Electron-beam deposition of zirconia films stabilized in high temperature phases by different oxides

Surface and coatings Technology. 49 (1991) 67—70

67

Electron-beam deposition of zirconia films stabilized in high temperature phases by different oxides Syed B. Qadri, E. F. Skelton, C. Kim, M. Z. Harford* and P. Lubitz U.S. Naval Research Laboratory, Washington, DC 20375 (U.S.A.)

Abstract Films of ZrO,—X,0

1 where X Al, In, and Sc and of Zr02—SnO, were deposited by simultaneous electron-beam evaporation of the respective metal oxides. Films deposited onto substrates at room temperature were initially amorphous, but were transformed and stabilized in either the tetragonal or cubic phases of Zr02 by annealing at appropriate temperatures over certain ranges of compositions. In the case of the Zr02—1n203 system, for example, the substrate was heated to 300 ~C during deposition and the Zr02 was recovered in the cubic phase for indium compositions varying between 3.5 and II at.%. However, in the Zr02—Sn02 system, the as-deposited film remained amorphous (even though the substrate was held at 300 °C). Results of the effect of composition on lattice parameters and phase stability are reviewed and discussed.

1. Introduction

2. Experimental details

Zirconia (Zr02) has been an important material in a variety of applications including thermal barrier and corrosion resistant coatings. For these applications it is desirable that Zr02 remains in a single bulk phase as the coating undergoes thermal cycling with temperatures reaching 1000 °C, in order to prevent cracking and other problems which may arise owing to large volume changes. At ambient conditions, Zr02 forms a crystal structure with a monoclinic lattice, which undergoes transformations to structures with tetragonal and cubic lattices as the temperature is increased, with accompanying large unit cell volume changes [1—3]. However, when ZrO2 is mixed with other oxides such as Al203, Sc203, Y203, CeO2, and 1n203, in appropriate mole fractions, the high temperature cubic or tetragonal form of Zr02 can be maintained under ambient conditions, In our previous studies [4—6], we have shown that the electron-beam evaporation technique has a significant advantage over the magnetron sputtering technique for film production as it does not require a conducting target, and as a consequence oxide targets can be readily used and changed. In this paper we will review our previous work and also present our latest results on the Zr02—Sn02 system, and the multilayers of Zr02 with A1203 and Sc203.

Pellets of Zr02 and one of the other desired oxides (Al203, Sc2 03, In2 03 or Sn02) were placed in separate electron-gun hearths in the electron-beam evaporating system. The initial vacuum was about 10_s Torr which was decreased to l0~Torr during deposition. The rate of evaporation was adjusted by the deposition rate controllers to obtain the desired ratio of zirconium to the other metal in the final product. A quartz crystal monitor was used to assess the flux at the substrate, with the resulting deposition rates ranged typically between 0.25 and 6 A s’. Once a deposition rate was determined, the shutters were opened for exposing the substrates. Fused quartz substrates were used and heated in some cases to 300 °Cduring the deposition. The composition was estimated using energy dispersive X-ray fluorescence techniques. The crystal structure was determined using standard X-ray diffraction techniques involving a rotating anode X-ray generator with a copper target and a Rigaku diffractometer.

*Sachs/Freeman Associates Inc., Landover, MD 20785 (U.S.A.).

3. Results and discussion 3.1. Zr02—A12 03 system In order to compare the electron beam deposition technique results with those of our previously reported findings using magnetron sputtering [7, 8], films of various zirconium and aluminum atomic ratios were deposited onto quartz substrates and subsequently equilibrated at room temperature. X-ray diffraction scans showed that the as-deposited films were amorphous, but crystallized upon annealing at 1000 °Cfor

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S. B. Qadri

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interesting aspect of the Zr02—A1203 system was the reduction in theA~ unit volume of the phase from 143.8 tocell 134.6 A~, as the monoclinic aluminum An

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24—48 h. The results were similar to those obtained for films deposited using magnetron sputtering as far as the particle sizes and the formation of the tetragonal and monoclinic phases of Zr07 were concerned, i.e. for films with Zr:Al ratios between 100:0 and 76:24, the monoclinic phase was obtained, whereas for ratios of 70:30 to 10:90, the tetragonal phase of ZrO, was stabilized. The particle size after annealing for 24 h at 1000 °C was measured to be of the order of 80—100 A using Scherrar’s formula [9] and the (Ill) diffraction peak of the tetragonal phase. Figure 1 shows the diffraction scan forMultilayers Zr02—Al203 of for Zr0a sample with Zr:Al ratio of 43:57. 2—A1203 were also deposited onto quartz substrates using the electron beam evaporation technique. Figure 2 shows the diffraction pattern for a sample with eight layers of Zr02—A1203 with thicknesses of 750 A and 1000 A, respectively. Mixed phases of the tetragonal and monoclinic Zr02 are observed with the monoclinic phase being predominant. Further work is in progress to study the effect of layer thickness on the nucleation and growth of Zr07 phases.

content was increased from 0 to 30 at.%. Once the tetragonal phase was stabilized, there was no measurable change in either the unit cell volume or the lattice parameter for higher aluminum compositions. This may suggest that the ZrO,—Al203 mixture was not forming solid solutions, but that the nucleation of tetragonal phase may be due to stress induced by the surrounding Al,03 matrix. It is also well known that Zr02 and Al,03 are not mutually soluble over the entire compositional range [10]. 3.2. ZrO,—Sc,03 system Films of Zr07—Sc203 were deposited onto quartz substrates with varying Zr:Sc ratios from 100:0 to 92:8. As-deposited films were all amorphous as determined by X-ray diffraction. The samples were annealed at 1000 ~C for periods ranging from 24 to 48 h. X-ray measurements showed that the nucleation of the monoclinic phase occurred for samples with less than 2 at.% Sc, mixed monoclinic and tetragonal phases for 2— 5 at.% Sc, and only the high temperature tetragonal or cubic phase for samples with 6—8 at.% Sc (Fig. 3). These results are in good agreement with the results of Ruh et a!. [II] for bulk samples which were grown from the melt. The unit cell volume for the phase 3 to 140.1 A3 monoclinic as the scandium decreased from 143.8 A composition increased from 0 to 3.5 at.%; the volume of the tetragonal unit cell for 7at.% Sc was l34A~. Since the tetragonal phase is stabilized with such a small percentage of scandium and because Sc,03 is soluble in ZrO,, this may suggest that a solid solution is being formed between the constituents.

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Fig. 2. 20 scan for a multilayer of ZrO,—Al,01 with eight layers of 750 A ZrO, and 1000 A Al,03, showing the mixed phases.

Fig. 3. Diffractometer scans of post annealed ZrO~—Sc,O3 with 2 at.% Sc (bottom curve) and 6 at.% Sc (top curve). The 2% sample has a single monoclinic phase pattern for ZrO,~the 6% sample has a tetragonal-phase pattern of ZrO,.

S. B. Qudri ci a!. / Zirconia films siahili:ed by oxides

Multilayers of Zr02—Sc2O3 were synthesized with eight layers having thicknesses of 2000 A and 62 A respectively. The diffraction pattern of the multilayer consisted of mixed phases with the monoclinic phase being predominant. In appearance the film was mechanically stable and did not show any sign of peeling. Work is in progress to deposit thin layers of Zr02 and Sc203 to obtain the tetragonal phase only. 3.3. Zr0~—In203 system Since In2 03 volatizes at 850 °C, it was desirable to deposit films with substrates maintained at elevated temperatures so that the films would be initially crystalline and would not require subsequent annealing. The substrates of fused quartz were held at 300 °C during deposition. Films deposited with less than 3 mol.Vo 1n203 showed predominantly the monoclinic phase, whereas films grown from 3.5—11.5 mol.% In203 all showed the cubic phase of Zr02 to be predominant (Fig. 4). The lattice parameter showed a monotonic variation with increasing molar per cent of In2 03 (Fig. 5), similar to the variation for the Zr02—Ce02 system of bulk samples reported by Duwez Odellstudy [12].isThe formation of the cubic Zr02 in theand present consistent with the phase diagram published by Morozova and coworkers [13, 14]. Since the ionic radii of zirconium and indium differ by only 3%, it is possible that solid solutions of Zr02—1n203 were being formed as the films were deposited, resulting in the nucleation of the cubic phase of Zr02.

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3.4. Zr02—Sn02 system Films of Zr02—Sn02 were deposited with the tin composition varying between 0 and 10 at.%. The substrates of fused quartz were held at 300 °C during deposition. As-deposited films were amorphous, in contrast to the Zr02—1n203 system where crystalline films were obtained under similar conditions. Subsequent

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Fig. 6. 20 scan of ZrO,—SnO, for a sample with 5 at.% Sn (bottom curve) and 10 at.% Sn (top curve). The monoclinic phase of Zr02 together with an unidentified set of peaks is seen for the 5% sample; the 10% sample shows a cubic-phase pattern of ZrO, and other peaks that could be identified as those from 5n104.

annealing at 1000 °Cfor 24—48 h showed evidence of the monoclinic phase for films with a tin composition of 5 at.%. In addition to the monoclinic phase, there were peaks that could not be identified with any of the known tin oxide phases. Films with 10 at.% Sn showed the formation of the cubic phase of Zr02 in addition to other diffraction peaks that could be indexed to the Sn304 structure (Fig. 6). More work is in progress to understand the origin of these phases with different tin contents.

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4. Conclusions

28(deg.) Fig. 4. Diffractometer scan of Zr02—1n201 with 10.2 mol.% 1n207 film showing peaks from the cubic phase of Zr02.

Using the electron beam evaporation technique, we were able to deposit Zr02—Al203, Zr02—Sc203, Zr02—

70

S. B. Qadri ci a!. / Zirconia films stahili:ed by oxides

In203, and Zr02—5n02 and were able to stabilize the tetragonal or cubic phases of Zr02 over certain compositional ranges. The simplicity of this method allows us to coevaporate Zr02 with other stabilizing oxides and their multilayers for a variety of applications.

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5 S. B. Qadri, E. F. Skelton, M. Z. Harford, P. Lubitz and L. Aprigliano, J. Vac. Sc Technol. A, 8 (1990) 2344. 6 S. B. Qadri. E. F. Skelton, M. Z. Harford, R. Jones and P. Lubitz, J. Vac. Sci. Technol. A, 9 (1991) 510. 7 S. B. Qadri, C. M. Gilmore, C. Quinn, E. F. Skelton and C. R. Gossett, Phys. Rev. B, 39 (1989) 6234. 8 C. M. Gilmore, S. B. Qadri, C. Quinn, C. R. Gossett and E. F. Skelton, J. Vac. Sci. Technol. A, 5 (1987) 2085. 9 B. D. Cullity, Elements of’ X-ray Dii]raeiion, Addison-Wesley. Reading, MA, 1978, p. 284. 10 G. Cevales, Ber. Dew’. Keram. Ges., 45(5) (1968) 217. G. Cevales, in M. K. Resser (ed.), Phase Diagrams for (‘eramicisis. The American Ceramic Society. Columbus, OH, 1975, p. 135. II R. Ruh. H. J. Garrett. R. F. Domagala and V. A. Patel, J. Am. Ceram. Soc.. 60(1977) 399. 12 P. Duwez and F. Odell, J. Am. ceram. Soc., 33 (1950) 274. 13 L. V. Morozova, P. A. Tikhonov and V. B. Glushkova, Dok!. Akad. Nauk. SSSR (Phys. (‘hem.), 273 (1983) 140—144. 14 L. V. Morozova, V. P. Popov, P. A. Tikhonov and V. B. Glushkova. J. App!. Chem. USSR, 59 (1986) 2261 —2264.