THE EFFECT OF TIP RADIUS ON NANO-INDENTATION HARDNESS TESTS

Sub-micron indentation hardness tests on silicon and aluminum, both in their bulk forms and in layer-substrate combinations, are simulated by the finite element method using a blunt indenter. The blunt indenter is modeled by a cone with a spherical cap with various radii. Emphasis is placed on the effect of the tip radius on the hardness tests. Two methods for the determination of the contact area are employed: one measures the contact area from the residual indentation; the other uses the plastic depth and the area-depth relationship of the indenter. Indentation tests on bulk materials show weak tip and indentation size effects. For an aluminum layer on a silicon substrate, the hardness is relatively insensitive to the tip radius. On the contrary, for a silicon layer on an aluminum substrate, the hardness is much more sensitive to the tip radius. In order to obtain the hardness of this layer-substrate combination without the influence from the substrate, the tip radius to layer thickness ratio should be less than 1.25.