Fabrication Of Nickel, Copper Matrix Composite Coatings And Their Electrochemical Behaviors

The composite coatings can improve the properties of applied materials for various purposes and in the practice,have been wiedly used in different application field.The composite deposition tenicque can also be used to fabricate some special composite materials.Therefore,the researches about the plating technique,the properties and the deposition mechanics of the composite coatings are always hot-spots.The paper, based on the background of new material project for the war industry, abundantly studied the electro-deposition technique of Ni-SiC,Ni-Al₂O₃,Cu-SiC and Cu-ZrW₂O₈ composite coatings.The properties such as:the wear resistant,anti-high temperature oxidization and salt solution corrosion of Ni-SiC composite coating are measured and analysed in the paper.The influences of solid particles on the electro-crystallization process of metal deposition were also studied by using cyclic voltammetry,chronoamperometry and AC impedance spectra.The mechanics of metal co-deposition with solid particles were analysed in the paper.The paper has nine chapters in which six chapters relate to the experiment.All research work may be divided to three aspects:the first part mainly studied the basic theroies on the composition electro-deposition(see in chapter 3).The second part studied nickel matrix composite coatings,including the electro-chemical fundament of Ni-SiC and Ni-nano-Al₂O₃ composite deposition,nickel matrix composite plating technology and the properties of Ni-SiC composite coating.The third part mainly studied copper matrix composite coating, including the fabricating technology for Cu-SiC composite coating,the electrochemical fundament for Cu-SiC composite deposition and the fabricating technology of new type Cu-ZrW₂O₈ composite material.1,Study on the fundamental theory of composite deposition In the third chapter,the solid powder feature and the primary factors of electrification on powder surfaces were discussed simply,and a method was given for determinating the positive or negative charge on powder particle surface.By pH change in bath solution,the chapter mainly analysed and described the behaviors of the electric double layer formatting on particle surface through powder adsorbing some positive ions,especially hydrogen ions in solution.Metal cation adsorbed on particle surfaces was thought as a very important factor for promoting solid particles into the coating.With measuring the open circut potential, the chapter also discussed the influence of some factors such as:solid particle in solution,bath solution temperature,pH and magnetic stirring et al on the electric double layer of the electrode surface.The main factors on the composite deposition were discussed according to two-step adsorbing mechanics of Gugleilmi.The preliminary results are shown as follows:1) Solid powders such as TiO₂,SiC,nano-Al₂O₃ particles etc can adsorb some ions in solution,especially hydrogen ion so as to cause the pH change in solution.PZC of TiO₂ powder in Watts` solution is 7.0 or so.When the solution pH＜7.0,the positive charge is inferred on the particle surface.The reasons on the powder surface adsorbing ions in solution are considered as the nonhomogeneous surface and the undersaturated valence bond of the atoms on surface.The other is also thought that the ions at surface lattice point or the functional groups on surface are selectly dissolved into the solution.2) Some factors such as:the powder feature and content in solution, solution temperature,pH,magnetic stiring etc have some influences on the electric double layer of the electrode.The effect range of various factors is about 0.001～0.077 V in open circuit potentials.The powder has more effect to the electrode potential,the most influence from among them is the nano powder,about 0.077 V。3) According to two-step adsorbing mechanics of Gugleilmi,the over potential(or current density) is a important factor to the composite deposition.Under certain circumstances,It is approximately exponential relation between the over-potential and the particle content in the coating.2,Electro-crystallization behaviors of Ni-SiC co-depositionThe chapter 4 in the paper studied the electrochemical behaviors of nickel co-deposition with SiC particles by using cyclic voltammetry, chronoamperometry and AC impedance spectra and explored the influences of SiC particles on electrochemical reaction and the nucleation/growth mechanics of nickel electro-deposition.The surface morphologies of Ni-SiC coating at the initial stage of the electro-crystallization were observed with Scanning Electron Microscopy(SEM). The main results are shown as follows:1) Electrochemical tests show that nickel co-deposition with SiC particles may begin at applied potential -700 mV or so.Under higher over potential(applied potential:-770～830 mV),the deposition nucleation of Ni-SiC solution system in the initial stage follows the instantaneous nucleation of Scharifker-Hills mechanism with three-dimensional.The nucleation time of Ni-SiC solution system may gradually be shortened with the increase of over potential and be more shorter than that of pure nickel deposition,which reflects that SiC particles on the electrode surface may offer many favorable sites for nickel crystallization nucleation.AC impedance spectra confirm that SiC particles can decrease electric-charge transfer resistance of nickel electro-deposition process. SiC particles may also results in that the peak current of Ni-SiC co-deposition descends and the potential corresponding peak current is also defered.2) The observation with SEM found that it is not easy to nucleate on particle surface because of SiC powder with poor electrical conductivity. In addition,SiC powders on cathode surface decrease the effective area for hydration cation reduction reaction.It can be inferred the reasons of the promoting effect of SiC powder to the electro crystallization nucleation of metal matrix are that SiC particles on the electrode surface may augment actual surface for adsorbing ions in bath solution and at the meantime,the hydration cation reduction reaction in the strictured cathode region increase the concentration of the reduction product (adsorbing atoms) so as to promote the electrocrystallization nucleation /growth.3,Plating technology and properties of Ni-SiC coatingThe chapter 5 in the paper mainly studied that the influence of technology factors such as:the current density,solution pH,solid particle concentration in solution and additive agent etc on SiC particle content in the coating and the morphology of the composite coating by using Scanning Electron Microscopy.Here offered the optimal process conditions for plating Ni-SiC coating and analyse the microstructure of the composite coating with plating time.The paper also discussed the influences of gravity and solution fluxion on the surface morphologies of the composite coating.By regulating plated-conditions and controlling the microstructure of the coating,the paper explored to improve the bonding condition of the interface between Ni-SiC composite coating and the cathode matrix.The chapter studied the wear resistant,anti-high temperature oxidization and the corrosion-resistant in 3.5%NaCl solution of Ni-SiC coating by means of abrading or corrosion loss weight and oxidization gain weight and analysed the corrosion behviors of Ni-SiC composite coating in 3.5%NaCl+(H₂O₂) solution with the polarization curve and AC impedance spectra.The main results are shown as follows:1) With the increase of current density or electrolyte solution pH value,SiC particle content in the composite coating exhibits a peak value change and under the conditions of current density 3.3 A/dm²,pH value 3.8～3.9,SiC content may be the maximum.Proper addition agent in bath solution can effectively promote SiC particle content in the composite coating.2) The micro-hardness of the composite coating increases with SiC content in the coating.Controlling technological parameter and using multiple-step plating may reduce the internal stress in the thick coating and improve the bond strength between the cathode matrix and the composite coating so as to avoid the desquamation phenomena(result from excessive internal stress).Micro-hardness test implies that Ni-SiC composite material with gradient function may be fabricated by using multiple-step plating methode.3) Ni-SiC composite coating is superior in the wear resistance and anti- high temperature oxidation to pure nickel coating,and the corrosion resistance of Ni-SiC coating to 3.5%NaCl solution may be similar to that of pure nickel coating.With immersion time,the corrosion rate of Ni-SiC coating decrease gradually.The open circuit potential of Ni-SiC coating evidently moves to the positive direction and the corrosion current of Ni-SiC coating decreases after adding little H₂O₂ in 3.5%NaCl solution.4) EIS shows that at the intial stage of corrosion process there is only one time constant determined,and two time constants occur after prolonging immersion time,which may indicate that SiC particles improve the compactability of the composite coating(SiC particles obturate the hole in the coating).4,Plating technology and behaviors of Ni-nanoAl₂O₃ co-deposition At present,metal co- deposition with nano-size particle also is one of research hot spots in the composite deposition field.The chapter 6 in the paper discussed the dispersion of nano-size Al₂O₃ powder in bath solution and technic conditions for Ni-Al₂O₃ co-deposition.The chapter also studied the electrochemical behaviors of Ni-nanoAl₂O₃ co-deposition by means of cyclic voltammetry,chronoamperometry and AC impedance spectra and analysed the reduction process feature and the mechanism of nickel electrocrystallisition from the bath solution containing nano-size Al₂O₃ particles.The main results are shown as follown:1) Al₂O₃ powder content in the coating decreases with cathode current density or bath solution pH.In the range of bath temperature: 30～55℃,the temperature has little influence on Al₂O₃ powder content in the coatings.Suitable stiring with compressed air or magnetic force may be benefit to improve the content.2) Nickel co-deposition with Al₂O₃ particles may begin at applied potential -740 mV or so and its nucleation also follows Scharifker-Hills with three-dimensional.Under the condition of higher over potential (applied potential:-890 mV),Ni-Al₂O₃ co-deposition may meet instantaneous nucleation mechanism.The nucleation time of Ni-Al₂O₃ co-deposition also is shorter than that of pure nickel deposition,which indicates that Al₂O₃ particle offer favorable sites for nickel crystallization nucleation on electrode surface.In addition to,Al₂O₃ powder may also decrease virtual cathode surface and defere peak potential.But it is noteworthy that nano-Al₂O₃ powders do not apparently decrease Faraday resistance of nickel electro-deposition process,which distinguishs from the effect of SiC particles in co-deposition.3) The observation with SEM found that nano-Al₂O₃ powders are approximately homogeneous distribution in the coating except small quantity little agglomerated particles.The metal nickel matrix of Ni-Al₂O₃ coating represents a kind of"podlike" microstructure.5,Technology and fundament of Cu-SiC co-depositionOwing to have good properties such as:higher strength,wear resistant,at the meantime,good thermal conductivity and low expansion coefficient etc,Cu-SiC coating has wide application and good prospect in electronic industry.The chapter 7 mainly studied the technic conditions for fabricating Cu-SiC composite material and the influence of SiC particle on the behaviors of copper electro-crystallization.The results are shown as follows:1) The experiment shows that with the current density,SiC particles in the coating just increase a little and adding suitable addition agent (especially at low current density) may be good to promote SiC powder content in the coating.Under the low current density of 1 A/dm²,the content is 14.07%wt or so.It is also found that SiC powder soaked in NaOH solution can effectively promote SiC powder content in the coating.2) The hardness of the composite coating increases evidently with the content of SiC powder in the coating.When SiC particle content in coating is 14.07%wt or so,the hardness reaches 206.98 Hv。3) The cyclic voltammetry and chronoamperometry show that SiC powder in bath solution may inhibit copper electro-crystallization process. Under the step potential of 70 mV or so,the electrochemical impedance spectra of Cu-SiC system represent a depressed semicircle following an inductive reactance arc in low frequency section.With the increase of over-potential,the radius of the semicircle decreases gradually and the impedance value of Cu-SiC system descends evidencely.The impedance-potential curve indicates that the electro-crystallization deposition may occur at the potential 90 mV or so.Only one time constant emerges during the deposition process.6,Fabricating Cu-ZrW₂O₈ composite materialIn the chapter 8,Zirconium tungstate powder was fabricated by using two-step solid state sintering method and the co-deposition technic conditions for fabricating Cu-ZrW₂O₈ composite material with high thermal conductivity and low expansion coefficient were preliminarily studied in the test.In the end,the paper discussed the practicability and development trend of the composite materials fabricating by using electron-deposition methode.The main conclusions are shown as follows:1) ZrW₂O₈ powder was successfully synthesized by using 600℃+1170℃/24h sintering method.Two-step sintering process is simple for operation in practice and ZrW₂O₈ content is high in sintered powder.2) Cu-ZrW₂O₈ composite coating has been successfully prepared from acid electrolyte.Under the condition of current density 2 A/dm²,pH 1.1,solution temperature,25℃,ZrW₂O₈ content in the coating is about～25%vol.The content exhibits a little peak change with the plating time. Adding CTAB(cetyltrimethyl ammonium bromide) or CTAB+SDS (lauryl sodium sulfate) agent in bath solution can influence the surface morphologies and the smoothness of the composite coatings,and decrease zirconium tungstate particle content in the composite coating. The operating with the intermittent-plating fasion may be favorable to increase ZrW₂O₈ content in the coating.3) The electro-deposition method has good prospect for fabricating Cu-ZrW₂O₈ composite material.The thickness of the composite coating reachs to 1mm or so after plated at the current density of 10 A/dm² for 30 hours.The observation from the cross-ssection of the composite coating found that ZrW₂O₈ powder is inhomogeneous distribution in the coating. There are many coarse powder in the coating near to the inner or external surface,but a small amount minor ZrW₂O₈ particles is in the middle range of the coating.Therefore,it needs to further improve the powder content and uniformity in the future.