ICMCTF2013 Session A2-1: Thermal and Environmental Barrier Coatings

The paper will focus on strain tolerant zirconia based thermal barrier coatings. In contrast to the high roughness of bond coats used for atmospherically plasma sprayed TBCs the bond coating for the PS-PVD process has to be smooth to achieve sufficient bonding. In addition, the growth of the thermally grown oxide (TGO) on the bond coat as well as the growth rate of the TBC itself has to be adopted. For the optimized coatings excellent thermal cycling performance in burner rig facilities could be demonstrated. The coatings showed lifetime more than two times better than conventional APS TBCs. The reason for this excellent behavior will be discussed in detail.

Finally, an outlook for future applications of the flexible and powerful PS-PVD process especially in energy systems will be given.

As gas turbine operating temperatures are raised to reap the environmental and economic benefits of increased efficiency, the failure mechanisms that shorten the working life of TBCs are exacerbated and expanded. The current industry-standard TBC composition, yittria stabilized zirconia (YSZ), is inherently limited to reach the prospective goals of technology motivating the search for new compositions with improved capabilities. Zirconia co-doped with CeO₂ and TiO₂ has shown promise as an alternative ‘next generation’ TBC. Previous work showed a relatively large, non-transformable, single-phase tetragonal field stabilized at 1350°C . Compositions within this field have been manufactured into coatings by air-plasma spray. Phase and microstructural evolution of thermally sprayed freestanding TBCs upon isothermal aging are investigated as a function of time at 1316° C and 1427° C. Coating performance during thermal gradient tests and possible explanations for observed behavior will also be discussed.

The continued need for increments in gas turbine operating temperatures has necessitated developments in new thermal barrier materials and their processing. Of particular interest in recent years is the potential for Gadolinium Zirconate as a candidate TBCs to replace yttria stabilized zirconia. Gd₂Zr₂O7 in particular provide lower conductivity and resistant to environmental damage from ingested sand particles (categorized as CMAS for calcium magnesium alumino-silicate). Zirconates however, have several challenges including low fracture toughness, incompatibility with the thermally grown alumina and low erosion resistance. Multilayer concepts based on combinations of zirconia and zirconate have been developed to address the multifunctional requiremetns. This paper seeks advances in processing science and control for layer-by-layer optimization of coating microstructure and properties so as to meet multifunctional obligations. The paper will demonstrate the advantages of plasma spray for such layered coating system along with identification of critical challenges.

More than 650 million tons per year of CO -emission is generated by air traffic. In order to meet the future demands like reducing the CO -emission and the fuel consumption of gas turbines, the increase of turbine inlet temperature (TET) is necessary. However, state-of-the-art coated superalloy materials might not stand a further increase of TET. Therefore the development of new top coats is necessary.

Yttria stabilized Zirconia (YSZ) is usually used as ceramic top coat for gas turbine parts of the first and the second stage of the gas turbine. Investigations have shown the accelerated phase transformation and the intensified sinter effects of the YSZ top coat at temperatures between 1,200 °C and 1,300 °C, leading to changes of microstructure. Such modifications of the microstructure lead to higher thermal stresses and consequently reduce the lifetime. Furthermore, thermal conductivity λ of the top coat increases. For this reason the pyrochlore zirconates lanthanum zirconate (La₂Zr₂O₇) and gadolinium zirconate (Gd₂Zr₂O₇) as top coat get into focus because of their promising material properties like both the 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) between top coat and substrate is of key interest. In a first step morphology and coating thickness are determined using scanning electron microscopy (SEM). The SCL and DCL systems show a columnar microstructure with a coating thickness of about 150 µm. In a second step thermal conductivity of SCL and DCL systems is measured between 800 °C and 1,300 °C by means of laser flash technique. The third step is high-temperature X-ray diffraction measurements of SCL and DCL systems during heating and cooling between 800 °C and 1,300 °C at atmosphere. Finally, the TGO at the interface between top coat system and substrate is analyzed. Thickness of the TGO is determined by means of SEM, composition using chemical analysis and phases using X-ray diffraction. Thus, a correlation between morphology, architecture, coating material and the thickness of the TGO can give information about oxygen diffusion processes.