Papers by Author: J.M. Guilemany

Abstract: NiTi alloy is known to have a potential interest specially for its superelasticity and shape memory effect (SME). Most of the research undergone up to now investigates such properties in bulk materials. The few works related to overlays (coatings) use magnetron sputtering for thin films and Vacuum Plasma Spray (VPS) for thicker coatings. VPS is the mostly used thermal spray technique regarding the Ti oxidation problems which considerably affect the so desired SME. The present work compares the microstructure of VPS NiTi coatings with those of High Velocity Oxygen Fuel HVOF and a liquid cooling device coupled to a Atmosphere Plasma Spraying (APS+Q), with optimized spraying conditions trying to minimize Ti oxidation. Apart from their characterization by Scanning Electron Microscopy, the main purpose is to further study how particles are retained within the coating so that Transmission Electron Microscopy is also employed. This has allowed the observation of amorphous and nanocrystalline phases as well as to the change from cubic B2 NiTi lattice to metastable lattices.

Abstract: Hydroxyapatite (HA) coatings are widely used to improve the biocompatibility of titanium substrates. The main problem concerning these coatings is related with the bad adhesion characteristic of the ceramic/metal interface. A TiO2 interlayer can be used to improve the adhesion of the coating; however the TiO2 is a bioinert material. Then, a solution can be the use of a mixture of HA-TiO2 (60% wt HA- 40% wt TiO2) powder has been sprayed by using High Velocity Oxy-Fuel (DJH-2700) for different spraying distances. The adhesion of the coatings has been mechanically tested and the values have been compared with the values obtained for coatings that have been immersed in simulated body fluid solution (SBF) after 24 and 120 hours. The crystallinity degree of the HA plays an important role on it for the dissolution affecting the behaviour of the interface at the same time it is also important osteoclastic resorption and carbonate apatite precipitation.

Abstract: Thermal spray coatings provide good tribological and corrosion-resistant properties. Coatings with carbides or diborides improve resistance to oxidation. However, depending on the thermal spray conditions some carbide and diboride oxidation takes place. The aim of this study is to analyze the oxidation of TiC or TiC+TiB2 powders embedded into a stainless steel matrix. The starting powders were obtained by Self-propagating High-temperature Synthesis (SHS). Oxidation was studied at two temperatures, 700°C and 800°C. The results aid our understanding of the powders’ behaviour during coating and use. An open electric furnace was used, and the samples were analyzed using a SEM coupled with an EDS. Thermal treatment time ranged from 2 minutes to 9 hours. A continuous oxidized layer was observed for the TiC particles. This was not the case for TiB2. The decomposition/oxidation of TiB2 begins before the oxidation of TiC. An oxidized layer of around 4 m is formed at a temperature of 800°C after 690 s, when using a powder of 45 m mean size.

Abstract: There is an increasing interest in the last years for materials with nanometric grain size because of the enhanced properties that could be achieved when reducing to the nanometric scale. Three coatings using conventional, nanostructured and bimodal (mixture of conventional and nano) WC-Co powders were obtained through High Velocity Oxygen-Fuel (HVOF) technique. The powders were sprayed under different spraying conditions in order to improve trybological properties for nanostructured powders. The relationship between spraying conditions and decomposition has been studied using in-flight measurements and XRD techniques. The nanostructured coating showed more decomposition than the other ones during spraying process, but this decomposition was reduced through the new conditions. Nanostructured coating showed more hardness using producer conditions, but the bimodal coating showed better abrasive and friction wear resistance. Nanostructured and bimodal coatings also provided an enhanced corrosion protection to the substrate when compared with the conventional one. An important improvement in the abrasive wear resistance for the nanostructured coating was obtained with the new conditions, due to the reduction in the decomposition.

Abstract: It is well known that microstructure evolution during rapid solidification depends on the rate between undercooling and solidification velocity. Microstructural evolution in a nonequilibrium solidification process depends critically on the melt undercooling and growth (solidification) velocities of competing phases. One way of obtaining metastable structures is by Thermal Spray Technology. Metastable coatings can be produced starting from microstructured powders through Atmospheric Plasma Spray (APS) technique, followed by a quenching route. The initial powders are melted during the spraying and deposited over a substrate that is quenched with nitrogen feeders, producing metastable coatings. The obtained coatings were characterized using XRD, SEM, FESEM and TEM in the Thermal Spray Centre (CPT) of the University of Barcelona. The properties of such coatings were studied by hardness, obtaining promising results.