Variation of contact angles with temperature and time in the Al-Al 2 O 3 system
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I.
INTRODUCTION
T H E wettability of molten metals on ceramic substrates is usually determined by the sessile drop method. In the conventional method to obtain a sessile drop, a small piece of metal placed upon a polished substrate is heated to a temperature higher than the melting point of the metal. The contact angle, 0, is then taken as a measure of wettability. The wetting behavior of A1 on A1203 investigated by this conventional method has been reported to strongly depend on temperature/~-6~ On comparing the experimentally determined contact angles of A1 on A1203, it can be noted that there is a significant inconsistency among the data, particularly at temperatures lower than 1000 ~ At 900 ~ for instance, the values of 0 have been reported to range from 90 to 170 degY -6] The oxide surface layer formed on the molten A1, which inhibits the spreading of liquid drop, has been pointed out to be the main cause of data scatter.tt-4.7-t01 Indeed, the oxide surface layer on A1 is always thermodynamically stable under experimental conditions generally employed, because an extremely low oxygen partial pressure, for instance, lower than 10 -.4 Pa at 700 ~ ~HJ is necessary to maintain a metallic AI surface without an oxide layer. At temperatures higher than 900 ~ however, the oxide layer is knowntl.4.7.sl to be disrupted most probably by the gaseous phase formed by 4A1 (1) + AI203 (s) --~ 3A120 (v)
[1]
Other mechanisms suggested for the disruption of the oxide layer include a sharp increase in the solubility of oxide in AF3J and the stress at the A1/A1203 interface due to the thermal expansion mismatch5 s,~~ The relative consistency of the reported contact angles at high temperatures, for instance, 0 = 70 to 90 deg at 1100 oC,[~1 is thus a consequence of such disruption which provides an intimate contact between A1 and A1203.
The sessile drop can also be formed by squeezing out a molten metal through a narrow orifice and dropping it onto the substrate. It was suggested that the oxide surface layer could be disrupted when the molten A1 was squeezed through the orifice and/or upon impact with the substrate. By using this dosing method, WeirauchtV] measured the contact angles of A1 on A1203 and reported a considerably lower value than that observed by the conventional method, i.e., 0 = 47 deg at 900 ~ In addition, Camahan et aLtO2] observed a cyclic variation of contact angles at high temperatures, for instance, from 50 to 80 deg for every 10 minutes at 1350 ~ which resuited in a series of reaction rings. The decrease of 0 was explained in terms of Reaction [1], particularly at the rim of the drop, while subsequent increase was attributed to the contraction of AI drop due to the evaporationY .121 On the other hand, the change in A1203 surface structure can also affect the wettability of the A1-A1203 system. The structural change of sapphire surface in vacuum has been observed by low-energy electron diffraction, and the transformation temperature has varied from 1000 ~ to 1400 ~ depending on the experimental conditions e
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