The role of MgO on the structural properties of CaO–Na 2 O(MgO)–P 2 O 5 –CaF 2 –SiO 2 derived glass ceramics S.M. Salman * , S.N. Salama, H. Darwish, E.A. Mahdy Glass Research Department, National Research Centre, El-Behoos St., Dokki, Cairo 12622, Egypt Received 20 April 2009; received in revised form 1 June 2009; accepted 29 June 2009 Available online 17 July 2009 Abstract The effect of increasing MgO/Na 2 O replacements (on mole basis) on the crystallization characteristics of glasses based on the CaO– Na 2 O(MgO)–P 2 O 5 –CaF 2 –SiO 2 system were studied by using DTA, XRD, and SEM. The crystallization characteristics of the glasses, the type of crystalline phases formed and the resulting microstructure were investigated. The main crystalline phases formed after controlled heat-treatment of the base glass were diopside, wollastonite solid solution, fluoroapatite and sodium calcium silicate phases. The increase of MgO at the expense of Na 2 O led to decrease the amount of sodium calcium silicate phase. The Vicker’s microhardness values (5837–3362 MPa) of the resulting glass– ceramics were markedly improved by increasing the MgO-content in the glasses. The obtained data were correlated to the nature and concentration of the crystalline phases formed and the resulting microstructure. Crown Copyright # 2009 Published by Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Glass; Crystallization characteristics; Microhardness 1. Introduction Glass–ceramic materials are polycrystalline solids with a residual glassy matrix leading to a characteristic microstructure that allows achievement of a better performance to abrasiveness and an increased resistance compared to traditional glasses. The microstructure of such material can be also obtained by a sintering process where crystallization and densification of glass particles occur during firing [1]. Calver et al. [2] studied the influence of fluorine content on the crystallization behaviour of CaO–MgO–SiO 2 –P 2 O 5 –F  glassy system. They found that all glasses of composition (50.28  x) CaO–7.11 MgO–35.46 SiO 2 –7.15 P 2 O 5 –x CaF 2 , where (x = 0, 0.4, 4.77) were crystallized to glass ceramic containing apatite and wollastonite phases. Apatite crystallizes at lower temperature than wollastonite, the former favouring bulk nucleation and the latter exhibits surface nucleation. Mechanism of crystallization of apatite and wollastonite phases were studied by Galliano and Poroto [3] in a glass of composition 44.7 CaO–4.6 MgO–34 SiO 2 –16.2 P 2 O 5 and 0.5 CaF 2 wt.%. The glass was heat-treated at 749–851 8C to form apatite phase and then at 925–1025 8C where the wollastonite phase was crystallized. They also noticed that the initial crystallization of wollastonite was very fast and also increases with temperature. Salama et al. [4] prepared glass–ceramics from 33.32 CaO– 20.1 MgO–40.76 SiO 2 –4.36 P 2 O 5 –1.46 CaF 2 (mol%), with minor amounts of Na 2 O, B 2 O 3 and TiO 2 . They showed that diopside and fluoroapatite crystalline phases were the main phases developed. The product has linear expansion coefficient (25–500 8C) 10.0  10 6 /8C and microhardness value is 7085 MPa. Tulyaganov [5] stated that there is no evidence for the formation of solid solution phases or liquid immiscibility gaps between diopside and fluoroapatite phases. Kokubo et al. [6] succeeded to prepare a high strength glass– ceramics in the CaO–MgO–SiO 2 –P 2 O 5 system (A–W glass ceramics). They found that the crystallization of the parent glass in a bulk form led to the occurrence of large cracks. This was attributed to the precipitation of fibrous b-wollastonite after precipitation of fluoroapatite, however, crystallization of the same glass in a powder form, cracks-free glass–ceramic formed due to the crystallization of both apatite and wollastonite fine crystals throughout the glass article. Shyu and Wu [7] and Liu and Chou [8] have studied the crystallization of glass–ceramic, that www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 55–61 * Corresponding author. Tel.: +20 2 33370931. E-mail address: saadmoghazy@hotmail.com (S.M. Salman). 0272-8842/$36.00. Crown Copyright # 2009 Published by Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.06.021