Mechanical Strength of Thin Films of Metals

The tensile strength of silver films are determined as a function of their thickness. The films are electro-deposited on the complete cylindrical surfaces of small steel rotors and the rotor speeds necessary to throw them off are determined. Both the tensile strength and the adhesion of the films are obtained by using rotors of different radii. Also, the adhesion is measured by electrodepositing the films on the rotor in circumferentially disconnected patches which eliminate the hoop stresses. In most of the measurements, the adhesion is made vanishingly small by dipping the rotors in an albumin solution before electrodepositing the film and by thermal cycling. The tensile strengths of the films thicker than about 6×${10}^{-5\mathrm{}}$ cm are found to be independent of thickness and approximately equal to that of bulk silver at the corresponding temperature. For thickness from 6×${10}^{-5\mathrm{}}$ cm to 2.5×${10}^{-5\mathrm{}}$ cm the data show some scatter, but below 2.5×${10}^{-5\mathrm{}}$ cm the tensile strength increases many-fold. The region where the tensile strength increases very rapidly is slightly dependent upon the electrodeposition current but within experimental error is independent of film temperature.