Izuru Nishikawa

Evaluation of Adhesive Strengths of Plasma-Sprayed CoNiCrAlY Coatings Using an Indentation Method

Thermal barrier coating (TBC) on the element for high temperature service, such as a gas turbine blade, has become an indispensable technology. In this study, adhesive strengths of plasmasprayed CoNiCrAlY coatings were examined using an indentation method. In order to examine the effects of the spraying process on the adhesive strength of a sprayed coating, coatings were deposited by both atmospheric plasma spraying (APS) and low pressure plasma spraying (LPPS). The half number of CoNiCrAlY(LPPS) specimens were thermally aged for diffusion treatment. The load-displacement curves during the indentation were measured, and the delamination energy per unit area of delamination was estimated. The delamination load was high when the distance from an edge of a specimen was long, however the energy per unit delamination area was almost independent of the distance from the edge and was uniquely determined. The delamination energy of CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings were found to be approximately the same. The delamination energy of CoNiCrAlY(LPPS) with diffusion thermal treatment was widely scattered as compared with foregoing two coatings, however the energy of the CoNiCrAlY(LPPS) coating with diffusion thermal treatment was found to be about three times higher than those of both CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings. It was concluded that diffusion thermal treatment was effective in improving the delamination strength. The CoNiCrAlY(LPPS) coating with diffusion thermal treatment was also found to be effective in improving the fatigue fracture life of a thermal-barrier-coated material.

Surface Strain Monitoring Approach to Delamination Failure of Thermal Barrier Coated Type 304 Stainless Steel under Thermal Cycle Condition.

Thermal cycle tests of thermal barrier coated type 304 stainless steel were conducted. 8 mass% Y2O3-ZrO2, Al2O3, CoNiCrAlY, NiCr coatings were used for the thermal barrier coatings (TBCs). Surface strain behavior during the thermal cycle test was measured using the laser speckle strain! displacement gauge (SSDG) in order to examine the delamination damage process. Surface strain was found to decrease when interface delamination occurred. Fatigue processes were discussed with the results of both surface strain measurement and microscopic observation of the specimen. Cracking occurred in ceramics layer and the crack grew parallel to the interface between the ceramics layer and the bond coating layer in the ceramics layer near the interface. The number of cycles to delamination decreased with increasing of the maximum temperature and hold time at its temperature. LPPS bond coating was found to be effective in extending the thermal cycle fatigue life of thermal barrier coated material.