DAY 21: CERAMICS

Inorganic & Nonmetallic. Compounds between metallic and non-metallic elements

Traditional: Clay based bricks, tiles, china, porcelain, and glasses

Engineering: High tech materials useful in high strength applications, take advantage of the unique properties

NATURE OF CERAMICS Bonding is ionic and covalent mix. These are

extremely strong bonds. This leads to high melting point, high strength and high modulus of elasticity.

Crystallinity is complex. Different sizes of atoms. Charge conservation. This makes slip, so common in metallic crystals, practically impossible. Also leads to less efficient packing, and lower density.

GOOD PROPERTIES

Strength and Hardness High Temperature Resistance Resistance to Corrosion Low Density

CHALLENGE

Brittleness. The problem is that these materials have ductility, fracture toughness, and great flaw sensitivity.

BUT, we know that it is possible to use brittle materials, provided that we do so in an intelligent way.

WHY USE CERAMICS FOR STRUCTURAL APPLICATIONS, I.E. TURBOCHARGER ROTOR?

High temperature resistance (hot exhaust gases)

Low coefficient of thermal expansion (small blade tip clearance)

Low mass (low polar moment of inertia for fast spin up)

Durability (survive over 100,000 miles) Low cost (need willing purchasers)

http://global.kyocera.com/application/automotive/auto_engine/turbo.html

Table 1.1 Turbocharger Rotor Materials (data from Callister)

Property Silicon Nitride

Nickel Alloy

Specific Gravity 3.3 8-9

Fracture Stress(avg. at 1100 K)

300 200

Weibull Modulus

10-20 >50

Young's Modulus(GPa)

304 200

Toughness, KIC

(MPam)

4-6 60-75

Thermal Coef. of Exp.(cm/cm per C)

3.1 13

Figure 1.2 Comparison of wide-open throttle acceleration for ceramic and metal turbocharger rotors, 2.3 liter engine - Zephyr test vehicle. (Wachtman, 1989)

OVERVIEW OF ADVANTAGES OF CERAMICS temperature resistance high hardness low density corrosion resistance

SPECIAL DESIGN CONSIDERATIONS FOR CERAMICS brittleness difficulty of manufacture.