Ceramics and Composites: Properties and Bonding

Ceramics and Composites Overview

• Ceramics involve ionic or covalent bonds to join complex compounds.
• They exhibit hardness, brittleness, high melting points, low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths.
• Ceramics are categorized into traditional and engineering types.
• Traditional ceramics include bricks and tiles
• Engineering ceramics consist of pure or nearly pure compounds such as Aluminum oxide (Al2O3), Silicon carbide (SiC) & Silicon nitride (Si3N4).

Bonding Types in Ceramics

• Ionic bonds hold atoms together using charged ions.
• Electrostatic attraction between unlike charges gives most of the bonding.
• Ions pack densely
• Covalent bonds involve atoms sharing electrons with neighbors to create a fixed number of directional bonds.
• Atoms connect like Lego's and structures differ from ionic bonds
• Packing forms three-dimensional networks of chains or sheets.

Hardness and Brittleness

• Electrostatic forces in ionic bonds facilitate dislocation movement on some planes but hinder it on others.
• Covalent bonds have localized bonds that present a significant barrier to dislocation movement.
• The presence of small defects reduces the energy needed for material fracture, resulting in brittleness.
• Careful manufacturing process controls are needed because the defects can reduce the strength of a ceramic.

Ceramic Processing Steps

• Powder production is usually achieved by grinding (traditional ceramics) or using a method such as vapour-phase deposition (advanced engineering ceramics).
• Compacting or Pressing which involves pressing ceramic particles in dry or wet conditions into a die, forming "green" products.
• Firing or Sintering increases rigidity and strength and causes additional shrinkage as pore size reduces.

Slip Casting

• In drain casting, a porous plaster of Paris mold is used.
• For solid casting, a solid mold is used.

Applications of Ceramics

• Wear parts like seals, bearings, valves, and nozzles
• Cutting tools such as lathe tools and milling cutters
• Heat engines, including diesel components and gas turbines
• Medical implants for teeth and joints
• Construction materials, including highways, bridges, and buildings

High-Temperature Applications

• High strength and creep resistance allow for the design of engines and turbines that operate at higher temperatures.
• Oxidation resistance and greater hardness at elevated temperatures improve seals and bearings.
• Covalent bonds influence the strength and properties; Limited plasticity results in brittle fracture.
• Factors to consider include resistance to thermal shock, Creep & effects of varied atmospheres at high temperatures.

Composites

• Composites mix two or more materials on a macroscale, creating superior properties.
• Composites include particulate, laminate, and fibre categories

Factors Controlling Composite Behavior

• Properties of the components
• Size and distribution of the components
• Volume fraction of the components
• Shape of the components
• Nature and strength of the bond between the components

Rule of Mixture Equations

• Composite strength = (strength of component 1 x fraction component 1) + (strength of component 2 x fraction component 2)
• Sigma (c) = Sigma (1)V(1) + Sigma (2)V(2) and E(c) = E(1)V(1) + E(2)V(2), where sigma = strength, V = volume fraction, and E = Modulus of elasticity

Particulate Composites

• These composites improve a base material's mechanical properties by adding small particles with improved properties.
• Particles can be stronger/harder than the matrix (steels, heat-treatable aluminum alloys) or softer/tougher (rubberized polymers).

Increasing Strength and Hardness in Particulate Composites

• Embed fine, hard particles in a softer, tough matrix for improved strength & increased tensile strength and modulus, but often toughness drops.
• Evenly disperse fine, hard particles
• Rubber-toughened polymers like ABS use small rubber particles to increase toughness.
• Cracks intersect and stretch rubber particles, acting as springs to clamp the crack shut and increase the needed load.

Laminate Composites

• Simple laminated composites include plywood and laminated windscreens.
• Plywood plies laminate at 90 degrees to remove directional properties.
• Laminating glass windscreens with a polymer prevents shattering and increases strength and safety.
• Advanced laminates feature fibre-reinforced polymers laminated to aluminum or titanium honeycomb, for high stiffness

Fibre Composites

• Fibre composites enhance strength, stiffness, fatigue properties, and strength-to-weight ratio.
• The resulting structure include stiff, brittle fibres into a softer, ductile matrix.
• The matrix transmits the load and the fibres carry applied load.

Mechanics of Fibre Reinforcement

• The matrix binds the fibers, protects them from chemical damage, separates them, and prevents spread of brittle cracks.
• Matrix transfers and distributes the load to fibres; The fibre and matrix must be strongly bonded
• Strength of the fibres
• Orientation of the fibres with respect to the applied load
• Continuity of the fibres
• Properties of the matrix
• Strength and nature of the bond between the fibres and the matrix

Fibre Reinforcement Materials

• Glass fibers, carbon fibres & Kevlar (aramid) fibres are the common materials.
• Glass fibres are widely used for plastics due to their low cost and availablity.
• Carbon fibres meet the aerospace industry needs because they offer light weight, superior strength & stiffness.
• Kevlar is lighter than carbon fibres, offering lower modulus in compression than in tension.

Fibre Selection

• The matrix should wet fibres to reduce voids, Prevent detrimental reactions between materials
• The thermal expansion coefficients of fibers and matrix should be considered to prevent thermal stresses on the composite.

Composite Applications

• Composites offer varied properties, suitable for use in jet engine turbine blades and golf clubs.
• Aerospace: Their high strength-to-weight and modulus-to-weight composites make them great for the aerospace industry .
• Improvements involve increasing payload with the same amount of power via structural weight reduction.
• Automotive: Motor car uses include panels (fiberglass reinforced polyester SMC with corrosion and dent resistance); Structural parts (single leaf spring); High heat applications (engine based).
• Sports and Recreation: Carbon fibres are used for golf club shafts, ski poles, fishing rods, and tennis rackets.

Composite Processing Techniques

• Includes hand lay-up, spray-up, filament winding & pultrusion
• Hand Lay-Up Method: Mats or fabrics are placed against a form saturated with polymer resin, rolled for contact and cured.
• Spray-Up Method: Uses continuous-strand fibres fed through a chopper and spray gun, depositing fibers and resins into the mould.
• Filament Winding Method: Fibres wrap around a mandrel to build a solid/hollow shape, suitable for cylindrical parts.
• Pultrusion Method: Extrudes a polymer matrix around fibres, forming a simple shape with a constant cross section.

Failure Modes in Fibre Composites

• Fibre Fracture, with fibres breaking in one plane leading to the composite failing because the soft matrix cannot carry the load.
• Fibre Pull-Out, when the adhesion is insufficient.

Improving Fracture Toughness

• Improved fracture toughness is a feature of fibre composites, defined by the resistance to crack propagation.
• Methods to achieve this include plastic deformation of the matrix, fibre pull-out, and the presence of weak interfaces.