Bond Coat Alloy Research Articles

Thermal response of TBC systems subjected to non-uniform thermal shocks and periodic thermal load: Effect of Biot number and interface imperfection

This study analytically examines the temperature variations within thermal barrier coating (TBC) systems under diverse kinds of thermal loadings, such as concentrated/distributed convective thermal shocks and oscillating thermal loads. In this study, the TBC system comprises ceramic top coat (TC) and NiCrAlY alloy bond coat (BC). The transient behavior of the TBC system facing various kinds of non-uniform convective thermal shocks is elaborated via the dual-phase-lag (DPL) heat conduction model. The effect of imperfect bonding of the TC/BC interface on transient responses of the regions in the neighborhood of the TC/BC interface is studied by considering impermeable, partially permeable and completely permeable heat flux flows. Laplace and Fourier integral transforms are employed to derive the exact solutions of time-dependent temperature fields governing equations. The prominent effects of sudden thermal impulse and steady alternating thermal loading on the variations of temperature within the TC, BC and substrate are inspected. The analysis reveals that the kind of thermal load, loading parameters, thermal properties and thickness of ceramic TC directly affect the transient/periodic temperature fields inside the TBC constituents. Meanwhile, the interface imperfection and the Biot number of convective thermal load play an impressive role in the severity of thermal response of the TBC constituents.

On the oxidation and interdiffusion behavior of an AlCoCrFeNiY high-entropy alloy bond coat on a directionally solidified Ni-based superalloy

The oxidation and interdiffusion behavior of a novel AlCoCrFeNiY bond coat deposited on a directionally solidified Ni-based superalloy were systematically studied at 1050, 1100 and 1150 °C, and compared with a conventional NiCoCrAlY coating deposited on the same substrate. The AlCoCrFeNiY bond coat exhibits lower oxide growth rates due to its large columnar grains and low Al activity at the oxide scale/bond coat interface. Meanwhile, AlCoCrFeNiY has higher resistance to oxide spallation than NiCoCrAlY, which is attributed to the formation of a clean and defect-free metal/oxide interface. Significant interdiffusion occurs across the AlCoCrFeNiY/superalloy substrate interface. Our experimental evidence and thermodynamic modelling suggest that Fe accelerates interdiffusion and destabilizes the γ’ phase, thereby causing the formation of a thick and γ’-depleted interdiffusion zone. In addition, the AlCoCrFeNiY bond coat undergoes more Al depletion and subsequent β to γ transformation compared with NiCoCrAlY. Based on the findings in this work, a novel AlCoCrFeNiY/NiCoCrAlY double-layer bond coat was designed, tested and validated to achieve optimal balance between oxidation and interdiffusion.

Novel Al2CoCrFeNi high-entropy alloy coating produced using suspension high velocity air fuel (SHVAF) spraying

Metallic coatings of Al2CoCrFeNi high entropy alloy (HEA) were deposited using the suspension high velocity air fuel spray (SHVAF) process, towards exploring its viability as a bond coat in thermal barrier coatings. The relatively high Al content promoted a BCC + B2 phase-dominated coating structure, leading to enhanced mechanical properties. The oxidized microstructure exhibited a protective Al2O3 layer with characteristics comparable to conventional bond coat alloys.

Microstructure evolution of as-cast CoNiCrAl bond coat alloys after isothermal heat treatments

In this paper, the microstructure evolution of two as-cast CoNiCrAl bond coat alloys after various isothermal heat treatments were investigated. The CoNiCrAl bond coat alloys were prepared by vacuum induction melting and casting process. The as-cast alloys were subjected to heat treatments from 550 °C to 1300 °C for periods up to 100 h to study the effects of temperature on the microstructure and phase distribution. The microstructure of heat treated CoNiCrAl bond coat alloys was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The CoNiCrAl alloys exhibited a two phase γ + β structure, with the β phase acting as the primary phase. It was shown that the precipitation of needle-like secondary γ phase was slow at temperatures below 600 °C but became rapid above 900 °C. It was found that the morphology of the secondary γ phase changed from needle-like to rod-like at 1100 °C. A homogeneous γ + β two phase structure was obtained at 1300 °C even after 2 h heat treatment. It was revealed that the heat treatment temperature had a significant influence on the formation and distribution of the secondary γ phase. In addition, the precipitation of sub-micron Cr-rich particles occurred at 1300 °C in both CoNiCrAl alloys, which was due to the low solubility of Cr in the β phase and rapid coalescence of γ phase at elevated temperatures.

Multi-scale modelling of the mechanical behaviour of a CoNiCrAlY bond coat alloy during small punch testing

The present work is undertaken to determine the ability of the small punch test to determine mechanical properties of CoNiCrAlY, a two phase bond-coat material found in thermal barrier coatings. It utilises a multi-scale modelling methodology in which 2-dimensional representative volume elements of CoNiCrAlY and finite element models of the small punch test are developed. Effective Young’s moduli and Poisson’s ratios are obtained from the representative volume elements and are compared with those obtained from analytical homogenization through the Voigt, Reuss and Voigt-Reuss-Hill schemes. The volume fraction of the β-Nial phase in CoNiCrAlY is varied and the force–displacement responses of representative volume elements are contrasted with those from small punch test finite element models, through comparison of ratios of discrete fréchet distances. The results of the study estimated Young’s modulus and Poisson’s ratio to be 178.56 ± 7.65GPa and 0.249 ± 0.037 respectively, and for an increasing β-Nial phase volume fraction, the Young’s modulus and Poisson’s ratio decrease. Lastly, the results suggest that the small punch test is insensitive to variations in material elastic behaviour because its results are more influenced by the yield properties of a material.

Advanced MCrAlY alloys with doubled TBC lifetime

Thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades. However, the most commonly used MCrAlY bond coats which largely determine air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. APS TBC has been found predominate failure within top coat near interface/or at interface with thermally grown oxides (TGO). TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in the top coat or both. By focusing on the reduction of tensile stress in the top coats, three new bond coat alloys have been designed and developed. More than doubled TBC lifetime has been achieved by using the new alloys as bond coat. Extended thermal cycle lifetime is attributed to the unique properties of the new alloys, such as remarkably lower coefficient of thermal expansion (CTE), lower weight fraction of β phase, lower aluminum migration rate and the absence of mixed/spinel TGO oxides.

Microstructure and thermophysical properties of as-cast CoNiCrAl bond coat alloys at different Al contents

In this paper, the microstructure and thermophysical properties of two CoNiCrAl bond coat alloys with differing Al contents were investigated. Two CoNiCrAl alloys with compositions of Co-31.7%Ni-20.8Cr-10Al and Co-31.7%Ni-20.8Cr-12Al (all in wt%) were prepared by vacuum induction melting and casting. The microstructure of the as-cast CoNiCrAl alloys was characterised by scanning electron microscopy (SEM), X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD). The phase changes of the CoNiCrAl alloys as a function of temperature were studied by differential scanning calorimetry (DSC). Thermophysical properties such as thermal conductivity, thermal diffusivity and thermal expansion were evaluated. The as-cast CoNiCrAl alloys exhibited a two-phase structure, consisting of the Al-rich BCC β phase and Co and Ni-rich FCC γ phase. It was found that the β-phase fraction increased with the overall Al content in the alloy. A two-stage thermal expansion was noted for both CoNiCrAl alloys. In addition, it was shown that the CoNiCrAl alloy with the higher Al content exhibited a lower thermal expansion coefficient at high temperatures. This is because more β phase was formed in the CoNiCrAl-12 alloy, resulting in a reduced thermal expansion coefficient.

Effects of Stepped Heating on the Initial Growth of Oxide Scales on NiCrAlHf Bond Coat Alloy under Air and Water Vapor Atmospheres.

Temperature and atmosphere have a significant effect on the oxidation of MCrAlY (M = Ni, Co) bond coating. The initial growth behavior of the NiCrAlHf bond coat alloy was investigated at 1100 °C under different atmospheric conditions and using heating methods. A thick Al2O3 oxide layer and large HfO2 particles were observed, perhaps due to metastable oxide growth at low temperatures when using stepped heating. However, in air and water vapor atmospheres, the oxide scale was thinner and the HfO2 precipitates were smaller in stepped heating than in constant heating. The size and distribution of the HfO2 particles might have induced different microstructures, particularly voids within the oxide scale.

Refractory Metal Intermetallic Composites, High-Entropy Alloys, and Complex Concentrated Alloys: A Route to Selecting Substrate Alloys and Bond Coat Alloys for Environmental Coatings.

This paper considers metallic ultrahigh-temperature materials (UHTMs) and the alloying behaviour and properties of alloys and their phases by using maps of the parameters δ (based on atomic size), Δχ (based on electronegativity), and valence electron concentration (VEC), and discusses what connects and what differentiates material groups in the maps. The formation of high-entropy or complex concentrated intermetallics, namely 5-3 silicides, C14 Laves and A15 compounds, and bcc solid solutions and eutectics in metallic UHTMs and their co-existence with “conventional” phases is discussed. The practicality of maps for the design/selection of substrate alloys is deliberated upon. The need for environmental coatings for metallic UHTMs was considered and the design of bond coat alloys is discussed by using relevant maps.

Modelling the stress state of a thermal barrier coating system at high temperatures

Abstract A model is proposed to assess the time- and temperaturedependent creep and oxidation behavior of typical thermal barrier coating (TBC) systems consisting of EB-PVD manufactured partially stabilized ZrO2 and a plasma-sprayed MCrAlY bond coat (BC) on a typical Ni-base sheet alloy. The creep behavior of the BC was investigated by compression tests at the NiCoCrAlY alloy PWA 1370. With the objective of describing the formation of the so-called thermally grown oxide, an oxidation kinetic for the BC alloy is presented, which predicts the oxide thickness as a function of time and temperature. Using a finite element model of the complete TBC system, the influence of time- and temperature- dependent oxide growth and deformation behavior of the components on the stress development was studied. The obtained results show a development of compressive stresses above the peaks and of tensile stresses in the valleys of the rough TBC/BC interface. With this, the delamination stress between top coat and BC, which finally leads to failure by spallations, can be assessed.

High-temperature oxidation behavior of CuAlNiCrFe high-entropy alloy bond coats deposited using high-speed laser cladding process

A promising new bond coat using CuAlNiCrFe high-entropy alloy was proposed in this paper. The difference in pre-oxidation conditions between the traditional thermal-sprayed MCrAlY bond coat and CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding was investigated. The result confirmed that the CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding can complete the pre-oxidation faster and form a continuous α-Al2O3 thermally grown oxide (TGO), which shortens the initial oxidation stage, thereby avoiding the formation of other oxides and spinel structures. In the subsequent isothermal oxidation process, the block-like structure of the high-speed laser cladding layer and the sluggish diffusion effect of the high-entropy alloy work together to ensure the slow and continuous supply of aluminum element to TGO layer and obtain a low growth rate. Also, the diffusion between the bonding layer and the substrate is controlled at a low level, and the new CuAlNiCrFe bond coat exhibits excellent oxidation and diffusion resistance. With the consumption of aluminum, the phase structure of the high-entropy alloy bond coat changed from BCC to FCC, but it still maintained a stable simple solid solution structure.

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.

On the Microstructure and Isothermal Oxidation of the Si-22Fe-12Cr-12Al-10Ti-5Nb (at.%) Alloy.

Nb-silicide based alloys are new ultra-high temperature materials that could replace Ni-based superalloys. Environmentally resistant coating system (s) with αAl2O3 or SiO2 forming bond coat alloys that are chemically compatible with the Nb-silicide based alloy substrates are needed. This paper makes a contribution to the search for non-pesting bond coat alloys. The microstructure and isothermal oxidation at 800 °C of the silicide-based alloy Si-22Fe-12Cr-12Al-10Ti-5Nb (OHC2) were studied. The cast alloy exhibited macrosegregation of all elements. The microstructures in the cast alloy and after the heat treatment at 800 °C consisted of the same phases, namely TM6Si5, TM5Si3 (TM = transition metal), FeSi2Ti, Fe3Al2Si3, (Fe,Cr)(Si,Al), and an unknown phase of dark contrast. The latter two phases were not stable at 950 °C, where the TMSi2 was formed. There was evidence of endothermic reaction(s) below 1200 °C and liquation at 1200 °C. The alloy followed parabolic oxidation kinetics after the first hour of isothermal oxidation at 800 °C, did not pest, and formed a self-healing scale, in which the dominant oxide was Al2O3. The alloy was compared with other alumina or silica scale-forming intermetallic alloys and approaches to the design of bond coat alloys were suggested.

High-temperature creep of a CoNiCrAlY bond coat alloy

Creep tests of a CoNiCrAlY bond coat alloy were conducted in the temperature range from 1073 K to 1473 K. The alloy was prepared from a powder with a nominal chemical composition of 34.2% Co, 28.6% Ni, 21.2% Cr, 15.6% Al, 0.4% Y (atomic percent is given throughout) by consolidating it under a stress of 60 MPa and annealing it at 1473 K. The final composition of the alloy is 36.3% Co, 29.6% Ni, 21.0% Cr, 12.8% Al and 0.3% Y. The stress exponent n decreases with the increasing temperature from 3.9 at 1073 K to approximately 1 at 1473 K. Consequently, the activation energy of creep is stress dependent and ranges from 512 kJ/mol to 224 kJ/mol with increasing stress. Combined creep and constant strain rate data can be described by the Garofalo (sinh) equation. A comparison of the results with available creep data of free-standing bond coats is presented. Creep resistance of the present alloy clearly overwhelms the resistance of bond coat alloys previously summarized by Wood.

Microstructure evolution under high temperature deformation of CoNiCrAlY bond coat alloy

The evolution of microstructure and texture during high temperature uniaxial compressive deformation of two-phase (γ+β) CoNiCrAlY bond coat alloy was investigated. Uniaxial compression tests were carried out at various temperatures ranging from 1073 K to 1473 K with true strain rates from 5.0 × 10−3s−1 to 5.0 × 10−4s−1 up to a true strain of −1.0. It was concluded that dynamic recrystallization (DRX) occurred in both γ and β phases, even in two-phase alloys. Under all deformation conditions, the main component of the texture in the γ phase having fcc structure was {101}. As for the conditions of low Zener-Hollomon parameter Z in the γ phase, the texture was not developed. This is due to the formation of twin boundaries during grain boundary migration which may weaken the texture. In the β phase with the B2 structure, the main component of the texture was {111}. Even under the low-Z conditions of the β phase, the texture was not developed. Because the area fraction of the β phase in CoNiCrAlY alloy is approximately 25%, and because the β-phase grains were surrounded by γ-phase grains, migration of the β-phase grain boundaries should not occur.

Effects of temperature on the β-phase depletion in MCrAlYs: A modelling and experimental study towards designing new bond coat alloys

This paper investigates the effects of varied temperature conditions on the kinetics of β-phase depletion in MCrAlYs by DICTRA diffusion simulations and experimental observations. A commercially available MCrAlY composition and two modelled compositions were examined at temperature regimes of 1000 °C, 1100 °C and 1200 °C. A previously developed temperature-dependent oxidation model was used at the boundary to examine the β-phase depletion kinetics at different temperatures. The oxide growth kinetics and the corresponding Al flux were calculated based on the oxidation model. The MCrAlY phase equilibrium calculations were conducted in Thermo-Calc and the oxidation-diffusion simulations were carried out using the finite difference diffusion software, DICTRA. It was found that the MCrAlYs exhibit a predominantly dual-phase γ + β structure at 1000 °C and above. It was revealed that varying the temperature had a strong effect on the β-phase depletion in low Al-content MCrAlYs from DICTRA simulations and available experimental measurements. In addition, it was demonstrated from the diffusion simulations that MCrAlYs with high Al content exhibit an enhanced lifetime in maintaining the two-phase structure with less β-phase depletion at high temperatures due to the high Al and β-phase content.

Preparation of Dense Al<sub>2</sub>O<sub>3</sub> Film on 8YSZ Coating by Sol-Gel Method

The 8 wt.% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are widely used to protect hot superalloy components of gas turbine and jet engine. However, Cracks and pores in the 8YSZ coating would accelerate the oxidation of the bondcoat alloy and the corrosion of 8YSZ topcoat by molten salts and finally result in permanent failure of the TBCs. Hence, in this work, a sol-gel spin-coating method was employed to prepare a dense α-Al2O3 film on the surface of air-plasma-sprayed 8YSZ coatings, in order to block the paths of cracks and pores while keeping the stress tolerance of 8YSZ coatings. The influences of the polyvinylpyrrolidone (PVP) and alumina content of the sol on the microstructure of the α-Al2O3 film were investigated. It is found that a sol with PVP addition amount of 50 mg/mL and alumina solid content of 10% let to the achievement of a compact α-Al2O3 film. The positive results in this work would inspire more efforts to prepare dense films on TBCs and therefore improve their performance under service conditions.

Microstructural Analysis and Transport Properties of Thermally Sprayed Multiple-Layer Ceramic Coatings

Multilayer, graded ceramic/metal coatings were prepared by an air plasma spray method on Ti-6Al-4V, 4140 steel and graphite substrates. The coatings were designed to provide thermal barriers for diesel engine pistons to operate at higher temperatures with improved thermal efficiency and cleaner emissions. A systematic, progressive variation in the mixture of yttria-stabilized zirconia and bondcoat alloys (NiCoCrAlYHfSi) was designed to provide better thermal expansion match with the substrate and to improve thermal shock resistance and cycle life. Heat transfer through the layers was evaluated by a flash diffusivity technique based on a model of one-dimensional heat flow. The aging effect of the as-sprayed coatings was captured during diffusivity measurements, which included one heating and cooling cycle. The hysteresis of thermal diffusivity due to aging was not observed after 100-h annealing at 800 °C. The measurements of coatings on substrate and freestanding coatings allowed the influence of interface resistance to be evaluated. The microstructure of the multilayer coating was examined using scanning electron microscope and electron probe microanalysis.

An Overview of Using Small Punch Testing for Mechanical Characterization of MCrAlY Bond Coats

Considerable work has been carried out on overlay bond coats in the past several decades because of its excellent oxidation resistance and good adhesion between the top coat and superalloy substrate in the thermal barrier coating systems. Previous studies mainly focus on oxidation and diffusion behavior of these coatings. However, the mechanical behavior and the dominant fracture and deformation mechanisms of the overlay bond coats at different temperatures are still under investigation. Direct comparison between individual studies has not yet been achieved due to the fragmentary data on deposition processes, microstructure and, more apparently, the difficulty in accurately measuring the mechanical properties of thin coatings. One of the miniaturized specimen testing methods, small punch testing, appears to have the potential to provide such mechanical property measurements for thin coatings. The purpose of this paper is to give an overview of using small punch testing to evaluate material properties and to summarize the available mechanical properties that include the ductile-to-brittle transition and creep of MCrAlY bond coat alloys, in an attempt to understand the mechanical behavior of MCrAlY coatings over a broad temperature range.

An analytical approach to the β-phase coarsening behaviour in a thermally sprayed CoNiCrAlY bond coat alloy

This paper investigates the β-phase coarsening behaviour during isothermal heat treatment of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc γ-Ni matrix and bcc β-NiAl precipitates. The volume fraction of the γ-Ni and the β-NiAl phases were measured to be around 70% and 30% respectively, with grain sizes varying largely from 0.5 to 2 μm for both phases. Isothermal heat treatments of the free-standing coatings were carried out at 1100 C for times up to 250 h. The β-phase coarsening behaviour during isothermal heat treatments was analysed by quantitative metallography. It is shown that the coarsening behaviour of β phase in the CoNiCrAlY alloy followed the classical Lifshitz-Slyozov-Wagner (LSW) theory of Ostwald ripening. By incorporating a dimensionless factor which correlates with volume fraction of the β phase, a modified LSW model coupled with formulaic interfacial energy and effective diffusion coefficient of the CoNiCrAlY alloy was utilised to interpret the coarsening behaviour of the β phase. The coarsening rate coefficient obtained from the modified LSW model shows good agreement with the corresponding experimental result.