TiO 2 additives for in situ formation of toughened silicon nitride-based composites Ren-Guan Duan, Gert Roebben, Jef Vleugels, Omer Van der Biest * Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, Heverlee, B-3001 Louvain, Belgium Received 24 June 2002; received in revised form 4 March 2003; accepted 5 March 2003 Abstract This paper is focussed on the reinforcing phases developing in a Si 3 N 4 -based composite microstructure when sintering a powder mixture of 74 wt.% Si 3 N 4 , 20 wt.% TiO 2 , and 6 wt.% Y 2 O 3 . Micron- and submicron-sized as well as Al 2 O 3 and Al 2 O 3 + SiO 2 -coated submicron-sized TiO 2 powders were used to prepare these in situ composites consisting of Si 2 N 2 O, a-Si 3 N 4 , h-Si 3 N 4 , and TiN. The impulse excitation technique was used to evaluate the intergranular glass phase, which affects crack growth and high-temperature deformation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to investigate the reinforcing phases. The influence of the TiO 2 -additive powder characteristics on the microstructure and properties of the ceramics is reported. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon nitride; In situ composites; TiO 2 additive; Grain boundaries; Mechanical properties; Sintering 1. Introduction The properties of silicon nitride can be improved for particular applications by incorporating particles of a different phase. A proper dispersion of elon- gated Si 2 N 2 O grains can toughen silicon nitride ceramics [1,2]. To obtain a combination of high hardness, good fracture toughness, and low electri- cal resistivity, Si 3 N 4 –TiN composites have been investigated extensively [3–7] . An interesting method to obtain these particles is in situ synthesis [4], which enables the production of composites at low cost, with desirable microstructures and enhanced sinterability [8]. Despite extensive investigations on the in situ preparation of Si 3 N 4 –TiN and Si 3 N 4 –Si 2 N 2 O com- posites [3–7], synthesis of Si 3 N 4 –Si 2 N 2 O–TiN ceramics by means of in situ chemical reaction of TiO 2 and Si 3 N 4 is not yet explored. This paper is focussed on the preparation and characterisation of Si 3 N 4 -based composites with finely dispersed hard TiN and elongated Si 2 N 2 O grains. The influence of the TiO 2 additive powder characteristics on the microstructure and properties of these ceramic com- posites is discussed. 0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00282-9 * Corresponding author. Tel.: +32-16-321264; fax: +32-16- 321992. E-mail address: omer.vanderbiest@mtm.kuleuven.ac.be (O. Van der Biest). www.elsevier.com/locate/matlet Materials Letters 57 (2003) 4156 – 4161