Atomic Behavior of Oxygen Adsorption and Its Effect on the Mitigation of Metallic Adhesion

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A field ion microscope (FIM) has been used to investigate the behavior of oxygen adsorption on W-tip, and extended to the study of the effect of oxygen adsorption on adhesion of W to Au. The adsorption of oxygen on the central (011) was found to be less than those on the other planes with lower work functions. Owing to the highest work function of the [110], the adsorption occurred as the last site. Field emission current measurement and Auger electron spectroscopy on the adsorption of oxygen on tungsten showed that the coverage is unity at an exposure of 200 Langmuirs (L). An adhesion contact in ultrahigh vacuum of the FIM was carried out using a W-tip either with or without oxygen adsorption. It was found that adhesion force decreased with increasing oxygen exposures up to 60 L. With an oxygen exposure of 16 L, an intermediate adhesion resulted. Adhesion is thus dependent upon the coverage of oxygen. The depth of adhesion-induced lattice defects was studied using the field evaporation technique. Over 200 L, adhesion force was not detectable, nor was distortion of W atoms. The results obtained in this study could be extended to the problems involved in space tribology, especially to the mitigation of friction and wear of machine elements which function at an altitude of 500 km where oxygen exists.