Burggraaf, A.J.

Theunissen, G.S.A.M.

Winnubst, A.J.A.

English

University of Twente Research Information

J O U R N A L  OF M A T E R I A L S  S C I E N C E  L E T T E R S  8 (1989) 55-57  Segregation aspects in the ZrOz-Y203 ceramic system G. S. A. M. T H E U N I S S E N ,  A. J. A. W I N N U B S T ,  A. J. B U R G G R A A F  University of Twente, Faculty of Chemical Technology, Laboratory for Inorganic Chemistry, Materials Science and Catalysis, PO Box 217, 7500 AE Enschede, The Netherlands In solid solutions a difference is often present in chemical composition between bulk and interface of the materials. The difference frequently occurs because of segregation of impurities or main com- ponents to the interfaces. The composition of grain boundaries and of surfaces of ceramic materials often control fabrication behaviour such as sintering and grain growth [1]. Besides this, segregation of main components under the influence of water may also influence the thermal stability of tetragonal zirconia as was discussed by Winnubst and Burggraaf [2]. In most zirconia ceramics grain growth behaviour varies with composition. Different mechanisms have been proposed to explain this difference in grain growth. Lange [3] attributed this difference to the presence of different crystals phases in the ceramic (the cubic grains control the growth of the tetragonal ones). Jurado et al. [4] assumed that the larger grains in tetragonal zirconia had grown at the expense of the smaller ones. However, in our zirconia materials, with a bulk composition ranging from 4 to 8.9mo1% YO~.5, no cubic phase or abnormal grain growth was observed. The mechanisms mentioned above therefore do not account for the differences observed [5]. Another phenomenon, which may explain the dif- ference in grain growth rate is a decrease in grain boundary mobility through an "impurity" drag mech- anism [1]. This implies the presence of a segregation layer at the surface, which consists of impurities or solute atoms. In order to find an explanation for the difference in grain growth (and grain boundary conductivity) in zirconia materials doped with different amounts of yttria, the chemical composition of pure ultrafine grained (ZrO2)100 x(YO1.5)x ceramic interfaces was studied by means of Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). As starting materials for the synthesis of the (ZrOz)100_x(YOi.5) x ceramics (sample code ZYx) ultrafine-grained powders were used. These powders were prepared by the alkoxide or chloride method [5, 6], which yields a pure and sinter-reactive powder. After isostatic pressing at 400 MPa and consequently sintering at temperatures from 1100 to 1400°C a relative density of more than 95% could be obtained. The resulting grain sizes range from 0.1 #m with low yttria content to 0.4/~m with high (>  13 mol % YO~.5) yttria content. Before the surface analysis experiments were carried out, the specimens were carefully polished with A1203 and ultrasonically cleaned in ethanol. These speci- mens were submitted to a temperature treatment in an oxygen atmosphere for 5h at 600, 700 or 1000°C. As a standard, rods were prepared which fractured trans- granular under ultra-high vacuum. In this way surface contamination could be avoided and a fresh surface having the bulk composition is obtained. The Auger analyses were performed with a cylindri- cal mirror analyser, type 10-155 (Physical Electronics). The analyses were performed using a primary beam current of 0.25#A. No sample charging occurred under these conditions when clean specimens were used. The XPS measurements were carried out on a Kratos ES 200B spectrometer, using AI-K~ radiation (1487eV), in a vacuum chamber with a background pressure lower than 10 -8 torr. Calibration of the XPS spectra was performed using the CLs peak. In case of AES the spectra were examined in a semi quantitative way using the equation of Palmberg et al. [71: C,. = ( Ix /S~) /~  (I~/S~) (1) where C~ is the concentration (in at %) of element x, and I, and S~ are, respectively, the peak intensity and the relative elemental sensitivity factor of the Auger line of element x used for quantification. The yttrium and zirconium sensitivity factors are obtained from the standard spectra, the oxygen sensitivity was given a fixed value [7]. To quantify the XPS specta an equation similar to Equation 1 was used [8]. The method of analysis is described elsewhere [9]. To give a good interpretation of the Auger spectra, good knowledge of the correct position of the Auger peaks is necessary. Yttrium has an MNN Auger peak at 76 eV, zirconium an MNN peak at 147 eV and oxy- gen a KLL peak at 503 eV. In measuring the yttrium peak one has to be careful because of the presence of an LMM silicon peak on exactly the same position (76eV). This is important because in some cases a small amount of silicon (which segregates quite easily) is present in the ceramic. Therefore, part of the peak intensity at 76eV can be due to silicon. In order to eliminate the silicon part of the 76 eV peak a calcu- lation has to be made. This can be done by deducing the silicon intensity at 76 eV from the intensity of the free KLL silicon peak at 1606eV (oxide form; in elemental form the peak is situated at 1619 eV [7]). By using the sensitivity factors as stated in Table I, the transformation is performed as follows: Isi.76 = Isi.,606 Ssi,76/Ssi,16o6 (2) The sensitivity factors of silicon at 76 and 1606 eV are obtained from a standard SiO2 Auger spectrum [7]. In cases where silicon is found to be present, the yttrium 0261-8028/89 $03.00 + .12 © 1989 Chapman and Hall Ltd. 55 T A B L E  I Auger sensitivity factors (Ep = 3 keV) Element Oxide form Zr 0.10 Y 0.13 O 0.50 Si (1619 eV) 0.081 Si (76 eV) 0.373 T A B L E  II Chemical surface composition Y203-ZrO2 ceramics heat treated for 5 h as determined by AES. (Materials with com- positions between 4 and 9 wt % YOL5 have a tetragonal structure, the others have a cubic fluorite structure) Bulk composition Temperature Surface composition of sample treatment (K) x (from ZYx) ZY4 ZY4 ZY5* intensity is obtained by subtracting the silicon inten- zYs* sity from the total intensity at 76 eV. Because only the zY6.1 upper part of the Auger peak at 76 eV has been taken ZY6.1 into account, the calculated Is~ 76 has to be divided by ZY6.1 ' ZY8.9 a factor of  2 to obtain the yttrium intensity: ZY13 Iy  = /tot 76 - -  Isi.76/2 (3)  ZY13 ' ZY13 in which /tot,76 represents the measured peak height at ZY17 76 eV. However, in the XPS spectra a clear distinction zY17 ZY17* can be made between the Y3d and the Si2s peak. The Y3d ZY26.4 peak has its maximum at 161.6eV, Si2s at 157.8eV. ZY26.4 The Auger spectra of a polished surface of  ZY17 zY32 heat treated at T = 873 and 1273 K, respectively are shown in Figs la and b. From this it can be seen that the Y/Zr peak intensity ratio of the high-temperature treated specimen (Fig. lb) is higher than that of  the bulk (Fig. l a). The inaccuracy in the choice of  the baseline is smaller than the observed differences in the peak intensity ratio between the two spectra. Therefore, it can be concluded that yttrium enrich- ment occurs at the surface of  ZYl7 .  The yttrium enrichment at the surface is also indi- cated by a decrease of  the yttrium signal as function of the Ar ÷ ion-sputtering time. After 30 to 40 sec of ion sputtering the Y/Zr peak intensity ratio becomes con- stant (surface composition is then about ZY10). This ® o~x IAo Zr x 3 0  ® X21/2 - -  1/10 - Z r  dN dE 0 260 460 600 16t~ 1~o electron energy (eV) Figure t Auger spectra of  a polished surface of ZYI7 heat treated for (a) 5 h  at 873K or (b) 5h  at 1273k. 873 5 1273 31 873 6 1273 30 873 6 973 14 1273 30 1273 29 873 11 973 12 1273 34 873 17 1273 35 1273 31 873 28 1273 34 873 33 * Single crystals quantitative result, obtained from the observed sputter profile, is not very reliable because of  some reduction of Zr 4 + to zirconium metal (the maximum of  zirconum metal does not occur at the same energy as Zr 4+ in oxides). Therefore, the thickness of the yttrium- segregation layer cannot be determined with any precision, but is estimated to be of  the order of 2 to 4 nm [9]. The quantitative results of the measurements are given in Table II, using the sensitivity factors given in Table I. From this it can be seen that after tempera- ture treatment a pronounced yttrium enrichment takes place up to a composition of about ZY30. These results are already corrected for possible sili- con contamination if this is present. That the larger Auger peaks at 76 eV are not due to silicon is shown in the XPS spectra of  Fig. 2. In Fig. 2b no significant Y3d _z, tO  c -  [1. x @ I I I 155,0 160.0 165.0 BE (eV) Figure 2 XPS spectra of ZY specimen treated at 1237K for 5h. (a) ZY4 with silicon contamination; (b) ZY6.1 without silicon contamination. 56 presence of silicon is shown (the detection limit is about 0.2 at %). In AES (Fig. 1) the peak-to-peak height of silicon at 1606 eV (with a 30 × peak enlargement) should be at least 5 mm to be visible above the background level. In case of a peak height of 5mm (1606eV) a silicon intensity of less than 0.5mm is calculated at 76eV which, in general, causes a correction of only a few per cent in the total peak height at 76 eV (and therefore in the yttrium intensity). The large influence of a possible silicon contami- nation is shown in the following example. A sample was deliberately contaminated with silicon. In this particular case the real peak intensity of yttrium was about 20% lower than for the case ignoring silicon, where the total 76 eV peak intensity was ascribed to yttrium. The composition was calculated to be ZY42 (ignoring silicon) and ZY34 with silicon taken into account. In this case the silicon concentration at the surface was found to be 3 at %. All samples which received a temperature treatment of 1273 K showed an yttrium enrichment of about 30 at % to the surface (and probably also to the grain boundary [9-11]). This is independent of their crystal structure (both 100% tetragonal and 100% cubic materials show the same result) and bulk composition. This concentration therefore seems to be a saturation value. In the XPS spectrum yttrium enrichment is also observed. Using the sensitivity factors found by Wagner et al. [8], a surface composition of ZY20 is calculated (bulk composition ZY6.1). To calculate the exact composition, standards had to be made, which has not been done in this case. The AES results were confirmed by Steel and Butler [12], who lound with SPS on the (00 1) surface of ZY15 a concentration of 33 at % yttrium. These results mean that the yttrium enrichment factor (defined a s  Cy,surface/Cy,bulk ) for materials containing low amounts of yttria (i.e. tetrag- onal materials) is much larger than the (cubic) materials containing high percentages of yttria. A stronger segregation (higher enrichment factor) results in a steeper yttrium gradient between the surface and the bulk of the material, and therefore in a stronger "solute" drag. Probably a stronger "solute" drag results in a stronger retardation of the grain growth (which means a smaller grain size after sintering). This is what has been observed while sintering yttria- stabilized zirconia materials doped with various amounts of yttria. All of the materials started with the same initial crystallite size [5]. This might also explain why in PSZ ceramics containing both tetragonal and cubic phases the larger grains are cubic. Because the as-prepared single crystals (by skull- melting) show the same segregation behaviour as the ceramical materials, a retardation in domain growth in single crystals may also be possible. In conclusion: 1. After temperature treatment at 1273K for 5h (ZrO2)J00_x(YOL0x ceramics show a surface corn- position of about 30 to 34 at % yttrium, regardless of the bulk composition of the ceramic. The segregation layer has a thickness of a few atom layers (~  2 nm). 2. A similar surface composition of about 30 at % yttrium for ceramics containing low amounts of yttria (in the bulk) results in a larger yttrium enrichmevt factor than in ceramics containing larger amounts of yttria. The yttrium enrichment factor therefore increases from 1.5 to 7 with a bulk concentration of yttria decreasing from 26 to 4 at % YOI.5. 3. Grain growth is dependent on the amount of yttria in the bulk, which results for tetragonal yttria- stabilized zirconia in a slower grain growth than the cubic ones. 4. To obtain reliable results, special attention has to be paid to the presence of silicon. Acknowledgements We would like to thank Elcoma (Philips Electronic Industries Eindhoven) for use of their AES equipment. A. van de Berg is acknowledged for his assistance in performing the XPS measurements and discussion of the results. References 1. R. J. BROOK,  in "Treatise of  Materials Science and Tech- nology", Vot. 9, edited by F. F. Y. Wang (Academic Press, New York, 1976) p. 331. 2. A. J. A. W I N N U B S T  and A. J. B U R G G R A A F ,  Pro- ceedings of  the 1986 Zirconia III Conference, Advances in Ceramics (American Ceramic Society) (in press). 3. F. F. LANGE,  J. Amer. Ceram. Soc. 69 (1986) 240. 4. J. R. J U R A D O ,  C. M O U R E  and P. D U R A N ,  J. Phys. Coll, 47 (1986) C1-789. 5. A. J. A. WINNUBST,  G . S . A . M .  THEUNISSEN,  W. F. M. GROOT ZEVERT and A. J. B U R G G R A A F ,  "The Sintering Behaviour of  Fine Grained ZrO2-Y203 Ceramics",  Science of Ceramics, Vol. 14 (British Ceramic Society, Canterbury, 1987) (in press). 6. M. A. C. G. VAN DE G R A A F ,  J. H. H. TER MAAT and A. J. B U R G G R A A F ,  J. Mater. Sci. 20 (1985) 1407. 7. P. W. PALMBERG,  G. E. R | A C K ,  R. E. WEBER and N. C. McDONALD,  "Handbook  of  Auger Spectroscopy, Physical Electronics" (Edina, Minnesota,  1972). 8. C. D. WAGNER,  W. M. RIGGS,  L. E. DAVIS, J. F. M O U L D E R  and G. E. MU1LENBERG,  "Handbook  of  X-ray Photoelectron Spectroscopy" (Perkin-Elmer Corpor- ation, Minnesota,  1979). A. J. A. WINNUBST,  P . J . M .  KROOT and A . J .  B U R G G R A A F ,  J. Phys. Chem. Solids 44 (1983) 955. A. J. B U R G G R A A F ,  M. VAN HEMERT,  D, SCHOL- TEN and A. J. A. WINNUBST,  in "Reactivity of  Solids" (Elsevier, Amsterdam,  1985) p. 797. A. J. B U R G G R A A F  and A. J. A. WINNUBST,  in "Surface and Near-surface Chemistry of Oxide Materials",  edited by J, Nowotny and L. C. Dufour  (Elsevier, Amster- dam, 1988) pp. 449-477. B. C. H. STEELE and E. P. BUTLER,  in ~'British Ceramic Proceedings, 1985" (The Institute of  Ceramics, Stoke-on-Trent, 1985) p. 45. 9. 10. 11. 12. Received 26 February and accepted 28 April 1988 57 

Segregation aspects in the ZrO2-Y2O3 ceramic system

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