Journal of The Australian Ceramic Society Volume 52[1], 2016, 167 – 176 167 Influence of Combined CaO and CaCO 3 Additions on the Microstructure and Properties of ZTA Zhwan Dilshad Ibrahim Sktani 1 , Mani Maran Ratnam 2 and Zainal Arifin Ahmad 1,* 1) Structural Materials Niche Area, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 2) School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia E-mail address: srzainal@usm.my Availabale online at: www.austceram.com/ACS-Journal ABSTRACT The aim of this investigation is to optimize the combined effects of CaO and CaCO 3 additions on the mechanical properties and microstructure of ZTA. Previous work shows that addition of CaCO 3 alone led to in-situ formation of hibonite (CaAl 12 O 19 ) that improves ZTA fracture toughness. However, the hardness declined due to excessive porosity. Therefore, in the current investigation, CaO is added together with CaCO 3 into ZTA samples. ZTA samples were fully sintered at 1600 °C for 4 hours to ensure CaAl 12 O 19 formation. The XRD analysis showed more CaAl 12 O 19 formation while the FESEM showed a denser microstructure and closer to fully equiaxed CaAl 12 O 19 grains. The Vickers hardness of ZTA samples was improved due to the porosity reduction of the samples. Maximum hardness (1627 HV) is obtained from ZTA with the addition of CaO only. The fracture toughness is almost stable and it is affected by the amount and shape of CaAl 12 O 19 grains. Keywords: ZTA, CaAl 12 O 19 (Hibonite), Equiaxed grains, Porosity, Density, Vickers Hardness, Fracture Toughness. 1. INTRODUCTION ZTA is more preferable to pure alumina (Al 2 O 3 ) in cutting tool insert applications due to the better toughness, good wear resistance and chemical stability [1–11]. The superior mechanical properties of ZTA compared to pure Al 2 O 3 is governed by the effect of stress induced phase transformation toughened by the t-ZrO 2 phase [12,13]. However, due to brittle nature of these ZTA cutting inserts, an effort is needed to increase its toughness and reliability for machining operation. Therefore, researchers [9,14– 18] have tried to further improve ZTA toughness by in-situ formation of elongated grains through addition of many oxides (or carbonates or nitrides) during sintering process. The in-situ formation of elongated grains is preferred because of its freedom from health hazard and ease of processing, which is free from additional complex processes such as hot press and isostatic pressing. [19] In-situ formation of elongated grains is a cost efficient way to obtain better toughness [16,18–21]. The mechanism is either attributed to crack deflection mechanism [16,22–27] or crack bridging mechanism [7,28,29] or both [19,30–32]. For example, in situ formation of elongated CaAl 12 O 19 grains was successfully applied to improve Al 2 O 3 based ceramics’ toughness [24,33– 35]. To obtain CaAl 12 O 19 , either CaCO 3 or CaO has to be added to Al 2 O 3 based matrix [33–35]. Our previous research [36] found that fracture toughness of ZTA was significantly improved through the in- situ formation of CaAl 12 O 19 by using CaCO 3 . However, in-situ formation of CaAl 12 O 19 by using CaCO 3 was always accompanied with CO 2 emission according to Equation 1 during sintering process [24,29,37]. The emission of CO 2 creates pores in the ZTA microstructure thereby considerably reducing the hardness and limiting its application as cutting inserts [36]. CaCO 3 CaO + CO 2 (1) Therefore, to avoid the excessive pores formation, CaO can be used instead of CaCO 3 . However, CaO alone could not form perfectly elongated grains of CaAl 12 O 19. For example Nagaota et al. [34] found only little improvement of fracture toughness when CaO alone was added into Al 2 O 3 based ceramics. Therefore, to ensure the formation of CaAl 12 O 19