DAY 24: CERAMICS Design Concepts Review The Concept of Thermal Shock Resistance Review of Selected Engineering Ceramics Glass

QUICK REVIEW OF DESIGN CONCEPTS

Design to a minimum safe value is not possible!

We must design to an accepted probability of failure.

Weibull statistics is used instead of normal statistics in fitting data and computing probabilities.

The Weibull modulus indicates the extent to which the strength data is scattered. Want it as high as possible.

The size of the object being designed must be accounted for. Designs that work for small components won’t work for large.

THE IMPORTANCE OF FRACTURE MECHANICS

Recall that fracture mechanics allows predictions of whether a flaw in a known material will be stable.

Brittle materials are very prone to cracking and fracture.

Hence fracture mechanics is incorporated into the design of engineering ceramics.

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THERMAL STRESS What is thermal stress? Mismatched materials and uniform heating.

T > 0

1 < 21

2

Material 1 in compression. Material 2 in tension. Think about what would happen if there were a surface crack.

THERMAL SHOCK

Consider uniform material and nonuniform temperature. I took something hot out of the oven and poured cold water on it.

Hot, in compressionCold, in tension

Gradient is actually gradual.

What would happen to a crack about

right here?

COULD FAIL SUDENLY!

THERMAL SHOCK RESISTANCE

To have thermal shock resistance what’s needed.

1. Strength High2. Modulus of Elasticity Low3. Coefficient of Linear Expansion Low4. Thermal Conductivity High

TWO FORMULAS FOR THERMAL SHCOK RESISTANCE

Here’s the one in the notes.

1fRE

Here’s another one that is sometimes used.

E

kR f

VALUES OF R

From Notes:

REVIEW OF THE MAJOR FAMILIES:ALUMINA

Lowest cost Cheapest—Most Commonly used – excellent environmental resistance, room temperature strength. Not really a high temperature material

Alumina ceramic is the most widely used material out of a variety of fine ceramics.This material is applied to widely diversified industrial field for its superb material characteristics such as high electrical insulation, high mechanical strength, high wear and chemical resistance.

Uses: seals, liners, substrates for electronics, electrical insulators, body armor plates.

REVIEW OF THE MAJOR FAMILIES:ZIRCONIA

Toughest Highest Coefficient of Thermal Expansion

Close to Steel Best Low Temperature strength Not really a high temperature material Zirconia ceramic has high mechanical

strength and toughness at room temperature out of a series of engineering fine ceramics. Zirconia was the first material adopted to fine ceramic scissors or knife application. Its excellent surface smoothness has brought this material into parts for pump products.

WHY IS ZIRCONIA TOUGH?

Zirconia contains material that experiences a martensite type transformation – when it sees high enough stress.

Such as just in front of a growing microcrack. The transformed material expands, and is

compressed by the unexpanded untressed material in the vicinity.

The compression blunts the crack making it harder for it to expand. Thereby toughening the material.

REVIEW OF THE MAJOR FAMILIES:SILICON CARBIDE

Best Strength at Highest Temperatures Best Thermal Conductivity Best Corrosion Resistance Silicon carbide maintains its high mechanical

strength up to as high temperature as 1,400 C. Typical application is part for mechanical seal ring and pump due to higher chemical corrosion resistance than other ceramics.

Uses: seals, rings for harsh environments, also for high temperatures.

REVIEW OF THE MAJOR FAMILIES:SILICON NITRIDE

Best Thermal Shock Resistance Lowest Coefficient of Thermal Expansion Silicon nitride exceeds other materials in

thermal shock resistance. This material does not deteriorate at high temperature, therefore it's appropriate for automotive engine and parts for gas turbine, including turbocharger rotor, glow plug of diesel engine and hot plug. It is expected that the field range this material can be applied to will widely expand.

APPLICATIONS

GLASS

Glass is a very important family of ceramic materials. Common, cheap, very, very useful.

Plates, containers, optical cables, fibers, art! Glass is often based on SiO2, the oxide of

silicon. If you add other oxides, such as Na2O, or

Na2CO3, sodium carbonate, you get a much lower melting point. Other additives are CaO and MgO to make it more chemically stable.

This is soda-lime glass which is about 90% of the glass produced.

STRUCTURE AND PROPERTIES OF GLASS

Glass is amorphous. (We briefly mentioned amorphous metals.) Amorphous means that there is no discernable crystal structure. The consituents form a 3D network with strong bonds.

The name “glass” is applied to all such solids be they metal, ceramic or polymer.

One consequence is the transparency which is such a desirable feature in glass. (Due to time limits we will not dwell on optical properties.)

Another consequence in terms of mechanical behavior is brittleness.

GLASS IS AMORPHOUS

GLASS TRANSITION TEMPERATURE, TG

T

Specific Vol

Tg

Solidification of a crystalline material

Supercooled liquid

Glass

Note: slope change

M.P.

GLASS TRANSITION

Used not just for ceramic glasses but also for polymers.

TEMPERING GLASS

Heat to above Tg.Cool suddenly with blast of air or quench bath.Different cooling rates lead to the following stress

pattern: compression on the surface and tension in the interior. Outside cools first and rigidifies, holding inner portion in tension.

 TensionCompression

GLASS ARMOR

It is fundamentally made by layering a polycarbonate substance between ordinary

glass pieces in a process of lamination.

http://www.technical-discovery.com/2009/04/nature-of-bulletproof-glass.html

In the new paper, Ortiz and her colleagues, including MIT Dean of Engineering Subra Suresh, report that the shell of the hot vent gasotropod has several features that help dissipate mechanical

energy from a potential penetrating predatory attack. Of particular importance is its tri-layered shell structure, which

consists of an outer layer embedded with iron sulfide granules, a thick organic middle layer, and a calcified inner layer.

Most other snail shells have a calcified layer with a thin organic coating on the outside. In the scaly foot gastropod, simulations suggest that the relatively thick organic middle layer can absorb

much energy during a penetrating attack. It may also help to dissipate heat and thermal fluctuations exhibited near

hydrothermal vents.

http://www.rdmag.com/News/2010/01/Materials-Soldiers-may-one-day-defend-with-sea-snail-armor/