1 CRYSTALLINE CERAMIC BY ANJU CHANDAK DEEPSHIKHA 2 Crystalline Ceramics
As with plastics the amorphous ceramics tend to be transparent
The structural ceramics tend to be crystalline and show greater strength as well as stability at high temperature
3 AX Structure - CsCl
Note This is not a BCC structure. Cs Simple Cubic Crystal 4 AX Structure - NaCl 2- FCC interpenetrating lattices. 5 AX Structure - ZnS 6 Silicate Ceramics
Many polymorphsQuartz CrystobaliteTridymite
Low density Quartz 2.65g/cm3
7 Crystalline Crystabolite 8 Carbon
Pure carbon has many polymorphs with vastly varying properties. It also exists in the amorphous state.
Diamond Is similar to ZnS in structure
Graphite is considered to be a crystalline ceramic
Fullerenes C60 are a newly discovered polymorph - with interesting properties.
AX type crystal structure similar
to that of ZnS.
Each carbon atom is covalently bonded to four other C atoms in a diamond-cubic crystal structure.
The material is optically transparent and extremely hard (hardest natural material known) and durable.
In engineering applications cruder or industrial forms of diamond that are much less expensive than the gemstone forms are used as abrasives indentors and coatings (especially thin films) for a variety of applications.
Layers of hexagonally arranged and
covalently bonded C atoms.
Between layers weaker Van der Walls
bonds are active giving easy slip
on the 0001 crystallographic planes.
Excellent as a dry lubricant relatively high strength at elevated temperatures high thermal and electrical conductivity low thermal expansion resistance to thermal shock and good machinability.
Usage electrodes heating elements crucibles casting molds rocket nozzles and other applications.
11 Fullerenes C60
Molecular form of carbon with a
hollow spherical structure resembling
a geodesic dome (soccer ball.)
Called buckyballs after R. Buckminister
Fuller who pioneered the geodesic dome.
Discovered in 1985 and have since been found to occur naturally in several sources.
In the solid crystalline state C60 molecules pack together in a FCC unit cell arrangement with a lattice parameter a1.41 nm.
The pure solid material density is about 1.65 g/cm3 and it is relatively soft and is non-conducting since it has no free electrons.
12 Try It!
Calcualte the theoretical density of pure C60 based on a FCC unit cell as shown
Obtain Flexural Strength (Modulus of Rupture) Stiffness (Modulus of Elasticity) and Ductility.
Strength is often good Stiffness my be high but Ductility and affected properties are poor.
In crystalline ceramics dislocation motion is difficult because of the need to maintain electro-neutrality. Consequently plastic deformation is restricted.
15 Stress-Strain Behaviour 16 Hardness of Ceramics 17 Effect of Porosity on Stiffness Where Eo is the theoretical modulus of elasticity with no porosity and P is the volume fraction of porosity. 18 Effect of Porosity on Strength
Where so is the theoretical modulus of rupture with no porosity P is the volume fraction of porosity and n is an empirical material constant
The viscosity of the material at ambient temperature is relatively high but as the temperature increases there is a continuous decrease in viscosity.
When the viscosity has decreased to the point that the ceramic is a fluid it is considered to have melted.
At ambient temperature while it is still solid it is said to be in the glassy condition.
There is no distinct melting temperature (Tm) for these materials as there is with the crystalline materials.
The glass transition temperature Tg is used to define the temperature below which the material is a solid and defines a practical upper limit on service temperature.
21 Specific volume of amorphous and crystalline ceramics. 22 Viscous Behaviour in Amorphous Ceramics t shear stress h viscosity of material
Plastic deformation does not occur by dislocation motion in amorphous or non-crystalline ceramics such as glass.
Deformation is by viscous flow rate of deformation proportional to applied stress.
23 Ceramic Phase Diagrams
Note They are similar to metal alloy systems - except the temperatures are generally higher.
24 ANY QUERY 25 THANKS FOR YOUR ATTENTION
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