Papers by Author: F. Stergioudis

Abstract: The modification of the outer layer of Al-Si alloys doped with iron atoms was examined. The samples both were taken from automotive pistons and they also were prepared in the laboratory. In any case, they were used as basic raw materials for examination. It was found that the structure and properties of these alloys are very much dependent on the cooling rate, composition and heat treatment. The iron atoms are usually doped, in order to diminish and stabilize the thermal expansion coefficient of the alloys. However, this process makes the alloys to become harder and harder; thus leading the alloys to a degradation of their mechanical behavior at elevated temperatures. Optical Microscope observations, SEM analysis of the interactive surface layers and XRD measurements were performed on tested samples. At elevated temperatures a large homogenized zone extended to about 500 (μm) without an apparent distribution of the silicon dendrites or faceted crystals was formed. Moreover at the surface of the so called homogenized zone a layer of about 35 (μm) was formed inside of which iron atoms were migrated. This layer is mainly responsible for scuffing phenomena.

Abstract: The interaction of solid NH₄HCO₃ with iron, where the ammonia product has been adsorbed nondissociatively to iron surfaces at low temperatures [1] was investigated. The nitride clusters formed on steel substrates modified the surface morphology and characteristics of the substrate and influenced their adhesion during subsequent procedure of coating. In our case, efforts were made to decorate the steel substrate in order to influence the base metal reactivity towards boron and its ability to react and form stable compounds with boron [2]. Boride layers on steel are examined by means of SEM and XRD analysis. The decorated surface was observed by FTIR method.

Abstract: In the present work the phases of the zinc coatings deposited with hot-dip galvanizing, pack cementation and wire flame spraying are examined with Scanning Electron Microscopy and Transmission Electron Microscopy. The different phases which are observed are identified with the combined results of electron and X-Ray diffraction. From the results it is concluded that pack cementation coatings are consisted by two different layers while hot dip galvanized coatings are composed by the same phases and additionally two extra phases of the Fe-Zn phase diagram. Flame sprayed coatings are composed by pure zinc, in the form of thin lamellae, together with nanocrystaline zinc oxide which is formed from the oxidation of liquid metallic droplets during the spray procedure.

Abstract: In the herein work coatings containing NiCrBSi alloy were deposited on low carbon steel by flame spray technique. The microstructure and morphology of the coatings were examined by electron microscopy and X-ray diffraction analysis. The as sprayed coatings are extremely rough with characteristic lamellic formations and porosity while a nanometer oxide film was formed on the top of the coating. High temperature oxidation tests revealed the high resistance of these coatings when exposed in such environment. This is mostly attributed to the existence of the temperature resistant oxide scale on the surface of the coupons.

Abstract: Sterilization of the NiTi alloy in boiling water or steam causes passivation, which results in an amorphous 3.5 nm thick TiO2 layer on the surface. Between the surface and the matrix a transition layer of Ni2O3 and NiO was observed, using the X-ray photoelectron spectroscopy. Differences in sterilization conditions affect the amount of metallic nickel on the surface.

Abstract: One of the most effective methods for the protection of ferrous substrates from corrosion is zinc hot-dip galvanizing. Although this method has many advantages, it is characterized by a very negative effect on the environment. In the present work Zn coatings were formed with thermal spraying, pack cementation and fluidized bed reactor, which are friendlier to the environment. Their microstructure was characterized with X-ray diffraction and scanning electron microscopy, while their corrosion performance was estimated with exposure in a salt spray chamber. From this investigation it was deduced that CVD coatings are composed by two layers referring to Γ-Fe11Zn40 and δ-FeZn10 phase of the Fe-Zn phase diagram. By contrast the thermal coatings are very porous and composed by pure Zn. However, the corrosion performance of all coatings is similar. This conclusion is very important because it verifies that hot-dip galvanizing could be replaced by the other coating methods.

Abstract: This work aims to investigate the feasibility of Zn-Al deposition on low alloy steels at temperatures from 400 up to 440oC by pack cementation process aiming to increase their corrosion resistance. A series of experiments were undertaken to investigate the effects of pack powder composition and the deposition temperature of the process. It was observed that the parameters of zinc content and temperature affect only the coating deposition speed, but not the phase composition of the as produced coating. Al forms an overlying layer that seals the zinc coating. In any case, the deposition of successive layers of Zn and Al is feasible with pack cementation. The corrosion performance of Zn-Al coatings formed with alternative methods is already studied and proved to be resistant in harsh environments. So the herein studied coatings are expected to be corrosion resistant. Furthermore as Al is much more resistive than Zn, these coatings are more effective than pure Zn ones.

Abstract: Thick sheets of steel were galvanized in a bath containing Al additions. A Fe2Al5 layer is formed at the substrate steel sheet, which leads to a desirable transient inhibition of Fe-Zn reactions. Thus the more protective (eta) phase rich in zinc is favored. However an appreciable intergranular diffusion and a gradual formation of internal and surface oxide particles influence the growth and stability of the inhibition layer. The location of some oxide particles at the Fe2Al5 surface or inside of this layer, led to conclude that oxide particles might cause Fe-Zn outburst growths to form. This is because zinc diffuses along the oxide particle/Fe2Al5 interface. Moreover the mechanism of oxide formation causes a local depletion of the atoms concentration in the bath in the vicinity of the formed oxide. This in turn diminishes the probability of the formation of the Fe2Al5 layer. So the whole mechanism provides a fast diffusion bath for Zn, which reacts with Iron atoms forming Fe-Zn phases. The formation of the phases, were determined by XRD measurements (PHILIPS diffractometer CuKα radiation) while the dispersion of the elements was examined by SEM (20kV JEOL 840A equipped with an OXFORD ISIS 300 EDS analyzer.