Papers by Author: D.N. Tsipas

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: The evolution of the Fe-Si phase’s formation during siliconising of ferrous material was studied. The examined coatings were formed at 990 oC, via chemical deposition of Si in fluidized bed reactor (FBR). The substrate used for this purpose was carbon steel, ST37. Coatings of 10-50 μm thickness were obtained. Optical micrograph (OM) SEM analysis, as well as XRD measurements were used for coatings characterization. During an annealing process of the as formed coatings an ordering of the solid solution of α-FeSi was observed. The ordering process was found to be continuous and the phase gradually becomes super saturated. A narrow portion of the X-ray diagrams including the 110 and the 220 reflections of the α -Fe and α-Fe3Si was chosen for data elaboration.

Abstract: 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.

Abstract: Although structural kaolin based ceramics are attractive and useful materials, having good mechanical characteristics, low density, good corrosion and high temperature resistance, their use is restricted by their brittle behaviour. In order to improve their properties and mainly strength, toughness and high temperature performance, fibre composite ceramics have been developed. In the present work a series of kaolin-short random dispersed Grafil carbon fibre composites were produced and sintered in an inert atmosphere of Argon at 1000oC and 1300oC and characterised using various techniques. XRD analysis of the kaolin matrix at 1000oC showed that the crystalline phases were decomposed without the formation of mullite a fact which also reconfirmed by SEM examination. However at 1300oC mullite formation was well evidenced. XRD analysis of the fibres in “as received” showed that they have graphite structure which was also retained, as SEM examination revealed, after sintering. Examination of the Grafil fibres showed that they were quite uniform in length and diameter and retained their integrity after sintering at the examined temperatures. Examination the fractured composite surfaces showed only a weak bond between fibres and matrix and at the pull out areas the fibres were replicated in the matrix.

Abstract: In order to manufacture products of acceptable quality within predetermined specification limits, an evaluation of the manufacturing process through the use of capability indices is necessary. Capability indices are used in order to evaluate a process and to monitor the product characteristics and the results of quality improvement cycles. The three point bending strength of kaolin ceramic specimens produced by two different production processes (slip casting and hydroplastic extrusion) were investigated. It was found that under statistical control and normal distribution of strength values, the process capability indices Cp, Cpk, regardless of any disadvantages and the fact that they have limitations, can be combined with other techniques in the field of statistical process control and become valuable tools in process evaluation of ceramic products.

Abstract: In this paper we present some simple methods for the fabrication of closed and open cell Al metal foams. The closed cell Al metal foams were fabricated using liquid metallurgy. Their microstructure and morphological characteristics were studied using optical and electron microscopy techniques. The mechanical properties and their relationship to microstructure were established. A three dimensional model was developed relating the cell geometry to the mechanical behavior of Al closed cell foam material. The open cell Al foams were fabricated using sodium chloride powder sinters and liquid Al metal infiltration techniques. The potential of using these cellular structures in various systems is discussed.

Abstract: Tin dioxide is a wide band semiconductor, with interesting chemical physical and mechanical properties, used in a variety of industrial, domestic, medical and agricultural applications, including gas detectors, transparent conductors, solar cells, anti-static films, nanoelectronic devices etc. The variety of nanosized SnO2 production methods in the form of powders or layers (e.g. solid state, sol-gel, sputtering, laser ablation, template, solution precipitation, precursor oxidation, CVD, PVD, etc) are discussed.