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Spark plasma sintering (SPS) investigations were carried out on three sets of Co specimens: untreated, high energy mechanically (HEMT) pre-treated, and nanomodified powders. The microstructure, density, and mechanical properties of sintered pellets were investigated as a function of various pre-treatments and sintering temperatures (700–1000 °C). Fine-grained sinters were obtained for pre-treated Co powders; nano-additives tended to inhibit grain growth by reinforcing particles at grain boundaries and limiting grain-boundary movement. High degree of compaction was also achieved with relative densities of sintered Co pellets ranging between 95.2% and 99.6%. A direct co-relation was observed between the mechanical properties and densities of sintered Co pellets. For a comparable sinter quality, sintering temperatures for pre-treated powders were lower by 100 °C as compared to untreated powders. Highest values of bending strength (1997 MPa), microhardness (305 MPa), and relative density (99.6%) were observed for nanomodified HEMT and SPS processed Co pellets, sintered at 700 °C.

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The effect of Ni and Co metal microparticles (MPs) and nanoparticles (NPs) on the structural and mechanical properties of Fe + 0.5 % C steel powder alloy was analyzed. The results revealed that the modification of the alloy by (Ni, Co) NPs can lead to the formation of a fine-grained compact and less porous structure, hence, significantly improve the mechanical properties of the sintered material. MPs modified samples were found to be highly porous when compared to the control. The introduction of 0.5 wt.% Co NPs increased the hardness value of the alloy to 58 HRB, whereas 0.5 wt.% Co MPs reduced the hardness to 47 HRB. The most beneficial effect is observed with 0.5 wt.% Ni NPs addition, wherein the hardness value increased to 63 HRB when compared to 52 HRB of the control sample. The highest flexural strength of 313 MPa was observed for Ni NPs incorporated alloy, whereas the least flexural strength of 156 MPa was noticed for the alloy containing 0.5 wt.% Co MPs. The fracture study confirmed that (Ni, Co) NPs increased the degree of densification, whereas Co MPs additives lead to the formation of large pits and cracks, consequently, to the destruction of material by a brittle inter-granular mechanism. Thus, this study introduces the use of Ni and Co NPs as modifiers in Fe + 0.5 % C alloy via powder metallurgy.