Please cite this article in press as: Azeggagh N, et al. Hertzian contact damage in silicon nitride ceramics with different porosity contents. J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.01.031 ARTICLE IN PRESS +Model JECS-9993; No. of Pages 8 Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society xxx (2015) xxx–xxx Hertzian contact damage in silicon nitride ceramics with different porosity contents N. Azeggagh a , L. Joly-Pottuz b,∗ , D. Nélias a , J. Chevalier b , M. Omori c , T. Hashida c a Université de Lyon, INSA Lyon, LaMCoS CNRS UMR5259, F-69621 Villeurbanne, France b Université de Lyon, INSA-Lyon, MATEIS CNRS UMR5510, F-69621 Villeurbanne, France c Tohoku University, 6-6-11, Aza-Aoba, Aramaki, Aobaku, Sendai 980-8579, Japan Received 16 October 2014; received in revised form 19 January 2015; accepted 31 January 2015 Abstract This work investigates the Hertzian contact damage in Si 3 N 4 ceramics with 3 and 18% porosity processed by spark plasma sintering technique. Low densification was achieved by sintering at pressures lower than needed for a full density. The indentation tests were performed using diamond spheres of various sizes to evaluate the Hertzian contact damage at different scales. Surface damage was observed under electron microscopy while subsurface damage was examined using focused ion beam sectioning technique. Different failure modes were noticed, depending on the size of the volume solicited and the porosity content. At small scale, short cracks initiate from the existing pores then coalesce leading to a quasi-plastic failure mode. At larger scale and for high porosity content, surface ring and radial cracks usually observed for dense brittle materials vanish; the damage mode is mostly related to the fragile rupture of bridges between collapsed pores. © 2015 Elsevier Ltd. All rights reserved. Keywords: Si 3 N 4 ; Porous materials; Hertzian contact; Size effect; Contact damage 1. Introduction Dense silicon nitride materials are used to fabricate high- speed metal cutting tools, rolling element bearings and many other components for applications at low or high temperatures. 1,2 Most of the brittle materials including Si 3 N 4 contain two kinds of pores:(i) well-controlled and voluntarily introduced into the specimens by addition of a pore-forming agent to the starting powder or by decreasing the amount of sin- tering aid for applications such as catalyst supports, gas filters and biomaterials, 3,4 or (ii) uncontrolled and undesirable with random size distribution and morphology. This second type is mainly due to the processing and can considerably impact the mechanical properties of the component, 5 which in turns signif- icantly reduce its lifetime. 6 Over the last century, several techniques have been devel- oped to process Si 3 N 4 based materials from the starting mixtures ∗ Corresponding author. Tel.: +33 472437102; fax: +33 472437930. E-mail address: lucile.joly-pottuz@insa-lyon.fr (L. Joly-Pottuz). containing material powders and small amounts of rare- earth sintering elements. Nowadays, field assisted sintering technol- ogy (FAST) better known as spark plasma sintering technique (SPS) is emerging. 7–9 The advantage of this method over the conventional hot pressing and hot isostatic pressing techniques consists in sintering the starting powders at relatively lower temperatures and shorter processing times. 10 It results from the simultaneous application of a pulsed (on–off) direct current of a few thousand amperes and an uniaxial pressure during sintering. Although the considerable effort made to optimize the processing conditions (time, pressure, temperature) of this technique, final sintered specimens can present some residual porosity. Previous studies in liquid-phase-sintered alumina 11 and silicon nitride materials with 37% of porosity 12 highlighted the key role of the voids on the damage mode transition from tension driven cracks mode usually observed in dense brittle materials with fine microstructure to distributed shear and com- pression driven subsurface damage mode. 13 On the other hand, the authors reported a significant effect of the porosity on the dif- ferent mechanical properties (elastic modulus, flexural strength, hardness, etc.). To facilitate the damage observations, Hertzian http://dx.doi.org/10.1016/j.jeurceramsoc.2015.01.031 0955-2219/© 2015 Elsevier Ltd. All rights reserved.