Reading Assignment

14.4 Processing of Ceramics

Outline

General Characteristics

Crystalline, few free electrons, hard, brittle stiff, stronger in compression than tension.
Totally elastic
Complex compounds and solutions that contain both metallic and nonmetallic elements.
Highly resistant to abrasion, heat and staining.
Susceptible to spalling (thermal cracking)
Typically heated to incandescence during processing.
Typically hard and brittle, high strength, high melting points, low thermal and electrical conductivity.

Properties of Ceramics

High Electrical Resistance
- Used in electrical industry
Withstand High Temperatures
- Refractory Material
- Melting Points up to 7000F
Resist Wear
Cutting tool applications
Hard, Brittle, High Melting-point
Low Thermal Conductivity
Low Thermal Expansion
Good Chemical and thermal Stability
Good Creep Resistance
High Modulus of Elasticity
High Compressive Strength

Applications

Glass products
- Laboratory Glassware
- Window Glazing
  - Wired glass
  - Tempered glass (pre-stressed)
  - Laminated glass
- Fiber Optics
Advanced Ceramics
- Cutting Tools
- Orthopedic Implants
- Aerospace Components
Structural Ceramics
Abrasives
- Silicon Carbide
- Aluminum Oxide
- Diamond
- Cubic Boron Nitride
Electrical and Magnetic ceramics
Whiteware

Nature and Structure

Traditional Ceramics

Clay, silica, feldspar
Bricks, sewer pipe, tile, china, porcelain

Technical Ceramics

Oxides, carbides, nitrides

Glass

Amorphous, can be somewhat crystalline. Glasses are distinct from both liquids and crystals.
- Glasses lack a clear phase transition, but are in most ways are like solids
Types of glass
- Silica glass
- Soda-lime glass (cheapest; windowpanes bottles, etc.)
- Borosilicate glass (pyrex)
- Lead oxide glass (decorative cut crystal, optics)
- Aluminosilicate glass (high temperature applications)
- High-content silica glass& fused silica glass (same properties as quartz) expensive. Scientific applications.

History of Ceramics

Naturally occurring glasses

Obsidian
Fulgurites
Libyan desert glass

Man made glass

Dates to ca.5000 B.C.
Core formed
Blown glass
Cristallo

Stone and Structural Ceramics

One of the oldest building materials
Types of rock and stone
- Igneous rock – (L. ignis = fire) formed by solidification of magma. e.g., granite.
- Sedimentary rock – small particles cemented together. E.g., limestone
- Metamorphic rock – changed by heat, pressure, and water to produce a more compact and highly crystalline form (slate and marble

Types of Brick

Common & Face brick
Face brick
- Darker, denser, stronger, tighter tolerances
- Used on exterior of buildings
Common
- Absorbs more water by weight
- Basic building of structures
- Grades SW, MW, NW (frost/no frost resistance)
Also glazed brick, paving brick, and tile
Firebrick
- Lower density, more porosity
- Little or No mortar used
- Fired at higher temperatures

Casting Ceramics

Slip Casting (Drain Casting)

Slip

Slip is a clay suspension in water (aqueous ceramic slurry)
Mixed With:

Nonplastics
- Silica
- feldspar
- talc (reduces shrinkage, prevent cracking)
And Deflocculants (keep the slip from settling).

Slip Casting Process

First, molds are cleaned, assembled, and clamped together (often with elastic bands). Slip is prepared and poured into plaster of paris molds. The molds absorb water from the slip, which solidifies on the inside surface of the mold. When the slip has dried to create a sufficient wall thickness, the remaining liquid slip is poured out of the mold. The mold is opened and the “green” part is removed. The plaster molds must dry before they are used to mold another component.

The “green” part is air dried, optionally coated with a vitreous glaze, and then sintered (fired) in a furnace.

Making the Plaster Molds

Molds are made from a pattern that is somewhat oversized to allow for shrinkage. They are made from plaster, wood, clay or plastic. The shape of the pattern must allow for part shrinkage to avoid breakage. Molds are considered permanent, but usually only last for 20-50 cycles.

Solid Casting

Similar to slip casting, with the major difference being that all of the slip is allowed to solidify in the mold. Often extra slip is supplied to make up for shrinkage.

Centrifugal Casting

This process uses a rotating mold into which some ceramic material has been placed. In true centrifugal casting, the internal diameter of the component is controlled only by the amount of slurry introduced into the mold. Centrifugal casting at different RPMs can affect the porosity of the components.

Pressing

Dry Pressing

Ram Pressing

The RAM Process is the mechanical hydraulic plastic pressing of ceramic shapes with permeable dies. Gypsum cement (plaster) is commonly used, but more permanent materials have been developed and are being used where applicable. Instant release of the pressed shape is obtained by means of fluid pressure forced through the permeable die.

Isostatic Pressing

Granular powder or die pressed compacts are loaded into a flexible air-tight container, typically polyurethane, placed in a closed pressure vessel filled with liquid and compacted by increasing the pressure within the vessel.

The pressure change takes place throughout the liquid, thus exerting a uniform applied pressure over the entire surface area of the air-tight container. In this way, the material is uniformly compacted and will retain the general shape of the flexible container, and any internal tooling profile. www.dynacer.com

Glass Pressing

Extruding

Auger Fed Extrusion

Process Schematic

Piston Fed Extrusion