L06 Ceramics

Ceramics & Processing

Key Points

  • Definition: Ceramics are defined as ‘an inorganic, non-metallic solid’.

Types of Ceramics

Natural Ceramics

  • Stone is the oldest construction material, exemplified by the 5000-year-old pyramids.

Traditional Ceramics/ clay products

  • Often do not bear significant tensile stress.

Cement and Concrete

  • Cement consists of a mixture of:

    • Lime (CaO)

    • Silica (SiO2)

    • Alumina (Al2O3)

  • Sets when mixed with water.

  • Concrete is a blend of sand and stone held together by cement.

High-Performance Engineering Ceramics

  • Includes oxides, carbides, nitrides, silicates, used in tools, reactors, etc.

History of Ceramics

Timeline

  • 26,000 B.C.: Discovery of clay with mammoth fat and bone,It can be molded and dried in the sunto form a brittle, heat-resistant material . Birth of ceramic art.

  • 6,000 B.C.: Introduction of ceramic firing in Ancient Greece.The Greek Pottery used forstorage, burial and art.

  • 4,000 B.C.: The discovery of glass in Egypt, primarily for jewelry. It consisted of a silicate glaze over a sintered quartz body

  • 50 B.C. – 50 A.D.: Introduction of optical glass production (lenses and mirrors), window glassand glass blowing production begins in Rome

  • 600 A.D.: Creation of porcelain (first ceramic composite) by the Chinese, utilized in varied applications from electrical insulators to dinnerware. It is made by firing clay with quartz.

  • 1870’s: Development of refractory materials for high-temperature applications. Used in everything from building bricks to steel making furnaces.

Modern Innovations

Breakthroughs

  • 1960: Discovery of lasers and fiber optics technology. Fibre optic cable allows light pulses to carry info with low energy loss

  • 1965: Development of photovoltaic cells for solar energy conversion.

  • 1987: Discovery of a superconducting ceramic oxide, ideal for high-speed computers due to the high critical temp of 92K

Classification of Ceramics

Details on Traditional Ceramics

Components

  • Composed of:

    • Clay

    • Silica (flint)

    • Feldspar

Characteristics

  • Includes products for different uses such as dinnerware and roofing.

Advanced Ceramics Properties

Characteristics

  • Utilized for their unique electrical properties.

  • Examples include magnetic and optical ceramics, as well as dielectric ceramics.

Features

  • Provides enhanced mechanical properties under demanding conditions.

  • Notable applications:

    • Bioceramics used in hip prostheses.

    • Tribological ceramics (ballpoint pen tips)

Properties of Ceramics

General Properties

  • High melting point

  • Thermally insulating

  • Electrically insulating

  • High Stiffness

  • High Strength

  • Low ductility

  • Usually elastic

  • Very little plastic

  • Brittle

Strength and Stiffness

Mechanical Properties

  • Ceramics exhibit high stiffness and strength due to strong atomic bonds (ionic & covalent).

Hardness Characteristics

Applications of Hardness

  • Ceramics exhibit high hardness and resistance to localised plastic deformation.

  • Applications include:

    • Anti scratch coatings•

    • Wear plates

    • Artificial hip joints

    • Valves in water taps

    • Knives & scissor blades

    • Bearings

    • Thread guides in industrial looms

    • Cutting tools

    • Ballistic armour


High-Temperature & Chemical Resilience

Thermal Applications

  • ceramics have high-temperature and chemical resistance:

    • Thermal barrier coatings in jet engines

    • Engine cylinder linings

    • Break disks

    • Turbocharger rotors

    • Valves and valve seals in engines

    • Re-entry shields

    • Furnace insulation

    • Chemical glassware

    • Electrical insulating on national grid


Bonding Structures in Depth

Bond Characteristics

  • Highlighting how covalent bonds create directional structures while ionic bonds are closely packed for stability, illustrated with examples.

Defects in Ceramics

Flaw Impact on Strength

  • Ceramics inherently contain flaws that concentrate stress, leading to fractures at lower stresses than their theoretical strengths.

  • Each component has a critical flaw that, when stressed, propagates failure.

  • These critical flaws typically range from a few micrometres to tens of micrometres, often undetectable by standard non-destructive techniques.

  • Ceramics fracture rather than yield in response to stress so theoretical strengths can the observe in practice.

  • Strength prediction for components is challenging due to variability in the critical flaw sizes.

  • however you can Use of Weibull statistics to estimate the probability of ceramical component survivability under stress.

  • but other complicating factors are that the strength of ceramic depends on volume under load and time under load.

Summary of Key Concepts

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