A comparison study on the R-curve behavior of alumina/aluminum titanate composites prepared with different TiO 2 powders M. Sobhani a,⇑ , T. Ebadzadeh b , M.R. Rahimipour b a Faculty of Materials & Metallurgical Engineering, Semnan University, Semnan, Iran b Ceramic Department of Materials and Energy Research Center, Alborz, Iran article info Article history: Available online xxxx Keywords: R-curve Alumina toughening Nano composites Aluminum titanate abstract Rising crack growth resistance (R-curve) behavior of nano and micro alumina/20 wt.% aluminum titanate composites have been compared. Preparation of the composites was made using reaction sintering of the alumina and titania powders. Nano and micro sized TiO 2 powders were used in achieving the Al 2 TiO 5 as secondary phase. The toughness curves of the composites were determined using indentation-fracture method. Both of the micro and nano composites show a significant rising of toughness in comparison with the monolithic alumina. The K 1C value increased from the initial value of 2.6 MPa p m to 4.5, 5.1 and 5.5 MPa p m at the maximum load (P) for alumina, micro and nano composites, respectively. The microstructural observations demonstrated that the bridging and microcracking are the main toughening mechanisms in the alumina/aluminium titanate composites. Bridging mechanisms in the micro and microcracking phenomenon in the nano composites have the major responsibility of the R-curve behavior. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Flaw sensitivity of alumina (Al 2 O 3 ) based ceramics during the mechanical bearing is the main limitation of its widespread appli- cation. The main toughening mechanism can be basically divided into intrinsic and extrinsic methods. Intrinsic methods include resistance mechanisms to fracture developed ahead of the crack tip [1]. In fact intrinsic rising toughness techniques can enhance the initial critical crack length and then the final fracture strength, but the brittle fracture remains yet, without any permission to stable crack growing. The extrinsic toughening mechanisms make the crack-tip shielding which are largely active behind the crack tip during crack growth [2,3]. Therefore the extrinsic mechanism will lead to resistance-curve (R-curve) toughening behavior and is responsible to stable crack growth during fracture that is well known as flaw tolerance behavior and reduced sensitivity of the strength to the size of any processing or structural applications induced cracks [4]. Microcrack toughening and crack bridging mechanisms are the most important non-transformation toughen- ing mechanisms that can motivate the R-curve behavior [4–6]. Fortunately, aluminum titanate (Al 2 TiO 5 ) as a secondary phase for alumina matrix composites has a high anisotropic thermal expansion coefficient (TEC) in comparison with the limited anisotropy behavior of alumina [7]. Also decomposition of Al 2 TiO 5 to TiO 2 and alumina in the temperature range of 900–1200 °C, as the main problem of aluminum titanate, have been resolved by addition of Mg 2+ or Fe 3+ cations [8–10]. However, as a result of the thermal expansion mismatch between alumina and Al 2 TiO 5 grains compressive or tensile stresses, depending on the particular crystallographic orientation of the grains, would develop during cooling from the sintering temperature at the grain–matrix inter- faces. Considering the grain boundary directions and Al 2 TiO 5 grain size spontaneous microcracking might occur during cooling [11]. Therefore pullout of the released grains during crack-tip opening under tensile stress make bridging mechanism of the toughness enhancement by R-curve behavior [3,12]. The main objective of the present study is the improvement in the R-curve behavior of the alumina-20 wt.% aluminum titanate with a different source of the TiO 2 powders. Regarding to the previous study the minimum critical grain size (D cri ) of the Al 2 TiO 5 to spontaneous microcrack- ing decreased when the Al 2 TiO 5 grains abutted by alumina grains [11]. Hence, the size of the aluminum titanate grains was kept lower than the D cri using nanosized TiO 2 powders and its fracture toughness behavior has been compared with the micronized TiO 2 . 2. Experimental procedures The starting materials were a-Al 2 O 3 (MR70 with d 50 = 0.7 lm), nanosized TiO 2 powder (P25, Degussa-Evonik, Germany) with an http://dx.doi.org/10.1016/j.tafmec.2016.01.005 0167-8442/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +98 2333349976; fax: +98 2333654119. E-mail address: m.sobhani@semnan.ac.ir (M. Sobhani). Theoretical and Applied Fracture Mechanics xxx (2016) xxx–xxx Contents lists available at ScienceDirect Theoretical and Applied Fracture Mechanics journal homepage: www.elsevier.com/locate/tafmec Please cite this article in press as: M. Sobhani et al., A comparison study on the R-curve behavior of alumina/aluminum titanate composites prepared with different TiO 2 powders, Theor. Appl. Fract. Mech. (2016), http://dx.doi.org/10.1016/j.tafmec.2016.01.005