Surface Titanizing And Nickelizing Of Copper Using Double Glow Plasma Surface Alloying Technique

The paper is a basic applied research for developing novel wear-resistance, oxidation-resistance and corrosion-resistance surface alloyed layers on the pure copper by using the double glow plasma surface alloyed technique.Taking to improve the surface properties and service life as the goal and proceeding from the demand for retaining the good conductivity of pure copper, the authors have successfully prepared the Cu-Ti and Cu-Ni alloyed layers on the pure copper substrate by titanizing and nicklizing with the double glow plasma surface alloying technique, which confirms the feasibility of forming functional alloyed layers for specific purposes by this technique. Based on lots of experimental results, the optimum process parameters have been determined. The micro-morphology, phase structure and concentration distribution of surface alloyed layers are measured and analyzed by SEM, EDS, XRD and TEM. The formation course of alloyed layer, the effects of process parameters, mechanism of diffusion and solid solution and aging strengthening are investigated in detail. The oxidation-resistance at 400-700 °C and wear-resistance of Cu-Ti alloyed layer, the corrosion-resistance of Cu-Ni alloyed layer and the surface alternating resistivity are analyzed, compared with that of pure copper.The process parameters have the important effects on the solute element content anddistribution, total thickness and micro-morphology of alloyed layer. When the Ar pressure is in the definite range, its effect on the temperature change of specimen, that is, the change of Ar ion transmit energy to the specimen is not evident. In the range of test temperature, when the temperature is raised, the total thickness of alloyed layer is increased, and the surface Ti content of Cu-Ti alloyed layer is decreased, however, surface Ni content of Cu-Ni alloyed layer is heightened. The structure and morphology of Cu-Ti alloyed layer is concerned with the temperature tested.The comprehensive analysis for the micro-structure of Cu-Ti alloyed layers measured by SEM, XRD, TEM indicates that the structure consists of deposited layer + diffused layer when it is treated at 880*C, deposited layer + fused layer + diffused layer at 910°C, 925'C and 945 "C, and fused layer + diffused layer at 965 *C. The measurement results show that the Ti content from surface to 20 u m distance has a steep concentration distribution. The phase structures of alloyed layer formed at 925 °C and 965 °C are composed of CuTi+Cu4Ti+(Cu) solid solution, other precipitates such as Cu3Ti2, Cu2Ti and CuTi2 have not discovered. The diffraction pattern of TEM confirms that the structure of CU4T1 is identical with Ni4Mo (Dla type). Also, the experiments find out that the temperature of specimen is interrelated to source voltage and cathode voltage. Therefore, on the premise of guarantee the temperature, the Ar pressure, source voltage and cathode voltage should be chosen reasonably. The optimum Ar pressure is between 25Pa and 40Pa, and it is suitable that the ratio of source voltage and cathode voltage Vs/Vc is larger than 2.The calculation results from mathematics model based on the distribution law of Ar ions in the cathode fall show that the average energy of Ar ions comes up to 102eV order of magnitude under the condition of double glow plasma surface alloying process, which is higher than the sputtering threshold of the common elements. This energy range provides the guarantee for forming active particles by sputtering from the source electrode. Meanwhile, Ar ions, under the effect of cathode voltage, bombard and transfer the energyestablished by the oxidizing gains and oxidizing time follows parabolic rate law. The relationship among the constant k of parabolic rate law, On—Ti content in Cu-Ti alloyed layer, Cs—O2 concentration of surface and diffusion coefficient Do accords with the equation k2=CJ)o/ On- High On is the main reason of lowering the constant k.The results of micro-hardness measurement and wear test indicate that the surface hardness and wear-resistance of pure copper after titanized can get a big increase. The reason is that during the wear course phases CuTi and CujTi play the supporting role and soft a substrate after worn can keep the lubricating oil, which bring the lubrication effect into full play. Thus, as the load increases, the weight loss doesn't increase significantly.The experimental results of nickelizing on the pure copper with double glow plasma surface alloying technique show that 950-1050'C is the optimum temperature range. Cu-Ni alloyed layer has a good binding strength with Cu substrate. The Ni content from surface towards the Cu substrate present a gradient distribution, and the micro-structure changes gradually from the solid solution based on the Ni to that on the Cu. Nicklizing on the pure copper with the double glow plasma surface alloying technique has many merits such as high surface Ni concentration, efficient and so on.The test results on corrosion-resistance of Cu-Ni alloyed layer measured by electrochemical method in lmolHCl, 10%HNC>3 and lmoU^SCU solutions indicate that the self corrosion potential and corrosion potential shift positively, and corrosion current density and corrosion rate are lowered, that is, corrosion-resistance ability in enhanced, compared with that of pure copper. But their corrosion dissolution behaviors and corrosion mechanism in these three acids are not alike. In lmolHCl solution, the powder product is formed on the surface of pure copper during corrosion test, and the corrosion morphology on the surface of Cu-Ni alloyed layer presents the particle appearance. Although this particle appearance doesn't play the good protecting role, it decreases the corrosion rate and raises the ability of resisting the active dissolution of Cl*. In 10%HNC?3 solution, theto the surface of cathode electrode. Thus some point defects are formed on the cathode surface, which is favorable to the mutual diffusion between the atoms adsorbed and cathode atoms at the early depositing stage. The bombardment of Ar ions also makes the cathode temperature be elevated, which promotes the diffusion to go on. The calculation results from the regression equation of Ti concentration curves show that the mutual diffusion coefficients of Ti and Cu are ~~10"9cm2/s order of magnitude. As the Ti content increases, the mutual diffusion coefficient is increased, which has close relationship with the melting point of Cu-Ti alloyed layer. The Ti atom diffusion along the grain boundary is a few orders of magnitude higher than that of Ti in the crystal.The cross section morphology of Cu-Ti alloyed layer, after solid solution treated, presents the structure laminated, which indicates that the Ti element is re-distributed at the same time when the alloyed layer is quenched. In the micro-region of Ti content fixed, the micro-hardness appears two-peak phenomenon after solid solution and aging treated. The first peak hardness is the result of the mutual action between the spinodal decomposition and dislocations. The second peak hardness caused is due to the superlattice phase formed and grown hinders the motion of dislocation. As the aging temperature elevates, the first peak hardness shifts towards the left, which indicates that the mutual action between the spinodal decomposition and dislocation is advanced, and the second peak hardness reaches the maximum value after short aging time, which has a bearing on the growing kinetics of the spinodal structure. The experimental result measured by X-ray diffraction proves that the surface phase structure of Cu-Ti alloyed layer consists of CuTi+CatTi+ a after solid solution and aging treated.Oxidation experimental results show that the oxidation-resistance of Cu-Ti alloyed layer is improved. The reason lies in that high Ti concentration on the surface can form the well-distributed TiO2 layer. Thus the diffusion rate of oxygen in the oxidation layer is effectively lowered and the growing rate of oxidation layer is restrained. The curvespolarization curves of pure copper and Cu-Ni alloyed layer indicate that both corrosion potentials are close to each other, and the passivation doesn't exist. The dissolution current density presents the increasing trend, but this trend for Cu-Ni alloyed layer is a bit slow, which shows that its corrosion-resistance is a little better than that of pure copper. The morphology of pure copper presents the even corrosion in the area tested and that of Cu-Ni alloyed layer presents the porous powder appearance. In I111OIH2SO4 solution, the polarization curve of Cu-Ni alloyed layer emerges a typical activation-passivation feature, and the passivation potential and passivation current density are lowered compared with the pure copper, which show the ability of its corrosion-resistance is enhanced. It is observable from the morphology corroded that uneven corrosion on the surface of Cu-Ni alloyed layer occurs, that is, the galvanic corrosion is found where the corrosion current is higher than that in other area.Also, compared with the pure copper, the surface alternating resistances of Cu-Ni and Cu-Ti alloyed layers are increased, especially that of Cu-Ti alloyed layer. The reason is in relation to the electronic configuration of solute.