FABRICATION AND FRACTURE OF POLYCRYSTALLINE NIO.
Interim technical rept. no. 3,
HONEYWELL RESEARCH CENTER HOPKINS MINN
Pagination or Media Count:
High purity, sub-micron NiO powders have been obtained by the preparation and pyrolysis of nickel oxalate. The grain growth and densification of these sub-micron particles was studied under various sintering atmospheres and under various hot pressing conditions. Sintering produced 6-17 micron NiO with a density as high as 96 of its theoretical x-ray density of 6.805 gmcc. Such material in the as-machine state had a zero transverse strength of 23,700 psi. Localized concentrations of grain boundary porosity appeared to be the source of failure in sintered NiO. Hot pressing produced 2 micron NiO with less than 2 porosity. When a closed pore state was reached at about 3.5 porosity, gases were entrapped within the NiO which expanded and apparently introduced grain boundary microcracks into the NiO structure. Such material fractured at stresses as low as 5,000 psi. at room temperature. The heat treatment of such material removed these stresses and increased its strength to as high as 18,300 psi. Mechanical polishing increased the room temperature strength to 23,000 psi. and its 650C strength to 24,400 psi. No evidence for a rhomberhedral transformation in this fine grained, polycrystalline NiO was observed. Author