Determination of the relation between the process controlled variations of anisotropic void system and thermal conductivity of electron beam physical vapour deposited (EB-PVD) PYSZ thermal barrier coatings

Abstract

Thermal barrier coatings (TBCs) deposited by EB-PVD protect the turbine blades situated at the high pressure sector of the aircraft and stationary turbines increasing the combustion temperature and/or diminishing the air-cooling requirements. It is an important task to uphold low thermal conductivity in TBCs during long-term service at elevated temperatures. One of the most promising methods to fulfil this task and to improve the turbine efficiency is to optimize the TBC properties by tailoring its microstructure and/or material. The different kinds of pores in EB-PVD TBCs influence their thermal conductivity according to their size, shape, orientation and volume. These pores can be open (inter-columnar and between feather arms gaps) and closed (intra-columnar pores). Since such pores are located within three-dimensionally deposited columns and enclose large differences in their sizes, shapes, distribution and anisotropy, the accessibility for their characterization becomes very complex and requires the use of sophisticated methods. In this work, three different EB-PVD TBC microstructures were manufactured by varying the process parameters, yielding various characteristics of their pores. The corresponding thermal conductivities in as-coated and after annealing at 1100C/1h and 100h conditions were measured via Laser Flash Analysis Method (LFA). Analysis of the pores formation during processing and their individual effect on the thermal conductivity are made by employing an USAXS-measuring method as well as modelling on the corresponding specimens which is able to characterize the size, shape, volume and orientation of all pores at the EB-PVD TBCs. Evident differences in the thermal conductivity values were found for each microstructure in as-coated and after annealing conditions, as well as between them. Broader columns introduce higher values in thermal conductivity. In general, thermal conductivity increases after heat-treatment for all three microstructures investigated although those with smaller pore surface area shows smaller changes.