Impact of variable loading conditions on fretting wear

Fretting is considered as a specific type of reciprocating sliding. It is defined as a small displacement amplitude oscillatory motion between two solids in contact, usually induced by vibrations. Depending on the loading conditions (displacement amplitudes, normal loading), fretting causes damage by surface fatigue and wear induced by debris formation. To prevent such damage, numerous hard coatings have been developed which improve the wear resistance of contacts. However, one difficulty is to estimate how long it will be before the coating wears through. Studies have been conducted to analyze the effect of displacement amplitude, normal force or ambient atmosphere, but usually under constant loading conditions. Such a situation is far from real operating components, where elements are subjected to variable loadings implying variable displacement amplitudes. To predict the durability of a coating under variable fretting displacements, wear depth is quantified as a function of the maximum accumulated dissipated energy density by derivation from a global energy wear approach. This model is compared to TiC vs. alumina fretting experiments. Very good correlation is observed between the prediction and the wear depth, independently of the applied variable amplitude sequences. An equivalent “Miner-Energy” wear model is introduced which permits the durability of the coating to be estimated

Fretting wear of TiN PVD coating under variable relative humidity conditions – development of a “composite” wear law

Fretting is defined as a small oscillatory displacement between two contacting bodies. The interface is damaged by debris generation and its ejection from the contact area. The application of hard coatings is an established solution to protect against fretting wear. For this study the TiN hard coating manufactured by a PVD method has been selected, and tested against a polycrystalline alumina smooth ball. A fretting test programme has been carried out at a frequency of 5 Hz, 100 N normal load, 100 μm displacement amplitude and at five values of relative humidity: 10, 30, 50, 70 and 90% at a temperature of 296 K. The intensity of the wear process is shown to be significantly dependent on the environmental conditions. A dissipated energy approach has been employed in this study to quantify wear rates of the hard coating. The approach predicts wear kinetics under constant medium relative humidity in a stable manner. It has been shown that an increase of relative humidity promotes the formation of hydrate structures at the interface and modifies the third body rheology. This phenomenon has been characterised by the evolution of wear kinetics associated with a significant variation of the corresponding energy wear coefficient. Hence, a ‘composite’ wear law, integrating the energy wear coefficient as a function of relative humidity, is introduced. It permits a prediction of wear under variable relative humidity conditions from 10 to 90% within a single fretting test. The stability of this approach is demonstrated by comparing various variable relative humidity sequences

Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5Hz have been carried out under 50-150N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50µm to 200µm at a constant value of relative humidity of 50% at 296K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical 1 dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated

3D-anatomy and systematics of cocculinid-like limpets (Gastropoda: Cocculiniformia): more data, some corrections, but still an enigma

New material and new methodologies substantially widen the anatomical knowledge on cocculinid limpets. We first provide 3D-anatomies of Fedikovella caymanensis and Teuthirostria cancellata based on serial sections. Both species differ in several major points (mainly the gill-type and several features of the alimentary tract) from typical cocculinids, accordingly they are classified in a new clade, Teuthirostriidae fam. nov. Specimens studied by McLean and Harasewych (LA County Mus Contrib Sci 453:1-33, 1995) under Fedikovella beanii probably represent another species new to science. Additional investigations of original (type) section series of Cocculina laevis Thiele, 1904 (type species of Paracocculina Haszprunar, 1987) and of Cocculina radiata Thiele, 1904 (type species of Coccocrater Haszprunar, 1987) imply some nomenclatorial revisions: Cocculina cervae Fleming, 1948 is designated as type species of Pedococculina gen. nov. Anatomical characters confirm the subsequent placement of Cocculina viminensis Rocchini, 1990 into Coccopigya Marshall, 1986, whereas the original generic status of the whale-fall inhabitant Cocculina craigsmithi McLean, 1992 is confirmed despite the unusual habitat. The latter species probably has symbiotic bacteria in the midgut gland;if so this might be due to the environmental and feeding conditions at whale cadavers or hydrothermal vents. Contrary to Lepetelloidea, the Cocculiniformia cannot be included in Vetigastropoda. Recent molecular data support a sistergroup relationship of Cocculiniformia with Neomphalida, and we add the phenotypic perspective on this so-called Neomphaliones-hypothesis. In particular, more phylogenomic data are needed to specify the position of Cocculinida among the rhipidoglossate Gastropoda

Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices

The electromagnetic propagator associated with the localized collective magnetoplasmon excitations in a semiconductor superlattice with broken translational symmetry, is calculated analytically within linear response theory. We discuss the properties of these collective excitations in both radiative and non-radiative regimes of the electromagnetic spectra. We find that low frequency retarded modes arise when the surface density of carriers at the symmetry breaking layer is lower than the density at the remaining layers. Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes occurs.Comment: Revtex file + separate pdf figure

Mean parameter model for the Pekar-Fr\"{o}hlich polaron in a multilayered heterostructure

The polaron energy and the effective mass are calculated for an electron confined in a finite quantum well constructed of $GaAs/Al_x Ga_{1-x} As$ layers. To simplify the study we suggest a model in which parameters of a medium are averaged over the ground-state wave function. The rectangular and the Rosen-Morse potential are used as examples. To describe the confined electron properties explicitly to the second order of perturbations in powers of the electron-phonon coupling constant we use the exact energy-dependent Green function for the Rosen-Morse confining potential. In the case of the rectangular potential, the sum over all intermediate virtual states is calculated. The comparison is made with the often used leading term approximation when only the ground-state is taken into account as a virtual state. It is shown that the results are quite different, so the incorporation of all virtual states and especially those of the continuous spectrum is essential. Our model reproduces the correct three-dimensional asymptotics at both small and large widths. We obtained a rather monotonous behavior of the polaron energy as a function of the confining potential width and found a peak of the effective mass. The comparison is made with theoretical results by other authors. We found that our model gives practically the same (or very close) results as the explicit calculations for potential widths $L \geq 10 \AA$.Comment: 12 pages, LaTeX, including 5 PS-figures, subm. to Phys. Rev. B, new data are discusse

Structure and optical properties of silicon layers with GaSb nanocrystals created by ion-beam synthesis

We have studied the ion-beam synthesis of GaSb nanocrystals in Si by high-fluence implantation of Sb and Ga ions followed by thermal annealing. RBS, TEM/TED, RS, and photoluminescence (PL) were employed to characterize the implanted layers. It was found that the nanocrystals size increases from 5 to 60 nm in the samples annealed at 900 8Cup to 20–90 nm in the samples annealed at 1100 8C. An existence of significant mechanical stresses within implanted layers has been detected. The stress values have been calculated from the shift of the Si first order Raman band. For the samples annealed at 900 8C a broad band in the spectral region of about 0.75–1.05 eV is detected in the PL spectra. The nature of this PL band is discussed

Cryo-EM structures reveal intricate Fe-S cluster arrangement and charging in Rhodobacter capsulatus formate dehydrogenase

Metal-containing formate dehydrogenases (FDH) catalyse the reversible oxidation of formate to carbon dioxide at their molybdenum or tungsten active site. They display a diverse subunit and cofactor composition, but structural information on these enzymes is limited. Here we report the cryo-electron microscopic structures of the soluble Rhodobacter capsulatus FDH (RcFDH) as isolated and in the presence of reduced nicotinamide adenine dinucleotide (NADH). RcFDH assembles into a 360 kDa dimer of heterotetramers revealing a putative interconnection of electron pathway chains. In the presence of NADH, the RcFDH structure shows charging of cofactors, indicative of an increased electron load

Application of multiplexed ion mobility spectrometry towards the identification of host protein signatures of treatment effect in pulmonary tuberculosis.

RationaleThe monitoring of TB treatments in clinical practice and clinical trials relies on traditional sputum-based culture status indicators at specific time points. Accurate, predictive, blood-based protein markers would provide a simpler and more informative view of patient health and response to treatment.ObjectiveWe utilized sensitive, high throughput multiplexed ion mobility-mass spectrometry (IM-MS) to characterize the serum proteome of TB patients at the start of and at 8 weeks of rifamycin-based treatment. We sought to identify treatment specific signatures within patients as well as correlate the proteome signatures to various clinical markers of treatment efficacy.MethodsSerum samples were collected from 289 subjects enrolled in CDC TB Trials Consortium Study 29 at time of enrollment and at the end of the intensive phase (after 40 doses of TB treatment). Serum proteins were immunoaffinity-depleted of high abundant components, digested to peptides and analyzed for data acquisition utilizing a unique liquid chromatography IM-MS platform (LC-IM-MS). Linear mixed models were utilized to identify serum protein changes in the host response to antibiotic treatment as well as correlations with culture status end points.ResultsA total of 10,137 peptides corresponding to 872 proteins were identified, quantified, and used for statistical analysis across the longitudinal patient cohort. In response to TB treatment, 244 proteins were significantly altered. Pathway/network comparisons helped visualize the interconnected proteins, identifying up regulated (lipid transport, coagulation cascade, endopeptidase activity) and down regulated (acute phase) processes and pathways in addition to other cross regulated networks (inflammation, cell adhesion, extracellular matrix). Detection of possible lung injury serum proteins such as HPSE, significantly downregulated upon treatment. Analyses of microbiologic data over time identified a core set of serum proteins (TTHY, AFAM, CRP, RET4, SAA1, PGRP2) which change in response to treatment and also strongly correlate with culture status. A similar set of proteins at baseline were found to be predictive of week 6 and 8 culture status.ConclusionA comprehensive host serum protein dataset reflective of TB treatment effect is defined. A repeating set of serum proteins (TTHY, AFAM, CRP, RET4, SAA1, PGRP2, among others) were found to change significantly in response to treatment, to strongly correlate with culture status, and at baseline to be predictive of future culture conversion. If validated in cohorts with long term follow-up to capture failure and relapse of TB, these protein markers could be developed for monitoring of treatment in clinical trials and in patient care