The Effect of Tip Momentum on the Contact Stiffness and Yielding During Nanoindentation Testing

Thin films and variations in surface chemistry can cause large changes in the mechanical hardness and tribological properties of a surface. We present nanoindentation results which indicate that many of these variations are related to the mechanical deformation of atomic-size surface asperities. Our results show that the initial contact stiffness and the yield point load depends on the impact velocity of the indenter tip. This dependence was seen during nanoindentation testing of GaAs and tungsten in air, but the dependence was found to disappear when tungsten was tested in dilute HCl, thus suggesting that the velocity dependence is a strong function of surface chemistry in metallic systems. These results can be explained if the initiation of plastic yielding depends on the deformation of atomic-size surface asperities and ledges. Small changes in the mechanical properties of these surface features (elastic modulus and Burgers vector), and variations in the surface forces acting between the contacting bodies, will affect the generation of defects at the surface. For surface asperities with high mechanical strength the action of surface forces alone may not be sufficient to initiate yielding at the surface. However, a small increase in the impact velocity may partially deform the very smallest asperities, so permitting the action of surface forces to initiate plastic deformation on contact.