Pressureless sintering and mechanical properties of SiO 2 –Al 2 O 3 –MgO–K 2 O–TiO 2 –F (CaO–Na 2 O) machinable glass–ceramics B. Ashouri Rad, P. Alizadeh * School of Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Received 30 January 2009; received in revised form 19 February 2009; accepted 13 March 2009 Available online 15 April 2009 Abstract To develop a high strength machinable glass–ceramic through pressureless sintering, the glassy compositions were obtained by mixing a mica- based frit and a frit in the SiO 2 –CaO–Na 2 O system. According to XRD results, the glass compositions mainly crystallized into phlogopite and diopside after sintering. The optimum sintered glass–ceramic with desirable mechanical properties, machinability and sinterability was achieved by addition of 30 wt.% SiO 2 –CaO–Na 2 O glass powder to 70 wt.% mica glass composition. SEM results confirmed presence of needle-like diopside crystals which played a reinforcement role to the platelet phlogopite and glassy matrix combination. The measurements showed bending strength and fracture toughness enhanced up to 144.6 Æ 17.6 MPa and 1.7 Æ 0.2 MPa m 1/2 , respectively. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Diopside; B. Phlogopite; C. Mechanical properties; D. Glass–ceramics 1. Introduction Fluormica glass–ceramics are well-known materials and show a favorable combination of thermal, electrical and biomedical properties. These materials can be easily cut, drilled and turned with conventional tools. Laminated structure of mica crystals is, directly responsible for the desirable machinability because they cleave easily along the interfaces between layers while being machined [1–4]. Recently much interest has been focused on mica glass– ceramics but the mechanical strength of these materials is fairly poor. Accordingly, numerous attempts have been made to improve this disadvantage through modification of composi- tion. Most of the commercial machinable glass–ceramics are based on potassium fluorphlogopite. Uno et al. [5] investigated barium–fluorphlogopite with higher mechanical strength and toughness in comparison with conventional potassium– fluorphlogopite. Likely, several investigations have been conducted to produce calcium–mica with enhanced mechanical properties [6–8]. Utilization of high strength phases was adjusted to prepare high strength mica glass–ceramics. The strengthening effect of additives such as titania and celsian powders has been studied via sintering method. Herein, the strength successfully improved by suppressing the longitudinal growth of mica crystals [9]. Recent study revealed that incorporating suitable amounts of Y-PSZ into mica base glass– ceramic, could result in simultaneous improvement in bending strength and fracture toughness [10]. Fabrication of oriented mica containing glass–ceramics using extrusion or hot press was another approach carried out for fulfillment of this purpose [11,12]. Efforts have been continuing to overcome these problems and to develop new mica glass–ceramics that will exhibit better performance. In this respect the addition of second glass frit to mica-based frit and sintering the obtained mixtures has still attracted less attention. Recently, Alizadeh et al. [13] have presented the pressureless sintering of apatite-based frit (viz. SiO 2 –CaO–P 2 O 5 –MgO glass system) and mica-based glass mixture. In the current study we attempted to improve the mechanical properties of machinable glass–ceramics by final precipitation of high strength crystalline phases along with the cleavable www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 2775–2780 * Corresponding author. Tel.: +98 21 82884399. E-mail address: p-alizadeh@modares.ac.ir (P. Alizadeh). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.03.027