ICMCTF2014 Session A1-1: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

The study concerns diffusion aluminide coatings modified with Pd and Pt using a novel CHC-PVD technique as a cost-effective alternative for Pt-aluminide bond coatings conventionally used for EB-PVD Thermal Barrier Coatings.

While Pt-aluminide diffusion coatings provide excellent high temperature oxidation resistance and are commonly applied as bond coatings for EB-PVD TBCs new alternatives are being sought for due to high prices of Pt. In order to maintain high oxidation protection along with simultaneous cost reduction alternative Pd+Pt-aluminide bond coatings have been developed using a novel technique applying Physical Vapor Deposition method and high activity “out of pack” diffusion aluminizing. The obtained coatings with varying thicknesses of Pt and Pd were compared and studied using FEG-SEM and EBSD techniques as well as XRD. The microstructures of the coatings were studied after every technological step including annealing of Pd-Pt layers, diffusion aluminizing and heat treatment. Prior to YSZ deposition using EB-PVD the coatings were put under pre-oxidizing treatment in order to form a thin α-alumina TGO. The microstructure of the pre-oxidized TGO was studied in detail using high resolution S/TEM and TEM.

In the last 10 years, the Pt-rich γ-γ’ bond-coatings have been studied for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating (TBC) systems. To optimize their fabrication and durability, it is essential to investigate how the interdiffusion with the Ni-base superalloy substrate takes place and what are the factors which can influence it. This study deals with the interdiffusion modeling of a γ-Ni-Al alloy with a pure Pt coating. Heat treatments at 1100°C for 15 min to 10h were performed in-situ in a high temperature X-ray diffraction apparatus under primary vacuum. The α-NiPt(Al) phase with the L1₀ crystal structure formed very rapidly as it was already observed after a fast heating (40°C/min) and a 100 sec dwell at 1100°C. This implies the very fast Al diffusion to the surface. This α phase vanished after 45 min – 1h at 1100°C and the γ’-(Ni,Pt)₃Al phase appeared. A two-phase γ-γ’ microstructure was obtained and no significant evolution was observed from 2h30 to 10h. Cross-sections of annealed samples were analyzed by SEM, EDS and EPMA. It was observed that voids formed at the interdiffusion zone / base material interface. Concentration profiles and EDS maps confirmed that voids were located at the Pt diffusion front and in the Al depletion zone. Then, a finite-difference method was used to model the fluxes and the concentration profiles in the Ni-Al-Pt system by considering the chemical potential gradients. Experimental and calculated profiles were found to be in good agreement for the γ-phase systems. The fitted interdiffusion coefficients were in the range of those found in the literature. The diffusion paths were determined and compared with the predicted results. Simulations in the γ-phase were performed to predict the possible nucleation and location of the α-phase. The diffusion of vacancies was also considered in order to test its ability to predict the occurrence of Kirkendall voids.

Cast NiCrAl alloys, co-doped with Y, Hf, Ti and/or B, were evaluated at 1100°C and 1150°C in dry air, wet air (10 and 50% H₂O) and CO₂-10%H₂O in order to study the effect of boron on alumina scale growth and adhesion. Ti and B additions were combined to potentially form stable TiB₂ precipitates in the alloy. Cyclic (1-h cycle time) testing at 1150°C in air with 10%H₂O clearly showed improved scale adhesion with the addition of B after 500 cycles. After only 200 cycles at 1100°C, only minor differences in mass change were observed. Microstructural characterization is in progress including quantifying the effect of boron on internal oxidation and alumina microstructure using transmission electron microscopy (TEM).

The environment effects (10%H₂O+air vs. 10%H₂O+90%CO₂) were also studied at 1100°C on the oxidation behavior of co-doped NiCrAl alloys with no significant difference being observed. Further analysis will be carried out.

Keywords: NiCrAl, water vapor, CO₂, Reactive Elements, a-Al₂O₃, Boron

The microstructure of the studied CoNiCrAlY coating was observed to change dramatically due to the applied aluminizing treatment. As expected, the amount of the Al-rich beta NiAl phase increased considerably in the outer part of the coating. In addition, the Al-enrichment resulted in precipitation of Co and Cr rich phases and reduction in the amount of the matrix phase (gamma Ni solid solution). During high-temperature oxidation exposure, depletion of the studied overalumized coating did not result in formation of the gamma prime (Ni3Al) phase. Rather the sub-scale formation of the Co/Cr-rich phases was observed, which was found to affect the alumina scale growth. Therefore the effect of aluminizing on the microstructure of the studied CoNiCrAlY coating appeared to be different from that commonly observed with Ni-rich MCrAlY-coatings.

Development of thermally sprayed nanocoatings is attracting attention of the various researchers due to the improved performance of the nanocoatings as compared to its corresponding conventional coating. On other hand thermal spray process is best suited for the application of high temperature resistant coatings for oxidation, corrosion, erosion and wear. The present investigation deals with the high temperature oxidation performance of the thermally sprayed NiCrAlY nanocrystalline on superalloy substrate. The NiCrAlY nanocoatings deposited by LVOF technique on the superalloy substrate were oxidized cyclically at 900ᵒC upto 100 cycles. The weight change per unit area and its square were plotted against number of cycles to monitor the kinetics of oxidation. It was compared with the similar curves obtained by oxidation of the NiCrAlY conventional coatings sprayed by similar process. The oxidized coatings were further characterised by XRD, FESEM/EDS and X-ray mapping techniques. The NiCrAlY nanocotings found to be more resistant to the oxidation as compared to the conventional NiCrAlY coatings due to the formation of more adherent, uniform and compact oxide layer.

With the increasing requirement of improved fuel efficiency and reduced CO₂ emission, the operation temperature of components, like turbine blades and vanes, is being pushed to higher and higher levels in modern gas turbines. To maintain the high performance of the components at higher temperatures, high-temperature coatings are needed to provide the resistance against oxidation and corrosion. In a variety of coatings, such as bond-coats in thermal barrier coating system or overlays, MCrAlY coating (M usually for nickel and/or cobalt) is one such candidate, capable to be further developed by modifying its composition. For instance, alloying elements such as yttrium, silicon and tantalum, at small proportions, have been added into MCrAlY coatings to enhance their oxidation resistance and/or mechanical properties. The aim of this study is to investigate the corrosion behavior of the coatings with Ru, Mo and Ir during high temperature exposure. Coatings with and without Ru, Mo and Ir were denoted as alloyed coatings and reference coatings, respectively.

The reference and alloyed coatings were sprayed by high-velocity oxy-fuel process on a Ni-based superalloy IN792 to form overlays. The hot corrosion tests on those coatings were performed by cyclic exposures in a mixture of Na₂SO₄ and K₂SO₄ at 900°C. Thereafter, the surfaces and cross-sections of the tested samples after different cycles were investigated in scanning electron microscope (SEM) equipped with energy dispersive system (EDS) and wavelength dispersive system (WDS). The technique of X-ray diffraction (XRD) was used to distinguish the surface species formed during the corrosion process.

A variety of oxides, e.g. α-Al₂O₃ and Cr-Co rich spinels, were found on the coating surface after the hot corrosion process. The formation of the oxides was significantly affected by the coating composition. The surface of the reference Ni-based coating was mainly covered by a dense and nodule-shaped alumina. But a more complex morphology of isolated islands embedded in a plate-shaped matrix was found in the alloyed Ni-based coatings. In the Co-based coatings, alloying Ru, Mo and/or Ir promoted the formation of Cr-Co spinels even though Al₂O₃ was still the matrix oxide to cover the whole coating surface. It seems as if the addition of Ru, Mo and/or Ir in MCrAlY coatings plays a role on promoting the formation of non-α-Al₂O₃ oxides to respond to the hot corrosion attack.

Hot corrosion can occur in gas turbine engines in the presence of sulfur and salt in the temperature range of approximately 650°C to 900°C. At lower temperatures, the sulfates are not molten, and at higher temperatures, component temperatures are above the dew point of the sulfates and little sulfate deposition may occur. Hence, this type of attack has previously been observed in the lower temperature regions (below blade or vane platforms) of aero gas turbines as well as on blades and vane airfoils of marine gas turbines which typically operate at temperatures below those of aero gas turbines. In the lower portion of this temperature range (~650°C to ~750°C), the attack occurs by pitting and is referred to as Type II attack. Due to increasing temperatures in aero gas turbines, this type of pitting attack is now being seen in turbine disks. Previous experimental work has shown that low temperature corrosion can significantly decrease the low cycle fatigue (LCF) life of disk alloys. However, coatings used to protect blade alloys are not always suitable for disk alloys due to different mechanical properties of the different alloys and components.

The low-temperature, hot corrosion behavior of an advanced disk alloy has been examined in the temperature range of 650°C to 800°C. Tests have been performed with coupons coated with Na₂SO₄ or 60Na₂SO₄-40MgSO₄ salt mixtures and exposed to low gas flows containing various O₂/SO₂ concentrations (10-1000 ppm SO₂). The extent of attack has been determined by sample weight change or pit depth. Both uniform attack and pitting has been observed with various O₂/SO₂ concentrations. For the uniform, non-pitting attack, the corrosion increased with increasing temperature and increasing SO₂ concentration in the cover gas. A summary of the work to date including pit morphology and chemistry will be presented as well as coating requirements and future coating directions.

In many applications at high temperature, micro-structured coatings have been applied in order to protect structural materials against a wide range of different environments: oxidation, metal dusting, sulphidation, molten salts, steam, etc… The resistance achieved by the use of different kind of coatings have been optimum, and with late design such as TBC´s and FGM´s coatings. Although, the lifetime of them are related with inter-diffusion, and different CET as main degradation mechanisms.

In the case of supercritical steam turbines, many attemps have been made in terms of micro-structural coatings design, mainly based in aluminides, and another diffusion coating systems. In order to consider another alternatives to minimize those problems, nano-structured coatings, applied by PVD and HIPIMS-PVD based in Cr, Ti and Al design, have been applied onto high temperature structural materials in order to analyze their high temperature oxidation resistance in steam environments.

The gravimetric results obtained have been analysed upto 2.000 hours, jointly with the evaporation behavior analysed by TG-Mass spectromnetry. Excellent results have been achieved for the nano-structured coatings tested. Those results are comparables with the results obtained for micro-sctructured coatings, and in some case better for nano-structured coatings.

According to the results obtained, the nano-structured coatings have a potential application as protective systems in high temperature, for some applications will be proposed.

The oxidation protection of Ni-based superalloys under thermo-cyclic oxidizing service conditions relies on the mechanical and chemical stability of diffusion ß-NiAl coatings, which provide the necessary Al-reservoir for the protective oxide scale formation. Outwards diffusion of Al and the interdiffusion with the substrate causes Al-depletion in the coating and therefore the coating possesses different mechanical properties depending on the present Al concentration hence on the existing phases throughout the exposure. Moreover, owing to the interdiffusion with the substrate, the Al content of the substrate is increased and thereby the alloying elements such as Ta, Mo and W precipitate within the γ/γ^' matrix and build up the secondary reaction zone with altered microstructure.

Imposition of intrinsic and external mechanical stresses on the oxide scale and the coating can lead to mechanical failure, if a critical stress threshold is exceeded. Intrinsic mechanical stresses in the coating and the oxide scale determine the load bearing capability of the respective zone, since their magnitude defines the tolerable external stress. In this study, sources of intrinsic stress in the oxidation influenced zones of the composite material system have been studied.

Sources of intrinsic stress under thermo-cyclic oxidizing conditions for the coating are stresses arising from temperature changes, phase transformations and the stress relaxation due to creep deformation. In the case of oxide scale, the intrinsic stress-state depends on the stresses arising from temperature changes, oxide growth, internal oxidation and surface curvature.

Ni-based superalloys are usually employed in industrial applications where good mechanical performance, corrosion and oxidation resistance at high temperatures is required. The present study describes microstructure investigation in relation to process parameters of diffusion aluminizing of directionally solidified Ni superalloy and Ni alloy with VPA method. Estimating parameters such as time, temperature, chemical composition of granules, used chlorides or fluorides activators has been related to the evolution of microstructure. Vapor phase aluminizing process has been carried out at 900 - 1100°C. Aluminide coatings formed on a nickel-base superalloy have been characterized in their as-coated condition. The studies were focused on thickness, chemical composition, mechanism of coatings formation and diffusion processes. The microstructural characterization involved use of X-ray diffraction (XRD), electron microscopy (SEM), chemical analysis in microareas (EDS) and glow discharge optical emission spectroscopy (GDOS).