Effects of pre-cracking on small punch creep testing of a vacuum plasma-sprayed CoNiCrAlY coating

Surface and Coatings Technology

McCartney

2017

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.

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

“…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

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 ...

“…The load is applied on the pan, E, and is transmitted to the sample via the vertical rod, F. Tests are conducted at elevated temperatures controlled by the furnace, G. The displacement of the centre of the disc, d, was measured through the punch movements as a function of time and recorded by a LVDT transducer, H, to an accuracy of AE 1 mm. 27 In the present work, disc samples were set up at 21 C, 400 C, 450 C, 500 C and 600 C in air. At each temperature, a 100 N load was applied in increments every 30 min as a step to investigate the resultant stress and strain.…”

Section: Small Punch Multi-step Loading Testmentioning

confidence: 99%

Evaluation of the ductile-to-brittle transition temperature in a thermally sprayed CoNiCrAlY coating by small punch multi-step loading tests

Proceedings of the Institution of Mechanical Engineers, Part L:

Yang

Xiao

2016

Self Cite

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings of an approximate thickness of 0.5 mm. Small punch tests under multi-step loading conditions were performed between room temperature and 600 C on these samples to evaluate the ductile-to-brittle transition temperature. The microstructure of the coatings was characterised using a scanning electron microscope with energydispersive X-ray analysis. A two-phase structure consisting of fcc g-Ni and bcc b-NiAl was found to exist. The displacements obtained from small punch multi-step loading tests at each load increment were relatively small and similar at temperatures below 500 C but a significant increase in displacement was noted at 600 C. Fractographic investigation showed that the main fracture mode was dominated by extensive g matrix tearing at elevated temperatures. A distinct stress and strain behaviour was found at 600 C, indicating that the ductile-to-brittle transition temperature of this CoNiCrAlY coating occurred between 500 C and 600 C.