7/28/2019 PE - Module 4

1/15

4.1

MODULE IV

ADVANCED MATERIALS AND CERAMICS

SUPER ALLOYS

The term "super alloy" was first used shortly after World War II to describe a group of alloysdeveloped for use in turbo superchargers and aircraft turbine engines that required high performance at

elevated temperatures. The range of applications for which super alloys are used has expanded to manyother areas and now includes aircraft and land-based gas turbines, rocket engines, chemical, and

petroleum plants. They are particularly well suited for these demanding applications because of theirability to retain most of their strength even after long exposure times above 650C (1,200F). Their

versatility stems from the fact that they combine this high strength with good low-temperature ductilityand excellent surface stability.

Super alloys are important in high temperatue applications; hence they are also known as hightemperature or heat resistant alloys.

Super alloys are Nickel, Iron- Nickel and cobalt based alloys generally used at temperature above540oC (1000

oF). Iron cobalt and nickel are transition metals. The Fe-Ni base alloys are an extension of

stainless steel technology and are generally wrought, where as Co base and Ni base super alloys may be

wrought or cast depending on the applications/ composition.Melting points Ni- 1453 oC, Co- 1495 oC, Fe- 1537 oC.

Characteristics of Super alloys

1. Good resistance to corrosion at elevated temperature.

2. Good resistance to mechanical and thermal fatigue at elevated temperature.3. Good resistance to mechanical and thermal shock at elevated temperature.

4. Good resistance to creep at elevated temperature.

Types of Super Alloys

a). Iron Base super alloysGenerally contain from 32% to 67% iro, from 15% to 22% chromium and from 9% to 38% nickel.

Common alloys in this group are Incoloy series.

b). Cobalt base super allys

Generally contain from 35% to 65% cobalt, from 19% to 30% chromium and up to 35% nickel.Cobalt is white coloured metal that resembles nickel. These super alloys are not as strong as nickel base

super alloys, but they retain their strength at high temperatures.

c). Nickel- base super alloys

They are the most common types. They are available in a wide variety of compositions. Theproportion of nickel is from 38% to 76%; they also contain up to 27% chromium and 20% cobalt.

Common alloys in this group are the Hastelloy, Inconel, Nimonic, Rene, Udimet, Astroloy and Waspaloyseries. They have high strength and corrosion resistence at elevated temperature.

Inconel is a nickel-chromium alloy with tensile strength up to1400 MPa. Hastelloy is a nickel-molybdenum-chromium alloy.

Applications

1. Jet engines and gas turbines

http://www.docudesk.com/

7/28/2019 PE - Module 4

2/15

4.2

2. Reciprocating engines3. In tools and dies for hot working metals

4. In nuclear, chemical and petro chemical industries.

Nickel-based alloys can be either solid solution or precipitation strengthened. Solid solutionstrengthened alloys, such as Hastelloy are used in applications requiring only modest strength. In the most

demanding applications, such as hot sections of gas turbine engines, a precipitation strengthened alloy isrequired. Most nickel-based alloys contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts

of B, Zr, and C. Other common additions are Mo, W, Ta, Hf, and Nb (often still referred to as"columbium" although the name "niobium" was adopted by the International Union of Pure and Applied

Chemistry in 1950 after more than 100 years of controversy). In broad terms, the elemental additions inNi-base super alloys can be categorized as being i) forms (elements that tend to partition to the matrix,

ii) ' formers (elements that partition to the ' precipitate, iii) carbide formers, and iv) elements thatsegregate to the grain boundaries. Elements which are considered formers are Group V, VI, and VII

elements such as Co, Cr, Mo, W, and Fe. The atomic diameters of these alloys are only 3-13% differentthan Ni (the primary matrix element). formers come from group III, IV, and V elements and include Al,

Ti, Nb, Ta, Hf. The atomic diameters of these elements differ from Ni by 6-18%. The main carbideformers are Cr, Mo, W, Nb, Ta, Ti. The primary grain boundary elements are B, C, and Zr. Their atomic

diameters are 21-27% different than Ni.

TITANIUM & TITANIUM ALLOYS

Titanium is a low density element (60 % of the density of steel approximately) which can bestrengthened by alloying and deformation processing. It is non magnetic and has good heat transfer

properties. Coefficient of expansion is lower than that of steel. Coefficient of expansion about half ofAluminum. Higher melting point than steel passivates and thereby exhibit a higher degree of immunity to

attack by most of mineral acids and chlorides. Non toxic and biologically compatible with human tissues.The combination of high strength, stiffness, toughness, low density and good corrosion resistance

provided by various titanium alloys at very low to moderately elevated temperature. Allows weightsavings in aerospace structures and other high performance applications.

Titanium and its alloys are used primarily in two areas of applications.

1). Corrosion resistance service

Low strength, unalloyed titanium fabricated into tanks, heat exchangers or reaction vessels.2). High strength applications

High strength alloyed titanium processed in a selective manner depending on factors like thermalenvironment, loading parameters, available product forms, fabrication characteristics and

reliability requirements. Many titanium alloys have been custom designed to have optimumtensile, compressive and creep strength at selected temperatures and at the same time to have

sufficient workability to be fabricated in to mill products suitable for specific applications. Of thedifferent alloys Ti-6Al-4V has been used most widely. It combines mechanical properties with

inherent workability, fabric ability and commercial availability. It has become the standard alloyagainst which other alloys are compared when selecting a titanium alloy for selecting a particular

application.

Applications

7/28/2019 PE - Module 4

3/15

4.3

1. Rotating components in gas turbine engines it require titanium alloys that maximize strengthefficiency and metallurgical stability at elevated temperatures. These alloys also must exhibit low

creep rates along with predictable behavior in stress rupture and low cycle fatigue.2. Aerospace Pressure vessels- it requires optimized strength weldability and fracture toughness at

cryogenic to moderately elevated temperatures. For cryogenic applications the3. Biocompatible4.

The mechanical properties of titanium compare favourably with those of other implantable metalsand alloys

5. Low modulus results in a material that is less rigid and deforms elastically under applied loads6. Titanium has an extreme low toxicity and is well tolerated by both bone and soft tissue.7. Animal experiments have revealed that the materials may be implanted for an extensive length of

time; fibrous encapsulation of the implants is minimal to nonexistent. Histopathological

examinations have failed to reveal any cellular changes adjacent to titanium implants.8. Titanium does not cause hypersensitivity, which makes it the metal of choice in patients suspected

of being sensitive to metals

SHAPE MEMORY ALLOYS

Shape memory alloys are that after being plastically deformed at room temperature in various

shapes, they return to their original shapes up on heating. For example a piece of straight wire made ofthis material can be wound in to helical spring. When heated with a match, the spring uncoils and returns

to the original straight shape. A typical shape memory alloy is 55% Ni-45% Ti; other such alloys are Cu-

Al-Ni, Cu-Zn-Al, Iron-Manganese-Silicon and Nickel- Titanium.

Shape memory alloys have good properties such as good ductility, good corrosion resistance, andhigh electrical conductivity. They can be used to generate motion and/or force in temperature sensitive

actuators. Their behavior can also be reversible that is shape can switch back and forth repeatedly up onremoval and application of heat.

Applications:

Eyeglass Frames Bra Under wires Medical Tools Cellular Phone Antennae Orthodontic Arches Connectors Clamps and fasteners

The Shape memory effect is currently being implemented in:

Coffeepots The space shuttle Thermostats Vascular Stents Hydraulic Fittings (for Airplanes)

Advantages and Disadvantages

7/28/2019 PE - Module 4

4/15

4.4

Some of the main advantages of shape memory alloys include:

Bio-compatibility Diverse Fields of Application Good Mechanical Properties (strong, corrosion resistant)

There are still some difficulties with shape memory alloys that must be overcome before they can

live up to their full potential. These alloys are still relatively expensive to manufacture and machinecompared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this

means that while under the same loading conditions (i.e. twisting, bending, compressing) a steelcomponent may survive for more than one hundred times more cycles than an SMA element

SMA Advantages

1. Produce large recovery stresses2. Easily machined in to different shapes and sizes

3. Manufactured to desirable properties

4. Very effective for low frequency vibrations5. Easily embedded in to laminated compositesSMADisadvantages

1. Slow reaction time2. in effective at higher frequency ranges

3. Low energy efficiency conversion4. Un weldable and expensive for large scale projects.

5. May not be operating with large temperature levels.

SMART MATERIALSUntil relatively recent times, most periods of technological development have been linked to

changes in the use of materials (eg the stone, bronze and iron ages). In more recent years the driving forcefor technological change in many respects has shifted towards information technology. This is amply

illustrated by the way the humble microprocessor has built intelligence into everyday domestic appliances.However, it is important to note that the IT age has not left engineered materials untouched, and that the

fusion between designer materials and the power of information storage and processing has led to a newfamily of engineered materials and structures.

Most familiar engineering materials and structures until recently have been dumb. They have

been preprocessed and/or designed to offer only a limited set of responses to external stimuli. Suchresponses are usually non-optimal for any single set of conditions, but optimized to best fulfill the rangeof scenarios to which a material or structure may be exposed. For example, the wings of an aircraft should

be optimized for take-off and landing, fast and slow cruise etc. However, despite the partial tailoring ofthese structures by the use of additional lift surface, which we see deployed as each passenger aircraft

approaches an airport, such engineering components are not fully optimized for any single set of flightconditions.

Similarly, advanced composites such as glass and carbon fiber reinforced plastics, which are oftenthought to be the most flexible engineering materials since their properties (including strength and

stiffness) can be tailored to suit the requirements of their end application, can only be tailored to a singlecombination of properties.

7/28/2019 PE - Module 4

5/15

4.5

Materials that respond to changes in temperature, moisture, pH, or electric and magnetic fieldsare called smart materials. Smart materials are poised to emerge from the lab in a wide range of medical,

defence and industrial applications. These are the materials that functions as sensing and/ or actuatingmaterials. Passively smart materials possess self repairing or stand-by characteristics. Actively smart

materials utilize feed back. Smart materials reproduce biological functions in load bearing systems.

The three criteria in characterizing smart structures are,1. Definite purpose

2. Means to achieve that purpose3. Possess a biological pattern of functionality.

Common available smart materials are,

a). Actuating materials1. Electrorheological fluids (ER fluids)

2. Shape memory alloys (SMA) - (already discussed)b). Sensing materials

1. Fiber optic sensors (FOS)c). Dual purpose materials (Actuating & Sensing)

1. Magnetostrictive materials2. Piezoelectric Materials

1. ER fluids

Electro-rheological materials are fluids, which can experience a dramatic change in their viscosity

when exposed to an electric field. These fluids can change from a thick fluid (similar to motor oil) tonearly a solid substance within the span of a milli second when exposed to an electric field. The effect can

be completely reversed just as quickly when the field is removed. The composition of each type of smartfluid varies widely. The most common form of ER fluids can be as simple as milk chocolate or corn starch

and oil. ER fluids have mainly been developed for use in clutches and valves, as well as engine mountsdesigned to reduce noise and vibration in vehicles. These fluids show promise in shock absorbers,

dampers for vehicle seats and excersise equipment and optical finishing.Smart materials encompass not only solids but also fluids, electro rheological and magneto

rheological fluids that can change state instantly by the application of electric or magnetic field. It consistof dielectric particles submerged in an oily fluid. It responds to and electric field by the aligning of

dielectric particles.ER fluids are generally used in laminated composite sandwich beam structures for ease of

manufacturing and makes addition of ER fluid.

ER fluid advantages1. Quick response

2. Fairly good structural durability3. Can operate with both AC and DC voltages

4. Induced shear stress in largeER fluid disadvantages

1. Behavior is not well known2. Poor reproducibility

3. Due to settlement of dielectric particles4. Produce small forces

5. Requires large amount of materials.

7/28/2019 PE - Module 4

6/15

4.6

6. Creep is an issue7. Strength is relatively low

8. Requires large amounts of power.

2. Fiber Optic Sensors

When an optical fiber is subjected to perturbations of different kind, it experiences geometrical(size and shape) and optical (refractive index, mode conversion) changes to a larger or lesser extent

depending on the nature and magnitude of perturbation. In fiber optic sensing this response to externalinfluence is deliberately enhanced so that the resulting change in optical radiation can be used as a

measure of the external perturbation. So the optical fiber serves as a transducer and converts measurandslike temperature, stress, strain, rotation or electric and magnetic currents in to a corresponding change in

the optical radiation. Since light is characterized by intensity, phase, frequency and polarization, any oneor more of these parameters undergo a change. The usefulness of the fiber optic sensor therefore depends

up on the magnitude of this change and our ability to measure and quantify the same reliably andaccurately.

There are many variety of fiber optic sensors. Based on the modulation and demodulation a sensorcan be called as intensity, a phase, frequency, or a polarization sensor. Since detection of phase or

frequency in optics calls for interferrometric techniques, the latter are also termed as interferometricsensors.

Based on the application fiber optic sensors are classified as

Physical sensors- used to measure physical properties like temperature, stress etc. Chemical sensors- used for PH measurements, gas analysis, spectroscopic studies etc. Bio- medical sensors used in bio medical application like measurement of blood flow,

glucose content etc.

Fiber optic sensors are also classified as extrinsic or intrinsic sensors. In the former, sensingtakesplace in a region outside of the fiber and fiber essentially served as a conduit for the to and

fro transmission of light to the sensing region. In intrinsic sensor one or more of the physicalproperties undergo a change and this change is measure of the external perturbation.

FOS advantages

1. Small size and light weight2. High sensitivity

3. Corrosion resistant4. Wide frequency band width

5. Simultaneous SENSING OF NUMEROUS PARAMETERS6. High tensile strength and fatigue life

7. quick response and immune to electromagnetic interference

FOS disadvantages1. May need to isolate from un wanted parameters2. Availability of optical sources

3. Cost and availability of instrumentation4. long term stability is relatively un known

5. low awareness of fiber optic technology

7/28/2019 PE - Module 4

7/15

4.7

3.Magnetostrictive materials

This refers to material quality of changing size in response to a magnetic field and conversely,producing a voltage when stretched.These materials show promise in applications ranging from pumps

and valves, to aerospace wind tunnel and shock tube instrumentation and landing gear hydraulics, to biomechanics force measurement for orthopedic gait and petrography, sports, ergonomics, neurology,

cardiology and rehabilitation.It is implemented as an actuator with an applied magnetic field. Magnetic field is generally created

by running a current through wire loop. It is used as a sensor by producing a magnetic field when strained.The behavior of magnetostrictive material is shown in figure.

Advantages1. Fast response time

2. High curie temperature3. Relatively high strain and force capabilities compared to piezoelectric4. No aging effects

5. Operate over large temperature range6. Low voltage operation

Disadvantages1. Low tensile strengths

2. Brittle3. Costly due to rare earth metals involved.

4. Large magnetic field required5. Equipment intensive

4. Piezo Electric materials

Piezo electric materials have two unique properties, which are interrelated. When a piezoelectricmaterial is deformed, it gives of a small but measurable electric discharge. Alternately, when an electric

current is passed through a piezo electric material it experiences a significant increase in size ( up to 4 %change in volume).

Maximum strain

Strain

Field

7/28/2019 PE - Module 4

8/15

4.8

Piezo electric materials are mostly used as sensors in different environments. They are often usedto measure fluid compositions, fluid density, fluid viscosity, or the force of an impact. An example of

piezo electric material in everyday life is the air bag sensor in your car. The material senses the force of animpact on the car and sends an electric deploying the air bag.

Advantages

1. Compact and light weight2. Displacement proportional to applied voltage

3. Operate over large temperature range4. Fast response to applied voltage

5. No moving parts6. No wear and tear on the element.

7. Function at high frequencies.8. Excellent stability.

Disadvantages1. Brittle due to crystalline structure

2. Produce small strain compared to SMA and magnetostrictives3. Cannot withstand at high shear and tension.

4. Material does age5. Can lead to instability

6. High voltages, high temperatures, large stresses.

Applications of Smart Materials

There are many possibilities for such materials and structures in the man made world. Engineeringstructures could operate at the very limit of their performance envelopes and to their structural limits

without fear of exceeding either. These structures could also give maintenance engineers a full report onperformance history, as well as the location of defects, whilst having the ability to counteract unwanted or

potentially dangerous conditions such as excessive vibration, and effect self repair. The Office of Scienceand Technology Foresight Programme has stated that `Smart materials ... will have an increasing range of

applications (and) the underlying sciences in this area ... must be maintained at a standard which helpsachieve technological objectives', which means that smart materials and structures must solve engineering

problems with hitherto unachievable efficiency, and provide an opportunity for new wealth creatingproducts.

CERAMIC MATERIALS

Introduction

Ceramic Materials are defined as those containing phases that are compounds of metallic and nonmetallic elements.

Classification

I. Fundamental Classification:i) Abrasives : Alumina, carborandum

ii) Pure oxide ceramics : MgO, Al2O3, SiO2

7/28/2019 PE - Module 4

9/15

4.9

iii) Fire clay products : Bricks, tiles, porcelain, etc.iv) Inorganic glasses : window glass, lead glass etc.

v) Cementing materials : Portland cement, lime, etc.vi) Stone : Granite, sand stone, etc.

vii) Minerals : Quartz, calcite, etc.viii) Refractories : silica bricks, magnetite, etc.

II. Structural Classification:i) Crystalline ceramics : single phase like MgO or multi-phase from the MgO to Al2O3

binary system.ii) Non-crystalline ceramics: natural and synthetic inorganic glasses.

iii) Glass bonded ceramics: fire clay products- crystalline phases are held in glassy matrix.iv) Cements: crystalline or crystalline and non crystalline phases.

Generally ceramics are devided in to two

1. Traditional ceramics;- Whiteware- Tiles- Bricks- Pottery- Abrasive wheels

2. Engineering or high tech ceramics:- heat exchangers- cutting tools- semiconductors- prosthetics

Advantages of ceramic materials

1. The ceramics are hard , strong and dense2. High resistance to the action of chemicals and weathering..3. Possess high compression strength compared with tension.4. They have high fusion points.5. They offer excellent dielectric properties.6. They are good thermal insulators.7. They are resistant to high temperature creep.8. Availability is good.9. Good sanitation.10.Better economy.

Applications of ceramics

1. White wares are largely used in Tiles, Sanitary wares,

7/28/2019 PE - Module 4

10/15

4.10

Low and high voltage insulators, High frequency applications, Chemical industry as crucibles, jars and components of chemical reactors. Heat resistant applications as parameters, burners, burner tips and radiant heater

supports

2. Newer Ceramics (e.g. Borides, carbides, nitrides, single oxides, mixed oxides, silicates,metalloid and intermetallic compounds) which have the high hardness values and heat andoxidation values are largely used in

Refractories for industrial furnaces. Electrical and electronic industries- as insulators, semiconductors, dielectrics, Ferro-

electric crystals, piezo electric crystals, glass, porcelain alumina, quartz and mica, etc.

Nuclear applications- as fuel elements, fuel containers, moderators, control rods andstructural parts. Ceramics such as UO2, UC, and UC2 are employed for all thesepurposes.

Ceramic metal cutting tools- made from glass free Al2O3. Optical applications- Ytralox, a comparative newcomer in the ceramic material field is

useful since it is as transparent as window glass and can resist very high temperature.3. Advanced Ceramics (e.g. SiC, Si3N4, ZrO2, B4C, SiC, TiB2, etc.)

These are utilized in,

Internal combustion engines and turbines as armour plates. Electronic packaging. Cutting tools Energy conversion, storage and generation.

Properties of ceramic materials

I. Mechanical Properties:-1. The compressive strength is several times more than the tensile strength.2. Non Ductile/ brittle- stress concentration has no effect on compressive strength.3. It possesses ionic and covalent bonds which impart high modulus of elasticity.4. As compared to metals more force is required to cause slip in diatomic ceramics5. Below recrystallisation temperature, non crystalline ceramics are fully brittle. The

cleavage failures occur along crystallographic planes and propagation of cracks takesplace at high speeds.

6. High rigidity at high temperature.

GLASSES

Glass is any substance or mixture of substances that has solidified from the liquid state without

crystallization. Glass is material made by fusion of mixture of silica, basic oxides and a few othercompounds that react either with silica or with basic oxides.

Structure- glass is a random arrangement of molecules, great majority of which are oxygen ions boundedtogether with the network forming ions of silicon, boron or phosphorus.

7/28/2019 PE - Module 4

11/15

4.11

Constituents of Glass1. Silica:2. Sodium or Potassium Carbonate3. Lime4. Manganese Dioxide5. Cullet.6.

Coloring substances.

Properties of glass

1. No definite crystalline structure.2. No sharp melting point.3. Absorbs, refracts or transmits light.4. Affected by alkalis.5. An excellent electric insulator at elevated temperature.6. Extremely brittle.7. Available in beautiful colours8. Not affected by air or water.9. Not attacked by ordinary chemical reagents.10.Possible to weld pieces of glass by fusion.

Compressive strength - 600 to 1200 MN/m2

Tensile strength - 27 to 700 MN/ m2

Classification of Glass

1. Soda-lime or crown glassComposition: sand 75 parts, lime 12.5 parts, soda 12.5 parts, alumina 1 parts and Waste

glass 50 to 100 parts.

It is cheapest quality of glass Easily fusible at comparatively low temperature.

Application:-Window glass, plate glass, bottle glass.

2. Flint GlassComposition: sand 100 parts, red lead 70 parts, potash 32parts, waste glass 10 parts.

It contains varying proportion of lead oxide. It liquefies at lower temperature.

Application: - electric lamps, thermometers, electron tubes, laboratory apparatuses,

containers for foods, etc.3. Pyrex or heat resistant glass

7/28/2019 PE - Module 4

12/15

4.12

Both soda lime and flint glasses will not with stand at high temperature. But in Pyrex thebasic oxides are eliminated and inclusion of boron oxide makes the glass very resistant to

heat, attack of acids, shock, electric arc etc.

Composition: silica 80 parts, boron oxide 14 parts, sodium oxide 4 parts, alumina 2 parts,with traces of potassium oxide, calcium oxide and magnesium oxide.

Special Types of Glass

1. Annealing Glass to reduce the brittleness they kept in an annealing furnace to cool veryslowly, the longer the annealing period, the better the quality of glass.

2. Sheet Glass it is made by blowing glass into hollow cylinder, splitting the cylinder andfinally flattening it over a plane surface. Used for doors and windows. Composition: 100 parts

sand, 35 parts lime stone or chalk, 50 parts waste glass.3. Plate glass it is made by pouring white hot glass over an iron table and rolling it to a uniform

thickness under heavy roller. It is stronger and more transparent. Composition: white sand 100parts, soda carbonate 33 parts, slaked lime 14 parts, manganese peroxide 0.15 part and waste

glass 100 parts. Uses: used for making looking glass, wind screens of motors, car skylights,and glass houses. It is also used for sales counters and table tops.

4. Fluted glass- when there are corrugations on one side of the plate glass then it is known asfluted glass. The other side is wavy but smooth. Light is admitted with out glare of the sun.

Uses: when in need of privacy without obstruction of light.5. Ground Glass- it is made either by grinding one side or by melting powdered glass up on it.

This glass is mostly translucent. Also known as frosted or obscured glass. Uses: used insituations where light is required without transparency. Used for window pans and bathroom

ventilators.6. Wired Glass- it is glass with wire netting or similar strengthening material embedded init

during manufacture. Also known as reinforced glass. Used for fire resisting doors, roofs etc.

In ceramic science the word glass signifies any amorphous component of ceramic mixture. Moregenerally, glass is a transparent silica product which may be amorphous or crystalline depending on heat

treatment. Glasses may be either inorganic or organic. Vitreous materials or inorganic glasses are thefusion products which during solidification form a liquid state failed to crystallize. Glass properties

are unfavorably affected if crystals develop in glass liquid. During the cooling process, the glasses exhibitno discontinuous change at any temp. And only a progressive increase in viscosity is noticed. In fact glass

is a hard liquid.Glass forming constituents

Silica which is obtained from high purity silica sand is the most widely used glass formingconstituent. Other glass formers are the oxides of boron, Vanadium, Ge and P

The oxide components added into a glass batch may be subdivided as1. Glass formers

2. Intermediates3. Modifiers

Glass formers or network formers include oxides such as SiO2, B2O3, GeO2, P2O5, V2O5 andAl2O3 which are indispensable in the formation of glass since they form the basis of the random 3D

network of glasses. Intermediate include Al2O3, Sb3O2, ZrO2, TiO3, PbO, BeO and ZnO. These oxides areadded in high proportions for linking up with the basic glass network to retain structural continuity.

7/28/2019 PE - Module 4

13/15

4.13

Modifiers include MgO, Li2O, BaOCaO, SrO, Na2O & K2O. These oxides are added to modify theproperties of glasses\The other additives in glass are the fluxes which lower the fusion temperature of the

glass batch and render the molten glass workable at reasonable temp.

Structure and properties of glasses

The structure of glasses is quite similar to that to the liquids except that there are highly viscousand non relaxing. There exists among atoms a short range order but also a long range order. Hence glasses

are generally regarded as super cooled liquids that are unstable w.r.t. their crystalline phases.Te inorganic glasses are built up from an irregular 3D network of polyhedra, much like the structure of

silica, feldspar and the zeolites having the same base units. Oxides like GeO2, P2O5, Sb2O3, B2O3, As2O3oxides impart properties as viscosity, easy fracture, thermal conductivity and coefficient of expansion of

glassesGlass practically deforms by the mechanism of viscous flow and not by step which is found to

occur in crystalline materials. Te rate at which viscous deformation can occur, of course dependsprimarily upon the amount of stress applied and 2o upon composition and structure.

The rate of viscous flow in glass at ordinary temp is extremely low. So if glass is subject to asuddenly applied load bonds within the structure of the glass are broken. Consequently it is more likely to

fail in a brittle manner. However glass may flow under its own weight over a long period of time

CERAMIC FABRICATION PROCESS

Several techniques are available for processing ceramics in to useful products. Generally theprocedure involves the following steps.

1. Crushing or grinding of raw materials in to very fine particles.2. Mixing them with additives to impart certain desirable characteristics.

3. Shaping, drying and firing the materials.The first step in processing ceramics is the crushing of the raw materials and it is generally done in

a ball mill, either dry or wet. Wet crushing is more effective because it keeps the particles together andprevents the suspension of the fine particles in air. The particles then be sized, filtered and washed.

The basic shaping processes for ceramics are1. Casting

2. Plastic forming3. Pressing

1. Slip Casting

Slip casting is also called drain casting. A slip is a suspension of colloidal ceramic particles in animmiscible liquid, which is generally water. In this process, the slip is poured in to a porous mold made of

plaster of Paris. The slip must have sufficient fluidity and low enough viscosity to flow easily in to themold.

After the mold has absorbed some of the water from the outer layers of the suspension, it is

inverted, and the remaining suspension is poured out. The top of the part is then trimmed, the mold isopened and the part is removed.

In some applications, components of the product (handles of the cup and pitchers) are made

separately and then joined, using the slip as an adhesive. Molds may also consist of multicomponents. Ironand other magnetic materials are removed using inline magnetic separators.

For solid ceramic parts, the slip is supplied continuously in to the mold to replenish the absorbedwater. The suspension is not drained from the mold. At this state the part is soft solid or semi rigid. The

higher the concentration of solids is in the slip, the less water has to be removed. While the ceramic partsare green, they may be carefully machined. Because of the delicate nature of the compacts machining is

done manually or with simple tools.

7/28/2019 PE - Module 4

14/15

4.14

Advantages

1. Large and complex parts such as plumbing ware, art objects and dinnerware can be made by slipcasting.

2. mold and equipment cost are low.Disadvantages

1. air entrapment can be significant problem during slip casting.2. dimensional control is poor.

3. production rate is low.

2. Plastic formingPlastic forming are also called as soft, wet or hydro plastic forming. It can be carriedout by various

methods, such as extrusion injection molding and jiggering. Plastic forming tends to orient the layeredstructure of clay along the direction of material flow and so tends to cause anisotropic behavior of the

material, both in subsequent processing and in the final properties of the ceramic product.In extrusion the clay mixture, containing 20% to 30% water is forced through a die opening by

screw type equipment. The cross section of the extruded product is constant there are limitations to wallthickness for hollow extrusions. Tooling cost is low and production rates are high. The extruded products

may be subjected to additional shaping operations.

3) Powder Pressing

Powder pressing is used to fabricate both clay and non clay compositions, including electronic andmagnetic ceramics as well as some refractory brick products.

In essence, a powdered mass, usually containing a small amount of water or other binder, iscompacted in to the desired shape, by pressure. The degree of compaction is maximized and fraction of

void space minimized by using coarse and fine particles mixed in appropriate proportions. There is noplastic deformation of the particles during compaction as there may be with metal powders. One function

of the binder is the lubrication of the powdered particles as they move past one another in the compactionprocess.

There are three basic powder- pressing procedures:i) Uniaxialii) Isostaticiii) Hot pressing

For uniaxial pressing, the powder is compacted in a metal die by pressure that is applied in a singledirection. The formed piece takes on the configuration of die and platens through which the pressure is

applied. This method is confined to the shapes that are relatively simple; however, production rates arehigh and process is inexpensive.

For Isostatic pressing, the powdered material is contained in a rubber envelope and the pressure isapplied by a fluid, isostatically. More complicated shapes are possible than with uniaxial pressing;

however, the isostatic technique is more time consuming and expensive.For both Uniaxial and isostatic procedures a firing is required after the pressing operation. During

firing, the formed piece shrinks and experiences a reduction in porosity and an improvement inmechanical integrity. These changes occur by the coalescence of the powder particles in to a more dense

mass in process termed Sintering. Sintering is carried out below the melting temperature so that a liquidphase is normally not present.

With hot pressing, the powder pressing and heat treatment are performed simultaneously thepowder aggregate is compacted at an elevated temperature. This procedure is used for metals that do not

form a liquid phase except at very high and impractical temperature; in addition it is utilized when highdensities with out appreciable grain growth are desired. This is an expensive fabrication technique that has

7/28/2019 PE - Module 4

15/15

4.15

some limitations. It is costly in terms of time, since both mold and die must be heated and cooled duringeach cycle. In addition, the mold is usually expensive to fabricate and ordinarily has a short life time.