Important efforts to develop high temperature coatings are being carried out at SNECMA to increase turbine efficiency and performance in aeroengine. The protection of components against very high temperature and environmental degradation is a major issue in modern gas turbines. The extreme operating conditions lead to the development of various coating technologies. The deposition of aluminium on the surface of the Ni-based superalloy is one of the most commonly employed technique. The oxidation resistance of this intermetallic bond coat system (β-NiAl) is based on its ability to form a protective alumina scale at high temperature. Platinum additions in aluminide coatings and incorporation of reactive element dopants (Y,Zr,Hf,Si) are known to improve both hot corrosion and oxidation resistance. The effect of various impurities, such as S or C, is also of particular interest because this could strongly affects the coating oxidation performances.

Thermal Barrier Coating systems (TBCs) have also proven their capability to protect the hottest section of gas turbine engines. This multilayer system allows to increase the component operating temperature, up to several hundreds of degrees, in the combustor and turbine sections. Considerations in deciding whether a part should be coated or uncoated include the choice of the substrate composition, the performance requirements and the cost target. The lifetime of the TBC is directly affected by the adhesion of the interfacial alumina scale. Spallation is particularly damaging because the turbine blade is no longer protected from very high temperature. The expertises of field returned parts contribute to improve the understanding of the unexpected degradation mechanisms (CMAS, Erosion, FOD, phase transformation, rumpling, voiding.....) and allow to promote the development of new TBC systems with higher performance and reliability.

We have performed in-situ measurements of growth strains in a-Al₂O!sub 3 scales, grown on α-phase Ni-20Al-20Pt-Hf (at. %) alloys with Hf concentrations varied between 0 - 1 %. Strain measurements were obtained, using 21.6 keV synchrotron x-radiation exploiting a single-exposure sin2? Method. Samples were first heated to 1150 °C and were held isothermally until the ? -> a phase transition was complete. Kinetics of the ? -> a phase transition were monitored at 1150°C by measuring the time dependence of ? And a-phase intensities and strains. The transformation rate systematically slows with substrate Hf concentration. We observe small or weakly compressive steady state stresses in scales formed on Hf-free alloys. Tensile stresses appear when alloys contain Hf. After the ? -> a transformation had occurred, the temperature was dropped to 1100°C; that imposed a compressive stress on the oxide, a consequence of the CTE difference between oxide and substrate. Oxide creep relaxation was then monitored at 1100°C. The creep rate, at 1100°C, was reduced when Hf was added to the alloy. Scale growth and relaxation mechanisms will be discussed.

Research was sponsored by the U.S. Dept. of Energy, BES, Materials Science, under contract DE-AC02-06CH11357 (ANL) and by the Office of Naval Research, contract No. N00014-02-1-0733.

Coatings have been fabricated by spraying Maxthal 211 (Ti₂AlC) powder with High Velocity Oxy-Fuel spraying on high temperature Ni-base alloys. Maxthal 211 is an alumina forming ternary carbide with a thermal expansion coefficient near that of alumina. These properties make this material suitable for high temperature applications because it produces a protective oxide that does not spall off during cycling. So far, Maxthal 211 has only been used in bulk form with good oxidation properties in air up to 1450°C.

This work is an initial oxidation study of Maxthal 211 coatings. The coatings have been analyzed both prior to and after oxidation with different characterization techniques, mainly scanning electron microscopy and X-ray diffraction. Coating properties such as phase content, adhesion etc. will be presented and discussed. To show the properties of this material over a wider temperature range, the results will also include long-term exposures at lower temperatures (700-900°C) for the bulk material.