Abstract
Development of thermal barrier coatings (TBCs) manufactured by suspension plasma spraying (SPS) is of high commercial interest as SPS has been shown capable of producing highly porous columnar microstructures similar to the conventionally used electron beam–physical vapor deposition. However, lifetime of SPS coatings needs to be improved further to be used in commercial applications. The bondcoat microstructure as well as topcoat–bondcoat interface topography affects the TBC lifetime significantly. The objective of this work was to investigate the influence of different bondcoat deposition processes for SPS topcoats. In this work, a NiCoCrAlY bondcoat deposited by high velocity air fuel (HVAF) was compared to commercial vacuum plasma-sprayed NiCoCrAlY and PtAl diffusion bondcoats. All bondcoat variations were prepared with and without grit blasting the bondcoat surface. SPS was used to deposit the topcoats on all samples using the same spray parameters. Lifetime of these samples was examined by thermal cyclic fatigue testing. Isothermal heat treatment was performed to study bondcoat oxidation over time. The effect of bondcoat deposition process and interface topography on lifetime in each case has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.
Highlights
A thermal barrier coating (TBC) system is designed to protect a gas turbine component from high temperatures and harsh environments
The samples were first cold mounted in low viscosity epoxy resin, sectioned using a cutting disk, and cold mounted again in Bondcoat Surface Topography
Precise measurement of bondcoat surface topography and careful assessment of the measurement data are essential to understand TBC failure mechanisms and subsequently enhance their lifetime. It has been shown in earlier work that only Ra as a roughness parameter is not sufficient to characterize a surface as surfaces with similar Ra can have different density of peaks, etc. and influence TBC lifetime (Ref 11)
Summary
A thermal barrier coating (TBC) system is designed to protect a gas turbine component from high temperatures and harsh environments. It has been shown in earlier works that a smoother bondcoat results in a higher thermal cyclic lifetime of the SPS TBC, perhaps due to the compact columnar topcoat microstructure created due to the low surface roughness as well as better adhesion of the topcoat due to a higher contact surface area (Ref [14, 16]).
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