Indentation hysteresis of glassy carbon materials

Indentation hysteresis during both pointed and spherical indentation is a common feature of the observed force-displacement response of glassy carbon materials. Field and Swain proposed a method of analysis of this behaviour with spherical indenters in the form of classic indentation stress-strain curves. They also proposed that the response resulted from limited reversible slip of the graphite-like nanocrystalline structure of these so-called glassy carbon materials. The present work extends the previous study by investigating the influence of the nanocrystalline grain size and the hysteretic response with indenters of sharper apical angle. It is found that the extent of the hysteresis is dependent upon the grain size as is the contact stress for the initiation of yielding. The critical strain for the onset of non-recoverable hysteretic response is clearly identified with sharper-apical-angle indenters. This irreversibility of the hysteretic response is discussed in terms of an analysis proposed by Brown whereby the critical limit for the strain for reversible hysteretic behaviour was related to the percolation limit for plastic shear strain sites within a material.