The Effect of Creep on the Residual Stress in Vapour Deposited Thin Films and Thermally Grown Oxide Scales

Thermally grown oxide scales can provide high-temperature oxidation protection if they are slow growing, sound and adherent to the substrate. In thermal barrier coatings (TBCs), failure occurs near or at the interface between the metallic bond coat and its thermally grown alumina layer. Oxide scales consisting mainly of alumina were grown on duplex ( ZrO 2/bondcoat) and functionally-graded ( ZrO 2/ Al 2 O 3/bondcoat) systems through static heat treatment at 1050°C. Electron microscopy was used to analyze the microstructures of oxide layers grown on various TBC systems. Different thickness of grown oxide scales resulting in the systems will be discussed in the light of the effectiveness of alumina interlayer acting as an additional oxidation barrier. Spallation observed at higher temperature for functionally-graded (FG) system is believed to be due to phase transformation of γ-alumina to α-alumina, and hence inducing additional residual stress causing a premature failure. The effect of incorporating alumina in the FG system, in terms of minimizing the thermal residual stress at the interfaces, was evaluated from the standpoint of Finite Element Modeling.

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Diffusion of Oxygen in Thermally Grown Oxide Scales

Defect and Diffusion Forum ◽

10.4028/www.scientific.net/ddf.289-292.405 ◽

2009 ◽

Vol 289-292 ◽

pp. 405-412 ◽

Cited By ~ 3

Author(s):

Sébastien Chevalier

Keyword(s):

Grain Boundary ◽

Oxygen Isotopes ◽

Oxygen Diffusion ◽

Boundary Diffusion ◽

Thermally Grown Oxide ◽

Grain Boundary Diffusion ◽

Diffusion Experiment ◽

Oxide Scales ◽

Growth Mechanisms ◽

Thermally Grown

High temperature reactivity of materials under oxidizing atmospheres is based on the formation of protective oxide scales. The protectiveness is obtained when the thermally grown oxide scales are dense, continuous and adherent to the metallic substrates (even during thermal shocks); as a matter of fact, the growth of the scale has to be governed by diffusion of species across the growing scale. The diffusing species are coming from the substrate (metallic ions) and/or from the oxidizing atmosphere (oxygen ions). The understanding of growth mechanisms can be reached by making two stage oxidation experiments, using oxygen isotopes. The experiment consists in oxidizing first the metallic substrate in 16O2, evacuating the oxygen after the desired time, and replacing it by 18O2. The distribution of oxygen isotopes given by secondary ion mass spectrometry (SIMS) across the oxide scale informs about the growth mechanisms: anionic transport, cations diffusion or mixed diffusion processes. The use of marker experiment is able to determine the oxygen diffusion coefficients within the growing scales. In this case, a thick scale is grown under 16O2, followed by a shorter diffusion experiment in 18O2. The distribution of 18O isotope across the scale by plotting ln (18O intensity) versus x (depth in oxide) allows determining the oxygen effective diffusion coefficient in the scale, according to the classical Fick’s law solution. A more accurate analysis of these profiles can provide information about bulk and grain boundary diffusion of oxygen. The results can be related to kinetics according to Wagner’s theory. The proposed work consists firstly in making a state of the art review about oxygen diffusion in thermally grown oxide scales, and secondly in connecting the so-obtained outcome (effective, bulk and grain boundary diffusion) to kinetics results. The proposed oxides are chromia, alumina and zirconia.

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