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Research Papers

The Influence of Transient Thermal Gradients and Substrate Constraint on Delamination of Thermal Barrier Coatings

[+] Author and Article Information
C. A. Johnson

GE Global Research Center,
Niskayuma, NY 12309

J. W. Hutchinson

School of Engineering and Applied Sciences,
Harvard University,
Cambridge, MA 02138
e-mail: hutchinson@husm.harvard.edu

The elastic energy in a thermally grown oxide layer in a multilayer TBC can contribute significantly to the energy release rate when the plane of delamination lies under the oxide layer, and it would have to be accounted for in a multilayer simulation.

Even with contact, the crack can be open at the tip with nonzero mode I component; an example is given in Ref. [4]. If frictional effects are not large, the energy release rate given by the procedure laid out here usually provides a good approximation, and the crack is dominated by mode II.

The notation here follows that of Ref. [5]; however, the numbering of the layers has been reversed.

The discussion concerns only the mode mix. The expression for the energy release rate, Eq. (21), is exact.

1Corresponding author.

Manuscript received August 13, 2012; final manuscript received August 31, 2012; accepted manuscript posted September 29, 2012; published online October 19, 2012. Editor: Yonggang Huang.

J. Appl. Mech 80(1), 011002 (Oct 19, 2012) (13 pages) Paper No: JAM-12-1386; doi: 10.1115/1.4007727 History: Received August 13, 2012; Revised August 31, 2012; Accepted September 29, 2012

A systematic study of factors affecting the delamination energy release rate and mode mix of a thermal barrier coating attached to a substrate is presented accounting for the influence of thermal gradients combined with rapid hot surface cooling. Transient thermal gradients induce stress gradients through the coating and substrate, which produce overall bending if the substrate is not very thick and if it is not constrained. Due to their influences on the coating stresses, substrate thickness and constraint are important aspects of the mechanics of delamination of coating-substrate systems, which must be considered when laboratory tests are designed and for lifetime assessment under in-service conditions. Temperature gradients in the hot state combined with rapid cooling give rise to a maximum energy release rate for delamination that occurs in the early stage of cooling and that can be considerably larger than the driving force for delamination in the cold state. The rates of cooling that give rise to a large early stage energy release rate are identified.

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References

Figures

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Fig. 3

Elliptical contours for constant delamination energy release rate for various G˜ = 2G(1-ν2)/(E2h2(1+ν2)) including depiction of rapid and slow cooling trajectories for a substrate constrained against bending. (a) With no thermal gradient in the hot state. (b) With a significant thermal gradient in the hot state. Even in the absence of a thermal gradient in the hot state, a bilayer subject to rapid cooling of the coating can experience a large energy release rate driven by the temperature drop of the coating relative to the substrate before the substrate has had a chance to undergo much cooling.

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Fig. 2

Elliptical contours of constant delamination energy release rate with the normalized temperature drop of the substrate on the horizontal axis and the normalized temperature drop of the coating surface relative to the substrate on the vertical axis. The horizontal axis reflects the thermal strain mismatch between the coating and the substrate while the vertical axis reflects the effect of rapid cooling of the coating. Full details are given in the text. The curve for the constrained case applies to any set of bilayer parameters. The curve for the unconstained case applies only to a bilayer with parameters specified by Eq. (8).

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Fig. 1

The TBC bilayer with a coating on top of a substrate. Constrained and unconstrained conditions are depicted. The heat transfer coefficients are H1 at the bottom surface and H2 at the top surface. Delamination is analyzed for both the constrained and unconstrained cases.

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Fig. 4

Transient temperature distribution for a bilayer with properties (Eq. (8)) with h1 = 3.5 mm and h2 = 0.75 mm subject to the JETS scenario in Eqs. (9) and (10)

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Fig. 5

Transient stress variations at four locations within the intact bilayer (well ahead of the delamination crack tip) with properties (Eq. (8)). The bilayer is subject to the JETS scenario specified by Eqs. (9) and (10). (a) No bending constraint. (b) With bending constraint. The transient temperature distributions are those in Fig. 4. In all cases in this paper, the stress in the coating in the hot state is taken to be zero. The stress in the constrained substrate in the hot state does not contribute to the delamination energy release rate, and it is taken to be zero in this figure.

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Fig. 6

The elasticity problem for computing the mode mix of the delamination crack ψ for the unconstrained bilayer subject to thermal stresses in (c). (a) The resultant forces and moments in each layer due to the thermal stress in the intact bilayer. (b) Equal and opposite resultant forces and moments that cancel the loads in (a) and that produce the stress intensity factors for problem (c).

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Fig. 7

(a) Transient variation of the delamination energy release rate and (b) the mode mix for a bilayer with properties (Eq. (8)) subject to the JETS scenario in Eqs. (9) and (10). Results for both constrained and unconstrained bending are shown. The variations of the temperature and stress distributions are those in Figs. 4 and 5. Included for both cases is the variation of the energy/area in the coating UCOATING well ahead of the delamination crack tip. As is evident, UCOATING supplies a good approximation to G.

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Fig. 8

The effect of the coating modulus E2 on the delamination energy release rate for an unconstrained bilayer specified by Eq. (8) and subject to the JETS scenario in Eqs. (9) and (10)

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Fig. 9

The effect of the substrate thickness h1 on the delamination energy release for the bilayer whose other properties are specified by Eq. (8). The coating thickness is fixed at h2 = 0.75 mm. Results for both constrained and unconstrained bending are shown for cooling given by Eq. (10). In all cases, the initial hot state temperatures of the interface and coating surface have the values associated with the reference JETS case: Tint(0) = 1013.9 °C and T2sur(0) = 1425 °C, corresponding to a fixed hot state heat flux, q = 0.822 MW/m2.

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Fig. 16

The effect of a delay t0 in switching on a high level of the heat transfer coefficient H2 at the coating surface on the delamination energy release rate for the reference unconstrained bilayer with properties from Eq. (8). For t≤t0, H2 = 200 Wm-2K-1 and for t > t0, H2 = 1500 Wm-2K-1; H1 = 200 Wm-2K-1 for all t > 0. The initial steady-state hot state temperature distribution is specified by T1sur(0) = 870 °C and T2sur(0) = 1425°C. The cooling gas temperatures, T1gas = 38 °C and T2gas = 38 °C are switched on at t=0. The curve for t0=0 is the reference JETS case for unconstrained bending.

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Fig. 15

The effect of the rate of switching on the cooling gas on the delamination energy release rate and mode mix as dependent on the time scale t0 defined in Eq. (26) for the reference unconstrained bilayer with properties from Eq. (8). The heat transfer coefficients used in these simulations, H1 = 988 Wm-2K-1 and H2 = 4698 Wm-2K-1, with hot state gas temperatures, T1gas(0) = 38 °C and T2gas(0) = 1600 °C, are consistent with the initial steady-state hot state surface temperatures, T1sur(0) = 870 °C and T2sur(0) = 1425 °C. Starting at t = 0, the gas temperature impinging on the coating surface is reduced to 38°C with an exponential decay (Eq. (26)) characterized by t0. The gas temperature on the substrate surface is maintained at 38°C throughout.

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Fig. 10

The effect of varying the substrate thermal diffusivity κ1 on the delamination energy release for the unconstrained bilayer whose other properties are specified by Eq. (8). The initial steady-state hot state temperature distribution is unaffected by κ1 and is the same as the reference JETS case with T1sur(0) = 800 °C and T2sur(0) = 1425 °C, corresponding to a fixed hot state heat flux, q = 0.822 MW/m2. The substrate diffusivity has a significant effect on the cooling rate of the substrate and, therefore, on the rate of approach to G in the cold state. The effect of substrate diffusivity on the peak G in the early stages of cooldown is less pronounced.

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Fig. 11

The effect of the coating thickness h2 on the delamination energy release for the unconstrained bilayer whose other properties are specified by Eq. (8). The substrate thickness is fixed at h1 = 3.5 mm. In all cases, the initial hot state temperatures of the interface and substrate surface are fixed at the values associated with the JETS reference case: Tint(0) = 1013.9 °C and T1sur(0) = 870 °C, corresponding to a fixed hot state heat flux, q = 0.822 MW/m2. Cooling is specified by Eq. (10).

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Fig. 12

The effect of the initial hot state coating surface temperature T2sur(0) on the delamination energy release rate and mode mix for the reference unconstrained bilayer in Eq. (8) subject to JETS cooling (Eq. (10)). The initial temperature of the surface of the substrate is T1sur(0) = 870 °C for all the simulations.

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Fig. 13

Effect of increasing the heat transfer coefficient at the substrate surface H1 on the delamination energy release rate and mode mix for the unconstrained bilayer (Eq. (8)) subject to the otherwise unchanged cooling scenario in Eq. (10). The JETS reference case has H1 = 200 Wm-2K-1 and H2 = 1500 Wm-2K-1. In all cases, T1sur(0) = 870°C and T2sur(0) = 1425 °C.

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Fig. 14

Effect of the heat transfer coefficient at the coating surface H2 on the delamination energy release rate and mode mix for the unconstrained bilayer (Eq. (8)) subject to the otherwise unchanged cooling in Eq. (10). The JETS reference case has H1 = 200 Wm-2K-1 and H2 = 1500 Wm-2K-1. In all cases, T1sur(0) = 870 °C and T2sur(0) = 1425 °C.

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Fig. 17

The elastic energy/area U in the layer of the coating of thickness h below the surface well ahead of the crack tip. This simulation is for the unconstrained reference bilayer in Eq. (8) subject to the JETS scenario in Eqs. (9) and (10). The energy release rate for a crack within the coating propagating parallel to the interface a distance h below the surface can be approximated by U.

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