Materials Science Forum Vols. 595-598

Abstract: K38 nanocrystalline coatings with various amounts of yttrium addition were deposited by magnetron sputtering. Cyclic oxidation tests were conducted at 800-1000oC in air in order to reveal the effect of reactive elements on the oxidation behavior of nanocrystalline material. The results indicated that the influence of yttrium was not observable at 800oC. At 900 and 1000oC, addition of 0.1 wt.% Y decreased the growth rate of scale, while 0.5 wt.% Y addition significantly increased the oxidation rate of nanocrystalline coating.

Abstract: Surface and interfacial properties of borosilicate glass/ceramic systems have been investigated using the sessile drop method. The purpose is to compare and understand the reactivity of the sealing glass in C / SiC and SiC / SiC composites. A hot wall reactor has been designed to measure the variation of the contact angle and the spreading kinetic according to the temperature (500 to 1100°C) and the atmosphere (Ar, Ar + O2 and Ar + H2O). Chemical and morphological analyses underline (i) the strong reactivity between the liquid and the ceramic, (ii) the influence of the infiltration process and (iii) the strong influence of the oxidizing agent on the wetting behaviour of the glass/ceramic systems.

Abstract: Chromia scales grown on several chromia-forming metallic substrates in various conditions were characterised by photoelectrochemistry (PEC), highlighting the presence of two semiconductor phases signed by their respective bandgaps (3.0 and 3.5 eV) with variations of semiconduction type (n, p and insulator, more or less doped). The protective character of the scale was clearly demonstrated when the highest bandgap phase (3.5 eV), identified as the external subscale was close to an insulator.

Abstract: Water vapour interacts with growing chromia scales in several different ways. Formation and volatilisation of Cr2O2(OH)2 is shown to account quantitatively for chromium loss from thin alloy foils reacted with air-steam mixtures over periods of 103 h. In the shorter term, water vapour is shown to refine the grain structure of Cr2O3 scales grown on Ni-25Cr. Scaling kinetics are at the same time accelerated by an additional, larger contribution to diffusion by a grain boundary species, either OH- or H2O. A slight increase in scaling rate observed at low water vapour partial pressures in H2/H2O gases is thought to be due to hydrogen doping.