Stability of nano-/microsized particles in deionized water and electroless nickel solutions.

A major problem in the co-deposition of nano- and microsized particles within electroless NiP coatings is particle dispersion in the electroless nickel solution because of the strong tendency of particles toward agglomeration and sedimentation. The stability of colloidal Al(2)O(3), CeO(2), and BN particles and Al(2)O(3)CeO(2) and Al(2)O(3)BN particle mixtures in deionized water and electroless nickel solution was investigated by zeta potential measurements and sedimentation tests. Dispersions of Al(2)O(3) and CeO(2) particles showed good stability in deionized water with zeta potential values of 55 and 39 mV, respectively. BN dispersion in deionized water was found to be relatively unstable at pH 4 with zeta potential values of -13 mV, but at higher pH (i.e., pH 5.5), the values decreased up to about -40 mV. When the dispersions were made in electroless nickel solution, a significant decrease of the zeta potential values was observed for both single particles and mixtures of particles, indicating a change in the surface charge from high positive to low negative with detrimental effects on dispersion stability. Further, the findings suggested that the stability of particle mixtures is dominated by one type of particle, i.e., the Al(2)O(3)CeO(2) dispersion is governed by the single CeO(2) particles, whereas the Al(2)O(3)BN dispersion is governed by the Al(2)O(3) particles. All the zeta potential measurements were in line with the results of the sedimentation tests (i.e., low zeta potential values corresponded to short settling times, whereas high zeta potential values corresponded to long settling times).