Role of impurities on the spark plasma sintering of ZrCx–ZrB2 composites

The study of the spark plasma sintering (SPS) of ZrCx and ZrCx–ZrB2 composites was carried out considering the effect of experimental parameters such as the applied load, the temperature and the heating time. In addition, the role of the main impurities, detected in the raw materials, on the sintering behaviour has been explored by Transmission Electron Microscopy (TEM). The analyses of monoliths and composites showed up the liquid phase formation from silica impurities and the complete structural reorganisation of free carbon into the graphite form during the sintering treatment. It is also shown that within composites, the plastic strain is preferentially accommodated by ZrB2 crystals

Purification of hot-pressed ZrCO into ZrC by a laser treatment

International audiencePellets of zirconium oxycarbide ZrC0.82O0.14, obtained by hot pressing of zirconia and zirconium carbide, were irradiated by an ytterbium-doped fibre laser in argon atmosphere (incident power density = 24.7 kW cm-2, beam diameter 0.7 mm). The surface of the samples, heated at 3300 C and more, i.e. near the melting, released its oxygen, leading to the carbide ZrC0.75 despite the presence of traces of oxygen in the cell of treatment (PO2 estimated around 1 Pa). A mechanism is proposed for explaining this result, based on the thermodynamical stability of the carbide, higher at these temperature and oxygen pressure than the oxide. Oxygen of the oxycarbide evolved in the form of the gaseous species ZrO, while the grains of the oxycarbide, converted into the carbide, grew from 2 to 20 lm. Similarly, the traces of dioxygen present inside the treatment cell react with the carbide, giving ZrO (gas), and do not form any oxidised solid phase. This opens interesting future prospects in the field of the production of oxygen-free zirconium carbide powders

Behaviour of alumina coated 304L steel in a waste to energy plant

International audiencePieces of 304L stainless steel, alumina-coated by plasma spraying, were placed during one week at about 1170 K in the furnace of theWaste-to-Energy plant of Limoges, France. The resistance against corrosionwas only slightly improved compared to non-coated alloy. The origin of this unsatisfactory result lies in the cracking of the coating that occurred when the pieces were introduced in the furnace, due to the great difference in the Coefficient of Thermal Expansion (CTE) of alumina and 304L steel. However, the plasma-sprayed alumina coatings themselves appear as impervious enough to provide anefficient protection of this alloy against the very corrosive atmosphere of the incinerator

304L stainless steel oxidation in carbon dioxide: An XPS study

International audienceFrom the early beginning of the oxidation of 304L stainless steel in carbon dioxide at 1273K (1 min, for a weight gain of 0.02mgcm−2), the surface of the alloy was entirely covered by oxides: magnetite Fe3O4, chromia Cr2O3 and traces of wüstite Fe1−xO. Later on, for weight gains approaching 1mg cm−2, magnetite remained at the outer interface, with traces of hematite (Fe2O3), above a thick layer of wüstite Fe1−xO. Magnetite and wüstite may favour adhesion of thermal plasma protective coatings such as alumina

Alumina plasma spraying on 304L stainless steel: Role of a wüstite interlayer

International audienceExcellent adhesion values (>70MPa) of alumina coatings achieved by Atmospheric Plasma Spraying (APS) were obtained on pre-oxidized stainless steel (304L), covered with a continuous layer of wüstite (Fe1−xO) surmounted by a very thin magnetite layer. This is due to epitaxial relationships between alumina, magnetite and wüstite, as shown by Transmission Electron Microscopy (TEM), giving a "crystallographic bonding"

Kinetics of some iron-based alloys in industrial CO2 : formation of wüstite phase

International audienceThis work compares the oxidation behaviour of three iron-based substrates (C40E, Invar® and 304L) in CO2 industrial gas, in order to determine the conditions for producing wüstite (Fe1-xO), on the basis of kinetic and morphologic studies. For the three alloys at the beginning of the reaction, wüstite formed under 105 Pa of CO2 following a rate law ≥ ≥ ∃ RT 220000 10.2 P e dt d( m/S) CO2 . For 304L, formation of a spinel phase chromite briefly preceded it during a first step. Magnetite appeared for long times of experiment, in the case of Invar® and 304L. This can be explained by the stopping of iron outer diffusion. These results are discussed according to the literature and thermodynamic data. They open new fields for coating these alloys by plasma spray processes

Revêtement plasma de l acier 304L par l alumine pour usage en milieux gazeux agressifs

Un procédé de revêtement plasma, récemment développé sur un acier C40E, a été étendu à un acier inoxydable 304L. Ce procédé, alternatif au sablage, consiste en la pré-oxydation contrôlée sous CO2 du substrat pour former de la wüstite Fe1-xO, puis à la mise en œuvre du dépôt d alumine. L excellente accroche du dépôt sur les alliages pré-oxydés été confirmée pour deux types de granulométries d alumine projetée, et une étude cinétique a déterminé les conditions de formation de la wüstite. Les granulométries utilisées conduisent à une microstructure comparable. Néanmoins, la tenue du dépôt, testée dans une centrale énergie-déchets, est catastrophique. A cause de la porosité du revêtement, les gaz agressifs atteignent en effet le substrat dont l oxydation conduit au décollement du revêtement. Le mécanisme d oxydation sous gaz agressifs semble analogue à celui sous CO2 où les décollements de la couche d oxydes se produisent au-delà de très faibles gains de masse (environ 1,15 mg.cm-2).A plasma spraying process, recently developed on C40E steel, has been extended to a 304L stainless steel. This process, alternative to sand blasting, consisted first in the controlled pre-oxidation of alloy substrates under CO2 giving a wüstite phase Fe1-xO. The plasma deposit of an alumina coating on so-treated substrates led to a very good bonding of the alumina layer. The two alumina powders used, with different grain sizes, provided similar coating microstructures, while the kinetic study allowed determining the conditions for the formation of wüstite interlayers. However, the pieces were destroyed when placed in waste-to-energy plants. Indeed, the gases penetrated toward the alloy through the coating porosity, and its consecutive oxidation resulted in a coating buckling. The oxidation mechanism under aggressive gases seems to be close to that under CO2, where it was shown that the breaks inside the oxides layer appear for very small weight gains (around 1.15 mg cm-2).LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Oxidation of stainless steel 304L in carbone dioxide

International audienceOxidation of 304L stainless steel in a carbon dioxide atmosphere at 10 5 Pa has been studied. Between 1193 and 1293 K the oxidation kinetics exhibit first a rapid increase, then a parabolic behaviour with apparent activation energy of (209 ± 8) kJ mol 1 and obeys a Langmuir pressure law. After 1.15 mg cm2 , the kinetics become almost linear. The reaction products are chromia at the grain boundaries, wüstite (Fe1xO) on the surface for weight gains greater than 0.30 mg cm2 and chromite. The very complex reaction mechanism takes into account random buckling for weight gains >1.15 mg cm