The influence of percolation during pulsed electric current sintering of ZrO 2 –TiN powder compacts with varying TiN content K. Vanmeensel a , A. Laptev b , O. Van der Biest a , J. Vleugels a, * a Department of Metallurgy and Materials Engineering (MTM), K.U. Leuven, Kasteelpark Arenberg 44, B-3001, Heverlee, Belgium b Department of Mechanical Engineering, Donbass State Engineering Academy, 72, Shkadinova Street 72, UA-84313, Kramatorsk, Ukraine Received 14 June 2006; received in revised form 13 October 2006; accepted 22 October 2006 Available online 8 January 2007 Abstract A series of ZrO 2 –TiN composite powder compacts with varying TiN content was densified using the field assisted sintering technique, also known as spark plasma sintering or pulsed electric current sintering (PECS). The TiN content was varied between 35 and 90 vol.% in order to obtain an electrical conductive composite material that can be shaped by electrical discharge machining. The influence of the TiN content on the densification behaviour was investigated experimentally, whereas its influence on the temperature and current dis- tribution in the PECS tool set-up was simulated using a previously developed finite element model. The predicted temperature distribu- tion was confirmed experimentally using a double pyrometer set-up, one focusing on the outer die wall surface and one on the bottom of a borehole in the upper punch. The changing thermal and electrical properties of the sintering ZrO 2 –TiN powder compacts were calcu- lated using mixture rules. Using a double pyrometer set-up, a clear relationship could be verified experimentally between the changing electrical properties of the sintering compact and the temperature redistribution in the punch/die/sample set-up during the PECS process. The homogeneity of sintering inside the PECS equipment is discussed in detail and suggestions are made in order to promote a more homogeneous sintering process. Carbon felt, acting as a thermal insulator, was placed around the die in order to minimize the radiation heat losses and to minimize the thermal gradients during heating and the dwell period at maximum temperature. The mechanical and electrical properties of the different composite materials are discussed as functions of the TiN content. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Pulsed electric current sintering (PECS); Spark plasma sintering (SPS); Ceramic composites; Thermo-electrical properties; Temperature and current distribution 1. Introduction During the last decade, the applicability of zirconia to induce toughening by the stress-induced transformation of the tetragonal to monoclinic ZrO 2 phase in the stress field of propagating cracks, a phenomenon known as trans- formation toughening [3,4], has been intensively investi- gated. Recent developments in zirconia composites are focused not only on the improvement of toughness, strength and hardness, but also on the possibility for mass production and manufacturing cost reduction. A successful approach is to incorporate electrically conductive rein- forcements such as TiB 2 [5], WC [6], ZrB 2 [7], TiC [8], TiCN [8] and TiN [11] into the zirconia matrix. The incor- poration of a certain content of these conductive reinforce- ments makes the composite electrically conductive enough to be machineable by electrical discharge machining (EDM), thus avoiding the expensive cutting and grinding operation for component shaping. Electrical resistivity threshold values for EDM are reported to be in between 100 and 300 X cm [9,10]. Traditionally, these composites are densified by hot pressing [11]. This study investigates the possibility to pro- cess ZrO 2 –TiN composites by PECS. In a previous study [2], it was pointed out that the changing thermal and elec- trical properties of a sintering ZrO 2 –TiN (60/40) composite powder compact should be taken into account in order to 1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.10.042 * Corresponding author. Tel.: +32 16 321244; fax: +32 16 321992. E-mail address: Jozef.Vleugels@mtm.kuleuven.be (J. Vleugels). www.actamat-journals.com Acta Materialia 55 (2007) 1801–1811