| According to electrocrystallization and dispersion strengthened theory, nano-composite electrodeposition technology is a valuable new surface intensification technology to obtain nano-particle reinforced metal matrix composites allowing to co-deposit some solid particles and inert particles with matrix metal, having the advantages of short and simple process, easy control and easy transition from experiment study to industrial production, possessing wide application prospects on the parts surface of metallurgy, chemical, tobacco and mechanic industry. Compared with direct current electrodeposition, by means of utilizing the relaxation of pulse current, pulse electrodeposition can improve cathode activation polarization and decrease concentration polarization, which avoids the shortage of single direction and persistence of direct current, therefore, it is more favorable for preparing out the metal matrix composites with excellent performance and has already attracted a great deal of attention in the field of composite electrodeposition.On the basis of technological advantage of pulse electrodeposition and cooperating advantage of multi-component materials, Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites were obtained on common carbon steel surface, possessing uniform and stable microstructures, higher microhardness, better wear resistance, corrosion resistance and high temperature oxidation resistance. The composition design optimization, dynamic optimization and process optimization were carried out. The initial growth behavior and deposition mechanism of pulse electrodeposition, the crystallization course, interface bonding type, microhardness, abrasivity, high temperature oxidation and chemical corrosion behavior and mechanism of the composites were researched, analyzed and characterized, the results are as follows:1. The effects of electrolyte compositions, process conditions, single and double pulse parameters on pulse electrodepositing Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites were investigated, the samples were characterized with chemical compositions, elements distributions, deposition rate, microhardness and surface morphologies, the better process parameters have been optimized as follows:1) The electrolyte compositions are as follows:NiSO4·H2O:70g/L, Na2WO4-2H2O:100g/L, NaH2PO2·H2O:6g/L, H3C6H5O7·H2O:120g/L, CTAB:6mg/L, SiO2 (Average size:30nm):20g/L and CeO2 (Average size:30nm):10g/L.2) The process conditions are as follows:pH:5.5, T:60℃, mechanical stirring speed: 1000r/min, ultrasonic power:400W and ultrasonic time:30min.3) The single pulse parameters are as follows:single pulse frequency:1000Hz, single pulse duty cycle:50% and single pulse peak current density:40A/dm2,4) The double pulse parameters are as follows:positive duty cycle:10%, negative duty cycle:30%, positive pulse working time:300ms, negative pulse working time:40ms, positive average current density:15A/dm2 and negative average current density:1.5A/dm2.2,The initial growth behavior of pulse electrodepositing Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites was probed into, the results are shown that at the beginning of pulse current, the deposition behavior has obvious selection, it only prefers to deposit and form growth points on some positions with lower overpotential, including defect and crystal boundary, so the composition fluctuation is very obvious. But with the pulse electrodeposition going on, the non-continuous deposition turns into the continuous deposition gradually.3,The deposition mechanism of double pulse electrodepositing Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites was discussed, the physical growth model was fabricated, the results are shown as follows:1) Among the electrolyte system composed of NiSO4, Na2WO4, NaH2PO2 and H3C6H5O7, co-deposition of W and P can be realized under the function of Ni2+, the deposition type belongs to inductive co-deposition. At this time, a great number of atomic beam forms on cathode surface by means of the co-deposition of Ni, W and P atom.2) CeO2 and SiO2 nano-particles having been activated by cationic surfactant are easy to enrich on cathode surface, among these nano-particles, some nano-particles having absorbed Ni, W and P atom form some new atomic beam.3) Atomic beam consisted of Ni, W and P atom grows up continuously and forms some amorphous small particles. Meanwhile, other atomic beam with the center of CeO2 and SiO2 nano-particles grows up continuously and forms amorphous small particles too. Most nano-particles are imbedded into the atomic beam or amorphous small particles.4) The positive pulse current improves cathodic overpotential and electric field force, strengthens electrostatic attraction of cathode for solid nano-particles, increases the expanding driving force of atomic beam in three-dimension and the forming probability of atomic beam, avoids continuous growing on a few atomic beam and amorphous small particles. While negative pulse current inhabits the fast growth of atomic beam and amorphous small particles and surface swelling of the composites, improves the uniform distribution of deposition thickness, meanwhile, it is favorable to form some new atomic beam and further turn into the new growing point during the course the positive pulse.5) Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites prepared by double pusle electrodeposition are composed of a large amount of CeO2 and SiO2 nano-particles and some amorphous small particles and display amorphous structure as-deposited.4,The crystallization course of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites was researched by means of the analysis of phase structure, crystallinity and grain sizes, the results are as follows:1) The composites are mainly amorphous state as-deposited. Increasing heat treatment temperature leads to the increase of crystallinity, in the meantime, some Ni3P stable phase and Ni2P, N5P2 metastable phases begin to precipitate and the structure has changed into the mixed-crystal state. When heat treatment temperature is at 500℃, Ni2P and N5P2 metastable phases have disappeared and changed into Ni3P stable phase, the crystallinity improves further and the phase structure tends towards stability.2) During the crystallization course, the sizes and average sizes of Ni grains increase in various degree on the different crystal face with (111), (220), (311) and (222). While the structures of CeO2 and SiO2 nano-particles do not change, that inhabits growing up of amorphous small particles and is helpful to improve hot stability of the composites.5,Interface bonding type of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites was studied, the results are displayed that there are no structure defects such as cavity, crack and little crackle in the interfacial combination. Heat treatment temperature promotes the interfacial combination, a part of intemelt phenomenon occurs and close metallurgical bonding forms.6,The microharness and abrasivity of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites were investigated, the results are as follows:1) The microhardness increases with the rise of heat treatment temperature or heat treatment time and reaches the highest value at 400℃×3h, while the change of the abrasivity is just opposite. The highest microhardness with 1169Hv and the lowest abrasivity with 0.69mg·m-2·h-1 have been obtained.2) Under the same heat treatment temperature and heat treatment time, the microhardness and wear resistance of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites are obviously better than that of the Ni-W-P alloys.7,The high temperature oxidation behavior and mechanism of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites were discussed, the results are as follows:1) When oxidation time is controlled at 1h, the oxidation kinetics curve of the composites between oxidation weight gain rate and oxidation temperature accords with the oxidation kinetics equation△w=-0.04876+0.03398eT/166.27753. While when oxidation temperature is controlled at 300℃, the oxidation kinetics curve between oxidation weight gain rate and oxidation time accords with the oxidation kinetics equation△w=-0.01084+0.07004t. The high temperature oxidation resistance of the composites is better than that of Ni-W-P alloys.2) When oxidation temperature is lower 400℃, the amorphous small particles of the composites do not grow up obviously. CeO2 and SiO2 nano-particles embedded into Ni-W-P matrix metal can play a role of pinning and mechanical shielding and improve high temperature oxidation resistance, the surface oxidation films of the composites are still continuous and compact, and there are no crackle, strip and falling out.8,The chemical corrosion behavior and mechanism of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites were studied, the results are as follows:1) In NaCl and HCl corrosion media, under the same corrosion media concentration and corrosion time, the corrosion resistance of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites is better that of Ni-W-P alloys.2) In NaCl and HCl corrosion media, the corrosion mechanism of Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites is mainly interface corrosion around the boundary of amorphous small grains. While the corrosion mechanism of Ni-W-P alloys is mainly dot corrosion and crevice corrosion.9,Finally, the performance index between Ni-W-P/CeO2-SiO2 nano-particle reinforced metal matrix composites and other metal matrix composites and hard chromium is compared, the application prospect of the metal matrix composites is also analyzed.The projects are supported by National Natural Science Foundation of China, Applied Basic Research Plans of Yunnan Province, Scientific Research Fund of Yunnan Provincial Education Department, Scientific Research Fund and Analysis Measurement Research Fund of Kunming University of Science and Technology.
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