Microstructure Evolution And Corrosion Resistance Variation Of Ni-Cu-P Amorphous Coating During Crystallization Process

Ni-P based amorpous coatings,prepared through electroless plating technique,have a number of distinguished properties such as high hardness,high electrical conductivity,high thermal conductivity,good anti-fouling performance,excellent electromagnetic shielding property and corrosion resistance.Therefore,they have be widely used in electronics,chemical,energy,ocean,aerospace and other industrial fields.The incorporation of Cu into Ni-P coating could further improve its functional performance,in particular thecorrosion resistance.However,the corrosion resistance mechanism of Ni-P based amorphous coatings is not fully understood,and the influence of Cu on the corrosion behavior should be investigated in detail.The microstructure and properties of Ni-P based amorphous coatings would be changed significantly when usd in a high environment due to their thermodynamic instability.However,the microstructural evolution and phase transition mechanism of the Ni-P based amorphous coatings during heat treatment process are not clear.In this study,the evolution of the microstructure and properties for the Ni-P based amorphous coatings during crystallization was comprehensively investigated,which would provide more insights for understanding the corrosion resistance mechanism.Four kinds of amorphous coatings with high P contents but different Cu content were prepared by electroless plating.The influence of Cu on the microstructure and properties of the coatings was carefuly analyzed by keeping a consistent P content inside the coatings.The microstructural evolution of the Ni-Cu-P amorphous coatings during crystallization was investigated by XRD and TEM.The results showed that the size of the ordered atomic cluster in the as-deposited Ni-Cu-P amorphous coatings was larger than that in the as-deposited Ni-P coating.With the increase of Cu content,the size of the atomic ordered cluster in the as-deposited Ni-Cu-P amorphous coating was decreased firstly and then showed an opposite trend,which is due to that Cu addition influenced the solubility of P in the Ni-P supersaturated solid solution.During crystallization process nanocrystalline Ni was precipitated firstly inside the Ni-Cu-P amorphous coating,Then,metastable phases such as Ni5P4,Ni12P5 and Ni5P2 were precipitated.The precipitation sequence of the metastable phases was controlled by their formation enthalpy.It was easier for the metastable phase with a relative smaller formation enthalpy to precipitate in Ni-Cu-P coating.Eventually,the metastable phases were transformed into stable Ni(Cu)and(Ni,Cu)3P phases.The metastable phase with relatively bigger Gibbs free energy difference between the stable phase transformed first.The number and the size of the nano-sized precipitates in the Ni-Cu-P coatings were relatively smaller than that formed in Ni-P coating under the same heat treatment condition.Compared to the Ni-P coating,the complete crystallization of the Ni-Cu-P coatings required higher heating temperature and longer duration time.This is due to that the Cu atoms could block the diffusion and aggregation of P atoms,and therefore the crystallization of Ni-Cu-P amorphous coating was inhibited.The thermal properties of the coatings were investigated by differential scanning calorimetry(DSC).The results showed that the incorporation of a small fraction of Cu(1-4 at.%)remarkably promoted the thermal stability of the as-deposited Ni-Cu-P amorphous coating.The phase transformation temperatures and the crystallization activation energies of the Ni-Cu-P coatings were higher than that of the Ni-P coating by 20? and 150 KJ/mol respectively.The thermal stability of Ni-Cu-P coating was gradually increased with the increased heating temperature from room temperature to 300?.This is because the content of Cu in the amorphous matrix was increased with the precipitation of NixPy phases,which enhanced the inhibition effect of Cu on the crystallization.The hardness of the coatings was studied by nanoindentation measurement.The results showed that the hardness of the Ni-Cu-P coatings with Cu content of 1?4 at.%was 30?120 Mpa higher than that of the Ni-P coating.During crystallization,the hardness of the Ni-Cu-P coating increased gradually with the increased heating temperature.The increased hardness was attributed to the increased size of the ordered cluster and the precipitated metastable phases as the temperature was below 300? heat treatment.At 400?.the hardness of the coating increased significantly due to the precipitation hardening of the large amounts of intermetallic compounds.During the low temperature(200? and 300?)relaxation process,the variation of precipitated nanocrystals in type,size and number density resulted into a small fluctuation of the coating hardness.The corrosion resistance and passivation behavior of the Ni-Cu-P coating were evaluated by electrochemical mesurement.The results showed that with the increase of Cu content the corrosion resistance of the Ni-Cu-P coating was enhanced firstly and then deteriorated.The Ni-Cu-P coatings showed better corrosion resistance than that of Ni-P coating in 3.5 wt.%NaCl,5 wt.%H2SO4 and 10 wt.%NaOH solution although the cluster size of the Ni-Cu-P coating was relatively bigger than that of the Ni-P coating.This is attributed to that the incorporation of Cu increased the corrosion potential,improved thermodynamic stability and promoted the passivation behavior of the Ni-Cu-P coating in the corrosive mediums.With the increase of heat treatment temperature,the passivation behavior of the Ni-Cu-P coating was weakened gradually and disappeared finally in 3.5 wt.%NaCl and 5 wt.%H2SO4 solution.Under the comprehensive influence of its microstructure and passivation properties,the corrosion resistance of the Ni-Cu-P coating was enhanced firstly and then deteriorated during crystallization.The Ni-Cu-P coating annealed at 300? for 1 h achieved the best corrosion resistance.The corrosion resistance,semiconductor properties and chemical composition of the Ni-Cu-P passive films in acid,alkaline and neutral corrosive media were investigated by EIS,Mott-Schottky curve and XPS respectively.The results showed thatthe Ni-Cu-P passivation film showed a better corrosion resistance than that of the Ni-P passivation film in 3.5 wt.%NaCl solution,This is because the Ni-Cu-P passivation film showed p and n bipolar semiconductor characteristics,while the Ni-P passivation film showed only p type semiconductor characteristic.In 5 wt.%H2SO4 solution,the Ni-Cu-P coating had a relative wider passivation interval than that of Ni-P coating.In 10 wt.%NaOH solution,under the condition of 1h passivation time,the semiconductor type of the passivation film varied from p-n bipolar semiconductor to p type.The corrosion resistance of the Ni-Cu-P passivation film in 10 wt.%NaOH solution was improved firstly and then deteriorated due to the competitive influene between the content of Cu oxides/hydroxides and the semiconductor properties.