Sodium corrosion of the protective SiO₂ surface scale is a potentially serious problem for Si₃N₄ and SiC components in gas turbine engines. The objective of this program was to develop corrosion-resistant chemical vapor deposition (CVD) mullite coatings to protect Si-based ceramics from hot corrosion. Mullite was selected as a protective coating material due to its high temperature stability, good corrosion resistance, and thermal expansion match with Si₃N₄ and SiC. Chemical vapor deposition provides a means of depositing a thin, dense mullite (3Al₂O₃-2SiO₂) layer with good adherence. Mullite was deposited by CVD using AlCl₃ and SiCl₄ as precursors. The effect of reagent concentrations was found to be significant. With the liquid SiCl₄ source at ambient temperature, the minimum flow rate of SiCl₄ (in the Ar carrier gas) resulted in CVD mullite coatings with a Si content that was higher than the stoichiometric value. The mullite coatings with a high Si content displayed decreased corrosion resistance. After refrigerating the SiCl₄ source to temperatures of 0 and -10°C, mullite coatings with high Al contents were deposited. The results of coating development and hot corrosion testing will be discussed.

Research sponsored by the Advanced Automotive Materials Program, DOE Office of Transportation Technologies, under contract DE-AC05-96OR22464 with Lockheed Martin Research Corporation.

Mathematical modeling of high temperature oxidation and diffusion is a purpose of this paper.

The mathematical model of diffusion and oxidation is described by diffusion equations of alloying elements and oxidant with oxidation processes taking into consideration by mass source term. The diffusion and oxidation mathematical model is integrated by numerical methods (the finite difference method). It permits one to take into consideration the most general peculiarities of the problem formulation: - the multilayer, the relationships between coefficients and coordinate, time and concentration.

To determination the main parameters of the model (diffusion, mass transfer, mass uptaking coefficients, etc.) was used the inverse problem solutions [1]. Inverse problem calculations were carried out with using experimental data from short-time tests. This is allowed to tuning of model parameters by means of experimental data.

Determined coefficients were used for the modeling of high temperature oxidation and diffusion apply to few alloys, coatings and its system. The element concentrate distribution and oxide thickness were calculated using determined coefficients for experimental data. The experimental data and calculated results of concentration distribution are in good correspondence. paragraf The usage of model and inverse problem solution for tuning the model parameters allowed to do the prediction of the diffusion and oxidation states of specimen made from nickel based alloy with and without metal-ceramic coatings at the high temperatures during 50000 h treatment.

1. Krukovsky, P.G. A Universal Approach to Solution of Inverse Heat Transfer Problems (Method and Software). Proceedings of the 30th 1995 National Heat Transfer Conference, Vol 10, ASME, New York, 1995, pp. 107-112.

Functionally gradient materials are a new concept for high temperature mateirals, which consist of ceramics on one side, metal on the other and an intermediate layer whose structure, composite and morphology chagne continuously from ceramic to metal in micro-scale. Hot salt corrosion was employed on functionally gradient materials. This kind of corrosion can cause serious problems in gas turbines and combustion chambers.

In the present work, the lasma spray technique was used to deposite powders such as NiCrAl, NiCrAl + MgZrO₃ and MgZrO₃ on the substrate. We selected a certain compsotion for cermet layer FGM samples were dipped in Na₂So₄+K₂So₄, Na₂SO₄+NaCl, Na₂SO₄ + V₂O₅ containing environment at temperature of 600^oC. The microstructure of the coatings were evaluated by means of an optical microscope and SEM. The results show that corrosion rate presents a parabolic function depending on time. It is also obtained that the most corrosive media is V₂O₅ environment. Morever, the corrosion resistance of FGM is much higher than TBCs considerably.

Functionally gradient materials are coating systems which are used to increase performances of high temperature components in the hot gas turbine section of gas turbine engines, diesel engines, in aerospace and aircraft applications. These coatings consiste of a transition from metallic bond layer to cermet and from cermet to ceramic layer.

In this study, oxidation behaviour of plasma-spray coated functionally gradient materials on AISI 304 L stainless steel substrates were investigated. The plasma spray process was employed to deposit metallic, cermet and ceramic powders such as NiCrAl, niCrAl + MgCrO3 and MgZrO3 on the substrate. We selected a certain composition for each sample and cermet layer. Oxidation tests were carried out at temperature of 800 and 1000^oC. The microstructure of the powders, coatings and oxidized coating were examined by means of an optical microscope, SEM and X-Ray Diffractometry. The results show that the oxide film thickness and oxidation rate at 1000°C are higher than at 800°C considerably.