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It was shown through two experiments that submicron-size pits formed on the surfaces of a dense fused quartz substrate and a Y 2 O 3 -ZrO 2 ceramic thin film that were coated with gold and then heat-treated in air at 1200 °C. The formation of the pits is believed to be caused by the positive capillary pressure (approximately 22 MPa) generated in the molten gold droplets ( < 1 /Am in diameter) that were formed as the gold coating melted.

The role of a negative liquid capillary pressure has been well recognized and exploited in ceramic sintering. In comparison, the effect of a positive liquid capillary pressure has so far drawn little attention. A liquid phase with a negative capillary pressure lowers the chemical potential of a solid that is in contact with the liquid phase, whereas a liquid phase with a positive capillary pressure raises the chemical potential. Such a positive liquid capillary pressure generally exists in any liquid phase resting on a flat solid substrate. In the present work we have shown through two experiments that submicron-size pits can be formed on the surfaces of two solid substrates that were coated with gold and then heat-treated in air at 1200 °C. In the first experiment, an yttria-stabilized-zirconia (YSZ) film was prepared by spin-coating a sol-gel solution on a fused quartz substrate (25 X 25 mm 2 ). The solution was prepared following Sakurai et al.'s method.1 The specimen was first heat-treated to 500 °C to obtain a crystallized YSZ film, and then part of the specimen was coated with gold. All gold coatings were applied with a SEM gold-coating apparatus (Ladd 30800) under the conditions of 2.5 KV, 20 mA, and 120 s. To isolate the effect of other variables, a single specimen was used. The specimen was divided into three sections: (A) without gold coating, (B) with one gold coating, and (C) with two gold coatings. The specimen was then heat-treated in air at 1200 °C for 2 h. From a previous experiment, it was found that the crystallization of the YSZ film started between 400 °C and 450 °C and was completed between 450 °C and 500 °C. The crystalline structure was cubic, and no further phase transformations were detected on subsequent heat treatment to 1300 °C. The appearance of the YSZ film after heat treatment at 500 °C is smooth and crackfree; no grains and pores can be clearly identified from SEM observation (Fig. 1; the edge of the film was intentionally selected for focusing). The film after the 1200 °C heat treatment can be divided into three regions according to their features: (A) without gold coating, (B) with one gold 2484

http://journals.cambridge.org

J. Mater. Res., Vol. 9, No. 10, Oct 1994

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FIG. 1. The crystallized YSZ film after heat treatment at 500 °C: smooth and featureless (the edge of the film was selected for focusing).

coating, and (C) with two gold coatings. In region (A), the YSZ grains have become clearly visible and the grain size is approximately 50 nm (estimated from the SEM micrograph); the grains are uniform in size and are close