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Lubricants, Volume 4, Issue 3 (September 2016) – 12 articles

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Review
On Monitoring Physical and Chemical Degradation and Life Estimation Models for Lubricating Greases
Lubricants 2016, 4(3), 34; https://doi.org/10.3390/lubricants4030034 - 13 Sep 2016
Cited by 20 | Viewed by 2358
Abstract
Degradation mechanisms for lubricating grease are categorized and described. An extensive survey of the available empirical and analytical grease life estimation models including degradation monitoring standards and methods are presented. A summary of the important contributions on grease degradation is presented. Full article
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Article
Towards Accurate Prediction of Unbalance Response, Oil Whirl and Oil Whip of Flexible Rotors Supported by Hydrodynamic Bearings
Lubricants 2016, 4(3), 33; https://doi.org/10.3390/lubricants4030033 - 08 Sep 2016
Cited by 5 | Viewed by 2537
Abstract
Journal bearings are used to support rotors in a wide range of applications. In order to ensure reliable operation, accurate analyses of these rotor-bearing systems are crucial. Coupled analysis of the rotor and the journal bearing is essential in the case that the [...] Read more.
Journal bearings are used to support rotors in a wide range of applications. In order to ensure reliable operation, accurate analyses of these rotor-bearing systems are crucial. Coupled analysis of the rotor and the journal bearing is essential in the case that the rotor is flexible. The accuracy of prediction of the model at hand depends on its comprehensiveness. In this study, we construct three bearing models of increasing modeling comprehensiveness and use these to predict the response of two different rotor-bearing systems. The main goal is to evaluate the correlation with measurement data as a function of modeling comprehensiveness: 1D versus 2D pressure prediction, distributed versus lumped thermal model, Newtonian versus non-Newtonian fluid description and non-mass-conservative versus mass-conservative cavitation description. We conclude that all three models predict the existence of critical speeds and whirl for both rotor-bearing systems. However, the two more comprehensive models in general show better correlation with measurement data in terms of frequency and amplitude. Furthermore, we conclude that a thermal network model comprising temperature predictions of the bearing surroundings is essential to obtain accurate predictions. The results of this study aid in developing accurate and computationally-efficient models of flexible rotors supported by plain journal bearings. Full article
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Article
Tribological Performance of MoS2 Coatings in Various Environments
Lubricants 2016, 4(3), 32; https://doi.org/10.3390/lubricants4030032 - 07 Sep 2016
Cited by 17 | Viewed by 3009
Abstract
Molybdenum disulfide (MoS2) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition [...] Read more.
Molybdenum disulfide (MoS2) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS2 coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS2 coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper. Full article
(This article belongs to the Special Issue Tribofilms and Solid Lubrication)
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Article
Synthesis and Tribological Behavior of Ultra High Molecular Weight Polyethylene (UHMWPE)-Lignin Composites
Lubricants 2016, 4(3), 31; https://doi.org/10.3390/lubricants4030031 - 31 Aug 2016
Cited by 2 | Viewed by 3106
Abstract
In this paper, we report the synthesis and characterization of ultra-high molecular weight polyethylene (UHMWPE)-lignin composites. During this study four different compositions, namely UHMWPE, UHMWPE-13 wt. % lignin, UHMWPE-25 wt. % lignin and UHMWPE-42.5 wt. % lignin were fabricated by hot pressing. Detailed [...] Read more.
In this paper, we report the synthesis and characterization of ultra-high molecular weight polyethylene (UHMWPE)-lignin composites. During this study four different compositions, namely UHMWPE, UHMWPE-13 wt. % lignin, UHMWPE-25 wt. % lignin and UHMWPE-42.5 wt. % lignin were fabricated by hot pressing. Detailed microstructural studies by scanning electron microscopy (SEM) showed that UHMWPE and UHMWPE-13 wt. % lignin had a uniform microstructure, whereas UHMWPE-25 wt. % lignin and UHMWPE-42.5 wt. % lignin samples were riddled with pores. UHMWPE and UHMWPE-13% lignin showed comparable flexural strengths of ~32.2 MPa and ~32.4 MPa, respectively. However, the flexural strength dropped drastically in UHMWPE-25 wt. % lignin and UHMWPE-42.5 wt. % samples to ~13 MPa and ~8 MPa, respectively. The tribology of UHMWPE-lignin composites is governed by the tribofilm formation. All the compositions showed similar µmean values and the specific wear rates (WR) decreased gradually as the concentration of lignin in UHMWPE was increased. Full article
(This article belongs to the Special Issue Green Tribology)
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Article
The Tribological Properties of Multi-Layered Graphene as Additives of PAO2 Oil in Steel–Steel Contacts
Lubricants 2016, 4(3), 30; https://doi.org/10.3390/lubricants4030030 - 31 Aug 2016
Cited by 37 | Viewed by 3467
Abstract
Multi-layered graphene was prepared by supercritical CO2 exfoliation of graphite. As the additives of polyalphaolefin-2 (PAO2) oil, its tribological properties were investigated using four-ball test method. The friction reduction and anti-wear ability of pure lubricant was improved by the addition of graphene. [...] Read more.
Multi-layered graphene was prepared by supercritical CO2 exfoliation of graphite. As the additives of polyalphaolefin-2 (PAO2) oil, its tribological properties were investigated using four-ball test method. The friction reduction and anti-wear ability of pure lubricant was improved by the addition of graphene. With a favorable concentration, the graphene was dispersive. The PAO2 oil with 0.05 wt % graphene showed better tribological properties than that for the other concentration of graphene additives. It could be used as a good lubricant additive for its excellent tribological characteristics, and the multi-layered graphene can bear the load of the steel ball and prevent direct contact of the mating metal surfaces. However, a higher concentration would cause the agglomeration of graphene and weaken the improvement of tribological properties. Full article
(This article belongs to the Special Issue Green Tribology)
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Article
Experimental and Numerical Simulation of the Dynamic Frictional Contact between an Aircraft Tire Rubber and a Rough Surface
Lubricants 2016, 4(3), 29; https://doi.org/10.3390/lubricants4030029 - 17 Aug 2016
Cited by 9 | Viewed by 3702
Abstract
This paper presents a numerical simulation of an aircraft tire in contact with a rough surface using a variable friction coefficient dependent on temperature and contact pressure. A sliding facility was used in order to evaluate this dependence of the friction coefficient. The [...] Read more.
This paper presents a numerical simulation of an aircraft tire in contact with a rough surface using a variable friction coefficient dependent on temperature and contact pressure. A sliding facility was used in order to evaluate this dependence of the friction coefficient. The temperature diffusion throughout the tire cross-section was measured by means of thermocouples. Both frictional heating and temperature diffusion were compared to numerical two- and three- dimensional simulations. An adequate temperature prediction could be obtained. In future simulations, wear should be taken into account in order to have a more accurate simulation especially in the case of high pressures and slipping velocities. A 3D finite element model for a rolling tire at a velocity of 37.79 knots (19.44 m/s) and in a cornering phase was investigated using a variable friction coefficient dependent on temperature and pressure. The numerical simulation tended to predict the temperature of the tire tread after a few seconds of rolling in skidding position, the temperature of the contact zone increases to 140 °C. Further investigations must be carried out in order to obtain the evolution of the temperature observed experimentally. The authors would like to point out that for confidentiality reasons, certain numerical data could not be revealed. Full article
(This article belongs to the Special Issue Numerical Simulation of Static and Dynamic Friction)
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Article
Formation of Anti-Wear Tribofilms via α-ZrP Nanoplatelet as Lubricant Additives
Lubricants 2016, 4(3), 28; https://doi.org/10.3390/lubricants4030028 - 05 Aug 2016
Cited by 27 | Viewed by 2980
Abstract
Effective tribofilms are desirable to protect mechanical systems. In the present research, we investigated the formation of a tribofilm through the use of α-ZrP (Zr(HPO4)2·H2O) as an additive. Experiments were conducted on a base oil where 0.2 [...] Read more.
Effective tribofilms are desirable to protect mechanical systems. In the present research, we investigated the formation of a tribofilm through the use of α-ZrP (Zr(HPO4)2·H2O) as an additive. Experiments were conducted on a base oil where 0.2 wt% of the additive was used. Experimental results showed a 50% reduction in friction and a 30% reduction in wear when compared to the base oil containing 0.8 wt% ZDDP. Spectroscopic characterization indicated that the tribofilm consists of iron oxide, zirconium oxide, and zirconium phosphates. The worn surface was seen to be smooth which renders it desirable for bearing systems. Full article
(This article belongs to the Special Issue Nanoparticles and Ionic Liquids in Lubrication)
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Article
Tribofilms Forming in Oil-Lubricated Contacts
Lubricants 2016, 4(3), 27; https://doi.org/10.3390/lubricants4030027 - 14 Jul 2016
Cited by 5 | Viewed by 2042
Abstract
The subject of the present paper is the characterization of third bodies of run-in systems. By means of continuous friction and wear measurement, lubricated steel-steel and steel-aluminum contacts were evaluated. Microstructure, chemical composition and response of the materials to shear were analyzed by [...] Read more.
The subject of the present paper is the characterization of third bodies of run-in systems. By means of continuous friction and wear measurement, lubricated steel-steel and steel-aluminum contacts were evaluated. Microstructure, chemical composition and response of the materials to shear were analyzed by XPS/AES and focused ion beam technique. After a proper running-in, both systems developed a third body. The third body differs significantly from the base materials. In addition to adapted microstructure and near-surface chemistry, the third body exhibited a substructure characterized by a near-surface zone that accommodates shear and a second, deeper region that ensures strengthening. Full article
(This article belongs to the Special Issue Tribofilms and Solid Lubrication)
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Article
The Friction Reducing Effect of Square-Shaped Surface Textures under Lubricated Line-Contacts—An Experimental Study
Lubricants 2016, 4(3), 26; https://doi.org/10.3390/lubricants4030026 - 11 Jul 2016
Cited by 34 | Viewed by 3897
Abstract
Surface texturing has been shown to be an effective modification approach for improving tribological performance. This study examined the friction reduction effect generated by square dimples of different sizes and geometries. Dimples were fabricated on the surface of ASP2023 steel plates using femtosecond [...] Read more.
Surface texturing has been shown to be an effective modification approach for improving tribological performance. This study examined the friction reduction effect generated by square dimples of different sizes and geometries. Dimples were fabricated on the surface of ASP2023 steel plates using femtosecond laser-assisted surface texturing techniques, and reciprocating sliding line contact tests were carried out on a Plint TE77 tribometer using a smooth 52100 bearing steel roller and textured ASP2023 steel plates. The tribological characterization of the friction properties indicated that the textured samples had significantly lowered the friction coefficient in both boundary (15% improvement) and mixed lubrication regimes (13% improvement). Moreover, the high data sampling rate results indicated that the dimples work as lubricant reservoirs in the boundary lubrication regime. Full article
(This article belongs to the Special Issue Green Tribology)
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Article
A Tribological Assessment of Ultra High Molecular Weight Polyethylene Types GUR 1020 and GUR 1050 for Orthopedic Applications
Lubricants 2016, 4(3), 25; https://doi.org/10.3390/lubricants4030025 - 30 Jun 2016
Cited by 13 | Viewed by 2993
Abstract
The wear properties of biomaterials have been demonstrated to have a high importance within orthopedic bearing surfaces. This study performed a comparison of the wear between the two main grades of Ultra High Molecular Weight Polyethylene types GUR 1020 and GUR 1050 articulating [...] Read more.
The wear properties of biomaterials have been demonstrated to have a high importance within orthopedic bearing surfaces. This study performed a comparison of the wear between the two main grades of Ultra High Molecular Weight Polyethylene types GUR 1020 and GUR 1050 articulating against Cobalt Chromium. Such a high capacity wear comparison has not been reported elsewhere in the scientific literature. Under an identical testing protocol it was found that GUR 1020 had a wear factor of 3.92 ± 0.55 × 10 6 ( mm 3 / Nm ) and GUR 1050 had a wear factor of 3.64 ± 0.39 × 10 6 ( mm 3 / Nm ) , with a non-statistical significant difference of p = 0.052. These wear factors correlate closely with those observed from other screening wear studies and explant analysis. Full article
(This article belongs to the Special Issue Wear Testing of Biomaterials)
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Article
MD Sliding Simulations of Amorphous Tribofilms Consisting of either SiO2 or Carbon
Lubricants 2016, 4(3), 24; https://doi.org/10.3390/lubricants4030024 - 29 Jun 2016
Cited by 28 | Viewed by 2695
Abstract
The sliding behaviors of two simplified tribofilms with amorphous structure consisting either of SiO2 molecules or C atoms were simulated by molecular dynamics modeling. The objective was to identify mechanisms explaining the experimentally observed lubricating properties of the two amorphous films. The [...] Read more.
The sliding behaviors of two simplified tribofilms with amorphous structure consisting either of SiO2 molecules or C atoms were simulated by molecular dynamics modeling. The objective was to identify mechanisms explaining the experimentally observed lubricating properties of the two amorphous films. The impacts of layer thickness, normal pressure, temperature and different substrate materials were studied systematically, while the sliding velocity was kept constant at 30 m/s. While the layer thickness was not critical, all the other parameters showed special effects under certain conditions. Normal pressure impeded void formation and could even eliminate voids if applied at high temperature. Stick-slip sliding was changed to smooth sliding at high temperature due to void healing. Considering the carbon film, high friction forces and shearing of the entire film was observed with diamond substrates, whereas interface sliding at low friction forces and an amorphous layer of iron mixed with carbon was observed if the supporting substrates consisted of α-Fe. Both films show a decrease of friction forces and smooth sliding behavior at elevated temperature, corresponding well to the tribological behavior of an advanced nanocomposite sliding against a steel disc under severe stressing conditions when high flash temperatures can be expected. Full article
(This article belongs to the Special Issue Tribofilms and Solid Lubrication)
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Article
Synthesis, Characterization and Tribological Evaluation of New Generation Materials for Aluminum Cold Rolling Oils
Lubricants 2016, 4(3), 23; https://doi.org/10.3390/lubricants4030023 - 28 Jun 2016
Cited by 3 | Viewed by 2027
Abstract
The present concept of being globally “green” puts additional demands on lubricants. They are to be biodegradable and ecofriendly. Therefore, in a search for alternate lubricants meeting the above demands, we have synthesized biodegradable new generation esters using alcohols such as 2,2-dimethyl-1,3-propane diol [...] Read more.
The present concept of being globally “green” puts additional demands on lubricants. They are to be biodegradable and ecofriendly. Therefore, in a search for alternate lubricants meeting the above demands, we have synthesized biodegradable new generation esters using alcohols such as 2,2-dimethyl-1,3-propane diol and 2,2-diethyl-1,3-propane diol and fatty acids like caproic and 2-ethyl caproic in presence of indigenous ion exchange resin catalyst. The synthesized esters were analyzed and characterized for their physico chemical properties. In addition, with a view to finding out the possibility of using these esters as aluminum cold rolling oils, their lubricity characteristics, biodegradability and toxicity were also investigated. The products were found to have good potential for use in biodegradable aluminum cold rolling oils meeting IS: 14385-2002 specification. Full article
(This article belongs to the Special Issue Green Tribology)
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