Yield strength predictions from the plastic zone around nanocontacts

Using a simplified version of Johnson's core model analysis of the plastic zone, one may determine the plastic zone size around a contact or, alternatively, determine the yield strength by measuring the plastic zone. The theoretical model contains three parameters: indentation load, yield strength and zone size so that knowing any two gives the third. This is experimentally demonstrated for a series of single crystals (Fe-3wt%Si, tungsten, zinc) and polycrystals (1100-0 Al, copper and 2024-T6 aluminum). In addition, two of these are evaluated in several work-hardened states. Plastic zone sizes are estimated principally by atomic force microscopy and Zygo interferometry imaging with some verification by transmission electron microscopy. On the theoretical side, verification of the relationship is obtained by elastic-plastic numerical analysis of a bi-material system based upon an Fe-3wt%Si single crystal with a thin oxide film. It is shown that in general predictions are reasonable down to nanometer level contacts except for two cases where an indentation size effect may dominate. The proposed relationship is suggested to be an alternative measure of yield strength compared to the often cited value of H/3, particularly at light contacts.