ICMCTF2012 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt+Pd electroplating, followed by vapor phase aluminizing at 1050˚C. Cyclic oxidation tests were performed at 1100˚C in 23h cycles. Microstructural and phase analysis conducted using SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings. Based on the results a microstructural degradation scheme of the investigated coatings is presented.

MCrAlY type (M = Ni, Co) coatings are commonly used as overlay coatings and bond coats (BC`s) for thermal barrier coatings (TBC`s) in aero engines and industrial gas turbines. It is known that the life time of thermal barrier coatings is crucially affected by the properties of the thermally grown oxide (TGO), which is formed during high temperature service at the TBC/BC interface. The most relevant properties of the oxide scale are growth rate, adherence to the BC, and composition [1]. Several investigations have shown that the TGO-properties depend strongly on the parameters of vacuum heat treatment commonly applied to the MCrAlY coated components prior to TBC deposition [2-4]. Variation of the heat treatment parameters can result in formation of different types of oxide scales on BC`s with nominally the same composition, which leads to different lifetimes of the TBC.

In the present study the influence of vacuum parameters and atmosphere composition on the phase equilibrium at the MCrAlY surfaces during heat treatment are investigated. For this purpose free standing MCrAlY coatings (manufactured via VPS or HVOF) with rough and polished surfaces were exposed at 1100 °C for times between 1 and 5 hours in different atmospheres (<10^-5 mbar, 10^-4 mbar, 10^-3 mbar with Argon gas, and 10^-3 mbar with synthetic Air). The surface scale composition and morphology were analysed by secondary neutral mass spectrometry (SNMS), Raman spectroscopy, X-Ray diffraction, optical metallography, and SEM equipped with EDX and Cathodoluminescence detectors. It has been found that the composition of the rough as well as the polished surfaces depends on the atmosphere and pressure conditions during heat treatment. It is shown that the phase composition at the MCrAlY coating surface is mainly governed by two competing processes, i.e. Cr evaporation and Y oxidation. The latter reaction has been observed to depend strongly on the Y reservoir in the coating.

References

[2] A. Gil, V. Shemet, R. Vassen, M. Subanovic, J. Toscano, D. Naumenko, L. Singheiser, W. J. Quadakkers, Surf. Coat. Technol. 201 (2006) 3824.

[3] U. Schulz, O. Bernardi, A. Ebach-Stahl, R. Vassen, D. Sebold, Surf. Coat. Techno. 203 (2008) 160.

[4] N. M. Yanar, E. S. Pettit, G. H. Meier, Metallurgical Mat. Transactions 37A (2006) 1563.

The role of water vapor on thermal barrier coating (TBC) performance has been investigated for both Pt diffusion and MCrAlY-type bond coatings. The addition of 10% water vapor reduced the average TBC lifetime on MCrAlY and MCrAlYHfSi bond coatings by ~30% compared to dry O₂, but did not affect the lifetime of Pt diffusion coatings. However, the oxide scale formed on Pt-diffusion coatings had a higher average thickness for the specimens oxidized in wet air compared to dry O₂. In both cases, the scale formed beneath the yttria-stabilized zirconia top coat was thicker than on the uncoated side of the specimen. Additional characterization will be presented on both coating systems at higher water vapor contents. Model MCrAl specimens also will be examined to further understand the role of water vapor on scale growth and microstructure.

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Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

The water vapor content of the exhaust in land-based turbines will increase when coal-derived synthesis gas (syngas) or hydrogen are used to replace natural gas. To investigate the perceived detrimental role of water vapor on thermal barrier coating (TBC) lifetime, coupons of single crystal superalloy X4 were sprayed with NiCoCrAlY and NiCoCrAlYHfSi bond coatings by the high velocity oxy fuel (HVOF) process and yttria-stabilized zirconia (YSZ) top coats by air plasma-spray (APS). Furnace cycling was conducted at 1100°C in order to induce TBC failures in <1000h. The bond coatings with Hf and Si produce at least a 20% higher lifetime in all cases. For both bond coatings in 1h cycles, the average TBC lifetime dropped ~30% in air with 10% water vapor compared to cycling in dry O₂. As a potential strategy to improve TBC performance, two versions of X4 with Y and La additions were similarly coated and cycled in 10% water vapor. However, the average TBC lifetimes were statistically similar to the base X4 superalloy. Initial experiments using 50% water vapor have not shown a decrease in lifetime compared to 10% water vapor. To better simulate base-load turbine duty, TBC lifetime in 100h cycles and 10% water vapor also is being investigated. As expected, the average TBC lifetime was much higher than with 1h cycles at 1100°C. Characterization of the failed TBC coatings will be presented quantifying the effect of water vapor on the thermally grown scale thickness and morphology.

MoSi₂-based composites have a high melting point, relatively low density and excellent oxidation resistance at high temperatures. This makes them useful materials for high temperature applications such as heating elements, burner nozzles and tubes in laboratory and industrial heating furnaces. It is well known that the oxidation resistance of molybdenum disilicides relies on the formation of a protective silica scale, formed at high temperatures due to the selective oxidation of Si. Although MoSi₂ composites show excellent corrosion resistance in oxidizing atmospheres, the SiO₂ scale is decomposed in reducing environments due to the evaporation of SiO(g). One way to avoid this problem is to partially substitute silicon with aluminium yielding a MoAl_xSi_2-x material. MoAl_xS_i2-x composites form a stable and adherent alumina scale at high temperatures and the material can therefore be used in oxidising, reducing and inert atmospheres up to almost 1600°C. In this study, a detailed investigation of the microstructure of the oxide film formed on a MoAl_xSi_2-x based composite (Kanthal Super ER) during oxidation in the temperature range 900 – 1500°C, has been carried out, aiming at understand the material’s oxidation behaviour.

The isothermal exposures were carried out in horizontal tube furnaces for up to 1000h. The composite consists of three phases, Mo(Si,Al)₂, Mo₅(Si,Al)₃ and alumina. The samples formed relatively pure α-Al₂O₃ scales during the oxidation in the whole temperature range. The Al supply to the growing alumina scale is mainly provided by the Mo(Si,Al)₂ phase in the bulk material. The outward Al transport is counterbalanced by inward diffusion of Si into Mo(Si,Al)₂. Therefore an Al poor Mo₅(Si,Al)₃ phase forms directly below the scale. In addition, an Al depletion gradient forms in the composite.

Platinum-modified aluminide coatings are widely used in both aero-engines and land-based gas turbine because they are essential to the overall life-time of the TBC system, which is of great interest in the industry as for the last two decades environmental and economic issues have raised over. It prevents the superalloy from detrimental oxidation at high temperature by forming a dense and adherent layer of alumina on its surface, and it improves the adhesion of the Thermal Barrier in TBC systems. Coatings investigated in this study are deposited by platinum electroplating followed by a low activity CVD aluminizing. The coating can be described by two layers: an outer β-NiAlPt single-phase layer approximately 50 μm thick and an interdiffusion zone referred to as IDZ. The coating resistance is usually assessed using standard cyclic oxidation tests with 1 hour at the maximum temperature during which the weight of samples is recorded by thermogravimetric methods. Net Mass Gain curves are then obtained and give an overview of the lifetime of the coating or allow the cross comparison of different coatings.

This paper reports upon results of an on-going investigation on degradation mechanisms of a platinum-modified nickel aluminide coating deposited on a Ni-base single crystal superalloy AM1 used by SNECMA for advanced blades. More particularly, a novel approach of the lifetime of platinum-modified coatings is proposed trough the detailed study of IDZ evolution and its correlation with Net Mass Gain curves of coatings. The effect of different thermal transients, hold time duration, maximum temperatures as well as thermomechanical loadings on the IDZ evolution has been studied and a close link with the degradation of the coating has been found. Interrupted tests were performed to assess the kinetics of the IDZ evolution and it was monitored using cross section analyses of specimen combining optical microscopy and electron microprobe analysis.

The interdiffusion zone shrinks while the hold time at high temperature increases and has been modeled by simple diffusion laws. The direct relation between IDZ evolution and the coating degradation make it simple to deduce a promising lifetime model for the studied coating.

Using a number of examples in the present paper it is shown how the presence of the base material influences the phase composition and microstructure of the coatings. In particular the effects of carbon and Ti incorporated from the superalloy substrate into the coating are elucidated. Furthermore, it is shown that the porosity formation at the bondcoat/superalloy interface can be in many cases attributed to interdiffusion induced phase transformations rather than to the Kirkendall effect, which is commonly claimed to be responsible for the observed porosity. The analytical studies by SEM are complemented with numerical thermodynamic calculations using the software packages Thermocalc and Dictra.

Nickel-based superalloy components in the hot sections of commercial gas turbine engines are often protected by aluminide coatings due to their ability to function in oxidative and corrosive environments. However, the microstructures of these coated systems are metastable and change in service due to interactions with the environment and interdiffusion with the underlying substrate. The extent of these changes depends critically upon coating microstructure, chemistry, and the environment that the coated component operates in. This presentation highlights the influences of chemical composition, post-deposition annealing, and isothermal oxidation at 1050°C on the microstructures and properties of NiAl-Cr-Hf and NiAl-Cr-Zr overlay bond coatings. In particular, the results indicated that coatings containing Hf exhibit lower oxidation mass gains for oxidation times of less than 100hrs as compared to the Zr containing samples, but higher mass gains above 100hrs of oxidation. This presentation also highlights the effect of processing parameters, such as the addition of a seed layer, on microstructures and coating properties. The results indicate that the addition of a Ni seed layer can improve the adhesion between the substrate and coating.