Mechanical Properties

1.2 Elasticity

Alumina shows the deformation behavior of a typical brittle solid, which is linear elasticity to failure. Elastic deformation is instantaneous when stress is applied, and is completely reversible when it is removed. In a tensile test of a rod or bar of alumina, the strain is linear with stress to failure; the slope of this stress-strain curve is the Young’s modulus. Values of various moduli and Poisson’s ratio for pure, dense poly­crystalline alumina are given in Table 4 (from [15]). These values are considerably higher than those for most other oxides, as a result of the strong (high energy) alumi­num-oxygen bonds in alumina. The various elastic constants of single crystal alumina are given in Table 5. As the temperature increases the elastic moduli decrease (as shown in Table 6) because of the increase in atomic displacements as the temperature increases, and consequent reduced bond strength.

Table 4 Elastic properties of polycrystaffine alumina

at room temperature [15], moduli in GPa

Young’s modulus 403

Shear modulus 163

Bulk modulus 254

Poisson’s ratio 0.23

Table 5 Elastic properties of single crystal alumina at room temperature [2, 15] in GPa

C11

498

S11

23.5

C12

163

S12

7.2

C13

117

S13

3.6

C14

-23.5

S14

4.9

C33

502

S33

21.7

C44

147

S44

69.4

Table 6 Temperature dependence of Young’s modulus for polycrystalline alumina [2] Temp. (°C) Young’s modulus (GPa)

25

403

500

389

1,000

373

1,200

364