Lubricated sliding wear behaviour of aluminium alloy composites

Interest in aluminium alloy (Al-alloy) composites as wear resistant materials continues to grow. However, the use of the popular Al-alloy-SiC composite can be limited by the abrasive nature of the SiC, leading to increased counterface wear rates. This study reports new Al-alloy composites that offer high wear resistance, to a level similar to Al-alloy-SiC. Aluminium alloy (2124, 5056) matrix composites reinforced by nominally 15 vol.% of Cr3Si, MoSi2, Ni3Al and SiC particles were prepared by a powder metallurgy route. The aluminium alloy matrix was produced by gas atomisation, and the Cr3Si, MoSi2 and Ni3Al were prepared by self-propagating high temperature synthesis (SHS), while the SiC was from a standard commercial supply. Following blending, the particulates were consolidated by extrusion, producing a homogenous distribution of the reinforcement in the matrix. Wear testing was undertaken using a pin-on-ring configuration against an M2 steel counterface, with a commercial synthetic oil lubricant, at 0.94 m/s and a normal load of 630 N, corresponding to initial Hertzian contact pressures of 750–890 MPa (the exact value depending on the material properties). Specific wear rates at sliding distances exceeding 400 km were in the range 4.5–12.7 × 10?10 mm3/Nm. The monolithic alloys gave the highest specific wear rates, while the MoSi2 and Cr3Si reinforced alloys exhibited the lowest. The worn surface has been analysed in detail using focused ion beam (FIB) microscopy to determine the sub-surface structural evolution and by tomographic reconstruction of tilted scanning electron microscopy (SEM) images, to determine the local worn surface topography. Consequently, the wear mechanisms as a function of alloy composition and reinforcement type are discussed.<br/

Fe nano-particle coatings for high temperature wear resistance

Oxidational wear continues to present an economic challenge for the replacement of components subject to high temperature fretting and sliding contacts in applications such as gas turbine engines. At elevated temperatures, low friction oxide ‘glaze’ layers can form and act as an interface between the contact and the substrate material. Whilst desirable, the glaze is formed from wear debris and often consumes the underlying substrate material. In order to induce rapid formation of low friction oxide layers without a severe ‘running-in’ period, nano particles of Fe in the range 5-10nm were deposited on ground flat ended pin and plate 080M40 substrates using a terminated gas condensation PVD process, to a thickness of 600nm. Coatings were tested in a reciprocating geometry at a fixed stroke length of 0.4mm, frequency of 31Hz and 40N normal load (1MPa contact stress) and at ambient, 300°C and 540°C. At ambient temperature the coated surfaces exhibited higher friction but lower wear compared to the uncoated substrates, whereas at elevated temperatures, the coated surfaces exhibited slightly lower steady state dynamic friction coefficients, and minimal changes in wear depth after a short incubation period. SEM of the worn surfaces indicated that hard oxide plateaus were responsible for the load bearing contact area at elevated temperatures. Cross sectional FIB, TEM and SIMS confirmed that at elevated temperatures, the nano-particle coating induced rapid formation of a nano-crystalline porous surface oxide film of mixed composition which protected the substrate from severe wear during the running-in period

Results of a UK industrial tribological survey

During the summer of 2012, the National Centre for Advanced Tribology at Southampton (nCATS) undertook a UK-wide industrial tribological survey in order to assess the explicit need for tribological testing within the UK. The survey was designed and implemented by a summer intern student, Mr Simon King, under the supervision of Drs John Walker and Terry Harvey and supported by the director of nCATS, Professor Robert Wood. The survey built upon on two previous tribological surveys conducted through the National Physical Laboratory (NPL) during the 1990’s. The aim was to capture a snapshot of the current use of tribological testing within UK industry and its perceived reliability in terms of the test data generated. The survey also invited participants to speculate about how UK tribology could improve its approach to testing. The survey was distributed through the nCATS industrial contact list, which comprises of over 400 contacts from a broad spectrum of commercial industries. The Institute of Physics (IOP) tribology group also assisted by distributing the survey to its membership list. A total of 60 responses were received for the survey, out of which 39 had fully completed the questionnaire. Participants came from a broad spread of industrial backgrounds, with the energy sector having the highest representation. Only 40% of respondents were dedicated tribologists/surface engineers, again reflecting the multi-disciplinary nature of the field. It was found that the companies that had the highest annual turnover also appeared to expend the most on tribology. The majority of respondents indicated that as a percentage of turnover tribology accounted for less than 1%, however the lack of hard figures only for tribology make this a conservative estimate. The greatest concern in relation to tribology of those who responded was the cost; however the influence of legislation and product reliability were also driving factors. Abrasive wear was still considered the number one tribological wear mechanism, with sliding contacts ranking as the most common type of wear interface. Metallic and hard coated surfaces were the most commonly encountered type of material suffering from tribological wear phenomena. Laboratory scale testing was a significant part of introducing a new tribological component, however component specific testing was considered the most reliable form of testing a new component over standardised test geometries. Overall there appeared to be much potential for improving the reliability of tribological test data, with most respondents indicating that simply more testing was not the best perceived approach to improving tribological data but rather more reliable, representative tests with improved knowledge capture. Most companies possessed an internal database to assist them with tribological information; however, many also expressed a strong desire for the use of a commercial or national database, although the format this might take was less clear. Opinions appeared split as to whether there would be a collective willingness to contribute to a centralised database, presumably on the grounds on the sensitivity of data

A spinor approach to Walker geometry

A four-dimensional Walker geometry is a four-dimensional manifold M with a neutral metric g and a parallel distribution of totally null two-planes. This distribution has a natural characterization as a projective spinor field subject to a certain constraint. Spinors therefore provide a natural tool for studying Walker geometry, which we exploit to draw together several themes in recent explicit studies of Walker geometry and in other work of Dunajski (2002) and Plebanski (1975) in which Walker geometry is implicit. In addition to studying local Walker geometry, we address a global question raised by the use of spinors.Comment: 41 pages. Typos which persisted into published version corrected, notably at (2.15

Strong Electron-Phonon Coupling in Superconducting MgB2_2: A Specific Heat Study

We report on measurements of the specific heat of the recently discovered superconductor MgB$_2$ in the temperature range between 3 and 220 K. Based on a modified Debye-Einstein model, we have achieved a rather accurate account of the lattice contribution to the specific heat, which allows us to separate the electronic contribution from the total measured specific heat. From our result for the electronic specific heat, we estimate the electron-phonon coupling constant $\lambda$ to be of the order of 2, significantly enhanced compared to common weak-coupling values $\leq 0.4$. Our data also indicate that the electronic specific heat in the superconducting state of MgB$_2$ can be accounted for by a conventional, s-wave type BCS-model.Comment: 4 pages, 4 figure

Surface and capillary transitions in an associating binary mixture model

We investigate the phase diagram of a two-component associating fluid mixture in the presence of selectively adsorbing substrates. The mixture is characterized by a bulk phase diagram which displays peculiar features such as closed loops of immiscibility. The presence of the substrates may interfere the physical mechanism involved in the appearance of these phase diagrams, leading to an enhanced tendency to phase separate below the lower critical solution point. Three different cases are considered: a planar solid surface in contact with a bulk fluid, while the other two represent two models of porous systems, namely a slit and an array on infinitely long parallel cylinders. We confirm that surface transitions, as well as capillary transitions for a large area/volume ratio, are stabilized in the one-phase region. Applicability of our results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical Review E; corrected versio

Symmetry Properties on Magnetization in the Hubbard Model at Finite Temperatures

By making use of some symmetry properties of the relevant Hamiltonian, two fundamental relations between the ferromagnetic magnetization and a spin correlation function are derived for the $d (=1,2,3)$-dimensional Hubbard model at finite temperatures. These can be viewed as a kind of Ward-Takahashi identities. The properties of the magnetization as a function of the applied field are discussed. The results thus obtained hold true for both repulsive and attractive on-site Coulomb interactions, and for arbitrary electron fillings.Comment: Latex file, no figur

The relationship of leaf photosynthetic traits - V-cmax and J(max) - to leaf nitrogen, leaf phosphorus, and specific leaf area: a meta-analysis and modeling study

Great uncertainty exists in the global exchange of carbon between the atmosphere and the terrestrial biosphere. An important source of this uncertainty lies in the dependency of photosynthesis on the maximum rate of carboxylation (Vcmax) and the maximum rate of electron transport (Jmax). Understanding and making accurate prediction of C fluxes thus requires accurate characterization of these rates and their relationship with plant nutrient status over large geographic scales. Plant nutrient status is indicated by the traits: leaf nitrogen (N), leaf phosphorus (P), and specific leaf area (SLA). Correlations between Vcmax and Jmax and leaf nitrogen (N) are typically derived from local to global scales, while correlations with leaf phosphorus (P) and specific leaf area (SLA) have typically been derived at a local scale. Thus, there is no global-scale relationship between Vcmax and Jmax and P or SLA limiting the ability of global-scale carbon flux models do not account for P or SLA. We gathered published data from 24 studies to reveal global relationships of Vcmax and Jmax with leaf N, P, and SLA. Vcmax was strongly related to leaf N, and increasing leaf P substantially increased the sensitivity of Vcmax to leaf N. Jmax was strongly related to Vcmax, and neither leaf N, P, or SLA had a substantial impact on the relationship. Although more data are needed to expand the applicability of the relationship, we show leaf P is a globally important determinant of photosynthetic rates. In a model of photosynthesis, we showed that at high leaf N (3 gm−2), increasing leaf P from 0.05 to 0.22 gm−2 nearly doubled assimilation rates. Finally, we show that plants may employ a conservative strategy of Jmax to Vcmax coordination that restricts photoinhibition when carboxylation is limiting at the expense of maximizing photosynthetic rates when light is limiting