1,721,272 research outputs found
Using self-assembly techniques for green lubrication solutions in tribological contacts
No full textThis thesis presents a novel lubrication solution for silicon nitride hybrid bearings developed through the use of polymer brush technology, specifically brushes created using surface initiated atom transfer radical polymerisation. This work also details a novel testing regime utilising custom colloidal probes to replicate, for the first time, this hybrid bearing under atomic force microscopy in both dry and lubricated conditions. Due to their promising tribological properties polymer brushes have the potential to be a lubrication solution for the hybrid bearing system where current lubrication solutions are not tailored to the surfaces and contain harmful components such as sulphur and phosphorus. Polymer brush systems have generated considerable interest in the academic community as a possible new greener lubrication solution. To further understand the mechanism by which an effective polymer brush can be employed in a tribological contact this study was initiated. As the first known study to investigate the effect of the polymer brushes on the silicon nitride-steel contact, previous literature findings have been reapplied to a novel material for a novel application. Grafting from the silicon nitride surface ensures that less additive competition will occur. Poly(methyl methacrylate) (PMMA) brushes were chosen for the reduction of steric hindrance within the polymer chain therefore allowing a higher density brush and better load carrying capacity in a tribological sense. These brushes act synergistically with a poly-alpha-olefin, a high quality base oil lubricant present in the type of engine where these hybrid bearing operate. The synergy here refers to the swelling effect in which the anchored macromolecule and base oil work as one to repel the asperity contacts, reducing friction whilst the brush system protects itself. The formation of polymer brushes on a silicon nitride surface utilises atom transfer radical polymerisation (ATRP) and activators regenerated by electron transfer (ARGET) coupled with a surface initiation step. Initiating from the surface allows a strong covalent bond to the contact surface ensuring stability when the final brush is subjected to physical interactions, the main advantage being that by adding monomer molecules individually in situ the steric interaction of the chain-chain iterations in the growing brush is reduced so denser films can be formed, especially with small molecules such as MMA. By applying recent developments such as ARGET synthesis of the polymer brush is made much easier, as this technique allows reactions to occur with limited amounts of oxygen present as well as reducing the quantity of the copper complex needed for the reaction. By investigating the chemical and mechanical properties of the polymer brush with techniques such as ellipsometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS), it is possible to suggest explanations for the tribological properties of the polymer brush system. Detailed XPS analysis shows that bromine is still present at the surface, a key indicator that the functional sites of the polymer process are still available to be bonded for increasing the chain length while also indicating that fewer termination reactions had occurred. With a lack of silicon visible in the XPS sample spectra it is clear that the polymer has achieved good surface coverage and should therefore exhibit better tribological characteristics. By using novel, custom made stainless steel colloidal probes, it has been possible for the first time to replicate the hybrid contact on the nanoscale, which allows high quality testing by accurately replicating the materials in contact and thus an effective evaluation of the lubrication solution. The importance of the polymer thickness, measured by ellipsometry, and the liquid in which they are solvated, is clearly elucidated by testing in multiple fluids, when highly synergetic fluids like the poly-alpha-olefin result in a significant reduction in friction whereas poor solvents like water can even be detrimental when compared to the bare surfaces in contact. In the worst case scenario under the highest load using the novel probes the lubricated polymer brush reduced the friction force successfully from 3.3 nN to 1.3 nN when compared to the bare nitride surface. Preliminary work has been completed in respect to the transition from the nanoscale to the macroscale, and the polymerisation reaction has been scaled from 1 cm2 silicon nitride wafers up to a 10 cm diameter silicon nitride discs. One of the reasons why the polymerisation can be scaled in such a way is due to the ARGET technique which allows polymerisations to occur in the presence of limited amounts of air. A tribological study of these PMMA modified disc surfaces using a pin-on-disc setup shows favourable results and on average a reduction of friction of 15% when comparing PMMA modified surfaces with unmodified ones in an oil lubricated environment
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications
No full textSelf-assembled monolayers (SAMs), after originally being investigated due to their functions in changing surface wettability, have been significantly developed over the years. Many types of SAMs have been developed on a variety of substrates. However their formation mechanism, rate and quality are found to be influenced by many factors. A range of SAMs including single- and multi- component are included in this review with focus on the nano and macro tribological properties. More recently, surface initiated polymer brushes, i.e. macromolecular assemblies attached to a substrate, have emerged to be an alternative and promising method for surface modification. The ability to tether these macromolecules to tribological contacts is key to their resistance to shear under loaded contacts. This review also covers atom transfer radical polymerisation (ATRP) and the role of this technique in developing new lubrication solutions. Particular care has been taken to include the development of lubrication solutions for silicon nitride due to the importance of this material as an engineering ceramic. This paper reviews the state-of-the-art development of SAMs and polymer brushes especially the potential opportunities and challenges in applying them in tribological contacts as a lubrication solutio
On the impact of turbulent inflow and crack-induced blunt trailing edge on the flow field and far-field noise of an airfoil
Full text linkAn experimental investigation is carried out to characterize the physical mechanisms by which a trailing-edge crack, idealized as a rectangular cavity to represent delamination damage, affects boundary layer development, coherent vortex shedding, and far-field noise of a National Advisory Committee for Aeronautics 0018 airfoil. Both clean and turbulent inflow conditions are considered to isolate the role of inflow disturbance in modifying these mechanisms. The primary objective is to gain insight into how a geometrical discontinuity at the trailing edge alters the coupled aerodynamic and aeroacoustic behavior. Far-field acoustic measurements and near-wake velocity field data are obtained in the anechoic wind tunnel at Delft University of Technology. Acoustic data from a phased microphone array (from prior work) are combined with new velocity field measurements using particle image velocimetry. The results reveal that increasing crack size leads to enhanced near-wall velocity gradients and stronger coherent vortex shedding, resulting in higher tonal noise levels, particularly at higher frequencies. Normalized tonal frequencies agree with the empirical prediction model of Brooks, Pope, and Marcolini for blunt trailing-edge noise, affirming the relevance of this model even in the presence of geometric imperfections. Under turbulent inflow, the coherent structure scale diminishes slightly, and the tonal frequency increases in the trailing-edge noise spectrum, indicating that inflow turbulence modifies the vortex shedding dynamics and should be accounted for in predictive models. This study is a first step toward understanding and modeling trailing-edge noise in the presence of structural damage, under varying flow conditions
Polymer brushes for silicon nitride-steel contacts: a colloidal force microscopy study
No full textA greener lubrication solution based on self-assembling methods for steel on silicon nitride hybrid contacts is investigated in this study. Surface initiated atom transfer radical polymerisation (ATRP) is employed to produce oleophilic polymer brushes based on methyl methacrylate (MMA). This paper presents the synthesis and characteristics of poly(MMA) brushes that have been designed to synergize with a PAO lubricant whilst forming a strong covalent bond with the silicon nitride surface. By utilising activators regenerated by electron transfer (ARGET) the amount of catalyst needed is reduced to ppm levels allowing polymerisations to take place in a limited amount of air. The initiators and the polymer brushes formed on silicon nitride surfaces are characterised using XPS, contact angle, gel permeation chromatography and atomic force microscopy. The lubricating effects of the polymer brushes under dry and swollen states are evaluated using lateral force microscopy with a steel colloid
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