1,720,967 research outputs found
Plasma spray deposition and high temperature characterization of ZrB2-SiC protective coatings
No full textRefractory metal borides are the object of special interest for aerospace applications requiring properties of chemical and mechanical resistance in ultra high temperature, such as nose and leading edges of re-entry space vehicles. The main objective of the research is the fabrication and characterization of plasma sprayed zirconium diboride-silicon carbide composite coatings and free-standing components for high temperature applications. High and low pressure plasma spray in a controlled atmosphere were selected as manufacturing techniques for the deposition of ceramic coatings. Fine ZrB2 and SiC precursors were agglomerated and preconsolidated into spherical, hollow powders for better flowability and silicon carbide thermal protection during the interaction with the plasma. Coatings and free-standing tubular specimens were fabricated and tested for high temperature behaviour. Thermogravimetric analysis, surface morphology investigation and high temperature X-ray diffraction showed that the addition of approximately 25% SiC induces a mechanism of self-protection of the ceramic material during heat treatment in oxidizing environments up to approximately 2100 K. (C) 2002 Elsevier Science B.V. All rights reserved
Plasma spray deposition of ultra high temperature ceramics
No full textAn innovative, proprietary methodology was developed to produce, by plasma spraying deposition, a ceramic composite containing SiC particles dispersed in a ZrB2 matrix. With such a technique both coatings and free standing parts were fabricated. In spite of the well known difficulty to obtain plasma sprayed coatings containing SiC, characterisation results evidenced that the used process did not affect the phase composition. Thermal and mechanical behaviour of the obtained material have been extensively characterized, in order to assess its applicability to specific space missions. In a first phase, tests were carried out on laboratory scale samples. Scope of the second phase of the activity, which is currently running, is the validation of the developed material in simulated operative conditions, by demonstrative components testing in a PWT (plasma wind tunnel) facility. A preliminary test campaign has been successfully completed, demonstrating that UHTC plasma sprayed coatings are able to withstand high temperature oxidising conditions. (c) 2006 Elsevier B.V. All rights reserved
Parametric study of an HVOF process for the deposition of nanostructured WC-Co coatings
No full textNanocrystalline WC-Co coatings were deposited by high velocity oxyfuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermochemical and gas-dynamical properties of gas and particle flows within the combustion flame were identified in various operating conditions by computational fluid-dynamics (CFD) simulation. Significant improvements of the mechanical properties of the coatings were obtained: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness
Effect of Grain Size and Distribution on the Shielding Effectiveness of Transparent Conducting Thin Films
No full textThis paper presents a morphological and functional characterization of nanostructured thin films featuring high radio frequency shielding effectiveness and high optical transparency in the wavelength range 400--1500 nm. The film morphology is analyzed at the micro- and nanoscales by processing the images acquired by a scanning electron microscope. A software tool developed for this purpose analyzes the statistical distributions of the film surface grains. Fitting models and experimental evidences are presented in order to describe and predict the correlations between the film morphological and functional properties. The adopted approach and measurement methods are developed to model and optimize a particular transparent conducting oxide but can be easily extended to similar materials, deposition processes, and applications
Silicon carbide based plasma sprayed coatings
No full textCoatings containing up to 66 vol.% of silicon carbide were deposited by plasma spray. Potential applications can be found in the protection of CMC (ceramic matrix composite) against wear and high temperature oxidation. It is well known that SiC cannot be deposited by thermal spray because it decomposes before melting. To face this problem, a mixture of SiC and ZrB2 was deposited, since those two compounds form a eutectic phase, at a temperature lower than the one of SiC decomposition. Coating microstructure was characterized by XRD, SEM, and EDS, confirming the presence of SiC in the deposited layer and the formation of the eutectic phase during spraying. Samples of the coatings were exposed in air at high temperature, in the range of 650 degrees C to 1700 degrees C. The oxide scale was investigated by means of XRD, SEM, EDS and WDS. It was constituted by a SiO2 layer, which includes islands of ZrO2. Test results showed the good potential of the material investigated to be used as a protection against the high temperature oxidation. (C) 2012 Elsevier B.V. All rights reserved
Effects of heat treatments on oxidation resistance and mechanical properties of ultra high temperature ceramic coatings
No full textA critical point in the development of future generation space vehicles is the availability of materials able to withstand the extreme temperatures generated during orbital re-entry. UHTC (Ultra High Temperature Ceramics) materials, such as Zirconium diboride based ceramic composites, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB2 based materials possible candidates in designing thermal protection systems for the hottest structures of re-entry vehicles such as wing leading edges and nose-tip. An innovative, proprietary way to deposit ZrB2 based coatings by plasma spraying has been developed. In the present work, the influence of high temperature exposure on oxidation resistance and mechanical properties has been investigated. Several coatings constituted by different amounts of SiC particles dispersed in a ZrB2 matrix were produced; some samples were tested in as sprayed conditions, other ones were submitted to a high temperature heat treatment before testing. Oxidation resistance was determined by exposing samples in air at 2073 K for 180, 1800 and 2520 s. Oxidation kinetics were analyzed and modelled. Evaluation of flexural strength and elastic modulus by means of four-points bending tests were carried out on samples showing better oxidation behaviour; mechanical tests were performed also on samples exposed at a typical operative temperature (1800 s at 2073 K). Results showed that heat treatment does not affect oxidation resistance, whilst it significantly improves mechanical properties. (C) 2008 Elsevier B.V. All rights reserved
Zirconium diboride based coatings for thermal protection of re entry vehicles: Effect of MoSi2 addition
No full textThe possibility of improving the performances of new generation re-entry vehicles as well as reducing the mission costs is closely connected to the development of new thermal protection systems: innovative heat shield materials, able to withstand higher temperature during the lift re-entry manoeuvre, would permit to improve reusability and easiness of maintenance as well as to increase the manoeuvrability and the payload of vehicles. UHTC materials, and, among them, Zirconium diboride based materials, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB2 based materials possible candidates for thermal protection systems of atmospheric re-entry vehicles. The experimental activities described in the present work were focused on ZrB2 based coatings deposited by plasma spray, containing SiC and MoSi2 in different ratios: the work aimed at understanding the role of MoSi2 addition in the oxidation resistance. XRD analysis was carried out on starting powders and plasma sprayed coatings, in order to evaluate the transformations of crystalline phases induced by thermal deposition and to analyse the possible presence of amorphous phase; XPS was also used to study the coatings surface composition and to obtain information about chemical bonds in the materials. Oxidation behaviour was studied carrying out exposure tests up to 1800°C in static air to compare and model the oxidation kinetics of the different compositions: results show the better behaviour of materials containing MoSi2. Microstructure of as sprayed and oxidised samples was observed by means of SEM and analysed by EDS to understand the link between dispersion of Mo containing phases and the improved oxidation resistance. © 2010 Elsevier B.V
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
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