**1.2 Sintering**

256 Sintering of Ceramics – New Emerging Techniques

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

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,

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).

corrosion-resistant coatings.

**1.1.3 Machinable glass ceramic** 

dispersed uniformly throughout glass matrix.

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 influence on the sinter quality.

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

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 eliminated as grain boundaries merge.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 259

 Fig. 4. (a-b): Microstructure of sintered pellet of (a) Metallographic image of magnesium aluminum silicate glass ceramic (MAS-G4) (b) SEM image showing combinations of grains,

The polycrystalline ceramic's microstructure consists of numerous grains or crystallites that are defined by the long-range order of the crystal. These grains are separated by a grain boundary that may include single or multiple phases of sintered aid material. A grain boundary is the surface that separates the individual grains and is a narrow zone in which the atoms are not properly spaced. In addition, microstructure is likely to contain a certain amount of porosity left behind during manufacture. Sometime separate phases of material exist within the microstructure as particles as is commonly found in reaction-bonded ceramics (Swab, 2009). During the sintering process, an agent (sintering aid) is added to help in the bonding process and reduced the temperature that is required during the sintering step. These sintering agents lead to a reduction in the mechanical properties of ceramic because they form relatively soft grain boundaries with low melting temperatures. Smaller particles sinter much faster than coarse particles because the surface area is larger and the diffusion distances are smaller. Rate of sintering varies with temperature. Sintering processes can be divided into three large categories (Yu, et al., 2001; Moghadam, et al.1982):

The most important factors involved during sintering process are "process variables" (temperature, time and furnace atmosphere), "materials variables" (particle size, shape and structure), green density and dimensional changes. Variety of sintering methods are available for sintering the ceramic compacts i.e., standard pressure sintering, reaction-sintering, hot pressing, post-reaction sintering, recrystallization sintering, atmospheric pressure sintering, ultra-high-pressure sintering (Ring, T.A. 1996), chemical vapor deposition (CVD) (Mitchell,

The present work concerns only about solid phase sintering process. It is very difficult to sinter fully dense state of machinable magnesium aluminum silicate (MAS) glass ceramic

(2004) and isostatic hot pressing (HIP) (Olevsky, et al. 2009, Smothers, 2009).

grain boundaries and porosity.

i. Solid Phase Sintering ii. Liquid Phase Sintering iii. Gas Phase Sintering

**1.3 Aim of work** 

**1.2.1 Factors effect the sintering** 

Fig. 3. (a-c): Schematic representation of the neck formation during solid-sintering due to (a) powder compaction, (b) neck growth, and (c) neck growth developed by densification.

The description of such topic is beyond the scope of this chapter, however, a detailed account of these stages has been discussed somewhere else (Richerson, 1992; & German, 1996). The sintering process is an important tool in fabrication of various materials into useful products. Today, it is used to manufacture a wide range of products for consumers, electronics, transportation and biomedical systems, e.g., rocket nozzles, ultrasonic transducers, automobile engines, semiconductor packaging substrate, and dental implants. Sintering is not only used for high temperature materials but also for other materials that can be densified below 1000°C. For example, firing of glass-based substrate and of-screenprinted metallic inks or paste for microelectronic applications. Densification or shrinkage of the sintered part is very often associated with all types of sintering. However, sintering can take place without any shrinkage; expansion or no net dimensional change is quite possible. From the tooling point of view it is preferred to avoid very large amount of dimensional changes. The driving force for solid state sintering is the excess surface free energy. Sintering converts a compacted powder into a denser structure of crystallites joined to one another by grain boundaries. Grain boundaries vary in thickness from 100pm to over 1μm.They may consist of crystalline or vitreous second phases, or may be simply a disordered form of the major phase because of differing lattice orientation in the neighboring grains. Grain boundary is the border between two grains, or crystallites, in a polycrystalline material. Grain boundaries are generally not as dense as the crystals and, in the early stages of sintering at least, allow free diffusion of gas to and from the outside atmosphere. Typically, polycrystalline ceramic's microstructure is developed by solid state sintering as shown in Fig. 4(a-c).

Fig. 4. (a-b): Microstructure of sintered pellet of (a) Metallographic image of magnesium aluminum silicate glass ceramic (MAS-G4) (b) SEM image showing combinations of grains, grain boundaries and porosity.

The polycrystalline ceramic's microstructure consists of numerous grains or crystallites that are defined by the long-range order of the crystal. These grains are separated by a grain boundary that may include single or multiple phases of sintered aid material. A grain boundary is the surface that separates the individual grains and is a narrow zone in which the atoms are not properly spaced. In addition, microstructure is likely to contain a certain amount of porosity left behind during manufacture. Sometime separate phases of material exist within the microstructure as particles as is commonly found in reaction-bonded ceramics (Swab, 2009). During the sintering process, an agent (sintering aid) is added to help in the bonding process and reduced the temperature that is required during the sintering step. These sintering agents lead to a reduction in the mechanical properties of ceramic because they form relatively soft grain boundaries with low melting temperatures. Smaller particles sinter much faster than coarse particles because the surface area is larger and the diffusion distances are smaller. Rate of sintering varies with temperature. Sintering processes can be divided into three large categories (Yu, et al., 2001; Moghadam, et al.1982):

i. Solid Phase Sintering

258 Sintering of Ceramics – New Emerging Techniques

Fig. 3. (a-c): Schematic representation of the neck formation during solid-sintering due to (a) powder compaction, (b) neck growth, and (c) neck growth developed by densification.

sintering as shown in Fig. 4(a-c).

The description of such topic is beyond the scope of this chapter, however, a detailed account of these stages has been discussed somewhere else (Richerson, 1992; & German, 1996). The sintering process is an important tool in fabrication of various materials into useful products. Today, it is used to manufacture a wide range of products for consumers, electronics, transportation and biomedical systems, e.g., rocket nozzles, ultrasonic transducers, automobile engines, semiconductor packaging substrate, and dental implants. Sintering is not only used for high temperature materials but also for other materials that can be densified below 1000°C. For example, firing of glass-based substrate and of-screenprinted metallic inks or paste for microelectronic applications. Densification or shrinkage of the sintered part is very often associated with all types of sintering. However, sintering can take place without any shrinkage; expansion or no net dimensional change is quite possible. From the tooling point of view it is preferred to avoid very large amount of dimensional changes. The driving force for solid state sintering is the excess surface free energy. Sintering converts a compacted powder into a denser structure of crystallites joined to one another by grain boundaries. Grain boundaries vary in thickness from 100pm to over 1μm.They may consist of crystalline or vitreous second phases, or may be simply a disordered form of the major phase because of differing lattice orientation in the neighboring grains. Grain boundary is the border between two grains, or crystallites, in a polycrystalline material. Grain boundaries are generally not as dense as the crystals and, in the early stages of sintering at least, allow free diffusion of gas to and from the outside atmosphere. Typically, polycrystalline ceramic's microstructure is developed by solid state


#### **1.2.1 Factors effect the sintering**

The most important factors involved during sintering process are "process variables" (temperature, time and furnace atmosphere), "materials variables" (particle size, shape and structure), green density and dimensional changes. Variety of sintering methods are available for sintering the ceramic compacts i.e., standard pressure sintering, reaction-sintering, hot pressing, post-reaction sintering, recrystallization sintering, atmospheric pressure sintering, ultra-high-pressure sintering (Ring, T.A. 1996), chemical vapor deposition (CVD) (Mitchell, (2004) and isostatic hot pressing (HIP) (Olevsky, et al. 2009, Smothers, 2009).

#### **1.3 Aim of work**

The present work concerns only about solid phase sintering process. It is very difficult to sinter fully dense state of machinable magnesium aluminum silicate (MAS) glass ceramic

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 261

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

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

Chemical Composition (wt %) Calcination Ball

MAS -G1 36.62 12.73 16.87 13.93 1.91 17.91 950 24 40 72 MAS -G2 37.57 13.19 16.49 14.26 1.97 16.49 950 24 40 72 MAS -G3 37.72 13.23 16.05 13.98 2.48 16.54 950 24 40 72 MAS -G4 38.51 13.51 16.38 14.27 2.53 14.78 950 24 40 72 MAS -G5 39.28 15.71 15.52 13.59 8.25 7.65 950 24 40 72 MAS -G7 40.67 16.43 14.12 13.43 8.62 6.73 950 24 40 72 MAS -G8 36.34 24.82 13.23 11.32 3.53 10.76 950 24 40 72 MAS -G9 44.32 12.91 15.45 13.62 4.12 9.58 950 24 40 72 MAS -G10 41.75 14.15 16.92 13.54 4.46 9.18 950 24 40 72

Table 1. Chemical composition and reaction conditions for preparation of MAS glass

Experimental results showed the phase stability, thermal stability, compressibility, and sinterability of MAS glass ceramic materials. The crystallinity of MAS glass was studied by

Temperature (oC)

Time

Milling Soaking

(h) Time (h)

all these studies was 0.2 V and WINDETA software was used for data acquisition.

SiO2 Al2O3 MgO K2CO3 B2O3 MgF2

any loss in weight after washing off acids and bases.

MAS-G = Magnesium aluminum silicate glass

ceramic material by sintering route.

**3. Results and discussion** 

**3.1 XRD phase analysis** 

Specimen #

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