CERAMIC MATERIALS

Dr. N. Ramesh BabuAssistant Professor

Dept. of Metallurgical and Materials Engineering

National Institute of Technology 1

Ceramics derived from the original Greek word keramos, meaning 'burned stuff or 'kiln-fired material', has long been directly appropriate.

keramos, meaning "pottery", which in turn is derived from an older Sanskrit root, meaning "to burn".

Modern ceramics, however, are often made by processes that do not involve a kiln-firing step (e.g. hot-pressing, reaction sintering,glass-devitrification, etc.).

Introduction

GLASSES

CERAMIC MATERIALS

CLAY REFRACTORIES ABRASIVES CEMENTSGLASSESADVANCED CERAMICS

GLASS CERAMICS

STRUCTURALCLAY

PRODUCTS

WHITEWARES

FIRECLAY

SILICA

BASIC

SPECIAL

Ceramics may be generally classified, according to type or function, in various ways. In industrial terms, they may be listed as

Classifying ceramic materials in micro structural terms

General properties of ceramicsGeneral properties of ceramics

The modulus of elasticity of ceramics can be exceptionally high (Table 10.1). This modulus expresses stiffness, or the amount of stress necessary to produce unit elastic strain, and, like strength, is a primary design consideration.

However, it is the combination of low density with this stiffness that makes ceramics particularly attractive for structures in which weight reduction is a prime consideration.

General properties of ceramics

The constituent atoms in a ceramic are held together by very strong bonding forces which may be ionic, covalent or a mixture of the two.

As a direct consequence, their melting points are often very high, making them eminently suited for use in energy-intensive systems such as industrial furnaces and gas turbines.

Alumina primarily owes its importance as a furnace refractory material to its melting point of 2050°C.

The type of inter-atomic bonding is responsible for the relatively low electrical conductivity of ceramics. For general applications they are usually regarded as excellent electrical insulators, having no free electrons.

However, ion mobility becomes significant at temperatures above 500-600°C and they then become progressively more conductive.

The strength of ceramics under compressive stressing is excellent; In contrast, the the tensile strength of ceramics is not exceptional, sometimes poor, largely because of the weakening effect of surface flaws.

Ceramics consist largely of elements of low atomic mass, hence their bulk density is usually low, typically about 2000-4000 kg m-3. Ceramics such as dense alumina accordingly tend to become pre-eminent in listings of specific moduli.

The strong interatomic bonding means that ceramics are hard as well as strong. That is, they resist penetration by scratching or indentation and are potentially suited for use as wear-resistant bearings and as abrasive particles for metal removal.

Grinding of ceramics is possible, albeit costly.

During the consolidation and densification of a 'green' powder compact in a typical firing operation, sintering of the particles gradually reduces the amount of pore space between contiguous grains.

The final porosity, by volume, of the fired material ranges from 30% to nearly zero. Pore spaces, particularly if interconnected, also lower the resistance of a ceramic structure to penetration by a pervasive fluid such as molten slag.

On the other hand, deliberate encouragement of porosity, say 25-30% by volume, is used to lower the thermal conductivity of insulating refractories.

Ceramics are often already in their highest state of oxidation. Not surprisingly, they often exhibit low chemical reactivity when exposed to hot oxidizing environments.

Their refractoriness, or resistance to degradation and collapse during service at high temperatures, stems from the strong inter-atomic bonding.

However, operational temperatures are subject to sudden excursions and the resulting steep gradients of temperature within the ceramic body can give rise to stress imbalances.

As the ceramic is essentially non-ductile, stresses are not relieved by plastic deformation and cracking may occur in planes roughly perpendicular to the temperature gradient, with portions of ceramic becoming detached from the hottest face.

The severity of this disintegration, known as spalling, depends mainly upon thermal expansivity (α) and conductivity (k).

Silica has a poor resistance to spalling whereas silicon nitride can withstand being heated to a temperature of 10000C and then quenched in cold water.

1515

Pauling’s RulesPauling’s first rule states that coordination polyhedra are formed. coordination polyhedra are three-dimensional geometric constructions such as tetrahedra and octahedra. Which polyhedron will form is related to the radii of the anions and cations in the compound

Pauling’s second rule on the packing of ions states that local electrical neutrality is maintained

Pauling’s third rule tells us how to link these polyhedra together.

Pauling’s fourth rule is similar to the third, stating that polyhedra formed about cations of low coordination number and high charge tend to be linked by corners.

The fifth and final rule states that the number of different constituents in a structure tends to be small; that is, it is difficult to efficiently pack different-sized polyhedra into a single structure.

1616

Pauling’s Rules

1717

1818

1919

NaCl structure Cesium chloride structure

Zinc blende structure

23

Fluorite structure

24

The perovskite structure

25

26

Polymorphic forms of carbon

27

28

29

30

31

32

33

34

35

In double chains, two silicate chains are connected periodicallyby a bridging oxygen. Asbestos is such a double chain, with O/Si = 2.75

36

37

The combination of strength and a low coefficient of thermal expansion (approximately 3.2 x 10-6 °C-1 over the range 25-1000°C) in hot-pressed silicon nitride confer excellent resistance to thermal shock.

Small samples of HPSN are capable of surviving 100 thermal cycles in which immersion in molten steel (1600°C) alternates with quenching into water.

Scientific basis of SAILONS

Although silicon nitride possesses extremely useful properties, its engineering exploitation has been hampered by the difficulty of producing it in a fully dense form to precise dimensional tolerances.

Hot-pressing offers one way to surmount the problem but it is a costly process and necessarily limited to simple shapes.

The development of SAILONS provided an attractive and feasible solution to these problems. The material is based on upon the Si-Al-O-N system.

On the basis of structural analyses of silicon nitrides, it was predicted that substituting oxygen (O2-) in nitrogen (N3-) was a promising possibility if silicon (Si 4+) in the tetrahedral network could be replaced by aluminium (Al3+), or by some other substituent of valency lower than silicon.

SiAlONs properties and applications:·         low density, ·         high strength·         superior thermal shock resistance, ·         moderate wear resistance·         fracture toughness,·         mechanical fatigue and creep resistance,·         oxidation resistance.

Shot Blast Nozzles Milling Media Thermocouple Protection Sheaths Weld Location Pins Extrusion & Drawing Dies Cutting Tips Chemical & Process Industry Applications

Addition of yttrium oxide-as sintering aid

4242

Transformation toughening

The term CSZ refers to material with a fully-stabilized cubic (not tetragonal) crystal structure which cannot take advantage of the toughening transformation. It is used for furnace refractory's and crucibles.

FSZ ceramics have a high coefficient of thermal expansion, higher than that of pure zirconia. This, together with low thermal conductivity, leads to poor resistance against thermal shock. The mechanical strength and fracture toughness are also lower than those of PSZ and TZP ceramics.

The version known as tetragonal zirconia polycrystal (TZP) contains the least amount of oxide additive (e.g. 2-4 mol% Y2O3) and is produced in a fine-grained form by sintering and densifying ultra-fine powder in the temperature range 1350-1500°C; such temperatures are well within the phase field for the tetragonal solid

After cooling to room temperature, the structure is essentially single-phase, consisting of very fine grains (0.2-1 micron) of t-ZrO2 which make this material several times stronger than other types of zirconia-toughened ceramics.

Metastable tetragonal zirconia particles can also be used in other matrices such as Al2O3, SiC, Si3N4, TiB2, mullite, etc. in which case the materials are referred to as DZC ceramics (Dispersed Zirconia Ceramics). The most well known example of this group is zirconia toughened alumina (ZTA).

An intergranular distribution of the metastable phase results when conventional processing methods are used but it has also been found possible to produce an intragranular distribution. As with PSZ materials, the size of metastable particles and matrix grains must be carefully controlled and balanced.

In addition to these established applications, it has been found practicable to harness the structural transitions of zirconia, thereby reducing notch-sensitivity and raising fracture toughness values into the 15-20 MN m-3/2 band, thus providing a new class of toughened ceramics.

The other approaches to increasing the toughness of a ceramic by either (1) adding filaments or (2) introducing micro-cracks that will blunt the tip of a propagating crack.

Ceramic Brake Ceramic Brake DiscsDiscs

Engine Engine ComponentsComponents

Rotor (Alumina)

Gears (Alumina)

Silicon CarbideSilicon CarbideBody armour and other Body armour and other components chosen for their components chosen for their ballistic propertiesballistic properties..

Automotive Automotive Components of Components of Silicon CarbideSilicon Carbide

Chosen for its heat Chosen for its heat and wear resistanceand wear resistance

NitridesNitrides The important nitride ceramics are silicon nitride The important nitride ceramics are silicon nitride

(Si3N4), boron nitride (BN), and titanium nitride (Si3N4), boron nitride (BN), and titanium nitride (TiN) (TiN)

Properties: hard, brittle, high melting Properties: hard, brittle, high melting temperatures, usually electrically insulating, TiN temperatures, usually electrically insulating, TiN being an exception being an exception

Applications: Applications: – Silicon nitride: components for gas turbines, Silicon nitride: components for gas turbines,

rocket engines, and melting cruciblesrocket engines, and melting crucibles– Boron nitride and titanium nitride: cutting tool Boron nitride and titanium nitride: cutting tool

material and coatingsmaterial and coatings

5656

The Structure of Glasses

A 3-co-ordinated crystalline network is shown at (a).

But the bondingrequirements are still satisfied if a random (or glassy) network forms, as shown at (b).

5757

5858

Radial distribution function [RDF; the quantity is ρ(r)]. ρ(r) = atom density in a spherical shell of radius r from the center of any selected atom.

5959

6060

6161

6262

6363

6464

6666

6969

The four-point loading method is often preferred because it subjects a greater volume and area of the beam to stress and is therefore more searching. MoR values from four-point tests are often substantially lower than those from three-point tests on the same material. Similarly, strength values tend to decrease as the specimen size is increased. To provide worthwhile data for quality control and design activities, close attention must be paid to strain rate and environment, and to the size, edge finish and surface texture of the specimen.

Geometry of a compact tensile specimen used in fracture toughness tests

Fracture Toughness Test

The picture is of a Single-Edged, Notched Beam configuration set up for three-point loading. Various-sized beams can be used on the same fixture. Also note that the lower pins are free to roll as the beam increases in length as it bends. Two little springs hold the roller pins in place until the sample is loaded.

Electrical properties

Superconductivity

Rochelle salt (NaKC4H4O6 · 4H2O), potassium dihydrogen phosphate(KH2PO4 ), potassium niobate (KNbO3 ), and lead zirconate–titanate (Pb[ZrO3 ,TiO3 ]).

Piezoelectric materials are utilized in transducers, devices that convert electrical energy into mechanical strains, or vice versa.

Familiar applications that employ piezoelectrics include phonograph pickups, microphones, ultrasonic generators, strain gages, and sonar detectors.

Soft Magnetic Materials - Ferromagnetic materials are often used to enhance the magnetic flux density (B) produced when an electric current is passed through the material.

Applications include cores for electromagnets, electric motors, transformers, generators, and other electrical equipment.

Data Storage Materials - Magnetic materials are used for data storage.

Permanent Magnets - Magnetic materials are used to make strong permanent magnets

Power - The strength of a permanent magnet as expressed by the maximum product of the inductance and magnetic field.

Magnetic materials

Ferromagnetism - Alignment of the magnetic moments of atoms in the same direction so that a net magnetization remains after the magnetic field is removed.

Ferrimagnetism - Magnetic behavior obtained when ions in a material have their magnetic moments aligned in an antiparallel arrangement such that the moments do not completely cancel out and a net magnetization remains.

Diamagnetism - The effect caused by the magnetic moment due to the orbiting electrons, which produces a slight opposition to the imposed magnetic field.

Antiferromagnetism - Arrangement of magnetic moments such that the magnetic moments of atoms or ions cancel out causing zero net magnetization.

Hard magnet - Ferromagnetic or ferrimagnetic material that has a coercivity > 104 A . m-1.

Calculate the total magnetic moment per cubic centimeter in magnetite. Calculate the value of the saturation flux density (Bsat) for this material.

Magnetization in Magnetite (Fe3O4)

Figure 19.15 (b) The subcell of magnetite.

Hexagonal ferrites (BaO-6Fe2O3)

Soft magnetic ceramic materials

Hard magnetic ceramic materials