Solid State lonics 32/33 (1989) 709-713 North-Holland, Amsterdam SURFACE REACTIVITY OF YTTRIA-DOPED ZIRCONIA WITH OXYGEN J. NOWOTNY, M. SLOMA * and W. WEPPNER Max-Planck-lnstitutJ~r Festk6rperforschung, D- 7000 Stuttgart 80, FederalRepublic of Germany Received 27 May 1988; accepted for publication 26 August 1988 The interaction between gaseous oxygen and the surface of yttria-stabilizedzirconia (9 mol%) was studied by in situ surface potential measurements at 780°C under Po., ranging between 1.74× 102 and 4.3 X 104 Pa. Different surface treatments of both single- and poly-crystalline specimens were applied. The studies involved both oxidation and reduction experiments. The expo- nent of oxygen pressure determined from the surface potential measurements is discussed in terms of the defect chemistry of the near-surface region. The oxygen exponent is 1/4 for polycrystalline samples annealed at 1300°C and then quenched to 780°C. This corresponds to the formation of doubly ionized oxygen vacancies. Subsequent annealing at 780°C for 150 h results in an increase of the oxygen exponent to 1/2 as a result of segregation of impurities leading to the formation of a sandwich-type low- dimensional near-surface layer. The layer is composed of a top layer enriched predominantly in Ca and A1and a sublayer enriched in Y. Polishing of single crystals with 0.1 ~tm diamond powder leads to the formation of a surface phase which exhibits a decrease of work function in oxygen. The surface phase disappears after about 80 h of annealing at 780°C. Then the sample behaves similarly as the polycrystalline specimen. 1. Introduction Segregation to interfaces has a significant, often controlling, effect on materials properties. It has been documented that bulk impurities, even at low con- centration, may segregate to surfaces and grain boundaries changing substantially their chemical composition. Accordingly, the extent of segregation is of direct importance for the processing and op- eration of ceramics involving the process of ionic transport and sintering. Despite the practical importance of zirconia in sci- ence and technology little is known about its surface properties. The main problem in surface studies of this material by using the commonly available sur- face sensitive techniques arises from the high resis- tivity and the resulting charging effects. Auger electron microscopy (AES) surface analysis of Axelsson et al. [ 1 ] has shown that the surface of undoped zirconia is enriched in Si. The enrichment in Si is favored by heating zirconia in the presence of water vapour. It has been argued that subsequent * On leave of from The Institute of Metallurgy, Academy of Mining and Metallurgy, 30-059 Cracow, Poland. treatment of zirconia in oxygen leads to the forma- tion of a ZrSiO4 thin surface layer. Studies of yttria-doped zirconia reported by Burg- graafet al. [2,3 ] have shown that yttrium Segregates to the grain boundaries of 8.5 mol% yttria-stabilized ZrO2. It has been demonstrated that the outer sur- face layer (2-4 nm in thickness) is enriched with yttria. The enrichment factor is about 1.5. The Y segregation may be blocked by more favorable seg- regation of Bi [2,3]. It has been observed that dis- solution of Bi into yttria-stabilized zirconia leads to the formation of the monoclinic phase along grain boundaries of the bulk cubic phase. The reported grain boundary enrichment factor of Bi is about 10 while a strong depletion of yttria is observed. It has been argued that the formation of the monoclinic grain boundary precipitate has a negative effect on the ionic conductivity of the material [2]. Yttria-stabilized tetragonal zirconia has recently received much attention because of its exceptional mechanical properties. Also its ionic conductivity is higher than in the cubic phase in spite of the smaller number of oxygen vacancies [ 4 ]. Substantial changes in the chemical composition and microstructure of the surface of this material have been observed in- 0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )