1,721,718 research outputs found
Electrodeposited nanocrystalline Ni-Co and Co-Ni-P coatings for hard chromium replacement
No full textThis thesis describes the preparation and characterisation of environmentally friendly and low-cost nanocrystalline Ni-Co coatings and Co-Ni-P coatings to replace hard chromium coatings for anti-wear and anti-corrosion applications. nanocrystalline Ni–Co coatings with different cobalt contents were electrodeposited.The investigation on the role of tribofilms and wear debris in the tribological behavior sliding against AISI-52100 stainless steel under unlubricated conditions shows that the tribofilms containing iron from the counterparts were formed on the worn surface of the coatings (less than 60 at.% Co), which exhibited high coefficients of friction and wear rates. In contrast, no tribofilm or iron transfer from the pin was found on the Co-rich coatings (more than 70 at.% Co), which exhibited a dramatic friction reduction of 50 % and improved wear resistance. The wear debris contains a mixture of face-centred cubic (fcc) metallic phase and fcc oxidised phase, irrespective of the coating composition. The oxidised debris cannot form an efficient lubricative film to promote separation of the sliding surfaces. Ni-Co coatings exhibited the active-passive polarisation behaviour in 3.5 % NaCl solution. The corrosion resistance of Ni-Co coatings needs to be further improved in order to replace hard chromium for anti-corrosion applications. A new approach to fabricate single-layer Ni-Co coatings with high cobalt content onto mild steel substrates has been developed by optimising of additives (saccharin and 2- butin-1,4-diol (BD)). The present method is more feasible in industry with a competitive cost compared to other techniques, e.g. developing graded coatings and applying pulse current waveforms. The effect of saccharin and BD on the properties of the coatings were investigated, including surface morphology, grain size, crystalline texture, hardness and tribological performance against a steel counterpart. The coating microstrain could be manipulated from tensile to compressive and the fibre texture could be modified from the (10¯10) for hexagonal close-packed (hcp) structure to (0002)hcp / (111)fcc. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via grain size and texture analyses. The coating from the bath with an optimised additive content had high hardness (500 HV) due to its reduced grain size (11±2 nm) and improved tribological properties due to the high proportion of hcp structure. The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Ni-Co coatings with different porosities were obtained by controlling the concentration of nickel sulphate and nickel chloride within electroplating baths. The coatings with the grain size in the range of 11-23 nm had varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect is taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Ni-Co coatings with 77±2 at% cobalt. The present model has been applied to other porous nanocrystalline coatings, and the Hall-Petch relationship is maintained. In order to further improve the microhardness, wear resistance and corrosion resistance of Ni-Co coatings to match the properties of hard chromium, a new Co-Ni-P coating has been developed by combining the precipitation hardening of Ni-P alloys with the lubricity of cobalt-rich Ni-Co coatings. The evolution of composition and microstructure, hardness, thermal stability and tribological properties have been investigated. The local pH near the cathode played an important role in the change of the microstructure from nanocrystalline to amorphous along the growth direction as the phosphorus content increased from 7 at.% to 26 at.%. The highest microhardness (980 HV) and the lowest wear rate (an order of magnitude lower than that of hard chrome coatings under the same dry sliding conditions) were achieved by annealing the coatings at 400 °C facilitating precipitation hardening. Furthermore, the coefficients of friction of both the as-deposited Co-Ni-P coating and the heat-treated samples were approximately 0.3, only half of that of hard chrome coatings. The roll-like debris found on the worn surfaces of the coating annealed at 500 °C were oriented perpendicularly to the sliding direction. The Co-Ni-P coating annealed at 400 °C exhibited improved anti-corrosion properties, which can be attributed to the formation of a protective oxide layer
Feasibility study of a micro-electrospray thruster based on a porous glass emitter substrate
No full textElectrospray thrusters utilize a strong electric field to extract and accelerate charged particles contained at a needle tip in a liquid state[1].Electrospray thrusters are one of the few types of electric propulsion systems that can offer high specific impulses (i.e. >1000 seconds) whilst still being scalable down to fit within a 1UCubeSat form structure. These small electrospray thrusters, here termed micro-electrospray thrusters are attracting considerable interestd due to this Nanosatellite system suitability[2].The thrust though from an individual down scaled micro-electrospray emitter is relatively small, of the order of0.1μN[3].Given this small value, it is necessary to ‘multiplex’ the thrust by having an array of hundreds or indeed thousands of emitters[4–7].To create these large arrays many different manufacturing methods have been investigated,including micro electro mechanical system (MEMS)methods[8,9], electrochemical etching[10,11],laser ablation[12–14],computer numerical control (CNC) machining[3,15,16]and additive manufacturing[17].Many of these techniques result in an electrosprayemitter design with an emitter tens of micrometres in heightand with emitter diameters ofthe order oftenmicrometresif not smaller. This results in a ‘postage stamp’sized emitter array, which in itself is considerably smaller than perhaps what is required onboard a CubeSat. Alternatively somewhat larger emitters may produce greater charged particle current per emitter, andtherefore greater thrust.To investigate the feasibility of a larger emitter design, thispaper studies athrusterusing anemittermanufactured fromsintered porous glassusingaCNC machining technique.The emitter manufactured is of a pyramidal shaped design, with an emitter height of 5mmandside surface anglesrangingfrom10 ̊to30 ̊.Five different prototy peemitter shapes are manufactured, investigating the feasibilityofmanufacturingsingleemitters using the method. Initial experimental results are illustrated, demonstratinggood operationparticularlyin a bipolar, alternating voltage,mode. Initial Time-of-flight measurements would seem to indicate that the emitter was operating in a purely ionic regime, with a thrust of the order of9.25μNper emitter and a specific impulse of the order of 4550secondsat 3443 V with 60μA emission currentfrom theon e emitter. This value of thrust is particularly high from one emitter, with onereas on suspected being the occurrence of multiple emission sites at the emitter tips during the electrospray process. Prototype designs ofarray of emitters, and also more spike like emitters, are presented with the manufacturing demonstrating the reasonable promise of this very low cost, simple and quick to iterate manufacturing process
Alternative tribological coatings to electrodeposited hard chromium: a critical review
No full textThe traditional importance of hard chromium electroplating in surface engineering is recognised and the key features of this well-established technology are summarised. Despite the high hardness, corrosion protection and wear resistance of chromium electrodeposits, a number of alternative coating compositions and application techniques have been developed for specific applications in tribology. Environmental challenges associated with hard chromium electroplating are highlighted and the need to develop and evaluate alternative coatings is stressed. Key examples of the alternative coatings are described, including their method of application, microstructure and tribological performance in controlled service environments. Research needs requiring rapid development are highlighted. A summary is given of the most competitive coatings and those having the potential to match the performance of hard chromium in selected applications are identified
Electrodeposition of nanocrystalline nickel and cobalt coatings
No full textThe importance of nanostructured deposits and methods for achieving them are considered. The scope for nickel and cobalt nanocrystalline single metal deposits is summarised and developments in the last decade are highlighted. In each case, chemical composition, phase composition and crystal structure are included. Nanostructure, physical and mechanical properties (including corrosion resistance) v. bath type and composition (including pH and electrolyte additives) together with plating conditions, including current density, temperature and agitation are summarised. While the majority of published studies have featured nickel, the more expensive cobalt has also been considered due to its better corrosion and high temperature oxidation resistance in some environments. An overall comparison between the two metals is made. The major applications of nanocrystalline nickel have been in wear and corrosion resistant coatings, particularly as a hard chromium replacement, but speciality emerging uses include catalytic electrode surfaces in water electrolysis. Future research needs are briefly liste
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
An electron microscopy study of the effect of Ce on plasma sprayed bronze coatings
No full textThe Cu-Al eutectoid alloy is an excellent material for mould due to its superior low friction. The conventional sand casting technique, however, is not feasible to fabricate high Al bronze because of high hardness and brittleness. Plasma arc spray has been used to produce high Al/Fe bronze coatings for mould. The inherent impurities such as H, O, N, S during the spray, however, may affect the coating's mechanical strength. One approach is to utilise the active rare earth Ce to clean up these impurities. The study is to investigate the effect of Ce on the microstructure, which has few reported in the literature<br/
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