Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings

Abstract

This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.

α-Si3N4 dendrites and whiskers fabricated using electron irradiation technology

“…The power density can reach millions of watts per square inch. 21,22 In general, gaseous SiO will be generated by a solid-solid reaction between Si and SiO 2 at high temperature. 23 Additionally, the solid aBN will split into boron atoms (B*) and nitrogen atoms (N*) with high activity by the bombardment of the electron beam.…”

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

“…EB-PVD first forms a layer of fine equiaxed crystals on the substrate, and then forms texture and columnar crystals on it, as shown in Fig. 68 [6].…”

“…Different from APS TBCs, the ceramic layer prepared by EB-PVD has a typical columnar structure feature. The columns are perpendicular to the substrate, regularly distributed, and weakly bonded, which remarkably improves the strain tolerance of the coating [5,6]. Therefore, the EB-PVD coating has a better anti-spalling ability, especially under thermal cycling.…”

“…Cross-sectional microstructure of EB-PVD coatings. Reproduced with permission from Ref [6],. © The Author(s) 2021.…”

Atomistic Simulation of Physical Vapor Deposition of Optical Thin Films

“…The structure and properties of the growing films significantly depend on the energy of the particles arriving to the substrate. In low-energy methods (thermal evaporation, electron-beam evaporation) the parameters of the flow of the emitted particles from the target are defined using the continual methods [38]. For instance, the mass flow rate is defined using the Hertz-Knudsen theory [39].…”