Ceramic and Glass Materials

1 Alumina 9

5.3 Fatigue

The strengths of crystalline and glassy oxides decrease with time under a constant

applied load. This static fatigue is usually modeled with a power law equation between

times to failure t when a sample is subjected to an applied stress s:

logt = c −nlogs (5)

in which c is a constant and the stress exponent n is a measure of the susceptibility
of the material to fatigue. The larger the n value the more resistant the material is to
fatigue. Typical values of n for silicate glasses are 13 or lower [21]; for alumina an n
value of about 35 was found [21], showing that alumina has much better fatigue
resistance than most other oxides under ambient conditions.
This fatigue in oxides results from reaction with water, which can break the cation–
oxygen bonds in the material; for example in alumina:

Al − O − Al + H 2 O = AlOH + HOAl (6)

Thus, when the ambient atmosphere is dry the fatigue failure time is long, and as the
humidity increases the fatigue time decreases.

5.4 Hardness

The hardness of a material is measured by pressing a rod tip into a material and find-

ing the amount of deformation from the dimensions of the resulting indentation.

Hardness measurements are easy to make but hard to interpret. The stress distribution

under the indenter is complex, and cracking, elastic and anelastic deformation, fault-

ing, and plastic deformation are all possible around the indentation. Alumina is one

of the hardest oxides. On the nonlinear Mohs scale of one to ten, alumina is nine and

diamond is ten, but diamond is about a factor of three harder than alumina. Some

approximate Knoop hardness (elongated pyramidal diamond indenter) values for

alumina are given as a function of temperature in Table 9, and in Table 10 for some

hard ceramics [22, 23]. It is curious that the hardness of alumina decreases much

more than the strength as the temperature is increased.

Table 8 Effect of porosity on the

bend strength of polycrystalline

alumina at 25°C from [2]

Porosity (%) Strength (MPa)

0 269

10 172

20 110

30 76

40 55

50 47