Abstract
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.
Highlights
In service, thermal barrier coatings (TBCs) are subject to significant thermal gradients and occasional events involving rapid cooling of the hot surface
It will be shown that substrate thickness and constraint significantly affect the driving force for delamination, and it will be argued that they must be considered in evaluating coating delamination, both in service and in laboratory tests
The examples consider in this paper with representative TBC substrate thicknesses and a coating thickness on the order of 1mm revealed that the maximum energy release rate for the constrained substrate is typically twice that for the unconstrained substrate
Summary
Thermal barrier coatings (TBCs) are subject to significant thermal gradients and occasional events involving rapid cooling of the hot surface. The combination of thermal gradients and rapid cooling produces a transient, non-uniform stress distribution through the coating-substrate multilayer. The aims of this paper are two-fold: (i) To realistically characterize transient temperature and stress distributions in thermal barrier coating systems under rapid cooldown situations. (ii) To determine the transient energy release rate and mode mix for coating delaminations as dependent on the initial thermal gradient, the rate of cool-down, the thickness of the substrate and the extent to which the substrate is constrained against bending. The paper builds on earlier work of Evans and Hutchinson [1] wherein simplified representations of transient cooling were considered for coatings on thick substrates that allowed no bending. Many TBCs have three or more layers, the system considered in this paper is a bilayer comprised of a single coating bonded to a substrate. Under steady-state conditions with T1gas and T2gas prescribed to be independent of time, the uniform heat flux and temperature distribution are q
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