| Electrodeposited copper has been used as a transition layer on metal components or intermediate layer in a metal multi-layer. In addition, it is applied widely to replace aluminium as the interconnect material for ultra-large integrated circuits. On the other hand, electrodeposited nickel has become the coating for the purpose of protection and decoration. So far, electrodeposited copper and nickel have attained more and more attention in mechanical, chemical and electronic industry, etc. To be applied in severe conditions, metal matrix composites (MMCs) are developed by electrodeposition of metal with solid particles. When solid lubricant particles, such as graphite, MoS2, PTFE, are incorporated into metal matrix, self-lubricating MMCs are obtained. Cu- and Ni-graphite composites have been used in bearings and sliding contacts, in which lubricating oil and grease cannot be used. Self-lubricating MMCs can effectively solve the wear problems in special environments, such as vacuum, high and ultra-low temperature. Furthermore, corrosion behavior of MMCs must be considered in corrosive media. On the basis of the above, Cu- and Ni-graphite composites are prepared by electrodeposition technique, and then an in-depth study on microstructure, electrocrystallization mechanism and properties is carried out.This paper aims to obtain MMCs in the electrolyte with graphite particles in suspension. Graphite particles are distributed uniformly in metal matrix, and graphite content in the composites can be changed. The microstructure of the electrodeposits is examined, and then electrocrystallization mechanism is studied in detail. More attention is paid to the examination of wear properties and corrosion behavior of the composites. The main contents and conclusions are shown as follows:Electrodeposition parameters, such as graphite particle concentration, current density, agitation rate, have an important effect on graphite content in Cu-graphite composites. The results show that formamide (HCONH2) is in favor of the codeposition of graphite particles with copper matrix. Graphite content in the composites increases with an increase of particle concentration, but the variation of graphite content is very small over particle concentration of 20g·l-1. However, graphite content decreases with an increase of current density from 1A·dm-2 to 10A·dm-2. There is a maximum value of graphite content at agitation rate of 100r·min-1. As for Ni-graphite composites, the effect of graphite particle concentration and agitation rate on graphite content is similar with that on Cu-graphite composites, but graphite content reaches the maximum at current density of 5A·dm-2.After the composites are prepared successfully, the microstructural characteristics of electrodeposited metals and MMCs are discussed, and then an attempt is done to discover the effect of current density and graphite particle concentration on the microstructure. It can be found that pyramid crystals on the surface of electrodeposited copper are formed at current densities of 1 A·dm-2 and 3 A·dm-2, and the preferential orientation of (200) plane appears. Nodular grains at current density of 10 A·dm-2 show the preferential orientation of (220) plane. The sub-structure size of electrodeposited copper decreases, and microhardness increases with an increase of current density. As for Cu-graphite composites, the microstructure with cauliflower-like morphologies is produced, and exhibits the strongest peak of (220) plane. Moreover, surface sub-structure becomes smaller with an increase of graphite content. On the other hand, surface morphology of electrodeposited nickel consists of big pyramids surrounded by many smaller pyramids, which is provided with strong preferential orientation of (200) plane. When graphite particles are embedded in nickel matrix, flake-like or granular microstructure with preferential orientation of (111) plane is formed.Electrocrystallization mechanism of copper is studied using potentiodynamic polarization curves and electrochemical impedance spectroscopy before and after graphite particles are added to the electrolytes. It can be known that polarization occurs at the potential ranging from 0V to–0.45V for the electrolyte with graphite particles in suspension, but depolarization happens for the stirred electrolytes. Electrocrystallization process is affected by the diffusion of copper ions. In comparison with the electrolytes without graphite particles, copper electrocrystallization mechanism is not changed after graphite particles are added to the electrolytes. Charge transfer resistance (Rct) decreases with lower potential, but adsorption resistance increases. As for the electrodeposition of nickel and Ni-graphite composites, graphite particles don't have an effect on electrocrystallization mechanism, whereas charge transfer resistance related to the reduction of nickel ions decreases.Friction and wear properties of the composites are tested using a wear tester in the dry conditions. The results show that the incorporation of graphite particles into copper matrix reduces significantly friction coefficient and wear rate. Friction coefficient decreases from 0.3 for copper to 0.16 for Cu-graphite composites. When load and rotating rate increase, friction coefficient and wear rate increase. Wear debris of copper is composed of the oxides of copper and iron, indicating that it is ground and mixed mechanically. However, the composition of wear debris generating from the Cu-graphite composites is copper, graphite and a few of copper oxides. The transfer materials adhere to the surface of the counterparts in the form of nodular structure. Therefore, adhesive wear and oxidative wear occur for copper, and the main wear mechanism of Cu-graphite composites is a combination of abrasive wear, adhesive wear and delaminating wear. As for Ni-graphite composites, friction coefficient decreases from 0.41 for nickel to 0.13 for the composites. In addition, a smooth layer of the transfer materials is smeared on the counterpart. Both adhesive wear and oxidative take place for nickel, but delaminating wear is the main wear mechanism for Ni-graphite composites.In engineering applications corrosion resistance of MMCs must be taken into account before exposure to corrosive environments. Accordingly, corrosion behavior of electrodeposited copper and Cu-graphite composites is studied using cyclic voltammetry and electrochemical impedance spectroscopy in 3.5%NaCl solution. Two oxidation peaks and one reduction peak appear in cyclic voltammograms. Two oxidation peaks result from the formation of CuCl and Cu (II), respectively; the reduction peak corresponds to the reduction of copper chloride. The corrosion product of Cu2O results in the second reduction peak at low potential. Adsorption resistance (Rads) and adsorption capacitance (Cads) are formed in impedance spectra, and Rads becomes higher with longer immersion time. When current density increases, the electrodeposited copper with (220) plane preferential orientation has better corrosion resistance than that with the preferential orientation of (200) plane. As for electrodeposited copper, corrosion process initiates at grain boundaries by means of pitting corrosion. Corrosion occurs at grain boundary and defects for Cu-graphite composites. Compared with copper, corrosion resistance of Cu-graphite composites is much improved.On the basis of electrochemical impedance spectroscopy, electrodeposited nickel has better corrosion resistance than Ni-graphite composites. As for nickel and the composites after immersion in 3.5%NaCl solution, Rads and Cads come into being. In addition, Rads of Ni-graphite composites increases with more immersion time. Corrosion resistance of nickel increases after immersion in NaCl solution, but that of the composites depends on immersion time and graphite content.In the present work, electrodeposition process, microstructure, electrocrystallization mechanism, wear behavior and corrosion resistance of Cu-graphite and Ni-graphite composites are studied systematically. All these studies can provide some help for development and application of self-lubricating MMCs. In addition, they lay the foundation for further research.
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