**1.1 Ceramic materials**

Ceramic materials are complex compounds and containing both metallic and non-metallic elements. Typically, ceramics are very hard, brittle, high melting point materials with low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths (Barsoum, 1997; Minh et al., 1995). Ceramics are of tremendous interest primarily because of their wide range of applications in high temperature environments; they are also extensively used in fuel technology (Koshiro et al, 1995), oxygen sensor (Ciacchi, et al. 1994), magnets ceramics (Valenzuela,2005) , all electronic equipments including integrated-chips, capacitors and digital alarms (Miller,& M. R. Miller (2002), telecommunication (Bhargava, A.K. 2005), abrasives (Callister, 2007) , ceramic crystal-glass (Carter, & Norton. (2007), ceramic insulators are widely used in the electrical power transmission system (Chowdhury, 2010), ceramic superconductors (David E. C.&, Brece K.Zoitos, 1992) and other pharmaceuticals industries (Rice, & R. W. Rice, (2002) etc.

Ceramic materials can be classified into four main groups (Rajendran, V. 2004):


The structures of ceramics fall into two main groups:

• Simple crystal structures: containing ionic or covalent bonds, or a mixture of the two. Examples are magnesium oxide, which is an ionic compound with cubic structure, and silicon carbide, with covalent bonds and a tetrahedral structure like diamond. Alumina has a close packed hexagonal structure, with a mixture of covalent and ionic bonds,

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Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 253

technological applications such as microelectronic substrates and packaging, optically transparent components, biomedical implants, catalytic supports, membranes and sensors, as well as for the matrix of composite materials. (Partridge, 1994; Manfredini, et al., 1997). In addition, they have been used in protection layers, sewing-thread in industry, ceramic tiles and also to develop into a very promising substrate material of computer hard disk (Novaes et al., 1994)**.** The universal glass ceramics are Lithia-alumina-silica (Li2O-ZrO2-SiO2-Al2O3, LZSA) systems (Montedo et al., 2009) that have zero thermal expansion co-efficient, and

The majority commercial glass-ceramic products are formed by highly automated glassforming processes and converted to a crystalline product by the proper heat treatment. Glass-ceramics can be prepared through a powder processing methods, in which glass frits are sintered and crystallized. The range of potential glass-ceramic compositions is therefore particularly wide, requiring only the ability to form a glass and control its crystallization. Glass-ceramics can provide advantages over conventional glass or ceramic materials, by combining the simplicity and exibility of forming and inspection of glass with improved and often unique physical properties in glass ceramic. They possess highly uniform microstructures, with crystal sizes (<10mm), this homogeneity indicates that their physical properties are highly reproducible. The glass-ceramics are fully densied with zero porosity. They generally are at least 50% crystalline by volume and often are >90% crystalline (Strand, 1986). Glass-ceramics are also called as pyrocerams, vitrocerams, devitrocerams, sitalls, slag-

Glass-ceramic materials are fabricated (green body) by means of either bulk or powder processing methods. In these methods, glass-ceramic products are melted and fabricated to shape in the glass state. Most forming methods are rolling, pressing, spinning, casting, and blowing. The product is then crystallized using a heat treatment designed for that material. This process, known as ceramic processing, usually comprise of a low temperature hold to

**Growth**

Fig. 1. (a-b): Crystal growing process (a) Nucleation and crystal growth as a function of temperature (b) heat treatment cycle showing crystal growth during sintering as a function

**Temperature**

**Nucleation** 

**Time** 

**Sintering**

**(b)**

**Crystal Growth**

**(a)**

hence no thermal shock problem.

ceramics, melt-formed ceramics, and devitrifying frits.

**1.1.2 Ceramic processing and crystal growth** 

**Temperature** 

**Nucleation**

**Growth Rate**

of time.

with one-third of the potential aluminum sites vacant in order to suit the valency requirements of the two elements ( (Mostaghaci, H. 1996).


In advance ceramic, extremely pure microscopic raw materials are used, with strict control of compositions and manufacturing conditions, in order to achieve a more uniform microstructure. Interest in modern ceramic continues to grow because these materials have a unique set of properties which no other family of materials can match. These include high hardness, heat resistance, ability to withstand corrosive atmosphere, resistance to abrasion, ability to sustain compressive loads at elevated temperatures, and an inexpensive and abundant availability of raw materials. Except for their brittleness, the ceramics stand out well against metals and plastics and can be applied as thermal barrier coatings to protect metal structures, wearing surfaces, or as integral components by themselves. These new ceramics are called either "new ceramics or fine ceramics" or frequently called "highperformance ceramics" as described by (Shinroku, 1986). Advance ceramic materials are being used in many new applications and have been proposed for numerous components which may result in significant growth of current and new business (Doremus, 1994; Lewis, 1989; Michael, B. 2007).

#### **1.1.1 Glass ceramics**

Glass ceramic materials are fine grained polycrystalline solid containing residual glass phase formed by melting of glass and forming into products by the way of controlled crystallization of a specially formulated glass (Shackelford, & Doremus, 2008). These are primarily silicates containing oxides such as Al2O3, TiO2, LiO2, and others. In amorphous form, the glasses are transparent. Glasses can be made to transform into a polycrystalline state by a suitable heat-treatment process, called devitrification. An initiator, such as TiO2, is added to begin the nucleation of ceramic crystals. The product is called glass ceramic. Glass ceramics are members of ceramic family and an important electroceramic type, were first investigated in the 1940s by Stookey at Corning Glass, (Pannhorst, 1995**)** are extensively exploited as electrical insulators in electronic industries and have a wide variety of

• Complex silicate structure**s:** The majority of ceramic materials, i.e., derived from clay, sand or cement, contain the silicon in the form of silicates. The arrangements are involving both chains of silicate ions (SiO4)2–, double chains and links in sheet form. • In advance ceramic, extremely pure microscopic raw materials are used, with strict control of compositions and manufacturing conditions, in order to achieve a more uniform microstructure. Interest in modern ceramic continues to grow because these materials have a unique set of properties which no other family of materials can match. These include high hardness, heat resistance, ability to withstand corrosive atmosphere, resistance to abrasion, ability to sustain compressive loads at elevated temperatures, and an inexpensive and abundant availability of raw materials. Except for their brittleness, the ceramics stand out well against metals and plastics and can be applied as thermal barrier coatings to protect metal structures, wearing surfaces, or as integral components by themselves. These new ceramics are called either "new ceramics or fine ceramics" or frequently called "high-performance ceramics" as described by (Shinroku, 1986). Advance ceramic materials are being used in many new applications and have been proposed for numerous components which may result in significant growth of

current and new business (Doremus, 1994; Lewis, 1989; Michael, B. 2007).

1989; Michael, B. 2007).

**1.1.1 Glass ceramics** 

In advance ceramic, extremely pure microscopic raw materials are used, with strict control of compositions and manufacturing conditions, in order to achieve a more uniform microstructure. Interest in modern ceramic continues to grow because these materials have a unique set of properties which no other family of materials can match. These include high hardness, heat resistance, ability to withstand corrosive atmosphere, resistance to abrasion, ability to sustain compressive loads at elevated temperatures, and an inexpensive and abundant availability of raw materials. Except for their brittleness, the ceramics stand out well against metals and plastics and can be applied as thermal barrier coatings to protect metal structures, wearing surfaces, or as integral components by themselves. These new ceramics are called either "new ceramics or fine ceramics" or frequently called "highperformance ceramics" as described by (Shinroku, 1986). Advance ceramic materials are being used in many new applications and have been proposed for numerous components which may result in significant growth of current and new business (Doremus, 1994; Lewis,

Glass ceramic materials are fine grained polycrystalline solid containing residual glass phase formed by melting of glass and forming into products by the way of controlled crystallization of a specially formulated glass (Shackelford, & Doremus, 2008). These are primarily silicates containing oxides such as Al2O3, TiO2, LiO2, and others. In amorphous form, the glasses are transparent. Glasses can be made to transform into a polycrystalline state by a suitable heat-treatment process, called devitrification. An initiator, such as TiO2, is added to begin the nucleation of ceramic crystals. The product is called glass ceramic. Glass ceramics are members of ceramic family and an important electroceramic type, were first investigated in the 1940s by Stookey at Corning Glass, (Pannhorst, 1995**)** are extensively exploited as electrical insulators in electronic industries and have a wide variety of

requirements of the two elements ( (Mostaghaci, H. 1996).

with one-third of the potential aluminum sites vacant in order to suit the valency

technological applications such as microelectronic substrates and packaging, optically transparent components, biomedical implants, catalytic supports, membranes and sensors, as well as for the matrix of composite materials. (Partridge, 1994; Manfredini, et al., 1997). In addition, they have been used in protection layers, sewing-thread in industry, ceramic tiles and also to develop into a very promising substrate material of computer hard disk (Novaes et al., 1994)**.** The universal glass ceramics are Lithia-alumina-silica (Li2O-ZrO2-SiO2-Al2O3, LZSA) systems (Montedo et al., 2009) that have zero thermal expansion co-efficient, and hence no thermal shock problem.

The majority commercial glass-ceramic products are formed by highly automated glassforming processes and converted to a crystalline product by the proper heat treatment. Glass-ceramics can be prepared through a powder processing methods, in which glass frits are sintered and crystallized. The range of potential glass-ceramic compositions is therefore particularly wide, requiring only the ability to form a glass and control its crystallization. Glass-ceramics can provide advantages over conventional glass or ceramic materials, by combining the simplicity and exibility of forming and inspection of glass with improved and often unique physical properties in glass ceramic. They possess highly uniform microstructures, with crystal sizes (<10mm), this homogeneity indicates that their physical properties are highly reproducible. The glass-ceramics are fully densied with zero porosity. They generally are at least 50% crystalline by volume and often are >90% crystalline (Strand, 1986). Glass-ceramics are also called as pyrocerams, vitrocerams, devitrocerams, sitalls, slagceramics, melt-formed ceramics, and devitrifying frits.

#### **1.1.2 Ceramic processing and crystal growth**

Glass-ceramic materials are fabricated (green body) by means of either bulk or powder processing methods. In these methods, glass-ceramic products are melted and fabricated to shape in the glass state. Most forming methods are rolling, pressing, spinning, casting, and blowing. The product is then crystallized using a heat treatment designed for that material. This process, known as ceramic processing, usually comprise of a low temperature hold to

Fig. 1. (a-b): Crystal growing process (a) Nucleation and crystal growth as a function of temperature (b) heat treatment cycle showing crystal growth during sintering as a function of time.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 255

very ne phase separation on reheating. The isolated phase, which can be a metal, titanate, zirconate, phosphate, sulde, or halide, is structurally incompatible with the host glass and is usually highly unstable as a glass. Nucleation is followed by one or higher temperature treatments to promote crystallization and development of the desired microstructure. Most commercial glass-ceramic products are formed by highly mechanized glass-forming processes and converted to a crystalline product by proper heat treatment. Glass-ceramic materials have been prepared through powder processing in which glass frits are sintered and then crystallized (Grossman, 1974) the range of potential glass-ceramic compositions is therefore extremely broad, requiring only the ability to form a glass and control its crystallization. The production of glass-ceramics from powdered, using conventional ceramic processes such as spraying, slip-casting, or extrusion, extends the range of possible glass-ceramic compositions by taking advantage of surface crystallization. In these materials, the surfaces of the glass grains serve as the nucleating sites for the crystal phases. The glass composition and processing conditions are selected such that the glass softens proceeding to crystallization and undergoes viscous sintering to full density just before the crystallization process is completed. Given these conditions, the final crystalline

microstructure is basically the same as that produced from the bulk process.

Fig. 2. (a-c): Crystal growing in aluminosilicate ceramic materials (a) Aluminum deficient

The precursor glass powders may be produced by various methods, the simplest being the milling of quenched glass to an average particle size of 3-15mm. Sol-gel processes (Yuan, et al., 2010; Chen, et al., 2010; and Lei, B. 2010) in which highly uniform, ultrane amorphous particles are grown in a chemical solution, may be preferable for certain applications. Such devitrifying frits are employed significantly as sealing frits for bonding glasses, ceramics,

silicalite-I (b) High silica aluminosilicate (c) sodium aluminosilicate.

induce nucleation, followed by one or higher temperature holds to promote crystallization and growth of the primary phase. Because crystallization occurs at high viscosity, product shapes are typically preserved with little or no shrinkage (1-3%) or deformation during the ceramming. Nucleation generally begins with phase separation, whereby an amorphous, homogeneous glass unadulterated into two immiscible phases of different compositions. Controlled devitrification is only possible for certain glass compositions and generally takes place in two stages: formation of submicroscopic nuclei, and their growth into macroscopic crystals. These two stages are called nucleation and crystal growth as shown in Figure 1(a-b) (Hölland,et al., 2002; Rincón, 1992 ).

Nucleation occurs when a small piece of soil forms from the liquid. The solid must achieve a certain minimum critical size before it is stable. Growth of crystal occurs as atoms from the liquid are attached to the tiny solid until no liquid remains. Devitrification means the formation of crystals on or inside of amorphous glass, typically due to a delayed cooling cycle. The presence of local crystalline inclusions supports the glass and makes it more flexible, reducing the presence and severity of micro-cracks and acting as crack stoppers. The heat treatment that supports the growth of these native crystals during the glass formation is called ceramming and it is a two step process. Ceramming is a controlled crystallization of the glass that results in the formation of tiny crystals that are uniformly distributed throughout the body of the glass structure. The size of the crystals, as well as the number and rate of growth is measured by the time and temperature of the ceramming heat treatment. There are two branch of ceramming process; crystal nucleation and crystal growth as shown in Fig. 2(a-c) **(**Steffestun, & Frischat, G.H. 1993; Durrani et al., 2005; Cattell, M.J. 2006). Each phase take place because the glass body is detained at exact temperature for a definite length of time. Crystals have a propensity to develop in a mixture of glass when it is held at a specific temperature, called the crystal nucleation temperature. This means that when apprehended at the crystal nucleation temperature, multiple seed crystals begin to grow throughout the glass body. The longer the glass is kept at this temperature, the more seed crystals will form. Ideally, a glass ceramic will be strongest when there is a very large number of a small crystal distributed uniformly all over its mass. Once a seed crystal forms, it will also begin to grow larger at this temperature, but somewhat slowly. If the temperature of the glass body is held at the crystal nucleation temperature for a very long time, a very large number of crystals of broadly varying size will form. The initially the seed will be largest, while the crystals that have lately begun to grow, will be the smallest. In order to better control the art of the finished product, the ideal glass ceramic will have crystals of a small and uniform size. Any form of devitrification in a glass structure will produce one degree of opacity. Large crystals are flat to make the glass opaque, while small crystals uniformly scattered throughout the structure have less of an impact on the optical qualities of the finished product. Thus it is of advantage to keep the temperature at the point of maximum seeding for a finite length of time in order to allow several tiny seed crystals to nucleate, and then to stop the nucleation process and support the ones that have formed to grow to appropriate size.

The glass ceramic materials with tailor made properties can be fabricated by controlling nucleation and crystal growth process. Although some glass compositions are selfnucleating, generally some nucleating agents are added to the batch to promote phase separation and internal nucleation. These melt homogeneously into the glass, but promote

induce nucleation, followed by one or higher temperature holds to promote crystallization and growth of the primary phase. Because crystallization occurs at high viscosity, product shapes are typically preserved with little or no shrinkage (1-3%) or deformation during the ceramming. Nucleation generally begins with phase separation, whereby an amorphous, homogeneous glass unadulterated into two immiscible phases of different compositions. Controlled devitrification is only possible for certain glass compositions and generally takes place in two stages: formation of submicroscopic nuclei, and their growth into macroscopic crystals. These two stages are called nucleation and crystal growth as shown in Figure 1(a-b)

Nucleation occurs when a small piece of soil forms from the liquid. The solid must achieve a certain minimum critical size before it is stable. Growth of crystal occurs as atoms from the liquid are attached to the tiny solid until no liquid remains. Devitrification means the formation of crystals on or inside of amorphous glass, typically due to a delayed cooling cycle. The presence of local crystalline inclusions supports the glass and makes it more flexible, reducing the presence and severity of micro-cracks and acting as crack stoppers. The heat treatment that supports the growth of these native crystals during the glass formation is called ceramming and it is a two step process. Ceramming is a controlled crystallization of the glass that results in the formation of tiny crystals that are uniformly distributed throughout the body of the glass structure. The size of the crystals, as well as the number and rate of growth is measured by the time and temperature of the ceramming heat treatment. There are two branch of ceramming process; crystal nucleation and crystal growth as shown in Fig. 2(a-c) **(**Steffestun, & Frischat, G.H. 1993; Durrani et al., 2005; Cattell, M.J. 2006). Each phase take place because the glass body is detained at exact temperature for a definite length of time. Crystals have a propensity to develop in a mixture of glass when it is held at a specific temperature, called the crystal nucleation temperature. This means that when apprehended at the crystal nucleation temperature, multiple seed crystals begin to grow throughout the glass body. The longer the glass is kept at this temperature, the more seed crystals will form. Ideally, a glass ceramic will be strongest when there is a very large number of a small crystal distributed uniformly all over its mass. Once a seed crystal forms, it will also begin to grow larger at this temperature, but somewhat slowly. If the temperature of the glass body is held at the crystal nucleation temperature for a very long time, a very large number of crystals of broadly varying size will form. The initially the seed will be largest, while the crystals that have lately begun to grow, will be the smallest. In order to better control the art of the finished product, the ideal glass ceramic will have crystals of a small and uniform size. Any form of devitrification in a glass structure will produce one degree of opacity. Large crystals are flat to make the glass opaque, while small crystals uniformly scattered throughout the structure have less of an impact on the optical qualities of the finished product. Thus it is of advantage to keep the temperature at the point of maximum seeding for a finite length of time in order to allow several tiny seed crystals to nucleate, and then to stop the nucleation process and support the ones that have formed to

The glass ceramic materials with tailor made properties can be fabricated by controlling nucleation and crystal growth process. Although some glass compositions are selfnucleating, generally some nucleating agents are added to the batch to promote phase separation and internal nucleation. These melt homogeneously into the glass, but promote

(Hölland,et al., 2002; Rincón, 1992 ).

grow to appropriate size.

very ne phase separation on reheating. The isolated phase, which can be a metal, titanate, zirconate, phosphate, sulde, or halide, is structurally incompatible with the host glass and is usually highly unstable as a glass. Nucleation is followed by one or higher temperature treatments to promote crystallization and development of the desired microstructure. Most commercial glass-ceramic products are formed by highly mechanized glass-forming processes and converted to a crystalline product by proper heat treatment. Glass-ceramic materials have been prepared through powder processing in which glass frits are sintered and then crystallized (Grossman, 1974) the range of potential glass-ceramic compositions is therefore extremely broad, requiring only the ability to form a glass and control its crystallization. The production of glass-ceramics from powdered, using conventional ceramic processes such as spraying, slip-casting, or extrusion, extends the range of possible glass-ceramic compositions by taking advantage of surface crystallization. In these materials, the surfaces of the glass grains serve as the nucleating sites for the crystal phases. The glass composition and processing conditions are selected such that the glass softens proceeding to crystallization and undergoes viscous sintering to full density just before the crystallization process is completed. Given these conditions, the final crystalline microstructure is basically the same as that produced from the bulk process.

Fig. 2. (a-c): Crystal growing in aluminosilicate ceramic materials (a) Aluminum deficient silicalite-I (b) High silica aluminosilicate (c) sodium aluminosilicate.

The precursor glass powders may be produced by various methods, the simplest being the milling of quenched glass to an average particle size of 3-15mm. Sol-gel processes (Yuan, et al., 2010; Chen, et al., 2010; and Lei, B. 2010) in which highly uniform, ultrane amorphous particles are grown in a chemical solution, may be preferable for certain applications. Such devitrifying frits are employed significantly as sealing frits for bonding glasses, ceramics,

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 257

Simmons, et al, 1982; studied the effect of fluorine content and its source on the crystallization of MAS materials. Identified predominant crystalline phases in their study were fluorite, norbergite or fluorophlogopite depending on heat treatment, fluorine concentration etc. Common preparation methods have been discussed in literature (Hattori, T. et al. 1982) but due to special category and technological importance of the material, the crucial process has either been vague or missing. Recently, magnesium aluminum silicate (MAS) glass ceramic systems have been synthesized by sintering route (Durrani, et al., 2010 & Hussain, S.Z et al., 2010). Therefore, the preparation of MAS materials acquires special importance for meeting indigenous requirement. In the light of this fact, preparation of machinable MAS glass ceramic was undertaken using sintering

Generally, when ceramic powders are formed and then heated (green compact) part, there is a certain temperature below melting point at which they begin to burn, and in most cases there is shrinkage or expansion resulting in densification, phenomenon is called sintering. (Moulson et al. 1992; Rahaman, 2005). The goal of the sintering process is to convert highly porous compacted powder into high strength bodies. Sintering may be considered the process by which an assembly of particles, compacted under pressure or simply confined in a container, chemically bond themselves into a coherent body under the influence of an elevated temperature. The temperature is usually below the melting point of the major constituent. Much of the difficulty in defining and analyzing sintering is based on the many changes within the material that may take place simultaneously or consecutively. In the sintering process the temperature of the granulated sinter compound is raised to temperatures between 1000oC and 1450oC to achieve partial fusion. During the heating and cooling cycle different species react with each other to produce certain phases. Molten material is produced which crystallizes or solidifies into various phases that bond the microstructure together. Therefore, sinter consists of an assembly of various phases of varying chemical composition and morphology. Each of different phases has a unique

Sintering is a complex process and for any given metal and set of sintering conditions there are likely to be different stages, driving forces and material transport mechanisms associated with the process. Sintering or firing of pure oxide ceramic requires relatively long time and high temperature because the diffusion proceeds in solid state. The complete sintering process is generally considered to occur in three stages: (i) initial stage, (ii) intermediate stage, (iii) and final stage. There is no clear-cut distinction between the stages since the

The reduction in surface energy can be used to explain the three main stages of solid-state sintering, (Wang, Y., 2008) is shown in Fig. 3(a-c). In the first stage, atoms migrate towards the points of contact between particles to form necks as this filling process reduces the surface area and the surface energy. In the second stage, the grain boundaries grow because, as atoms are removed from the grain boundary and diffuses towards the neck, this causes the centers of particles to mutually converge. In the final stage, the grain is slowly

processes that are associated with each stage tend to overlap each other.

route.

**1.2 Sintering** 

influence on the sinter quality.

eliminated as grain boundaries merge.

and metals. Other applications include co-red multilayer substrates for electronic packaging, matrices for fiber-reinforced composite materials, refractory cements and corrosion-resistant coatings.

### **1.1.3 Machinable glass ceramic**

Machinable glass ceramics based on magnesium aluminum silicate (MgO-Al2O3-SiO2, MAS) glass ceramic system have technological importance due to their applications for high voltage and in ultra high vacuum (Hattori, T. et al., 1982, Emad M El-Meliegy, 2004). An important group of these materials are mica-containing glass-ceramics. The crystal phase generated in the mica containing glass ceramics is called fluorophlogopite (KMg3AlSi3O10F2) receive wide application due to their high machinability, which results in an increased versatility of the products and numerous possibilities for industrial application.These materials not only have strange feature of machinability but also have potential use till date are in electronic and semiconductor industry (precision coil formers & high voltage insulators), laser industry (spacers, cavities and reflectors in laser assemblies), high vacuum industry (thermal breaks in high temperature processing equipment, coil supports), aerospace and space Industry (retaining rings on hinges, windows and doors of NASA's space shuttle, supports and components in several satellite borne systems) and also in nuclear industry (fixtures and reference blocks in power generation units). It is noted (Hench et al., 1993) that glass ceramics can also be used for biomedical purpose including dental materials field. The MAS glass ceramic can be machined into complicated shapes and precision parts with ordinary metal working tools, quickly and inexpensively, and it requires no post firing after machining, no frustrating delays, no expensive hardware, no post fabrication shrinkage, and also no costly diamond tools to meet the required specifications. MAS ceramic material exhibits non-wetting, zero porosity, withstands high temperatures up to 1000ºC and has high dielectric strength. MAS materials are being used in nuclear technology (Bozadzhiev L.S.; 2011) in the production of proto type components, used in medicines for the axles of mechanisms providing energy for implanted cardiostimulators and also used in the production of welding jets or as holders for welded components. MAS have potential application in spacers, headers and windows for microwave tube devices, sample holders for microscopes and aerospace components (Goswami et al., 2002; Baik, et al. 1995; Boccaccini, 1997). Properties of MAS glass ceramics such as hardness, machinability, conductivity depend upon the composition and microstructure. Machining of these materials can be carried out to precise tolerances and surface finish with conventional tools. Factual reason of good machinability character of MAS lies in unique microstructure of inter-locking array of plate like mica crystals, dispersed uniformly throughout glass matrix.

MAS glass ceramic materials have been prepared by controlled crystallization in which a large number of tiny crystals rather than few bigger single crystals have been grown (Margha, et al., 2009). Controlled crystallization or heat treatments generally consist of a two-stage heat treatment, namely a nucleation stage and crystal growth stage. In the nucleation stage, small nuclei are formed within the parent glass. After the formation of stable nuclei, crystallization taking place by growth of a new crystalline phase. The nucleation and crystallization parameters of glasses are very significant in the preparation of glass-ceramics with desired microstructures and properties (Abo-Mosallam, 2009). Simmons, et al, 1982; studied the effect of fluorine content and its source on the crystallization of MAS materials. Identified predominant crystalline phases in their study were fluorite, norbergite or fluorophlogopite depending on heat treatment, fluorine concentration etc. Common preparation methods have been discussed in literature (Hattori, T. et al. 1982) but due to special category and technological importance of the material, the crucial process has either been vague or missing. Recently, magnesium aluminum silicate (MAS) glass ceramic systems have been synthesized by sintering route (Durrani, et al., 2010 & Hussain, S.Z et al., 2010). Therefore, the preparation of MAS materials acquires special importance for meeting indigenous requirement. In the light of this fact, preparation of machinable MAS glass ceramic was undertaken using sintering route.
