High temperature bending creep behavior of a multi-cation doped a/b-SiAlON composite Alper Uludag ˘ * , Dilek Turan Anadolu University, School of Civil Aviation, Iki Eylul Campus, 26470 Eskisehir, Turkey Received 2 August 2010; received in revised form 20 August 2010; accepted 29 October 2010 Available online 8 January 2011 Abstract SiAlON ceramics with high hardness and high toughness can be made through designing a/b-SiAlON composites. An important advantage of a-SiAlON phase is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of a-SiAlON phase during sintering. But, the thermal stability of the a-SiAlON phase is an important concern for a/b-SiAlON composites especially at high temperatures. The use of different types of single or multiple cations during fabrication directly affects resultant microstructures and mechanical behavior of a/b-SiAlON composites. In this study, the creep behavior of a multi-cation (Y, Sm and Ca) doped a/b-SiAlON composite, in which aluminum-containing nitrogen melilite solid solution phase was designed as intergranular phase, was investigated by four-point bending creep tests under stresses from 50 to 150 MPa and at temperatures from 1300 8C to 1400 8C in air. The stress exponent was determined to be 1.6  0.13 at 1400 8C and the creep activation energy was calculated to be 692  37 kJ/mol 1 . Grain boundary sliding coupled with diffusion was identified as the rate-controlling creep mechanism for the a/b-SiAlON composite. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Creep; a/b-SiAlON; Aluminum-containing nitrogen melilite 1. Introduction Silicon nitride (Si 3 N 4 ) is one of the major structural ceramics that has been developed following many years of intensive research. It possesses high flexural strength, high fracture resistance, good creep resistance, high hardness and excellent wear resistance [1]. These excellent properties offer great potential in structural applications [2]. On the other hand, other important structural material SiAlONs generally consist of two crystalline phases: a-SiAlON and b-SiAlON which are isostructural with a-Si 3 N 4 and b-Si 3 N 4 , respectively [3]. An important advantage of a-SiAlONs is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of a-SiAlON phase during sintering. As a-SiAlON and b-SiAlON phases are completely compatible, SiAlON ceramics with both higher hardness and higher toughness can be achieved through designing of a/b- SiAlON composites [4]. Meanwhile, the combination of SiAlON phases has a great influence on mechanical properties of SiAlON ceramics. Due to the different microstructure and composition, a and b-SiAlON have distinct contribution to creep resistance [5,6]. Klemm et al. pointed out that creep resistance was enhanced with the increase of a-SiAlON content in the study of four point bending creep of a/b-SiAlON composite using YAG as intergranular phase. They attributed the improved creep resistance to the thinner grain boundaries between a-SiAlON grains and a strong creep resistance of skeleton [5]. The two common methods for improving high-temperature properties of Si 3 N 4 based ceramics are (i) increasing the softening point of the amorphous phases above the temperature range to which the ceramic will be exposed and (ii) crystallization of the amorphous phases, so as to eliminate the softening point entirely [2,7]. Therefore, one of the main aspects in designing SiAlONs with improved high temperature properties would be to use additives that would provide a good liquid phase sintering behavior and at the same time would crystallize well to refractory phases after sintering. In this study, Y–Sm–Ca multi cation system was chosen to produce hard and www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 37 (2011) 921–926 * Corresponding author. E-mail addresses: alperuludag@anadolu.edu.tr (A. Uludag ˘), dtetik@anadolu.edu.tr (D. Turan). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.11.012