Phase partitioning and epitaxy of Zr(Al)O 2 thin films on cubic zirconia substrates
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Thin films of ZrO2-Al 2 O3 were grown on cubic-Zr(Y)O2 substrates by a liquid precursor route. Phase formation and epitaxy of these films was studied as a function of heat-treatment temperature and time. The following sequence of phases was observed in these thin films: Precursor —• f-(Zr,Al)O2 —• f-(Zr,Al)O2 + y-(Al,Zr) 2 O 3 —2, a - A ^ O g , and trace amounts of f-ZrO2. The lattice parameters for the f-ZrO2 (a = 5.0827 A and c= 5.1908 A) and m-ZrO 2 (a = 5.1510 A, b = 5.1991 A, c = 5.3170 A, and /3 = 99.205°) were determined by Rietveld refinement of the x-ray peak positions. Figure 2(a) shows XRD patterns close to the high intensity, (002) substrate peak for thin films heated between 11000 °C and 1400 °C. Figure 2(b) shows similar patterns for films heat-treated at 11000 °C for different
J. Mater. Res., Vol. 10, No. 7, Jul 1995 Downloaded: 14 Mar 2015
IP address: 139.86.13.152
P. K. Narwankar et al.: Phase partitioning and epitaxy of Zr(AI)C>2 thin films
periods up to 200 h. To determine the surface normal orientation for the films, XRD patterns were simulated assuming Gaussian peak shapes and using peak positions measured on powders. Comparison of Figs. 1 and 2 shows that the (Zr, A1)O2 films annealed at temperatures ^12000 °C for ssl h have the tetragonal phase after cooling to room temperature. Annealing the films above 1200 °C, or for extended periods (^48 h) at 1100 °C, produces m-ZrO 2 on cooling. The same XRD patterns show that the t-(Zr, A1)O2 films are oriented with their a-axis normal to the film/substrate interface and the m-ZrO 2 films have their c-axis normal to the interface. B. Scanning electron microscopy For heat-treatment temperatures less than 1200 °C, the films appeared smooth and featureless in the field emission SEM. The film topography became clearer with an increase in temperature. Secondary electron images for films annealed for 1 h at 1300 °C and 1400 °C are shown in Fig. 3. Secondary electron images for films annealed at 1100 °C for different periods are shown in Fig. 4. Topographical features of the films annealed for less than 48 h at 1100 °C were difficult to observe due to the apparent nanocrystalline nature of the surface. The bright features in Figs. 3 and 4 correspond to ZrO 2 grains, and the dark features correspond to A12O3 grains. Increasing the annealing time or temperature produces an increasing amount of A12O3 on the film surface. At high temperatures A12O3 grains have two different morphologies, as seen in Figs. 3(b) and 3(d). Some A12O3 grains are irregular in shape and others have a long, prismatic morphology. A careful examination also reveals that the long, prismatic grains from angles of 0°, 60°, or 120° between one another. C. Transmission electron microscopy Figure 5(a) is a bright-field (BF) image of a film heat-treated at 1100 "C/l h, and Fig. 5(b) is a crosssection schematic to help explain the field of view. The TEM image reveals that the top portion of the film, denoted as A, is polycrystalline. The inset in Fig. 5(a) is a selected area diffraction patte
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