Mechanical and viscoelastic properties of materials by instrumented indentation.

Abstract

The mechanical properties of small volumes of material have received increasing attention in the past decade due to the extensive use of films and coatings in microelectronic devices and as protection against wear and corrosion. The mechanical properties of thin films of a given material are often substantially different from those of the identical bulk material. Instrumented, or ultra-low load, indentation instruments are capable of measuring the elastic, plastic and time-dependent properties of small volumes of materials. A two stage area model has been developed to predict the variation in measured hardness with depth of penetration for soft films on hard substrates. The model is able to predict the variation for depths of penetration less than and greater than the film thickness. The model incorporates constraints on the film hardness based on the uniaxial compression of a flat cylindrical disk. Friction, or adhesion, at the film/substrate interface causes the film hardness to increase as the depth of penetration increases. However, the film hardness is not allowed to increase beyond the substrate hardness. The model is compared to experimental hardness versus depth data for three different film/substrate systems with different levels of adhesion. Time-dependent properties of materials are obtainable from instrumented indentation tests by measuring the force and displacement as a function of time. Indentation creep experiments on a linear viscoelastic material, amorphous selenium, were used to establish the relationship between indentation strain rate and the strain rate measured in conventional creep tests. Equations for determining viscosity from indentation tests were also obtained. Finally, it was shown that stress relaxation functions for viscoelastic materials may be obtainable from indentation creep experiments.