CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 39 (2013) 7869–7877 Effect of YF 3 on the phase stability and sinterability of hydroxyapatite—Partially stabilized zirconia composites Anıl Aykul a , Isil Kutbay a , Zafer Evis b , Metin Usta a,n a Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze, Kocaeli 41400, Turkey b Department of Engineering Sciences, Middle East Technical University, Ankara 06800, Turkey Received 23 January 2013; received in revised form 13 March 2013; accepted 14 March 2013 Available online 26 March 2013 Abstract Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF 3 and HA–PSZ composite containing 5 wt% PSZ without YF 3 were sintered in air at 900 1C, 1100 1C and 1300 1C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF 3 showed desirable thermal stability below 1300 1C and besides various amounts of CaZrO 3 , any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 1C, composites with YF 3 showed higher thermal stability than the composites without YF 3 . On the other hand, pure HA started to decompose and TCP was observed at 1300 1C. Composites with YF 3 showed improved thermal stability than the composite containing 5 wt% PSZ without YF 3 and pure HA at lower sintering temperatures such as 900 1C and 1100 1C. However, it was observed that the increasing amount of YF 3 addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF 3 showed the highest relative density among all of the composites with or without YF 3 . & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sintering; C. Thermal properties; D. Apatite; D. YF 3 1. Introduction Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HA) is a biocompatible and bioactive ceramic [1–4]. HA has an efficient influence on repairing damaged bone because of its similarity to inorganic part of bone [5–7]. However, there are several restrictions for the usage of HA due to its poor mechanical properties [8–13]. Various methods [7,10,12–17] were used to improve the mechanical properties of HA. Among these methods, reinfor- cing of HA with another ceramic [14–16,18–20] such as zirconia (ZrO 2 ) which has high strength and fracture tough- ness, is a promising method [2,3,5,6,8–10,17,21–25]. How- ever, there are some restrictions for reinforcing HA with ZrO 2 . One of them could be the polymorphic transformation of ZrO 2 from tetragonal to monoclinic phase during the cooling process of sintered HA–ZrO 2 composites. This phase transformation mechanism introduces large volume change into composite matrix. This change is sufficient to exceed elastic and fracture limits, and consequently to cause cracking of the ceramics [17]. Therefore, additives such as CaO, MgO and Y 2 O 3 can be dissolved in ZrO 2 to minimize this phase transformation that can cause large volume change. When 8% Y 2 O 3 was added to ZrO 2 , fully stabilized zirconia was obtained instead of mono- clinic phase at room temperature (RT). However, when the lower amount of Y 2 O 3 exists in ZrO 2 , partially stabilized zirconia (PSZ) occurs at RT. In contrast, small degree of this volume expansion provides higher fracture toughness, which is called transformation toughening mechanism [2]. According to previous works, it was found that the excellent mechanical properties were obtained for approximately 3% addition of Y 2 O 3 into ZrO 2 [26]. HA decomposes to β-TCP approximately at 1200 1C and α-TCP at  1300 1C as seen in Reaction 1 [27–30]: Ca 10 (PO 4 ) 6 (OH) 2(1-x) -3Ca 3 (PO 4 ) 2 +CaO+(1-x)H 2 O (1) Decomposition of HA seen in Reaction 1 takes place well below 1300 1C and HA starts to decompose to β-TCP in the range of 1100–1150 1C and α-TCP in the range of 1150– 1250 1C in the presence of ZrO 2 [12,31,32]. When bioinert ZrO 2 is added to HA as a reinforcing compound, it can 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.2013.03.048 n Corresponding author. Tel.: +90 262 605 2655. E-mail address: ustam@gyte.edu.tr (M. Usta).