Electroless Ni–P coating on W–Cu composite via three diffe

P u b l i s h e d b y M a n e y P u b l i s h i n g (c ) I O M C o m m u n i c a t i o n s L t d

Electroless Ni–P coating on W–Cu composite via three different activation processes

L.Hao *1,2,J.Wei 1and F.X.Gan *1,2

To obtain qualified electroless Ni–P coating on W–Cu composite,the influences of three different activation processes,chemical activation,chemical activation and strike nickel plating,and anodic activation and strike nickel plating,on the adhesion of electroless Ni–P coating to W–Cu composite have been studied in this paper.The adhesion between the Ni–P coating and the W–Cu composite has been estimated by scratch,file and bend tests.The results indicate that the samples,pretreated by anodic activation and strike nickel plating,have the qualified adhesion.The samples that pretreated by chemical activation and chemical activation and strike nickel plating do not have the qualified adhesion.SEM shows that the surface morphology of Ni–P coating is smooth,no hemispherical clusters appeared and XRD indicates that it has an amorphous structure.

Keywords:Electroless,Ni–P coating,W–Cu composite,XRD,SEM

Introduction

Tungsten–copper (W–Cu)composites are promising materials for manufacture of electric contact parts,welding electrodes and thermal management devices.However,the W and Cu exhibit mutual insolubility or negligible solubility,so the W–Cu composite shows very poor sinterability,even by liquid phase sintering above the melting point of the Cu phase.1–4Defects appear in the structure (pores,copper lakes and tungsten agglom-erates)and the properties are sensitive to the defect,5especially in corrosive surroundings.These defects con?ne its application in other areas and to obtain the compact surface of W–Cu composite,the present study is mainly focused on the improvement of preparation procedures of W–Cu composite.6–12

To overcome defects as mentioned above,an attempt is made on coating metal deposit on this composite by the electroless Ni–P coating process,which provides distinct advantages,for instance the uniformity and hardness of deposits on complex shapes,low cost and simple equipment.13,14The electroless Ni–P coating has been widely used to fabricate metallic ?lms as engineer-ing protection coatings or functional ?lms.15,16Fabrication techniques for very thin and uniform ?lms have become very important with the progress of microelectronics and microelectronic packaging industry in recent years.17,18As far as we know,less work has been done on electroless Ni–P coating on the W–Cu composite.Once quali?ed Ni–P coating is obtained on W–Cu composite,it can ?nd wide applications in the

industries of aviation,aerospace,electronics,petroleum,chemistry,machinery,textiles and automotives owing to its high corrosion resistance,wear resistance and uniform coating thickness.19

Nevertheless,W–Cu composite is classi?ed as a substrate dif?cult to plate Ni–P coating owing to its high potential.Electroless coating on W–Cu composite has many challenges in the processing of plating and there are limited reports on W–Cu composite.The most dif?cult part of plating them is developing an appro-priate activation process,once a suitable undercoating is in place and many desired metals can be plated.The process of electroless Ni–P coating onto high potential substrate such as W–Cu composite requires the use of a catalyst to initiate the chemical reaction.20,21Thus,it is necessary to seed the surface with Ni(0)clusters,which should be able to initiate the electroless plating.22Insuf?ciently catalytic surfaces either do not plate or give poor plating adherence.To make W–Cu composite catalytic,an activation process is usually introduced before electroless plating.In the present paper,the Ni–P coating was deposited on the W–Cu composite by using three different activation processes;in addition,the morphology and structure of the Ni–P coating are also characterised.

Experimental procedure

Electroless Ni–P coating plating process

In this work,the metallic sheets of W–Cu composite with a composition of 88W–12Cu (wt.%)and dimen-sions of 2561062mm were used for electroless Ni–P coating.W–Cu composites were polished with abrasive papers from 0to 6#,cleaned with alkaline solution at 70u C for 30min and then,rinsed with water.They were put into different activation solutions for pretreatment.The electroless plating process is shown in Fig.1and the

1

Department of Environmental Engineering,College of Resource and Environmental Science,Wuhan University,Wuhan 430079,China 2

Environmental Corrosion Center,Institute of Metal Research,Chinese Academy of Sciences,Shenyang 110016,China

*Corresponding authors,email chinahaolong@76a58818a76e58fafab00366 and fxgan@76a58818a76e58fafab00366

?2009Institute of Materials,Minerals and Mining

Published by Maney on behalf of the Institute Received 22April 2008;accepted 29May 2008

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DOI 10.1179/174329408X326371

P u b l i s h e d b y M a n e y P u b l i s h i n g (c ) I O M C o m m u n i c a t i o n s L t d electroless Ni–P coating bath,the compositions of which are given in Fig.2,and the speci?c activation processes (Process A presents chemical activation;Process B presents chemical activation and strike nickel plating;Process C presents anodic activation and strike nickel plating)are illustrated in Fig.3.The compositions of the three activation processes are shown in Table 1.Deposition was done in a 500mL bath,maintained at 88u C.The samples were plated for 1h with a Ni–P

coating thickness of 12–13m m determined by measuring the weight of W–Cu composite before and after plating.(i)surface cleaning:to attain clean surface,the substrates were degreased by immersion in acetone for 5min,cleaned with alkaline solu-tion at 50–60u C for 30min and then,rinsed with deionised water (ii)pickling in acid:as exposed to the air,there may be oxidation film on the W–Cu composite surface and the acid pickling is necessary in gaining an unoxidised surface.The unoxidised surface was gained at room temperature for 1min by etching in a solution of 10vol.%H 2SO 4solution (iii)chemical activation:as the pickling in acid step can only remove the puffing oxidation film on

the W–Cu composite,the chemical activation

can provide a further unoxidised surface (iv)strike nickel plating:as the W–Cu composite belongs to high potential substrate,the process of electroless Ni–P coating onto it requires the use of a catalyst to initiate the chemical reaction.Thus,it is necessary to seed the surface

with Ni(0)clusters that should be able to initiate the electroless plating.To make W–Cu compo-

site catalytic,an activation step is usually

introduced before electroless plating (v)heat treatment at 180u C:H 2is produced in the electroless Ni–P coating process,and the H 2may enter the Ni–P coating and lead to its poor quality such as hydrogen crack.To avoid this,the samples are annealed at 180u C for 1h and furnace cooled.

Characterisation

Surface morphology was determined by means of scanning electron microscopy (X-650).XRD measure-

ments were made for the deposits in as plated conditions

using a diffractometer (Lab-6000).The coating adhesion was assessed using scratch,?le and bend tests.In the

scratch test method,six parallel cuts were made on the

test panel using a cutting tool with six cutting edges spaced 1or 2mm apart.According to GB/T 9286(eqv ISO 2409),23the quality of adhesion is ranked by

different numbers ranging from 0to 5.Excellent adhesion is assigned by 0in which no coating could be removed from the sample surface.When the removed coating is .65%,the adhesion quality is assigned by 5showing a very poor adhesion.One can rank the cross-cut adhesion number as excellent (0),very good (1),good (2),moderate (3),poor (4)and very poor (5).

Results and discussion

Adhesion and activation processes

Table 2shows the results of activation processes on the adhesion force between Ni–P coating and W–Cu composite checked by scribe,?le and bend tests.

The 1Schematic diagram of electroless plating procedures using different processes Table 1Bath composition and operating condition of different activation processes

Bath compositions and operating conditions Process A Process B Process C

Chemical activation Chemical activation and strike nickel plating Anodic activation and

strike nickel plating

H 2SO 4(98%),mL L 21100––250–

HCl (37%),mL L 21–300200–200

NiCl 2?6H 2O,g L 21––300–300

Temperature,u C 60–70Room temperature Room temperature Room temperature Room temperature Current density,A dm 22––333

Reaction time,min 3–43–43–43–4

3–4

P u b l i s h e d b y M a n e y P u b l i s h i n g (c ) I O M C o m m u n i c a t i o n s L t d

process C shows that the quali?ed adhesive force lies in the thin Ni ?lm formed on the sample surface by electrodeposited nickel.The corrosive product produced in the activation processes A and B may be the main reason for the unsatisfactory adhesion force owing to the dif?culty in controlling the reaction temperature and time.

Process A:chemical activation

After the chemical activation,the electroless Ni–P

coating can be obtained on the W–Cu composite,but there is a sharp potential change between the W–Cu composite and the Ni–P coating.The sharp potential change may be the main reason for the poor adhesion and this fact can explain why an interlayer between the W–Cu composite and the Ni–P coating is necessary.The speci?c activation processes can be seen in Fig.3.

Process B:chemical activation and strike nickel plating

Although the adhesion of the Ni–P coating obtained

from Process B is better than that obtained from Process A,the adhesion cannot be ranked as (0).The strike nickel can be functioned as an interlayer between the W–Cu composite and the Ni–P coating,but the strike nickel interlayer does not have a strong adhesion to the substrate,because the corrosive product is puf?ng in nature.The speci?c activation processes can be seen in Fig.3.

Process C:anodic activation and strike nickel plating

For nickel activation,Ni 2z ions are adsorbed on its surface when the substrate is immersed into the activation solution.These Ni 2z ions can be reduced by Cl 2to form active nuclei for the electroless Ni–P coating.The strike nickel can be functioned as an interlayer between the substrate and the Ni–P coating.The speci?c activation processes can be seen in Fig.3.

Although the electroless Ni–P coating can be obtained from the processes A–C,the process C can be the optimal activation process for the Ni–P coating esti-mated by adhesion tests.Therefore,the activation process is anodic activation and strike nickel plating.The samples for characterisation are just prepared from the process C.

The typical components of an electroless Ni–P coating bath are a source of nickel ions,a reducing agent,a complexing agent,a stabiliser or inhibitor and source of energy.This bath produces a binary alloy composed of nickel and phosphorus (Ni–P).24The mechanism of electroless Ni–P coating is based on the dehydrogena-tion of the reducing agent (hypophosphite in the present experiments).In an acidic hypophosphite deposition bath,the mechanism would involve the following

H 2PO {2z H 2O ?HPO 2{3z H z

z 2H ad

(1)Ni 2z z 2H ad ?2H z z Ni ;

(2)H 2PO {2z H ad ?P z H 2O z OH

{(3)2H ad ?H 2:

(4)

The overall reaction is expressed by two equations

H 2PO {2z H 2O z Ni 2z ?3H z z HPO 2{

3z Ni ;(5)H 2PO {2z 3H ad ?H 2O z OH {

z P ;z H 2:

(6)

This mechanism also accounts for the co-deposition of nickel and phosphorus along with the pH dependence of deposition and the generation of hydrogen gas.It also explains the autocatalytic nature of the process and the action of active surface.The electroless Ni–P coating is an autocatalytic reaction,which requires a catalytic substrate for nucleation and growth.

Morphology and microstructure of Ni–P coating

It has been well known that the physical properties of electroless coating depend on their structural characters.Therefore,it is very important to investigate the crystallographic structures of the electroless coating.25The microstructure and surface morphology were determined by means of SEM and XRD.As can be seen from Fig.4a ,the surface of electroless Ni–P coating on W–Cu composite is very smooth with no hemi-spherical clusters appeared and it consists of very tiny nodular crystals showing no cracks,pitting and other defects.

Fig.4b shows the XRD patterns of deposited coating by electroless Ni–P coating.As shown in Fig.4b ,a broad diffraction peak appears at 2h 540–50u ,indicating that the structure of Ni–P coating is in an amorphous state.This structure shows the absence of the defects in crystalline alloy such as grain boundaries,dislocation,stacking faults and segregation,26and it can been contributed to the good corrosion resistance of the Ni–P coating to corrosive solution.

Conclusions

Although the electroless Ni–P coating can be obtained from the processes A–C,the process C can be the optimal pretreatment for the Ni–P coating estimated by

Table 2Evaluation of adhesion between Ni–P coating

and W–Cu composite Activation process No.Scribe test File test Bend test Estimation of adhesion Process A 1(5)(4)(4)(5)2(5)(4)(5)(5)3(5)(5)(5)(5)Process B 1(3)(3)(4)(3)2(4)(3)(4)(4)3(4)(3)(3)(3)Process C

1(0)(0)(0)(0)2(0)(0)(0)(0)3

(0)

(0)

(0)

(0)

3Schematic diagram illustrating three different activation

processes

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adhesion tests.Therefore,the activation process is anodic activation and strike nickel plating.After this step,the surface of the W–Cu composite is catalytically active towards metal deposition in the electroless Ni–P coating solution.It could be possible to carry out such electroless plating of Ni–P coating on a high potential composite by providing a nickel interlayer.During the nickel activation process,the nickel ions are adsorbed onto the W–Cu composite surface and reduced into the metallic nickel.In the following step of electroless Ni–P coating,inpidual Ni catalytic centres would reduce nickel ions to neutral nickel atoms and form uniformly distributed nucleation centres of nickel islands.The coated metal particles nucleate and grow around the catalytic centres and become self-catalytic after initial growth with catalytic centres having been covered.Adhesion tests show that the electroless Ni–P coating has a quali?ed adhesive force with W–Cu composite.SEM shows that the surface morphology of Ni–P

coating is smooth,no hemispherical clusters appear and XRD indicates that it has an amorphous structure.

Acknowledgements

This work was supported by the Materials and Environmental Electro-chemistry Research Lab in Wuahan University,China.The authors also wish to acknowledge the help rendered by Dr Huazhu,Dr Xuhui Mao and Miss Guifen Fang in collecting the data.

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4a scanning electron microscopy image and b XRD pat-tern of electroless Ni–P coating on W–Cu composite by activation process C

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