CeramicsIntroductionTraditional ceramics tend to be fragile and very brittle due to them being quite porous. Most ceramics are electrically insulating. Clay when added with water becomes hydro plastic, which is easily formed. There are ceramics, which are covalently bonded, in reality most ceramics are ionically bonded. A sintering process that converts the material in a non-hydro plastic material with sufficient strength and rigidity. Glass is an amorphous ceramic and it falls under a big family of ceramics – Silicates. There are the engineering ceramics, which have superconductivity, exhibit magnetism, they can also exhibit some order in the crystal structure and change their dimensions and there are also ceramics, which can be used as bearing materials and surfaces. Most ceramics are compound which contain carbon.

Typical PropertiesHigh hardness associated with their very strong bonding. The atoms within a ceramic are bonded very well together. Traditional ceramics are largely brittle however nowadays a lot of ceramics through very controlled process can achieve high toughness. Electrons within the ceramic are free to move, they cannot transport electricity from one side to another – they are electrically insulating. They also have good thermal insulating properties – not all of the ceramics display such a property. They are chemically stable – good resistance to chemical attacks. At least they are not susceptible to electrochemical corrosion. They are generally durable and non magnetic. There are ceramics that display extremely high magnetism.

Can be crystalline and amorphous. In ceramics generally the bonding includes both ionic and covalent. Most ceramics include a mixture of ionic and covalent bonding. The equation gives us the percentage ionic character

The crystal structure of the ceramic is going to be affected by the magnitude of the electric charge of the ions. The cations are the components that are giving the electron while the anions are the components that are accepting the electron. All the surrounding anions must be in contact with the cations for the structure to be stable. The radius of the ions depends on the valence and the number of anions surrounding the cation.

Ceramic crystal structures AX type where the number of cations is equal to the number of anions.

*Know how to sketch certain structures such as Flourite unit cell.

Silicates Silicate materials can be amorphous and some also crystalline. An amorphous ceramic is a random structure, the ion constituting the material are placed in a random fashion there is a no long-range order. A crystalline ceramic is ordered, a repeat order throughout the material. The atoms are in an ordered configuration.

The basic building block is the SiO4 tetrahedron. The tetrahedron on its own is not neutral- 8 negative charges and 4 positive. There are certain silicate material called island silicates – can be considered as consisting of islands of these individual tetrahedra that do not share any oxygen atoms (unbridged) and the various individual tetrahedra are bonded together via interstitial cations. Typical island silicates are garnets.

Linear silicates have two bridging oxygens and two non-bridging oxygens. They are joined together by interstitial cations that ionically bond linear silicates together.

Sheet or layered silicates have 3 bridging oxygens and 1 non-bridging oxygen. Each 2 silicon cations would have 5 silicon anions - Si2O5 with a net negative charge of 2- since there are 8 positive charges and 10 negative charges. Example: clays.

*A 3D crystal structure of SiO2 (quartz) see slide 29. For quartz all the oxygen atoms are bridged. For each silicon there are 4 halves thus SiO2. Silica based materials are considered to be covalently bonded but silicates are considered to be mainly covalent but then sheet silcates, island and linear are joined together via ionic bonding.

Glass is an amorphous silicate material. Different glasses are all based on the silica tetrahedra.

SiO2 can exist in an amorphous state (fused silica is used where the application temp. is high, where the optical properties are important). Three polymorphs (quarts, cristobalite, tridymite) are all composed of crystalline silica. Quartz in itself has a relatively low density around 2.6g/cm3. It is used a lot for its appearance and also for its hardness (kitchen tops over 95-96% quartz). Its very popular since when it is given a charge it can vibrate at a very specific frequency that is very precise and can be translated in to an electrical signal that is why it is used in watches (Piezoelectric effect).

Amorphous silicates also called fused or vitreous. These are materials, which do not have any order within their structure. The silica tetrahedra are arranged in a random fashion. In pure silica (amorphous) all the oxygens are bridged thus making the material rigid, highly viscous and having a high melting temperature. Adding other oxides such as calcium oxide and sodium oxide modifies commercial glasses. These oxides modify the network by leaving interstitial cations. Other oxides that produce cations are also added to replace the silca cations - these are called intermediates. Network formers - other oxides that form networks like silica. Example: Boron oxide. Adding oxides lowers the melting temperature and changes the viscosity of the glass.

Sheet/layered silicates. Sheets of silica tetrahedra are formed by sharing 3 out of the 4 oxygen ions. Kaolinite clay is a sheet silicate material. This silicon cation is occupying the space between 4 oxygen anions and thus we call it a tetrahedral sheet. It has a net negative charge largely associated with the oxygen protruding

out of the sheet. A tetrahedral sheet bonded to an octahedral sheet with an anion midplane. For kaolinite clay the net negative charge is balanced by an adjacent sheet that is composed by Al2(OH)4 has 6 positive charges and 4 negative charges therefore 2+. Each Al is surrounded by 6 anions (OH) and this is called an octahedral sheet. For kaolinite clay in the presence of water the H2O between the sheets makes the material hydroplastic. Not all sheet silicate materials are hydroplastic an example is mica. It is very popular and is used in furniture. It is also wear resistant. Mica contains potassium. Aluminum in mica replaces one of the silicon cations and having potassium between the sheets bonds strongly the two sheets together making the material non-hydroplastic.

Schematic Diagram: TOT k TOT The tetrahedral and octahedral sheets are neutral.

Deformation in CeramicsThere are 2 main classes: crystalline and amorphous. Crystalline are composed of ions placed in a regular manner. As stress is applied the bonds are stretched leading to elastic deformation. To plastically deform slip must occur. Especially in ionic ceramics the structure is made out of ions and slip is not easy since repulsion occurs. Very few slip systems in these materials and the charged ions repel each other as they come close to each other making it difficult for slip to occur resulting in a brittle material. For covalent ceramics the component atoms within the material are strongly bonded together through covalent bonds. For slip to occur a lot of force is required which exceed the fracture strength of these ceramics. For non crystalline ceramics there are no slip planes, no order, so definitely slip cannot occur in amorphous ceramics. They can deform in the same fashion as a liquid. The higher the energy of the bonds, the higher the viscosity. Viscous flow would not happen near RT because the viscosity of materials near RT is extremely high. The viscosity of these amorphous materials is extremely high on the tune of 1014 Pas, the non crystalline material won’t be able to deform plastically.

Fracture in CeramicsA ceramic material is composed of ions and as other materials it contains defects such as pores, micro cracks at the surface, internal micro cracks. As stress is applied the ceramics would not plastically deform and thus the defects would act as stress concentrators. This leads to premature failure. Thus, the fracture strength depends on the manufacturing process, on the heat treatment (ex: sintering), on the geometry and on the size of the component (a larger component displays a lower fracture strength because of the higher probability of finding a flaw).

With ceramics their ability to resist fracture is best described via fracture toughness. Fracture toughness such as plane strain is measured by pulling a block of material with a crack in tension. a is the crack length(external crack) a is half the crack length (internal crack)y is a dimensionless value that depends on the specimen and the crack geometryk (MPam1/2)

kic=sigma y (pi a)1/2

When the RHS gets bigger than the kic, the crack will propagate and the component will fail catastrophically.

A ceramic exposed to a static load and after some time the ceramic fails. This is very common with silicate materials. It is commonly referred to static fatigue. The ceramic is in an environment under a static stress, and the environment (ex: water) will sharpen existing cracks within the material by interacting with the ionic bonds of the crack. This is displayed especially in silica glasses. It is a brittle failure and it is catastrophic.

The brittle fracture of ceramics can be explained through the aspect that these ceramics

Do not plastically deform, The presence of flaws within the material act as stress concentrators –

cause failure at even lower stresses and The fracture strength depends not only on the material but on the

manufacturing processes, heat treatments, geometry and size of components.

For a SiN material the frequency distribution shows a range of fracture strengths because it is the probability of finding a defect.

The smaller the volume of material the lower the probability to find a flaw. In compression the flaws within a ceramic do not act as stress concentrators therefore in compression the strength is higher.

Ceramics usually display up to 0.001 strain before they fail. Due to the brittle behavior, the properties, the stress-strain characteristics are obtained using a flexural test. Given the difficulty in preparing a ceramic component a flexural test is normally used since the component is either a bar or a rod. Another reason why a flexural test is used is that the specimen is just supported and not gripped therefore avoiding breaking of the specimen. Since ceramics fail with a very small amount of strain, a slight misalignment could affect the result. The flexural strength (modulus of rupture) and the tensile strength are related. In a flexural test, from the neutral axis upwards the specimen is under compression while from the neutral axis downwards the specimen is under tension. Therefore, failure is more likely to occur at the bottom. In a tensile test the specimen is under tension only and thus it will exhibit less strength than in the flexural test.

Defects in Ceramics Bulk defects act as stress concentrators and affect the strength of the

ceramic. Planar defects also act as stress concentrators and affect the properties

the strain in particular. Linear defects such as dislocations in crystalline ceramics Point defects such as vacancies or interstitial atoms:

Frenkel defect where a cation leaves a vacancy and then resides somewhere nearby.

Schottky defect creates a vacancy pair where a cation and anion migrate to a free surface. They migrate together to keep the charge balanced: charge neutrality. Are possible in ax compounds.

Stoichiometric defect – the defect does not alter the chemical formula of the compound.

Anti-structure disorder occurs where the ceramics are predominantly covalent, the cation will reside in anion position and vice versa

Non-stoichiometric defect where one of the ions can take multiple valances.

Electronic defects occur in semiconductors such as ceramics like GaAs (Gallium Arsenide). These defects are increased by doping to form a p type or an n type semiconductor.

Hardness and PorosityCeramics are used as coating on metals and grit for polishing – abrasive materials (sand paper). Higher porosity tends to negatively impact the properties of the material: lowers strength, toughness, thermal and shock resistance. As porosity is increased in alumina both the fracture and the strength decrease.

GlassIt is a class of ceramics that is very important mainly due to its optical properties and is used in the form optic fibres and lenses. Additives are added to the pure silica to control the viscosity temperature relationship for that particular glass. Different types of glass include: soda-lime glass, pyrex etc.

The specific volume vs. temperature of glassAt high temperature the glass is liquid and the viscosity is low. As the temp decreases the density decreases and at the tg the atoms become much less mobile and thus the glass becomes solid. Above tg the glass is called supercool glass.

Slide 47 all glasses are amorphous except for glass-ceramic. Fused silica is composed entirely of silica. It has a very high melting temperature and is very difficult to process. It also has a low thermal coefficient of expansion therefore it has a much higher resistance to thermal shock and it is very expensive. It is transparent to UV and is used a lot in UV lamps.

Vycor is very expensive. It is high in silica content. It is more chemically resistant than normal glass. It consists of 4% network former. Used a lot in lab ware.

Pyrex – Borosilicate contains a lot of boron oxides, which control the viscosity and make the material resistant to thermal shock. Used in ovens. Much easier to form compared to fused silica.

Soda lime or normal glass, it has a working temperature which is must lower than the rest, a low melting temperature and cheap to produce artifacts from it. Does not have good thermal shock resistance.

Viscosity-temperature curves. This relationship is related to how these materials will flow around the melting temperature. The viscosity of water is 10-

3. The melting point- the temp at which the glass would have a viscosity of

10Nsm-2

The working range-the glass can be very easily deformed. Glass is formed in this region. Materials like fused silica are much more difficult and expensive to work since the working temperature is much higher. *see graph

The softening point-the temperature at which the glass can be handled without deformation

The annealing point-the glass is held at the annealing point so that any residual stresses in the glass would be eliminated.

The strain point-below this temperature fracture will occur before any plastic deformation takes place

The network formers, additives affect the viscosity temperature relationship.

Glass forming Pressing – thick pieces pressed in dies Blowing – a gob of glass is given a preliminary shape and blown into a die.

It is a fast and rather cheap process. Drawing – pulling the glass through a die. Easily performed with soda

lime. Sheet forming- float process- a process in which glass sheets are

produced. A furnace filled with the required ingredients and melted, this molten glass flows on tin and surface tension effects will encourage the glass to spread over the bar of liquid tin. The heater heats the glass at a temperature within the working range of the glass in order to enable the glass to flow and form a flat polished surface. The glass is then cut into several sheets. Glass sheets used to be very expensive

Fibre drawing – raw material will also include modifiers, placed inside a furnace, molten, homogenized, refined and goes into channels-forehearths- each die will have hundreds of holes where glass comes out as fibres which are wound on various spools, The velocity of the spool is much higher than the velocity of the glass coming out resulting in tension in fibres.

Annealing and temperingAnnealing to relieve the thermal stresses. Tempering is very different from annealing and has nothing to do with the tempering of steels. The heat treatment process leads to compressive stresses at the surface and tensile stresses at the

core. During cooling the surface of the glass will shrink since it cools first. As the core cools down it tries to shrink and it compresses the surface. The core of the sheet will be in tension while the surface of the sheet will be in compression. This makes the glass stronger since as the glass is being bent, any crack in the surface has to first overcome the residual compressive strength.

Glass ceramics are not entirely amorphous. Produced by the crystallization of inorganic glass by heat treatment. Titanium dioxide acts as a nucleant that promotes crystallization during heat treatment. Much more resistant to fracture, since the start material is glass which leads to less deformations than if it were powder. Have a low coefficient of thermal expansion. Most of the glass used on oven doors is made out of a glass ceramic. Even more the glass covers on electric hobs are made out of glass ceramic. As the temp is increased the crystalline phase has a characteristic where it shrinks while the amorphous phase expands thus they balance out each other.

At the working range the material is formed. Up to this pint it is still amorphous. At the nucleation crystallites start to form and the atoms form a crystalline phase. The lower the temperature the more nucleation sites that is why the nucleation temperature is low.

ClayClays are hydro plastic when water is added i.e. easily deformed at RT. Another characteristic is that when sintered or fired they start to melt over a range of temperatures. Non-plastic ingredients are added:

Quartz Fluxes – feldspars

Slip Casting is a very cheap way to produce ceramics. The final product is normally glazed so that it would seal and be non porous. Plaster of paris as the mold.

Abrasives and ErodentsSince ceramics are hard materials they are used as abrasives. Ceramic abrasives are used in grinding wheels. Other ceramics are used as erodents such as alumina and garnets. Garnets tend to break on impact and thus exposing sharp edges after every impact. This makes them reusable

Pressing and SinteringCommon techniques to produce ceramic components. The starting material will be a powder ceramic that is mixed with a binder (ex: polymer). When pressed, the green part is then placed in a furnace, the binder is allowed to escape. The most conventional way is uniaxial pressing. Other techniques include hot pressing or hot isostatic pressing where pressure is applied all around the part.