Metallic-adhesion experiments reported in the literature have shown that softer metal couples could weld more easily than harder, and that metal couples of hexagonal atomic structure are more difficult to weld than those of cubic structure. Because of the presence of contamination in all of these studies, however, it has not been clear whether these tendencies are intrinsic properties of the metals, or simply due to a difference in the sensitivity of these materials to such contamination. Previous studies by the authors have shown that the principal barrier to adhesion between soft metals at room temperature is due solely to contamination. This work is presently extended to a metal of high hardness, a molybdenum-molybdenum adhesion couple, and to a couple of hexagonal atomic structure, titanium-titanium. Both metal couples were found to be most difficult to prepare with ultraclean surfaces; however, when this was accomplished, cold welding was observed at room temperature to approximately one-half the bulk strength of titanium and one-quarter of the bulk strength of molybdenum, respectively. The greater difficulty encountered when pressure welding harder metals or couples of hexagonal atomic structures, was shown to be due to contamination effects, and is not an inherent property of these materials. A few atomic layers of contaminant are sufficient to prevent adhesion of these metals under lightly loaded conditions. Contact resistance has been demonstrated as a useful tool for the estimation of the degree of cleanliness of the metal surfaces.
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