Navegando por Autor "Vilar, Rui"

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Erosion by solid particle impingement : experimental results with cast-iron, laser-treated surfaces.
(1997) Costa, Adilson Rodrigues da; Vilar, Rui
The surfaces of cast-iron specimens have been modified by CO2 laser processing in order to enhance their hardness, providing different surfaces for wear behaviour studies. Various laser heat treatmentswere performed in order to generate distinctmicrostruc- tures. Based on previous tests, interaction time and power density usedwere 0.5 s and 0.4 kW/mm2, respectively. Alloyingwith pure Cr allowed microstructure and the chemical composition of the material to be modified in order to attain the desired properties. Chromium^iron carbides, (Cr,Fe)7C3, formed ``in situ'', during laser surface melting, by reaction with carbon of the cast iron, change the wear properties. These carbides were detected by conventional X-ray diffraction. The expected effect of a carbide-rich structure on the erosion behaviour is to decrease themass loss by erosionmechanisms. Erosion was done by SiC erosive blast impin- gement tests. In this method the SiC particles were accelerated in a compressed air stream along a nozzle and projected against the surface to be eroded. After the laser treatment, the erosion behaviour of material was evaluated by measuring the weight loss. The wear test was complemented by SEMmicrostructural observations. Experiments performed in this work show that, in erosion, the mass loss of material does not depend on hardness as proposed by the accepted tribological theories. The worst behaviour was observed for laser-treated surfaces. Embedding of erodent particles plays a decisive role in blast erosion performance of the surfaces studied.
Niobium and chromium rich coatings tailored by laser alloying : XRD analysis at high temperatures.
(2002) Costa, Adilson Rodrigues da; Craievich, Aldo; Vilar, Rui
Laser treatment technologies have been widely used to modify superficial layers of different materials. In this work we prepare Nb and Cr rich coatings according to laser alloying technique using cast iron as substrate material. Nb and Cr are intensive used in order to overcome challenges like good chemical and mechanical performance at high temperatures. Following laser alloying the surface-modified samples were submitted to an “in situ” XRD analysis under controlled high temperature and atmosphere. The phase transitions registered point to transformations that do not implies formation of fragile phases or cracks induced by high volumes modifications.
Phase transitions in Nb rich coating produced by laser alloying : a synchrotron radiation diffraction study.
(2001) Costa, Adilson Rodrigues da; Craievich, Aldo; Vilar, Rui
A great variety of studies have been performed in order to develop new materials with high performance in many adverse conditions such as wear and corrosive environment coupled with high temperatures and high stress levels. A strong scientific and technical effort is currently dedicated to develop wear and corrosion resistant surfaces to provide components having good behavior in such extreme working conditions, we have chosen laser alloying and prepared Nb rich coatings upon a nodular cast iron microstructure. The new tailored material is in fact a composite having a very hard coating covering a traditional iron base material. If the material is submitted for thermal cycling, it is imperative to know the phase transformations during heating/cooling cycles, as well as, during long periods of permanence at high temperatures. According to these guidelines, we performed an X-ray diffraction (XRD) study using the XRD beam line of the National Synchrotron Light Laboratory (LNLS, Brazil). The analysis of the XRD pattern indicates the presence of niobium and niobium carbides (Nb, NbC, Nb2C) from room temperature up to 750°C.