Chapter7_ceram

Chapter7_ceram - Chapter Chapter 7 Ceramic Materials in...

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hapter 7 Chapter 7 Ceramic Materials in Aero Applications
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eading Material Reading Material • Callister p. 414 – 454 general • Callister p. 460 – 470 advanced ceramics
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Engineering Ceramics ngineered ceramics are pure or nearly pure • Engineered ceramics are pure or nearly pure compounds of oxides, carbides and nitrides he crystal structure of advanced ceramics is The crystal structure of advanced ceramics is complex. Bonding is frequently a combined ionic/covalent form • Ceramics are brittle and have high strength and hardness at elevated temperatures. They have a w coefficient of thermal expansion and low low coefficient of thermal expansion and low conductivities (some conduct at high T)
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lassification ig 13.1 p. 461 Classification Fig 13.1 p. 461
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Recall: Mechanical and Physical Properties • Typically metals and ceramics are crystalline • Glasses (including metallic glasses) are amorphous. • Ionic bonds present in most commercial ceramics makes slip difficult – Strong +/- bonds must be broken – Like ions must pass one another (Coulombic repulsion) – Slip vector (b) tends to be large
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Recall Stress – Strain Behaviour in eramics Ceramics • Hard, strong but lack toughness, – use K IC = Y σ ( π a) 1/2 , –K IC for ceramics is typically below 10 MPa m 1/2 – Note in the presence of a wet atmosphere, failure can occur below K IC due to a kind of stress corrosion effect that affects the ionic bonds around the crack tip.
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Flexural Strength • Typical stress-strain tensile tests are replaced by a three-point flexure test (p. 447) governed by ASTM C1161 (ambient temperature) sually flexural strength is reported in the • Usually flexural strength is reported in the texts, 3F /2bd 2 ectangular section) σ fs = 3 F f L /2bd (rectangular section) σ fs = 3 F f L / π R 3 (circular section) here F the load at fracture L distance where F f is the load at fracture, L distance between supports
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hree- oint bending for Three point bending for σ fs F b d /2 R L/2 L/2
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lexural Stress - train Curve Flexural Stress Strain Curve Flexural strength vs. strain No plastic flow - rittle brittle E is still the slope
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hermal Conductivity Thermal Conductivity • Material Thermal conductivity (W/m K) y( ) •N i 9 0 i 2 Ti 22 • Al 238 u 00 • Cu 400 •A l 2 O 3 16-40 • ZrO 2 0.8-4 • Polymers 0.03-0.3
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ecall: Thermal shock Recall: Thermal shock udden changes in temperature (especially Sudden changes in temperature (especially those causing tensile stresses) can cause failure as the transient thermal stress exceeds the fracture strength.
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Chapter7_ceram - Chapter Chapter 7 Ceramic Materials in...

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