Effect of Holding at High Temperature on Delamination Strength of 8mass%Y₂O₃·ZrO₂/CoNiCrAlY Thermal Barrier Coating

Abstract

Thermal barrier coating (TBC) was applied to Co based superalloy specimens, where bond coat was CoNiCrAlY deposited by high velocity oxygen fuel (HVOF) spraying and top coat was 8mass% yttria stabilized zirconia (YSZ) by atmospheric plasma spraying. After the specimens were heated and held at high temperature, tensile tests and edge indent tests of the specimens were carried out to measure interfacial fracture toughness and delamination energy of the coating, respectively. The results showed that Al₂O₃ oxide was formed along the interface between the top coat and the bond coat, and the thickness of Al₂O₃, B_Al, increased with increasing heating temperature T and holding time t. The growth behavior of Al₂O₃ layer was expressed by the equation: B_Al=A·exp(-Q/RT)·tⁿ, where Q is apparent activation energy, R is gas constant, and A and n are constants. The interfacial fracture toughness and the delamination energy increased with increasing B_Al when it was less than about 2μm, while when B_Al exceeded 8-10μm, both of them decreased drastically. By using a critical oxide layer thickness where the delamination strength was still maintained, a combination of operation temperature and time during which the delamination of coating would not occur was determinated.