| 1. |
- Alfredsson, Bo, et al.
(författare)
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An Elastic-Plastic Model for Single Shot-Peening Impacts
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 52:2, s. 231-251
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Tidskriftsartikel (refereegranskat)abstract
- A model was developed for impacts of elastic perfectly plastic spherical particles with impact velocities up to 250 m/s. The model is based on the two master curves, for normalized pressure and projected contact area c (2), which both are functions of the representative strain I > at maximum impact. The model and its parameters were fitted to finite element results for elastic perfectly plastic and strain rate-independent materials. It was applied to a wide range of materials with different ratio between yield stress and elastic properties, different ball sizes and impact velocities. The impact model predicted the results from finite element method for contact radius, maximum impact depth in both target and ball as well as remaining impact depth in target and ball. The remaining impact depth was determined from elastic spring back with Hertzian and quadratic pressure at maximum impact. The rebound velocity was also estimated by following the load-deformation path during spring back. If the strain rate-compensated yield stress was used for the master curve parameters, then the model predicted the impact results also for the investigated strain rate-dependent materials.
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| 2. |
- Alvarez-Asencio, Rubén, et al.
(författare)
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Tribological Properties Mapping : Local Variation in Friction Coefficient and Adhesion
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 50:3, s. 387-395
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Tidskriftsartikel (refereegranskat)abstract
- Tribological properties mapping is a new technique that extracts friction coefficient and adhesion maps obtained from lateral atomic force microscope (LAFM) images. By imaging the surface systematically as a function of load, a series of images can be tiled, and pixelwise fitted to a modified Amontons' Law to obtain friction coefficient and adhesion maps. This removes the ambiguity of friction contrast in LAFM imaging which can be a function of the load used for imaging. In ambient laboratory, air and tetradecane, a sample of Vancron(A (R))40, commercial powder metallurgical tool alloy containing nitrogen, have been scanned using a standard silicon cantilever in order to obtain tribological data. The tribological properties mapping provides unique information regarding the heterogeneous alloy microstructure as well as shedding light on the tribological behavior of the alloy.
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| 3. |
- Bergseth, Ellen, et al.
(författare)
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Effect of Gear Surface and Lubricant Interaction on Mild Wear
- 2012
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 48:2, s. 183-200
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Tidskriftsartikel (refereegranskat)abstract
- In this study, a twin-disc test machine was used to simulate a rolling/sliding gear contact for three surface finishes, each run with two types of lubricants, thus seeking to develop insight into the tooth flank/lubricant tribological system. The test disc surfaces were case-carburised before the surfaces were produced by: transverse grinding followed by a mechanical abrasive polishing process; transverse grinding only; and transverse grinding followed by preheating as a final finishing step (intended to enhance the build-up of an easily sheared surface boundary layer using a sulphur additive). The twin-disc contact was lubricated with an ester-based environmentally adapted lubricant or a polyalphaolefin-based commercial heavy truck gearbox lubricant. To obtain information about the composition of chemically reacted surface layers, the specimens used were analysed using glow discharge-optical emission spectroscopy. The results indicate that the interactions between different surface finishes and lubricants have different impacts on friction behaviour, wear and the reacted surface boundary layer formed by the lubricant. Running a smooth (polished) surface with the appropriate lubricant drastically reduces the friction. Surface analysis of the ground surfaces gives clear differences in lubricant characteristics. The commercial lubricant does not seem to react chemically with the surface to the same extent as the EAL does. Micropitting was found on all ground discs with both lubricants, though at different rates. The highest amount of wear but less surface damage (i.e. micropits) was found on the preheated surface run with the commercial lubricant.
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| 4. |
- Björling, Marcus, et al.
(författare)
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Friction reduction in elastohydrodynamic contacts by thin-layer thermal insulation
- 2014
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 53:2, s. 477-486
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Tidskriftsartikel (refereegranskat)abstract
- Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatings
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| 5. |
- Björling, Marcus, et al.
(författare)
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The effect of DLC coating thickness on elstohydrodynamic friction
- 2014
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 55:2, s. 353-362
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Tidskriftsartikel (refereegranskat)abstract
- The application of surface coatings has been shown to reduce friction in elastohydrodynamic lubrication (EHL), not only in the mixed and boundary regime when asperity interactions occur, but also in the full film regime. Several studies suggest that the full film friction reduction is due to a violation of the no-slip boundary condition and thus slip is taking place between the solid and the liquid. Another hypothesis proposes that the full film friction reduction is due to the low thermal conductivity of diamond-like carbon (DLC) coatings. In this work, two DLC coatings with the same composition, but different thicknesses, are investigated with uncoated steel specimens as a reference, all with the same surface roughness. Friction tests in a ball-on-disk machine show that both coatings reduce friction compared to the uncoated reference case in full film EHL. The thicker coating is significantly more effective at reducing friction than the thinner one at a maximum friction reduction of 41 % compared to 29 % for the thinner coating. Moreover, contact angle measurements, surface energy measurements, and spreading parameter calculations show no statistically significant differences between the two coatings, suggesting that the friction reduction capabilities of coatings in full film EHL cannot be described by solid-liquid interactions alone. The difference in friction reduction between the specimens in this work is mainly attributed to different thermal properties.
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| 6. |
- Björling, Marcus, et al.
(författare)
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The influence of DLC coating on EHL friction coefficient
- 2012
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 47:2, s. 285-294
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Tidskriftsartikel (refereegranskat)abstract
- High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Björling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used
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| 7. |
- Gåård, Anders, 1977-, et al.
(författare)
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Influence of Tool Material and Surface Roughness on Galling Resistance in Sliding Against Austenitic Stainless Steel
- 2012
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Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 46:2, s. 179-185
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Tidskriftsartikel (refereegranskat)abstract
- Transfer and accumulation of adhered sheet material, generally referred to as galling, is a major cause for tool failure in sheet metal forming. In the present work, the galling resistance of three different tool materials was evaluated in lubricated sliding against austenitic stainless steel using a SOFS tribometer. All tool materials were prepared to four different surface roughnesses, ranging from a polished surface with R (a) = 0.05 mu m to a ground surface with R (a) = 0.3 mu m. The overall best performance was obtained for polished nitrogen alloyed powder metallurgy (PM) tool steel, where galling was detected only at the highest load evaluated, 700 N. However, for both the D2 type tool steel and nodular iron, best performance was observed for the surface possessing a surface roughness of 0.1 mu m. The improved galling resistance for the rougher surfaces was related to filling of grinding scratches with sheet material during the initial stage of sliding, prolonging the development of protruding sheet material on the tools surface. Similar trend was not observed for the PM steel, which was related to width of the scratches originating from the surface preparation, in relation to tool microstructure.
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| 8. |
- Heinrichs, Jannica, et al.
(författare)
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Initiation of Galling in Metal Forming : Differences Between Aluminium and Austenitic Stainless Steel Studied In Situ in the SEM
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 50:3, s. 431-438
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Tidskriftsartikel (refereegranskat)abstract
- High friction and transfer of work material to tool surfaces constitute important industrial problems in forming of many metals and alloys. However, it is very hard to gain a deeper understanding of these phenomena by studying real forming operations. In this paper, we have tried to gain fundamental understanding by avoiding as much as possible of the complexity of real forming. This has been realised by studying the friction and material transfer between well-defined tool material surfaces; uncoated and DLC-coated tool steel, and a needle shaped austenitic stainless steel tip, in situ in the SEM. The tool materials were tested in two conditions; well polished and well polished with local intentional scratches. It was found that work material was immediately transferred to the tool steel surface. When passing an intentional scratch, the local transfer was on a much larger scale, and the friction was higher, but the effect was mostly local. For the polished DLC-coated surface, almost no work material was transferred and the friction was low. An intentional scratch in the polished DLC surface barely influenced the galling behaviour. The present results are discussed in the light of previously published results from an analogous study with aluminium as work material.
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| 9. |
- Heinrichs, Jannica, et al.
(författare)
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Mechanisms of transfer of aluminium to PVD-coated forming tools
- 2012
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 46:3, s. 299-312
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Tidskriftsartikel (refereegranskat)abstract
- Cold forging is a group of methods effectively used in a number of industrial applications to form aluminium. Tool life and complexity of the possible shapes are mainly restricted by galling; the uneven tool surfaces caused by lumps of transferred material lead to surface damage on successive parts formed and/or problems in successive forming operations, and high stresses occurring when forming complex shapes. Earlier investigations have shown that improved surface roughness of an uncoated steel tool decreases the amount of adhered aluminium on the tool, but does not decrease the friction in the long run. Testing has also shown that when using conventional forming tools it is very important to have sufficient lubrication to avoid galling. Addition of an optimized low-friction PVD-coating decreases the galling tendency and increases the tolerance to insufficient lubrication and lubricant film breakthrough, as long as the coating surface is smooth, whilst other similar coatings have little or no effect. The present article further investigates the mechanism behind the transfer of aluminium to the coatings. All coatings were found to be intact after testing, and so the differences were concluded to lie in the chemical properties of the coating and, if these are favourable, then in surface roughness on the micro scale.
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| 10. |
- Jelagin, Denis, et al.
(författare)
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Nonlocal Frictional Effects at Indentation of Elastic Materials
- 2013
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 51:3, s. 397-407
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Tidskriftsartikel (refereegranskat)abstract
- Indentation of elastic materials is investigated numerically using the finite element method. Large deformation theory is relied upon for accuracy. The study focuses on nonlocal frictional effects on relevant indentation quantities in the microindentation regime. The indentation quantities investigated include both local and global ones. It is shown that nonlocal frictional effects are small when global quantities are at issue, as is the case when conventional (Coulomb) theory of friction is used, also when these features are introduced at the ridges of a Vickers indenter where stress gradients are substantial. These effects are, however, shown to be of importance for particular indenter geometries as far as local field variables are concerned.
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| 11. |
- Jenei, István Zoltán, et al.
(författare)
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Correlation Studies of WS2 Fullerene-Like Nanoparticles Enhanced Tribofilms : A Scanning Electron Microscopy Analysis
- 2013
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 51:3, s. 461-468
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Tidskriftsartikel (refereegranskat)abstract
- The beneficiary effects of tungsten disulphide (WS2) inorganic fullerene-like nanoparticles (IFLNPs) in the lubrication industry were shown in recent years. However, their successful incorporation into lubricants (oils, greases) is not straightforward. In practice, the lubricant contains several components for different purposes, e.g. reducing the oxidization of the oil (antioxidant), minimizing the wear rate (anti-wear additive), dispersants, etc. These additives can contain chemically active compounds, which under the lubrication process (where locally extreme conditions can develop: high pressure and flash temperatures) can change the chemistry in the contact zone and block the beneficial effects of the inorganic nanoparticles. In this investigation, poly-alpha-olefin (PAO) is being used as base oil in which the WS2 nanoparticles and different additives are mixed. A ball-on-disc sliding test revealed that certain additives inhibit the nanoparticles to reduce friction (less than 5 % decrease in friction coefficient), while in other cases, the friction reduction was above 50 %. The comparison is being made between PAO + additive and PAO + additive + IFLNPs. Scanning electron microscope and energy dispersive X-ray spectroscopy were used to investigate the elemental composition of the tribofilms formed on the wear marks. Further analysis was made in order to reveal correlations between elemental compositions of the tribofilms and external parameters such as the friction coefficient and wear rate. For instance, a strong correlation between tungsten content of the tribofilm and the friction coefficient was found.
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| 12. |
- Larsson, Per-Lennart
(författare)
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On the Determination of Biaxial Residual Stress Fields from Global Indentation Quantities
- 2014
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 54:1, s. 89-97
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, quantitative relations for the determination of surface residual stresses, using sharp indentation testing, are presented. The relations are based on previous results for equi-biaxial residual fields but further developed to apply also for a general situation. The present analysis relies on theoretical methods, but the results are validated using previous experimental and numerical findings. Cone indentation of classical Mises elastoplastic material behavior is assumed throughout the investigation for clarity but not out of necessity. Further development for a complete characterization of a general residual stress field is discussed in some detail.
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| 13. |
- Lauridsen, Jonas, et al.
(författare)
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Microstructural and Chemical Analysis of AgI Coatings Used as a Solid Lubricant in Electrical Sliding Contacts
- 2012
-
Ingår i: Tribology letters. - : Springer Verlag (Germany). - 1023-8883 .- 1573-2711. ; 46:2, s. 187-193
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Tidskriftsartikel (refereegranskat)abstract
- AgI coatings have been deposited by electroplating on Ag-plated Cu coupons. Electron microscopy shows that the coatings consist of weakly agglomerated AgI grains. X-ray diffraction, differential scanning calorimetry, thermogravimetry, and mass spectrometry show that the AgI exhibits a reversible transformation from hexagonal to cubic phase at 150 A degrees C. AgI starts to decompose at 150 A degrees C with an accelerating rate up to the AgI melting temperature (555 A degrees C), where a complex-bonded hydroxide evaporates. Ag pin-on-disk testing shows that the iodine addition to Ag decreases the friction coefficient from 1.2 to similar to 0.4. The contact resistance between AgI and Ag becomes less than 100 mu I (c) after similar to 500 operations as the AgI deagglomerates, and Ag is exposed on the surface and remains low during at least 10,000 reciprocating operations. This makes AgI suitable as a solid lubricant in electrical contacts.
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| 14. |
- Nyberg, Harald, et al.
(författare)
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Calculated Trajectories of Curling Stones Sliding Under Asymmetrical Friction : Validation of Published Models
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 50:3, s. 379-385
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Tidskriftsartikel (refereegranskat)abstract
- In the sport of curling, stones are slid across an ice sheet, aimed towards a target area. A sliding stone does not move in a straight line, but follows a curled trajectory, deviating in a direction determined by its rotation. As yet, no satisfactory explanation for this motion has been presented, although many attempts have been made. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, typically such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In this paper, the motion of a rotating curling stone sliding over ice is calculated, for different assumed distributions of the coefficient of friction in the contact between stone and ice, using a numerical method. It is shown that no redistribution of the friction, no matter how extreme, can explain the observed motion of a real curling stone.
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| 15. |
- Nyberg, Harald, et al.
(författare)
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Tribochemical formation of sulphide tribofilms from a Ti-C-S coating sliding against different counter surfaces
- 2014
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Ingår i: Tribology letters. - : Springer Science+Business Media B.V.. - 1023-8883 .- 1573-2711. ; 56:3, s. 563-572
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Tidskriftsartikel (refereegranskat)abstract
- Tribochemically active Ti-C-S coatings are nanocomposite coatings containing a S-doped titanium carbide, from which S can be released in a tribological contact. This work studies tribochemical reactions between a Ti-C-S coating and various counter surface materials, and their effect on the tribological performance. Tribological tests were performed in a ball-on-disc set-up, using balls of five different materials as sliding partners for the coating: 100Cr6 steel, pure W, WC, 316-L steel and Al2O3. For W balls, a WS2 tribofilm was formed, leading to low friction (down to A mu = 0.06). Furthermore, increasing normal load on the W balls was found to lead to a strong decrease in A mu and earlier formation of the low-friction WS2 tribofilm. Similar WS2 and MoS2 tribofilms were, however, not formed from WC- and Mo-containing 316-L balls. The performance when using WC and Al2O3 balls was significantly worse than for the two steel balls. It is suggested that this is due to sulphide formation from Fe, analogous to formation of anti-seizure tribofilms from S-containing extreme pressure additives and steel surfaces. The tribochemical activity of Ti-C-S coatings, with the possibility of S release, is thus beneficial not only for pure W counter surfaces, but also for Fe-based sliding partners.
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| 16. |
- Olsson, Erik, et al.
(författare)
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On the Appropriate Use of Representative Stress Quantities at Correlation of Spherical Contact Problems
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 50:2, s. 221-232
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Tidskriftsartikel (refereegranskat)abstract
- Correlation of contact problems is discussed in a detailed manner with focus on spherical contact. The finite element method is used to determine appropriate stress quantities, representative stresses, aiming at a general description of contact quantities such as mean contact pressure, and the size of the contact area. It is shown that the mean contact pressure can be well described by a single master curve, while this is not so for the size of the contact area. The latter feature is explained partly by a pronounced effect from elastic deformation, but is also shown that large deformation effects can have a substantial influence on correlation attempts. The analysis is restricted to classical Mises elastoplasticity, but the results can also serve as a guideline for similar attempts when using more advanced constitutive modeling. An obvious application of the present results concerns material characterization by indentation testing.
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| 17. |
- Pitenis, A. A., et al.
(författare)
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In vacuo tribological behavior of polytetrafluoroethylene (ptfe) and alumina nanocomposites : The importance of water for ultralow wear
- 2014
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 53:1, s. 189-197
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Tidskriftsartikel (refereegranskat)abstract
- Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10-6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10-7 mm3/(Nm)
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| 18. |
- Rowe, K. G., et al.
(författare)
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Lessons from the lollipop : Biotribology, tribocorrosion, and irregular surfaces
- 2014
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Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 56:2, s. 273-280
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Tidskriftsartikel (refereegranskat)abstract
- Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.
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| 19. |
- Rydin, Andreas, et al.
(författare)
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On the Correlation Between Residual Stresses and Global Indentation Quantities : Equi-Biaxial Stress Field
- 2012
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 47:1, s. 31-42
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Tidskriftsartikel (refereegranskat)abstract
- Sharp indentation test have been studied very frequently in recent years and the fundamental knowledge of the mechanics of such test has reached some maturity. Such studies also include the correlation between residual stresses and the global properties, i.e., hardness and size of the contact area. The investigations presented have been based on experimental, theoretical, and numerical methods and as a result, the basic features of the problem are fairly well understood but quantitative relations, for the determination of residual stresses using sharp indentation, have been proven to be less accurate and accordingly not suitable for a practical situation, in particular so at predominantly compressive residual stresses. It is therefore the aim of the present study to investigate this matter in some detail and to determine possible mechanisms for the difference in indentation behavior between tension and compression and, with this as a background, determine relations suitable for a quantitative determination of the residual fields. The present analysis is based on theoretical and numerical methods and in the latter case, the finite element method is relied upon. Classical Mises elastoplastic material behavior is assumed throughout the investigation.
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| 20. |
- Shah, Faiz Ullah, et al.
(författare)
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Boron in Tribology : From Borates to Ionic Liquids
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 51:3, s. 281-301
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Forskningsöversikt (refereegranskat)abstract
- Boron compounds are widely used in a range of tribological applications such as friction modifiers, antioxidants, antiwear additives, and in many cases as environmentally friendly lubricants. The chemical nature and structure of boron compounds provide multifunctionality. They are used as (1) solid lubricants such as boric acid and hexagonal boron nitride, (2) liquid lubricants such as ionic liquids, (3) lubricant additives such as borate derivatives of various organic and inorganic compounds, and (4) coatings such as cubic boron nitride and different metal borides. Boron is also one of the most favorable elements for coatings and thin films in biotribological and biomedical applications. This review outlines the growing role of boron in lubrication over the past several decades, summarizes the main findings, and identifies future challenges related to boron chemistry.
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| 21. |
- Shah, Faiz Ullah, et al.
(författare)
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Novel alkylborate-dithiocarbamate lubricant additives : Synthesis and tribophysical characterization
- 2012
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 45:1, s. 67-78
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Tidskriftsartikel (refereegranskat)abstract
- Boron-based lubricant additives have recently received significant attention, because of their wear-reducing and frictional properties and low pollution. At the same time, dithiocarbamate complexes with different metals have a long history of being used as multifunctional additives to lubricants. In this study, novel, environmentally friendly additives containing alkylborate and dithiocarbamate groups with alkyl or methylbenzyl substitutes in one molecule were studied. Tribological tests were performed with the additives admixed in a mineral oil using steel-on-steel contacts in a four-ball tribometer. Borate derivatives of different dithiocarbamate ligands were synthesized by several step reactions to investigate tribochemical properties of boron, sulfur, and nitrogen combined in one selected compound. Viscous liquid products were characterized by multinuclear 1H, 13C, and 11B NMR spectroscopy. The surface morphology and the elemental composition of the tribofilms were investigated using an optical profiler and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). It was found that some of these novel compounds provide better antiwear performance and similar frictional properties compared with a commercially available ZnDTP package. Traces of sulfur in the tribofilms formed with both 0.2 and 1.0 wt% of alkylborate-dithiocarbamate compounds in a mineral oil were detected with EDS.
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| 22. |
- Shi, Yijun, et al.
(författare)
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Friction and wear behavior of CF/PTFE composites lubricated by choline chloride ionic liquids
- 2013
-
Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 49:2, s. 413-420
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Tidskriftsartikel (refereegranskat)abstract
- The use of ionic liquids (ILs) as lubricants has received increasing attention in recent years. The use of ILs, however, is limited by the corrosion problem and their potential toxic property. Here we present the results of our initial study on the tribological properties of carbon fiber (CF)-filled polytetrafluoroethylene (PTFE) composites, which have an excellent chemical resistance property, lubricated by choline chloride ILs. The difference between choline chloride ILs and water and hydraulic oil as lubricants was studied at the same time, as was the effect of the anion on the lubricating property of choline chloride ILs. The worn surface and transfer film of CF/PTFE composites were studied by scanning electron microscopy. Our results indicate that the lubricating property of choline chloride ILs is much better than that of water and hydraulic oil. The friction coefficient and wear rate of CF/PTFE composites lubricated with ILs were approximately 60 and 50 % lower than those under the dry friction condition. Among the three kinds of ILs tested, the best tribological properties of the CF/PTFE composites were found for those sliding in the mixture of 1,2-propanediol and choline chloride. The worn surface and transfer film of CF/PTFE composites were also much smoother than those under the dry friction, water lubrication, and hydraulic oil lubrication conditions.
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| 23. |
- Simmons, Gregory F, et al.
(författare)
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Synthetic lubricants in hydrodynamic journal bearings : experimental results
- 2011
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 42:1, s. 109-115
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Tidskriftsartikel (refereegranskat)abstract
- Synthetic lubricants and additives have seen many major improvements in recent years. However, very littleis known about the performance peculiarities of these new lubricants in actual machines. To fill this gap, anew full-scale hydro-dynamic journal bearing test rig has been constructed to evaluate the behavior ofconventional and new bearing designs, synthetic lubricants and variations in operating parameters. This testrig’s bearing has diameter 180 mm with measuring capabilities including continuous film thickness and filmpressure as well as temperature. The new machine was used to compare a number of synthetic lubricants tomineral based lubricants, finding that performance of the synthetic lubricants was superior to their mineralbased counterparts of much higher viscosity grade. These tests showed that high viscosity index (VI) syntheticlubricants had higher viscosity in the region of maximum pressure and lower viscosity elsewhere in the bearingthan similar mineral based lubricants. This reduction in viscosity in low pressure zones was found to producea measurable reduction in friction and power loss in the bearing system. This paper provides comparativeperformance results of several formulations of current and future turbine oils from measurements of losses,oil film thickness, and temperature under a range of operating parameters. Lubricants tested include ISOVG68 and VG32 mineral based turbine oils (VG68 and VG32), ISO VG32 synthetic ester based oil (SE32),two ISO VG22 synthetic ester based oils (SE22 and SV22), and ISO VG15 synthetic ester based oil (SE15).It was found that SE32 and VG68 provided similar performance at lower speeds while SV22 provided similarperformance to VG68 at the highest speed. Likewise, SE22 and SV22 provided similar performance toVG32 at low speeds while SE15 provided similar performance to VG32 at medium to higher speeds.Generally, the results demonstrate the potential for replacing mineral based lubricants with high performancesynthetic lubricants of significantly lower viscosity grade without sacrificing bearing safety.
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| 24. |
- Skedung, Lisa, et al.
(författare)
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Finger Friction Measurements on Coated and Uncoated Printing Papers
- 2010
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 37:2, s. 389-399
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Tidskriftsartikel (refereegranskat)abstract
- A macroscopic finger friction device consisting of a piezoelectric force sensor was evaluated on 21 printing papers of different paper grades and grammage. Friction between a human finger and the 21 papers was measured and showed that measurements with the device can be used to discriminate a set of similar surfaces in terms of finger friction. When comparing the friction coefficients, the papers group according to paper grade and the emerging trend is that the rougher papers have a lower friction coefficient than smoother papers. This is interpreted in terms of a larger contact area in the latter case. Furthermore, a decrease in friction coefficient is noted for all papers on repeated stroking (15 cycles back and forth with the finger). Complementary experiments indicate that both mechanical and chemical modifications of the surface are responsible for this decrease: (1) X-ray photoelectron spectroscopy measurements show that lipid material is transferred from the finger to the paper surface, (2) repeated finger friction measurements on the same paper sample reveal that only partial recovery of the frictional behaviour occurs and (3) profilometry measurements before and after stroking indicate small topographical changes associated with repeated frictional contacts.
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| 25. |
- Svahn, Fredrik, et al.
(författare)
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Formation of Low-Friction Particle/Polymer Composite Tribofilms by Tribopolymerization
- 2011
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 41:2, s. 387-393
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Tidskriftsartikel (refereegranskat)abstract
- Inorganic fullerene-like (IF) solid lubricant particles based on MoS(2) or WS(2) have recently gained attention in various tribological applications, for instance incorporated in coatings, porous materials, greases and dispersed in oils. However, their effect in oil can be limited often due to inadequate penetration into the contact zone. Meanwhile, tribopolymerization of monomer additives in oil have proven to be efficient to reduce wear, but without significantly reducing the friction. This investigation combines these two lubrication techniques in order to form particle/polymer composite tribofilms, aimed to give low friction and wear in high-pressure sliding contacts. A cyclic amine, caprolactam, was used as the monomer together with IF as well as normal 2H WS(2) particles. Zn-DTP was used as a reference antiwear additive and poly-alfa-olefin (PAO) was used as the base oil. Reciprocal ball-on-flat sliding test results found that monomer plus particles reduced the friction by 10-50%, depending on sliding speed and type of particles, compared with particles or monomer alone. And the scattering between different tests was also considerably lower. The wear rate was also substantially reduced to a level similar to that of Zn-DTP. The particle/polymer composite tribofilms were generally much larger than the actual contact area and its thickness varied from below a few nanometres in the contact centre to several micrometers in the outer parts. Consequently, the reduced friction is believed to come from two effects: one being a trapping of particles in the contact zone, which reduces the boundary friction level; and the other a shift in lubrication regime towards EHD-lubrication due to reduced contact pressures accompanied with the formation of the composite tribofilm.
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| 26. |
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| 27. |
- Totolin, Vladimir, et al.
(författare)
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Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid Additive in Alkylborane-Imidazole Complexes
- 2014
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 53:2, s. 421-432
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Tidskriftsartikel (refereegranskat)abstract
- The assessment of ionic liquids (ILs) as lubricants in several tribological systems has shown their ability to provide remarkable reduced friction and protection against wear, whether they are used as additives or in the neat form. However, their corrosion and limited solubility in non-polar hydrocarbon oils represent the bottleneck-limiting factors for the use of ILs as lubricants. Therefore, in order to tackle these problems, mixtures of alkylborane-imidazole complexes with one halogen-free IL as additive were used in this study. The knowledge of the additive-surface interactions and hence the understanding of tribological properties are an important issue for lubricant formulations and were also investigated in this work. Thus, combination effects between two ionic liquid additives, a halogenated and a halogen-free one, were evaluated by a ball-on-disc-type tribometer under boundary lubrication conditions. Effective friction reduction and anti-wear properties have been demonstrated in tribological investigations when adding between 0.7 and 3.4 wt% of the halogen-free IL into base fluid composed of alkylborane-imidazole complexes. X-ray photoelectron spectroscopy analyses of the steel specimens were conducted to study the correlation between tribological properties and chemical surface composition of the boundary films formed on the rubbing surface. This work suggests potential applications for using halogen-free ILs as additives for synthetic ionic liquid lubricants
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| 28. |
- Wahlström, Jens, et al.
(författare)
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A cellular automaton approach to numerically simulate the contact situation in disc brakes
- 2011
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 42:3, s. 253-262
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Tidskriftsartikel (refereegranskat)abstract
- Since brake wear is an important contributor to the concentration of airborne particles in urban atmospheres, it is important to increase our understanding of the origin of these particles. The contact situation between the pad and disc is complicated. Metal fibres in the pad (or other hard materials) form stable contact plateaus, which carry the main part of the load. A flow of wear particles in the boundary layer between the pad and disc partially stack up against these plateaus (and increase their area), and some will escape from the contact and become airborne. The purpose of this article is to investigate the possibility to numerically simulate the contact situation in the boundary layer between the pad and disc and the amount of wear that leaves the contact using a cellular automaton approach. To do so, the contact pressure, pad temperature and wear are determined in that order. Based on these results, the creation, growth and destruction of contact plateaus are simulated using a cellular automaton model. Finally, the amount of wear that leaves the contact is determined. The simulated behaviour of the contact situation correlates qualitatively with experimental observations found in the literature.
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| 29. |
- Wahlström, Jens, et al.
(författare)
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A Pin-on-Disc Study Focusing on How Different Load Levels Affect the Concentration and Size Distribution of Airborne Wear Particles from the Disc Brake Materials
- 2012
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 46:2, s. 195-204
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Tidskriftsartikel (refereegranskat)abstract
- Airborne wear particles originating from disc brakes are one important contributor to the concentration of airborne particles in urban environments. It is therefore of interest to improve the knowledge of these particles. The purpose of this article is to investigate the concentration and size distribution of the airborne wear particles generated from the contact between a low-metallic pad material and a grey cast iron disc at different load levels. This is done on model level with a pin-on-disc machine that allows the cleanliness of the air surrounding the test specimens to be controlled, and thus the airborne portion of the wear particles to be studied separately. The concentration and size of airborne wear particles were measured online during testing with four particle instruments. In addition, airborne wear particles were collected on filters during the tests and afterward analysed using SEM. Trimodal size distributions with peaks around 280, 350 and 550 nm were registered during running-in for all load levels. After running-in bimodal size distributions with peaks around 350 and 550 nm were registered for all load levels with the exception of the highest load level where multimodal size distributions were registered. At the two highest load levels the concentration of ultrafine/fine particles showed an increase up to a factor hundred indicating a change in wear mechanism. SEM images show ultrafine, fine and coarse airborne wear particles.
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| 30. |
- Wahlström, Jens, et al.
(författare)
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Size, Shape, and Elemental Composition of Airborne Wear Particles from Disc Brake Materials
- 2010
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 38:1, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- During braking, both the rotor and pads experience wear, generating particles that may become airborne. In field tests, it is difficult to distinguish these particles from others in the surrounding environment, so it is preferable to use laboratory test stands to study the amount of airborne wear particles generated. The purpose of this work is to investigate the possibility of separate, capture, and analyze airborne wear particles generated by a disc brake in a disc brake assembly test stand. This test stand used allows the cleanliness of the air surrounding the test specimens to be controlled and thus the airborne portion of the wear particles to be studied separately. One pair each of low-metallic (LM) and non-asbestos organic (NAO) brake pads was tested against grey cast iron rotors. Before testing, the elemental contents of the brake materials were analyzed using glow discharge optical emission spectroscopy (GDOES). The concentration and size of airborne wear particles were measured online during testing. In addition, airborne wear particles were collected on filters during the tests and afterward analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The analyzed wear particles contained elements such as iron, titanium, zinc, barium, manganese, and copper. Both the low-metallic and non-asbestos organic type of brake pads tested display a bimodal size distribution with peaks at 280 and 350 nm. Most of the airborne particles generated have a diameter smaller than 2.5 mu m.
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| 31. |
- Westerberg, Lars-Göran, et al.
(författare)
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Free-surface grease flow on a rotating plate
- 2014
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 56:2, s. 317-325
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Tidskriftsartikel (refereegranskat)abstract
- Grease lubrication is traditionally used in a great variety of mechanical systems such as rolling bearings, seals, and gears where it has been shown more advantageous than oil, mainly due to its consistency allowing the grease to stay inside the system and not leak out. Knowledge of the flow dynamics of grease is important for the understanding and prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. Free-surface effects play an important role in rolling bearings and open gears as the configuration normally is filled with about 30 % grease to avoid heavy churning. In this study, an analytical model of the stationary uniform flow on a rotating disc is developed and validated with experiments. The model results in the velocity profile for the flow in the thin fully yielded viscous layer in connection to the surface as well as an expression for the plug flow region on top of the viscous layer. Furthermore, the depth-averaged velocity is derived as is the shear stress value on the plate. From the latter, follows a condition for the grease to start moving and in turn yielding an expression for the viscous layer thickness as a function of the grease yield stress value, grease density, angular velocity, and radial position. In addition, an expression of the layer thickness containing the ratio between the flow rate and the layer width which in turn can account for effects not included in the model such as wall slip and surface adhesion and thus add another degree of freedom into the model. Experiments with two different greases having NLGI grade 1 and 2, respectively, shows it is possible to obtain a good fit with the analytically obtained thickness using the rheological parameters for actual greases.
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| 32. |
- Zhang, Chen Hui, et al.
(författare)
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EHL properties of polyalkylene glycols and their aqueous solutions
- 2012
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Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 45:3, s. 379-385
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Tidskriftsartikel (refereegranskat)abstract
- Polyalkylene glycols (PAGs) are a type of synthetic lubricants widely used as compressor lubricants, gear oils, hydraulic fluids, and metal working fluids. The PAGs with typical molecular structure can dissolve in water, which makes it a candidate for the base stock of water-based lubricants. Till now, most of the investigations on the water-based lubrication have focussed on the additives. In this work, the potential of PAGs aqueous solutions to replace water as base stocks has been investigated. Four types of PAGs with different molecular weight and their aqueous solutions with different concentrations were studied to reveal their elastohydrodynamic lubrication (EHL) behavior. It has been found that the PAGs solutions can form EHL film like traditional oils. The film-forming capability depends on the viscosity, the pressure-viscosity coefficient, and the molecular weight of PAGs. The results indicate that the PAGs aqueous solution can be employed as base stocks of water-based lubricant
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| 33. |
- Zhu, Yi, et al.
(författare)
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Friction Between Wheel and Rail : A Pin-On-Disc Study of Environmental Conditions and Iron Oxides
- 2013
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Ingår i: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 52:2, s. 327-339
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Tidskriftsartikel (refereegranskat)abstract
- The coefficient of friction between railway wheels and rails is crucial to railway operation and maintenance. Since the wheel-rail system is an open system, environmental conditions, such as humidity and temperature, affect the friction coefficient. Pin-on-disc testing was conducted to study the influence of environmental conditions and iron oxides on the coefficient of friction between the wheel and rail. The iron oxides were pre-created in a climate chamber. The surfaces of the tested samples were analysed using X-ray diffraction, scanning electron/focused ion beam microscopy, and Raman spectroscopy. Results indicate that the coefficient of friction decreases with increasing relative humidity (RH) up to a saturation level. Above this level, the coefficient of friction remains low and stable even when the RH increases. In particular, when the temperature is low, a small increase in the amount of water (i.e., absolute humidity) in the air can significantly reduce the coefficient of friction. At high humidity levels, a water molecule film can keep the generated haematite on the surfaces, counterbalancing the effect of rising humidity.
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