Effect of nanocrystalline binder on the microstructure and mechanical properties of ultrafine Ti(CN) cermets

The influence of highly deformed nanocrystalline binder ingredient on the overall microstructure and mechanical properties of ultrafine grade Ti(C_0.5N_0.5) cermet was investigated. Nanocrystalline Ni and Ni-aluminides viz. NiAl and Ni₃Al, synthesized by mechanical milling/alloying, were blended to the cermet powder prior to sintering. The mechanically milled nano-Ni contained a mixture of f.c.c. and hexagonal Ni phases. The cermet with nanocrystalline Ni showed a considerably improved microstructure over cermets with commercial coarse grade Ni and exhibits a high Vickers hardness of 16.1 GPa, along with a good fracture toughness value of 9 MPa m^1/2. However, no significant change in the mechanical properties could be detected in the case of nanocrystalline NiAl and Ni₃Al binder addition. The enhanced properties of the cermets containing nano-Ni were attributed to the finer particle size of the hard phase after sintering along with improved particle size distribution caused by rapid dissolution, which prevents excessive coalescence of the hard particles. On the other hand, the presence of high porosity in the cermets containing nano-NiAl and -Ni₃Al apparently nullified any such enhancement.