Available online at www.sciencedirect.com Journal of the European Ceramic Society 30 (2010) 1027–1034 Characterization of pop-in phenomena and indentation modu polycrystalline ZrB 2 ceramic Stefano Guicciardi ∗ , Cesare Melandri, Frederic Tullio Monteverde ISTEC-CNR, Via Granarolo 64, I-48018 Faenza (RA), Italy Received 13 May 2009; received in revised form 2 October 2009; accepted 20 October 2009 Available online 20 November 2009 Abstract Low-load nanoindentation tests were carried out on a polycrystalline ZrB 2 -based ceramic. Pop-in phenomena were observed when inden marks were placed in the interior of the ZrB 2 grains. Both pop-in loads and pop-in extents were statistically distributed with a mutu correlation. The critical shear stresses at pop-in were in good agreement with the theoretical shear strength of ZrB 2 . The experimental pop-in extents were also compared to a simplified model developed for homogeneous dislocation nucleation. The influence of the indentation modulus was derived from the model of Delafargue and Ulm (2004) 50 and compared to the experimental results. Some results definitely influenced by the polycrystalline structure of the investigated ceramic. © 2009 Elsevier Ltd. All rights reserved. Keywords: Mechanical properties; Plasticity; Nanoindentation; Borides 1. Introduction Zirconium diboride (ZrB 2 ) is a refractory ceramic belong- ing to the Ultra High Temperature Ceramics (UHTC) family currently under study foraerospace applications. 1,2 To the authors’ knowledge only one paper has been published on the nanoindentation behaviour of this material. 3 Nanoindenta- tion tests are of paramount importance in order to characterize the mechanical behaviour of a material on a very small scale when only tiny portions of volume are stresses as it is the case in applications involving wear or contact. During a series of low-load nanoindentation tests on a ZrB 2 ceramic we have recently observed that the load–displacement curves displayed a sudden pop-in at the beginning of loading. This kind of phe- nomenon has been noted in metals, 4–7 semi-conductors 8–10 and ceramics. 11–16 Many studies have reported that before the pop- in the unloading curve would superimpose to the loading curve indicating that till that point no dissipative mechanisms, i.e. plas- ticity,took place. After the pop-in, instead, hysteresis in the loading–unloading cycle appeared. In an aluminium film, in situ ∗ Corresponding author. Tel.: +39 0546 699720; fax: +39 0546 46381. E-mail address: stefano.guicciardi@istec.cnr.it (S. Guicciardi). TEM analysis has clearly shown the appearance of disloc in correspondence of the pop-in. 17 The most widely accepted explanation of pop-in is homogeneous dislocation nuclea 18 This is also supported by atomistic simulations of indent in perfect crystals. 19,20 There are however some experimen- taland theoretical evidences that this event can be triggered by heterogeneous dislocation nucleation when the mater face presents surface steps or ledges. 21,22 Other experimental results seem instead to indicate a stress-rate dependent nisms. For a 4H SiC, a direct relationship was found betw the stress-rate and the pop-in load 23,24 butthe opposite was observed on specific crystallographic planes for sapphire. 25 However, experimental results have been presented in which the load at pop-in was shown to be independent on the lo ing rate in several metals and semi-conductors. 9,26 Finally, non-dislocation driven explanations were recently propo pop-in phenomena such as phase transformation in GaAs 27,28 and twinning in sapphire. 25,29 One of the main support to the theory of homogeneous dislocation nucleation is that the shearstress at which pop-in occurs is usually in very good agreement with the theoretical shear strength of the indented material. Though the form in which engineering materia most often employed is polycrystalline, the main part of ies on incipient plasticity was focused on monocrystals t 0955-2219/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2009.10.014