| 1. |
- AlMotasem, Ahmed Tamer, et al.
(författare)
-
Atomistic insights on the wear/friction behavior of nanocrystalline ferrite during nanoscratching as revealed by molecular dynamics
- 2017
-
Ingår i: Tribology letters. - New York : Springer. - 1023-8883 .- 1573-2711. ; 65:3, s. 101-
-
Tidskriftsartikel (refereegranskat)abstract
- Using embedded atom method potential, extensive large-scale molecular dynamics (MD) simulations of nanoindentation/nanoscratching of nanocrystalline (nc) iron have been carried out to explore grain size dependence of wear response. MD results show no clear dependence of the frictional and normal forces on the grain size, and the single-crystal (sc) iron has higher frictional and normal force compared to nc-samples. For all samples, the dislocation- mediated mechanism is the primary cause of plastic deformation in both nanoindentation/nanoscratch. However, secondary cooperative mechanisms are varied significantly according to grain size. Pileup formation was observed in the front of and sideways of the tool, and they exhibit strong dependence on grain orientation rather than grain size. Tip size has significant impact on nanoscratch characteristics; both frictional and normal forces monotonically increase as tip radii increase, while the friction coefficient value drops by about 38%. Additionally, the increase in scratch depth leads to an increase in frictional and normal forces as well as friction coefficient. To elucidate the relevance of indentation/scratch results with mechanical properties, uniaxial tensile test was performed for nc-samples, and the result indicates the existence of both the regular and inverse Hall-Petch relations at critical grain size of 110.9 angstrom. The present results suggest that indentation/scratch hardness has no apparent correlation with the mechanical properties of the substrate, whereas the plastic deformation has.
|
|
| 2. |
- Bair, Scott, et al.
(författare)
-
Comment on “History, Origins and Prediction of Elastohydrodynamic Friction” by Spikes and Jie
- 2015
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 58:1
-
Tidskriftsartikel (refereegranskat)abstract
- Progress in the classical field of EHL has for decades been paralyzed by the assumption that shear thinning should be indistinguishable from the shear dependence of the viscosity of a liquid heated by viscous dissipation and that the parameters of this simple shear dependence can be obtained from the shape of a friction curve. In the last few years, by abandoning this assumption and employing real viscosity measured with viscometers, there has been revolutionary progress in predicting film thickness and friction. Now, Spikes and Jie conclude that the previous assumption has as much merit as the use of viscosity measured in viscometers. This suggestion may be popular among those who wish to ignore viscometer measurements in favor of extracting properties from friction curves. However, within the subject article, there are numerous misstatements of fact and misrepresentations by omission, and the recent progress using real viscosity is not acknowledged. The debate has degenerated into a friction curve fitting competition which is not helpful. The great progress of the last few years would not have been possible using the concepts and methods espoused in this article
|
|
| 3. |
- Bennett, A. I., et al.
(författare)
-
Contact Measurements of Randomly Rough Surfaces
- 2017
-
Ingår i: Tribology letters. - : Springer Science and Business Media, LLC. - 1023-8883 .- 1573-2711. ; 65:4
-
Tidskriftsartikel (refereegranskat)abstract
- This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).
|
|
| 4. |
- Bennett, A. I., et al.
(författare)
-
Deformation Measurements of Randomly Rough Surfaces
- 2017
-
Ingår i: Tribology letters. - : Springer Science and Business Media, LLC. - 1023-8883 .- 1573-2711. ; 65:4
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap.
|
|
| 5. |
- Björling, Marcus, 1983-, et al.
(författare)
-
DLC and Glycerol : Superlubricity in Rolling/Sliding Elastohydrodynamic Lubrication
- 2019
-
Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 67:1
-
Tidskriftsartikel (refereegranskat)abstract
- Low friction is one of the most important parameters for the development of machine components and machines with high efficiency. Many of the common machine components of today such as gears, rolling element bearings and cam-followers are defined by their non-conformal contacts leading to high-contact pressures, typically 1–4 GPa. The lubrication of such contacts is usually called elastohydrodynamic lubrication (EHL). Diamond-like carbon (DLC) coatings and glycerol have individually been shown to produce low friction in boundary, mixed and full film lubrication. A few studies have been conducted using both glycerol and DLC-coated surfaces to achieve even lower friction in pure sliding boundary-lubricated contacts. However, the literature is lacking studies of how the combination of glycerol and DLC performs in non-conformal rolling/sliding contacts where many common machine components operate. Such a study is presented in this article where a ball-on-disc test rig is used to investigate the performance of the combination of DLC and glycerol at pressures up to 1.95 GPa at various entrainment speeds and slide-to-roll ratios. The investigation shows that the DLC-glycerol combination provides very low friction values, in some cases, below the superlubricity threshold of 0.01, possibly shown for the first time at such high pressure in a non-conformal rolling/sliding contact. The low friction mechanism in full film lubrication is a combination of the low pressure-viscosity and high temperature-viscosity sensitivity of glycerol in combination with thermal insulation of the DLC coating and is presented as thermally assisted liquid superlubricity.
|
|
| 6. |
- Broitman, Esteban
(författare)
-
Indentation Hardness Measurements at Macro-, Micro-, and Nanoscale: A Critical Overview
- 2017
-
Ingår i: Tribology letters. - : SPRINGER/PLENUM PUBLISHERS. - 1023-8883 .- 1573-2711. ; 65:1
-
Forskningsöversikt (refereegranskat)abstract
- The Brinell, Vickers, Meyer, Rockwell, Shore, IHRD, Knoop, Buchholz, and nanoindentation methods used to measure the indentation hardness of materials at different scales are compared, and main issues and misconceptions in the understanding of these methods are comprehensively reviewed and discussed. Basic equations and parameters employed to calculate hardness are clearly explained, and the different international standards for each method are summarized. The limits for each scale are explored, and the different forms to calculate hardness in each method are compared and established. The influence of elasticity and plasticity of the material in each measurement method is reviewed, and the impact of the surface deformation around the indenter on hardness values is examined. The difficulties for practical conversions of hardness values measured by different methods are explained. Finally, main issues in the hardness interpretation at different scales are carefully discussed, like the influence of grain size in polycrystalline materials, indentation size effects at micro-and nanoscale, and the effect of the substrate when calculating thin films hardness. The paper improves the understanding of what hardness means and what hardness measurements imply at different scales.
|
|
| 7. |
- Cha, Yingying, et al.
(författare)
-
Airborne Wear Particles Generated from Conductor Rail and Collector Shoe Contact : Influence of Sliding Velocity and Particle Size
- 2016
-
Ingår i: Tribology letters. - : Springer-Verlag New York. - 1023-8883 .- 1573-2711. ; 64:3
-
Tidskriftsartikel (refereegranskat)abstract
- The mechanical wear of train components is one of the main sources of airborne particles in subway air. A certain contribution is suspected to derive from third-rail systems due to the sliding of two metallic surfaces between conductor rail and collector shoe during operation. In this study, a pin-on-disc apparatus was used to simulate the friction between such two sliding partners (shoe-to-rail). Airborne particles generated from the sliding contact were measured by particle counters (a fast mobility particle sizer spectrometer and an optical particle sizer) and were collected by an electrical low-pressure impactor for physical and chemical analysis. Interface temperature for each test was measured by a thermocouple. The influence of sliding velocity and temperature on particulate number concentration, size distribution, and chemical composition was investigated. Atomic absorption spectroscopy, cyclic voltammetry, and energy-dispersive spectroscopy measurements were carried out to determine the chemical compositions. Results show that increasing sliding velocity results in a higher temperature at the frictional interface and a higher concentration of ultrafine particles. The ratio of manganese to iron surface oxides increased strongly with smaller particle size. A copper compound was observed in some particle samples, probably gerhardite (Cu2NO3(OH)(3)) formed due to high temperature.
|
|
| 8. |
- Deng, Liang, 1986-, et al.
(författare)
-
Development of a Tribological Test Programme Based on Press Hardening Simulations
- 2017
-
Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 65:2
-
Tidskriftsartikel (refereegranskat)abstract
- Press hardening is widely utilized to form ultra-high-strength steels characterized by a high strength-to-weight ratio for automotive components. Press hardening processes include heating boron–manganese steels to austenite phase, forming the steels at a high temperature, and cooling the formed blanks until the martensite phase is reached . However, press hardening processes lead to severe contact conditions between the blank and the tools including contact pressure, relative sliding, and high temperatures, which result in tool wear and increased maintenance cost. The contact conditions that occur in the stamping tool are difficult to study on site. Additionally, simplified tests, such as pin on disc and ball on disc, are insufficient to reproduce press hardening conditions in laboratory environments . The aim of this study includes developing a tribological test with press hardening conditions in which tool steel pins continuously slide on fresh and hot boron–manganese steel strips. The test programme mimics press hardening conditions with respect to sliding distance, sliding velocity, contact pressure, and surface temperature that were studied based on finite element (FE) simulations of a press hardening experiment. Furthermore, a FE simulation of the tribological test is established and it provides contact temperature in the pin tip with a high accuracy. A tribological test is used to study friction and mass loss with variational pressures and velocities that represented typically variational contact conditions in the press hardening. The tribological test is also used to obtain correlations between the tribological behaviours and process parameters in press hardening including pressure and sliding velocity.
|
|
| 9. |
- Deng, Liang, 1986-, et al.
(författare)
-
Experimental Evaluation of Galling Under Press Hardening Conditions
- 2018
-
Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 66:3
-
Tidskriftsartikel (refereegranskat)abstract
- Severe adhesion, also referred to as galling, is a critical problem in press hardening, especially in stamping tools used for hot forming of Al–Si-coated ultra-high strength steel. Galling is known to develop rapidly on the tool surface and it negatively affects the quality of the formed products. Earlier research on this topic has focused on the galling initiation. However, studies on the galling development during extended sliding and the corresponding quantitative measurement still lack depth. In the present study, a tribological test is established to study the galling development under press hardening conditions. The tribological test set-up aims to simulate extended sliding between the Al–Si-coated boron steels and the tool die material. The contact conditions in the interface are studied by a numerical model of the tribological test. The friction coefficients and material transfer are discussed taking into account the variation of the different test conditions. Using the results from the tribological tests, the galling simulation is performed in the numerical model. A geometry-updated sample based on the galling (transferred material build-up) height is simulated and the consequent pressure fluctuation is obtained in the numerical model. This contributes to the explanation of the severe transferred material accumulation during the test.
|
|
| 10. |
- Harris, Kathryn L, et al.
(författare)
-
Janus Blocks : A Binary System Wear Instability
- 2016
-
Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 63:1
-
Tidskriftsartikel (refereegranskat)abstract
- In this manuscript, a simple binary model is devised that describes the wear behavior of two blocks coupled under a constant, dynamically partitioned normal load. In this simple system, the frictional force is reacted by two independent springs and the blocks are allowed to move and wear independently based on system dynamics and kinematics. The only coupling between the blocks occurs through the partitioning of the applied normal load, which uses a pair of springs in parallel to model elasticity. This system is found to preferentially wear one of the blocks until two disparately unique conditions of steady wear are reached in the system: (1) a condition in which the partitioning of the load between the blocks yields equal wear and thus steady partitioning of the load and (2) a condition in which the pair of blocks go to zero wear by having one block not sliding but carrying all of the load and the other block completely slipping but carrying none of the load. These “Janus blocks,” the simplest of binary spring–block systems, begin life in a nominally identical state and then their behavior bifurcates, producing runaway or irregular wear. The onset of this instability can initiate from any differences in load partitioning, spring constants, friction coefficient, or wear rates (no matter how small).
|
|
