Effect of interfacial species on shear strength of metal-sapphire contacts

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Abstract
The interfacial shear strength of the metal‐insulator system has been studied by means of the coefficient of static friction of copper, nickel, or gold contacts on sapphire in ultrahigh vacuum. The effect on contact strength of adsorbed oxygen, nitrogen, chlorine, and carbon monoxide on the metal surfaces is reported herein. It was found that exposures as low as 1 L of O2 on Ni produced observable increases in contact strength, whereas exposures of 3 L of Cl2 lead to a decrease in contact strength. These results imply that submonolayer concentrations of these species at the interface of a thin Ni film on Al2O3 should affect film adhesion similarly. The submonolayer concentrations of these species are present on the metal surface as chemisorbed phases, without the formation of surface films of metal compounds. Thus, chemical reaction of metal compounds (oxides, for example) with Al2O3 is excluded as a source for the increase in interfacial strength. The atomic mechanism by which these surface or interface phases affect interfacial strength is not yet understood.

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