Materials Science and Engineering A 443 (2007) 60–65 Thermal conductivity investigation of zirconia co-doped with yttria and niobia EB-PVD TBCs D.S. Almeida a,∗ , C.R.M. Silva a , M.C.A. Nono b , C.A.A. Cairo a a Comando-Geral de Tecnologia Aeroespacial-Divis˜ ao de Materiais AMR-CTA, P¸ ca. Marechal do Ar Eduardo Gomes, 50, Cep. 12228-904, S.J. Campos-SP, Brazil b Instituto Nacional de Pesquisas Espaciais, Laborat´ orio Associado de Materiais e Sensores LAS-INPE, Av. dos Astronautas, 1758, Cep. 12227-010, S.J. Campos-SP Brazil Received 16 February 2006; received in revised form 17 July 2006; accepted 26 September 2006 Abstract A technique used to improve the life cycle and/or the working temperature of the turbine blades uses ceramic coatings over metallic material applied by electron beam-physical vapor deposition (EB-PVD). The most usual material for this application is yttria doped zirconia. Addition of niobia, as a co-dopant in the Y 2 O 3 –ZrO 2 system, can reduce thermal conductivity. The purpose of this work is to evaluate the influence of the addition of niobia on the microstructure and thermal properties of the ceramic coatings. This new formulation will, in the future, be able to become an alternative to the composition currently used by the aerospace field in EB-PVD thermal barrier coatings (TBC). A significant reduction of the thermal conductivity, measured by laser flash technique, in the zirconia ceramic coatings co-doped with yttria and niobia when compared with zirconia–yttria coatings was observed. © 2006 Elsevier B.V. All rights reserved. Keywords: EB-PVD; TBC; ZrO 2 –Y 2 O 3 ; ZrO 2 –Y 2 O 3 –Nb 2 O 5 ; Thermal conductivity 1. Introduction The great advantage of coatings is that it is possible to modify its response to the environment by changing only the superfi- cial part of the component, thus providing completely different properties. Some of the obtained benefits are: reduction of main- tenance costs, increase of the working temperature, reduction of thermal loads, resistance increase to erosion and corrosion and reduction of the high temperature oxidation [1]. The electron beam-physical vapor deposition (EB-PVD) pro- cess enables to attain coatings with unique properties. The pro- cess parameters are adjusted so that the deposit has a columnar grain structure perpendicular to the interface. This morphology maximizes the resistance to strains that arise from differences in thermal expansion coefficients. Other advantages are: aerody- namically favorable smooth surface, better interaction with the substrate, greater thermal cycle tolerance and, hence, greater lifetime comparativeness with the plasma spray process [2–12]. ∗ Corresponding author. Tel.: +55 12 3947 6470; fax: +55 12 3947 6405. E-mail address: dsa62@yahoo.com (D.S. Almeida). Dense zirconia based materials already exhibit low thermal conductivity. The introduction of a stabilizer, required to avoid the detrimental effect of tetragonal to monoclinic phase trans- formation, is accompanied by the incorporation of a substantial amount of vacancies providing an efficient source of phonons scattering [3]. When a trivalent oxide, e.g., Y 2 O 3 , is added to ZrO 2 as a stabilizer, a certain amount of lattice defects, e.g., oxygen vacancies and negatively charged solutes, are produced in the ZrO 2 lattice [12]. The addition of Ta 2 O 5 , Nb 2 O 5 and HfO 2 to bulk Y 2 O 3 – stabilized tetragonal ZrO 2 increases transformation, as for exam- ple the tetragonal (t) to monoclinic (m) transformation temper- ature, of the resulting zirconia ceramics. The enhanced trans- formability is related to the alloying effect on the tetragonality (c/a—cell parameters ratio) of stabilized tetragonal ZrO 2 , sub- sequently, by adding these oxides the tetragonal distortion of the cubic lattice is increased. The increase in the tetragonality, due to alloying, is consistent with the increase in the fracture hard- ness and the increase in the t to m transformation temperature [12–16]. Contrary to trivalent oxides, pentavalent oxides are positively charged when dissolved in the ZrO 2 lattice, the addition of these 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.072