| Commercially pure, water-atomized iron powders were coated by autocatalytic nickel deposition that allows full coating on individual particles. For a thin deposit of approximately hundreds of nanometers, original irregular particle morphology was retained. Alternatively, thick deposits up to tens of micrometers could alter morphology to nodular structure. The coating was proven beneficial to ejection behavior during powder consolidation by reducing the ejection pressure and eliminating stick-slip phenomenon, whereas the sintered strength was comparable or better than that from the conventional elemental powder admixing process.;Preliminary studies with the concentration of sodium hypophosphite (NaH 2PO2) as a reducing agent and powder loading (solid content) of the depositing bath were carried out and were chosen at 0.2 M and 100 g/l, respectively, for subsequent experiments with pH, temperature, and duration time of coating. In this process, phosphorus from hypophosphite was codeposited with nickel. It was observed that, at pH 5, the deposition rate was at its highest and decreased with increasing pH. At 65°C, a low level of nickel and phosphorus resulted but then increased with increasing temperature. All coating experiments were carried out for 2 to 10 minutes in a batch process. It was found that coating time beyond 5 minutes has little effect on nickel content but it smoothened the deposit morphology.;Surface morphology was greatly affected by pH and temperature, one showing an opposite trend to the other. A low pH resulted in a smooth coating whereas a high pH yielded a nodular deposit. High magnification photomicrographs showed that an individual nodule is in fact a cluster of nanocrystallites.;All coated powders were further admixed with 0.5 wt % zinc stearate and compacted as per ASTM standard B331. Upon ejection, the pressure was recorded. The ejection behavior was viewed through an ejection profile (pressure vs. displacement). Powders with nodular deposit, e.g. coating at pH 7.15, showed the ejection pressure as high as 99 +/- 2 MPa, exceeding 86 +/- 5 MPa of the uncoated powder. However, powders coated at pH 4 yielded only 40 +/- 3 MPa for the same compaction condition. Further analysis concluded that, in addition to surface morphology, phosphorus content played a crucial role in the reduction of ejection pressure. |
