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Research Papers: Applications

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

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

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

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

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

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

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

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

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

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

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

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

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

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

Commentary by Dr. Valentin Fuster

Technical Brief

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

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

Commentary by Dr. Valentin Fuster

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