Abstract
In the previous study, the relative density (Ds) of the hydrogen-sintered compact obtained from Cu, Cu-10.8 mass%Sn or Cu-10.8 mass%Ni fine powder of 5-15 μm used for injection molding was found to tend to saturate at 95-97% and this phenomenon was suggested to be caused by the high equilibrium gas pressure ratio of H2O/H2 in the reduction reaction of the oxides, although the content of oxygen retained in the sintered compact was as small as 0.02 mass%. In this study, the cause for the above phenomenon was investigated in more detail.
(1) The effect of reduction time (0-32.4 ks) at 773 K in H2 gas on Ds or the oxygen content was not observed for each powder, suggesting that the limited amounts of oxides near the center of the powder particles are not easily reducible at low temperatures. (2) The addition of 1 mass%Al2O3 powder inhibited the crystal grain growth of the sintered compact, but the Ds also saturated at about 95%, indicating that the phenomenon is not related to the grain boundary acting as the sink of atomic vacancy. (3) The sintering in vacuum resulted in the complete densification. (4) Taking account of these results and the calculated value of the H2O gas pressure in the isolated pores, the cause for the above phenomenon was considered to be due to the high value of the equilibrium gas pressure ratio of H2O/H2 as suggested in the previous study.
