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  • 1
    ISSN: 1573-2711
    Keywords: static friction ; surface force apparatus ; confined liquid lubricant ; history-dependent shear response ; deformation threshold
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We studied the shear response of a confined lubricant layer on approach of the transition to sliding with a surface force apparatus modified for oscillatory shear. In a given experiment, we found that the transition to sliding occurred always around the same deformation amplitude although the shear stress needed to initiate sliding varied up to a factor of two depending on sample history. This suggests the concept of deformation-controlled switching from rest to sliding. The elastic spring-constant, in the stick state, weakened with increasing deformation amplitude. This decrease can be described by a power law when plotted versus the distance to a critical deformation amplitude. The build-up of solid-like behavior after sliding stopped was also gradual and was consistent with a logarithmic time dependence. We suggest a model relating the gradual decrease of stiffness to weakening of the boundary layer, specifically to destruction of some elastic links between molecules or between molecules and the solid surfaces. Static friction (the force that must be overcome at the onset of kinetic motion) is proportional to the number of such links formed during the time of stick.
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  • 2
    ISSN: 1573-2711
    Keywords: scanning force and friction microscopy (FFM) ; nanotribology ; electrochemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The local chemical properties of the contacting asperities in a real tribosystem are of crucial importance for the resulting macroscopic tribo-behavior. Thus, the lateral forces acting on the tip of a standalone scanning force and friction microscope have been investigated as a function of controlled surface chemistry, realized by potentiostatic control of the sample. The results obtained show a clear dependence of nanoscale friction behavior upon changes in the electrochemical state of the system.
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  • 3
    ISSN: 1573-2711
    Keywords: chloroform ; extreme-pressure lubrication ; additives ; surface chemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Carbon tetrachloride is an extremely good extreme-pressure (EP) lubricant additive at low concentrations (〈3 wt% chlorine) since it can react to form a high-melting-point Fe3C antiseizure layer. In contrast, small hydrogen-containing additive molecules (CH2Cl2, CHCl3) decompose to form FeCl2 which melts at ~940 K and limits the maximum seizure load to ~3500 N as measured hi a pin and v-block apparatus. However, both thermodynamic calculations and results of a Mössbauer analysis of an iron foil heated in CHCl3 at 830 K indicate that iron carbide can be formed from chloroform. In addition, it is also found in that case that a plot of seizure load versus concentration, after initially forming a plateau, once again increases with higher additive concentrations (〉4 wt% chlorine) in accord with the idea that a higher melting point carbide film can be formed. It has been shown previously that asymptotes in the plot of removal rate versus applied load correspond to melting of the interfacial anti-seizure film. When using 9.0 wt% chlorine from chloroform as the additive, a drastic increase in removal rate is found at an interfacial temperature of ~940 K corresponding to the melting of FeCl2 and an additional asymptote is evident at ~1500 K due to the melting of Fe3C in accord with the thermodynamic and Mössbauer results.
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  • 4
    ISSN: 1573-2711
    Keywords: lubrication ; solid lubrication ; high-temperature lubrication ; ceramics ; silicon nitride ; silicon oxycarbide ; Raman spectroscopy ; photoluminescence ; fluorescence spectroscopy ; tribochemistry ; surface chemistry ; carbon ; friction polymers ; ethylene ; acetylene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract While lubrication of hot-pressed silicon nitride surfaces at temperatures between 350 and 650°C by carbons deposited from carbonaceous gases and vapors has been demonstrated to be very effective (friction coefficients of less than 0.02 and negligible wear), a chemical mechanism has not yet been established. Tribometer experiments followed by surface spectroscopies of Raman, fluorescence, Auger and XPS are consistent with a heterogeneous mechanism of vapor decomposition. Laser-induced photoluminescence determined with a Raman spectrometer using 514 nm argon ion excitation showed a nearly 1 ∶: 1 correspondence between the intensity of the broad maximum near 4000 cm-1 with carbon adsorption and lubrication efficiency: surfaces of low fluorescence could not be lubricated by ethylene or similar gases. Carbon was found to adsorb preferentially on wear tracks, which also exhibited very high fluorescence. Wear track surfaces generally exhibited a lower N/Si surface composition ratio than the surrounding surfaces. The reduced nitrogen content could be responsible for dangling bonds and structural defects, which would result in more chemisorption and strong fluorescence.
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  • 5
    ISSN: 1573-2711
    Keywords: extreme pressure additives ; aluminum ; boundary-lubricated wear
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract To investigate the effects of phosphorus-containing additives not only on steel but also on aluminum surfaces comprising tribocouples, the effect of phosphates and phosphites on the wear of aluminum was evaluated under boundary lubrication conditions. The chemical reactivity of additives was investigated by differential thermal analysis. X-ray photoelectron spectroscopy and Auger electron spectroscopy were used to determine the tribochemical reaction between additives and aluminum. The active lubrication mechanism of P-containing additives with aluminum is proposed.
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  • 6
    ISSN: 1573-2711
    Keywords: friction ; wear ; diamond ; tribochemistry ; debris
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Molecular dynamics simulations have been used to examine the friction between two surfaces placed in sliding contact. These simulations show that the presence of methane-debris molecules squeezed between two hydrogen-terminated diamond (111) surfaces significantly reduces the friction compared to the same two surfaces in the absence of debris. These findings are in agreement with macroscopic experiments that have examined the friction of diamond on diamond in the presence of debris. In addition, the friction coefficients are significantly lower compared to the same system with methyl groups chemisorbed to one of the diamond surfaces.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Tribology letters 2 (1996), S. 99-111 
    ISSN: 1573-2711
    Keywords: erosion ; abrasion ; cast iron ; slurry erosion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Erosion resistance of four types of cast iron of different microstructures and graphite morphologies (viz., grey cast iron, compacted graphite iron, spheroidal graphite iron and austempered ductile iron) was evaluated in three different erosive media. Results indicate that austempered ductile iron has the highest erosion resistance in all three media, followed by spheroidal graphite iron, compacted graphite iron and grey cast iron, in that order. Graphite morphology has a significant effect on the erosion resistance of these irons in quartz-water and iron oxide-oil slurry. However, the matrix microstructure determines the erosion resistance of these irons in quartz-oil slurry. The parameter H/E (which is the ratio of the Brinell hardness number to Young's modulus of the material) has been found to be a good indicator of erosive wear in quartz-oil slurry.
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  • 8
    ISSN: 1573-2711
    Keywords: moisture absorption ; composites ; strains ; model ; wear
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Moisture absorption degrades interfacial strength and induces expansional strains in fibre-polymer composites. These strains subsequently affect the abrasive wear behaviour of composites subjected to boiling water treatment. A wear model is suggested, which quantitatively estimates the wear performance of boiling-water-treated glass-polyester composites. The theoretical model is in concurrence with experimental results.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Tribology letters 2 (1996), S. 189-197 
    ISSN: 1573-2711
    Keywords: scanning tunneling microscope ; ZDDP ; graphite ; structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We have studied the adsorption of zinc O,O-diisopropylphosphoro-dithioate (ZDDP) adsorption on graphite at room temperature by STM. Two different structures corresponding respectively to monomer and dimer were observed on this surface although only the dimer was found to exist in the crystalline state. Molecularly resolved images showed that the molecular packing is significantly different from that in the bulk crystalline state. The monomers stand on the substrate to form a regular structure with a unit cell of 4.6 × 6.5 Å2 (β = 110°). The dimers, imaged as elongated protrusions, lie on the substrate to form a regular structure with a unit cell of 6.5 × 9.0 Å2 (β = 125°). The coexistence of both monomer and dimer on graphite reflects competition between the molecule-molecule and molecule-substrate interaction for which the former favors the dimeric form and the latter favors the monomeric form.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Tribology letters 2 (1996), S. 173-187 
    ISSN: 1573-2711
    Keywords: tribometry ; high temperature ; single-crystal silicon ; polycrystalline silicon ; vacuum ; surface chemistry ; dangling bonds ; reconstruction ; adsorbates ; friction ; wear
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract SEM tribometric experiments were performed with Si(100) vs. Si(100) interfaces in mode-rate vacuum to 850°C. The results are compared with similar tests previously completed with fine-cauliflowered PCD (PCDfcf) mated against itself, and polished C(100)-textured polycrystalline diamond (PCDC(100)) sliding against Si(100). All data agree with a hypothesis connecting the thermal desorption of adsorbates and wear with the generation of dangling bonds on the sliding surfaces. Linking of the counterfaces by the free radicals appears to be the main cause of high adhesion and friction. The high friction can be drastically reduced by dissociative chemisorption of certain passivating gaseous species condensing at sufficiently low surface temperatures. Strong circumstantial evidence continues to mount for the incremental reduction in high temperature friction being caused by surface reconstruction. Deconstruction of the sliding surfaces and the reemergence of high friction eventually occurs on discontinued heating, until the adsorbates chemisorb on the cooled surfaces. There, the friction drops to a level determined by the characteristic shear strength of the interfaces and the wear-induced increase in the real area of contact. The maximum friction measured at high temperatures in vacuum, indicative of the most intensive interaction of dangling bonds at the interface, scaled only approximately with the 1.8 times strength of the C-C versus the Si-Si bonds. The 1.6 experimental ratio is lower than the theoretical, reflecting the broad distribution of dangling bond energies (densities of surface trap states) for PCD and even for polished Si(100). The wear rate of Si(100) sliding against itself is about four-orders-of-magnitude higher (~ 2 × 10-12 m3/(Nm)) than that of unpolished PCDfcf vs. itself (4 × 10-16 m3/(Nm)) or rough and unpolished PCDC(100) wearing its polished version (8.5 × 10-16 m3/(Nm)).
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