Influence of subsurface microstructure on the running-in of an AlSi alloy

The friction and wear behavior of a lubricated AlSi11Cu3 disk in contact with a 100Cr6 pin was studied by a radionuclide-assisted pin-on-disk tribometer. It is well known that shear forces change chemistry and microstructure of the near-surface material, thereby influencing friction and wear. To better understand the influences of the microstructure on the running-in behavior, disks with different silicon phase morphologies were tested under constant stressing conditions. Topography, chemistry and microstructure of pin and disk were characterized before and after tribological testing by white light interferometry, X-ray photoelectron. spectroscopy, Auger electron spectroscopy and focused ion beam microscopy. Wear of pins was measured with a radionuclide technique to resolve ultra-low wear rates. To characterize near-surface deformations, Ga ion markers were implanted and Auger electron spectroscopy was applied to follow their shear-induced displacements. To monitor subsurface shear, the deformation of columnar markers was analyzed. The results were discussed using Godet's third body model. Furthermore, the results allowed to us extend the model to systems operated under ultra-low wear rates

Provitamin A biofortification of cassava enhances shelf life but reduces dry matter content of storage roots due to altered carbon partitioning into starch

Storage roots of cassava (Manihot esculenta Crantz), a major subsistence crop of sub-Saharan Africa, are calorie rich but deficient in essential micronutrients, including provitamin A β-carotene. In this study, β-carotene concentrations in cassava storage roots were enhanced by coexpression of transgenes for deoxy-d-xylulose-5-phosphate synthase (DXS) and bacterial phytoene synthase (crtB), mediated by the patatin-type 1 promoter. Storage roots harvested from field-grown plants accumulated carotenoids to ≤50 lg/g DW, 15- to 20-fold increases relative to roots from nontransgenic plants. Approximately 85%–90% of these carotenoids accumulated as all-trans-β-carotene, the most nutritionally efficacious carotenoid. β-Carotene-accumulating storage roots displayed delayed onset of postharvest physiological deterioration, a major constraint limiting utilization of cassava products. Large metabolite changes were detected in β-carotene-enhanced storage roots. Most significantly, an inverse correlation was observed between β-carotene and dry matter content, with reductions of 50%–60% of dry matter content in the highest carotenoid-accumulating storage roots of different cultivars. Further analysis confirmed a concomitant reduction in starch content and increased levels of total fatty acids, triacylglycerols, soluble sugars and abscisic acid. Potato engineered to co-express DXS and crtB displayed a similar correlation between β-carotene accumulation, reduced dry matter and starch content and elevated oil and soluble sugars in tubers. Transcriptome analyses revealed a reduced expression of genes involved in starch biosynthesis including ADP-glucose pyrophosphorylase genes in transgenic, carotene-accumulating cassava roots relative to nontransgenic roots. These findings highlight unintended metabolic consequences of provitamin A biofortification of starch-rich organs and point to strategies for redirecting metabolic flux to restore starch production

Variations in strain affect friction and microstructure evolution in copper under a reciprocating tribological load

The microstructure of the materials constituting a metallic frictional contact strongly influences tribological performance. Being able to tailor friction and wear is challenging due to the complex microstructure evolution associated with tribological loading. Here, we investigate the effect of the strain distribution on these processes. High-purity copper plates were morphologically surface textured with two parallel rectangles—referred to as membranes—over the entire sample length by micro-milling. By keeping the width of these membranes constant and only varying their height, reciprocating tribological loading against sapphire discs resulted in different elastic and plastic strains. Finite element simulations were carried out to evaluate the strain distribution in the membranes. It was found that the maximum elastic strain increases with decreasing membrane stiffness. The coefficient of friction decreases with increasing membrane aspect ratio. By analyzing the microstructure and local crystallographic orientation, we found that both show less change with decreasing membrane stiffness

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Einlaufverhalten einer untereutektischen AlSi-Legierung unter Berücksichtigung des Randzonengefüges

Unter Berücksichtigung des Einlaufverhaltens konnte an tribologischen Systemen vergleichbarer Rauheit ein vom Randzonengefüge abhängiges tribologisches Verhalten gefunden werden. Die Unterschiede ließen sich mit Hilfe von Markerexperimenten auf unterschiediche Tiefen der plastischen Verformung zurückführen. Niedrigste Verschleißraten werden durch Randzonengefüge erzeugt, die eine Konzentration der Scherung an die Oberfläche ermöglichen

Multi-Phase Friction and Wear Reduction by Copper Nanopartices

Finely dispersed copper nanoparticles were added as an additive to fully-formulated engine oils. The copper additive was in colloidal form, with an inner core of Cu2+ atoms covered by surfactants to form stable reverse micelles that are completely dispersible in the base oil. The tribological process to form protective films at the metal surface is comprised of three phases. Phase I can be considered a physical process involving the build-up of polar molecules by absorption to produce a friction modifier film, whereas phases II and III have to be treated as mechanochemical processes comprising a combination of redox reactions and a third body formation. The tribological performance was investigated using atomic force microscopy, a microtribometer, a pin-on-disk tribometer in combination with continuous and high-resolution wear measurements with radionuclide technique, and high pressure stressing in a thrust roller bearing test rig. In addition, the nanostructure of the additive was characterized by atomic force microscopy. Finally, the chemical composition of the metal surface was analyzed using photoelectron spectroscopy

Influence of subsurface plastic deformation on the running-in behavior of a hypoeutectic AlSi alloy

The present publication elaborates on the influence of the initial microstructure preset by machining on the friction and wear behavior of a hypoeutectic AlSi9Cu3 alloy. During running-in the subsurface microstructure of tribological contacts is subjected to shear followed by plastic deformation. Aluminum disks were tested with a steel pin on a pin-on-disk tribometer with focus on running-in and sensitivity phenomena. The systems were operated in the ultra-low wear regime. Wear was measured continuously using Radionuclide Technique. The microstructure of the samples was characterized by nanoindentation, x-ray photoelectron spectroscopy and focused ion beam microscopy. The visualization of subsurface shear by markers in the disks showed a significant correlation of subsurface deformation and wear. With the experiments the relationship between machining and tribological behavior was analyzed and ways to optimize tribological systems were shown

The running-in corridor of lubricated metal-metal contacts

In this paper the question is raised whether the coefficient of friction and the wear rate of a lubricated metal-metal system after passing the running-in can be deduced from the initial friction power density this tribological system was subjected to. This contribution defines a running-in corridor as specific energetic range in which the tribological system is able to develop ultra-low wear rates and small coefficients of friction. It will be shown that this corridor is associated with the formation of the third-body. The running-in corridor has a certain width which depends on external tribological stressing conditions, on materials, lubricants and mainly on the initial coefficient of friction. Using two different material pairings it will be demonstrated how tribological systems can be taught to find the route into the running-in corridor. Furthermore, levers of optimization employing friction-modifying additives or appropriate final machining routines will be discussed. The results of this con