Effect of Low-Temperature Pore-Forming Additives on the Microstructural Evolution of Porous Nickel Prepared by Wet Powder Metallurgy

Porous nickel (Ni) coatings were fabricated by wet powder metallurgy using urea and ammonium bicarbonate (NH4HCO3) as low-temperature pore-forming additives. Ni powder was mixed with 3 wt.% carboxymethyl cellulose (CMC), coated onto Ni mesh substrates, and sintered in a hydrogen atmosphere at temperatures of 550–900 °C for 30–120 min. The microstructural evolution, phase composition, elemental distribution, and pore characteristics were systematically characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) analyses. The results demonstrate that sintering temperature governs the evolution of the porous structure more strongly than the investigated space holders. With increasing temperature, progressive neck growth, grain coarsening, and densification were observed, consistent with the transition from initial particle contact to the intermediate stage of solid-state sintering. An interconnected porous framework formed at 700 °C, identified as the selected condition due to its balance between interparticle bonding and pore preservation. XRD confirmed a single face-centered cubic (FCC) Ni phase after sintering, while BET analysis showed minor differences in specific surface area but noticeable variations in pore volume and pore-size characteristics. The limited influence of urea and NH4HCO3 is attributed to their decomposition at temperatures well below the effective sintering range of Ni. Consequently, the generated gases are likely to escape before a continuous metallic framework is established, resulting in only modest changes in the final pore architecture. These findings indicate that the porous structure is governed predominantly by diffusion-controlled solid-state sintering rather than by the direct pore-forming action of the investigated additives. This study highlights the importance of thermal compatibility between space holders and the sintering window of the metal matrix, providing new insight into the rational design of porous Ni materials prepared by wet powder metallurgy.