**2. Preparation of MAS glass ceramic by sintering route**

Magnesium aluminum silicate glass ceramic specimens (designated as MAS-G1-MAS-G10) were prepared through sintering route using three stage schedules i.e. calcination, nucleation and crystallization process using stiochiometeric amount of inorganic metal salts, such as oxides, hydroxide, fluorides and carbonates in weight percent as shown in **Table 1**. The starting raw materials used in the present study were Silica (SiO2, Fluka, 99.9%), aluminum oxide (Al2O3, BDH, 99.9%), magnesium oxide (MgO, Merck, 99.9%), potassium carbonate (K2CO3, Riedel-de Haen 99.9%), boron oxide (B2O3, BDH, 99.9%), magnesium fluoride (MgF2 BDH99.9%), ortho-phosphoric acid (H3PO4.BDH, 99.9%), acetone (CH3COCH3 BDH, 99.9%), hydrochloric acid (HCl, Marck), sodium carbonate (Na2CO3, BDH, 99.9%) and sodium hydroxide pellets (NaOH, Merck). All chemicals were of analytical reagent grade and used without further purification.

In a typical preparation, the initial charge was mixed thoroughly and calcined at 9500C for 24 h with a pre-determined heating schedule. Approximately 4-7wt% MgF2 was added to calcined charge and milled in a planetary ball mill for 40 h. The median particle size of the ball-milled sample was around 5.3μm. The fine ball milled charge was seasoned in 5% H3PO4 acid solution in acetone medium for 72h. Compacts of MAS (61×16×5 mm) were made using hydraulic press of load capacity ~ 25 tons/in2 (1 ton/in2 = 15.44 MPa). The compacts were sintered using two-step heating program. In first step the compact was heated up to 600-630oC for 2-4 h to ensure good nucleation and to initiate crystal growth. In second step, heating was carried out with different heating rates in the range of 15-60oC/h up to sintering temperatures of 950-1080oC. The sample was kept at the sintering temperature for a sufficiently long time to achieve the desired crystal growth. In order to measure the phase purity, crystallinity and morphology of synthesized products, the MAS materials were characterized using different analytical techniques i.e., TG/DTA, XRD/XRF and SEM. Phase purity and crystallinity of MAS specimens was determined by X-ray diffraction on a Rigaku Geiger flux instrument using Cu Kα radiation. For measurement of weight loss the combined TG-DTA thermal analysis was performed using Netzsch STA-409 thermoanalyzer. Coefficient of thermal expansion was observed on horizontal high temperature computer-controlled differential dilatometer (Netzsch, DIL, 409) with heating rate of 10°C/min up to 1100°C in static air. The particle size distribution of powder specimens was measured by laser particle size analyzer (SK-Laser Micron PRO-7000S). Density of MAS specimens was measured using Ultra Pycnometer 1000 (Quantachrome). Porosity was measured by radiographic technique using Real-time radiography Instrument (Model Isvolt HS Panta Sei Fert). Micro-structural features, elemental contents and porosity

materials with fluorophlogopite as the main crystalline phase. The primary objective of the present research work is to provide a simple sintering method for preparation of crystalline magnesium aluminum silicate glass-ceramic body with predominant fluorophlogopite crystal phase, which can be utilized as candidate material for machinable tools acquiring good resistance to attack by acids and alkalies. The variation in sintered densities, mechanism of phase transformation, microstructure changes and thermal expansion coefficient of MAS glass ceramic was also ascertained. It constitutes a part of our ongoing studies on MAS glass ceramic material in detail and all the results are based on techniques

Magnesium aluminum silicate glass ceramic specimens (designated as MAS-G1-MAS-G10) were prepared through sintering route using three stage schedules i.e. calcination, nucleation and crystallization process using stiochiometeric amount of inorganic metal salts, such as oxides, hydroxide, fluorides and carbonates in weight percent as shown in **Table 1**. The starting raw materials used in the present study were Silica (SiO2, Fluka, 99.9%), aluminum oxide (Al2O3, BDH, 99.9%), magnesium oxide (MgO, Merck, 99.9%), potassium carbonate (K2CO3, Riedel-de Haen 99.9%), boron oxide (B2O3, BDH, 99.9%), magnesium fluoride (MgF2 BDH99.9%), ortho-phosphoric acid (H3PO4.BDH, 99.9%), acetone (CH3COCH3 BDH, 99.9%), hydrochloric acid (HCl, Marck), sodium carbonate (Na2CO3, BDH, 99.9%) and sodium hydroxide pellets (NaOH, Merck). All chemicals were of analytical

In a typical preparation, the initial charge was mixed thoroughly and calcined at 9500C for 24 h with a pre-determined heating schedule. Approximately 4-7wt% MgF2 was added to calcined charge and milled in a planetary ball mill for 40 h. The median particle size of the ball-milled sample was around 5.3μm. The fine ball milled charge was seasoned in 5% H3PO4 acid solution in acetone medium for 72h. Compacts of MAS (61×16×5 mm) were made using hydraulic press of load capacity ~ 25 tons/in2 (1 ton/in2 = 15.44 MPa). The compacts were sintered using two-step heating program. In first step the compact was heated up to 600-630oC for 2-4 h to ensure good nucleation and to initiate crystal growth. In second step, heating was carried out with different heating rates in the range of 15-60oC/h up to sintering temperatures of 950-1080oC. The sample was kept at the sintering temperature for a sufficiently long time to achieve the desired crystal growth. In order to measure the phase purity, crystallinity and morphology of synthesized products, the MAS materials were characterized using different analytical techniques i.e., TG/DTA, XRD/XRF and SEM. Phase purity and crystallinity of MAS specimens was determined by X-ray diffraction on a Rigaku Geiger flux instrument using Cu Kα radiation. For measurement of weight loss the combined TG-DTA thermal analysis was performed using Netzsch STA-409 thermoanalyzer. Coefficient of thermal expansion was observed on horizontal high temperature computer-controlled differential dilatometer (Netzsch, DIL, 409) with heating rate of 10°C/min up to 1100°C in static air. The particle size distribution of powder specimens was measured by laser particle size analyzer (SK-Laser Micron PRO-7000S). Density of MAS specimens was measured using Ultra Pycnometer 1000 (Quantachrome). Porosity was measured by radiographic technique using Real-time radiography Instrument (Model Isvolt HS Panta Sei Fert). Micro-structural features, elemental contents and porosity

previously applied (Durrani et al., 2010; Hussain et al. 2010).

reagent grade and used without further purification.

**2. Preparation of MAS glass ceramic by sintering route** 

of sintered specimen MAS -G8 were observed using scanning electron microscope (SEM, LEO 4401). The specimen was fully polished, put onto aluminum stud, dried in air and then the specimen was coated with thin gold film for the SEM observation. For the measurement of micro-hardness of sintered specimens, indentation technique using Vickers diamond pyramid indentor on the micro hardness tester was used. Before measurements, the sample surface was polished with 3μ alumina powder to get good reflective surface. The measurement was done on the polished surface by applying 300g load for 15 sec. The effect of K2CO3 concentration on sintered density and mechinability of MAS specimens (MAS-K1- MAS-K10) at fixed amount of MgF2 (4-11%) and nucleation temperature (630oC) was also studied. The sintered specimens were treated with 5% hydrofluoric and hydrochloric acids for 24h at 95oC to observed the effect of these acids. Effect of 5% sodium hydroxide and sodium carbonate was also studied for 6 h duration at 95oC. The samples were weighed for any loss in weight after washing off acids and bases.

Impedance spectroscopy on pellets of MAS glass ceramic, which were sintered at 1040 and 1050oC temperatures was performed in the frequency range of 1 ≤ frequency ≤ 107 Hz at room temperature, using an alpha-N Analyzer (Novocontrol Germany). The surfaces of both sides of the pellets were cleaned properly and contacts were made by silver paint on opposite sides of the pellet, which were cured at 150oC (423K) for 3 h. Before the impedance experiments, the dispersive behavior of the leads were carefully checked to exclude any extraneous inductive and capacitive coupling in the experimental frequency range. The ac signal amplitude used for all these studies was 0.2 V and WINDETA software was used for data acquisition.


MAS-G = Magnesium aluminum silicate glass

Table 1. Chemical composition and reaction conditions for preparation of MAS glass ceramic material by sintering route.
