Synthesis and Characterization of Yttria-Stabilized Zirconia Nanoparticles Doped with Ytterbium and Gadolinium: ZrO 2 9.
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UCTION
SINCE the idea of applying a relatively thin ceramic coating on a metal substrate to reduce the effect of temperature on mechanical properties of the substrate in the late 1950s by NASA and US Air Force, thermal barrier coatings (TBCs) have been used in many applications and still provide the subject for numerous research works and improvements.[1–4] Usually, TBCs are composed of a yttria-stabilized zirconia (YSZ) top coating which comprises t-prime phase and MCrAlY bond coat (M = Ni, Co, Fe) to enhance the high-temperature durability of metal components in aviation as well as power generation gas turbines.[2,5,6] Pure zirconia (ZrO2) has three polymorphs, including monoclinic phase (m-ZrO2) at temperatures up to about 1175 C, tetragonal phase (t-ZrO2) in the temperature range of 1175 C to M. BAHAMIRIAN, M.R. RAHIMIPOUR and M. FARVIZI are with the Department of Ceramics, Materials and Energy Research Center, Karaj, Iran. S.M.M. HADAVI is with the Department of Materials Engineering, University of Tarbiat Modares, Tehran, Iran. Contact e-mail: [email protected] A. KEYVANI is with the Department of Metallurgy and Materials Engineering, Faculty of Technology and Engineering, University of Shahr-e kord, Shahr-e kord, Iran. Manuscript submitted September 11, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
2370 C, and cubic phase (c-ZrO2) in the temperature range of 2370 C to 2760 C.[7,8] Upon cooling, ZrO2 experiences cubic to tetragonal and tetragonal to monoclinic phase transformations. The tetragonal to monoclinic phase transformation upon cooling is accompanied by a 3 to 4 pct volume expansion which results in the destruction of zirconia coating.[9–11] There are two main approaches to stabilize high-temperature zirconia phase at room temperature. One approach is to incorporate dopant ions such as Y3+, Mg2+, Ce4+, Ca2+, and rare-earth elements into ZrO2 lattice.[7,12–15] The other approach to keep the tetragonal phase at room temperature is the reduction of the particle size of zirconia to some critical value.[16] The addition of dopant ions to ZrO2 extends the existence of cubic and tetragonal phases to low temperatures.[17] In fact, with increasing the contents of dopants, the temperature at which high-temperature zirconia phase (tetragonal or cubic phase) is phase-transformed to monoclinic phase decreases. For example, in the phase diagram of ZrO2-Y2O3, over the range of 2.5 to 7.5 mol pct Y2O3 (dopant) and above 600 C, there is a two-phase region including both cubic and tetragonal phases.[18] At room temperature, the equilibrium phases are monoclinic and cubic phases. In this situation, zirconia is termed as ‘‘partially stabilized zirconia.’’ With increasing Y2O3 (dopant) content, the cubic phase
becomes stable even at room temperature, where the material is called ‘‘fully stabilized zirconia.’’[17] Upon rapid cooling of the partially stabilized zirconia from high temperature to room temperature, a non-diffusional phase transformation occurs and forms a ‘non-transformable’ metastable tetragon
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