Time filter

Source Type

Home > Press > Computer simulation discloses new effect of cavitation: Steam bubbles in fast flowing fluids obviously also result from chemical surface properties; use for reducing wear in pumps and plain bearings Abstract: Researchers have discovered a so far unknown formation mechanism of cavitation bubbles by means of a model calculation. In the Science Advances journal, they describe how oil-repellent and oil-attracting surfaces influence a passing oil flow. Depending on the viscosity of the oil, a steam bubble forms in the transition area. This so-called cavitation may damage material of e.g. ship propellers or pumps. However, it may also have a positive effect, as it may keep components at a certain distance and, thus, prevent damage. Materials and friction researchers wanted to know which influence chemically different surfaces have on the flow behavior of a lubricant. In particular, they were interested in flow behavior in nanometer-sized lubrication gaps, a critical case close to boundary friction, i.e. shortly before the surfaces are in direct contact. For this purpose, they generated a mathematical model, in which they varied viscosity of the lubricant and surface properties of the walls. "We were very surprised to find cavitation in the transition area of the surfaces, i.e. at the boundary between oil-attracting and oil-repellent," Dr. Lars Pastewka and Professor Peter Gumbsch of KIT's Institute for Applied Materials report. Cavitation is a known and feared physical phenomenon due to its destructive force. "Existing cavitation models assume a certain geometry that causes cavitation, such as a constriction in a pump or a ship's propeller producing high flow rates," Pastewka explains. Here, Bernoulli's physical law applies, according to which static pressure of a fluid decreases with increasing flow rate. If static pressure drops below the evaporation pressure of the fluid, steam bubbles are formed. If pressure increases again, e.g. if the fluid flow rate decreases after having passed a constriction in a pump, the steam in the bubbles condenses suddenly and they implode. The resulting extreme pressure and temperature peaks lead to typical cavitation craters and significant erosion even of hardened steel. "This sudden implosion of steam bubbles, however, does not occur in most lubricated tribosystems," Dr. Daniele Savio says, who has meanwhile taken up work at the Fraunhofer Institute for Mechanics of Materials in Freiburg. "As the fluid gap between two contacting surfaces usually is very narrow, the cavitation bubbles cannot grow and, hence, remain stable. The cavitation bubble then has no destructive effect and even serves as a buffer that reduces wear and friction of the surfaces. It is therefore important to generate this positive effect in a controlled manner," he adds. The simulation model of Savio and his colleagues confirms that chemically alternating surfaces may lead to cavitation bubbles. Their publication in Science Advances starts from the question of whether cavitation is the rule or an exception in situations where a lubricant flows between two surfaces. "Usually, surfaces in engines or cylinder systems are never homogeneous, i.e. only oil-attracting or oil-repellent," Savio points out. "The effect calculated by us may therefore be encountered wherever alternating neighboring surface properties exist in lubricated engines and pumps." So far, cavitation has been considered a geometric effect resulting from shear forces, flow rate, and pressure differences exclusively. "It is a completely new finding that cavitation can also occur in transition areas of alternating surface properties," Pastewka emphasizes. By the specific adjustment of surface chemistry, the researchers are convinced, interaction between surface and lubricant can be improved considerably. In the model simulations, an improved surface separation by 10% was observed. "A distance increased by 10% means that normal forces and load carrying capacities of plain bearings can be increased," Savio adds. In any case, surface chemistry has to be re-evaluated as a design element in mechanical engineering, the scientists agree. About Karlsruhe Institute of Technology (KIT) Karlsruhe Institute of Technology (KIT) pools its three core tasks of research, higher education, and innovation in a mission. With about 9,300 employees and 25,000 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe. KIT - The Research University in the Helmholtz Association Since 2010, the KIT has been certified as a family-friendly university. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | December 5, 2016
Site: www.eurekalert.org

Based on simulations, friction properties of the two-dimensional carbon graphene were studied by scientists of Karlsruhe Institute of Technology (KIT) in cooperation with researchers of the Fraunhofer Institute for Mechanics of Materials IWM and scientists in China and the USA. In contact with monolayer graphene, friction is higher than in case of multi-layered graphene or graphite. Moreover, friction force increases for continued sliding. The scientists attribute this to the real contact area and the evolving quality of frictional contact. They report their results in the journal Nature. (DOI: 10.1038/nature20135) When interfaces of solids are in contact and move in opposite directions, friction occurs. Energy is converted into heat that is lost unused. In addition, friction causes erosion and wear. To reduce friction for metallic sliding elements and high contact pressures, e.g. in automobiles or industrial machines, substances of lamellar structure are frequently used as dry lubricants, as their particles easily slide on each other. One of the most widely applied dry lubricants is graphite, a natural form of carbon with a three-dimensional, layered structure. Graphite theoretically consists of several layers of graphene that are stacked on top of each other with a slight offset. Graphene is a modification of carbon with a two-dimensional structure: It consists of only one layer of carbon atoms that are arranged in hexagons similar to honeycombs. In nature, graphene does not exist as an isolated monolayer material, but it can be produced by several methods. Experiments have shown that monolayer graphene exhibits higher friction than multilayer graphene or graphite and that friction increases with continued sliding. So far, the reasons have not yet been understood. Scientists of the Institute for Applied Materials (IAM) and the Institute of Nanotechnology (INT) of KIT, together with researchers of the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Xi'an Jiaotong University/China, Tsinghua University in Beijing/China, Massachusetts Institute of Technology/USA, and the University of Pennsylvania/USA, reproduced the experiments by atomistic simulations and obtained new findings with respect to layer-dependent friction and the increase in friction force for graphene. Their results are presented in the journal Nature. In the simulations, the scientists made a silicon tip slide over graphene applied onto an amorphous, i.e. non-crystalline, silicon substrate. Previously, it had been assumed that friction between interfaces depends on the true contact area - the number of atoms within the range of interatomic forces - and increased friction of monolayer graphene had been attributed to the larger true contact area. The scientists of KIT and their colleagues now found that apart from the true contact area, also the evolving contact quality is important. Due to its increased flexibility, the thinner and less constrained monolayer graphene tends to readjust its configuration. Carbon atoms more strongly adhere to the atoms of the silicon tip and exhibit an increased synchronicity in their stick-slip behavior. Contacts on the atomic scale increase quantitatively in terms of the contact area and qualitatively in terms of friction force. "Our concept of evolving contact quality can be used to explain why friction of interfaces of a less constrained structure changes with time," Dr. Suzhi Li of IAM - Computational Materials Science of KIT, explains. Suzhi Li, Qunyang Li, Robert W. Carpick, Peter Gumbsch, Xin Z. Liu, Xiangdong Ding, Jun Sun & Ju Li: The evolving quality of frictional contact with graphene. Nature, 2016. DOI: 10.1038/nature20135 Karlsruhe Institute of Technology (KIT) pools its three core tasks of research, higher education, and innovation in a mission. With about 9,300 employees and 25,000 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe. KIT - The Research University in the Helmholtz Association Since 2010, the KIT has been certified as a family-friendly university. This press release is available on the internet at http://www. .


Pastewka L.,Fraunhofer Institute for Mechanics of Materials | Moser S.,Fraunhofer Institute for Mechanics of Materials | Gumbsch P.,Fraunhofer Institute for Mechanics of Materials | Gumbsch P.,Karlsruhe Institute of Technology | And 2 more authors.
Nature Materials | Year: 2011

Diamond is the hardest material on Earth. Nevertheless, polishing diamond is possible with a process that has remained unaltered for centuries and is still used for jewellery and coatings: the diamond is pressed against a rotating disc with embedded diamond grit. When polishing polycrystalline diamond, surface topographies become non-uniform because wear rates depend on crystal orientations. This anisotropy is not fully understood and impedes diamondĝ€™s widespread use in applications that require planar polycrystalline films, ranging from cutting tools to confinement fusion. Here, we use molecular dynamics to show that polished diamond undergoes an sp 3 -sp 2 order-disorder transition resulting in an amorphous adlayer with a growth rate that strongly depends on surface orientation and sliding direction, in excellent correlation with experimental wear rates. This anisotropy originates in mechanically steered dissociation of individual crystal bonds. Similarly to other planarization processes, the diamond surface is chemically activated by mechanical means. Final removal of the amorphous interlayer proceeds either mechanically or through etching by ambient oxygen. © 2011 Macmillan Publishers Limited. All rights reserved.


Greve L.,Volkswagen AG | Fehrenbach C.,Fraunhofer Institute for Mechanics of Materials
Journal of Power Sources | Year: 2012

A quasi-static mechanical abuse test program on cylindrical Lithium ion battery cells has been performed at a state of charge (SoC) of 0%. The investigated load cases involved radial crushing, local lateral indentation and global three-point bending of the cell. During the tests, the punch load, the punch displacement, the cell voltage and the temperature development of the cell have been monitored using an infrared camera and temperature sensors. After the test, the cells have been analysed using computer tomography. It is indicated that macroscopic jelly roll fracture on a global scale initiates the internal short circuits, revealed by a sudden decrease of the global mechanical load due to the rupture, followed by a drop of the measured voltage and immediate increase in cell temperature. A macro-mechanical finite element crash simulation model has been established for the cell housing and the jelly roll. The classical stress-based criterion after Mohr and Coulomb (MC) has been applied to predict fracture and the initiation of an internal short circuit of the jelly roll. The MC criterion correctly represents the punch displacement to fracture, where the predicted fracture locations correspond to the observed locations of the internal short circuits of the cells. © 2012 Elsevier B.V. All rights reserved.


Kadkhodapour J.,Isfahan University of Technology | Butz A.,Fraunhofer Institute for Mechanics of Materials | Ziaei Rad S.,Isfahan University of Technology
Acta Materialia | Year: 2011

A detailed analysis of the microstructure and failure mechanism of a dual-phase steel material as a function of strain was conducted. Accordingly, three tensile tests were performed and interrupted at different strain levels in order to investigate void nucleation, void growth and void coalescence. Scanning electron microscopy analysis revealed that void nucleation occurs by ferrite grain-boundary decohesion in the neighborhood of martensite grains. Further, void initiation could be observed between closely situated martensite grains. Martensite morphology and distribution has a significant impact on the accumulation of damage. The mechanism of failure was found to be influenced by deformation localization due to microstructural inhomogeneity. Based on the experimental observations and simulation results, a model describing the failure mechanism is proposed for dual-phase steel material. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


News Article | March 28, 2016
Site: phys.org

A cavitation bubble is formed in the lubricant between the oil-attracting (yellow) and the oil-repellent surface (black). When used as a buffer, it might reduce wear. Credit: KIT Researchers have discovered a so far unknown formation mechanism of cavitation bubbles by means of a model calculation. In the Science Advances journal, they describe how oil-repellent and oil-attracting surfaces influence a passing oil flow. Depending on the viscosity of the oil, a steam bubble forms in the transition area. This so-called cavitation may damage material of e.g. ship propellers or pumps. However, it may also have a positive effect, as it may keep components at a certain distance and, thus, prevent damage. Materials and friction researchers wanted to know which influence chemically different surfaces have on the flow behavior of a lubricant. In particular, they were interested in flow behavior in nanometer-sized lubrication gaps, a critical case close to boundary friction, i.e. shortly before the surfaces are in direct contact. For this purpose, they generated a mathematical model, in which they varied viscosity of the lubricant and surface properties of the walls. "We were very surprised to find cavitation in the transition area of the surfaces, i.e. at the boundary between oil-attracting and oil-repellent," Dr. Lars Pastewka and Professor Peter Gumbsch of KIT's Institute for Applied Materials report. Cavitation is a physical phenomenon feared due to its destructive force. "Existing cavitation models assume a certain geometry that causes cavitation, such as a constriction in a pump or a ship's propeller producing high flow rates," Pastewka explains. Here, Bernoulli's physical law applies, according to which static pressure of a fluid decreases with increasing flow rate. If static pressure drops below the evaporation pressure of the fluid, steam bubbles are formed. If pressure increases again, e.g. if the fluid flow rate decreases after having passed a constriction in a pump, the steam in the bubbles condenses suddenly and they implode. The resulting extreme pressure and temperature peaks lead to typical cavitation craters and significant erosion even of hardened steel. "This sudden implosion of steam bubbles, however, does not occur in most lubricated tribosystems," Dr. Daniele Savio says, who has meanwhile taken up work at the Fraunhofer Institute for Mechanics of Materials in Freiburg. "As the fluid gap between two contacting surfaces usually is very narrow, the cavitation bubbles cannot grow and, hence, remain stable. The cavitation bubble then has no destructive effect and even serves as a buffer that reduces wear and friction of the surfaces. It is therefore important to generate this positive effect in a controlled manner," he adds. The simulation model of Savio and his colleagues confirms that chemically alternating surfaces may lead to cavitation bubbles. Their publication in Science Advances starts from the question of whether cavitation is the rule or an exception in situations where a lubricant flows between two surfaces. "Usually, surfaces in engines or cylinder systems are never homogeneous, i.e. only oil-attracting or oil-repellent," Savio points out. "The effect calculated by us may therefore be encountered wherever alternating neighboring surface properties exist in lubricated engines and pumps." So far, cavitation has been considered a geometric effect resulting from shear forces, flow rate, and pressure differences exclusively. "It is a completely new finding that cavitation can also occur in transition areas of alternating surface properties," Pastewka emphasizes. By the specific adjustment of surface chemistry, the researchers are convinced, interaction between surface and lubricant can be improved considerably. In the model simulations, an improved surface separation by 10% was observed. "A distance increased by 10% means that normal forces and load carrying capacities of plain bearings can be increased," Savio adds. In any case, surface chemistry has to be re-evaluated as a design element in mechanical engineering, the scientists agree. Explore further: Seeing sound in a new light More information: D. Savio et al. Boundary lubrication of heterogeneous surfaces and the onset of cavitation in frictional contacts, Science Advances (2016). DOI: 10.1126/sciadv.1501585


Schmidt I.,Fraunhofer Institute for Mechanics of Materials
Computational Mechanics | Year: 2011

This article presents the details of a numerical technique for computing the macroscopic response of a material with a given micro-structure to arbitrary prescribed loading histories. The method uses classical concepts of homogenisation theory in combination with the finite element method and focusses on the computation of macroscopic yield surfaces and inelastic strain rates. It places no restrictions on themagnitude of deformation and allows arbitrary combinations of stress- or strain control including the prescription of histories of the Cauchy stress. The method is illustrated by analysing a model material, consisting of a non-linearly elastic matrix with stiff elasto-plastic inclusions, which exhibits macroscopically associative elastoplastic material behaviour with finite elastic strains. Yield surfaces and the directions of plastic flow after a prior finite simple shear deformation are computed for this material and are shown to be consistent with an additive decomposition of the Eulerian strain rate into elastic and plastic parts and asuitable formulation of the normality rule in Cauchy stress space; a novel version of the latter is derived which is valid for an arbitrary reference configuration. © 2011 Springer-Verlag.


Korner W.,Fraunhofer Institute for Mechanics of Materials | Elsasser C.,Fraunhofer Institute for Mechanics of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We present a first-principles density functional theory study of doped ZnO with focus on its application as a transparent conducting oxide, having both high optical transparency and high electrical conductivity. Investigated is the impact of grain boundaries on the physics of atomic defects, and especially the formation energies of oxygen vacancies, cation dopants Al and Ga, and anion dopants N and P are determined. The main goal is to obtain information about the positions of the defect levels generated by the different dopants in the electronic band gap. Because of the known deficiency of the local density approximation (LDA) to yield accurate values for band gap energies for insulators such as ZnO a self-interaction correction (SIC) to the LDA is employed. As atomistic supercell models which contain grain boundaries and dopants are quite large in size we implemented the SIC by means of SIC pseudopotentials which merely increase the computational costs, as compared to the LDA. The main result of our study is that grain boundaries do affect the formation energies for substitutional dopants significantly. Furthermore the position and shape of dopant-induced electronic energy levels at the grain boundaries are changed considerably with respect to the single crystal. This may help us to explain, for example, why N doping can lead to p conductivity at room temperature or why Al or Ga doping can increase the transparency. © 2010 The American Physical Society.


Korner W.,Fraunhofer Institute for Mechanics of Materials | Elsasser C.,Fraunhofer Institute for Mechanics of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We present a density functional theory (DFT) study of doped rutile and anatase TiO2 in which we investigate the impact of grain boundaries on the physics of atomic defects. The main goal is to obtain information about the positions of the defect levels generated by an oxygen vacancy, a titanium interstitial, cation dopants Nb, Al, and Ga, and an anion dopant N in the electronic band gap having in mind the application of TiO2 as a transparent conducting oxide (TCO) or its use in heterogeneous catalysis. Due to the known deficiency of the local density approximation (LDA) of DFT to yield accurate values for band gap energies for insulators such as TiO2, a self-interaction correction (SIC) to the LDA is employed. The main result of our study is that grain boundaries do affect the defect formation energies as well as the position and shape of the dopant-induced electronic energy levels significantly with respect to the single crystal. According to our study Nb doping may lead to n-conducting TiO2 whereas doping with N, Al, or Ga is not promising in order to achieve p-conducting TiO2. Furthermore an increase in the photoconductivity of TiO2:N and the colorlessness of TiO2:Al may be explained by our results. © 2011 American Physical Society.


Hashibon A.,Fraunhofer Institute for Mechanics of Materials | Elsasser C.,Fraunhofer Institute for Mechanics of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We present a first-principles study of the native point defects in the thermoelectric material Bi2Te3. Calculated formation energies of defects and electronic densities of states were analyzed in detail. The most prominent native point defects considered are vacancies and antisite defects on the Bi, Te1, and Te2 sublattices of the Bi2Te3 structure. Vacancies on all three sublattices are found to have much higher formation energies than antisite defects. The most dominant antisite defects are found to be BiTe1 at Bi-rich conditions, and TeBi at Te-rich conditions. These lead to the formation of resonant defect states at the top of the valence band and bottom of the conduction band, respectively. Hence they are expected to impact charge and energy transport in a profound way. Furthermore antisite defect pairs tend to form at nearest-neighbor distances, and lead to substantial changes in the electronic structure and hence in the thermoelectric properties of Bi2Te3. © 2011 American Physical Society.

Loading Fraunhofer Institute for Mechanics of Materials collaborators
Loading Fraunhofer Institute for Mechanics of Materials collaborators