Abstract
The relationship between surface strain behavior and the state of a subsurface delamination for a thermal-barrier-coated (TBC) material was described in this paper. Tension-compression fatigue tests were carried out at high temperatures (893K and 833K) and room temperature for ZrO_2-8Y_2O_3-sprayed and Al_2O_3-sprayed type 304 stainless steel specimens. Surface strain behavior during the fatigue test was measured using a laser speckle strain gauge. It was found that a substrate strain was fully reflected on a surface strain unless a subsurface delamination was initiated, while a surface strain was found to decrease when a subsurface delamination was initiated. All the surface strain behaviors were classified in five patterns, and they were related to the state of a subsurface delamination. The number of cycles to delamination was decided by the surface strain behavior. It was found that the fatigue fracture life was long when the fatigue delamination life was long for the specimen with a high strength bond-coating, while the fatigue fracture life was found to be short when the fatigue delamination life was long for the specimen with a low strength bond-coating. It was concluded that the bond-coating with high strength and high adhesive strength was effective in improving the fatigue fracture life of a thermal-barrier-coated material due to the restriction of a crack initiation into the substrate.
