Ceramics Lecture Notes

Ceramics

Introduction to Ceramics

• Ceramics are compounds formed between metallic and nonmetallic elements.
• They commonly include oxides, nitrides, and carbides.
• Examples:
  • Aluminum oxide (alumina, Al2O3)
  • Silicon dioxide (silica, SiO_2)
  • Silicon carbide (SiC)
  • Silicon nitride (Si3N4)
  • Clay minerals (e.g., porcelain)
  • Cement
  • Glass

Silicate Ceramics

• Silicates primarily consist of silicon and oxygen.
• Silicon and oxygen are the two most abundant elements in the Earth's crust.
• Soils, rocks, clays, and sand are largely composed of silicates.
• Silica (SiO_2) has a relatively high melting temperature of 1710°C.

Applications of Silicate Ceramics

• Early Ming Dynasty Bowls (14th century).
• Brick walls.
• Tile patterns.
• Electric fuses.

Applications of Advanced Ceramics

• Si3N4 gas turbine rotor.
• WC blast nozzle.
• MgO refractory bricks (furnace liners).
• Al2O3 structural parts.

Mechanical Properties of Ceramics

• Ceramics are generally stiff and strong, with stiffness and strength comparable to metals.
• They exhibit greater resistance to high temperatures and harsh environments compared to metals and polymers.

Elastic Behavior

• A linear relationship exists between stress and strain in ceramics.
• The elastic moduli of ceramic materials range from approximately 70 to 500 GPa.
• Porosity significantly influences the modulus of elasticity such as in Aluminum oxide.

Stiffness Comparison

• Comparison of stiffness (elastic modulus) at room temperature.

Flexural Strength

• Tensile tests are not commonly used for brittle ceramics due to:

  1. Difficulty in preparing specimens with the required geometry.
  2. Challenges in gripping brittle materials without causing fracture.
  3. Ceramics failing at approximately 0.1% strain, necessitating perfect alignment of tensile specimens.

• The three-point bending test is commonly employed.

• The stress at fracture in this test is termed flexural strength or fracture strength.

• Flexural strength equations:

  • Rectangular sample:
  • Circular sample:

Strength Comparison

• Comparison of strength (tensile strength) at room temperature.

Brittle Fracture

• Fracture occurs before plastic deformation under tensile load.
• The process involves crack formation and propagation across the material's cross-section.
• Measured fracture strengths are typically lower than theoretical predictions based on interatomic bonding forces.

Distribution of Fracture Strengths

• Fracture strength varies considerably among specimens of a specific brittle ceramic material.
• This variation is due to the dependence of fracture strength on the probability of flaws initiating cracks.

Resistance to Fracture

• Comparison of fracture toughness at room temperature.

Property Comparison: Ceramics vs. Metals vs. Polymers

*Key: ↑ Tendency to high values, Tendency to low values

Types and Applications of Ceramics

• Ceramic materials encompass:
  • Glasses
  • Clay products
  • Refractories
  • Abrasives
  • Cements
  • Ceramic biomaterials
  • Carbons
  • Advanced ceramics

Clay Products

• Clay-based products are divided into:
  1. Structural clay products: bricks, tiles, sewer pipes (emphasizing structural integrity).
  2. Whitewares: porcelain, pottery, tableware, china, plumbing fixtures (white after firing).

Diamond

• Exceptional physical properties:
  • Highest hardness among bulk materials.
  • Lowest sliding coefficient of friction.
  • Extremely high thermal conductivity.
  • Notable electrical properties.
  • Transparent in visible and infrared regions.
  • Widest spectral transmission range.

Refractory Ceramics

• Key properties:
  1. Ability to withstand high temperatures without melting or decomposing.
  2. Inertness and unreactivity in severe environments (e.g., hot, corrosive fluids).

Abrasive Ceramics

• Used to wear, grind, or cut other, softer materials.

Carbon Fibers

• Small-diameter, high-strength, high-modulus fibers used as reinforcement in polymer-matrix composites.

Ceramic Biomaterials

• Desirable properties for biomaterials:
  • Chemical inertness
  • Hardness
  • Wear resistance
  • Low coefficient of friction
  • Brittle fracture tendency
• Applications include implants, load-bearing, orthopedic applications, and femoral heads for hip replacements.

Glasses

• Non-crystalline silicates.
• High melting temperature.
• Used in containers, lenses, and fiberglass.

Annealed vs. Tempered Glass

• Annealed glass:
  • Untreated; possesses standard grains.
  • Weak and prone to chemical attack.
• Heat-strengthened glass: created by heating and slowly cooling glass.
• Tempered (safety) glass: created by quickly cooling heated glass.

Glass Laminating

• Involves placing a thin layer of ductile polymer between glass layers.
• The polymer holds the glass in place after fracture, such as in car windshields.

Glass Breakage Comparison

Lecture 18 Summary

• Definition of ceramics.
• Silicate ceramics.
• Mechanical properties of ceramics:
  • Elastic behavior
  • Flexural strength
  • Brittle fracture
• Types and applications of ceramics.