Study On Preparation And Performance Of Ni-Co-P Ternary Alloy Electroless Nano-composite Coatings On Al Alloys

Due to their good performance, aluminum alloys are widely applied in industries,however, their disadvantages, such as low hardness, poor wear resistance and corrosionresistance, are becoming more and more outstanding. Electroless plating compositecoatings can improve their properties. But up to now, the nickel-phosphor binary alloyhas been the most widely used matrix for composite coating; and meanwhile there arestill some probloms, such as the poor dispersion of nanoparticle in the bath; and theeffect of particle size, particle species on the coating performance are relatively fewstudied. So in this thesis, the prepration of electroless plating Ni-Co-P ternary alloycomposite coatings reinforced by Al2O3、Si3N4、SiC have been studied, and threeNi-Co-P/Al2O3、Ni-Co-P/Si3N4、Ni-Co-P/SiC composite coatings have been preparedsuccessfully after three-step ultrasound method was invented to improve thedispersibility of nano particles in the bath, then the morphology, composition,microstructure, adhesion, microhardness, wear and corrosion resistance of deposits werecharacterized by scanning electron microscopy, energy dispersive spectroscopy, X-raydiffraction, automatic scratch instrument, Vickers hardometer, high speed reciprocatingfriction tester and electrochemical workstation, respectively. Finally, the effects of theparticles concentration, annealing temperature, particle size, and particle type on thestructure and properties have been obtained. The main conclusions are listed as follows:①Ni-Co-P/Al2O3coatings co-deposited with6,9,12and15g/l Al2O3particles (60nm) were prepared, respectively. The results reveal that all the coatings are cellularstructures, and particles aggregations are found to occur on the12and15g/l particlesincorporated coatings. With the increase of the particles concentration, the plating speed,Al2O3content in coatings, hardness, friction coefficient, and corrosion resistance of thecoatings first increase, and then decrease, and the first four reach the maximum at12g/l,and the best corrosion resistance occurres at9g/l. The wear resistance increased withthe increase of the particles concentration (0-12g/l). The wear rate of the15g/l particlesincorporated coating is higher than other composite coatings when the friction distanceis beyond600m.②The Ni-Co-P/Si3N4coatings prepared at12g/l20nm-Si3N4concentration inthe bath were annealed at200，300，400，500℃, respectively. The results indicate thatabove300℃, the cellular structures get larger and adhesion between the substrate and coating is enhanced. The surface of coatings is oxidized at500℃. The crystallizationbehavior was found to appear in the annealled coatings and the phosphides emerged at200-400℃, which can account for the increase of the hardness of both Ni-Co-P andNi-Co-P/Si3N4. But the hardness of these two coatings decrease after annealing at500℃while the Ni-Co-P coating decrease more greater. The variation tendency of the wearresistance with the annealing temperature is similar to that of hardness. The corrosionresistance is improved at200℃, but decreases when the annealing temperaturecontinuously increases, however, it gets better again at500℃.③Ni-Co-P/Al2O3coatings co-deposited with SiC particles of different sizes (40、200、800nm) were successfully prepared, respectively. The results show that all thethree coatings are cellular structures, and the particle size has not obvious impact on thethickness of the coatings. The obvious aggregations are found to emerge on the coatingsco-deposited with40nm particles, and some pores appear on the coatings with200and800nm particles. The coating co-deposited with finer particles has less particles content,but greater number density, leading to finer grains, higher hardness and better wearresistance. The coating reinforced by200nm particles has the lowest friction coefficient.800nm particles are prone to come off from the coatings easily, causing severe wear.The coating co-deposited with finer particles is more compact, leading to bettercorrosion resistance. the particles④Ni-Co-P/Al2O3、Ni-Co-P/Si3N4and Ni-Co-P/SiC coatings with800nm particlesize were successfully prepared at12g/l concentration in the bath, respectively (denotedas NAL, NSN, NSC). The results suggest that the obvious aggregations appear on NALcoating. The different incorporated particles have not much impact on the thickness ofthe coatings, but affect P content clearly. The descending order of particles content inpercentage composition is NSN>NAL>NSC, but in number density is NSC>NAL>NSN.Three coatings have the same predominant peak but different grain size. The sequenceof the grain size and corrosion resistance is NAL>NSN>NSC. The order of hardness isNSC>NSN>NAL. NSC has the best wear resistance and lowest friction coefficient. Theorder of the wear resistance and friction coefficient is NSC>NAL>NSN andNAL>NSN>NSC, respectively.These findings obtained above illustrate the influence and mechanism of theparticle concentration, the annealing temperature, the particle size, and the particle typeon the structure and performances of the electroless nano-composite coatings, whichhas important scientific value and significance for further optimization of composite plating process, improvement the properties, and promotion the industrial applicationsof the electroless plating composite coatings technology.