Application of small punch creep testing to a thermally sprayed CoNiCrAlY bond coat

Journal of Alloys and Compounds

McCartney

2017

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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 freestanding 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 2 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.

“…EDX analysis showed them to be rich in oxygen and aluminium and they are which agrees well with previously reported work [15].…”

Section: Microstructural Analysissupporting

confidence: 93%

Section: Introductionmentioning

confidence: 99%

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

Journal of Alloys and Compounds

McCartney

2017

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“…low pressure plasma spraying (LPPS), vacuum plasma spraying (VPS) and, more recently, high velocity oxy-fuel (HVOF) thermal spraying [28][29][30][31][32][33][34][35][36][37][38][39][40]. It is generally recognised that the rate of Al loss and the β-phase depletion behaviour is closely related to the oxide growth kinetics at the oxide/coating interface.…”

Section: Introductionmentioning

confidence: 99%

Some aspects on modelling of the β-phase depletion behaviour under different oxide growth kinetics in HVOF CoNiCrAlY coatings

Surface and Coatings Technology

McCartney

2017

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In this paper, β-phase depletion behaviour of free-standing high velocity oxy-fuel (HVOF) thermally sprayed CoNiCrAlY coatings was studied. Microstructural analysis showed a twophase microstructure of γ-Ni matrix and β-NiAl secondary phase after heat treatment. Fine grains were found around the sprayed particle boundaries and coarse grains were retained as the original particle structure, with grain sizes varying from 2 to 0.5 µm or even less for both phases. The β-phase depletion behaviour was investigated during isothermal oxidation and was also modelled through diffusion calculations. A previously developed β-phase depletion model was utilised to study the evolution of β-phase depletion under different oxide growth kinetics. Three oxide growth models were tried: 1) Meier model, 2) thermogravimetric analysis (TGA) model, and 3) experimentally fitted oxide growth model. The oxide growth kinetics were converted to Al flux functions which were used as the boundary conditions in the DICTRA modelling. It is shown that the results obtained from the three models exhibit good agreements between the measured and predicted results for times up to 100 h at 1100 C, but discrepancies were noted at longer oxidation times. Further improvements on closely 2 modelling the oxidation kinetics and the effective diffusion behaviour are needed to minimise the discrepancies at longer oxidation times.

“…Additionally, a thermally grown oxide (TGO) forms at the interface between the top coat and the bond coat during service at elevated temperature due to the fact that oxygen permeates through the ceramic top coat and oxidises the bond coat. The durability of the overall TBC system is largely determined by the microstructural, chemical and mechanical characteristics of the bond coat [6][7][8]. Among different types of bond coats, MCrAlY overlay coatings, where M = Co, Ni or a combination of the two, are of great interest for their excellent oxidation resistance and good adhesion between the top coat and the superalloy substrate [9][10][11][12][13][14][15][16].…”

Section: Introductionmentioning

confidence: 99%

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

Surface and Coatings Technology

Jackson

Voisey

et al. 2016

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This paper investigates the β-phase depletion behaviour during oxidation of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) bond coats 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-Ni and bcc -NiAl, with grain sizes varying largely from 0.5 to 2 µm for both phases.Isothermal oxidation tests of the free-standing coatings were carried out at 1100 C for times up to 250 h. The phase depletion behaviour at the surface was measured and was also simulated using Thermo-Calc and DICTRA software. An Al flux function derived from an oxide growth model was employed as the boundary condition in the diffusion model. The diffusion calculations were performed using the TTNi7 thermodynamic database together with the MOB2 mobility database. Reasonable agreement was achieved between the measured and the predicted element concentration and phase fraction profiles after various 2 time periods. Grain boundary diffusion is likely to be important to element diffusion in this HVOF sprayed CoNiCrAlY coating due to the sub-micron grains. Keywords IntroductionThermal barrier coatings (TBCs) are widely used to protect high temperature components in turbine engines from harsh operating environments [1,2]. TBC systems consist of a ceramic top coat, a metallic bond coat and a superalloy substrate [3][4][5]. Additionally, a thermally grown oxide (TGO) forms at the interface between the top coat and the bond coat during service at elevated temperature due to the fact that oxygen permeates through the ceramic top coat and oxidises the bond coat. The durability of the overall TBC system is largely determined by the microstructural, chemical and mechanical characteristics of the bond coat due to interdiffusion of element between the MCrAlY and the superalloy substrate. This is not considered in the present paper but has been studied by others, e.g. [29][30][31][32][33][34][35][36][37][38][39][40][41]. In these research works, the emphasis has been on substrate/bond coat interdiffusion and specific comparisons between measured and predicted β-phase depletion at the oxide/bond coat interface have not been considered. On the other hand, several analytical models concerned with the oxidation of 4 two phase systems in which the secondary phase dissolves during oxidation have been reported, e.g. [42][43][44][45][46][47][48]. In these models, the secondary phase depletion behaviour during oxidation can generally be represented by the parabolic diffusion law, Eq. (1), ~√( 1) where is the width of the second phase depletion zone and is the oxidation time.Considering the significance of β depletion on the degradation of TBCs, therefore, the aims of the work reported in this paper were to investigate specifically the kinetics of β depletion at the oxide surface in free-standing MCrAlY coatings ...