DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

4.3.6. Hardness Test
Microindentation Vickers test on the 99.2% dense specimen yields an average
hardness of VPN120, with measured diagonal length d1 and d2 of the pyramid between
85µm and 90 µm, as shown in Table 4.2. This value suggests good mechanical behavior
similar to that achieved by casting or other powder metallurgy routes. An example of the
pyramid indent for hardness test is shown in Figure 4.8. Depending on processing history
and impurity content, Ni may exhibit different hardness values: a VPN71 has been
reported for annealed Ni tape 158 without mentioning carbon concentration; Ni 201 with a
carbon content at 0.02% by weight has a hardness equivalent to VPN79 and Ni 200 with
a carbon content at 0.15% by weight has a hardness equivalent to VPN104. 159 A hardness
value of VPN120 is reasonable for a Ni specimen with a 99.2% density and carbon
content less than 0.364% by weight. The exact form of carbon in Ni specimen is not
investigated, but nickel carbide (Ni3C) is only thermodynamically stable at temperature
greater than 500 °C. 160, 161 Thus, at ambient temperature, the Ni3C might exist in a
metastable state in the sintered nickel parts.
Table 4.2 Microindentation Vickers test measurement for Ni specimens.
Indent # 1 2 3 4 5 6 7 8 9
d1 (µm) 87.8 89.4 87.2 88.5 86.7 87.6 89.2 87.7 86.3
d2 (µm) 88.3 89.2 86.9 89.1 86.5 87.3 89.8 87.9 86.6
Hardness VPN 120 116 122 118 124 121 116 120 124
130