Delamination toughness of electron beam physical vapor deposition (EB-PVD) Y₂O₃–ZrO₂ thermal barrier coatings by the pushout method: Effect of thermal cycling temperature

Abstract

A pushout test method was used to quantify effect of thermal cycling temperatures on the delamination toughness of an electron beam physical vapor deposited thermal barrier coating (EB-PVD TBC). The delamination toughness, Γi, was related to the maximum thermal cycling temperature, Th, equal to 1000, 1025, 1050, and 1100 °C. The measured delamination toughness varied from 9 to 95 J/m2. At Th = 1000 °C, Γi attained a maximum value, larger than that of the as-deposited sample and decreasing with increased Th. During the thermal cycling tests, the thermally grown oxide (TGO) was formed between the TBC and the bond coat deposited onto the superalloy substrate. Inside the TGO layer, mixture of Al2O3 and ZrO2 oxides was observed close to the TBC side with nearly pure Al2O3 phases close to the bond-coat side. During the pushout test, delamination occurred at the interface of the mixture and pure Al2O3 layer with an exception for Th = 1100 °C specimens where delamination also occurred at the interface between the TGO and bond-coat layers. The effect of thermal cycling temperatures on the delamination toughness is discussed in terms of the microstructural change and delamination behavior.

Lifetime Assessment for Thermal Barrier Coatings: Tests for Measuring Mixed Mode Delamination Toughness

“…This direction of loading adds to the compression in the coating from the thermal expansion mismatch and therefore more closely mimics the delamination conditions experienced in service. Kagawa and colleagues have developed two related tests, the barb test 12 and the push-out test, 13 to measure delamination toughness of TBCs. In each test, the coating is subject to additional compression by forcing it, but not the substrate, against a hard block.…”

“…Like the aforementioned bridge test, the barb and pushout tests require highly refined specimen preparation and sophisticated testing which are likely to limit their use for routine toughness testing. Nevertheless, Kagawa and colleagues 13 have collected an extensive data set showing how the toughness Fig. 3.…”

“…3. Delamination toughness of an EB-PVD TBC on a NiCoCrAlY bond coat as a function of thermal exposure plotted as a function of the thickness of the TGO from Tanaka et al 13 The upper band of data is for isothermal exposure and the lower band is for cyclic thermal exposure. The TGO thickness increases with thermal exposure time in accord with the markers on the upper band of data.…”

Effects of Heat Exposure Time and Temperature on the Delamination Behavior of Air Plasma-Sprayed Thermal Barrier Coatings under Shear Loading

“…The pushout test method was used to measure the delamination behavior under shear loading conditions [19][20][21] . The substrate side of an APS-TBCs specimen was bonded using an epoxy-based adhesive and polished to form a pushout specimen.…”

“…However, due to the low driving force for crack propagation and the dif culty of applying the unique vertical load to the coating, symmetrical crack propagation does not occur often. Barb [14][15][16][17][18] , pushout [19][20][21] , and other shear tests 22) have also been developed to measure delamination toughness under shear loading. Changes of delamination toughness in different interfacial roughness at as deposited state and in different heating condition such as isothermal heat exposure and thermal cycling have been evaluated in the electron beam physical vapor deposited TBCs (EB-PVD TBCs) by barb and pushout tests.…”