Mechanical properties of thin alloy films: Ultramicrohardness and internal stress

On a series of Ag-Al, Ag-Au, Ag-Cu, Au-Cu, and Au-Fe thin films, made by vapor deposition, both ultramicrohardness and internal stress were determined as a function of the chemical composition of the deposits. In order to interpret these results microstructural investigations were performed using x-ray diffraction and transmission electron microscopy, supplemented by electrical resistivity measurements. The observed data on hardness and internal stress can be well understood with the help of the metastable phase diagram of the alloy films. In the case of single-phase solid solutions, internal stresses are tensile. Compound formation, phase decomposition, or ordering are accompanied with dramatic changes of the internal stresses. With indentation experiments at very low loads the hardness-concentration dependences were observed in the case of films having a thickness of 1 μm only. Metallic films in general are characterized by a very high defect concentration. Moreover, a dramatic reduction of the average grain size was observed upon alloying. The resulting hardness increase may be understood in terms of a mechanism implying the blocking of the motions of dislocations.