Nondestructive Evaluation of High-Temperature Oxidation Behaviour in Thermal Barrier Coatings(High Temperature Materials)

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Two-layer plasma sprayed coatings on a Ni base superalloy have been used in advanced gas turbine plants to improve their durability and efficiency. Generally, the coatings consist of an inner layer of a metallic bond coating (MCrAlY) and an outer layer of 8wt% yttria-stabilised zirconia (YSZ) as a thermal barrier coating (TBC). After aging, a thermally grown oxide (TGO) was formed at the interface between the YSZ and the MCrAlY. The thickness of the TGO increased with aging time. Due to the difference between the thermal expansion coefficients of the Ni base superalloy, the YSZ top coating and the TGO, the presence of the TGO layer leads to degradation of the TBC by mechanisms such as cracking, delamination or spalling. Therefore, the capability to carry out a nondestructive evaluation (NDE) of the formation and the growth of the TGO is important. An in-situ NDE was implemented by means of an impedance spectroscopy (IS) technique at 1000□. Meanwhile, a great deal of attention has been paid to the oxidation behaviour of the TBC/MCrAlY interface. However, the role of oxide formation in terms of the TBC top layer is still not clearly understood. In our previous work, we revealed that the oxidation behaviour of specimens with and without TBC is quite different, in spite of their being subjected to the same aging conditions. The oxidation behaviour of the TGO for both specimens was investigated using an IS technique. The results showed that the impedance of the specimen with TBC was smaller than that of the specimen without TBC, although the specimen with TBC had a thicker oxide layer than the one without TBC.
一般社団法人日本機械学会の論文
2001-10-20

Nondestructive Evaluation of High-Temperature Oxidation Behaviour in Thermal Barrier Coatings(High Temperature Materials)

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