Influence of surface roughness on the wetting angle
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In this paper the influence of surface roughness on contact angles in the system of liquid Al wetting solid surfaces of AI2O3 has been studied. It was observed that contact angles of liquid Al vary significantly on different rough surfaces of A12O3. A model is proposed to correlate contact angles with conventional roughness measurements and wavelengths by assuming a cosine profile of rough grooves with a Gaussian distribution of amplitudes. In comparison with the experimental results, the model provides a good estimate for describing the influence of surface roughness on contact angles of liquid Al on A12O3.
I. INTRODUCTION Interface strength between metal and ceramic may be significantly influenced by the wetting behavior on rough surfaces. Rough surfaces may increase the interface area which contributes to an increase of interface strength. On the other hand, a rough surface might not be completely wetted because of poor wetting and sharp grooves. These unwetted parts of the interface may be one of the main reasons for reducing the interface strength. An accurate measurement of contact angle is important, as the interface energy and the work of adhesion are related to it.1 However, the contact angle of liquid Al on A12O3 seems to vary significantly for different measurements.2 The commonly accepted argument for this inconsistency points to the influence of surface oxide layers around liquid Al. Differences in atmosphere or vacuum during wetting experiments contribute to the formation of the oxide layer. However, the deviations of contact angles still varied considerably in some experiments although the vacuum conditions were nearly the same.2 Indeed, A12O3 is very hard and difficult to be polished smoothly and since it is usually prepared by sintering, it is porous as well. In that sense the surface of A12O3 is intrinsically rough, an aspect that might affect the experimental values of the contact angle substantially. Despite its importance for (laser) processing and mechanical behavior, effects of surface roughness on the contact angle have not been sufficiently emphasized in the field of metal-ceramic interfaces. On the other hand, in the study of liquid polymer wetting on a solid, investigations on the effect of surface roughness have been carried out since 1936.3~5 According to a thermodynamic analysis, Wenzel3 derived the following formula: cos 6 = D cos 6th, where D is the roughness parameter which is defined as the average area ratio of real attached interface to its pro1984
J. Mater. Res., Vol. 10, No. 8, Aug 1995
http://journals.cambridge.org
Downloaded: 14 Jul 2014
jected part. In his treatment, the rough surface was supposed to be completely wetted by a liquid drop at the interface. However, if the rough surface consists of a sharp groove, the interface will be partly wetted. In addition, Cassie and Baxter6 derived another equation: cos 0 = (1 - F)cos 0 th - F, where F is the area fraction of an uncontacted solid-liquid interface on solid. However, the equations of Wenzel, Cassie, and Baxter have
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