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

Abstract

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 ...

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

“…For the compositional term, (1 − )/ ( − ) 2 , it can be determined from the elemental composition in each phase. Table 2 summarises the composition for γ and β phase calculated from the Thermo-Calc using TTNi7 database, which was proved to be consistent with the experimental phase composition in previous work [42]. Since Al is the main alloy element in the β phase and Co is the base element in the γ matrix, the compositional term then equals to 3.…”

“…Due to the small amount of yttrium in the powder (0.5 wt%) and its unavailability in the TTNi7 database, yttrium is thus neglected in the phase fraction calculations. Previous work [42] has shown that the measured phase compositions exhibit good agreements with the calculated phase compositions from the TTNi7 database and the interdiffusion modelling work between a Co-based MCrAlY coating and a Ni-based superalloy reported by Dahl et al…”

“…Taken the diffusion data from Karunaratne et al [31], D eff is found to be 1.19×10 -14 m 2 /s. The magnitude falls into the right range of the previously reported work [29,42]. It has to be emphasised that the D eff used here may not be the exact effective diffusion coefficient in this…”

“…Debonded coatings were heat treated in a vacuum with a nominal pressure of 6.0  10 -3 mbar at 1100 C for 2 h followed by air cooling to replicate the initial heat treatment given to the bond coat material in conventional TBC manufacture. Previous work has demonstrated that the initial heat treatment has the effect of reducing any coating porosity to a minimal level and allows the β phase to precipitate [42].…”

“…With sufficient thermal energy, β precipitates undergo coarsening, also termed Ostwald ripening, during which smaller particles dissolve and large particles grow [39]. The β-phase coarsening phenomenon has been observed in a few studies, but none of them investigated the coarsening characteristics [40][41][42]. Due to the importance of the particle sizes of β phases for the mechanical properties of MCrAlYs, an understanding of the coarsening process of β precipitates is essential for the design and application of MCrAlY coatings.…”