Large-scale molecular dynamics simulation of growth, microstructure and properties of plasma-sprayed coatings
Ceramic thermal barrier coatings (TBCs) are applied on actively cooled turbine blades to reduce the thermal load on the metallic blades and, hence to increase their service time. Large-scale molecular dynamics (MD) simulations are employed to study the growth of TBC by simulating the impingement of pre-heated or even premolten particles on a cooled substrate. The size of the particles is varied between 10 nm and approximately 30 nm, furthermore, the effect of their velocity and temperature on the microstructure of the forming layers is studied. To accomplish this, three to four particles are deposited next to each other, to see how the forming splats interact. Particular focus is put on the formation of the interfaces between several splats. It is intended to study three material systems: refractory metals, silicon as model for covalently bonded ceramics and zirconium oxide as an oxide ceramic that is used for TBC applications. For the first two systems well-characterized interatomic potentials are available, whereas a potential for the oxide system needs to be derived from literature data. These studies serve to characterize the microstructure that forms during the plasma spraying and, in particular, which atomic structure the resulting interfaces between splats attain during the coating process.
In the second part of the project, the microstructures resulting from the process simulations are used as a basis for MD simulations to characterize the heat conductivity through the formed microstructures. This will be accomplished by establishing a temperature gradient on a road-like geometry and calculating the heat flux between the ends of this rod. The effect of different microstructural defects, like nanovoids, nanocracks, grain boundaries and most-importantly the interfaces between different splats, on the heat conductivity will be characterized. From these studies, general scaling relation for the heat conductivity as a function of density and distribution of microstructural defects is derived. Such scaling relations can be used to model and to predict the properties of thick TBC as a function of their microstructure with continuum methods.