ICMCTF2011 Session A1-1: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

This paper reports upon the deposition of bond coatings with different compositions as well as their oxidation and degradation mechanisms at temperatures used for industrial turbine blades (900 - 950 C). Physical vapour deposition technique, magnetron sputtering, was used to deposit a range of Ni-Cr-Al-Co coatings on 10 mm diameter sapphire substrates. This was achieved through co-sputtering two targets: a Ni10Cr, Ni20Cr, Ni50Cr, Ni20Co40Cr or Ni40Co20Cr (target changed after deposition) and another pure Al target. About a hundred samples with varying compositions were produced by this method. The coatings were then isothermally oxidised in air for 500 hours in furnaces set at 900 and 950°C.

All samples were then assessed with pre- and post-exposure metrology (coating thickness, specimens weights) which showed that magnetron sputtering successfully deposited 20 to 30 µm thick coatings and allowed the calculation of oxide growths rates. Energy dispersive x-ray (EDX) analysis was used to characterise the exact composition of each sample. Additionally, x-ray diffraction (XRD) identified the oxides formed during exposure. The selective growth of protective chromia or alumina oxide (depending on the initial composition) was observed. This influenced the oxide scale’s growth rate indicating which coatings were more protective and allowing future optimisation of the bond coating to be planned.

Hot corrosion is a highly accelerated form of high-temperature oxidation caused by the presence of a salt deposit, commonly Na₂SO₄ or a molten reaction product from this deposit. Despite its longstanding prevalence in commercial applications, a thorough understanding of the manner by which compositional, microstructural, and environmental factors influence the hot corrosion behavior of alloys and coatings has proven elusive. For instance, the mechanism by which Pt enhances the hot corrosion resistance of β-NiAl coatings is not clear. Moreover, elements such as Cr and Co are nearly always found in β-NiAl coatings as a result of interdiffusion with the superalloy substrate, and the effects of such elemental additions on the hot corrosion behavior of these coatings have not been systematically investigated. In this study, cast NiAl alloys with small additions of Pt, Cr, and Co were exposed to Type I (900° C) hot corrosion conditions, and the amount of degradation was compared using SEM/EDS characterization coupled with measurements of the weight-change kinetics. Differences in hot corrosion resistance are explained by thoroughly investigating the nature of the corrosion products formed on these alloys during the early stages of 900°C oxidation and hot corrosion.

In the last 7 years, Pt rich γ- γ’ alloys and coatings have been studied by several research groups and have shown good oxidation and corrosion resistance. They can be considered as an alternative to β-(Ni,Pt)Al for bond coatings in thermal barrier coating system (TBC). Several studies have shown that Pt rich γ-γ’ alloys can be excellent alumina formers at high temperatures (1100°C-1200°C), depending on their composition. Moreover, an optimized doping of these alloys can lower significantly the growth rate of the alumina. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire TBC systems. In the present study, this technique was used to fabricate doped bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr and metallic additions such as Cu, Au or Ag. Thin films of these elements were deposited on the superalloy substrate by radio frequency PVD before SPS. The SPS step was followed by an annealing treatment to obtain the Pt-rich γ-γ’ phases in the coatings. These samples were then coated by EB-PVD with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with Pt rich γ-γ’ bond coatings were compared to standard TBC system composed of a β-(NiPt)Al bond coating. Thermal cycling tests were performed during 1000-1h cycles at 1100°C under laboratory air. Spalling areas were monitored during the oxidation test. Most of the Pt rich γ-γ’ samples exhibited a better adherence of the ceramic layer than the β-samples. After the oxidation test, cross sections have been prepared to characterize by scanning electron microscopy the thickness and the composition of the oxide scales, mostly alumina. In particular, the influence of the doping elements on the oxide scales formation was controlled. Moreover, compositions of the bond coats, still γ-γ’ after 1000-1h cycles, were analysed.

With the high price of Re, there has been significant interest in reducing the quantities present in single crystal superalloys. General Electric has developed a version of their second generation superalloy with the Re level reduced from 3% to 1.5%. Coupons of this material have been coated with simple aluminide, Pt-modified aluminide and Pt diffusion bond coatings. Cyclic oxidation testing was conducted to evaluate the reaction kinetics, surface roughening, scale adhesion and lifetime of a ceramic topcoat on this substrate compared to conventional second generation superalloys with similar coatings.

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Research sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program.

Pt-modified NiAl is widely used in aerospace industries where high temperature and high pressure are required. The additions of Pt have been shown to improve oxidation and hot corrosion resistance in NiAl coatings. However, the exact mechanisms have not been established. This study focuses on the beneficial role of Pt and Pd in improving Type I hot corrosion resistance of beta-NiAl coatings. Several Pt- and Pd-containing coatings are studied and compared with commercial PtAl and PGM-free NiAl baseline. The results indicate that Pt and Pd promote the formation of alumina, decrease beta-to-gamma prime transition rate, and reduce spallation. The differences between Pt and Pd will also be highlighted.