Sylvain Weber

Mullite based oxidation protection for SiC-C/C composites in air at temperatures up to 1900 K

For an industrial Si-SiC coated C/C material (reference material) the temperature dependence of the linear rate of mass loss is interpreted in the temperature range 773 < T < 1973 K. The Arrhenius plot of the thermogravimetrically determined oxidation rate shows four typical regimes. Only in the temperature range 1323 < T < 1823 K is the oxidation rate close to or lower than the limit for long-term application. Pulsed Laser Deposition (PLD) allows the ablation of nonconductive and high melting targets and the preparation of films with complex composition. High energy impact CO 2 laser pulses (j= 3.10 7 W cm -2 ) lead to melting and evaporation of the target material in a single step. Therefore the flux of the metal components is stoichiometric. Deposited green layers did not show IR peaks typical for mullite. After a short oxidation treatment (15 min at 1673K) the formation of mullite in the coating was completed as was confirmed by IR spectroscopy and XRD investigations. Thin PLD-mullite layers (900 nm) did not markedly improve the oxidation resistance of the reference material in the high temperature range 1473 < T < 1973 K. However, a preoxidation treatment of the substrate material and mullite coatings with a thickness of 2.5 μm improved the oxidation behaviour significantly. Because of SiO 2 formation at the mullite-SiC interface all samples exhibited a mass increase on oxidation. The inward diffusion of oxygen across the outer mullite-containing layer controlled the kinetics of the reaction as was deduced from 18 O diffusivity measurements in PLD mullite layers. The calculated oxidation rates resulting from the diffusion parameters in SiO 2 and mullite are close to the thermogravimetric data. For oxidation durations of three days only amorphous SiO 2 is formed at the mullite-SiC interface.

Cation transport in yttria stabilized cubic zirconia: 96Zr tracer diffusion in (ZrxY1–x)O2–x/2 single crystals with 0.15⩽x⩽0.48

Abstract For a wide range of stabilizer concentrations in yttria stabilized cubic zirconia (YSZ), Zr diffusion data extracted from published creep data and dislocation loop shrinkage data are discussed together with published Zr tracer diffusion data and our own data on Zr tracer diffusion in order to identify the most probable point defect responsible for Zr diffusion. From this evaluation, complex defects can be ruled out, as the single vacancy, VZr4′, fits best.

Combination of EBSP measurements and SIMS to study crystallographic orientation dependence of diffusivities in a polycrystalline material: oxygen tracer diffusion in La2 − xSrxCuO4 ± δ

Abstract In the present work we demonstrate that electron back scattering pattern (EBSP) measurements together with secondary ion mass spectrometry (SIMS) can be used to study the crystallographic orientation dependence of tracer diffusivities in polycrystalline materials. The crystal orientation of single grains was determined with a dedicated scanning electron microscope in the EBSP mode. 18 O isotope depth profiles obtained in a subsequent tracer diffusion experiment were measured by SIMS on single grains with known orientation. The experiments yielded oxygen tracer diffusivities as a function of crystal orientation, temperature and strontium concentration for La 2 − x Sr x CuO 4 ± δ ( x = 0 and 0.15) between 600 and 900 °C. A strong anisotropy of the oxygen diffusivity for the investigated dopant concentrations was found.

Magnetic and structural properties of ion nitrided stainless steel

The magnetic properties and crystalline structure of expanded austenite obtained by ion beam nitriding of AISI 316 steel are investigated. Magnetic force microscopy reveals that the nitrogen expanded austenite has two different layers, an outermost ferromagnetic layer and a paramagnetic layer beneath it. Superimposing the nitrogen concentration profile determined by secondary neutral mass spectrometry and the magnetic force microscopy image, one can see that the paramagnetic-ferromagnetic transition takes place at the inflection point of the nitrogen concentration profile at about 14±2 N at. %. Conventional and glancing angle x-ray diffraction suggests that nitrogen could occupy first tetrahedral interstitial positions (nitrogen-poor paramagnetic phase) and then, after saturation of Cr traps, octahedral interstitial positions (nitrogen-rich ferromagnetic phase). The ferromagnetic-paramagnetic transition is seen to be governed by Cr (traps)–N interactions.

Self-diffusion studies on 15N in amorphous Si3BC4.3N2 ceramics with ion implantation and secondary ion mass spectrometry

Nitrogen self-diffusion studies in amorphous precursor-derived Si3BC4.3N2 ceramics were carried out, using ion implanted stable 15N isotopes as tracers and secondary ion mass spectrometry for depth profiling. The analysis of the diffusion profiles in the range of 1500–1700 °C did not show the typical Gaussian broadening of the implantation profiles. Instead, we observed the occurrence of a high concentration region where the width of the implantation profile is nearly unchanged due to implantation damage, and a low concentration region where diffusion occurs. The experimentally determined diffusivities obey an Arrhenius behavior with an activation enthalpy of about H=7 eV and a pre-exponential factor D0 in the order of 5 m2/s which indicates a diffusion mechanism via vacancy-like defects.

Lanthanide transport in stabilized zirconias: Interrelation between ionic radius and diffusion coefficient

The diffusion of all stable lanthanides was measured both in calcia stabilized zirconia (CSZ) and in yttria stabilized zirconia (YSZ) in the temperature range between 1,286 and 1,600 degrees C. The lanthanide diffusion coefficients obtained increase with increasing ionic radius. The experimental activation enthalpy of diffusion is near 6 eV for CSZ and between 4 and 5 eV for YSZ and is not strongly affected by the type of lanthanide. The results were correlated with defect energy calculations of the lanthanide diffusion enthalpy using the Mott-Littleton approach. An association enthalpy of cation vacancies with oxygen vacancies of about 1 eV (96 kJ/mol) was deduced in the case of CSZ, while there is no association in the case of YSZ. Furthermore, the change in diffusion coefficients can be correlated to the interaction parameter for the interaction between the lanthanide oxide with zirconia: The higher the interaction parameter, the higher the lanthanide diffusion coefficient.

Cation diffusion in calcia stabilized zirconia (CSZ)

Abstract Tracer diffusivities of both cations in single crystalline calcia stabilized cubic zirconia (11 and 17 mol% CaO) were measured in the temperature range 1130°C < T < 1460°C. The calcium tracer diffusion is more than 10 times faster than the zirconium tracer diffusion. For both elements, the activation energy of tracer diffusion varies between 5.1 and 5.6 eV. The pre-exponential factors are higher than in the case of yttria stabilized zirconia (102−103cm2s−1 as compared to 1–10 cm2s−1). There is no significant dependency of the cation diffusivity on the stabilizer content, which indicates a diffusion mechanism via vacancy clusters (V4 г, 2V2∗o).

Nitrogen diffusion enhancement in a ferrous alloy by deuterium isotopic effect

Studies of nitrogen implantation in an iron alloy using photoemission electron spectroscopy, sputtered neutral mass spectrometry, and elastic recoil detection analysis, reveal an enhancement of nitrogen diffusion when deuterium replaces hydrogen in the gas. Compared to hydrogen, deuterium reduces NOx species on the surface (geometric barrier), increasing the nitrogen activity at the surface and consequently nitrogen diffusion into the solid solution.