machinable glass ceramics

Composition, Microstructure, Properties of Machinable Glass Ceramics

MMS 802 Ph.D Seminar report

Submitted in partial fulfillment of the requirements

of the degree of

Doctor of philosophy

By

HARSHAVARDHANA.N Roll.No: 10411413

Under the guidance of

Prof. PARAG BHARGAVA

(Department of Metallurgical Engineering and Materials Science)

Department of Metallurgical Engineering and Materials Science INDIAN INSTITUTE OF TECHNOLOGY BOMBAY

31March, 2011

CONTENTS

List of Figures i

List of Tables ii

Nomenclatures iii

Chapter 1. Introduction 1

1.1 Definition 1 1.2 Glass-ceramics 1 1.3 Machinability of Glass Ceramics 3 1.4 Advantages Machinable Glass ceramics 3 1.5 Application of machinable glass ceramics 4 Chapter 2 Literature Survey 5 2.1 Background of Invention 5 2.2 Composition of Machinable glass-ceramics 7 2.3 Preparation of Machinable glass-ceramics 7 2.4 Properties of machinable glass ceramics 11 2.5 Microstructure Study of Machinable Glass Ceramics 13 2.6 Machinability of Machinable Glass Ceramics 18 Chapter 3 Summary 21 References 23

List of Figures

1) Components from machinable glass ceramics 1

2) Heat treatment cycle for glass ceramics 2

3) Heat treatment cycle for a machinable glass-ceramic material 8

4) DTA diagram of the base glass 8

5) Schematic flow diagram of heat treatment process for machinable glass-ceramic 9

6) Crystal Structure of glass-ceramics at 586°C after 5 minutes 9

7) Crystal Structure of glass-ceramics at 910°Cafter 5 minutes 10

8) Crystal Structure of glass-ceramics at 940°C after 5 minutes 10

9) Crystal Structure of glass-ceramics at 950°C 10

10) Microstructure of machinable glass-ceramic by SEM and TEM 13

11) X-ray diffraction pattern of machinable glass-ceramics at various temperature

range. 14

12) X-ray diffraction pattern of machinable glass-ceramics subjected

to heat treatment 15

13) Microstructure and Energy Dispersed X-ray analysis of machinable

glass-ceramics. 15

14) Microstructure at 650oC after 1 hr of heating 16



17) Microstructure at 950oC 17

18) Microstructure of machinable glass ceramics with high aspect ratio 18

19) Tool wear vs Time of machining for machinable glass ceramics for

turning operation 20

i

List of Tables

1. Composition of Machinable glass-ceramics 7

2. Mechanical Properties 11

3. Thermal Properties 12

4. Electrical Properties 12

ii

Nomenclatures

Si Silicon

K Potassium

B Boron

Mg Magnesium

DTA Differential Thermal Analysis

XRD X-ray Diffraction

EDX Energy Dispersed X-Ray Diffraction

TEM Transmission Electron Microscope

SEM Scanning Electron Microscope

iii

1

Composition, Microstructure, Properties of Machinable Glass Ceramics

Chapter-1: Introduction

1.1. Definition: Machinable glass-ceramics is a white, opaque polycrystalline materials formed by the

controlled crystallization of glass. These glass ceramics can be easily machined into desired

shape using standard metalworking tools. [1]

Glass Ceramics + Machinability = Machinable glass-ceramics

Figure 1: Components from Machinable Glass Ceramics [2]

1.2. Glass-ceramics: The glass-ceramics are the polycrystalline materials formed by controlled

crystallization of glass It exhibits both the properties glasses and ceramics. These glass-

ceramics are produced by controlled crystallization process which results in exhibiting both

amorphous and crystalline phases.[1]

The production of glass-ceramics involves two main steps. .In the first stage of heat

treatment process, the batch is melted at 1700oC which results in formation of transparent

glass. During second stage of heat treatment, the glass-ceramics is produced by heating the

glass to a temperature range of 530oC to 760oC for the considerable period of time of about 8

hrs, which results in nucleation of crystals and followed by heat treatment to a temperature

2

range of 850oC to 1100oC for about 3 hrs, which results in the formation of randomly

oriented glass-crystals.. Thus by the end of this process, partly crystallized glass ceramics

structure is formed which has the application in many field. [1]

Fig 2:Heat treatment cycle for glass ceramics [3]

General properties of Glass ceramics are as follows [3]

Odourless, opaque white material

High temperature resistance

Non-porous

Dimensionally stable

Good insulator

A wide variety of glass-ceramic which are widely used are as follows [4]

Li2O x Al2O3 x nSiO2-System (LAS-System),

MgO x Al2O3 x nSiO2-System (MAS-System),

ZnO x Al2O3 x nSiO2-System (ZAS-System),

Glass-ceramics made of Lithium-Disilicate and

Machinable glass-ceramics

3

1.3. Machinability of Glass Ceramics:

Machinability is defined as the ability of the material to machine easily with the

acceptable level of surface finish and depth of cut. Machinability is difficult to predict as it

involves many variables such as microstructure of the materials, grain size, heat treatment,

chemical composition, fabrication method, hardness, yield strength, of the materials, tensile

strength of the materials etc. Further it dependents on the physical conditions such as

modulus of elasticity, thermal conductivity, thermal expansion, and work hardening. [5]

The machinability of material can be predicted based in the following methods

a) Tool life method:

Machinability of material can be predicted by measuring amount of wear on

the tool for the constant depth of cut and surface roughness.

b) Tool forces and power consumption method

Machinability of material can be predicted by calculating the total force and

total power consumption acting on the material for the constant depth of cut and

surface roughness.

c) Surface finish method

Machinability of material can be predicted by measuring surface roughness

value for the desire depth of cut and tool.[5]

1.4. Advantages Machinable Glass ceramics

It can operate at continuous usable temperature of 800 °C and with a peak

temperature of 1000 °C.

It is a low thermal conductivity and a good thermal insulator even at very high

temperatures

It also act as an excellent electrical insulator

Machinable glass ceramics is porous-free material and does not outgas when

baked out. This makes the machinable glass ceramics as an ideal material for

ultrahigh vacuum applications.

4

It has a very high strength, rigidity and creep limit.

It is radiation-resistant and is therefore used in nuclear engineering.

Machinable glass ceramics has a property to join or sealed to itself or other

materials in a number of ways through metallizing, brazing, fritting or using

epoxy resin.

It is white and can be bright-polished. Thus it is used in medical and optical

devices. [4,6,7]

1.5. Application of machinable glass ceramics:

Machinable Glass ceramics can be widely used in following areas [4,6,7]

Flight and aerospace applications

Used as spacers, headers and windows for microwave tube devices

Used as substrates for Field Ion Microscopes

Used in welding nozzles

Medical equipments.

Sample holder for microscope

Sealing glass.

High temperature applications

Used in stove windows, cookware and tableware etc

5

Chapter-2 - Literature Survey

2.1. Background of Invention

Sazmal (2008) et al have worked in crystallization and microstructural evolution of

commercial fluosilicate glass-ceramic in which the glass ceramics is characterized by using

advanced microscopy techniques. Further the microstructural characteristics and crystal

evolution based on melting, heat treatment and phase transformation of crystal was studied in

this journal.[11]

Denry1 (1999) et al have worked in preparation and characterization of a new

lithium-containing glass-ceramic in which he has compare the thermal properties and

microstructure of a new lithium-containing glass-ceramic to a experimental dental glass

ceramic. The chemical composition of both control and experimental glasses was determined

by electron microprobe analysis. The nucleation and crystallization temperatures were

determined by Differential Thermal Analysis (DTA). The glass specimens were submitted to

various heat treatments and analyzed by X-ray diffraction (XRD). Thus the results showed

that optimal crystallization of the experimental glass-ceramic was achieved after heat

treatment at 950°C for 30 min. Further it is observed that the microstructure of the

experimental glass-ceramic exhibited mica platelets randomly oriented and highly

interlocked.[16]

Balk et al (1995) have worked in comparative evaluation method of machinability for

mica-based glass-ceramics and machinability of mica glass-ceramics is evaluated using a tool

dynamometer. Several samples with different chemical compositions and microstructures

were tested in turning operations using TiCN cermet tools. Thus the cutting rate has been

studied to for the evaluation of machinability. The mechanical strength, surface roughness

and fracture toughness were measured to support the machining behavior.[17]

6

Saraswati et al (1992) have worked on glassed ceramics with K20-MgO-AI2O3-

MgF2-SiO2 composition through the sol-gel. Thus the resultant powder produced after heat

treatment is hot pressed into workable discs. Thus the glass-ceramic was found to be

machinable with conventional tools. Its physical and mechanical property is compare with

commercial macor and it is tabulated. Further the microstructure study is also made to

analyze the flexure strength of the given machinable glass ceramics.[15]

Toshio Hamasaki et al (1988) have worked on prepartion and characterized

machinable mica glass ceramics by Sol-gel process. The physical, chemical, electrical and

mechanical property of machinable glass ceramics are investigated and machinability of

ceramics are discussed.[19]

James et al (1987) have worked on preparation of mica based glass ceramics by using

the composition range as(in mole%): Al2O3 -1.5 to 15%, CaO - 22 to 55%, P2O5 - 28 to 65%,

SiO2 is upto 15.0%, Other Oxides is upto 15%. Two stages were used to prepare the

machinable glass ceramics. The first stage results in formation of glass and second stage is by

nucleating at elevated temperature to form a crystalline phase. [10]

David D. Grossman (1974) have worked on preparing machinable glass ceramics by

varying the composition K2O 6-9%, Li2O 2-4%, MgO 19-22%, SiO2 57-62%, F 6-8.5% with

the fluoromica comprises the principle crystal phase which formed from the molten metal.

Thus mechanical properties and machinability of the machinable glass ceramics is

calculated.[10]

George et al (1974) have worked on glass article wherein the predominant crystal

phase is synthetic fluromica. The composition are K2O - MgO –Al2O3- B2O3 - SiO2 - F with

microstructure consisting of very large 2D crystal having high aspect ratio. This feature is

easily cleavable in brittle matrix impart excellent machinability fracture energy.[20]

David D.Grossman (1973) have worked on preparation of tetrasilicic fluorine mica

glass ceramics from the controlled crystallization of glass containing basic oxide such as Si02

40-70%, MgO 8-20%, MgF3 8-15%, other oxides 5-35%. Thus these tetrasilicic fluorine mica

glass ceramics exhibits good machinability with steel tool, good mechanical strength, modern

thermal expansion and good chemical durability.[13]

7

2.2. Composition of machinable glass-ceramics

The general composition of machinable glass-ceramics are shown below [9,10, 11]

Starting Materials Oxide Constituent Wt %

Silica Gel in powder form

SiO2 45 to 70%

White Tabular Alumina

Al2O3 1.5 to 17%

MgO powder MgO 8 to 15%

MgF powder MgF 0 to 10%

K2CO3 K2O 0 to 20%

Boric Acid (H3BO3)

B2O3 7%

NH4F F 4%

Al2O3 promotes in internal crystal nucleation leading to the formation of

precipitate of AlPO4 crystal during the heat treatment process. When little Al2O3 is added to

glass-ceramics results in difficulty in crystallization and when more amount of Al2O3 results

in increasing in melting of the glass. Thus Al2O3 must added in a right proportion to obtain

AlPO4 precipitate which acts as major nucleating site for the formation of crystal phase.[11]

Further the addition of compound such as Na2O, K2O, Li2O, MgO, BaO and ZnO

to a small amount (0 to 5%) in glass ceramics results in reduction of liquidus temperature and

further modifying the viscosity of the melt.[11]

2.3. Preparation of Machinable glass-ceramics:

The machinable glass ceramics can be produced by heating the batch to the sufficient

high temperature followed by cooling results in the formation tetra silacic-mica glass

ceramics which is a simple quaternary system of K2O-MgF2-MgO-SiO3.In the first stage of

heat treatment process, the batch is melted at 1400oC which results in the formation of

opalescence glass. During second stage of heat treatment, the glass-Ceramics is produced by

8

is heating the glass to a temperature range of 560oC to 760oC for 8 hrs, which results in

nucleation of crystals and followed by heated to a temperature range of 850oC to 1100oC for

3 hrs, which results in formation of ceramics crystals. Thus results in formation of randomly

oriented tetra silicic-mica crystals.[10,13]

Fig. 3 : Heat treatment cycle for a machinable glass-ceramic material [14]

After completion of first stage of heat treatment process, Differential Thermal Analysis

(DTA) is carried on the base glass at sufficient temperature range of 40o-1100oC inorder to

predict the nucleating and crystallization temperature of the glass-ceramics.[8]

Fig. 4 : DTA diagram of the base glass [8]

9

Fig 5: Schematic flow diagram of heat treatment process for machinable glass-

ceramic[4,10,13]

The effect of heat treatment on crystallization is discussed below.

a) At the temperature slightly above the annealing point (i.e from 586oC to 900 oC),

there is a formation of fine scale phase which leads to the crystallization of spherical mica of

diameter 400Ǎ, which results in completely transparent material.[12]

Fig 6: Crystal Structure of glass-ceramics at 586°C after 5 minutes [12]

The above mentioned raw materials is mixed together in ball mill using acetone medium for 24 hrs. Thus results in formation of batch.

Melting of batch at the sintering temperature of 1500oC for 30Minutes followed by quenching in air results in formation of Glass.

Differential thermal analysis (DTA) on the base glass is carried at the temperature range of 40o-1100 o C in order to find the nucleating and crystallization temperature.

Conversion of glass into Glass Ceramics by nucleating heat treatment Process. In this process glass is heated to a temperature range of 560oC to 760oC for 8 hrs.

Further it is followed by crystallization heat treatment Proces in which the glass is heated to a temperature range of 850oC to 1100oC for 3 hrs.

Polished and etched with 12% HF solution for 5 min. using acetone medium for 24 hrs

Characterization using OM, SEM, TEM, EDS, XRD, DTA, Microhardness tester etc.

10

b) When the temperature increased further, results in the increase in microstructure of

spherical mica to 0.2µm. and thus the material becomes opaque. The Variation of the size of

mica with respect temperature is clearly shown in fig 2.5 [11]

Fig 7: Crystal Structure of glass-ceramics at 910°Cafter 5 minutes [11]

Fig 8: Crystal Structure of glass-ceramics at 940°C after 5 minutes [11]

Fig 9: Crystal Structure of glass-ceramics at 950°C [12]

11

Thus in case of machinable glass-ceramics of variable microstructures, it is found that the

machinability is inversely proportional to the mechanical strength. Very fine-grained crystal

will have high-strength which is relatively more difficult to machine, where as coarse-grained

crystal will have low-strength which is relatively more easy to machine. Thus by heat

treatment process, coarse grained mica is obtained which can be easily machined to obtain

the desired shape.[12]

2.4. Properties of machinable glass ceramics

2.4.1. Mechanical Properties [9]:

Property Values for machinable glass ceramics

Density 2.52 g/cm3

Porosity 0%

Young's Modulus (25°C) 66.9 GPa

Poisson's Ratio 0.29

Shear Modulus (25°C) 25.5 GPa

Modulus of Rupture (25°C) 94 MPa

Compressive Strength 345 MPa

Fracture Toughness 1.53 MPa m0,5

12

2.4.2. Thermal Properties [9]:

Property Values for Machinable Glass Ceramics

Coefficient of Thermal Expansion

74 x 10-7 / °C (@ -200 to 25°C)

93 x 10-7 / °C (@ 25 to 300°C)

114 x 10-7 / °C (@25 to 600°C)

126 x 10-7 / °C (@25 to 800°C) Continuous operating temp 800°C

Max. operating temperature 1000°C (no load)

Thermal Diffusivity (25°C) 7.3 x 10-7 m2 / s

Thermal Conductivity (25°C) 1.46 W/m°C

Specific Heat (25°C) 0.79 KJ / kg°C

2.4.3. Electrical Properties [9]:

Property Values for Machinable Glass Ceramics

Dielectric Constant (25°C) 6.03 (1 KHz)

Dielectric Loss Tangent (25°C) 4.7 x 10-3 (1 KHz)

Dielectric Strength (25°C) 40 KV / mm (at 0.254mm thickness)

Volume Resistivity Greater than 1016 Ω-cm

13

2.5. Microstructure Study of Machinable Glass Ceramics

By X-ray diffraction study, it is revealed that the phases in machinable glass ceramics are

fluorophlogopite (KMg3AlSi3O10F2), mullite (3Al2O3 .2SiO2), magnesium fluoride (MgF2)

and a significant amount of glass.[11]

Scanning Electron Microscope image (SEM) clearly shows the morphology of the crystal

phases comprises of fluorophlogopite laths (~1-50 µm), elongated mullite (1-3 µm) and

spheroidal magnesium fluoride (~1 µm in diameter). Further the Energy Dispersed X-Ray

(EDX) analysis clearly confirms the presence of the mullite and MgF2 and also reveals glass

is a homogeneous composition potassium aluminosilicate [11]

(a) (b)

(c)

Figure 10: Microstructure of machinable glass-ceramic by SEM (a & b) and TEM (c) [11]

14

X-ray diffraction study on the machinable glass-ceramics clearly reveals the crystallization

sequence at the various temperatures range. Thus after 1 h at 650oC, results in the formation

of chondrodite which can be identified by measuring the peak intensity. Further at 750oC

after 1 h, result in formation of norbergite and at 850oC after 1 h result in formation

fluorophlogopite and after 4 h at 950oC, there is a formation of only fluorophlogopite and

mullite remain [11]

Figure 11: X-ray diffraction pattern of machinable glass-ceramics at various

temperature range HT1, 1 h at 650oC; HT2, 1 h at 750oC; HT3, 1 h at 850oC and HT4, 4h at 950oC [11]

X-ray diffraction study on the machinable glass-ceramics subjected to heat treatment

at 4 h at 950oC and the two-step heat treatment (700oC for 2 h and 950oC for 4 h) revealing

that fluorophlogopite is the dominant phase and other minor phases are mullite and

magnesium fluoride etc [11]

15

Figure 12. X-ray diffraction pattern of machinable glass-ceramics subjected to heat treatment at 4 h at 950oC and the two-step heat treatment (700oC for 2 h and 950oC for

4 h) [11]

Energy Dispersed X-ray analysis (EDX) revealed the glass matrix (light contrast)

consist of potassium magnesium aluminosilicate and phase separated regions rich in Mg and

F with no K. The major component of the glass would be B2O3 and further it has K2O, Al2O3

and SiO2 thus results in producing potassium magnesium (boro)aluminosilicate glass [11]

(a) (b)

16

(c) (d)

Figure 13: Microstructure of machinable glass-ceramics showing phase separation (a) by SEM (b) TEM; (c) Energy Dispersed X-ray analysis (EDX) shows light contrast

continuous Mg, Al, Si, K,O-containing glass; (d)Energy Dispersed X-ray analysis (EDX) shows the darker isolated regions rich with Mg and F with no K [11]

At 650oC after 1 hr of heating crystals size is found to be less than 1 µm which results

in domination of microstructure presumably chondrodite, which is indicated by XRD pattern.

Thus the SEM image shows that there is a clustering of crystals taken place and these clusters

will act as a initial point of formation of the fluorophlogopite crystals [11].

Figure 14: Microstructure at 650oC after 1 hr of heating [11, 12]

At 750oC after 1 hr of heating the crystals of chondrodite was observed along with

formation of fluorophlogopite with a fan-like morphology. The morphology is due to the

formation of small cuboidal/spheroidal crystals with fluorophlogopite laths formation in

outwards direction. EDX analyses on this crystals revealed that they have the same

proportion as that of fully developed fluorophlogopite laths.[11]

17

Figure 15: Microstructure at 750oC after 1 hr of heating [11]

The SEM image shows, the microstructure consists of predominantly

fluorophlogopite laths uniformly throughout after 1 h for 850oC, with minor cuboidal mullite

[11]

Figure 16: Microstructure at 850oC after 1 hr of heating [11]

The Complete fluorophlogopite laths is formed by heating the sample to 950oC for the period

of 4 hrs. Thus temperature gets increased, results in increase in crystal growth.[11]

Figure 17: Microstructure at 950oC [11, 12]

18

2.6. Machinability of Machinable Glass Ceramics

The machinability as well as other mechanical properties of machinable glass-ceramics is

dependent on the following factors.[12]

Microstructure of mica

Degree of interlocking of mica crystals.

Two important factor that affects the degree of interlocking are [12]

Aspect ratio (plate diameter to the thickness)

Volume percentage of mica crystal.

Thus if the glass ceramics which containing 1/3 rd volume of mica, thus results in formation

of machinable glass ceramics. Thus, by making the volume percentage of mica crystal as a

constant, the aspect ratio of the crystal directly affects the machinability. Thus, the aspect

ratio need to be high enough to cause high degree of interlock.[12]

Figure 18: Microstructure of machinable glass ceramics with high aspect ratio [12]

Thus, the average mica plate diameter and thickness can be measured from the scanning

electron microscope. For extremely fine-grained mica glass-ceramics (plate diameters less

than 4.5 µm), the mechanical strength increases with the decrease in mica-plate diameter.

Thus, the resistance to the dislocation increases with the reduction in thickness value. [12]

19

Most machinable glass-ceramics have mica plate diameters in the order of 20 µm with some

varieties having mica crystals as large as 250 µm. Over this range in crystal size, the strength

is found to be inversely proportional to the flake diameter. The decrease in strength with

increase in size. Further the strength controlling flaws are also increased as the mica grain

size increases.[12]

For machinable glass-ceramics of variable microstructures, machinability is inversely

proportional to the mechanical strength. Very fine-grained crystal will have high-strength

which is relatively more difficult to machine, where as coarse-grained crystal will have low-

strength which is relatively more easy to machine. Thus, by heat treatment process, coarse

grained mica is obtained which can be easily machined to obtain the desired shape.[12]

Machinable glass ceramics can be machined to make precision components but its machining

characteristics are different to metals and plastics. Machinable glass ceramics consists of

interlocking plate-like mica crystals in a glassy matrix. These crystals acts as a barrier for

stopping microscopic fractures, thus machinability is reduced. During machining, the tool

pulverizes and tears the surface to produce a fine powder of crystals and glass. The crystals

are so small (i.e less than 20 µm) that, which results in a good surface finish. After

machining, the component is cleaned and ready for use with no further treatment.[9]

The various machining operation that can be preformed on machinable Glass Ceramics are as follows [9]

a) Sawing

Machinable glass ceramic can be sawed using carbide grit blade with a 30

m/min band speed, or a diamond or silicon carbide cut-off wheel.

B) Turning

Machinable glass ceramic can be machined using carbide tipped tools with the

suggested turning speeds is around 600 rpm for the diameter 5mm to 10mm rod.

Value of Feed rates will be around 20-30 mm/minute with a depth of cut value is 2-

4mm for roughing and less than 1mm for finishing.

Side and back rake angle, end and side relief angles should be around 5°. The

recommended side cutting edge angle is 15°-45° and the nose radius should be larger

20

than 0.8mm.Thread cutting can also be done at low spindle speeds with the depth of

cut value as 0.025-0.040mm per pass.

Figure 19 – Tool wear vs Time of machining for machinable glass ceramics for turning operation [12]

c) Milling

Machinable glass ceramic can be milled using typical head speeds of 1000–

1500 rpm with a chip load of 0.05mm per tooth. Depths of cut must be kept minimum

for the milling condition.

d) Drilling

Machinable glass ceramic can be drilled to a holes of diameter 5mm with a

spindle speed of 1000–1500 rpm and a feed rate of 20-30 mm/min. Important point is

to relieve the drill flutes constantly, especially for drilling small diameter holes.

e) Grinding and Polishing

Machinable glass ceramic can be grind using diamond grinding wheels for

obtaining the best results although silicon carbide and alumina wheels can be used.

Water is used as coolant.

21

Chapter 3 - Summary Machinable glass-ceramic is a class of ceramic material which is capability of being

machined to precise tolerances using conventional metalworking tools and equipment.

Machinable glass ceramic is a porcelain-like (in appearance) material composed of

approximately 55% fluorphlogopite mica in a glass matrix. It is non- toxic in nature. The

production of machinable glass-ceramics involves two main steps. .In the first stage of heat

treatment process, the batch is melted at 1700oC which results in formation of transparent

glass. During second stage of heat treatment, the glass-ceramics is produced by heating the

glass to a temperature range of 530oC to 760oC for the considerable period of time of about 8

hrs, which results in nucleation of crystals and followed by heated to a temperature range of

850oC to 1100oC for about 3 hrs, which results in the formation of randomly oriented glass-

crystals. Al2O3 present in machinable glass ceramics promotes in internal crystal nucleation

which leads to the formation of precipitate of AlPO4 crystal during the heat treatment

process. When little Al2O3 is added to glass-ceramics results in difficulty in crystallization

and when more amount of Al2O3 results in increasing in melting of the glass. Thus Al2O3

must added in a right proportion to obtain AlPO4 precipitate which acts as major nucleating

site for the formation of crystal phase. Further the addition of compound such as Na2O, K2O,

Li2O, MgO, BaO and ZnO to a small amount (0 to 5%) in glass ceramics results in reduction

of liquidus temperature and further modifying the viscosity of the melt. Glass-ceramics with

fine-grained crystal will have high-strength which is relatively more difficult to machine,

where as coarse-grained crystal will have low-strength which is relatively more easy to

machine. Thus by heat treatment process, coarse grained mica is obtained for obtaining the

better machinability.

The microstructure study is done at the various phases for determining the

structure of machinable glass ceramics at various conditions. By X-ray diffraction study, it is

revealed that the phases in machinable glass ceramics are fluorophlogopite

(KMg3AlSi3O10F2), mullite (3Al2O3 .2SiO2), magnesium fluoride (MgF2) and a significant

amount of glass. Energy Dispersed X-ray analysis (EDX) studies clearly reveals that the glass

matrix (light contrast) consist of potassium magnesium aluminosilicate and phase separated

regions rich in Mg and F with no K. The major component of the glass would be B2O3 and

22

further it has K2O, Al2O3 and SiO2 thus results in producing potassium magnesium

(boro)aluminosilicate glass. As the temperature increases from 650oC to 950 oC, there is a

change of phase from chondrodite to fluorophlogopite crystals. Thus at 650oC after 1 hr of

heating crystals size is found to be less than 1 µm which results in domination of

microstructure presumably chondrodite, which is indicated by XRD pattern. At 950oC after 1

hr of heating crystals size is found that the size of the mica increases to 20 µm which results

in domination of microstructure presumably fluorophlogopite crystals which are coarse in

nature which results in machinability of glass ceramics.

The machinability as well as other mechanical properties of machinable glass-ceramics is

dependent on the following factors such as microstructure of mica, degree of interlocking

aspect ratio (plate diameter to the thickness) etc. Thus strength is found to be inversely

proportional to the flake diameter. There is a decrease in strength with increase in size. Very

fine-grained crystal will have high-strength which is relatively more difficult to machine,

where as coarse-grained crystal will have low-strength which is relatively more easy to

machine. Thus coarse grained mica can be preferred for obtaining the desired shape.

23

References:

[1] Kirk-Othmer, “Encyclopedia of Chemical Technology”, John Wiley & Sons, Inc, p

626-643.

[2] www.EuropTec.com

[3] http://www.pgo-online.com/intl/katalog/macor_machinable_glass_ceramic.html

[4] http://en.wikipedia.org/wiki/Glass-ceramic

[5] http://en.wikipedia.org/wiki/Machinability.

[6] http://www.ceramic-substrates.co.uk/macor_ceramic.html

[7] http://www.plasticsintl.com/datasheets/MACOR.pdf

[8] Shibayan Roy (2004) ‘Microstructure evaluation of machinable mica based glass

ceramics for dental application’, International Symposium of Research Students on

Material Science and Engineering, December 20, 2004, Chennai, India

[9] http://www.technicalglass.co.uk/pdf/macor_machinable_ceramics.pdf

[10] James, Peter, Finlay, ’Glass-ceramics’, Patent No – W087/ 07256, December 3, 2004

[11] Sazmal E. Arshad, William E. Lee and Peter F. James (2002) ‘Crystallization and

microstructural evolution of commercial fluosilicate glass-ceramic’, July 16, 2001,

Glass Technol., 43C, 69-80

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