Densification and mechanical property of NiO-CoO and NiO-CoO-MgO system ceramics

The densification behavior and the mechanical properties of NiO-CoO and NiO-CoO-MgO systems were investigated. Al2O3 of 2 mass% was added in those systems as a sintering aid. Samples were prepared by hot isostatic press (HIP) treatment performed after a conventional pressureless sintering. The density of the NiO-CoO system did not depend on CoO content. On the other hand, the densification of NiO-CoO system was slightly prevented by the addition of MgO. Those samples have microstructure dispersed with (Ni, Co) Al2O4 or (Ni, Co, Mg) Al2O4 spinels in a NiO-CoO or NiO-CoO-MgO solid solution matrixes and the addition of CoO or MgO had no effect on the microstructure. In the NiO-CoO system, the bending strength increased, and the Vickers hardness increased with increasing the CoO content. Swelling around the Vickers mark, which is often observed in materials with plastic deformation ability, was observed. In the bending strength and the Vickers hardness of the NiO-CoO-MgO system increased with increasing the MgO content. These results suggested that the addition of CoO improved a brittle property of the NiO ceramic and the addition of MgO increased both the strength and the hardness of the NiO-CoO solid solution ceramic.