Structure-Property Relationship of Ceramic Coatings Produced by Laser Processing
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de-wetting process due to the formation of a more stable ceramic at interface. However, such an increment in contact angle has to the best of our knowledge never been observed experimentally [1-3,7-10]. In the treatment of reactive wetting, Aksay et al [6] and Laurent et al [7] attempted to introduce the negative free energy into the Young's equation of wetting. However, in principle the use of interface energy is valid only for an equilibrium state of wetting, but not for a dynamic situation like a reactive wetting, where the interface tension should be used which, of course, is not always equal to the interface energy . In summary, the scientific literature is still confusing and rather puzzling about the effect of chemical reaction on wetting with the mechanism of reactive wetting not yet established [11,12]. This paper attempts to unravel the confusion by introducing the effects volume changes may have on wetting phenomena. Here we report two wetting experiments: of liquid Al on Si0 2 and of liquid Ti on A120 3. According to the experimental results, a reactive wetting mechanism has been proposed, which can explain relevant results in literature. In addition some results of laser melt injection of SiO 2 particles into molten Al are presented [13-15]. EXPERIMENTS The surface roughness of the A120 3 substrate is about 0.01 gtm and the wavelength is 5 jtm measured by using a Stylus size of 5 gtm. The phase of A1203 is ox-Al 20 3 as determined by X-ray diffraction. The wetting experiments of Al on SiO 2 (quartz glass) and on Al 20 3 were 537 Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
done at 1023 K for 2 hours, 1123 K for 2 hours and at 1223 K for 1 hour in vacuo of 10' Pa. The wetting experiment of Ti on A120 3 was processed at about 1973 K for 5 minutes by Ar shielding. The contact angle was measured from the drop profile. Further, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were applied to study the surface and interface structures between metals and ceramics. RESULTS Table 1 lists the measured contact angles. The contact angle of liquid Al on Si0 2 is about the same as that of liquid Al on A120 3 at 1023 K, but much lower at higher temperature of 1223 K. The larger deviation in contact angles of liquid Al on Si0 2 is due to the non-spherical shape of the Al drop. The surface of the Al drop after reaction contains many Si precipitates, which may prohibit the formation of a spherical shape of the liquid drop. The EDS composition analysis indicated that Si0 2 reacted with Al and transformed to Al 20 3. The reacted surface of oxide shrunk and became very rough after the reaction (Fig. 1). Many fine Si fibers have been formed on the reacted surface of substrate. In some cases, liquid Al was in contact only with the Si fibers. The cross section of the reacted Al on Si0 2 indicates that A120 3 grains are largely separated by boundary cavities which are filled later by Al and Si. Many Si precipitates exist in the Al. The contact angle of liquid Ti on A12
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