Processing of Ceramics and Cermets

Processing of Ceramics and Cermets

1. Processing of Traditional Ceramics

Definition

Traditional ceramics are typically made from naturally occurring powdered minerals, shaped through particulate processing.

Examples

Pottery, porcelain, and bricks.

Steps

1. Preparation of raw materials: This involves reducing rocky lumps to powder, typically resulting in a plastic paste.
2. Shaping: Forming the ceramic using methods include slip casting, extrusion and pressing.
3. Drying: Removing water from the shaped part which is critical but can cause shrinkage.
4. Firing (Sintering): Heating the dried part to bond the particles into a hard mass.
5. Finishing: Machining and grinding to obtain final shape and dimensions.

Raw Materials Preparation

• Comminution: Reducing particle size using mechanical energy by impact, compression, and attrition. Most effective way to process brittle materials, such as ceramics.
  • Crushing: Reducing large lumps to smaller sizes. Multiple stages with reduction ratios of 3-6 times. Achieved by compression or impact.
    • Jaw Crusher: Uses toggles to crush lumps against a rigid surface.
    • Roll Crusher: Squeezes ceramic lumps between rotating rolls.
  • Grinding: Reducing small pieces to fine powder by abrasion, impact, or compaction using hard media.
    • Ball Mill: Hard spheres rotate with the stock inside a cylindrical container, using gravity to drop the grinding action.
    • Roller Mill: Stock compressed against a horizontal table by rollers.

Ceramic Paste Ingredients

1. Clay: Hydrous aluminum silicates, providing ideal forming characteristics when mixed with water.
2. Water: Creates plasticity for shaping.
3. Non-plastic raw materials: Alumina and silica, reducing shrinkage but may reduce plasticity.
4. Other ingredients:
   • Fluxes: Promote sintering by melting (vitrifying) during firing.
   • Wetting agents: Improve mixing of ingredients.

Shaping Processes

• Slip casting: Uses a slurry (clay-water mixture).
• Plastic forming: Uses plastic clay.
• Semi-dry pressing: Uses moist clay with low plasticity.
• Dry pressing: Uses basically dry clay (less than 5% water) with no plasticity.

Slip Casting

• Ceramic powders suspended in water (slip) are poured into a plaster of paris mold.
• Water is absorbed into the plaster to form a firm layer.
• Slip composition: 25% - 40% water.
• Variations:
  • Drain casting: Excess slip is drained after a semi-solid layer forms.
  • Solid casting: Time is allowed for the entire body to become firm.

Plastic Forming

• Mixture consistency: 15% - 25% water.
• Manual and mechanized methods.
• Manual methods use more water for easier forming.
• Mechanized methods use less water for stiffer clay.

• Hand modeling: Manipulating plastic clay to desired geometry.
  • Hand molding: Using a mold to define geometry.
  • Hand throwing: Using a potter's wheel for circular cross-sections.
• Jiggering: Wet clay slug is placed on a mold and shaped using a jigger tool.
• Plastic pressing: Clay slug is pressed between molds; vacuum removes moisture.
• Extrusion: Clay is compressed through a die to produce long, uniform cross-sections.

Semi-Dry and Dry Pressing

• Semi-dry pressing: Moist powder is pressed at high pressure.
• Dry pressing: Water content is < 5%. Dies are made of hardened tool steel or cemented carbide.
• No drying shrinkage occurs.
• Products: bathroom tiles, electrical insulators.

Clay Volume and Water Content

• Water is important for shaping but must be removed before firing.
• Shrinkage occurs during drying as water contributes volume.

Drying

• Two Stages:
  • Stage 1: Rapid evaporation from surface, volumetric shrinkage, with risk of warping and cracking.
  • Stage 2: Ceramic grains are in contact, little or no further volumetric shrinkage.

Firing

• Heat treatment (sintering) in a kiln.
• Bonds are developed between grains.
• Densification and porosity reduction occur.
• Additional shrinkage.
• A glassy phase forms among crystals, acting as a binder.

Glazing

• Application of a ceramic surface coating for impermeability and enhanced appearance.
• Process: Fire, apply glaze, then fire again.

2. Processing of New Ceramics

Manufacturing Sequence

1. Preparation of starting materials
2. Shaping
3. Sintering
4. Finishing

Preparation of Starting Materials

• Strength requirements are greater than for traditional ceramics.
• Smaller, more uniform powders are needed, as strength is inversely related to grain size.
• Powder preparation uses mechanical and chemical methods.

Shaping of New Ceramics

• Processes borrowed from powder metallurgy (PM) and traditional ceramics.
• Methods:
  • Slip casting
  • Extrusion
  • Dry pressing
  • Hot pressing
  • Isostatic pressing
  • Powder injection molding (PIM)

Hot Pressing

• Dry pressing at elevated temperatures; sintering occurs simultaneously.
• Higher densities and finer grain size.
• Reduced die life due to hot abrasive particles.

Isostatic Pressing

• Hydrostatic pressure compacts powders from all directions.
• Avoids non-uniform density.

Powder Injection Molding (PIM)

• Ceramic particles mixed with a thermoplastic, injected into a mold.
• Polymer acts as a carrier.
• Plastic binder is removed, and the ceramic part is sintered.

Sintering of New Ceramics

• Drying step omitted, as plasticity is not water-based.
• Functions are same as before:
  1. Bond individual grains into a solid mass
  2. Increase density
  3. Reduce or eliminate porosity

Finishing Operations for New Ceramics

• Purposes:
  1. Increase dimensional accuracy
  2. Improve surface finish
  3. Make minor changes in part geometry
• Abrasive processes needed.
• Diamond abrasives are used.

3. Processing of Cermets

Definition

Composite materials with ceramic particles embedded in a metallic binder.

Composition

Metal matrix composites, with metallic binder holding the bulk material together. Carbide particles typically constitute 80% - 95% by volume.

Binders for Cemented Carbides

• Carbide powders sintered with a metal binder for strength.
• Cobalt works best with $WC$.
• Binder percentage: 4% - 20%.
• Powders mixed wet in a ball mill and dried in a vacuum or controlled atmosphere.

Compaction

• Cold pressing for high production.
• Dies are oversized to account for shrinkage.
• $WC-Co$ liners reduce wear.

Sintering of $WC-Co$

• Binder yields a structure virtually free of porosity.
• Liquid phase sintering.
• Sintering temperatures: $1370-1425^{\circ}C$ ($2500-2600^{\circ}F$), below cobalt's melting point of $1495^{\circ}C$ ($2716^{\circ}F$).
• $WC$ gradually dissolves in $Co$ during sintering, reducing its melting point.
• Molten metal removes gases, rearranges $WC$ particles for closer packing, and results in densification and shrinkage.

Secondary Operations

• Required after sintering for dimensional control.
• Grinding with a diamond or other very hard abrasive is most common.
• Other operations:
  • Electric discharge machining
  • Ultrasonic machining