Continuous measurements of load-penetration curves with spherical microindenters and the estimation of mechanical properties

Elastic and plastic properties of metals and Young's modulus of ceramics are determined in the microindentation regime by continuous measurements of load versus depth of penetration with spherical indenters. Calibration procedures, usually applied in nanoindentation experiments, are not needed in the microregime where spherical indenters (rather than sharp indenters with microscopical spherical tips) can be manufactured. As indenters of larger diameters are used, the elastic response of the specimen can be probed during the loading stage of the indentation tests (and not only during unloading, as is the case with nanoindenters). Hence, an accurate determination of Young's modulus can be achieved without a prior knowledge of possible “piling up” or “sinking in” which may occur at the perimeter of the contact area. The contact response of materials is shown to undergo four distinct regions: (i) pre-Hertzian regime, (ii) Hertzian regime, (iii) small-scale plasticity, and (iv) large-scale plasticity. A general methodology for estimation of yield strength and hardening exponent of metals is proposed in the last regime.