Πλοήγηση ανά Συγγραφέα "David, K."
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Τεκμήριο Characterization of Al Metal Matrix Composites Produced by the Stir-Casting Method(2013-09) Anthymidis, K. G.; David, K.; Trakali, A.; Agrianidis, P.Composite materials which main constituent part is a metal are called Metal Matrix Composites (MMCs). The other compounds may be metals too, ceramics or even organics. They are well known for their excellent thermo-physical and mechanical properties. Reinforcement is used to improve different properties of the main material, such as wear resistance, hardness, fatigue resistance, friction coefficient, thermal conductivity and others. As a result, during the last years, MMCs have found a lot of application in automobile industry for the production of brakes and parts of engines and in aerospace industry for the production of structural components, as well as in electrical and electronic industry and in many other applications. MMCs can be produced by many ways, such as, powder blending and consolidation, foil diffusion bonding, electroplating, spray deposition, stir-casting and others. In this research stir-casting was used as processing technique for the production of Aluminum matrix composites reinforced by ceramic particles and iron. The morphologies of the produced composite materials were examined using optical and SEM microscopy. The compositions of their micro structural features were determined by EDX spectroscopy. The phases formed were determined by XRD techniques. In the tribological tests, under dry wear conditions, the as-produced composites materials showed significant increased resistance to wear compared to pure Al metal.Τεκμήριο Deposition of coatings containing Si and B on steels in a CVD fluidised bed reactor(2005-01) Agrianidis, P.; Anthymidis, K. G.; David, K.; Tsipas, D. N.Diffusion coatings are frequently used to protect materials in various aggressive environments and in order to improve their surface properties. Fluidized bed technology (FBT) has been successfully used for the deposition of different types of diffusion coatings by a C.V.D. process, e.g. aluminizing, chromizing, nitriding, carburizing. On the other hand, very little information exists on boronizing or siliconizing using FBT, although the method is simple, efficient and environmentally friendly and the boride coatings have been reported to have an excellent combination of properties, e.g. high fatigue strength and wear resistance and the silicide coatings are well known for their excellent corrosion resistance. In this paper the results of a process of boronizing and siliconizing of steels via C.V.D. in a fluidised bed reactor are presented. Coatings of different thicknesses were obtained at temperatures below 1000°C. The coatings were analysed by means of optical microscopy, as well as by x-ray diffraction (XRD) and Vickers microhardness, in terms of the coating’s composition, morphology, thickness, hardness and phase formation. The boride coatings showed significantly improved tribological properties under dry wear conditions.Τεκμήριο Determination of the Fatigue Behavior of Aluminide Coatings by Means of the Impact Testing Method(2007-09) David, K.; Anthymidis, K. G.; Agrianidis, P.; Tsipas, D.N.The impact testing is an efficient experimental method that enables the quantitative and qualitative determination of the fatigue resistance of mono- and multilayer coatings deposited on various substrates, which was not possible with the common testing methods previously available. In this paper the experimental assessment of the fatigue resistance of coatings working under cyclic loading conditions by means of the dynamic impact testing method is presented. The fatigue failure mode, such cohesive or adhesive, of the investigated coatings is determined using scanning electron and optical microscopy, as well as EDX analysis. Critical values of the stress components, responsible for distinctive fatigue failure modes of the coating substrate system are obtained and the fatigue limits of aluminide coatings are illustrated in simple diagrams containing the impact load versus the number of successive impacts that the examined aluminide-P91 system can withstand.Τεκμήριο Enhanced ULF radiation observed by DEMETER two months around the strong 2010 Haiti earthquake(2010-12) Athanasiou, M.; Anagnostopoulos, G.; Iliopoulos, A.; Pavlos, G.; David, K.In this paper we study the energy of ULF electromagnetic waves that have been recorded by the satellite DEMETER, during its passing over Haiti before and after a destructive earthquake. This earthquake occurred on 12/1/2010, at geographic Latitude 18.46o and Longitude 287.47o , with Magnitude 7.0 R. Specifically, we are focusing on the variations of energy of Ez-electric field component concerning a time period of 100 days before and 50 days after the strong earthquake. In order to study these variations, we developed a novel method that can be divided in two stages: first we filter the signal keeping only the very low frequencies and afterwards we eliminate its trend using techniques of Singular Spectrum Analysis, combined with a thirddegree polynomial filter. As it is shown, a significant increase in energy is observed for the time interval of 30 days before the strong earthquake. This result clearly indicates that the change in the energy of ULF electromagnetic waves could be related to strong precursory earthquake phenomena. Moreover, changes in energy were also observed 25 days after the strong earthquake associated with strong aftershock activity. Finally, we present results concerning the comparison in changes in Energy during night and day passes of the satellite over Haiti, which showed differences in the mean energy values, but similar results as far as the rate of energy change is concerned.Τεκμήριο Evaluation of the Fatigue Resistance of Chromium Slurry Coating Steel Substrate Compounds by Means of an Improved Impact Testing Procedure(2012) Anthymidis, K. G.; David, K.; Trakali, A.; Tsipas, D. N.Chromium coatings have been used for the protection of gas turbine blades in power plants during the last years due to their very good resistance against steam corrosion. Microhardness, scratch, adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of these coatings [1,2]. However, very limited information exists on their fatigue resistance. In this paper we present the experimental results of the impact testing examination of the above coatings. This experimental method is capable to assess the fatigue and the impact wear resistance of coatings working under cyclic impact loading conditions. From the experimental results it was concluded that the slurry Cr, Zr coatings deposited on P91 steel showed adequate fatigue strength for the above-mentioned kind of use.Τεκμήριο Fatigue resistance assessment of slurry coating steel substrate compounds under cyclic loading(2008-07) Anthymidis, K. G.; David, K.; Tsipas, D. N.Slurry coatings have been used for the protection of gas turbine materials in power plants during the last years. These coatings can be applied by spaying, brushing or dipping. The main constituent elements are silicon, chromium, potassium, borium, carbon and zirconium. They are characterized by high hardness and very good resistance against corrosion, erosion, abrasive and adhesive wear. To guarantee the reliability of coated steam turbines components used in power plants, the lifetime assessment of the coatings and their failure prediction become very important. Microhardness, scratch, adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of these coatings [F. Loeffler: Thin Solid Films, Vol. 339 (1999), p. 181]. However, the above testing methods do not model the dynamic cyclic fatigue. In this paper we evaluate the fatigue resistance of slurry coatings working under cyclic loading conditions by the impact testing method. The coating failure mode and its extent were assessed by SEM observations and EDX analysis. From the experimental results it was concluded that the Si,Cr,O,B,C coatings deposited on P91 steel substrate showed an improved fatigue strength compared to that of Si,Cr,O deposited on the same substrate.Τεκμήριο Production of Al Metal Matrix Composites by the Stir-Casting Method(2013-11) Anthymidis, Konstantinos; David, K.; Agrianidis, Pavlos; Trakali, Afroditit is well known that the addition of ceramic phases in an alloy e.g. aluminum, in form of fibers or particles influences its mechanical properties. This leads to a new generation of materials, which are called metal matrix composites (MMCs). They have found a lot of application during the last twenty-five years due to their low density, high strength and toughness, good fatigue and wear resistance. Aluminum matrix composites reinforced by ceramic particles are well known for their good thermophysical and mechanical properties. As a result, during the last years, there has been a considerable interest in using aluminum metal matrix composites in the automobile industry. Automobile industry use aluminum alloy matrix composites reinforced with SiC or Al2O3 particles for the production of pistons, brake rotors, calipers and liners. However, no reference could be cited in the international literature concerning aluminum reinforced with TiB particles and Fe and Cr, although these composites are very promising for improving the mechanical properties of this metal without significantly alter its corrosion behavior. Several processing techniques have been developed for the production of reinforced aluminum alloys. This paper is concerned with the study of TiB, Fe and Cr reinforced aluminum produced by the stir-casting method.Τεκμήριο Reinforcement of aluminum by TiB dissolution(2008-07) Agrianidis, P.; Anthymidis, K. G.; David, K.Interactions between solid materials and liquid aluminum lead to a dissolution of solid elements into aluminum, which in turn results in a subsequent growth of intermetallic and intermediate phases. It was established that the growth of the intermetallic phases could be governed by chemical reactions at the interfaces and by interdiffusion of the reacting elements through the different phases. Dissolution on the other hand mainly depends on thermodynamic conditions, experimental parameters such as temperature, stirring time, and reacting holding time and on the degree of the saturation of aluminum as well as on the chemical composition of the solid materials in the reaction zone. The above-mentioned factors play also an important role in the formation of the different phases during dissolution. Nevertheless, a non-uniform distribution of the solute elements may causes a local concentration of these elements into the liquid aluminum, which practically delays the process or alters the equilibrium of the growth of the phases. Thus it is crucial to control the dissolution conditions so that the instabilities induced at the solid materials/aluminum interface are limited. The main objective of this study was to investigate both the formation of intermetallic and intermediate phases in the reaction zone and to examine the development of the diffusion structures of pure aluminum reinforced with TiB particles and to investigate the mechanical properties of the as-produced composite materials.Τεκμήριο Reverse engineering techniques for cranioplasty: a case study(2008-04) Maravelakis, E.; David, K.; Antoniadis, A.; Manios, A.; Bilalis, N.; Papaharilaou, Y.This paper presents rapid prototyping and reverse engineering techniques applied to create an implant for the surgical reconstruction of a large cranial defect. A series of computed tomography (CT) images was obtained and purpose built software was used to extract the cranial geometry in a point cloud. The point cloud produced was used for: (a) the creation of a stereolithographic (STL) physical model for direct assessment of the cranial defect; and (b) the creation of a 3D mould model for the fabrication of the patient-specific implant.Τεκμήριο Siliconizing in a Fluidized Bed Reactor(2013-11) Anthymidis, Konstantinos; David, K.; Trakali, Afroditi; Tsipas, D. N.Fluidized bed technology has been used for surface engineering applications in the deposition of hard and/or corrosion resistant layers e.g. aluminizing, chromizing, nitriding, carburizing, boronizing. In the present paper we used fluidized bed technology (FBT) to deposit silicide coatings on steels. The silicide coatings are well known for their excellent corrosion resistance. Siliconizing in fluidized bed is a promising method to improve the corrosion resistance of steel without altering its other properties. The asproduced silicide coatings are uniform all over the steel substrate, with good thickness, adequate adherence and consisted only from the phase FeSiC.Τεκμήριο Wear of cutting tools used in milling treatments(2011) Anthymidis, K. G.; Balouktsis, A.; David, K.; Trakali, A.The usage of Computer Numerical Controlled Machines has been generalized over the last decades due to the increased demands for the production of mechanical parts with précised dimensions, higher production rates and products with better treated surface quality. It is well known that the duration of life of a cutting tool is one of the most important parameters during the cutting of metal parts, because it affects the cost of the manufacturing process substantially. Therefore, it is important to know accurately, the relation between the duration of life of the cutting tool and the conditions of the machinery such as cutting velocity, feed rate (fz), the depth of cut (radial and axial) etc. The purpose of this research is to conduct a proper number of cutting experiments in milling, measuring the wear of the cutting tools, in order to conclude in a mathematical model the wear cutting tool. This model can be implemented for the prediction of the cutting tool life, which is very important for the determination of the best cutting conditions. From the experimental diagrams we can come to conclusions for the course of the wear of the cutting tool in connection with the velocity of cutting, for the machinability of the materials used etc. The wear of the used cutting tools was determined by means of optical microscopy and stereoscopy.