Krishna Praveen Jonnalagadda

Stresses and Cracking During Chromia-Spinel- NiO Cluster Formation in TBC Systems

Thermal barrier coatings (TBC) are used in gas turbines to reduce the temperatures in the underlying substrate. There are several mechanisms that may cause the TBC to fail; one of them is cracking in the coating interface due to extensive oxidation. In the present study, the role of so called chromia-spinel-NiO (CSN) clusters in TBC failure was studied. Such clusters have previously been found to be prone to cracking. Finite element modeling was performed on a CSN cluster to find out at which stage of its formation it cracks and what the driving mechanisms of cracking are. The geometry of a cluster was obtained from micrographs and modeled as close as possible. Nanoindentation was performed on the cluster to get the correct Young’s moduli. The volumetric expansion associated with the formation of NiO was also included. It was found that the cracking of the CSN clusters is likely to occur during its last stage of formation as the last Ni-rich core oxidizes. Furthermore, it was shown that the volumetric expansion associated with the oxidation only plays a minor role and that the main reason for cracking is the high coefficient of thermal expansion of NiO.

Hot Corrosion Mechanism in Multi-Layer Suspension Plasma Sprayed Gd2Zr2O7 /YSZ Thermal Barrier Coatings in the Presence of V2O5 + Na2SO4

This study investigates the corrosion resistance of two-layer Gd2Zr2O7/YSZ, three-layer dense Gd2Zr2O7/ Gd2Zr2O7/YSZ, and a reference single-layer YSZ coating with a similar overall top coat thickness of 300-320 µm. All the coatings were manufactured by suspension plasma spraying resulting in a columnar structure except for the dense layer. Corrosion tests were conducted at 900 °C for 8 h using V2O5 and Na2SO4 as corrosive salts at a concentration of approximately 4 mg/cm2. SEM investigations after the corrosion tests show that Gd2Zr2O7-based coatings exhibited lower reactivity with the corrosive salts and the formation of gadolinium vanadate (GdVO4), accompanied by the phase transformation of zirconia was observed. It is believed that the GdVO4 formation between the columns reduced the strain tolerance of the coating and also due to the fact that Gd2Zr2O7 has a lower fracture toughness value made it more susceptible to corrosion-induced damage. Furthermore, the presence of a relatively dense layer of Gd2Zr2O7 on the top did not improve in reducing the corrosion-induced damage. For the reference YSZ coating, the observed corrosion-induced damage was lower probably due to combination of more limited salt penetration, the SPS microstructure and superior fracture toughness of YSZ.

A Study on Crack Configurations in Thermal Barrier Coatings

Spallation of thermal barrier coatings subjected to thermal fatigue occurs through cracking in or close to the metal ceramic interface. To better match an experimentally observed damage progression ...

Bending Fatigue of Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are ceramic coatings used in gas turbines to lower the base metal temperature. During operation, the TBC may fail through, for example, fatigue. In the present study ...

Hot corrosion behavior of multi-layer suspension plasma sprayed Gd2Zr2O7/YSZ thermal barrier coatings

This study investigates the corrosion resistance of double layer Gd2Zr2O7/YSZ, triple layer dense Gd2Zr2O7 / Gd2Zr2O7/YSZ and a reference single layer YSZ coating with a similar overall top coat thickness of 300–320 µm. All the coatings were manufactured by suspension plasma spraying (SPS), resulting in a columnar structure. Corrosion tests were conducted at 900°C for 8 hours using vanadium pentoxide and sodium sulphate as corrosive salts at a concentration of 4 mg/cm2. SEM investigations after the corrosion tests show that Gd2Zr2O7 coatings exhibited lower corrosion resistance than the reference material, YSZ. Reaction between the corrosive salts and Gd2Zr2O7 results in the formation of gadolinium vanadate (GdVO4) along the top surface and between the columns. While the stresses due to phase transformation of zirconia can be relieved to some extent by realigning of the columns in the top coat, it is believed that GdVO4 formation between the columns, along with low fracture toughness of Gd2Zr2O7 had resulted in lower corrosion resistance. Furthermore, the presence of a relatively dense layer of Gd2Zr2O7 on the top, as a preventive layer for salt infiltration, did not improve the corrosion resistance.

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.