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Chapter 7
Conclusions
In this thesis the structural stability at high temperature of Zirconia based TBCs, stabilized by Yttria and co-doped with Gadolinia and Dysprosia has been evaluated and discussed.
An extensive study about metastability in Zirconia/Rare Earth Oxides systems in general and Zirconia/Yttria systems in particular has been conducted with the intent of gathering enough data to build some guidelines to optimise, among the several possibilities, the co-dopant choice. A trend for eutectic temperature and some other critical points in Zirconia based systems phase diagrams has been found versus co-dopant ionic radius.
XRD patterns analisys of as-deposited and aged barriers on three different grades of alumina demonstrated that the nature of the substrate has an influence on the resistance to partitioning of the final material. It was found that a substrate with higher purity yields a product with higher resistance and that also the substrate crystallographic structure has an effect on the coating stability. In fact when this was deposited on single crystal alumina, it showed an improved resistance to destabilization compared to the polycrystal. In particular when 7YSZ was deposited on PC96, it was de-stabilised after 2 hours at 1450°C, when deposited on PC99 showed evidence of monoclinic structure after 135 hours at 1500°C and when deposited on SX monoclinic phase formation occurred after 862 hours at 1550°C. The addition of 2% of Gadolinia or Dysprosia resulted in as-deposited coating characterized by a mixed metastable t’ and t’’ phases that remained stable after 386 hour at 1550°C. The addition of 4% of Gadolinia as co-dopant yielded a coating which not only remained stable after 720 hours at 1550°C but also did not show any sign of diffusion process. This is believed to depend on the fact the point representing such composition at this temperature falls, in the phase diagram, in the cubic field and not anymore in the biphasic tetragonal+cubic field. This means that such a material would not be affected by partitioning any longer.
Structural analysis was conducted on the top surface and cross section of the manufactured specimens by means of ESEM. Micrographs taken from as-deposited samples confirmed the presence of well known and expected features in the barrier such as fine porosity, feathery structure and columnar
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arrangement of the grains. It was found that, when the vapour flow incidence angle from the ingot is in some way changed by the presence of physical obstacles in the deposition chamber, the resulting coating structure can be considerably affected in terms of growth direction of the columnar grains. On the other hand it was also shown that such variation in the TBC texture does not yield any significant modification in the barrier stability. This observation can be applied to the real life production of TBCs because, given the high geometrical complexity of typical coated items, such as turbine blades, it is sometimes inevitable for the vapour flow not to have a perfectly constant incidence direction on the surface of the part.
The observation of ESEM micrographs taken from aged specimens allowed to appreciate the modifications the barrier microstructure undertakes during heat treatment: coatings deposited on PC99 and SX almost completely lost their typical fine porosity and the prominent pyramidal tops at the exterior of the columns smoothed out developing pronounced surface undulations. Sintering of columns occurred leading to the formation of blocks of columns (with larger apparent diameters) separated by evident vertical cracks.
When PC96 was adopted, the top coat typical columnar organization after a 9 hour long heat treatment dramatically changed into a globular structure with round borders separated by large and irregular interstitial spaces and with no more preferential orientation.
Further investigations about such peculiar behaviour by EDS demonstrated that significant amount of Silicon moved from the substrate towards the coating layer during ageing supporting the idea that the drastic change in the barrier microstructure depended on the amount of impurities in the substrate.
The study of the data available in the literature about sintered ceramic materials contributed to the postulation of a theory to justify the peculiar appearance of the coatings deposited on PC96 and their lower high temperature stability. According to such theory the Silicon, present within the alumina substrate in the form of oxide, is more stable on the grain surface than on the bulk material and then it accumulates/disperses at grain boundaries to form a new phase with a melting point ranging from 800 to 1500°C. This means that such intergranular phase is in a liquid state at the ageing temperature and then migrates through the grain boundaries towards the coating layer where, upon cooling, eventually generates the glassy irregular intergranular phase around the globular Zirconia which has been observed in our specimens.
The lower stability of these barriers would depend both on the modified thermo-mechanical properties of the region where the glassy phase managed to penetrate and on the higher solubility of Yttria in the intergranular phase which would cause Yttrium depletion in the coating grains.
In conclusion it is worth to note that, if this theory proves to be correct, it would have an impact on the thermal barrier coating study not only from an academic point of view: it would also affect the practical criteria of choosing materials for
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turbine blades manufacturing. The reason being that, although the Thermally Grown Oxide (on which TBCs are deposited in real life applications) is generated by bond coat oxidation and not, as in the present study, by powder sintering, Silicon will be still present in the actual TBC substrate. It is, in fact, usually added to the bond coat in order to enhance its resistance to type II hot-corrosion and oxidation (Grunling and Bauer, 1982), (Goward and Cannon, 1988), (Nicholls, 2003).
