Effect of Oxygen on the Surface Tension of Liquid Co and Co-Fe Alloys and the Wettability of Al<SUB>2</SUB>O<SUB>3</SUB>

The surface tensions of liquid Co-O and Co-Fe-O alloys and the contact angles between liquid Co-O and Co-Fe-O alloys, and Al2O3 substrates were measured in Ar+H2+H2O mixtures at 1873 K by the sessile drop method. Solidified Co-O and Co-Fe-O alloys/Al2O3 substrates interfaces were examined by the metallographic observation and EPMA survey.Main results obtained were as follows:(1) The surface tensions of liquid Co-O and Co-Fe-O alloys decreased remarkably with addition of a small amount of oxygen, and were independent of the iron content when the oxygen content was less than about 0.06 mass%.(2) Excess surface concentration of oxygen in the surface layer of liquid Co was 17×10−6 mol/m2 in agreement with the reported values in the liquid Fe-O and Ni-O system, suggesting that the arrangements of the adsorbed oxygen on the surfaces of liquid Ni, Co and Fe were analogous to one another.(3) The wettabilities of Al2O3 by liquid Co-O and Co-Fe-O alloys increased with increasing oxygen content, especially in iron-rich alloys. This change in the wettability with the iron content is considered to be due to the difference in the affinity for oxygen between cobalt and iron.