Modifications of thermal barrier coatings (TBCs)

To develop highly efficient gas turbines, thermal barrier coating systems with a high reliability and a long lifetime under severe operating conditions are required. The failure of TBC-systems is caused by thermal cycling conditions, oxidation attack, and insufficient adhesion at the interface of the ceramic coating and the bond coat. Coating failure occurs mostly near the interface top coat-bond coat. Two modifications of a conventional duplex TBC-system consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO2 7 wt.% Y2O3-top coat, which is used as the reference system, are presented as follows. (i) By contouring the MCrAlY-bond coat with a laser, the stress distribution at the ZrO2-bond coat interface can be modified by forming folds within the laminate structure of the ceramic top coat and increasing the bonding area. TBC-systems containing a contoured bond coat show better thermal cycling behaviour. FEM-simulation of thermally induced stress shows an alternating stress distribution which is caused by the contoured bond coat interface. (ii) High-velocity oxygen fuel (HVOF)-sprayed MCrAlY layers are a new possibility to create homogeneous bond coats. Thermal barrier coatings with LPPS-(low pressure plasma sprayed) or HVOF-CoNiCrAlY bond coats are compared by investigating their porosity, roughness, and oxidation behaviour. The porosity is proportional to the roughness of the HVOF bond coats. The oxide content was examined by TEM and EDX analysis. HVOF-CoNiCrAlY bond coats show oxidation behaviour similar to coatings produced by LPPS. (C) 1997 Elsevier Science S.A.