Nanostructural Changes in a Ni/NiO Cermet During …
223
3 Results and Discussion
As-sintered NiO ceramic specimens (of mode 1) exhibit quite low strength (Table 1)
corresponding to weak bonds between nickel oxide particles. Truncated hexagonal
pyramid-shaped particles are seen on the specimen fracture surface (Fig. 2a). They
are mostly unbroken exhibiting a distinct intergranular fracture micromechanism
(Fig. 3a).
The intense reduction course of as-sintered ceramic material in hydrogen at 600 °C
under the pressure 0.15 MPa (mode 2) is displayed in the metallic nickel structure of
the former hexagonal NiO particles (Fig. 2b). Each spongy-like particle has smoothed
edges, as compared to the initial material. The particle shrinkage occurs due to the
appearance of numerous nanopores in them decreasing thus their contact surface area.
Also, the interparticle porosity increased, which, however, did not violate the material
integrity. As compared to the as-sintered ceramics, the material stiffness decreased
by 47% (see Table 1). This, however, did not cause the lowering of its strength. It was
concluded that the material strength is affected by the quality of the reduced nickel
as well as of interparticle contacts and grows to 141% (see Table 1). Completely
reduced grains of metallic nickel with small pores inside can be observed in the
structure of the material (Fig. 2c). By analyzing the specimen fracture surface, it was
noted the intergranular fracture micromechanism with signs of plastic deformation
(Fig. 3b).
Fig. 2 SEM a, b fractography and c structure of specimens in (a) as-sintered state and (b, c)
reduced in pure hydrogen according to mode 2 (Table 1)
223
3 Results and Discussion
As-sintered NiO ceramic specimens (of mode 1) exhibit quite low strength (Table 1)
corresponding to weak bonds between nickel oxide particles. Truncated hexagonal
pyramid-shaped particles are seen on the specimen fracture surface (Fig. 2a). They
are mostly unbroken exhibiting a distinct intergranular fracture micromechanism
(Fig. 3a).
The intense reduction course of as-sintered ceramic material in hydrogen at 600 °C
under the pressure 0.15 MPa (mode 2) is displayed in the metallic nickel structure of
the former hexagonal NiO particles (Fig. 2b). Each spongy-like particle has smoothed
edges, as compared to the initial material. The particle shrinkage occurs due to the
appearance of numerous nanopores in them decreasing thus their contact surface area.
Also, the interparticle porosity increased, which, however, did not violate the material
integrity. As compared to the as-sintered ceramics, the material stiffness decreased
by 47% (see Table 1). This, however, did not cause the lowering of its strength. It was
concluded that the material strength is affected by the quality of the reduced nickel
as well as of interparticle contacts and grows to 141% (see Table 1). Completely
reduced grains of metallic nickel with small pores inside can be observed in the
structure of the material (Fig. 2c). By analyzing the specimen fracture surface, it was
noted the intergranular fracture micromechanism with signs of plastic deformation
(Fig. 3b).
Fig. 2 SEM a, b fractography and c structure of specimens in (a) as-sintered state and (b, c)
reduced in pure hydrogen according to mode 2 (Table 1)