Fractographic and microstructural study of isothermally and cyclically heat treated thermal barrier coatings

The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been studied by scanning electron microscopy. The adhesion test have been made using the standard method described in ASTM 633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrates of Hastelloy X. The bond coat (BC) is of NiCoCrAlY type and the top coat (TC) consists of yttria partially-stabilised zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation 2) thermal cycling fatigue (TCF) and 3) burner rig test (BRT). The fracture surfaces of the adhesion tested specimens were characterised. A difference in fracture mechanism was found for the different heat treatments. Isothermal oxidation gave fracture mainly in the top coat while the two cyclic heat treatments gave an increasing amount of BC/TC interface fracture with increasing number of cycles. Some differences could also be seen between the specimens subjected to burner rig test and furnace cycling.

► Fractography and adhesion tests on isothermally and cyclically heat treated TBCs.
► Isothermal heat treatment increased, and cyclic decreased, adhesion of TBCs.
► Isothermal heat treatment gave fracture entirely in the TBC.
► Cyclic heat treatment gave larger amount of bond coat/TBC interface fracture.
► Fracture depends on defect morphology in thermal sprayed coating.