Kang Yuan

INTERSPLAT OXIDATION OF ATMOSPHERIC PLASMA SPRAYED MCRALY COATINGS

MCrAlY coatings and thermal barrier coatings (TBC) are commonly used in gas turbines to prolong the life of structural parts. The atmospheric plasma spray (APS) process yields coatings with a typic ...

Life Prediction of High-Temperature MCrAlY Coatings Based on Microstructural Observations

Thermal barrier coatings are commonly used in gas turbines for protection against high tem-perature and oxidation. Life prediction of oxidation protective coatingsmay be done bymicrostructure-based techniques such as -depletion based life criteria. In this study, a thermal barrier coating sys-tem, with an overlay NiCoCrAlY coating as bond coat, was oxidised up to 10000 h at 900 C. Themicrostructure was studied and related to Al depletion. It was found that a -depletion based lifecriterion could not be used for the studied coating composition and temperature as it would be tooconservative. A 0-depletion based model was instead suggested and supported by interdiffusion sim-ulation.

Microstructure-Based Life Prediction of Thermal Barrier Coatings

The widespread use of thermal barrier coatings (TBC) in gas turbines stresses the importance of accurate life prediction models for TBCs. During service, the TBC may fail due to thermal fatigue or through the formation of thermally grown oxides (TGOs). The current paper presents a Thermo-Calc/Dictra-based approach to life prediction of isothermally oxidised atmospheric plasma sprayed (APS) TBCs. The β-phase depletion of the coating was predicted and compared to life prediction criteria based on TGO thickness and Al content in the coating. All tried life models underestimated the life of the coating where the β-depletion-based model was the most conservative.

Influence of Precracked Diffusion Coating of Pt-Modified Aluminide on HCF Fracture Mechanism of IN 792 Nickel-Based Superalloy

High-cycle-fatigue (HCF) fracture mechanism of nickel-based superalloy IN 792 coated with Pt-modified aluminide outward-diffusion coating is studied with focus on the influence of coating cracks. It is found that cracking of the diffusion coating prior to HCF tests has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). By fractographic analysis, three types of fractural modes are established according to their crack initiations: internal, external and mixed. While external fractural mode is related to the propagation of existing cracks in the coating, internal facture initiates often at Ti-Ta-W-rich carbides and/or topological-close-packed (TCP) phases and grainboundaries in the superalloy. Increasing the thickness of diffusion coating or the amplitude stress promotes the fractural mode transition from internal/mixed to external. The influence of precracking of coatings on the HCF fracture mechanism can be qualitatively explained by its influence on the stress intensity factor.

Modeling the Diffusion of Minor Elements in Different MCrAlY—Superalloy Coating/Substrates at High Temperature

As demands of energy supply have been increased continuously and at the same time the uses of fossil fuel are limited and the greenhouse effect should be minimized, the gas turbine industries have been making efforts to increase gas turbine efficiency and to reduce emissions for power generations. One of the efforts is a continuous development of high temperature capacity of ceramic thermal barrier coatings (TBCs) and metallic MCrAlY overlays. The MCrAlY overlays are used as both protective coatings and bond coats to TBCs on underlying superalloy components in the gas turbines. During high temperature exposure, elemental diffusion occurs between the bond coat and the substrate which can affect the overall coating performance. The present study investigates the diffusion of minor elements like Re, Ta, Si, Mo and Ti in various MCrAlY overlays and superalloy substrates. An oxidation-diffusion model has been used to study the elemental diffusion. The diffusion process contains two stages: β depletion stage and the β depleted stage. In the stage when MCrAlY overlays exhibit γ + β microstructure, the diffusion of minor elements in the coatings was observed to be related to the β depletion rate. After that the diffusion of the minor elements did not show any clear dependence on the coating composition. The development principle of the elemental diffusion is discussed in detail.

Influence of Ru, Mo and Ir on the Behavior of Ni-Based MCrAlY Coatings in High Temperature Oxidation

To improve the oxidation/corrosion resistance of MCrAlY coatings (M for Ni and/or Co), elements like Y, Si and Ta have been added into the coatings in past decades. In this study the oxidation perf ...

Interdiffusion and Microstructure Simulation in Ni and Co Based Overlay Coatings on a Ni Based Superalloy at High Temperatures

In turbine engines, Ni or Co based alloys are used at high temperature, either as base materials, superalloys, or deposited on the surface of superalloys, as coatings. In the present study, two different MCrAlY overlay coatings, Ni and Co based, on a Ni based superalloy IN792 were aged for different times in air at three temperatures, 900°C, 1000°C and 1100°C. The aging processes were simulated by using DICTRA software by focusing on the interdiffusion behavior in the superalloy-coating systems. The results of simulation captured the main microstructural features observed and were used to analyze the diffusion behavior of alloying elements and the corresponding microstructure development. It was found that coating composition and temperature affected significantly the microstructure near the superalloy-coating interface, and their relations were mapped as a summary.