debris and wear particles, reducing friction and wear of mating components. There are numerous fabrication techniques that can be used to create microsized depressions on surfaces, but each has limitations in terms of control and scalability. In the present study, modulation-assisted machining (MAM) is demonstrated as a viable approach to produce such textures, offering a potentially cost-effective approach for scalable production of these features on component surfaces. In this work, the wear behavior of several textured surfaces created by MAM was studied using a ball-on-flat reciprocating tribometer. Textured and untextured alloy 360 brass disks were mated with stainless steel AISI 440C balls under lubricated conditions and variable sliding distance. The textured surfaces exhibited noticeably reduced wear under the longer sliding distances and the tribological performance of the surfaces depended on the size of the microdimples. Wear mechanisms are elucidated from the optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) observations and the implications for using such surfaces in practice are briefly discussed.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2018;140(6):064501-064501-5. doi:10.1115/1.4040115.

A finite element analysis was used to study the onset of plasticity of a coated sphere compressed by a rigid flat. This was done for soft coatings on a harder substrate. Generally, the results agree very well with the findings in the literature for the opposite case of an indentation of a rigid sphere into a coated flat, with a soft coating. In this case, a weakening prevails over the entire range of coating thicknesses, resulting in critical loads (at yield inception) lower than the critical load of the uncoated contact. However, a surprising strengthening effect was discovered, in this study, for very thin coating thicknesses resulting in a higher resistance to plasticity, compared to an uncoated sphere. This phenomenon resembles a mirror image of the opposite weakening effect which was reported for very thin hard coatings. These results may suggest that when very thin coatings are to be applied in a spherical contact, a softer, rather than harder, coating should be considered in order to increase the contact's resistance to plasticity inception.

Topics: Coatings , Stress , Plasticity
Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(6):064502-064502-7. doi:10.1115/1.4040154.

In this technical brief, we present detailed finite element simulations of a sealing system operating in quasi-static conditions, in the framework of the real piston actuator of a landing gear braking system. Numerical results show two peaks of the contact pressure on the rod, and demonstrate that this contact pressure remains larger than that in the fluid chamber. These numerical results are qualitatively validated by scanning electron microscopy (SEM) observations of a worn sealing system. Overall, this study shows the benefits of numerical simulation in geometrical design of sealing systems targeting a given contact pressure at the rod/seal interface.

Commentary by Dr. Valentin Fuster

Discussion

J. Tribol. 2018;140(6):065501-065501-1. doi:10.1115/1.4040013.

The authors have to be commended for a concise description of their noteworthy approach to solving the transient EHL problem [1]. The significance of the work can probably be summarized by three remarks: (1) the problem statement is comprehensive and precise, (2) the solution is very efficient (with no iteration required) and robust (despite the highly nonlinear nature of EHL problems), and (3) the solution is correct and technically meaningful, as proven by ample comparison to the experimental results. Based on the descriptions given in the paper and in Ref. [3], we independently implemented the algorithm using our own finite element models and came—within what can be assumed to be numerical differences—to identical results. Having done so, we can state that the given “recipe” is complete and ready for use. Care should be taken in the choice of initial conditions, as outlined by the authors. We would like to add that using the methodology promises the potential to fill the gaps in approximate solutions of the “normal approach” EHL problem. It seems that, although squeeze velocities are often non-negligible (e.g., in cam-and-follower problems), available approximate solutions for their analysis cover only the sliding and rolling aspects.

Commentary by Dr. Valentin Fuster

Closure

J. Tribol. 2018;140(6):066001-066001-2. doi:10.1115/1.4040014.

The authors thank Graf and Rienäcker for their kind words. It is encouraging to learn that the algorithm laid out earlier [1] and elaborated in this paper [2] has been independently and successfully implemented by others on the basis of those descriptions alone. It is also important to note that this additional implementation made use of their own solid and fluid finite elements not reported in either paper noted (or in Ref. [3]).

Commentary by Dr. Valentin Fuster

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