Hardness

The hardness of a material is measured by pressing a rod tip into a material and find­ing 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, fault­ing, 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 9 Knoop hardness of alumina as a function of temperature

T (K)

Hardness (kg mm 2)

400

1,950

600

1,510

800

1,120

1,000

680

1,200

430

1,400

260

1,600

160

Table 10 Knoop hardness values of some ceramics at 25°C from [2, 22]

Material

Hardness (kg mm 2)

Diamond

8,500

Alumina

3,000

Boron carbide

2,760

Silicon carbide

2,480

TopaZ (Al12Si6F10O2;

) 1,340

Quartz (SiO2)

820