Effect of ageing on microstructure changes in EB-PVD manufactured standard PYSZ top coat of thermal barrier coatings

Kurzfassung

The ceramic top coat of Thermal Barrier Coatings (TBC) manufactured by EB-PVD process is a crucial part of a system which protects the high pressure turbine blades of aero engines and stationary gas turbines. Under service conditions, turbine blades are exposed to temperatures above 1100°C, typically with short-term peaks. Ceramic top coat has a unique microstructure made up of individual primary columns, growing in a preferred crystal direction. This columnar growth produces an inter-columnar spacing exhibiting a weak bonding between the neighbouring columns, and providing the TBCs with high strain tolerance. Rotation of the substrates during vapour phase deposition produces an additional intra-columnar closed porosity. Furthermore, this causes formation of a feather-like sub-columnar structure at the periphery of each primary column due to over-shadowing, increasing total porosity and thus contributing to low thermal conductivity of the material. On thermal exposure during service, both inter- and intra-columnar porosity decrease as a result of sintering induced surface area reduction. The driving force for this process is the decrease in surface energy. As a result, thermal conductivity increases due to the alteration of the porosity distribution, size and morphology. In addition the in-plane strain tolerance decreases after the primary columns sinter together to form bridges at the contact points. These are two important factors which affect the performance of the entire system. Thus, analysis of the involvement of each pore family (feather-arm, intra- and inter-columnar pores) in thermal insulating capabilities of PYSZ TBC may provide a guideline for engine manufacturers in designing more efficient coatings with optimised thermal properties, in all probability solely by controlled adjustment of the process parameters. This work reports the thermal-induced microstructural changes in terms of total porosity at EB-PVD produced top coats before and after annealing at 900°C, 1000°C, 1100°C, 1200°C, 1300°C and 1400°C for 1h, as well as at 1100°C for 20min, 1h, 3h, 10h and 100h. For the analysis of the changes in pore surface area, methods such as SANS (Small Angle Neutron Scattering) and BET (Brunauer, Emmett and Teller) were applied. Alteration of surface area with time and temperature commonly gives an indication of the type of diffusion mechanism. Hence, kinetic and the activation energy of the mechanism are calculated and determined with the help of the data obtained by SANS and BET-analysis. Due to the presence of a large number of nano-sized closed porosity in the coatings, SANS measurements were crucial to distinguish those from the remaining open porosity. A mean porosity representative for pores smaller than 180 nm is obtained by interaction of neutrons diffracting parallel to the primary column axis.