The influence of MgF2 content on the characteristic improvement of machinable glass ceramics
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], [2], [3], [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], [6], [7], [8], [9], [10]. The mica containing glass–ceramics received wide application due to their high machinability, excellent exthetics, low thermal conductivity, high strength, durability, biocompatibility, ease of manufacture 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 SiO2–MgO–CaO–Na2O–K2O–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 Na2O 0–7.4 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 with fluorophlogopite as the main crystalline phase in the system K2O–B2O3–Al2O3–SiO2–MgO–F was investigated by Roy and Basu [2] for dental restoration purpose. Wange et al. [15] reported the microstructure behavior in MgO–Al2O3–SiO2–TiO2 glass–ceramics system using different heat treatment schedules. The effects of different nucleating agents like TiO2, ZrO2 or their mixture are well documented in literature [15], [16], [17], [18], [19], [20], [21], [22], [23]. Dargaud et al. [19] reported the role of Zr4 + as a nucleating agent in a MAS glass system. Ghasemzadeh and Nemati [24] investigated the effects of MgF2 with respect to the crystallization kinetics and microstructures in the SiO2–MgO–Al2O3–K2O–B2O3–F glass–ceramics system. They observed that the crystallization energy increases with addition of MgF2 and also machinability increases. Patzig et al. [25] reported the crystallization behavior of the glass–ceramics in MgO–Al2O3–SiO2–ZrO2 system.
In this paper the effects of MgF2 in the SiO2–Al2O3–MgO–K2O–B2O3 glass–ceramic system is presented. The glasses were prepared with different amount of MgF2 by melt quenching technique and then transformed to glass–ceramics by heat treatment scheduled. The crystallization behavior, activation energy, phase separation, microstructure and hardness have been reported.
Section snippets
Glass synthesis
Table 1 shows the compositions of the three different batches of SiO2–Al2O3–MgO–K2O–B2O3–MgF2 glass system. All raw materials, SiO2 (99.8%), Al2O3 (99.3%), MgO (99.9%), K2CO3 (98.5%), H3BO3 (99.5%) and MgF2 (99.9%) are purchased from M/S Merck Specialties Private Limited, India. Batches are prepared by using weighted amounts of materials and then mixed in an attrition mill and then melted at 1500 °C for 2 h in an alumina crucible in air using an electrically heated furnace with intermittent
DSC study
The glass to glass–ceramics conversion is a phase transformation process; in which several crystalline phases are involved and proper heat controlled treatment is necessary. The purpose of the differential scanning calorimetry (DSC) study is to determine the glass transition temperature (Tg) and crystallization peak temperature (Tp). Fig. 1(a)–(c) shows the differential scanning calorimetry (DSC) thermographs at a heating rate of 10 °C min− 1 for all three glass specimens. Table 2 shows the
Conclusions
The crystallization of fluorophlogopite in the SiO2–Al2O3–MgO–K2O–B2O3–MgF2 glass system was investigated by systematically changing the amount of fluorine (MgF2), a network modifier. It was noted that with the increase in MgF2 content the glass transition temperature (Tg) and first crystallization peak temperature (TpI) decreased whilst the second crystallization peak temperature (TpII) increased. All the glass specimens exhibit CTE in the range of (6.34–6.40) × 10− 6 K− 1 (50–400 °C). The stability
Acknowledgments
The authors would like to thank the UPE scheme of University Grants Commission and the Center for Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta for the financial support. One of the authors, Debasis Pradip Mukherjee thanks the Center for Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta, Kolkata, India, for providing him the Senior Research Fellowship (SRF).
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Present address: Glass Technology Group, Department of Build Environment and Energy Technology, Linnaeus University, Sweden.