Preparation Of Nano-sized Alumnia Particles By Mechanical Method And Its Application In Electroless Nano-composite Coating

Alumina(Al2O3) is one of important ceramics in industries, which has excellentperformance such as hardness, strength, heat resistance and corrosion resistance.Nano-particles of alumina are widely used in various applications like functional ceramic,composite material, bio-material, electronic material due to the superior characteristics,compared to the micro-sized material. In this dissertation, we synthesized electrolessNi-P-Al2O3nano-composite coatings via the incorporation of finely dispersed nano-sizedalumina particles, which were prepared by mechanical milling in a high energy densitystirred beads mill, and analyzed the wear and corrosion resistance enhancement of theelectroless coatings with nano-Al2O3particles as well.In Chapter2, the effects of parameters (i.e., plating temperature, Ni2SO4content inplating bath, NaH2PO2·H2O content in plating bath in process of electroless Ni-P coating) onthe micro-hardness, deposition rate and phosphorus content in the coating were investigatedvia a designed orthogonal experiment. The effect of heat treatment on the structure andmicro-hardness was also evaluated. The results show that the plating temperature has asignificant effect on the micro-hardness, and the content of Ni2SO4in plating bath has aneffect on the phosphorus content of coating and depositing rate. The micro-hardness of Ni-Pcoating could reach760HV under the optimum conditions (i.e., Ni2SO4of40g·L-1，NaH2PO2·H2O of35g·L-1and the plating temperature of80℃). The deposition rate couldreach20.0μm/h under the optimum conditions (i.e., Ni2SO4of30g·L-1，NaH2PO2·H2O of40g·L-1and the plating temperature of70℃).In Chapter3, nano-sized alumina particles (α-Al2O3) were wet prepared by a highenergy stirred bead mill. The specific surface area of the nanoparticles was measured via anitrogen adsorption method. The particle size of the ground products was determined byX-ray transmission particle size analyzer and acoustic particle sizer, respectively. The particlemorphology was examined by scanning electronic microscopy (SEM). The crystal lattices ofparticles were analyzed using X-ray diffraction (XRD). The results show that the groundproduct of α-Al2O3obtained in the mill has the average size of10-15nm with the specific surface area of132m2/g, which could be available for the subsequent codeposition of theelectroless Ni-P nano-composite coatings. It was also indicated that the particle size decreasedrapidly with the increase of the grinding time up to6hours. The main crystal planes of theground particles were damaged to some extent, but still remained α-phase.In Chapter4, Ni-P-Al2O3nano-composite coatings were deposited from the platingsolution containing nano-sized alumina (α-Al2O3) particles. The phase structure, morphologyand elemental composition of Ni-P-Al2O3nano-composite coatings were characterized byX-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersivespectroscopy (EDS), respectively. Effect of the nano-particle concentration in the solution onthe hardness, wear and corrosion resistance of the as-deposited and the heat-treated coatingswere investigated. The results show that the incorporation of nano-sized alumina particles canenhance the hardness and wear resistance of Ni-P electroless coatings. However, thisincorporation can affect the corrosion resistance. The hardness of the composite coatingincreased with the increase of the alumina nanoparticle concentration in the plating bath.When the alumina nanoparticle concentration increased to2g/L in the plating bath, themicro-hardness increased to1016HV and the friction coefficient was in the range of0.02-0.04.In addition, the mechanical properties of the electroless nano-composite coatings by theincorporation of various alumina nanoparticles prepared by conventional chemical methodand mechanical milling were investigated. It was indicated that the hardness of the compositescoatings obtained with the mechanically ground nanoparticles is as the same as that obtainedwith the chemically synthesized nano-particles. This illustrates that the nanoparticles preparedby mechanical milling method can be used for the electroless nano-composite coatings insteadof the nanoparticles prepared by chemical methods.The final chapter of this dissertation gave some outlooks in this aspect.