Influence of temperature on phase stability and thermal conductivity of single- and double-ceramic-layer EB-PVD TBC top coats consisting of 7YSZ, Gd2Zr2O7 and La2Zr2O7

Yttria Stabilized Zirconia (YSZ) is usually used as ceramic top coat for gas turbine blades and vanes. Investigations have shown the accelerated phase transformation and the intensified sinter effects of the YSZ top coat at temperatures between 1200 degrees C and 1300 degrees C, leading to changes of microstructure. Such modifications of the microstructure lead to higher thermal stresses and consequently reduce the lifetime. Furthermore, thermal conductivity lambda of the top coat increases. For this reason lanthanum zirconate (La2Zr2O7) and gadolinium zirconate (Gd2Zr2O7) as top coat get into focus because of their high phase stability up to their melting points and the lower thermal conductivity compared to YSZ. Within this work single- (SCL) and double-ceramic-layer (DCL) top coats consisting of 7 wt.% yttria stabilized zirconia (7YSZ), La2Zr2O7 or Gd2Zr2O7 are deposited by means of Electron Beam-Physical Vapor Deposition (EB-PVD). Aim of this work is on the one hand the investigation of temperature-dependent phase behavior and change of thermal conductivity of SCL and DCL top coats. On the other hand the influence of different top coat materials and architectures on the growth of thermally grown oxide (TGO) is of key interest. In a first step morphology and coating thickness were determined using scanning electron microscopy (SEM). The SCL and DCL systems show a columnar microstructure with a coating thickness of about 150 mu m. In a second step thermal conductivity of SCL and DCL systems was measured between 400 degrees C and 1300 degrees C by means of laser flash technique. The third step contained X-ray diffraction measurements of SCL and DCL systems after atmospherical isothermal oxidation at 1300 degrees C. Finally, the TGO at the interface was analyzed. The TGO composition was determined by means of energy dispersive X-ray spectroscopy (EDX). The TGO phase was identified by using X-ray diffraction. Thus, by a correlation between morphology, architecture, coating material and TGO behavior information about oxygen diffusion processes have been obtained. (C) 2013 Elsevier B.V. All rights reserved.