CERAMICS
Introduction Ceramics can be defined as inorganic crystalline materials. They are the compounds between metallic and non‐metallic elements. Basically they are the most natural materials. Such traditional ceramics are used to make bricks, sanitary ware, refractories, abrasives, etc. On the other hand, ceramics are also used in most sophisticated applications such as computer chips, sensors, etc. The word “Ceramics” is derived from Keramikos, which means burnt stuff in Greek since desirable properties of ceramics are normally achieved through a high temperature heat treatment process (firing). Oxides, nitrides, carbides of metals/non‐metals are some examples for ceramic materials. The atomic bonding in ceramics is mixed, ionic and covalent, the degree of ionic character depends on the difference of electronegativity between the cations (+) and anions (‐). Common characteristics of ceramic materials are listed here: Generally ceramics are hard, brittle and possess high melting point. They are electrically and thermally insulators. Optically they can be opaque, semi‐transparent, or transparent. They are inert in most of the environments. They are generally porous and hence light (low density). Most of the ceramics are stable even at high temperatures. Ceramics have exceptional strength under compression. The wear resistance of ceramics is high.
Ceramic crystal structures
The atomic bonding in ceramics is mixed type, i.e., a mixture of ionic and covalent types. The degree of ionic character depends on the difference of electronegativity between the cations (+) and anions (‐). For example, CaF2 has about 89% ionic bonds whereas SiC has only 12% ionic character. Ionic radii and electrical neutrality are two important factors to be considered in order to understand crystal structures of ionically bonded solids. Charge balance dictates chemical formula (Ca2+ and F‐ form CaF2). Relative sizes of the cations and anions are important since cations want maximum possible number of anion nearest neighbors and vice‐versa. These topics are already discussed (during the study of crystal structures).
Application of ceramics
Ceramics are used in a wide range of technologies such as refractories, spark plugs, capacitors, sensors, abrasives, magnetic recording media, etc. For example, the space shuttle makes use of ~25,000 reusable, lightweight, highly porous
ceramic tiles that protect the aluminum frame from the heat generated during re‐entry into the Earth’s atmosphere. These tiles are made from high‐purity silica fibers and colloidal silica coated with borosilicate glass. Ceramics can appear in nature as oxides. They are present even in human body (bones and teeth). Ceramics are also used as coatings (glazes, enamels, etc.). Alumina and silica are the most widely used ceramic materials. The compressive strength is typically ten times the tensile strength. Hence, in structural applications, ceramics are used against compressive loads.
The transparency to light of many ceramics enables them to use in numerous optical applications (windows, photographic cameras, telescopes, etc).
Good thermal insulation enables them to use in ovens, the exterior tiles of the shuttle orbiter, etc.
Good electrical isolation enables them to use ceramics are used to support conductors in electrical and electronic applications.
Good chemical inertness enables them to use applications in reactive environments.
Ceramics are often used to provide protective coatings to other materials. Thin films of many complex and multi‐component ceramics are used in modern electronic components.
Fibers are produced from ceramic materials for several uses – as reinforcement in composite materials, for weaving into fabrics, for use in fiber‐optic systems, etc.
Ceramic oxides are used as magnetic and dielectric materials (ferrites and ferroelectric materials).
The table given in the next page gives numerous applications of ceramic materials in diverse areas.
Classification of ceramics
In fact ceramics and glasses represent some of the earliest and most environmentally durable materials for engineering. In addition they also represent some of the most advanced materials developed for aerospace and electronic industries. This diverse collection of engineering materials can be studied under three main categories, viz., crystalline ceramics, glasses and glass – ceramics.
Basically ceramics can be classified as natural (or traditional) ceramics and advanced ceramics. Traditional ceramics are those based on clay (china, bricks, tiles, porcelain), glasses where as advanced / new ceramics are modern day materials used extensively in electronic, computer, aerospace industries. Another way of classifying the ceramics is on the class of chemical compounds such as
oxides, carbides, nitrides, sulfides, fluorides, etc. However, the most elegant way is to classify ceramics by their function (refer the table given below).
A glass is an inorganic nonmetallic material that does not have a crystalline structure. Such materials are said to be amorphous. These are the non‐crystalline solids with compositions comparable to the crystalline solids. Examples of glasses range from the soda‐lime silicate glass in soda bottles to the extremely high purity silica glass in optical fibers. Glass‐ceramics are the crystalline ceramics that are initially formed as glasses and then crystallized in a controlled manner.
Some of the important and most widely used ceramic materials are summarized here:
Silica (SiO2): One of the most widely used ceramic material which is essential ingredient in glasses, refractories, abrasives, etc. Many of the traditional ceramics fall into the category of silicates. In the form of long continuous fibers, it is used to make optical fibers. Powders of silica are used in tyres, paints, etc. Silica shows polymorphisms with three hexagonal and one fcc polymorphs. These are known as low quartz (up to 573°C, hexagonal), high quartz (between 573 and 867°C, hexagonal), high tridymite (between 873 and 1470°C, hexagonal) and high cristobalite (between 1470 and melting point – 1723°C, fcc).
Alumina (Al2O3): It is also a widely used ceramic and used in variety of applications ranging from refractories to electronic packages. It possesses low thermal conductivity, high hardness and chemical stability. It has a good strength even at high temperatures. It is used as insulators in spark plugs.
Zirconia (ZrO2): It is an important oxide ceramic. There are three polymorphs of zirconia – Monoclinic zirconia (stable up to 1150°C), tetragonal (1150 – 2300°C) and cubic (above 2300°C). The tetragonal‐monoclinic phase transformation is accompanied with 4% volume change. If this transformation is allowed in an uncontrolled manner during service, spontaneous failure of zirconia ceramic can occur. This problem is countered by doping the zirconia with oxides of calcium, yttrium or magnesium, which stabilizes the structure (partially stabilized zirconia)
Barium titanate (BaTiO3): Most widely used electronic ceramic. It is a ferroelectric material. High dielectric constant of barium titanate made it ideal material for capacitors.
Boron carbide (B4C): Very hard but light weight and hence used in applications where good abrasion resistance is required. It is used in nuclear shielding, bulletproof armour plate, etc.
Cordierite (2MgO.2Al2O3.5SiO2): Used to make honeycomb structure in catalytic converters to carry a dispersion of nano‐sized metal particles.
Lead zirconium titanate (PZT, PbxZr1‐xTiO3): Most widely used piezoelectric material, finding application in gas igniters, ultrasound imaging devices, etc.
Silicon carbide (SiC): Exhibits excellent oxidation resistance (even at temperatures > 1200°C). Hence it is used as coating for metals, carbon‐carbon composites and other ceramics to protect in extreme temperature conditions. It is also used as abrasive in grinding wheels and heating element in furnaces.
Zinc oxide (ZnO): Used as an accelerator in the vulcanization of rubbers. It is also used in paints, skin ointments, etc.
Magnetic oxides: Magnetic ceramics are complex oxides that belong to one of the categories – spinels (cubic ferrites), garnets (RE ferrites) or hexagonal ferrites.
Yttrium Aluminium Garnet (YAG, Y3Al5O12): These crystals are used for making Nd‐YAG lasers.
Chinaclay (Kaolinite, Al2O3.2SiO2.2H2O): A traditional ceramic. Mica (K2O.3Al2O3.6SiO2.2H2O): A widely used insulation material. Asbestos (3MgO.2SiO2.2H2O): Another ceramic used in number of industrial appliocations.
123 superconductors: Some of the oxide ceramics are finding applications as high (critical) temperature superconductors. YBa2Cu3O7‐x or 123 superconductor is a typical example for this class of ceramics.
MagCarb (Magnesium carbonate, MgCO3): A ceramic mineral, popularly known as magnesite. It contains about 43% MgO. It is commonly used in pottery bodies, glazes, glass, etc. and also as reinforcing agent translucent filler in rubber and plastic compounds. It also acts as flame retardant in plastics. Being a refractory, it is used to make bricks for the cement and metal industries. However, it decomposes at 900°C and loses its CO2.
Carbon: It is not a ceramic. Carbon exists in various polymorphic forms: sp3 diamond and amorphous carbon, sp2 graphite and fullerenes/nanotubes, one dimensional sp carbon. Carbon (Diamond) has diamond‐cubic structure. It is one of the strongest/hardest materials known. It has high thermal conductivity (unlike ceramics). It is transparent in the visible and infrared, with high index of refraction. It can be doped to make electronic devices. It is a metastable material (transforms to carbon when heated). On the other hand, Carbon (Graphite) has a layered structure with strong bonding within the planar layers and weak, van der Waals bonding between layers. Easy interplanar cleavage property of graphite finds applications as a lubricant and for writing (pencils). It is a good electrical conductor and chemically stable even at high temperatures. The prime applications include furnaces, rocket nozzles, welding electrodes, etc.
Processing of ceramics
Ceramics melt at relatively higher temperatures and exhibit brittle behaviour under tension. Therefore, they cannot be processed by using conventional routes such as casting, thermo mechanical forming, etc. Generally ceramic materials are processed using powder metallurgy route. The basic steps involved in the processing of ceramics are schematically presented in the figure given in the next page. A powder is a collection of fine particles. Crushing and grinding are conducted to reduce the particle size of the minerals. Ball milling further reduces the size of particles and blends different powder ingredients. In this, a cylindrical vessel containing grinding media (steel/alumina/zirconia spheres) is fed with powder ingredients and it is rotated. Collisions between the grinding media and the ceramic powder ingredients lead to size reduction and efficient blending. Sol‐gel process is a chemical technique that is used to produce large quantities of high purity ceramic powders. A sol is a dispersion of colloidal matter. It is converted into a gel and ultimately into a useful product (such as thin films, powders, etc.). Calcination refers to the heating of a mineral/intermediate product in order to decompose or to remove moisture. In leaching, acids and alkalies are used to dissolve a mineral.
Forming/shaping of powders
Thermal treatment/ sintering
Secondary processing
Final product
Powder preparation
Ceramic powders prepared by using various techniques are shaped using different methods depending on the requirement. The figure given in the next page shows few such techniques.
When the ceramic powders are needed in the form of soft agglomerates, a technique known as spray drying is used. In this, a slurry of ceramic powder is sprayed through a nozzle into a chamber in the presence of hot air. Compaction and sintering form the most common methodology in the formation of ceramic products. Compaction is nothing but application of force (uniaxial/isostatic) to compact the ceramic powders (into the required shape) to form green ceramic. Very large pieces are produced by using cold isostatic pressing (CIP) where pressure is applied using oil. Sintering involves different mass transport mechanisms that result into densification. In some cases, parts are produced under conditions in which sintering is conducted using applied pressure (hot pressing). In hot isostatic pressing (HIP), pressure is applied isostatically using oil.
A technique known as tape casting is used for the production of thin ceramic tapes. In this, a slurry containing ceramic particles, solvent, plasticizers and binders is made to flow under and onto a plastic substrate. The tape is then dried. Slip casting is another technique that uses an aqueous slurry of ceramic powder (known as slip). It is poured into plaster of Paris (PoP – CaSo4.2H2O) mould. As the water from the slurry begins to move out by capillary action, a thick mass builds along the mould wall. After sufficient thickness is built, excess slip is drained and casting is removed after partial drying. This green ceramic is then sintered at high temperature. Figure given below shows the various steps involved in slip casting.
Extrusion is a popular technique used for making furnace tubes, bricks, tiles and insulators. A viscous mixture of ceramic powders, binder and other additives is fed to an extruder to get a continuous green product which is cut, dried and
sintered. In injection moulding, ceramic powder is mixed with a thermoplastic plasticizer and other additives and injected into a die. The polymer contained in the green ceramic is burnt off and rest of the ceramic body is sintered at high temperature. Sintering commonly refers to the processes involved in the thermal treatment of powder ceramic compacts carried out at elevated temperatures (> 0.5TM).
Refractories A group of ceramic materials capable of withstanding high temperatures for prolonged periods of time are common called as ceramic refractories. There are three types based on their chemical behaviour as indicated below: Acidic Refractories ‐ Common acidic refractories include silica, alumina, and fireclay (an impure kaolinite).
Basic Refractories – These are the refractories based on MgO (magnesia, or periclase). These are more expensive than the acid refractories.
Neutral Refractories ‐ These refractories, which include chromite and chromite‐magnesite, can be used to separate acidic and basic refractories, preventing them from attacking one another.
Other refractory materials such as graphite, zirconia (ZrO2), zircon (ZrO2 ∙ SiO2), and a variety of nitrides, carbides, and borides are known as special refractories. The table given here lists the compositions of common refractory ceramics.
Mechanical behaviour and plastic deformation of ceramics Ceramics are brittle. (For brittle fracture stress concentrators are very important). Measured fracture strengths are significantly smaller than theoretical predictions for perfect materials due to the stress risers. Fracture strength of ceramic may be
greatly enhanced by creating compressive stresses in the surface region (similar to shot peening, case hardening in metals). The compressive strength is typically ten times the tensile strength. This makes ceramics good structural materials under compression (e.g., bricks in houses, stone blocks in the pyramids). In crystalline ceramics slip (dislocation motion) is very difficult. This is because ions of like charge have to be brought into close proximity of each other. Therefore there is a large barrier for dislocation motion. In ceramics with covalent bonding slip is not easy as well (covalent bonds are strong). But in non‐crystalline ceramics, there is no regular crystalline structure and hence no dislocations. Therefore, materials deform by viscous flow, i.e. by breaking and reforming atomic bonds, allowing ions/atoms to slide past each other (like in a liquid).
Glasses Glass is a non‐crystalline inorganic material having composition similar to many ceramics (mainly made up of silicates). It is a metastable material that has hardened without crystallizing. Solidification of glass from molten state is gradual, through a viscous stage (viscosity is increasing with decreasing temperature), without a clear melting temperature. The specific volume does not have an abrupt transition at a fixed temperature but rather shows a change in slope at the glass‐transition temperature (see the figure given below). Important properties of glass are: transparency, hardness, insulation, chemical inertness, corrosion resistance and high brittleness. Due to these attractive properties they are used in number of structural and special applications.
Silicate glasses are the most widely used category of glasses. Fused silica (pure SiO2) has a high melting point and undergoes small dimensional changes during cooling. However, commercial glasses contain number of other oxides with SiO2. The oxides used in the glasses are generally fall into three categories as given in the following table:
Category Characteristics Examples
Glass formers/ network formers
The oxides that form glass by themselves.
SiO2, B2O3, GeO2, P2O5, V2O3
Intermediates The oxides that do not form glass by themselves but substitute the glass former in the network structure.
TiO2, ZnO, PbO2, BeO, Al2O3
Modiefiers The oxides that break up the network structure and thereby cause the glass to devitrify.
Na2O, MgO, CaO, PbO, Y2O3
A brief summary commercial glasses is presented here: Fused / vitreous silica: It is a high purity silica (≥ 99% SiO2). It has high melting point and can withstand high temperatures up to 1000°C. It is used for applications like furnace windows, crucibles, etc.
Soda‐lime glass: Major share of the glass industry is involved with soda lime glass. The typical composition is: SiO2 71‐73%, Na2O 12‐14% and CaO 10‐12%. It has lower melting range (softening point: 800 – 1500°C). It is inexpensive and hence very common glass in use. However, it has poor resistance to chemical attack and thermal stresses. It is used in application such as windows, containers, etc.
Borosilicate glass: It is also known as Pyrex glass. Typical composition is: SiO2 81%, B2O3 12%, Na2O 5%, Al2O3 2%. It possesses good formability, low expansion, good thermal shock resistance and excellent chemical stability. Hence, it is used to make lab wares, cooking wares, etc.
Lead glass: This glass contains significant quantity of lead oxide and used for decorative applications, lenses and radiation windows. The composition of this glass is SiO2 63%, PbO 20%, Na2O 8% and K2O 6%.
E‐glass: It is also known as fiber glass. It contains about 55% SiO2, 15% Al2O3, 20% CaO, 10% B2O3 and a small quantity of Na2O (< 1%). It is insulative and corrosion resistant. It is the most commonly used glass fiber in composites.
The characteristic temperatures in the processing of glasses are defined in terms of viscosity as shown in the following figure:
Melting point: viscosity = 100 Poise, above this temperature (below this viscosity) glass is liquid.
Working point: viscosity = 104 Poise, glass is easily deformed. Softening point: viscosity = 4x107 Poise, maximum T at which a glass piece maintains shape for a long time.
Annealing point: viscosity = 1013 Poise, relax internal stresses (diffusion). Strain point: viscosity = 3x1014 Poise, above this viscosity, fracture occurs before plastic deformation.
Glass forming operations are generally carried out between softening and working points. On the basis of above characteristic points, three ranges are identified for the processing of glasses, viz., liquid range (< 100 poise viscosity), working range (1500 – 107 poise) and annealing range (1013 – 1015 poise). Liquid range: Sheet and plate glasses are generally manufactured when they are molten state. Rolling the molten glass through water cooled rolls produces glass sheets. Float‐glass process produces a glass sheet with very smooth surface finish. In this technique, the molten glass is made to float
through a bath of molten tin. Casting large parts and drawing of glass fibers are also done in the liquid range. Figure given below shows the techniques for manufacturing sheet and plate glass.
(a) Rolling the glass and (b) floating the glass on molten tin
Working range: Containers, bulbs are formed in this range by using the processes such as pressing/drawing/blowing. In the working range, glass is formable but not “runny”.
Annealing range: In this range, glass parts are annealed to reduce the residual stresses introduced during forming.
Toughening of glasses Glasses are generally brittle. However, toughness of glass can be increased by introducing residual compressive stresses in the surface. This is known as tempering. Tempered glass is capable of withstanding higher tensile stresses and impact than the ordinary glass. It is used in home windows, refrigerator shelving, ovens, furniture and many other applications where safety is important. On the other hand, annealed glasses are stress‐free and used to make the laminated glass (a polymer is sandwiched between two annealed glass pieces) which is used in automobile windshields. Glass tempering can be done in two ways, viz., thermal and chemical means. In thermal tempering, glass is heated above the glass transition temperature but below the softening point and then quenched in an air jet or oil bath. The interior, which cools slower than the outside, tries to contract while in a plastic state after the exterior has already become rigid. This causes residual compressive stresses
on the surface and tensile stresses inside. In fracture, a crack has first to overcome the residual compressive stress, making tempered glass less susceptible to fracture. The alternate technique, known as chemical tempering, involves chemical exchange of larger ions (K+) for the surface Na+ ions. The compressive stressing of the silicate network produces chemically strengthened glass.
Glass‐ceramics These are the crystalline materials derived from amorphous glasses. Hence, the combine the properties of crystalline ceramics and non‐crystalline glasses. These materials have very good mechanical strength, toughness, low thermal expansion coefficient and high temperature corrosion resistance. In addition, they have the formability and density of glasses. Glass‐ceramics are used for making cooking utensils, ceramic tops for stoves, and also in advanced fields like communications, computers and optical applications. The production/heat treatment schedule for a typical glass‐ceramic system (Li2O‐Al2O3‐SiO2) is shown below.
The composition of the above glass‐ceramic is: Li2O 4%, Al2O316%, SiO2 74% and TiO2 (nucleating agent) 6%.