Application of USAXS Analysis and Non-interacting Approximation to Determine the Influence of Process Parameters and Ageing on the Thermal Conductivity of Electron-Beam Physical Vapor Deposited Thermal Barrier Coatings

Abstract

Thermal barrier coatings (TBCs) deposited via Electron-Beam Physical Vapour Deposition (EB-PVD) are used to reduce the temperature on the metal blades situated in the high pressure section of aircraft and stationary turbines. One of the most promising methods to improve the efficiency of these turbines is to optimize the TBCs properties through upgrade of their microstructure. The EB-PVD TBCs contain open (inter-columnar gaps and gaps between feather arms) and closed pores (intra-columnar pores) whose spatial and geometrical distribution can be altered by changing the process parameters, influencing additionally their thermal conductivity. Since such pores display large variations in size and accessibility, and exhibit high shape and distribution anisotropies, their characterization requires complex measuring methods. In this work, three different microstructures were manufactured by varying the process parameters. Their corresponding thermal conductivities were measured via the Laser Flash Analysis Method (LFA) in both the as-coated and after annealing (1100°C/100h). Analysis of the pore formation during processing was carried out by ultrasmall-angle x-ray scattering (USAXS), supported with a modelling programme developed by researchers at APS and NIST, which enables the characterization of the size, shape, volume and orientation of each of the pore populations in EB-PVD TBCs. Measurements and modelling results indicate that microstructures containing a higher volume fraction of fine anisotropic intra-columnar pores, and having larger oblate voids between the feather-arms with their planes oriented more closely with the substrate plane, correspond to lower thermal conductivity values. Inter-columnar gaps do no significantly contribute to lowering the thermal conductivity. On heat-treatment, the larger gaps between the feather-arms break up into arrays of nano-sized globular voids, and the anisotropic, elongated shapes of fine intra-columnar pores become also globular and less effective in reducing the thermal conductivity.