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Elucidating the mechanism of bond coat cavitation under CMAS-infiltrated thermal barrier coatings

Wessels, Kaylan Marie
Degree Grantor:
University of California, Santa Barbara. Materials
Degree Supervisor:
Matthew R. Begley and Carlos G. Levi
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Materials science
Thermal barrier coatings
Bond coats
Dissertations, Academic and Online resources
Ph.D.--University of California, Santa Barbara, 2016

Thermal barrier coating (TBC) systems are essential in advanced gas turbine engines, which require higher operating temperatures for increased efficiency and performance. As turbine operating temperatures increase, TBC systems are vulnerable to degradation by ingested siliceous deposits referred to as CMAS (calcium-magnesium-alumino-silicates). These melt during engine operation and infiltrate the porous ceramic topcoat, compromising its strain tolerance. The ensuing stiffening can lead to delaminations through (i) the topcoat, (ii) the TBC/TGO interface, and notably (iii) a recently identified mechanism that involves cavitation within the bond coat. While bond coat void formation is not a new phenomenon, the correlation between CMAS infiltration of the TBC and bond coat cavitation has not been studied previously. This degradation is characterized by spallation of the TBC, revealing a heavily pitted metal substrate.

Examination of specimens displaying signs of this type of damage suggests that creep deformation of the bond coat driven by stresses associated with the stiffened TBC lead to void nucleation and growth, typically at residual grit particles or second phases in the interdiffusion zone. Once the voids grow large enough to compromise the integrity of the bond coat, the TBC delaminates and eventually spalls, leaving behind a thermally unprotected surface. It is notable that the delamination occurs in the ductile bond coat layer, rather than in the brittle ceramic topcoat, as is typical of CMAS degradation.

Two manifestations of bond coat cavitation have been identified: in the first manifestation, channel cracking of the TBC provides the stress concentrations that drive cavitation, while in the second, uplift of the TBC in the form of blistering accompanies void growth. Analytical and finite element models are used to understand the resulting stress states motivated by the two distinct manifestations and the evolution of the stresses during thermal cycling and high temperature dwells. The modeling work is complemented by novel laboratory testing including laser gradient experiments with applied uniaxial tension.

The models suggest that through-thickness thermal gradients and local lateral thermal gradients, either operating synergistically or separately, can drive cavity formation in platinum aluminide bond coats. The stresses induced by the opening of channel cracks scale with coating thickness, coating stiffness, and tensile stress in the coating. These stresses can develop upon cooling from a stress-free condition and can relax with cavity growth and the associated creep, but are regenerated with thermal cycling, a necessary criterion in the formation of bond coat cavities.

Laboratory experiments designed to replicate bond coat cavitation led to additional forms of damage, including but not limited to the damage and loss of the ceramic coating system prior to the requisite number of thermal cycles. Several accelerated testing methods, including laser gradient testing with an applied uniaxial tensile stress and thermal gradient burner rig testing, were able to produce bond coat cavitation. The formation of bond coat cavitation during isothermal furnace cycle testing with CMAS-stiffened TBCs occurs in lieu of rumpling, which occurs when the TBC is not stiffened by CMAS during the same testing conditions. The cavitation resulting from testing highlights the prevalence of this mode of degradation when other traditional damage mechanisms, such as coating delamination and spallation, are precluded.

Physical Description:
1 online resource (298 pages)
UCSB electronic theses and dissertations
Catalog System Number:
Inc.icon only.dark In Copyright
Copyright Holder:
Kaylan Wessels
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