ICMCTF2017 Session A2-1: Thermal and Environmental Barrier Coatings

Modeling efforts for Rare Earth (RE) silicate Environmental Barrier Coatings (EBCs) typically utilize properties of dense homogeneous materials, yet EBCs are often processed using an Air Plasma Spray (APS) process which results in heterogeneous phase distribution, porosity, and splat microstructures. In this paper, properties of dense homogeneous yttrium disilicate processed by Spark Plasma Sintering (SPS) are compared to those of APS yttrium disilicate. Specifically, phase stability during thermal cycling, thermal expansion, and stability in high temperature steam are compared. Phase changes during repeated temperature cycling were characterized with X-ray diffraction. APS phase mixtures evolved during cycling whereas SPS yttrium disilicates were more phase stable. Thermal expansion of both materials was characterized up to 1400°C using dilatometry. Coefficients of Thermal Expansion (CTE) for APS yttrium disilicate varied significantly without a prior annealing treatment. Once annealed, the CTEs for the SPS material were significantly more consistent than those of the APS material. The silica depletion depth for yttrium disilicate samples was measured after steam-jet exposures at 1200°C for times of 60 to 250 hours and steam velocities between 150-180 m/s. The silica depletion rate was lower in dense homogeneous SPS yttrium disilicate compared to the more porous and heterogeneous APS materials. Cross sectional analysis of APS specimens showed both silica depletion to greater depths at splat boundaries and localized yttria-rich areas with minimal silica depletion. The implications of the heterogeneity of APS RE silicate phases and microstructures for EBC applications will be discussed.

The effects of composition, temperature and environment on the performance of vacuum plasma sprayed (VPS) NiCoCrAlYHfSi bond coatings (BC) on directionally-solidified (DS) 247 substrates with air plasma sprayed Y₂O₃-stabilized ZrO₂ has been investigated. Four compositions were investigated including the base composition and additions of Ti, B and Ti+B. The addition of B did not improve the average coating lifetime in 1-h cycles at 1100 °C in air with 10% H₂O in either case, however, the addition of Ti caused a decrease in lifetime. Photo-stimulated luminescence spectroscopy was used to map residual stresses in the thermally-grown Al₂O₃ scale. To study performance near the operating temperature of BCs in land-based turbines, the effect of water vapor was studied at 900 °C in laboratory air and air with 10% H₂O for 10-500 h cycles. Water vapor had little effect on the measured parabolic rate constants at 900 °C and a comparison of the oxide microstructures will be reported.

Research sponsored by the U. S. Department of Energy, Office of Fossil Energy’s Turbine Program.

Protective metallic nickel aluminide (NiAl) diffusion coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials. Aluminising provides a cost effective alternative and allows coating of complex shaped components. However, the composition of the substrate and the specimen geometry influence the final coating microstructure during the aluminising process and govern the high temperature behaviour of the coating system during subsequent exposure.

The formation of a protective alumina scale and the diffusion from the coating into the substrate result in the loss of Al and thereby the dissolution of the ß-NiAl phase in the coating. The extent of interdiffusion depends on the coating and substrate composition. The compatibility of a given type of a coating with its base material substantially influences the performance of a coated material during service. Potentially detrimental precipitate phases may form especially in the interdiffusion zone (IDZ) during the aluminising process and during subsequent service depending on the composition of the base material. Evaluation of the material’s high temperature behaviour requires extensive experimental testing. Computational methods can substantially reduce the extensive experimental efforts required for coating evaluation and qualification.

In the current work, an in-house developed coupled thermodynamic and kinetic computational model was employed to predict the microstructural evolution in nickel aluminide (NiAl) diffusion coatings on Ni-base wrought and cast alloys during the coating (aluminising) process as well as during the subsequent high temperature service. Three Ni-base alloys (602 CA, IN718 and Rene80) were chosen to evaluate the influence of specimen thickness and alloy composition on the performance of nickel aluminide (NiAl) coatings. Specimens of different thicknesses (0.1-2 mm) were coated with nickel aluminide via chemical vapour deposition (CVD) on 602 CA and IN718 and via a slurry coating process on Rene80. The specimens were discontinuously exposed for 1000 h at 1100 °C in laboratory air. Specimens were removed from the furnace at 100, 300 and 1000 h.

Element concentrations and phase distribution were obtained by scanning electron microscopy (SEM). Phases were identified by energy/wavelength dispersive X-ray spectroscopy (EDX/WDX) and electron backscatter diffraction (EBSD). The modelling results were validated with experimental data. The computational approach assists in estimating the lifetime of the coating and provides a tool to predict microstructural changes in coating systems as a function of alloy/coating composition, time and temperature.

Thermal barrier coatings (TBC) reduce the thermal load of gas turbine components. State-of-the-art TBCs consist of partially yttria stabilized zirconia (PSZ) and are deposited by thermal spray techniques (e. g. atmospheric plasma spraying) or electron beam physical vapor deposition (EB-PVD). The resulting microstructure is dependent on the process and strongly influences the coating´s properties and time to failure. Columnar structures exhibit a higher lifetime and are thus favored.

This talk investigates an alternative deposition technique (gas flow sputtering - GFS), which also gives rise to columnar microstructures. The aim of this work is to gain a fundamental understanding of the influence of crucial process parameters such as substrate temperature and bias voltage on the microstructure, and the suitability of GFS coatings for the use as TBCs.

PSZ coatings were deposited on polished FeCrAlY-Alloy substrates and described utilizing WDX, SEM, FIB and XRD.

The substrate temperature has been identified to be a crucial parameter. Between 500 and 800 °C, four different types of columnar microstructures are defined based on XRD pattern and morphology. The growth direction of the columns changes from <111> to <100> accompanied by a change in column shape from triangular to four-sided. The principal shape of the columns is furthermore explained using a growth model.

Applying bias voltage at a given substrate temperature does not give rise to a new type of microstructure but alters the microstructure defined by the substrate temperature. While low bias values lead to more regular columns, high bias voltages lead to further densification and compressive stresses, rendering these conditions unsuitable for TBC manufacturing.

The current demand for higher gas turbine engine operating temperatures has brought into attention the hot corrosion attack to thermal barrier coatings (TBCs) due to siliceous debris infiltration into gas turbines. The debris are commonly composed of calcium-magnesium-alumino-silicates (CMAS) which are carried in environmental pollution in the form of sand, fly ash, volcanic ash, among others. Previous studies in standard 7YSZ coatings deposited by EB-PVD techniques have shown a significant relation of coating micro-structure and CMAS infiltration depth. In the previous study a physical model was proposed to estimate CMAS infiltration into EB-PVD coatings by simplifying the micro-structure as a concentric pipe (kernel pipe) with open channels (feather arms) which distribute the CMAS reducing the infiltration time into the TBC. Therefore, it is of highly importance to understand the kinetics of CMAS infiltration since by refining the deposition parameters (deposition pressure, rotation, etc.) CMAS infiltration can be reduced. The current work correlates experimental results obtained for short term CMAS infiltration performed at 5 min. using a synthesized CMAS source with theoretical results using the proposed “concentric pipe model”. The calculated results are in good agreement by closely predicting the infiltration time for a given depth when compared with short term infiltration experiments. Additionally, the results were compared with the “open pipe” infiltration model which has been previously used in literature for CMAS infiltration estimation. The results also show a significant error in infiltration prediction for the open pipe model which proved a more realistic approach for CMAS infiltration using the proposed concentric pipe model.

The oxidation behaviors of CrN, AlCrN, and AlTiN PVD-cathodic-arc coatings were evaluated isothermally at 800^oC, 900^oC and 1000^oC for a duration of 5hr in ambient atmosphere using 304 stainless steel substrates. A novel substrate design was employed where all substrate edges were chamfered at both faces to coat both surfaces and edges. The oxidation rate measurement of all-surface-coated samples were conducted using in-situ thermogravimetric apparatus. The coating composition, morphology, and hardness were analyzed before and after the oxidation test using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscope (SEM).

XRD results for CrN revealed the evolution of various temperature-dependent oxides forming within and beneath the coating including Cr₂O₃, Fe₂O₃, and Cr₂O after the oxidation tests. The diffraction peaks for CrN coating (1.8 um in thickness) disappeared entirely after the oxidation test at 1000^oC suggesting complete transformation of the CrN layer into mixed oxide layer. The specific weight gain curves showed continuous increase with exposure time at all temperatures examined. The oxidation resistance of AlCrN coating exhibited significant improvement as compared to CrN coating as can be evident by comparing the oxidation kinetic curves of the two coatings owing to the formation of the more protective Al₂O₃ in addition to Cr₂O₃. The coating provided effective protection at 800^oC and 900^oC as can be inferred from the oxidation rate curves. The thickening of AlCrN layer, the absence of oxidation product at the interface between the coating and the underlying substrate, and the asymptotic oxidation rate behavior suggest persistence of protection of AlCrN coating up to 1000^oC.

The major oxidation products of AlTiN coating over 900-1000^oC comprised of TiO₂ and Al₂O₃. The oxidation kinetics showed initial rapid growth stage and a delayed linear behavior. The initial stage may be attributed to the formation of porous TiO₂ with non-continuous Al₂O₃. The delayed linear oxidation rate is similar to the oxidation rate of AlCrN coating at 1000^oC. This has been explained in terms of the dominance of the growth of continuous Al₂O₃ layer in both coatings at high temperature.