Effect of Bond Coat layer Delamination Strength on High Temperature Fatigue Fracture Strength of Thermal Barrier Coated Steel

Abstract

Fatigue tests were conducted at both 893K and room temperature for Thermal Barrier Coated (TBC) type 304 stainless steel. Surface strain during the fatigue test was monitored to examine the delamination of a subsurface layer using a laser speckle strain gauge. The strength of an APS-bond-coating was so low that multiple cracks occurred, while the strength of a LPPS-bond-coating was so high that a single crack occurred. In case of the specimen with a low strength APS-bond-coating, the fatigue fracture life of an Al_2O_3-sprayed specimen which bond-coating was delaminated was longer than that of a ZrO_2-8Y_2O_3-sprayed specimen which bond-coating was not delaminated. The reason was that the crack initiated at a bond-coating penetrated into a substrate for a non-delaminated specimen. In case of the specimen with a high strength LPPS-bond-coating, the fatigue fracture life of a specimen with a high adhesive strength thermal-aged-bond-coating was longer than that with a non-thermal-aged-bond-coating and the fatigue lives of these specimens were longer than those with APS-bond-coatings. The improvement of the fatigue fracture life was caused by the restriction of a crack initiation into the substrate due to the LPPS-bond-coating with a relatively higher strength. It was concluded that a bond-coating with high strength and high adhesive strength was effective on the improvement of fatigue fracture lives of plasma sprayed materials.