The influence of MgF 2 content on the characteristic improvement of machinable glass ceramics Debasis Pradip Mukherjee a,1 , Atiar Rahaman Molla b , Sudip Kumar Das a, ⁎ a Department of Chemical Engineering, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009, India b Glass Science and Technology Section, Glass Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata 700032, India abstract article info Article history: Received 21 August 2015 Received in revised form 10 November 2015 Accepted 26 November 2015 Available online 4 December 2015 The influence of MgF 2 on the various properties like crystalline behavior, microstructure phases, hardness etc. in the SiO 2 –Al 2 O 3 –MgO–K 2 O–B 2 O 3 glass system has been investigated. Three batches of glass system were synthesized and characterized by differential scanning calorimetry (DSC), coefficient of thermal expansion (CTE), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) equipped with energy dispersion X-ray spectroscopy (EDS). DSC study reveals that with the increase in MgF 2 content the glass transition temperature (T g ) and first crystallization peak temperature (T p I ) decreased whilst the second crystallization peak temperature (T p II ) slightly increased. The CTE of the glasses is found to be in the ranges 6.34–6.40 × 10 -6 K -1 (50–400 °C). The activation energy (E c ) and frequency factor (υ) both increase with increasing MgF 2 content. The three-dimensional crystal growth is observed. The mica crystals are identified as fluorophlogopite, the predominant crystal phase for all the three glass specimens heat treated at 1050 °C. Vickers hardness values decrease with increasing amount of fluorine content and it gives better machinability. © 2015 Elsevier B.V. All rights reserved. Keywords: Glass–ceramics Crystallization Microstructure Hardness Machinability 1. Introduction Glass–ceramics (GC) are attractive materials for engineering purpose including electronic, semiconductor, laser, high vacuum, aerospace and space industry and also bio-medical purpose including bone, dental, and tissue engineering applications [1–4]. Developments in this field in recent years are in the form of both new materials and processing techniques. The main types of novel glass–ceramics that are being researched for potential as biomedical application [3] are the following: (i) fluorosilicate glass–ceramics (sheet silicates, e.g. fluormica and chain silicate, e.g. fluorrichterite and fluorcanasite) have good mechanical properties and highly anisotropic crystalline microstructure; (ii) aluminosilicate glass–ceramics (apatite mullite) exhibit exceptional stability, good chemical durability and resistance to thermal shock; and (iii) silicate glass–ceramics composed of alkali and alkaline silicate crystal, e.g., enstatite. Glass–ceramics are partially crystallized glasses that are produced by proper nucleation and the growth of crystals in the glass matrix phase. Properties of these glass– ceramics depend on the amount of final crystals, their distribution and residual glass composition. The crystal phase formation is a function of heat treatment time, heating rate, presence of nucleating agent etc. [5–10]. The mica containing glass–ceramics received wide application due to their high machinability, excellent exthetics, low thermal con- ductivity, high strength, durability, biocompatibility, ease of manufac- ture and high wear resistance [2,3]. A machinable glass–ceramic can be turned, milled, drilled and tapped with using normal tools used for machining metals without breaking. These glass–ceramic materials have highly interlocked mica crystals in the glass matrix and facilitate microfracture along the weak mica-glass interfaces and mica basal planes; hence microfracture can easily propagate from crystal to crystal [11]. Goswami et al. [12] synthesized magnesium–aluminium–silicate (MAS) machinable glass ceramics for fabrication of insulators/spacers for high voltage applications under high vacuum conditions by sintering and glass route. They concluded that the MAS glass ceramics from the glass route were better in respect to the surface finish, less porous, higher density and electrical breakdown strength to those prepared by sintered route. Denry and Holloway [13] investigated the effect of magnesium content (as MgO 12–18 wt.%) on the microstructure and crystalline behavior in the SiO 2 –MgO–CaO–Na 2 O–K 2 O–F glass– ceramics system. They observed that microstructure consists of interlocked acicular crystals and at highest magnesium content mica phase and fluorrichterite coexist. Later [14] they investigated the effect of sodium content (as Na 2 O0–7.4 wt.%) on their crystalline behavior and thermal properties. They reported that sodium free glass ceramics consist of hexagonal mica crystal and other composition shows needle-shaped fluorrichterite crystals in addition to mica and diopside crystals. Machinable mica based glass–ceramics materials Journal of Non-Crystalline Solids 433 (2016) 51–59 ⁎ Corresponding author. Tel.: +919830638908. E-mail address: drsudipkdas@vsnl.net (S.K. Das). 1 Present address: Glass Technology Group, Department of Build Environment and Energy Technology, Linnaeus University, Sweden. http://dx.doi.org/10.1016/j.jnoncrysol.2015.11.031 0022-3093/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol