Yoshifumi Okajima

Effect of Powder Injection on the Interfacial Fracture Toughness of Plasma-Sprayed Zirconia

Adhesive strength of the plasma-sprayed thermal barrier coating is one of the most important parameters which influence their durability and reliability during service. While many methods exist to measure the adhesive strength, in general, they require cumbersome and time-consuming specimen preparation. Furthermore, considerations of the adhesion strength from the point-of-view of fracture toughness or for that matter, their systematic correlation to both processing variances are limited. Consequently, there is an opportunity to both simplify the measurement procedure and establish correlations among methods and linkages between processing parameters and interfacial fracture toughness. In this paper, we report results on adhesion strength of plasma-sprayed yttria-stabilized zirconia (YSZ) coating on aluminum substrates based on both interfacial indentation test (to measure interfacial fracture toughness) and the modified tensile adhesive test. Carrier gas flow for powder injection into the plasma torch was systematically varied to introduce variances in particle melting with concomitant impact on the measured adhesive strength. The results indicate the correlation between the particle melting index and the measured interfacial fracture toughness.

Interfacial Fracture Toughness and Residual Stress of Thermally Sprayed Coatings

Residual stresses generated in coatings during thermal spraying plays an important role on the interfacial adhesion. Furthermore, their magnitude and distributions are affected by both materials and processing parameters. Thus, it is of interest to evaluate their evolution and influence on the adhesive behavior of thermally sprayed coatings. Given the complexity in both the process and dimensions, it is difficult to extract both residual stresses and adhesion strength from a single specimen. In this study, the residual stresses were determined using an in situ thermo-elastic curvature method which not only provides a non-destructive method of characterizing stress but also allows for its layer by layer evolution during deposition. Concurrently, the interfacial indentation test is proposed to characterize the adhesion strength of plasma-sprayed coatings. Through a combination of these experiments, the interfacial fracture toughness of the plasma-sprayed ZrO2 coatings deposited on Al substrates were evaluated. In order to study the effects of the residual stresses, samples were sprayed under various processing conditions while the indentation tests on resultant coatings were also compared to the standard tensile adhesion tests. These test results indicate a certain correlation between the residual stresses and the interfacial fracture toughness of thermally sprayed coatings.

Development of the Advanced TBC for High Efficiency Gas Turbine

Since 2004, Mitsubishi has been pursuing a 1,700°C gas turbine as part of the Japanese National Project [1][2]. One of the most important key technologies for the target is thermal barrier coatings (TBCs) which are capable of improving cooling efficiency of hot parts. With increasing the turbine inlet temperature, TBCs surface temperature is also rising up. In addition, the temperature gradient through TBCs thickness must steepen as a result of keeping metal temperature. Both have a significant effect on durability such as spallation and erosion of TBCs. To evaluate these issues, thermal cycle test and hot erosion test were introduced. After the screening of those component tests, the advanced TBCs coated in the first unit of M501J were verified at our pilot plant called T-point. Sound condition for row 1 blades and vanes had been confirmed after over 3 year operation.