Ceramics 3z2v45

Ceramics  They are inorganic and nonmetallic materials.  Most ceramics are compounds between

metallic and nonmetallic elements for which the interatomic bonds are either totally ionic, or predominantly ionic but having some covalent character.  The term “ceramic” comes from the Greek word keramikos, which means “burnt stuff,” indicating that desirable properties of these materials are normally achieved through a high-temperature heat treatment process called firing

Ceramics Phase Diagram Phase diagrams have been experimentally

determined for a large number of ceramic systems. For binary or two-component phase diagrams, it is frequently the case that the two components are compounds that share a common element, often oxygen. These diagrams may have configurations similar to metal–metal systems, and they are interpreted in the same way

Comparison metals and ceramics Metals

Ceramics

Al2O3–Cr2O3  One of the relatively simple ceramic phase diagrams is

that found for the aluminum oxide–chromium oxide system.  This diagram has the same form as the isomorphous copper–nickel phase diagram consisting of single liquid and single solid phase regions separated by a two-phase solid–liquid region having the shape of a blade.  It exists for all compositions below the melting point of Al2O3 inasmuch as both aluminum and chromium ions have the same charge as well as similar radii (0.053 and 0.062 nm, respectively).  Furthermore, both Al2O3 and Cr2O3 have the same crystal structure.

Al2O3–Cr2O3

MgO–Al2O3  The

phase diagram for the magnesium oxide– aluminum oxide system is similar in many respects to the lead–magnesium diagram.  There exists an intermediate phase, or better, a compound called spinel, which has the chemical formula MgO–Al2O3 .  Even though spinel is a distinct compound [of composition 50 mol% Al2O3 –50 mol% MgO (72 wt% Al2O3–28 wt% MgO)].  It is represented on the phase diagram as a singlephase field, that is, there is a range of compositions over which spinel is a stable compound.

MgO–Al2O3 Thus,

spinel is nonstoichiometric for other than the 50 mol% Al2O3 –50 mol% MgO composition. Furthermore, there is limited solubility of Al2O3 in MgO below about 14000C at the left-hand extremity of diagram, which is due primarily to the differences in charge and radii of the and ions (0.072 versus 0.053 nm).

MgO–Al2O3

ZrO2-CaO  Another important binary ceramic system is that for

zirconium oxide (zirconia) and calcium oxide (calcia).  The horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which composition the compound CaZrO3 forms. It is worth noting that one eutectic (22500C and 23 wt% CaO) and two eutectoid (10000C and 2.5 wt% CaO, and 8500C and 7.5 wt% CaO) reactions are found for this system.  ZrO2 phases having three different crystal structures exist in this system—namely, tetragonal, monoclinic, and cubic. Pure ZrO2 experiences a tetragonal-to-monoclinic phase transformation at about 11500C

ZrO2-CaO

SiO2 –Al2O3 Commercially, the silica–alumina system is

an important one since the principal constituents of many ceramic refractories are these two materials. The polymorphic form of silica that is stable at these temperatures is termed cristobalite

SiO2 –Al2O3  Silica and alumina are not mutually soluble in

one another, which is evidenced by the absence of terminal solid solutions at both extremities of the phase diagram  Also, it may be noted that the intermediate compound mullite, 3Al2O3-2SiO2 exists, which is represented as a narrow phase field.  Furthermore, mullite melts incongruently at 18900C.  A single eutectic exists at 15870C and 7.7 wt% Al2O3

SiO2 –Al2O3

Types of Ceramics Whitewares Refractories Glasses Abrasives Cements

Whitewares Crockery Floor and wall tiles Sanitary-ware Electrical porcelain Decorative ceramics

Whiteware: Bathrooms

Whitewares

Refractories Firebricks for furnaces and ovens. Have high Silicon or Aluminium oxide content. Brick products are used in the manufacturing plant for iron and steel, nonferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemical industries.

Refractories  Used to provide thermal protection of other materials in

very high temperature applications, such as steel making (Tm=1500°C), metal foundry operations, etc.

 They are usually composed of alumina (Tm=2050°C)

and silica along with other oxides: MgO (Tm=2850°C), Fe2O3, TiO2, etc., and have intrinsic porosity typically greater than 10% by volume.

 Specialized refractories, (those already mentioned) and

BeO, ZrO2, mullite, SiC, and graphite with low porosity are also used.

Refractory Brick

Amorphous Ceramics (Glasses)  Main ingredient is Silica (SiO2)  If cooled very slowly will form crystalline

structure.  If cooled more quickly will form amorphous structure consisting of disordered and linked chains of Silicon and Oxygen atoms.  This s for its transparency as it is the crystal boundaries that scatter the light, causing reflection.  Glass can be tempered to increase its toughness and resistance to cracking.

Glass Types Three common types of glass: Soda-lime glass - 95% of all glass, windows containers etc. Lead glass - contains lead oxide to improve refractive index Borosilicate - contains Boron oxide, known as Pyrex.

Glasses Flat glass (windows) Container glass (bottles) Pressed and blown glass (dinnerware) Glass fibres (home insulation) Advanced/specialty glass (optical fibres)

Glass Containers

Crystalline Ceramics Good electrical insulators and refractories. Magnesium Oxide is used as insulation material in heating elements and cables. Aluminium Oxide Beryllium Oxides Boron Carbide Tungsten Carbide. Used as abrasives and cutting tool tips.

Abrasives Natural (garnet, diamond, etc.) Synthetic abrasives (silicon carbide,

diamond, fused alumina, etc.) are used for grinding, cutting, polishing, lapping, or pressure blasting of materials

Cements Used to produce concrete roads,

bridges, buildings, dams.

Advanced Ceramics  Advanced ceramic materials have been developed over the past

half century

 Applied as thermal barrier coatings to protect metal structures,

wearing surfaces, or as integral components by themselves.

 Engine applications are very common for this class of material which

includes silicon nitride (Si3N4), silicon carbide (SiC), Zirconia (ZrO2) and Alumina (Al2O3)

 Heat resistance and other desirable properties have lead to the

development of methods to toughen the material by reinforcement with fibers and whiskers opening up more applications for ceramics

Advanced Ceramics Structural: Wear parts, bioceramics, cutting

tools, engine components, armour. Electrical: Capacitors, insulators, integrated circuit packages, piezoelectrics, magnets and superconductors Coatings: Engine components, cutting tools, and industrial wear parts Chemical and environmental: Filters, membranes, catalysts, and catalyst s

Engine Components

Rotor (Alumina)

Gears (Alumina)

Turbocharger Ceramic Rotor

Ceramic Brake Discs