ELECTRODEPOSITION OF AMORPHOUS Ni-P ALLOY COATINGS

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Microstructure of Cu-based Amorphous Composite Coatings on AZ91D Magnesium Alloy by Laser Cladding

To improve the sliding wear resistance of AZ91D magnesium alloy, Cu-based amorphous composite coatings made of CuaTTi34Zr11Nis and Cu47Ti34Zr11Ni8＋20 wt pct SiC powders were fabricated on AZ91D magnesium alloy by laser cladding, respectively. SEM （scanning electron microscopy）, EDS （energy dispersive X-ray spectroscopy）, XRD （X-ray diffraction） and TEM （transmission electron microscopy） techniques were employed to study the phases of the coatings. The results show that the coatings mainly consist of amorphous phase and different intermetallic compounds. The reason of formation of amorphous phase and the function of SiC particles were explained in details.

Amorphous coatings deposited on aluminum alloy by plasma electrolytic oxidation

Amorphous [Al-Si-O] coatings were deposited on aluminum alloy by plasma electrolytic oxidation (PEO). The process parameters, composition, micrograph, and mechanical property of PEO amorphous coatings were investigated. It is found that the growth rate of PEO coatings reaches 4.44 μm/min if the current density is 0.9 mA/mm2. XRD results show that the PEO coatings are amorphous in the current density range of 0.3 - 0.9mA/mm2. EDS results show that the coatings are composed of O, Si and Al elements. SEM results show that the coatings are porous. Nano indentation results show that the hardness of the coatings is about 3 - 4 times of that of the substrate, while the elastic modulus is about the same with the substrate. Furthermore, a formation mechanism of amorphous PEO coatings was proposed.

High-temperature tribological properties of Ni-P alloy coatings deposited by electro-brush plating

High-temperature tribological properties of Ni-P alloy coatings processed by electro-brush plating on 20CrMo steel have been investigated. A baU-on-disc configuration was employed and 4 mm diameter Si3N4 balls were used as static counterpart. All the wear tests were carried out at 450℃ for 180 rain without lubricants. The electro-brush plating Ni-P coating is amorphous in as-deposited condition, and it becomes polycrystalline with the formation of Ni and Ni3P after heat treatment at 450℃for 1 h. The friction coefficient of the Ni-P coating is just 50% of that of the 20CrMo steel at the friction temperature of 450℃. A mild adhesive wear mechanism was found for the electro-brush plating Ni-P coating tested at 450℃, whereas for the 20CrMo steel at the same temperature a mixed adhesive and abrasive wear mechanism was observed.

Fabrication and microstructure of Fe-based amorphous composite coatings by laser cladding

Fe-based amorphous composite coatings were fabricated on AISI 1045 steel by laser cladding. The results of the X- ray diffraction and transmission electron microscopy analyses .show the coating is composed of an amorphous phase in majority and a nanocrystaUine phuse in m,inority. Phase composition of the coating changes along the depth of the coating. The reasonable scanning speed for fabricating an amorphous composite coating is 3 500 mm/min when the laser power is 4 800 W and the laser beam diameter is 2 mm. If the scanning speed is lower than 3 500 mm/min, the intensity of the two main diffraction peaks in X-ray diffraction patterns of the coatings decreases with the scanning speeds increasing. At the same time, a broad halo peak emerges and enlarges. High laser power and fast scanning speed are the essential conditions of amorphization. The coating exhibits high microhardness.

Molybdenum based amorphous and nanocrystalline coatings prepared by high velocity oxy-fuel spraying

The high velocity oxy-fuel(HVOF) based thermal spray process has developed as a potential advantageous approach for fabricating various kinds of functional coatings.In this article,the coatings of Mo-based alloy were synthesized using the HVOF process.The microstructure and the mechanical properties of the HVOF-processed coatings were investigated using SEM,TEM,XRD,and hardness and wear tests.Annealing treatment was applied to the as-sprayed coatings to develop the microstructure and its effect on the microstructure and mechanical properties of the coatings was examined.It is found that the HVOF-processed Mo-based alloy coatings are comprised of an amorphous splat matrix embedded with nano-sized crystalline particles.Annealing at temperatures over 950 ℃ results into crystallization of the amorphous matrix.The mechanical properties of the as-sprayed coatings are enhanced with annealing temperature up to 750 ℃ and from 950 to 1050 ℃,keeps constant between 750 and 950 ℃,and reduce over 1050 ℃.The change of the mechanical property with the microstructure was illustrated in the study.

Investigation on a Non-cyanide Plating Process of Ni-P Coating on Magnesium Alloy AZ91D

In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coating on Mg alloy AZ91D, the process of copper transition coating plated in the cyanides bath can be replaced. A new bath composed of NiSO4 was established by orthogonal test. The results show that zinc transition coating can increase the adhesion and protect the Mg alloy substrate from the bath corrosion. The optimal plating bath composition is NiSO4·6H2O 20 g/L, NaH2PO2·H2O20g/L and C6H8O7·H2O 2.5 g/L, and optimal bath acidity and optimal plating temperature are pH 4.0 and 95℃, respectively. The present process flow is composed of ultrasonic cleaning→alkaline cleaning→acid pickling→activation→double immersing zinc→electroplating zinc→electroless nickel plating→passivation treatment. The present non-cyanide process of electroless nickel plating is harmless to our surroundings and Ni-P coating on Mg alloy AZ91D produced by present process possesses good adhesion and corrosion resistance.

Effect of Heat Treatment on Structure and Electrochemical Properties of Electro-deposited Ni-P Amorphous Alloy

The influences of heat treatment time on structure and the electrochemical behavior of electro-deposited amorphous Ni-P alloy coating were studied. The results show that the as-plated deposits have an amorphous structure, and the deposits crystallize into Ni12P5 and Ni3P at 260 °C, and the corrosion resistance of the as-plated deposits are better than that of the deposits after heat treatment.

Process-structure-property relationship for plasma-sprayed iron-based amorphous/crystalline composite coatings

This study explores the fabrication of Fe-based amorphous/crystalline coating by air plasma spraying and its dependency on the coating parameters(plasma power,primary gas flow rate,powder feed rate,and stand-off distance).X-ray diffraction of the coatings deposited at optimized spray parameters showed the presence of amorphous/crystalline phase.Coatings deposited at a lower plasma power and highest gas flow rate exhibited better density,hardness,and wear resistance.All coatings demonstrated equally good resistance against the corrosive environment(3.5wt%NaCl solution).Mechanical,wear,and tribological studies indicated that a single process parameter optimization cannot provide good coating performance;instead,all process parameters have a unique role in defining better properties for the coating by con-trolling the in-flight particle temperature and velocity profile,followed by the cooling pattern of molten droplet before impingement on the substrate.

MONTE-CARLO SIMULATION FOR ATOMIC DEPOSITION OF AMORPHOUS ELECTROLESS Ni_(80)P_(20) COATING

Atomic growth process and structure of Amorphous Electroless Coating have been studied, using Monte-Carlo simulation method. The simulation results of amorphous Ni80P20 coating show that PDFs are in accordance with practical values. The migrations of adatoms in coating's growth are different from that of solidification of amorphous materials. In some cases, the migrated adatoms in the process of growth of amorphous coating are not enough to occupy all vacancies and traps, so the amorphous coating is micro-porous. The immovable probability k and the largest migration distance of adatoms, which lie on the electroless bath components, affect the PDF, volume density and microporosity remarkably.

TEMPERATURE DEPENDENCE UPON VISCOSITY OF Ni-P ALLOY ON AMORPHOUS FORMATION

The viscous flow of amorphous alloys near T_gis closely related to the viscosity of liquid alloys. On the basis of the measured surface tension and density for Ni-P binary alloys,the correla- tion between the viscosity of the melt and viscous flow process of the amorphous alloy is inves- tigated.The relationship between viscosity and temperature for Ni-P alloys going from liquid to amorphous solid may be described by the following formula:

APPLICATION OF MAXIMUM ENTROPY METHOD TO CALCULATE RDF OF Ni-P AMORPHOUS ALLOY FROM X-RAY DIFFRACTION DATA

Experiments clarified that application of the maximum entropy method to calculate the RDF (radial distribution function)of the Ni-P amorphous alloy from fewer data of its X-ray diffraction data shows a higher resolution and good consistent with known results.It is be- lieved that this method is available for the analysis of amorphous structure.

Influence of laser scanning speed on Cu-Zr-Al composite coatings on Mg alloys

To improve the surface properties of magnesium alloys, a study was conducted on Cu-Zr-A1 composite coatings on AZ91HP magnesium alloy by laser cladding. The influence of laser scanning speed on the microstructures and properties of the coatings was discussed. The coatings consist of amorphous phase, CusZr3, and Cul0ZrT. With the increase of laser scanning speed, the amorphous phase content of the coatings increases and reaches 60.56wt% with the laser scanning speed of 2.0 m/min. Because of the influence of laser scanning speed on the amorphous and crystal phases, the coatings show the maximum elastic modulus, hardness, and wear resistance at the laser scanning speed of 1.0 m/min. At the laser scanning speed of 2.0 m/min, the coatings have the best corrosion resistance.

High-temperature Compressive Performance of Mg Alloy Foams Coated with Ni-P Layer

Mg/Ni hybrid foams were fabricated by the electroless method.The Ni-P(Nickel-Phosphorous)coatings were deposited on the surface of closed-cell Mg alloy foams.The composition,microstructure and phases of the Ni-P coatings were characterized by scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD),respectively.The compressive tests were performed on the Mg/Ni hybrid foams at 400℃using the Mg alloy foams as a reference.The experimental results show that the yield strength,plateau stress and energy absorption capacity of the closed-cell Mg alloy foams at high temperature were improved by the Ni-P coating.And there are four main modes for the Mg/Ni hybrid foam failure at 400℃,i e,shearing in cell wall,bending in cell edge,shedding and cracking in Ni-P coating.

Ni–P coating on AZ31 magnesium alloy and its crystallization

Amorphous Ni-P coating was plated on AZ31 magnesium alloy via the electroless plating technique, and the plated alloy was subsequently annealed. X-ray dif- fraction （XRD）, scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, and differential scanning calorimetry （DSC） were used to characterize the coating. The results show that the hardness of the coating is much higher than that of bare magnesium alloy, which further increases after crystallization. The electrochemical polarization and salt spray tests show that the coating exhibits a much higher corrosion resistance than that of the bare magnesium alloy. Moreover, the crystallized coating still exhibits a much stronger corrosion resistance than that of the bare magnesium alloy, although its corrosion resistance is lower than that of the as-plated one.

Stability of electrodeposited amorphous Ni-Fe-P alloys

The stability of the electrodeposited amorphous Ni-Fe-P alloys was studied by DTA,DSC,XRD and improved four-ball wear tester in order to clear its applied scope.The results show that the element content has influence on the stability of amorphous Ni-Fe-P alloy,in which the crystallization temperature increases with Fe content,and the increase of P content delays the appearance of stable crystallization phases and recrystallization.There exist 6 exothermal reactions during heating the amorphous Ni69Fe8P23 alloy continuously.The activation energies of exothermal reactions at 248,303,322,350,376 and 442 ℃ are 131.5,111.6,237.8,253.6 and 238.5 kJ/mol,respectively.The amorphous Ni60Fe22P18 alloy crystallizes when the heating temperature is beyond 250 ℃.The stable crystallization phases consist of Ni(Fe)and Ni3P-type compounds Ni3P,Fe3P,(Fe,Ni)3P.The pressure and fraction have influence on the stability of amorphous alloy.Rubbing above the critical pressure crystallization will take place on the fractional surface.The crystallization phases due to pressure and fraction are different from those due to heating.It is the crystallization that increases the wear resistance of Ni-Fe-P coating under higher pressure.

Formation mechanism of amorphous Ni-Fe-P alloys by electrodeposition

The formation mechanism of the amorphous Ni-Fe-P coating was studied by analysis of the forming thermodynamics, dynamics, and crystallography of the amorphous alloy. The results show that, in the initial stage of deposition a thin 'crystal epitaxial growth' layer first forms, and then transforms to amorphous gradually. The cross section in Ni-Fe-P coatings by electrolytic etching exhibits a banded structure of alternate dark and light bands. It is proposed that the banded structure is caused by a change in the P content with thickness,which is due to alternated depletion and enrichment of [OH-] in the diffusion layer resulting from the generation and evolution of hydrogen gas. The amorphous Ni-Fe-P coating will be formed in proper composition, high nucleation rate and strongly hindered growth of the crystal nucleus. Amorphous Ni-Fe-P alloys form as islands, and grow up by layer.

Wear mechanism of electrodeposited amorphous Ni-Fe-P alloys

The wear mechanism of amorphous Ni-Fe-P coating was discussed. The wear resistance of the amorphous Ni-Fe-P coatings was tested on a Timken wear apparatus, and the wear track of the amorphous Ni-Fe-P coatings as-deposited and heated at various temperatures was observed by SEM. The results show that the wear resistance reaches a maximum value at NaHPO2·H2O concentration of 5 g/L, and heating at 400 ℃. The wear mode of the coating will change with the heating temperature increasing from pitting+plowing at 200 ℃ to pitting at 400 ℃, and to plowing at 600 ℃. The pits on the worn surface of the amorphous Ni-Fe-P coating result from the tribo-fatigue fracture. The cracks of spalling initiate at pits and propagate at certain angle with the sliding direction on surface, and then extend into sub-surface along the poor P layers or the interface between layers. Finally under repeated action of the stress in the rubbing process the cracks meet and the debris forms. The generation of the pits and spalling is related with the internal stress, brittleness and layer structure of the amorphous Ni-Fe-P coating.

Electroless Ni-P plating on Mg-Li alloy by two-step method

Pretreated Mg-Li alloy sheets were pre-plated in a NiCO3？2Ni（OH）2？4H2O solution to form a thin Ni-P alloy film and then plating in a NiSO4？6H2O solution was carried out to obtain a protective coating.The surface morphology,structure and corrosion resistance of the coating were studied.The results showed that a flat,bright and compact plating layer,which was integrated into the matrix metal,was obtained.The P content of the Ni-P coating reached 13.56%（mass fraction）.The hardness value of the Ni-P coating was about HV 549.The polarization curve showed that the corrosion potential of the Ni-P coating reached ？0.249 V（vs SCE）.A long passivation region was found on the polarization curve,and this phenomenon indicated that the coating has an excellent anti-corrosion property.

An in situ measure method to study deposition mechanism of electroless Ni-P plating on AZ31 magnesium alloy

An in situ method was designed to measure a continuous open circuit potential （OCP） curve of AZ31 magnesium alloy and to observe the morphology variation of Ni-P coating during the process of the electroless plating. The deposition mechanism of the electroless Ni-P plating on AZ31 Mg alloy was studied by OCP curve, scanning electron microscopy （SEM）, and energy dispersion spectroscopy （EDS）. The process of electroless Ni-P plating contains the coating formation stage and the coating growth stage. The formation stage includes three procedures, i.e., the nucleation and growth of Ni crystallites, the extension of the coating in two-dimensional （2D） direction and the coalescence of the coating along three-dimensional （3D） direction. SEM investigations demonstrate that the spherical nodules of the Ni-P coating are not only formed during the coating growth stage, but also generated in the initial deposition stage of electroless Ni-P plating. The variation of the coating rates at different deposition stages corresponds to the deposition mechanism of their respective deposition stage.