ICMCTF1998 Session A1: Coatings to Resist High-temperature Corrosion

The oxidation behaviour of HVOF-sprayed MCrAlY (M=Ni,Co) coatings was examined. The coatings were isothermally oxidized in synthetic air at temperature between 850^° and 1050^°C for different times.

The HVOF MCrAlY coatings have a microstructure similar to the ODS alloys. During the thermal spraying the aluminium and yttrium were partially oxidized and a fine oxide dispersion was formed in the coating. The oxidation rate of the HVOF MCrAlY coatings was 3-4 times slower than that of the vacuum-plasma sprayed coatings. It seems that the fine oxides hinder the diffusion of the elements from the bulk material and modify the oxidation mechanism. The formation of the oxide dispersion in the coating influenced also the adherence of the oxide scale.

The structure of the oxide scale was examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

The resistance of alumina scales to cracking and spalling under the influence of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coatings, and bond coats for thermal barrier coatings. However, the relative magnitudes of the stresses and their distribution are often not known. In this study several x-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both during oxidation and after cooling to room temperature. The corresponding stresses are being calculated using appropriate elastic constants. Preliminary results were presented in this conference in 1997. This presentation will highlight the results obtained during the intervening year. These include the observations that:

1. Growth stresses are high in alumina formed on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys.

2. Yttrium additions to alumina-forming alloys do not result in lower growth stresses in the scales.

3. Growth and thermal stresses can be relaxed by plastic deformation of both the alloy and oxide.

The implications of these results with regard to alumina adhesion will be discussed.

For achieving increased efficiencies the gas inlet temperatures of advanced land based gas trubines have dramatically been increased in the past decade. Thermal, mechanical and environmentally induced stresses in land based gas turbine applications have become more and more similar to those in aero turbines. Thus, blade life is limited mainly by thermo-mechanical fatigue and oxidation rather than by creep and sulphidation as was often the case in the past.

Protection against oxidation induced damage of the blade materials is obtained by the application of Ni(Co)CrAlY coatings which are applied by thermal spray processes. The oxidation resistance of these coatings relies on protective alumina surface scales which form on the coating surfaces upon exposure to the oxidizing environment at the high service temperatures of around 900°C. In the present study isothermal and cyclic oxidation tests were carried with a number of Ni(Co)CrAlY-coating compositions applied by Vacuum Plasma Spraying. The coating compositions were systematically varied in Co-, Cr- Al and Re-contents. The oxidation tests were carried out in air and in Ar.O₂ in the temperature range 900 - 1100°C. Before and after exposure the specimens were examined by optical metallography, SEM, energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). The results showed that in describing the oxication properties of the Ni(Co)CrAlY coatings it is of great importance to take into account that the coating alloys are multiphase. Depending on the exact coating compositon the microstructure can consists of γNi, γ'-Ni₃Al, β-NiAl, α-Cr and.or α-CrCo, whereby the mentioned phases are solid solutions with variable compositions tather than stoichiometric compounds. The amount and distribution of these phases affect the coatings oxidation properties in at least two ways: 1) oxides of different composiitons can be formed on the various phases in the early stages of oxidation. The composition and distribution of these transient oxides can strongly affect the long time oxidation behaviour and 2) phase transformations can occur upon temperature changes (e.g. during start up and cool down of the turbine) which can significantly affect the thermal expansion coefficient of the coatings and therfore also the adherence of the alumina based surface scales.

From the obtained results guidelines for coating compositions which are optimized in respect to long term oxidation protection as well as to applications of Ni(Co)CrAlY's as bond coat for thermal barrier coatings can be derived.