NiP/SiC composite coatings: the effects of particles on the electrochemical behaviour

NiP/SiC (17 at.% P) composite coatings were prepared by electrodeposition from a Brenner type plating bath containing SiC particles. Cyclic voltammetry and immersion time were used to evaluate the electrochemical behaviour of these coatings. The results showed that the presence of SiC particles in NiP alloy increased corrosion properties, because the exposed area of the metallic matrix was reduced due to its recovering by SiC particles. However, the current densities developed by the NiP/SiC composite coatings increased with increasing amount of incorporated particles, and such effect is more remarkable for small particles, after heat treatment and when the systems are polarized

Functionalization of synthetic talc-like phyllosilicates by alkoxyorganosilane grafting

A range of talc-like phyllosilicates were prepared via a hydrothermal synthesis performed at five different temperatures from 160 to 350 °C. The organization of the lattice and the degree of crystallinity of the new materials were evaluated by different techniques such as XRD, FTIR, solid-state 29Si NMR, TEM, FEG-SEM and TG-DTA. When synthesized at low temperature the material presents high degree of hydration, low crystallinity and flawed structure. This was attributed to stevensite-talc interstratified product present in the samples. The stevensite/talc ratio and the hydration decrease in the talc-like phyllosilicate samples when the hydrothermal synthesis temperature increases and so the crystallinity becomes higher. A thermal treatment at 500 °C allowed a significant flaw reduction in talc-like phyllosilicate structure; the synthesized sample at 350 °C and heat treated presents a structure close to that of talc. The different talc-like phyllosilicates were grafted covalently by two organoalkoxysilane reagents, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (IM2H) and 2-hydroxy-4-(3-triethoxysilylpropoxy)-diphenylketone (HTDK). The grafted amounts of the hybrids, determined by elemental analysis and confirmed by thermogravimetric data, are dependent on the hydrothermal synthesis temperature and organoalkoxysilanes; they become smaller when the synthesis temperature increases and when HTDK is used. FTIR and solid-state 13C CP MAS NMR were applied to characterize the grafted organic groups. So, in this work it is shown that by choosing the hydrothermal synthesis temperature or by performing an additional annealing it is possible to adjust the amount of defects in the structure of talc-like phyllosilicates which seems to be strongly correlated to the grafting performance

Development of nickel phosphorus coatings containing micro particles of talc phyllosilicates

The present work aims to characterized nickel phosphorus coatings co-deposited with talc particles on steel. The NiP-talc composite deposits were developed to serve as hard coatings with a lubricating effect at 600 C. This process, which is free of hexavalent chromium, could provide a reliable substitute for the electrodeposition of hard chromium coating used in industrial applications. Local responses to static and dynamic mechanical loading have been obtained by nano- and microhardness, microtensile and nanoscratch testing. The hardness and stiffness values slightly decrease when the amount of talc increases for untreated coatings. In contrast, a 420 C heat treatment leads to high hardness and Young’s modulus values due to crystallization. Moreover, a 600 C heat treatment lowers these values through overageing. A 420 C heat treatment greatly improves the adherence and the cohesion of the coatings containing talc

Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

A new promising and versatile process based on the sol–gel transformation has been developed to deposit yttria-stabilised thermal barrier coatings. The non-oriented microstructure with randomly structured pore network, resulting from the soft chemical process, is expected to show satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. First stage of the research consists of optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce, as much as possible, the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermo-mechanical stresses and generate detrimental propagation resulting in spallation. Cyclic oxidation tests are performed using a cyclic oxidation rig instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigate

Heat Treated NiP–SiC Composite Coatings: Elaboration and Tribocorrosion Behaviour in NaCl Solution

Tribocorrosion behaviour of heat-treated NiP and NiP–SiC composite coatings was investigated in a 0.6 M NaCl solution. The tribocorrosion tests were performed in a linear sliding tribometer with an electrochemical cell interface. It was analyzed the influence of SiC particles dispersion in the NiP matrix on current density developed, on coefficient of friction and on wear volume loss. The results showed that NiP–SiC composite coatings had a lower wear volume loss compared to NiP coatings. However, the incorporation of SiC particles into the metallic matrix affects the current density developed by the system during the tribocorrosion test. It was verified that not only the volume of co-deposited particles (SiC vol.%) but also the number of SiC particles per coating area unit (and consequently the SiC particles size) have made influence on the tribocorrosion behaviour of NiP–SiC composite coatings

The sol–gel route: A versatile process for up-scaling the fabrication of gas-tight thin electrolyte layers

Sol–gel routes are often investigated and adapted to prepare, by suitable chemical modifications, submicronic powders and derived materials with controlled morphology, which cannot be obtained by conventional solid state chemistry paths. Wet chemistry methods provide attractive alternative routes because mixing of species occurs at the atomic scale. In this paper, ultrafine powders were prepared by a novel synthesis method based on the sol–gel process and were dispersed into suspensions before processing. This paper presents new developments for the preparation of functional materials like yttria-stabilized-zirconia (YSZ, 8% Y2O3) used as electrolyte for solid oxide fuel cells. YSZ thick films were coated onto porous Ni-YSZ substrates using a suspension with an optimized formulation deposited by either a dip-coating or a spin-coating process. The suspension composition is based on YSZ particles encapsulated by a zirconium alkoxide which was added with an alkoxide derived colloidal sol. The in situ growth of these colloids increases significantly the layer density after an appropriated heat treatment. The derived films were continuous, homogeneous and around 20 μm thick. The possible up-scaling of this process has been also considered and the suitable processing parameters were defined in order to obtain, at an industrial scale, homogeneous, crack-free, thick and adherent films after heat treatment at 1400 °C

Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings

The effect of doping on the structural, morphological and thermal properties of ZrO2–XO1.5 (X=Y, La, Sm, Er) solid solutions for thermal barrier (TBC) applications was investigated. Oxide powders of various compositions from 9.7 to 40 mol% XO1.5 (X=Y, La, Sm, Er) were synthesised by the sol–gel route. The structural analysis of the powders was performed using X-ray diffraction analysis coupled with Rietveld refinements and the measurement of their specific surface area with the BET method. For each rare earth dopant, the morphology of the powders varies from monoliths to agglomerates of thinner particles when the doping amount increases. In order to determine the specific heat, the thermal diffusivity at room temperature and the thermal expansion coefficient of some selected compositions, DSC, laser thermal diffusivity and hightemperature dilatometry measurements were performed on samples densified by Spark Plasma Sintering. Working thermal characterisation indicated that zirconia doped with 30 mol% SmO1.5 and ErO1.5 have better insulation properties and a lower thermal expansion coefficient than our reference YSZ ceramic. These various compositions are very promising for the elaboration of multilayer TBCs by the sol–gel process

Improvement of barrier properties of a hybrid sol-gel coating by incorporation of synthetic talc-like phyllosilicates for corrosion protection of a carbon steel

Sol–gel coatings for corrosion protection of metals are a good alternative to toxic chromate treatments. The present work focussed on the incorporation of inorganic fillers in a sol–gel coating to improve the barrier properties of the film. Talc-like phyllosilicates obtained by hydrothermal synthesis at 160°C, 260°C and 350°C, called T160, T260 and T350 respectively, were selected as inorganic fillers. The synthetic materials showed talc lamellar structure but, in contrast with natural talc, their smaller size (about 300 nm) and their hydrophilic character allowed easier dispersion of the particles in the sol–gel matrix. Electrochemical impedance measurements performed on the sol–gel coatings deposited on XC35 carbon steel showed that the incorporation of T260 and T350 at a concentration of 20 g L− 1 strongly enhanced the barrier properties of the coating by comparison with the filler-free system. As a consequence, the corrosion protection of the metal substrate was improved

Les revêtements sol-gel pour l’anticorrosion

La voie sol-gel est une méthode de synthèse mais également un procédé de dépôt en phase liquide. Depuis plusieurs années, la communauté scientifique travaillant sur la technique de dépôt par voie sol-gel est impliquée dans des problématiques de durabilité des matériaux. Des travaux montrent les potentialités intéressantes de cette technique pour des applications anticorrosion et font émerger des solutions prometteuses pour le remplacement des couches de conversion chromatées. Ce procédé de « chimie douce » permet d’élaborer des revêtements hybrides ou céramiques sous forme de couches minces. Dans les procédés développés pour l’anticorrosion, les sols sont le plus souvent constitués de précurseurs de type alcoxydes de silicium, zirconium, ... fonctionnalisés ou non. Ce sol peut être déposé par dip-coating ou spray-coating sur différents substrats pour former un film mince qui, au cours du processus d’hydrolyse et de condensation, se transforme en gel. Ce gel, défini comme une structure tridimensionnelle, résulte d’une agrégation homogène de sorte que macroscopiquement ce milieu renfermant du solvant semble monophasé. Lorsque dans une étape ultérieure le solvant est éliminé par un étuvage à basse température, l’architecture du gel est détruite et le composé hybride obtenu sous forme de couche mince dense est appelé xérogel. Parmi les revêtements issus de la voie sol-gel destinés à la protection contre la corrosion, les plus avancés en terme de développement industriel sont des revêtements protecteurs de type barrière renfermant un promoteur d’adhérence pour peinture. Les travaux sur les couches anticorrosion dites actives renfermant des inhibiteurs de corrosion sont nombreux et s’orientent plus récemment sur la recherche d’architectures de couches tendant à optimiser leur efficacité dans le temps. Afin de montrer dans cette présentation le grand intérêt de ce nouveau procédé pour l’obtention de couches anticorrosion, nous faisons état de récents travaux issus de la littérature