Ceramic materials-structures, Composites, Processing and uses

1. INTRODUCTION

Ceramics include natural stones; clays and porcelains; electric insulators; abrasives; glass; and cement. These materials are hard and brittle; they do not conduct electricity and are often transparent. These properties are the consequences of a filled valence band. The chemical bond is covalent for elemental solids and ionic or mixed covalent-ionic for compounds. The nature of the chemical bond controls the crystal structure of the materials. These bonds are generally stronger than in metals and give the solid a high melting temperature that prevents casting from a melt. Ceramics cannot be machined the way metals are. Ceramic pieces are fabricated by forming a paste, consisting of the ceramic powder and water, into a near-final shape and solidifying it by firing. Firing causes sintering of the ceramic particles through the diffusion of atoms or molecules. Glass is an amorphous silicon oxide with additions of sodium, magnesium, or boron. These additions form positive ions that neutralize the oxygen atoms and allow a disordered structure. Glass does not have a melting point but increases its viscosity, upon cooling, to values so high that the glass cannot be deformed at room temperature. The fabrication of glass objects makes use of its high viscosity.

Ceramics are compounds between metallic and nonmetallic elements for which the interatomic bonds are either ionic or predominantly ionic.
The term ceramics comes from the Greek word keramikos which means ‘burnt stuff’.
Characteristic properties of ceramics are, in fact, optimized through thermal treatments.
They exhibit physical properties that are different from that of metallic materials. Thus metallic materials, ceramics, and even polymers tend to complement each other in service.
Most Ceramics have a crystal structure.
Ceramics are used extensively in the electrical industry because of their high electrical resistance.

Characteristics of Ceramics:

High temperature stability
High hardness
Brittleness
High mechanical strength
Low elongation under application of stress – low thermal and electrical conductivities.
Ceramics are used extensively in the electrical industry because of their high electrical resistance.

Classification of Ceramics:

Traditional ceramics such as stones, clay products, refractories, and abrasives
Synthetic high-performance ceramics (including semiconductors)
Glass
Cement and concrete

Stone: Stone and rock are ceramics that have always been important building materials. The popular ones are granite, limestone, and marble. Granite is a mixture of feldspar (aluminosilicate containing sodium, potassium, or calcium), quartz (SiO2) and mica (aluminosilicate). Stones are agglomerates of crystallites held together by a glassy network Their brittle nature facilitates quarrying and polishing their low coefficients of thermal expansion (-8 x 10-6°C-I), relatively low density (-2.5-2.7 g/cm3), and low tendency to absorb water are beneficial attributes in structural applications.

Clay: Clay Products The term ceramics has traditionally meant fired whiteware and structural clay products. Their common ingredient is clay, which varies widely in chemical, mineralogical, and physical characteristics. Clay basically consists of aluminosilicate layers in the form of tiny crystalline platelets that readily slide over each other. Electrical neutrality is maintained by the sharing of oxygen atoms and by hydrogen, sodium, and potassium or magnesium ions.

Refractories: Refractories are used for thermal insulation, crucibles, and hardware in all kinds of casting operations as well as in high-temperature processing and heat treatment furnaces. Whether they line the immense blast furnaces used in steelmaking or line molds for casting turbine blades, they must withstand direct contact with molten metals and glasses (e.g., slags) without decomposing or cracking. Refractory oxides used for these general purposes most commonly include fireclays containing alumina-silicates, magnesium oxide, relatively pure silica, and zirconia (ZrO2).

Abrasives: Abrasives consist of natural silicon oxide (flint); corundum, a natural form of aluminium oxide; synthetic silicon carbide (carborundum) or diamond dust. Abrasive ceramic materials are critical in the grinding, lapping, and polishing of parts requiring high dimensional tolerances. The sharp abrasive grains must be hard enough to penetrate the material being cut. They are commonly bonded to grinding wheels or to paper and cloth (e.g., sandpaper), but are also used in loose form or embedded in pastes and waxes.

Synthetic High Performance Ceramics:

Modern, high-performance ceramics are synthesized; they include alumina (Al203); silica (Si02); other oxides, such as Ti02, Zr02, Na20, Li20; carbides WC, TiC, SiC, BC; nitrides: Si3N4, TiN, BN; and borides TiB2. Since they are compounds of metals with the lighter elements N, C, and O their density is usually lower than that of metals.

Glass:

Glass is one of the most versatile materials and also one of the oldest. Obsidian, a common volcanic glass, was widely used for arrowheads, spearheads, and knives in the Stone Age.

The basic component of glass is silicon oxide, and its basic structural element is the SiO4-4 tetrahedron shown in Figure 1. Besides Si02' glass contains elements such as sodium, potassium, boron or lead that are dissolved as ions. These atoms are structure modifiers. They attach themselves to oxygen atoms at the corners of the tetrahedra; forming an ionic bond, they transfer an electron to the oxygen and remove the need for the corner to be shared with another tetrahedron. The result is that the rigid crystalline structure is no longer necessary; the tetrahedra form an irregular, amorphous structure shown in Figure 2. Thus, silica glass is a disordered arrangement of SiO4 tetrahedra.

FIGURE-1

Fabrication of the Glass objects:

The fabrication of glass objects exploits the increasing viscosity of glass as its temperature is lowered. Viscosity determines virtually all of the melting, forming, annealing, sealing, and high-temperature heat treatments of glass.

Various methods that are used in fabrication of the glass objects are as follows

Pressing
Blowing Casting
Rolling
Float moulding

Processes to form and shape glass. (A) Pressing a glass container (B) Blowing a bottle. (C) Centrifugal casting of a tube: (D) Plate glass manufacture by the Pilkington method.

Pressing :

An example of pressing involves the forming of glassware by compressing it in a mould with a plunger. The process (Figure A) resembles closed-die forging of metals and compression moulding of polymers. Parts with weights up to 15kg are pressed using pressures of 0.7MPa (100psi).

Blowing:

Hand blowing of glass is practised today much as it was in antiquity. Glass is gathered on the end of an iron blowpipe and the glassblower shapes the "gathers" with lungpower or the use of compressed air. Artistic glassware is hand-blown while continually rotating the glass. The largest pieces reach weights of 15kg, lengths of almost 2m, and diameters of lm Glass containers, light bulb envelopes, and many other mass-produced objects are machine-blown. Gobs of glass are delivered to the blank mould where they are performed either by blowing or pressing. The blank is blown in the blow mould to finish the operation (Figure B).

Casting :

In this process, glass is poured into or on moulds, tables, or rolls. The largest piece ever cast was the Mount Palomar reflector, a cylinder measuring 5.08m (200in) in diameter and 0.457m thickness. Other products routinely cast are radiation-shielding window blocks and television and cathode ray tubes. The latter are centrifugally cast, and as the molten gobs are spun the glass flows up to create a uniform wall thickness (Figure C).

Rolling and Float Molding :

Both of these methods produce plate glass. In continuous processes, shown in Figure below, raw materials are fed in at one end of very large horizontal furnaces (with capacities of 1,000 tons) where successive melting and refining of glass occurs. At the other end, the glass is fed into a pair of cooled rollers, and the emerging ribbon of solid glass is conveyed on rollers through an annealing lehr. Plate glass nearly 4m wide and lcm thick can be rolled at rates of over6 m/min. The surfaces of rolled plate glass must be further ground and polished.

Cement and Concrete:

In tonnage consumed, cement and concrete far exceed that of steel, wood, and polymers combined. They are the essential ingredient in some of the largest structures built in this century such as high-rise buildings, airport runways, and dams. Concrete is composed of cement, aggregates (sand, gravel, crushed rock), and water. Cement, a generic term for concrete binder, is the key ingredient. The most important binder for concrete is Portland cement, which is produced from an initial mixture of 75% limestone (CaCO3), 25% clay, assorted aluminosilicates, and iron oxides and alkali oxides. This mixture is ground and fed into a rotary kiln (a large cylindrical rotating furnace) together with powdered coal. Progressive reactions at temperatures extending to 1,800°K breakdown clays, decompose the limestone to yield quicklime (CaO), and fuse them to produce clinker (pellets) 5-10mm in size. After cooling, the latter is mixed with 3-5 wt% gypsum (CaSO4) and ground to the powder that is Portland cement.

Portland cement is composed of the following identifiable compounds:

Calcium oxide (lime)CaO usually denoted by C
Silicon oxide (silica) i02 usually denoted by S
Aluminum oxide (alumina) Al203 usually denoted by A

Some three quarters by weight is composed of

Tricalcium silicate (C3S) (i.e., 3CaO-SiO3 or Ca3SiO5)
Dicalcium silicate (C2S) (i.e., 2CaO-Si02).
Tricalcium aluminate (C3A) (i.e., 3Ca0-Al203).

Different proportions of these and other ingredients are blended to produce cement that either set slowly or more rapidly, liberate less or more heat of hydration, or are intended to resist degradation by water containing sulfates. Water is added to the cement, which solidifies through chemical reactions that are complex and not entirely understood. Cement does not harden by drying, but the water is incorporated into the solid by a chemical reaction. Cement does not shrink when hardening.

Crystal Structures of Ceramics:

The crystal structure of ceramics is determined by their composition and the type of their chemical bonds. In covalent ceramics, the directions of the chemical bonds shape the crystal structure. In ionic materials, the structure is largely controlled by the sizes of the ions and the electric charges they carry. Since ceramics are compounds that sometimes have a large number of atoms per formula, we must expect their structures to be rather complex. In fact, they are not all well established yet. Both crystalline and noncrystalline states are possible for ceramics. The crystal structures of those materials for which the atomic bonding is predominantly ionic are determined by the charge magnitude and the radius of each kind of ion.

Silicates: Silicates are composed primarily of silicon and oxygen. These two are the most abundant elements in the earth’s crust.
Silicates structure is more conveniently represented by means of interconnecting SiO4-4 tetrahedra as shown below.
As in the figure, four oxygen atoms are situated at four corners of the tetrahedron with silicon at the centre.
This is the basic unit of the silicates.

Based on the sharing of corners of SiO4-4 tetrahedron with an oxygen atom, below are different types of silicates.

a) Silica:

It is a three-dimensional network that is generated when every corner oxygen atom in each tetrahedron is shared by adjacent tetrahedra.
Chemically, the most simple silicate material is silicon dioxide, or silica (SiO2).
This material is electrically neutral and all atoms have stable electronic structures.
Under these circumstances, the ratio of Si to O atoms is 1:2, as indicated by the chemical formula.

b) Silica Glasses:

If the above silica exists as a noncrystalline solid or glass, which is having a high degree of atomic randomness, such a material is called fused silica or silica glass.
It is a liquid in characteristic, such a material is called fused silica, or vitreous silica.

2) Carbon:

Carbon materials do not really fall within any one of the traditional metal, ceramic, polymer classification schemes.
However, we choose to discuss these materials in this chapter since graphite, one of the polymorphic is sometimes classified as a ceramic, and the crystal structure of diamond also a polymorph
Carbon exists in various polymorphic forms, as well as in the amorphous state.

Here brief introduction on diamond, graphite, the fullerenes, and carbon nanotubes are given below

a) Diamonds:

Each carbon bonds to four other carbons, and these bonds are totally covalent.
This is appropriately called the diamond cubic crystal structure, which is also found for other elements like germanium, silicon, and grey tin those are in Group IVA of the periodic table.

Properties:

Physical: Extremely hard (the hardest known material)
Electrical: Has a very low electrical conductivity
Due to its crystal structure and
The strong inter-atomic covalent bonds.
Thermal: Has a high thermal conductivity for a nonmetallic material,
Optical: Transparent in the visible and infrared regions of the electromagnetic spectrum

Applications:

Used as gemstones.
diamonds are utilized to grind or cut other softer materials.
Diamond films are having many uses like
The coating on the surfaces of drills dies, bearings, knives etc. to increase surface hardness.
The surface of machine components such as gears, optical recording heads and disks, and as substrates for semiconductor devices.

b) Graphite:

Graphite is also a polymorph of carbon. It has a crystal structure distinctly different from that of diamond and is also more stable than a diamond in normal conditions.

Structure:

The graphite structure is composed of layers of hexagonally arranged carbon atoms
Within the layers, each carbon atom is bonded to three coplanar neighbour atoms by strong covalent bonds.
The fourth bonding electron participates in a weak van der Waals type of bond between the layers.
Because of this weak vander waals bonds between the layers, it easy to achieve inter-planar cleavage.
This gives rise to the excellent lubricative properties of graphite.
Also, the electrical conductivity is relatively high in crystallographic directions parallel to the hexagonal sheets.

Properties:

Physical:High strength and good chemical stability at elevated temperatures
Thermal:
High thermal conductivity
Low coefficient of thermal expansion
High resistance to thermal shock
High adsorption of gases, and good machinability.

Applications:

Graphite is commonly used as heating elements for electric furnaces
As electrodes for arc welding, in metallurgical crucibles, in casting moulds for metal alloys and ceramics.
For electrical contacts, brushes and resistors.
In air purification devices.

c) Fullerenes:

Another polymorphic form of carbon was discovered in 1985. It exists in discrete molecular form and consists of a hollow spherical cluster of sixty carbon atoms; a single molecule is denoted by C60.

Structure:

Each molecule is composed of groups of carbon atoms that are bonded to one another to form both Hexagon (six-carbon atom) and Pentagon (five-carbon atom) geometrical configurations.
These are arrayed in such a way that no two pentagons share a common side.
The molecular surface is like a soccer ball.

Properties:

As a pure crystalline solid, this material is electrically insulating.
But with proper impurity additions, it can be made highly conductive and semiconductive.

Processing of Ceramics:

Many crystalline ceramics cannot be cast because of their high melting temperature; often, they dissociate chemically before they melt. Because of their high hardness and lack of ductility, they cannot be shaped by plastic deformation. The starting material in the fabrication of ceramic objects is a paste composed of small solid particles, water, and often a binder. The paste is easily shaped at room temperature by various methods and acquires a modest strength after drying. A ceramic piece formed at room temperature is called a green. Its final solidification and hardness are acquired by firing.

The very specific character of ceramics – high-temperature stability – makes conventional fabrication routes unsuitable for ceramic processing.
Inorganic glasses, though, make use of lower melting temperatures, most other ceramic products are manufactured through powder processing.

Ceramic powder processing consists of powder production by milling/grinding, followed by fabrication of green product, which is then consolidated to obtain the final product

Typical ceramic processing routes are