journal of materials processing technology 206 ( 2 0 0 8 ) 221–230 journal homepage: www.elsevier.com/locate/jmatprotec Densification behaviour of nanocrystalline hydroxyapatite bioceramics S. Ramesh a,∗ , C.Y. Tan a , S.B. Bhaduri b , W.D. Teng c , I. Sopyan d a Ceramics Technology Laboratory, University Tenaga Nasional, 43009 Kajang, Selangor, Malaysia b School of Materials Science & Engineering, Clemson University, Clemson, SC 29634, USA c Ceramics Technology Group, SIRIM Berhad, 40911 Shah Alam, Selangor, Malaysia d Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia, Malaysia article info Article history: Received 6 June 2007 Received in revised form 10 November 2007 Accepted 6 December 2007 Keywords: Hydroxyapatite Bioceramic Sintering Mechanical properties abstract The sinterability of nanocrystalline hydroxyapatite (HA) particles by microwave sinter- ing was compared with conventional pressureless sintering. The results revealed that microwave heating was effective in producing a useful HA body in a very short sinter- ing cycle without disrupting the HA phase stability. The maximum hardness of 7.21GPa and 6.38 GPa was obtained for HA sintered at 1050 ◦ C by the conventional method and 1150 ◦ C by microwave sintering, respectively. The maximum fracture toughness measured for the microwave-sintered and conventional-sintered HA was 1.45MPam 1/2 at 1050 ◦ C and 1.22 MPam 1/2 at 1000 ◦ C, respectively. Although the relative density of microwave-sintered HA was slightly lower than the conventional-sintered HA throughout the sintering regime employed, taking into account of the heating and soaking periods, the time taken by microwave sintering to achieve a relative density of 96.5% was about 3% of the time con- sumed for samples sintered by the conventional heating. Microwave heating was found to be an effective technique to produce a useful HA body for clinical applications without causing grain coarsening. © 2007 Elsevier B.V. All rights reserved. 1. Introduction Hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 (HA) material has been clin- ically applied in many areas of dentistry and orthopaedics because of its excellent osteoconductive and bioactive prop- erties which is due to its chemical similarity with the mineral portion of hard tissues (Hench, 1998; Afshar et al., 2003; Suchanek and Yoshimura, 1998; Aoki et al., 1987; Metsger et al., 1982). One of the major concern with HA is the low fracture toughness, i.e. <1 MPam 1/2 of the sintered body (Gabriel Chu et al., 2002; Muralithran and Ramesh, 2000). As a result various ∗ Corresponding author. Tel.: +60 3 89287282; fax: +60 3 89212116. E-mail address: ramesh@uniten.edu.my (S. Ramesh). studies have been carried out to improve the mechani- cal properties of sintered HA (Suchanek and Yoshimura, 1998; Rodr´ ıguez-Lorenzo et al., 2002; Kokubo et al., 2003). These include the development of new powder process- ing/synthesis techniques and composition modification. The ultimate goal is to identify the most appropriate synthesis method and conditions to produce well-defined particle mor- phology (Suchanek and Yoshimura, 1998; Aoki et al., 1987; Metsger et al., 1982; Gabriel Chu et al., 2002; Muralithran and Ramesh, 2000; Rodr´ ıguez-Lorenzo et al., 2002; Kokubo et al., 2003) that is sinter active at low temperatures to produce body that exhibits high mechanical properties. 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.027