Protruded Four-Point Bending Testing Method and Characterization of Near Interface Cracking in Thermal Barrier Coatings

Abstract

A protruded four-point bending testing method has been developed to characterize the crack initiation of thermal barrier coatings (TBC) near the interface. Two types of protruded TBC specimens, with and without a reinforcement attached on the top of the protruded TBC, were prepared from in-service used transition ducts made of TBC (6% Y2O3 stabilized ZrO2) and bond coatings (NiCoCrAlY) plasma-sprayed over a superalloy substrate. In the unreinforced protruded TBC specimen tests, pre-existing TBC cracks extended in the transverse direction while near interface TBC cracking did not occur. The reinforced protruded TBC specimen hindered the transverse TBC cracking and allowed the formation of TBC cracks adjacent to the oxidized TBC/bond coating interface in a similar mode to in-service TBC spalling. The onset of TBC cracks was identified by a change in the loading rate in the elastic deformation regime. The local stress distribution at the edges of the reinforced protruded TBC was analyzed using finite element analysis. The critical local tensile stress for the initiation of TBC cracks near the interface was estimated for the in-service used transition duct. The near interface TBC cracking behavior in the protruded TBC tests is discussed in light of the applied and residual stress distribution.