Development of Intermixed Zones of Alumina/Zirconia in Thermal Barrier Coating Systems

Abstract

The mechanisms whereby intermixed zones of alumina and zirconia are formed at the interface between the metallic bond coat and the ceramic top coat (yttria-stabilized zirconia) in thermal barrier coating systems have been investigated. The results lead to the following mechanism for the formation of the zones. The predominant mechanism for intermixed zone formation involves formation of a metastable alumina polymorph (h or c) during TBC deposition, with a significant amount of zirconia dissolved in it. The outward growth also begins to incorporate zirconia particles, which initiates the formation of the intermixed zone. Upon thermal exposure, the metastable TGO continues to grow outward, extending the intermixed zone, and eventually transforms to the equilibrium a-Al 2 O 3 . The transformation to a-Al 2 O 3 results in an increase in the volume fraction of zirconia in the intermixed zone as it is rejected from solution. Once the aAl 2 O 3 appears, subsequent TGO growth produces a columnar zone of the TGO without a second phase. When a-alumina was preformed on the bond coat, prior to TBC deposition, no intermixed zone was formed for Pt-modified aluminide bond coats.

Beneficial Effect of Pt and of Pre-Oxidation on the Oxidation Behaviour of an NiCoCrAlYTa Bond-Coating for Thermal Barrier Coating Systems

“…However, in [23] the surface preparation before thermal barrier deposition by EB-PVD was not mentioned and the oxidation was carried out at a relatively low temperature. Our observation that Pt did not favour h-Al 2 O 3 formation is also in contradiction with the study of Stiger et al [69] where a continuous mixed zone was observed in TBC systems composed of a Ptmodified NiCoCrAlYTa bond-coating (a mixed zone is considered by many authors as being the result of the presence of transient alumina before thermal barrier deposition [69][70][71][72][73] [27].…”

Improvement of the isothermal oxidation resistance of CoNiCrAlY coating sprayed by High Velocity Oxygen Fuel

“…Failure of the TBC coating critically depends on the characteristics of the TGO layer. If this layer is composed of a continuous, compact and adhesive alumina (Al 2 O 3 ) scale, it acts as a diffusion barrier; moreover, the alumina scale can suppress or reduce the growth of other detrimental oxides and protect the substrate from further oxidation, improving the system service life [4][5][6][7]. Many researches showed that TBC failures can be correlated to the reaching of a TGO critical thickness [8,9].…”

On TGO creep and the initiation of a class of fatigue cracks in thermal barrier coatings

“…9, 10). 3 A competition between the imposed axial force and the growth strain can be also observed. In the first portion of the TGMF cycle, the applied load is less than 50 MPa, which is not enough to overcome the compressive stresses in the TGO due to the growth stresses.…”

“…However, final spallation occurs during cooling. 3 The local trend of evolution of the TGO stresses differs slightly between the first cycle and the following cycles. This is due to that the state of stress in the system changes significantly during the first cycle.…”

“…the majority ofTBC failures are associated with the TCO. More spe<:ifically, failure ofTBCs common ly occurs at the bond coat/ TCO interface or in the TCO/ top coat interface [3].…”