Ceramics: Bonding, Properties and Classification

• Ceramics are complex compounds joined by ionic or covalent bonds.
• Ceramics features hardness and brittleness.
• Ceramics have high melting points.
• Ceramics have low electrical and thermal conductivity.
• Ceramics possess good chemical and thermal stability.
• Ceramics exhibit high compressive strengths.
• Ceramics are divided into two groups: traditional and engineering ceramics.
• Traditional ceramics include bricks and tiles.
• Engineering ceramics include aluminum oxide, silicon carbide and silicon nitride.

Ionic Bonding

• In ionic bonds, atoms are held together as charged ions (particles).
• Electrostatic attraction between unlike charges gives most of the bonding.
• Ions pack densely to maximize the proximity of plus and minus charges.

Covalent Bonding

• In covalent bonds, atoms bond by sharing electrons with neighbors, forming a fixed number of directional bonds.
• This forms three-dimensional networks of chains or sheets.

Hardness and Brittleness

• Electrostatic forces in ionic bonds make dislocation movement easy on some planes but difficult on others.
• Localized bonds in covalent bonds present an enormous resistance to dislocation movement.
• Ceramic hardness arises from an enormous resistance to dislocation movement
• Brittleness in ceramics is linked to extremely small defects like cracks, voids, and inclusions, reducing the energy needed to fracture the material.
• Manufacturing processes require careful control because defects can drastically reduce strength.

Ceramic Component Processing

• Most ceramic products are made by compacting powders or particles into shapes and heating them to bond the particles.
• Three basic processing steps include:
  • Powder Production: Traditional ceramics use grinding, while advanced ceramics use methods like vapor-phase deposition.
  • Compacting or Pressing: Ceramic particles are pressed in dry or wet conditions into a die, forming "green" shaped products.
  • Firing or Sintering: Enhances rigidity and strength, causing additional shrinkage as pore size reduces
  • Slip casting is another method.
• Applications include:
  • Wear parts: silicon carbide, alumina
  • Cutting tools: silicon nitride
  • Heat engines: silicon carbide, silicon nitride, zirconia
  • Medical implants: bioglass, alumina, Zirconia
  • Construction: advanced cermets and concrete

High Temperature Applications of Ceramics

• High strength and creep resistance allows design of engines and turbines that can operate at higher temperatures and more efficiently.
• Oxidation resistance and greater hardness at elevated temperatures gives better wear resistance.
• Properties depend on the strength of the covalent bond relative to metallic bonds, but leads to brittle fracture due to little plasticity.
• Factors to evaluate when using ceramics:
  • Resistance to thermal shock.
  • Creep.
  • Effect of different atmospheres at high temperatures.

Composites

• Composites are created by combining two or more materials on a macroscale to achieve superior properties.
• Types of composites include particulate, laminate, and fiber composites.
• Overall behavior depends on:
  • Components' properties.
  • Size and distribution.
  • Volume fraction.
  • Shape.
  • Bond nature and strength.

Rule of Mixture

• Since composites consist of identifiable components possessing individual properties, strength and moduli can be expressed.
• Composite strength = (strength of component 1 x fraction component 1) + (strength of component 2 x fraction component 2) +

Particulate Composites

• Particulate composites improve base material mechanical properties by adding small particles of another material.
• Particles can be stronger and harder than the matrix (steels and heat-treatable aluminum alloys) or softer and tougher (rubberized polymers).
• To increase strength and hardness involves embedding fine, hard particles in a softer matrix.
• Rubber toughened polymers like ABS use small rubber particles to increase toughness. These particles act as springs that clamp cracks shut.

Laminate composites

• Simple forms include plywood and laminated windscreens.
• Laminating removes wood's directional properties by laminating piles at 90°
• A polymer adhesive such as polyvinyl butyral, is used to stick 2 pieces of glass
• The strength will increase of the one plate, prevent shattering and improve safety factor.
• Advanced materials use fiber-reinforced polymers laminated to aluminum or titanium honeycomb for high stiffness and strength.

Fiber Composites

• Fiber composites enhance strength, stiffness, fatigue properties, and strength-to-weight ratio by using strong, stiff, brittle fibers in a softer, more ductile matrix.
• The matrix transfers load to the fibers, which carry most of the applied load.
• The matrix binds fibers and protects surfaces, separating fibers to prevent brittle cracks from spreading.
• Common fiber reinforcements include glass, carbon, and Kevlar (aramid) fibers.
  • Glass fibers: Widely used due to low cost and availability for plastics reinforcement.
  • Carbon Fibers: Developed for light aerospace materials with superior strength and stiffness.
  • Kevlar Fibers: Lighter than carbon fibers, but exhibit lower modulus in compression.

Considerations for Selecting Fibers

• Matrix should wet the fibers to reduce voids at the interface.
• Avoid detrimental reactions between matrix and fiber materials.
• Manage difference in thermal expansion coefficients to prevent excessive thermal stresses.

Composite Applications

• Composites offer a wide variety of properties and applications.
• Aerospace Industry: Strength-to-weight and modulus-to-weight make them prime candidates.
• Automotive Industry: Used as panels, structural parts like leaf springs (unidirectional glass reinforced epoxy reducing 80% weight), and high-temperature engine applications.
• Sports and Recreational Industry: Carbon fibers are widely used for golf clubs shafts, ski poles, fishing rods, tennis rackets, squash rackets and parts od gliders etc.

Composite Processing

• Various methods by application and materials
• Methods include hand lay-up, spray-up, filament winding and pultrusion.
• Two failure modes in fiber composites include:
  • Fiber fracture where fibers break in one plane, causing composite failure.
  • Fiber pull-out where fibers are pulled out of matrix due to insufficient adhesion

Fracture Toughness in Fibre Composites

• Fracture toughness can be improved together with high strength
• Fracture toughness is crack propagation resistance and it can be increase by:
  • Plastic deformation of the matrix.
  • Fibre pull-out.
  • Presence of weak interfaces, fibre-matrix separation and deflection of crack.

Non-Destructive Testing (NDT)

• NDT tests materials and components to determine the existing state or quality of a material, with a view to acceptance or rejection without destroying them or impairing their designed use.
• An area where NDT is primarily used is the detection of faults in materials.
• Faults may be external or internal, such as surface finish, machining marks, hardening cracks, blow holes, overlaps, fish-tails, piping, micro-segregation and hydrogen embrittlement.
• Test methods:
  • Visual Testing
  • Hydrostatic Testing
  • Dye Penetrant Testing
  • Magnetic Particle Testing
  • X- & Gamma Ray Radiography
  • Ultrasonic Testing
  • Eddy Current Testing

Visual Testing

• Visual Testing is examination with the eye, aided by magnifying glass, borescope and light source
• Used to detect cracks.
• Reliability depends on the ability and experience of the inspector.
• Optical inspection probes permit visual inspection of limited access areas such as rigid (2-20mm) and flexible (4-15mm).

Hydrostatic Testing

• Defects revealed by the flow of gas or liquid into or through the defects.
• Finds leak by filling inner tube with gas > pressure, leaks located by bubble formation when immersed in water.
• To check for leakage in welded pressure vessels, piping or valve, seals ends, applies 1.5-2 times the working pressure
  • Leakage detected by water/gas seepage or changes in liquid/gas pressure.

Dye Penetrant Testing

• Penetrant inspection detects small cracks or discontinuities which cannot be found by normal visual inspection depends
• Testing allows highly penetrating liquid to seep into any discontinuities in the material is applied.
• Used to detect defects which are open to the surface of a homogenous material.
• Equally suitable on all homogeneous materials (metals, plastics, glass, ceramic), orientation independent, and requires no sophisticated equipment.
• Penetrants separated by the 2 basic groups, visible and fluorescent and they are then subdivided into three gropus (water-washable, post emulsified, solvent removable)
  • Visible: has bright dyes
  • Fluorescent: florescent under filtered ultra-violet (black light)
  • Water-Washable penetrates employ penetrant which is soluble in water (least sensitve)
  • Post-Emulsified: penetrant which is not soluble (oil based). The penetratant does not contain an emulsifier allowing it to seep into surface defects.
  • Sovlent removable: not water soluble/easily emulsified. Reqiures suitable solvent

Basic Procedures for Water-Washable System

• Cleaning the Test Surface.
• Applying the Penetrant.
• Removing the Excess Penetrant.
• Applying the Developer.
• Inspection and Interpretation.

Precautions for Penetrant Solutions

• Keep the penetrants and solvents off clothes and skin which may cause chemical skin irritation.
• Adequate ventilation is required to avoid vapour.
• Most materials in portable visible penetrant process are highly flammable.
• Do not look directly at the black-light source as it will glow the viewer's sight to become hazy.

Magnetic Particle Inspection

• Used to detect discontinuities at or near the surface in ferromagnetic materials
• Presence of a leakage field, and therefore the presence of discontinuities, detected by the application of finely divided ferro-magnetic particles to the surface.
• Methods of Generating magnetic field include (1) Longitudinal Magnetisation and (2) Circular Magnetisation.

Longitudinal Magnetisation

• Permanent Magnets or Yoke-type Electro-magnets where poles of horse-shoe/magnetic yoke are placed on steel surface, magnetic flux is set up. Cracks lying between 90° and 45° to the flux path can be detected if flux strength is sufficient.
• Coil (Solenoid): Achieved by fixed/portable current-carrying coils which encircle the part and induce a longitudinal field.

Circular Magnetisation

• Direct Induction achieved by applying the electrical contacts at each end/side of the area being inspected to allow current to flow directly.
• Prod Contacts may be used if part is too lagre.

Current Requirement

• The current is affeceted by the (1) metal's permeability, (2) the shape, (3) thickness and (4) type of discontinuity.

Magnetic Particles

• Wet: Consist of a suspension of light petroleum
• Dry: Powder form variety of colours used via shaker, spray bubbls etc.

Demagnetisation

• Removes residual magnetism by subjecting it to alternating (reversing, diminishing magnetic field)
• Residual magnetism can cause
  • Adverse effecs on instruments
  • Interference in future machining by adherence on tools
  • Adherence of mag particles to wearing parts
  • Induction of stray voltages in adjacent circuits

X & Gamma Ray Radiography

• Radiography defined as formation of images on fluorescent screens/photographic material by short wave length radiation (X/Gamma) Rays
• Basic methid: X-Radiography nad Gamma rays radiogrphy
  • X-Radiography are a form of electromagnetic radiation, having veru shirt wavelengths that are produced where high energy electrons travel from a filament on a cathode with high voltage to strike a target on anode. Only about 1% electron energy coverts to radiant
  • Gamma radiography - Emitted by radioactive substance decompsotion and have shorter wave lengths

Film Density

• The film darkness that is known. The amount of light transmited thru film
• A film density of 2.0 transmits 1% of the incident light
• Definition in radiograph - Sharpness/clearness
  • Film unsharpness dependent on radiation/film energy
  • Geometric on specimen in the focale' spot

Radiographic Sensitivity

• smallest discontinuity that can be detected
• Controlled by
  • Source/FF be as small as possible
  • Source Distance as large as possible
  • Specimen to film min
  • Rad rays direct perendicular
  • Film spceimen parallel planes
  • 1% of thickness

Image Quality Indicator(IQI)

• IQI used to quantify sens
  • Wire IQI- Set of wires/diff diamaters
  • SteoHole IQI- Hole diameer equal tep thickeness and sensitiviey s smal hole that can be read

Ultrasonic Testing

• Ultrasonic flaw detection uses ultrasound above audio freq
• Intensity isn diminished after traveling with material irdusinty vs X rays pass w/ 0 loss
• Ultrasnoic energy cann ot pass in medtal or air
• Generated from the piezo-electric materials such as Quartz Crysta, Barium Titanate and Lithium Sulphate and involves (1) Pulse-Echo and (2) through-Transmissons
  • Pulse ECHO: travel throught specimen until reflected back.Refelcted sound makes amplitude to cathode
  • Through Transmission: Two sperarate tradsucer that measure variation
  • the discontinuiry/area reduces amplitude

Eddy Current Testing

• Definted by circulating electrical current induced in a conductin arteicle by alternating magentic field.Mag changes current
• Pattern changes via presence of defects Magnet/ mag affects immdedeance col prozimity to discocvery from a defc
• This change gives indication/ struct diff Skin effect
• Apilcation and metle soring and void defects and measuing thicnkess EMII/NDTSchool Factors
• Magnitute

Test coils and probes

• Concentric- Complete sirronds Point probe on 807 surface Inside tobe