Dispersion Behavior Of Fine Particles In Electroless Nickel Solution

Electroless nickel(EN) composite coatings containing sorft particles(Mo S2, etc.) or hard particles(diamond, etc.) feature excellent tribological and wear properties, which make them feasible to be used in a wide range of applications. The potential benefits of such particles reinforcement depend on the good dispersion of them in EN plating bath and then in the nickel coating matrix. Currently, these poposes can be realized by using some surfactants to improve the wettability and dispersion stability of the particles, especially for water-repellent materials, in the EN solution. The existence of surfactant in the composite plating systems, however, carries some negative effects on the improvement of the properties of the EN composite coatings. To overcome this issue, an approach of surface coating on particle was proposed to substitute for the usage of surfactant. The mayor research work and results are as followed:Surface coatings on diamond and MoS2 particles were prepared by using the heterogeneous nucleation process. The surface morphology, thermal stability and phase structure of the coated particles were examined by using SEM, TG/DSC and XRD. Zeta potential measurements and sedimentation tests of the coated and uncoated particles in the EN solution were conducted. Results show that the diamond and MoS2 particles are coated with Al(OH)3 layer successfully, whose surface morphology have changed significantly. Both of the as-deposited coating and its product after calcination at 350 ℃, a transition phase of Al2O3, show an amorphous structure. The suspended stability of the coated particles in the EN solution is enhanced due to the increase of zeta potentials.The contact angles of the coated and uncoated particles for diamond and Mo S2 were measured by using a thin layer wicking method, and their surface free energy and components were calculated by using YGGF equation. A new parameter of the ratio of+γ/ γ?is suggested to evaluate the change of hydrophilicity/hydrophobicity for the surface before and after coating. Results indicate that water contact angle of the coated MoS2 particles decreases from 70.8 ? to 55.2 ?, and that of the coated diamond particles increases from 44?to 48.5? slightly. The surface energy and its acid-base component of the uncuated MoS2 are very similar to that of Talc, while that of the coated MoS2 and the coated diamond are very similar to that of alumina. The ratio of+γ/ γ?is suitable to both two types of particles.Interaction between fine particles in the EN solution and their influence factors were studied based on the DLVO theory and XDLVO theory. The calculations of prediction were verified by using sedimentation tests and particle size distribution measurements. Results illuminate the particle dispersions of the EN solution can be predicted by using their interaction potential curves. The fineparticles in the EN solution and its low dilutions show attractive interaction at any separate distance because there are not any potential barriers on their curves. As a result, fast sedimentation rates occur due to the particles agglomeration. As the increase of the EN solution dilution, the particle can keep themselves separate alone and extremely slow sedimentation rates occur in the EN dilutions because enough positive potential barriers exist on their curves. A stable dispersion of diamond requires the EN solution be diluted at least 100 times, and that of MoS2 requires at least 200 times. The coated particles, however, can not be obtained stable dispersions even although in the EN dilutions. A decrease of the attraction for the coated particles in the EN solution would be helpful to realize the dynamic dispersion in a stirring bath.Dynamic interactions between particles and the flow of EN solution were discussed based on the theory of turbulence. By comparing the adhersion strength of agglomeration and the turbulence stress arise from stirring in a bath, whether the agglomeration could be broken or not can be predicated.Both of the static and the dynamic strength of the agglomeration are considered. Results show that the predication applying dynamic strength of agglomeration is in accordance with the experimental results. The stability of agglomeration in flow is relevant to the primary size of particle, diameter of agglomeration and stirring rate. The breaking tendency of the agglomeration composed of the coated particles tends to be easier than that of the uncoated ones.The EN composite coatings embedded with the coated MoS2 particles were prepared without using any surfactants. The surface morphology, microstructure compactness, friction coefficient and wear properties of the composite coatings were evaluated. Results imply that the tribological and wear properties of them depend on their surface features, which are relevant to the stirring rate during their preparation and coating amount of the embedded particles. The coated particles is mainly located at the inner and surface of cellular body of the composite coating, while the uncoated one at the bottom.Ni-P-MoS2/Al2O3 composite coatings show a low frication coefficient and a low wear loss compared to Ni-P-MoS2 composite coating.