Spherulitic crystallization of apatite–mullite glass-ceramics: Mechanisms of formation and implications for fracture properties Kenneth T. Stanton a, ⁎, Kevin P. O'Flynn a , Stephen Kiernan a , Julian Menuge b , Robert Hill c a School of Electrical, Electronic and Mechanical Engineering, University College Dublin, Belfield, Dublin 4, Ireland b School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland c Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom abstract article info Article history: Received 9 April 2009 Received in revised form 1 July 2010 Available online 3 August 2010 Keywords: Crystal growth C2863; Glass ceramics G160; Mechanical properties M120; Fracture F200 Apatite–mullite glass-ceramics crystallize from glass of generic composition SiO 2 Al 2 O 3 P 2 O 5 CaO CaF 2 to form an osseoconductive apatite phase existing as spherulites within a mullite matrix. To further investigate the formation of these apatite spherulites, two glass-ceramic systems, both known to form apatite and mullite, were heat treated to varying degrees of crystallinity. Microscopical investigation and mechanical testing was performed on the samples. The results allow us to show the effect of these spherulites on the mechanical properties of the material and elucidate evidence for a previously hypothesized mechanism describing their formation. Mechanical testing was used to determine the effect that this has on the indentation fracture toughness, K Ic, idt . Following heat treatment and fracture testing, samples were prepared and viewed using optical imaging to determine crack interactions with the spherulites. Results showed that these crystal/glass boundaries have a significant influence on the mechanics of crack propagation and as such, K Ic, idt is unreliable for partially cerammed glasses due to the differences between results obtained from indentation measurements performed on the glass, boundary and crystal. Following full ceramming of the glass, a marked increase in K Ic, idt above that of the glass was observed. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Biocompatible metal alloys such as Ti6Al4V have found wide- spread use as structural orthopaedic materials due to their favourable properties, in particular their biocompatibility, high fatigue strength, and a relatively low modulus of elasticity. However, one of the major drawbacks of Ti6Al4V when used in vivo is its inability to bond easily with surrounding hard tissue [19]. To overcome this, it can be coated in a bioactive material to aid osseointegration. A number of different coating materials have been previously examined: calcium phos- phates including: hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) [13,16], silica gel [18], bioglasses [9], and glass-ceramics [11]. These materials defy typical fibrous encapsulation, instead creating a strong bone- material bond by forming a surface apatite layer similar to bone and subsequently bonding to the bone through this layer [22]. The most commonly used coating material is synthetic HA. However, although HA has the advantage of being highly bioactive, it has a number of significant drawbacks, particularly a lack of strength after sintering [2]. Bioactive glass-ceramics are an alternative to synthetic HA for use in vivo both in restorative dental applications and bone implantation [11]. Glass-ceramics are prepared by the controlled crystallization of suitable metastable glass compositions. Careful choice of the parent glass composition and heat treatment regime allows for the production of a wide range of resultant microstructures. Glass- ceramics that form apatite crystals, chemically analogous to hydroxy- apatite found in human bone and teeth, have generated significant interest due to their bioactive nature and possible applications in prostheses [30]. Such ceramics can be used as castable dental implants or as enameled coatings for orthopaedic implants [29]. In particular, apatite based glass-ceramics have been shown to exhibit high fracture toughness, K Ic , values of more than 3 MPa ffiffiffiffi m p ; this has been attributed to the highly acicular nature of the apatite crystals formed [2]. However, it has been noted that these crystals form within spherulitic apatite regions that can have a significant effect on fracture paths [28]. The present work examines the formation of these crystals in apatite– mullite glass-ceramics and what implications they have for fracture toughness. It is useful to summarise the different crystal growth theories as applied to similar materials before proceeding further. Once a stable crystal nucleus has formed and begun to grow there are a number of possible crystal growth mechanisms and these determine the final crystal morphology. Lewis et al. [17] have reviewed crystal growth in Journal of Non-Crystalline Solids 356 (2010) 1802–1813 ⁎ Corresponding author. E-mail address: kenneth.stanton@ucd.ie (K.T. Stanton). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.006 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol