0
Research Papers

Delamination Susceptibility of Coatings Under High Thermal Flux

[+] Author and Article Information
Z. Xue, J. W. Hutchinson

School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138

A. G. Evans

Department of Mechanical Engineering, University of California, Santa Barbara, CA 93106

J. Appl. Mech 76(4), 041008 (Apr 22, 2009) (7 pages) doi:10.1115/1.3086590 History: Received March 06, 2008; Revised January 26, 2009; Published April 22, 2009

Delamination of coatings initiated by small cracks paralleling the free surface is investigated under conditions of high thermal flux associated with a through-thickness temperature gradient. A crack disrupts the heat flow thereby inducing crack tip stress intensities that can become critical. A complete parametric dependence of the energy release rate and mode mix is presented in terms of the ratio of the crack length to its depth below the surface and coefficients characterizing heat transfer across the crack and across the gaseous boundary layer between the surface and the hot gas. Proximity to the surface elevates the local temperature, which in turn, may significantly increase the crack driving force. A detailed assessment reveals that the energy release rates induced by high heat flux are capable of extending subsurface delaminations in thermal barrier coatings, but only when the modulus has been elevated by either calcium-magnesium-alumino-silicate (CMAS) penetration or sintering. Otherwise, the energy release rate remains well below the toughness.

FIGURES IN THIS ARTICLE
<>
Copyright © 2009 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 9

Energy release rate computed numerically for shallow cracks at various depths for BC=0 and BG=1 for q0=0.4 MW/m2 and E=200 GPa (H=1 mm, k=1 W/m K, ν=0.2, and α=11×10−6/K). Critical flaw sizes are indicated based on a representative mode I toughness, ΓTBC.

Grahic Jump Location
Figure 10

Delaminations in the TBC on an engine shroud (5). The delaminations just below the surface occur within the CMAS infiltrated regions, which give rise to a significantly increased Young’s modulus.

Grahic Jump Location
Figure 11

(a) Cut above the crack creating a simply connected region. (b) Resultant force/length and moment/length required to eliminate displacement discontinuity across the cut in (a).

Grahic Jump Location
Figure 1

Three problems analyzed in the paper. The coefficient of heat transfer across the crack is denoted by hC in each of the three problems. The heat transfer coefficient across the gas boundary layer at the surface of the thermal barrier coating in Problems II and III (shown as a shaded layer) is denoted by hG. The temperature of the hot gas above the boundary layer is TG. In all three problems, the vertical heat flux in the absence of the crack is denoted by q0.

Grahic Jump Location
Figure 2

Effect of heat conduction across an isolated crack on energy release rate (9). The normalizing value G0(0) is the energy release rate for the nonconducting crack given by Eq. 10. Pure mode II pertains (ψ=90 deg).

Grahic Jump Location
Figure 3

Amplification of energy release rate for shallow cracks, as predicted by the asymptotic result for the energy release rate of a crack just below the surface as dependent on the Biot number governing heat transfer through the gas boundary layer, BG∗=hGa/k. The crack length is 2a and the depth below the surface is d. The crack is nonconducting (hC=0) and has mode mix ψ=52.1 deg. The normalizing value, G0(0), pertains to the isolated crack experiencing the same overall heat flux given by Eq. 10.

Grahic Jump Location
Figure 4

Normalized energy release rate computed numerically for nonconducting crack of length 2a located a distance d below the surface for various Biot numbers, BG∗=hGa/k, characterizing the gaseous boundary layer at the surface. The normalizing value, G0(0), pertains to the isolated crack experiencing the same overall heat flux given by Eq. 10, i.e., the limit d/a⪢1.

Grahic Jump Location
Figure 5

Mode mix, ψ, computed numerically for nonconducting crack of length 2a located a distance d below the surface. The curve applies to all Biot numbers, BG∗=hGa/k, characterizing the gaseous boundary layer at the surface. This result also applies for Problem II for any combination of BC∗ and BG∗.

Grahic Jump Location
Figure 6

Normalized energy release rate computed numerically for limit (hG=∞ and Tsurface=TG) with no gaseous boundary layer. The Biot number governing heat transfer across the crack is BC∗=hCa/k, and G0(BC∗) is given by Eq. 12. The mode mix, ψ, is plotted in Fig. 5.

Grahic Jump Location
Figure 7

Normalized energy release rate computed numerically for BC∗=hCa/k=0.2 and various BG∗=hGa/k with G0(BC∗) given by Eq. 12. The mode mix, ψ, is plotted in Fig. 5.

Grahic Jump Location
Figure 8

Energy release rate and mode mix in Problem III computed numerically for cracks at various depths for BC=0 and BG=1. HS/H=3, ksubstrate/k=100, Esubstrate/E=5, νsubstrate=0.3, and νTBC=0.2.

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In