indentation, with loads typically in the range of a few Newtons to 9.8 N (1 kg-f) and
occasional data for high-toughness ceramics as high as 98 N (10 kg-f). At small
indentation loads, problems arise from the load dependence of hardness and from
measurement uncertainty due to the small indentation size. At higher loads, cracking and
fracture become problems in some cases, making measurement impossible. Typical hard
ceramics have Vickers hardness in the 10-30 GPa range [QUI98]. Teeth particularly are
considered ceramic composites. Typical hardness values for enamel range from 2.9-3.9
GPa and dentin 0.6 GPa [FOR91, WIL92].
-^ "12 "__f n
Figure 4-7 Vickers microhardness testing.
The square pyramidal shape of the Vickers indenter creates a smaller deeper
impression, although more likely to crack than Knoop indentations. ASTM standard E
384, Microhardness of Materials, covers Vickers hardness; C 1327 is the new standard
for Vickers hardness of advanced ceramics and recommends a load of 9.8 N. The
universal standard [ASTOO] for this calculation is described in equation 4-7.
Vickers Hardness (HV) = 1854.4 P/d2 (4-7)
where P is the load in grams force
d is the mean diagonal length of indentation in jn,
assuming standard face angle of indenter tip is 136