| As the rapid development of micrifabrication technology, the application of metallic nano-oxides in devices has received much more attention, resulting in the preparation being the focus of research recently. The sonochemical synthsis, which takes the advantages of the local high temperature and pressure, microjet and shock wave arising from the acoustic cavitation phenomenon to prepare metallic nano-oxides(i.e. CuO, Cu2 O, NiO, and Al2O3) with novel structure, is adopted in this dissertation. Among them, as one kind of hard particles, α-Al2O3 can codeposite with the electroless Ni-P coating to increase the mechanical properties of the substrate significantly. However, the α-Al2O3 particles are positively charged in the acidic solution, and exhibit very low negative potential in the acidic plating solution of concentrated sulfate, resulting in the poor dispersibility of α-Al2O3 nanoparticles. This is not conductive to obtain the Ni-P-Al2O3 coatings with high performance. In this dissertation, we aimed to increase the potential of α-Al2O3 particles through the modification of the alumina precursor, and obtain the composite coating with excellent mechanical property and corrosion resistance by increasing the dispersibility of α-Al2O3 particles in the plating solution with surfactant.Firstly, nanoparticles of Cu O and Cu2 O with various morphologies were synthesized by sonochemical technique. The Cu2 O nanoparticles were prepared via the deoxidation of the CuO nanoparticles by ascorbic acid. The effect of NaOH concentration on the morphology, structure and optical property of CuO and Cu2 O was discussed. With nickel sulfate and urea as raw materials, the NiO2.45C0.74N0.25H2.90 precursor was prepared under the ultrasonic radiation, and hierarchically porous NiO was obtained after calcination. The surface characteristic and optical property of NiO were further discussed. The alumina precursor prepared by the sonochemical technique was consisted of amorphous aluminum hydroxide(Al(OH)3) and ammonium aluminium carbonate hydroxide(AACH). The complete transformation to α-Al2O3 was occured at 1100°C for 2 h. The precursor was modified by sodium dodecyl benzene sulfonate(SDBS), which overcame the hard aggregates successfully. The SDBS concentration not only affected the particle size, but also had an impact on the zeta potential of α-Al2O3. The median diameter and zeta potential of α-Al2O3 were 25 nm and-31.3 mV respectively as the SDBS concentration was 9 wt.%. Then the α-Al2O3 nanoparticles were used to codeposite onto the substrate with Ni-P matrix.Secondly, the parameters of electroless Ni-P plating were optimized. The NiSO4 and NaH2PO2 concentration, plating temperature and pH were 40 g/L,30 g/L,75°C and 5.50, respectively according to the orthogonal test. The effects of heat treatment temperature on the microstructure and microhardness were also discussed. The effects of DL-malic acid, lactic acid and citric acid concentration on the phosphorous content, deposition rate, surface morphology and microhardness were investigated by the single factor analysis method. The composition of complexing agent was DL-malic acid: 10 g/L, lactic acid: 10 g/L and citric acid: 5 g/L. The buffering capacity of plating solution, phosphorous content, deposition rate, microhardness, wear and corrosion resistance of coatings using(NH4)2SO4, NH4 F and NH4HF2 as buffer agent respectively were compared. The results showed that the buffering capacity of NH4 F was best. The maximum microhardness 898.4 HV and deposition rate 7.65μm/h were obtained as NH4 F concentration was 20 g/L, and consequently the wear and corrosion resistance were also best. The adhesion of deposit and substrate was good for coating using NH4 as the buffer agent.Thirdly, the modified α-Al2O3 nanoparticles were dispersed by 0.15 g/L tween-20 combining with cetyl trimethyl ammonium bromide(CTAB), sodium dodecyl sulfate(SDS) and polyacrylic acid(PAA), respectively. The distribution of α-Al2O3 particles in the deposits was the most uniform with CTAB as the surfactant. The content of modified α-Al2O3 particles in the deposits increased to a maximum value(approx. 13.01%) with 0.5 g/L CTAB, and consequently the maximum microhardness of approximately 1157.4 HV was obtained. In addition, the effect of ball milling time and Al2O3 concentartion in the plating solution on the performance of Ni-P-Al2O3 composite coating were also discussed. The perforemance of composite coating was the most excellent as the ball milling time was 2 h and Al2O3 concentartion was 5 g/L. Compared with the composite coating codeposited with unmodified α-Al2O3 particles, the performance of composite coating codeposited with modified α-Al2O3 was improved significantly.Finally, as the corrosion resistance of Ni-P-Al2O3 composite coating was worse, the duplex coatings were prepared based on the excellent corrosion resistance of Ni-P coating, in order to improve the corrosion resistance of composite coatings. The Ni-P/Ni-P-Al2O3 duplex coatings was composed of the Ni-P coating as bottom coating and the Ni-P-Al2O3 composite coating as surface coating. The effect of the deposition time of Ni-P coating, ball milling time, CTAB concentration and Al2O3 concentration in the plating solution on the performance Ni-P/Ni-P-Al2O3 coatings were invesitigated. The maximum microhardness was 1766.2 HV, as the deposition time of Ni-P coating was 60 min, CTAB concentration was 10 wt.%, ball milling time was 2 h and Al2O3 concentration was 5 g/L. The wear and corrosion resistance of Ni-P/Ni-P-Al2O3 duplex coatings were also better than that of Ni-P-Al2O3 composite coating. Moreover, the Ni-P/Ni-P-Al2O3 duplex coatings were deposited according to the new process, which avoided the oxidation of Ni-P coatings during the change of plating bath. The Ni-P-Al2O3 composite coating was deposited on Ni-P coating by the addition of the Al2O3 slurry direct into the Ni-P plating solution used for the deposition of bottom coating. The porosity of Ni-P-Al2O3 composite coatings was increased because of the incorporation of Al2O3 particles. The Ni-P coating was deposited on Ni-P-Al2O3 composite coatings. As the Ni-P coating was deposited for 30 min, the microhardness of Ni-P-Al2O3/Ni-P duplex coating was 1298.2 HV. The wear and corrosion resistance were more excellent than that of Ni-P-Al2O3 and Ni-P/Ni-P-Al2O3 coatings.In this dissertation, the α-Al2O3 nanoparticles were distributed in the plating solution uniformly, which was conductive to prepare Ni-P-Al2O3 composite coatings with excellent mechanical properties. Meanwhile, the duplex coatings were developed by the combination of different layer with excellent properties, which provided an important reference for the preparation of electroless coatings possessing both excellent wear and corrosion resistance. |
