CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 40 (2014) 6211–6217 The influence of in-situ formation of hibonite on the properties of zirconia toughened alumina (ZTA) composites Zhwan Dilshad Ibrahim Sktani a , Ahmad Zahirani Ahmad Azhar b , Mani Maran Ratnam c , Zainal Arifin Ahmad a,n a Structural Materials Niche Area, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia b Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia, 50728 Gombak, Selangor, Malaysia c School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Received 27 September 2013; received in revised form 16 November 2013; accepted 16 November 2013 Available online 3 December 2013 Abstract This paper reports an investigation on the impact of in-situ formation of CaAl 12 O 19 (hibonite) on the microstructure and mechanical properties of zirconia-toughened-alumina (ZTA). Various amounts of CaCO 3 (0–13 wt%) were added to ZTA to form CaAl 12 O 19 . Samples were sintered at 800 1C for 4 h to obtain CaO from decomposition of CaCO 3 , and then at 1600 1C for 4 h to produce elongated CaAl 12 O 19 grains. Analyses of samples were done using XRD and FESEM. The higher the amount of CaCO 3 added, the higher the amounts of CaAl 12 O 19 and pores observed. The toughness value increased with the increase of CaAl 12 O 19 for a critical range, and subsequently decreased; the reverse trend was observed for the hardness and density results. The sample with 0.5 wt% CaCO 3 addition produced the highest toughness (6.3 MPa m 1/2 ), reasonable hardness (1568.6 HV), density (4.11 g/cm 3 ), and porosity (1.29%) values. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: ZTA; Hibonite (CaAl 12 O 19 ); In-situ formation; Fracture toughness 1. Introduction Alumina (Al 2 O 3 ) based ceramics are good candidates for cutting tool applications owing to their excellent properties such as high hot hardness, good corrosion resistance, high insulation, process flexibility, and high chemical resistance. However, their application as cutting tools is restricted due to their brittleness [1–9]. Therefore, it is necessary to improve this property so as to increase their fracture resistance and toughness. Since the 1980s, there has been great interest in the toughening of Al 2 O 3 -based ceramics via in-situ formation of a second phase during the sintering process. These in-situ products contribute to a highly anisotropic growth habit and toughen the ceramic compo- sites. The in-situ method has many advantages compared to the method of adding second phases into Al 2 O 3 -based matrices. The advantages include lower cost and ease of processing to avoid the use of complicated methods like hot pressing, isostatic pressing, and colloidal processing. Besides that, it also reduces hazards and promotes a more practical way of achieving elongated grains [10–16]. In-situ formation of elongated grains toughening has been successfully applied for Al 2 O 3 -based ceramics in previous works [10–19]; the results show that it is an effective way to achieve better toughness and mechanical properties. The formation of elongated grains or secondary phases in the microstructure improves fracture toughness of Al 2 O 3 -based cera- mics due to crack bridging mechanism [11,16,20–22] and crack deflection mechanism [1,6,14,23] or both [12, 15,17,18,24]. Hexaaluminates are best examples of in-situ toughened materials. They are obtained from reactions between oxides (or after calcinations from carbonates, nitrates, etc.) and Al 2 O 3 [1,16,20,23–25]. Oungkulsolmongkol et al. [1] produced elongated grains of strontium hexaaluminate (SrAl 12 O 19 ) with good mechanical properties from the reaction between SrO and Al 2 O 3 . Chen et al. [11] discussed in-situ formation of many www.elsevier.com/locate/ceramint 0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.076 n Corresponding author. Tel.: þ60 45996127; fax: þ 60 45941011. E-mail address: zainal@eng.usm.my (Z.A. Ahmad).