Characterisation of air plasma sprayed TBC coating during isothermal oxidation at 1100 degrees centigrade

Purpose: The purpose of the research was characterization of failure modes of air plasma sprayed thermal barrier coatings system in isothermal oxidation test conditions at 1100 degrees centigrade. Design/methodology/approach: The research allowed the identification of microstructural changes that take place during oxidation test of TBC system in area of ceramic top-layer, bond coat and superalloy substrate. The examinations were conducted on specimens after 50, 120, 500, 1000, 1500 and 2000 h of exposition. The scope of the research encompassed a microstructural analysis with the use of macro and micro investigation - LM, SEM microscopy. Findings: It was found that the lifetime of commercially quality TBC system is related to progress of degradation in area of bond coat and ceramic top layer. During isothemal oxidation, the bond coat in the TBCs was oxidized to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and bond coat of NiCoCrAlY alloy. At the moment of oxidation acceleration, the cracks initiation and propagation was observed in the mixed oxide layer near the YSZ. The crack propagation inducted the delamination and spallation of top coat after 1000 h oxidation. Research limitations/implications: The discussed research proves that main reason of degradation in TBC system is related to progressive development of cracks at the interface of the thermally grown oxide with bond coat, within the top coat or at its interface with TGO. Practical implications: The results obtained allow the determination of the degree of life-time lost of the investigated TBC system and specification of the time of safety operation. Originality/value: The results obtained are valuable contribution to durability characterization of TBC systems. They enable the identification of the degradation mechanisms, which enhances the durability and safety of high temperature operation.