engineering properties of en coatings

8/11/2019 Engineering Properties of en Coatings

1/24

THE

NGIN RING

PROP RTI S

OF

ELE TRO

L SS NI K L

O TINGS

E l e c t r o l e s s n i c k e l i s a

term

used t o d e s c r i b e p l a t i n g o f a

nickel phosphorus c o a t i n g onto

a

s u i t a b l e

s u b s t r a t e

by

chemical

r e d u c t i o n . Unlike e l e c t r o p l a t e d c o a t i n g s

e l e c t r o l e s s

n i c k e l

i s a p p l i e d without an e x t e r n a l l y a p p l i e d e l e c t r i c c u r r e n t .

I n s t e a d

t h e

c o a t i n g i s d e p o s i t e d onto a p a r t s

s u r f a c e by

reducing n i c k e l i o n s t o m e ta ll i c n i c k e l with sodium

hypophos-

p h i t e . This chemical p ro ce ss a vo id s many o f

t h e

problems

a s s o c i a t e d

with

most

m e t a l l i c

c o a t i n g s

and

provides

d e p o s i t s

w i t h many

unique

c h a r a c t e r i s t i c s .

As a p p l i e d


n i c k e l

c o a t i n g s

a r e

uniform hard

r e l a t i v e l y

b r i t t l e

l u b r i o u s

e a s i l y

s o l d e r a b l e and h i g h l y

c o r r o s i o n r e s i s t a n t . They can

be p r e c i p i t a t i o n hardened

t o

very

high l e v e l s through t h e use o f lo w tempera ture

t r e a t

ments producing wear

r e s i s t a n c e equal t o t h a t

o f commercial

hard chrome c o a t i n g s . T h is c om b in at io n makes

t h e

c o a t i n g

wel l

s u i t e d f o r many

severe

a p p l i c a t i o n s and o f t e n al lows

t o

be

used

i n

p l a c e of more expens ive o r l e s s

r e a d i l y

a v a i l

a b l e a l l o y s .

The engineering

p r o p e r t i e s

o f e l e c t r o l e s s

n i c k e l

d e p o s i t s

and

how

they r e l a t e t o t h e use o f t h e

c o a t

i n g a r e d i s c u s s e d i n t h e fol lowing s e c t i o n s .

STRU TUR

Hypophosphite

reduced

e l e c t r o l e s s n i c k e l i s one of t h e

very

few m e t a l l i c g l a s s e s

used

as an

engineering

m a t e r i a l .

Depend-

ing onthe formula t ion o f t h e p la ti n g s ol u ti o n commercial

c o a t i n g s may

c o n t a i n

5 to

12 p e r c e n t

phosphorus

d i s s o l v e d

i n

n i c k e l and as much as 0.25 p e r c e n t o t h e r e lements . The

s t r u c

t u r e o f t h e s e

c o a t i n g s depends

upon t h e i r co mp ositio n. C oating s

c o n t a i n i n g up t o 5 p e r c e n t phosphorus c o n s i s t of c r y s t a l l i n e S

n i c k e l with

phosphorus

i n

s o l i d

s o l u t i o n .

Those

with

phosphorus

c o n t e n t s between

5 and

p e r c e n t c o n t a i n

a

mixture o f

and S

phases

and

a r e

p a r t l y c r y s t a l l i n e . Coat ings c o n t a i n i n g

more

than p e r c e n t phosphorus c o n s i s t only o f

phase

n i c k e l

phosphorus and a r e normally amorphous to x - r a y s . These high

phosphorus d e p o s i t s

have no c r y s t a l

s t r u c t u r e o r s e p ar a t e

phases

2 3

E l e c t r o n d i f f r a c t i o n s t u d i e s

o f

d e p o s i t s

have

confirmed t h e i r l a c k o f c r y s t a l s t r u c t u r e a t magnif ica-

t i o n s up

t o

150 000

4


2/24

The c o n t i n u i t y

o f e l e c t r o l e s s

n i c k e l

c o a t i n g s

a l s o

depends upon

~ t : h e i r

c o mp ositio n. C oa tin gs c o n t a i n i n g more t h a n

10

p e r c e n t phosphorus and

l e s s

t h a n

0.05

p e r c e n t o f

i m p u r i t i e s

a r e

t y p i c a l l y c o n t i n u o u s .

A

c r o s s

s e c t i o n a l

view o f one o f t h e s e

~ o t i n s i s shown i n F i g u r e

S te el s ub st ra te

C o p p e r o v e r p l a t e

75

llm

t h i c k

coat i ng

p h o s p h o ru s

c o a t i n g s and e s p e c i a l l y t h o s e a p p l i e d

from

b a t h s s t a b i l i z e d o r b ri gh te n ed w i th heavy m e t a l s

o r

s u l f u r

compounds

a r e o f t e n

p o r o u s .

These d e p o s i t s c o n s i s t

o f

columns

s e p a r a t e d b y

c r a c k s and

h o l e s . The p r e s e n c e o f such

d i s c o n t i n u i t i e s

h as

a

s e v e r e e f f e c t on t h e

d e p o s i t s

p r o p e r -

t:i;ces:YLespecia lly on i t s

d u c t i l i t y and c o r r o s i o n

r e s i s t a n c e .

Q ~ e X a m p l e

o ~

t h i s

t y p e

o f d e p o s i t

i s

shown

i n

F i g u r e

2

1 2 5 6

F I G U R E I

Cross s e c t i o n a l

view o f a 75

llm

t h i c k

E l e c t r o l e s s Nickel

; d ~ p o s i

t.

',,400X m agni fi cat i on.

E t ched in

n i t a l .

~ C r e s s e c t i o n a l

viE; w o f

a 25

llm

thick e l e c t r o l e s s

n i c k e l cont ai ni ng

approximately 8

p e r c e n t phosphorus

an d

0.15

p e r c e n t

cadmium

and

l e a d .

400X

m agni fi cat i on.

Etched

in 2

n i t a l .

FIGURE

2


3/24

Lower phosphorus

and

heavy

meta l

s t ab i l i z ed depos i t s

a l so

f requent ly

appear to have a l aminar

s t ruc tu r e pa r a l l e l

to

t h e i r

subs t r a t e One

of t h i s

type

o f coa t ing i s shown in

Figure

3. These l amina t ions

r e su l t

from

v ar ia tio ns in

th e phosphorus con ten t

o f the d i f f e r e n t

l ayer s o f th e c oa tin g which in tu rn a re due

to

changes

in

th e

pH

or

s t ab i l i z e r

con ten t

o f

th e ba th

dur ing

p l a t i ng The more soph i s -

t i c a t ed

complexing and

s t ab i l i z i ng system s u sed to

app ly h igh phos-

phorus

depos i t s

e l imina te these

va r i a t i on s

and

produce

th e

more

homogeneous s t ruc tu r e shown in Figure

2

4 5 7

FIGURE 3

Cross

sect ional

view of

a laminar

15 thick electro

less

nickel

containing 7 to 9 percent phosphorus

and small amounts of

sulfur .

The

tu l ip shaped area is the

resul t of

a par t ic le of sand or

blast ing

media

embedded

in

the

deposit

during

plat ing.

The coating was

subse-

quently l apped, which removed the

protrusion produced

by the

par t ic le

1000X magnification.

Etched

in

1

oxa li c a cid a t 6 volts .

As e l e c t r o l e s s n i cke l depos i t s

a re

hea ted to

t empera tu res abov

220

to

260C

420

to

500F)

s t r uc t u r a l

changes begin

to

occur

;

F i r s t cohe ren t and then

d i s t i n c t

pa r t i c l e s o f nicke l phosphi te

Ni

3P)

form

within th e

a l l oy Then

a t t empera tu res

above 320C

600

oF)

th e d ep os it

begins

to

c ry s t a l l i z e

and to

l ose

i t s amor-

phous cha rac t e r With con t inued hea t ing th e nicke l phosph i te

pa r t i c l e s

conglomerate

and

a two phase a l loy forms. With coa t -

ings con ta in ing more

than

8

pe rcen t phosphorus

a matr ix o f Ni3P

forms, while

alm ost pure

n icke l i s th e p re dom inat e phas e in

lower

phosphorus

depos i t s

These changes

cause

a rap id inc rease

in

th e

hardness

and

wear r e s i s t ance

o f

th e co atin g

bu t

cause ts co r -

ros ion

r e s i s t ance

and

duc t i l i t y to

be

reduced .

A c ross

s ec t i ona l

view o f a fu l l y

hardened,

coa t ing i s shown in Figure 4

1

2 5

7,8.

Heat ing

a l so

causes

th e

depos it to shr ink

and

can

r e su l t

in

crack ing

through

th e co atin g

to

th e

subs t r a t e

9


4/24

FIGURE 4

ross se t ion l

vi w

o f a 5

t h i c k ,

c o a t i n g a f t e r h e a t

t r e a t

ment

a t

400C f o r 4 hours .

lOOOX

m a g n i f i c a t i o n .

Etched

i n Cr03 a t

6

v o l t s .

w i t h

prolonged

h e a t t r e at m e nt

a t h igh t empe ra tu re s

phosphorus

from

t h e r e g i o n n e a r t h e

i n t e r f a c e

w i l l

d i f f u s e

i n t o t h e sub

s t r a t e .

This can be h e l p f u l i n

o b t a i n i n g

adhesion on

p a s s i v e

m e t a l s l i k e t i t a n iu m o r s t a i n l e s s s t e e l . On

mild

s t e e l , a

n i c k e l - i r o n i n t e r m e t a l l i c l a y e r between t h e

c o a t i n g

and t h e

s u b s t r a t e can

be produced

by

4 hour h ea t t r ea tm e nt s

a t

temp

e r a t u r e s above 650C 1200

o F .

This

l a y e r

provides

a

very

wear r e s i s t a n t

s u r f a c e , and i n

some

environments

w i l l i n c r e a s e

t h e

c o r r o s i o n r e s i s t a n c e o f t h e

c o a t i n g

2 5 7 8.

INTERNAL STRESS

The

i n t e r n a l

s t r e s s

i n


n i c k e l c o a t i n g s

c o n s i s t s

o f two components--a thermal s t r e s s

due t o

t h e

d if f e re nc e i n

thermal

expansion

between t h e c o at i n g and t h e s u b s t r a t e and

a s t r u c t u r e s t r e s s

due t o

s t r u c t u r a l mismatch w i t h i n

t h e

d e p o s i t

caused by

non-homogeneity.

Both a r e

p r i m a r i l y a

f u n c t i o n

o f

t h e

c o a t i n g s composi t ion. As i l l u s t r a t e d by

Figure SID on

s t e e l t h e s t r e s s

i n

c oa ti ng s c on ta in in g

more

t h a n 10 p e r c e n t phosphorus i s n e u t r a l o r

compress ive.

with

lower

phosphorus

d e p o s i t s , however t e n s i l e s t r e s s e s o f 15

t o

MPa

t o

6 k s i

o c c u r .

The

high

l e v e l

o f

s t r e s s

i n

t h e s e c o a t i n g s promotes t h e i r c r a c k i n g

and

p r s t y ~


5/24

FIGURE 5

Effect

of

phosphorus

content on the i n t e r n a l

s t r e s s of


n icke l d epos it s on

s t e e l .

15

C

o

1

0

l

5

w

Tensile

a:

0

Compressive

5

z

a:

w

1

z

15

5

6

7

8

9 1

11 12

13

PHOSPHORUS CONTENT

PERCENT

The

s t r u c t u r a l changes during h e a t t r e a t m e n t a t temperatures

above 220C

420F , cause

a vo lume tr ic s hr in kage o f

up

t o

4 t o 6 p er ce nt w ith in e l e c t r o l e s s n i c k e l d e p o s i t s . This

i n c r e a s e s

t e n s i l e

s t r e s s

o r

reduces

compressive

s t r e s s i n

t h e

c o a t i n g s 2 , 8 , 9 .

Deposi t

s t r e s s

i s a l s o i n c r e a s e d by t h e

c o - d e p o s i t i o n

o f

o r t h o p h o s p h i t e s

o r

contaminants , o r by

t h e ~ r s n o f

excess

complexing a g e n t s i n

t h e

p l a t i n g

s o l u t i o n 9 1 . Even smal l

q u a n t i t i e s

o f

some meta l s can produce a severe i n c r e a s e i n

s t r e s s .

For

i n st a n ce , t h e a d di ti o n

o f

only 5

mg/l

o f bismuth

and

antimony

t o

most

b a t h s

w i l l

cause

t h e d e p o s i t

s t r e s s

t o

i n c r e a s e t o as much as 350 MPa

50

k s i t e n s i l e . High l v l s ~

o f i n t e r a l s t r e s s a l s o reduce

t h e

c o a t i n g s d u c t i l i t y and

i n c r e a s e i t s

p o r o s i t y

2 ,9

.

UNIFORMITY

One e s p ec ia ll y b e n ef ic ia l p r o p e r t y o f e l e c t r o l e s s n i c k e l i s

i t s

uniform

c o a t i n g

t h i c k n e s s . With e l e c t r o p l a t e d c o a t i n g s ,

t h i c k n e s s can

vary s i g n i f i c a n t l y

depending

upon

th e

p a r t s

c o n f i g u r a t i o n

and

i t s

proximity

t o t h e anodes. Not

only

can

t h e s e

v a r i a t i o n s e f f e c t

t h e

u l t i m a t e

performance o f t h e c o a t in g ,

but they

can a l s o cause a dd it i o na l f i n is h in g t o

be

r e q u i r e d

a f t e r p l a t i n g .


6/24

With

e l ec t ro l e s s

n icke l

the pla t ing

r a t e and coa t ing t h i ck -

ness

a re

th e

same

on any sec t ion o f

the

pa r t exposed to f resh

pla t ing

so lu t ion Grooves s lo t s b lin d h ole s and even th e

i n s ide of tubing wi l l have th e

same

amount o f coa t ing

as

the

ou t s ide o f a

par t

This

i s

i l l u s t r a t ed by Figure 6 which shows

th e uniform t h i ckness on th e i n t e rna l threads

of

a

smal l

spray

nozz le

FIGURE 6

example

of

the uniformity

of

e lec t ro less nickel coatings

Unlike the copper overplate

the 25 vm thick e lec t ro less

nickel

coating

reproduces

the

prof i le of

the

in te rna l threads

of

the

par t

The

substrate

i s

leaded

s tee l

lOOX magnificat ion

Etched in

picra l

Because

o f

i t s uni fo rmi ty

of ten

th e ove ra l l

f in i sh ing

cos t of

a pa r t w il l be l e s s

with

e l ec t ro l e s s n icke l than

wi th

e l ec t ro -

p la ted

coa t ings even though th e ma te ria l c os t of

th e

process

i s h igher For example th e subs t i t u t i on o f e l ec t ro l e s s

n icke l

fo r hard chromium

on

many

o f the cy lin de rs and

ro l l s used in

th e

pr in t ing

and t e x t i l e

i ndus t r i e s

has

not

only

reduced

th e

cos t o f p la t ing by 4 percen t

but

also has allow ed 55 percen t

o f

th e g rin din g t ime to be saved

1 2


7/24

~ ~ ~ ~ ~

With

e l e c t ro l e s s n icke l c oa ti ng th ic kn es s

can be con t ro l l ed

to

s u i t

the app l i ca t ion .

Coat ings

as th in as ~ ~ m

0 .1

mil

a re commonly

app lied fo r

e lec t ron ic components , while

those

as t h i ck as 75 to

125

~ m

3

to

5

mils a re t yp i ca l fo r corros ive

environments .

Coat ings

t h i cker than 250 10

mils

are used

fo r sa lvage o r

r epa i r

of

worn or

mismachined pa r t s

5.

ADHESION

The adhesion of e l e c t ro l e s s n icke l coa t ings to

most meta ls

i s exce l l en t . The i n i t i a l replacement r e ac t i on which

occurs with ca t a ly t i c meta l s toge ther with th e assoc ia ted

ab i l i t y

o f the

ba ths to

remove submicroscopic

so i l s

al lows

the

depos i t

to

e s t ab l i sh meta l l i c as

wel l

as mechanical

bonds with th e

subs t ra te . The

bond

s t reng th

of

coa t ings to proper ly

cleaned s t e e l

has been

found

to be 400

MPa 60 ks i

o r morel

3 The

adhesion

to

aluminum

and

a lumi

num a l loys

i s

l e s s bu t

usual ly exceeds 300

MPa

40 ksi 2 ,5 ,7 :

With non-ca ta ly t i c or passiv e m eta ls

such

as

s t a i n l e s s

s t e e l an

i n i t i a l replacement reac t ion does not

occur

and

adhesion i s

reduced.

w ith proper pre t rea tment and ac t iva

t i on however , th e

bond s t r eng th

of the coa ting

normal ly i s

a t le a s t 140

MPa

20 ksi 2,5 ,7 .

The adhesion to

copper

a loys

i s

usua l ly

between 300 and

350

MPa 40 and 50 ks i 2 .

With meta l s such as

aluminum

i s common p ra ct ic e to bake

pa r t s

a f t e r p la t ing

fo r 1 to 4

hours a t 130+ 200C

270

to 400F to increase th e adhesion o f the coa ting . These

t rea tments

r e l i eve

hydrogen

from

th e

pa r t and

the

depos i t and

provide

a very minor

amount

of codif fus ion between the coa t

ing

and

subs t ra te .

They

a re

most

usefu l

where

pre t rea tment

has been

l e s s

than adequate and adhesion i s

marginal .

With

p ro p er ly a pp li ed coa t ings

baking

wi l l have only a minimal

e f f e c t

upon bond s t r eng th 2,5,8

MELTING POINT

Elec t ro less n icke l

i s

an eu tec t i c a l loy with a

wide

mel t ing

range.

Unlike a pure compound,

does not

have a t rue

melt ing po in t .

This

i s

i l l u s t r a t ed

by th e pha se d ia gram fo r

nickel-phosphorus a l loys shown

in Figure

~


8/24

Jlom t:P

If J }

I

I I

11/SZ

0\

\

S a l m ~ k ~

~

0

~ ~

:;;;

;;;:

tl

+

fl15

~ .

ffltJ

0

I

i fill

\

SC JI ;elre \

I

~ \ i _

O f ~

j

\

l 1IJ

18

/

6

88tJ

I

00

110


9/24

~

.0

.6

.4

~

2

.0

a

~

6

,

7

FIGURE 8

Effect o f d ep os it

phosphorus

content

on

the densi ty

of

e lec t ro less nickel .

2

hosphorus content

{

The the rmal and e l e c t r i c a l proper t i e s of

these

coa t ings

a l so

vary

with

composi t ion .

For

high

phosphorus coa t ings

e l e c t r i c a l r e s i s t i v i t y and thermal

conduc t l

v i ty a re genera l ly about 9 0 ~ ~ c m and 0.08

Wjcm OK

4 .6

Btuj

f t -hr -oF

respec t ive ly lB 19 .

Accordingly

th e se coa t ings a re

s i gn i f i can t l y

l e s s conductive than conven ti ona l conductor s

such as

copper .

Because of

the

r e l a t i ve ly

th in l ayer s

used

however fo r most

app l i ca t ions rho r09is tance

of

e l e c t ro l e s s

n icke l i s

not

s i gn i f i can t .

coa t ings

are

being success fu l ly

used

fo r

such

app l i c a t i on s

as exchanger

tubing

and

e l e c t r i c a l

swi tches

and

con tac t s .

Heat t r e a tment s

prec ip i t a t e

phosphorus

from th e a l loy and

can

increase the c ondu ct iv it y o f

e l ec t ro l e s s

n icke l by 3 to

4

t imes 2 7. The

formula t ion of

th e p la t i ng

so lu t ion

can

a l so

a f f e c t

conduc t iv i ty .

Tests

with

ba ths

complexed with

sodium ace ta t e and with succin ic ac id showed

e l e c t r i c a l

r e s i s t i v i t i e s o f 61

and 804

~ ~ m

r espec t ive ly2 .

Phosphorus

con ten t also

has

a

s t rong

e f f e c t on

th e

thermal

expansion

of

e l e c t ro l e s s n icke l . This i s shown in Figure 9

which i s based on depos i t

s t r e s s measurements

on

d i f fe ren t

subs t ra t es 10.

The

coe f f i c i en t

of

t he rma l e xp an sio n o f

coa t ings

i s approximately

equal to

t h a t of s t e e l .


10/24

_ _ ~

FIGURE 9

D

osp orus content

5

8

j

E ffec t o f

heat t reatment

a t

di f feren t

temperatures

on the hardness of

Electroless

Nickel.

Temperature OC

For

some

app l i ca t ions

high

temperature t rea tments cannot

be

to lora ted

because

o f

pa r t warpage o r because o f t h e i r

e f f e c t

on the subs t r a t e For

t hese

i s

sometimes p oss ib le to use

longer t imes and

lower t empera tures to ob ta in th e des i red

hardness This i s

i l l u s t r a t ed by Figure 13, which shows the

e f f ec t

of d i f f e r en t

tr ea tment p er io ds on th e hardness o f

coat ings

s.

Treatments

a t

340C 650F fo r

4

to

6

hours and

a t 290C

550F fo r 10 to

12

hours a re

commonly

used

fo r e l ec t ro l e s s

n icke l

depos i t s

These

can pro duce h ard ne ss v alu es of

950

to 1000 VHNIOO Treatments a t 260C

500F

are also

occasional ly

used , al though the r e su l t i ng

hardness i s

lower.

At t empera tures of 230C 450F and

below, only a minimal

increase

in

hardness i s

obta ined Accordingly such

t r e a t -

ments

a re

only

r a r e l r used,

except

fo r

hydrogen r e l i e f

o r

adhesion improvement


14/24

FIGURE

13

0

:--


15/24

Because

o f t h e i r

high hardness

e l ec t ro l e s s

n icke l

coat ings

have e x c el le n t r e si st an c e to wear and abras ion

both

in th e

as depos i ted and

hardened cond i t ions . Laboratory

t e s t s have

shown fu l ly

hardened

coa t ings to have w ear re s i s tance

equa l

to hard

chromium

under

both dry

and l ub r i ca ted

cond i t ions .

This

i s i l l u s t r a t e d

by Table 1

which shows th e r e s u l t s

o f

t y p i c a l Taber Abraser Wear

t e s t s o f

e l ec t ro l e s s

n icke l

coa t ings

an d com pares

them

t o e le ct ro p la te d

n icke l

and

chromium 26 ,27

28.

The e x c e l l e n t

re s i s tance of e l ec t ro l e s s

n ick e l

of ten

al lows

to

r ep lace high a l loy

mate r i a l s and

hard chromium.

T BLE

1

COMP RISON

OF THE

T BER BR SER

RESISTANCE

OF DIFFERENT ENGINEERING COATINGS

CO TINJ

HE T

TRE lMENT TWI, mg/1000

Cycles 1)

Watts

Nickel

on

25

Electroless Ni-9 P None

17

Electroless

Ni-9 P

300

oC/l

hr

10

Electroless

Ni-9 P 500

oC/l

hr

6

Electroless

Ni-9 P

650

oC/l

hr 4

Electroless

Ni-5 B on

9

Electroless

Ni-5 B

400

oC/l

hr 3

Hard Chromium

on

3

1)

Taber

Wear

Index

CS-IO

abraser wheels

1000

gram

load

de te rm in ed a s a vera ge w eig ht loss per 1000 cycles

for

to ta l te st

of 6000 cycles.

Tests with

e l ec t ro l e s s

n icke l c oa te d v ee b lo ck s in a Falex

Wear Tes te r have confi rmed a s i m i l a r

re la t ion

between hea t

t r ea tmen t and wear and shown th e co a ting to be more r e s i s -

t a n t than hard

chrome

under l ub r i ca ted wear cond i t ions .

This

i s

i l l u s t r a t e d by Table 2 fo r an

e l ec t ro l e s s

n icke l

con ta in ing approximate ly 9 percen t

phosphorus .


16/24

TABLE 2

COMPARISON OF THE FALEX WEAR RESISTANCE

OF CHROMIUM AND ELECTROLESS NICKEL COATINGS

COATII G

PLATED V B LOCKS

UNPLATED

STEEL

PINS

Heat

Hardness Wear

ear

mg

1)

Treatment

lHN

mg 1)

Chromium

None

1100

0.5

1.9

Electroless

Nickel

None

590

6.6

0.2

Electroless

Nickel

290

oC/2

hrs 880

1.2

0.1

Electroless

Nickel

290

oC/16

hrs

1050 0.4

0.1

Electroless

Nickel

400

oC/l

hr

1100 0.5

0.2

Electroless Nickel

540

oC/l

hr

750

1.4

0.1

1

Falex Wear Test under

white o il lubricant .

a t 60 HRC

hardness.

180

kg 400

lb

for 40 minutes with

Unplated pins were SAE 9310 s teel

The e f f e c t

o f

phosphorus con ten t

upon th e

wear

exper ienced

by


n icke l coa t ings under l ub r i ca ted cond i t ions

i s

summarized in

Figure

15.

These ro t a t i ng b a l l

t e s t s

showed t h a t

a f t e r h e a t t r ea tmen t high phosphorus depos i t s l i ke

i

pro

vide

th e b e s t r e s is ta n c e to adhesive wear

29 3 0 .

5

;

>

0

E

e

,

0

>

0

8

6

e : eposite

2

l

r

1

1

8

6 250

c

{480 of} for 1

::

2

400 c 750

of

for 1

1

1

,

6

2

1

I

.

0 00

o

0 00

I.

o

o

E

0 0

0.0

0 0

0 0

e

ll

0 00

0 00

E ffect of phosphorus

content

on

the

wear of

e lec t ro less nickel

coatings in r ota ti ng

ba l l t e s t s .

FIGURE

15

12

Phosphorus content


17/24

FRICTIONAL PROPERTIES

The f r i c t i o n a l c ha ra c t e r i s t i c s o f e l e c t ro l e s s nicke l

coa t ings

are exce l l en t . Thei r phosphorus con ten t

prov ides

a

n a tu r a l

l ub r i c i t y which he lps

to

minimize hea t

bui ldup

and reduces

scor ing and ga l l ing

and which

can be

very

use fu l fo r

ap p l i c a t i o n s

such as p l as t i c

molding .

The c oe f f i c i e n t o f f r i c t i on fo r e l e c t ro l e s s nicke l versus

s t e e l

i s about

0.13

fo r lub r ica ted

condi t ions

and 4 fo r

u n lu b r ic a te d c o n d it io n s . This i s approximately 20 percent

lower

than

chromium, one ha l f

o f

t h a t o f s t e e l

and

much

lower

than aluminum o r s t a in l e s s s t e e l .

The

f r i c t i ona l

p ro pe rt ie s o f

th es e co atin gs

vary

l ttl with

e i t h e r

phosphorus

content

o r

with h ea t t re atm e nt

2

, 2 6 ,30 .

SOLDERABILITY AND WELDABILITY

Elec t ro less

n icke l

coa t ings

can be

eas i ly

soldered and

are

commonly

used in e le ctro nic

a p pl ic at io n s to

f a c i l i t a t e solder ing

o f

l i g h t

meta ls l ike

aluminum.

For

mos t c ompo ne nt s,

mildly

ac t iva ted

ros in RMA

f lux i s

spec i f i ed t oge the r with convent ion-

a l t i n l e a d

s o ld e r .

Prehea t ing the

component to 100

to 110C

2100 to 230F

w i l l

improve

th e

ease and speed o f jo in ing .

with moderately oxidized su r faces such as th ose resu l t ing from

steam

ag ing

a c ti va te d r os in RA

f lux

i s usua l ly

req uired to

ob ta in

wet t ing

of th e

coa t ing

2

, 31

Welding

o f

e l e c t ro l e s s

n icke l coated

components

i s more d i f f i c u l t

due to

th e low welding p o i n t o f th e

a l l o y

and because phosphorus

can

d if fu se in to

an

embr i t t l e

s t e e l .

Some

success

has

been

re -

por ted using s p ec i a l high pur i ty s t a in l e s s s t e e l e lec t rodes and

i n e r t

gas

sh ie ld ing .

with

p ip ing high n icke l backup r ings

are

also sometimes used

2

CORROSION RESISTANCE

Elec t ro less n icke l i s a b a r r i e r coa t ing t pro tec t s

i t s

subs t ra t e by

sea l ing

t o ff from th e environment , ra the r

than

by s a c r i f i c i a l ac t i o n . Because of

i t s

amorphous

na ture

and p a s s i v i t y however,

th e c or ro sio n

res i s t ance

o f th e

coa t ing

i s ex ce l l en t and in many

environments

supe r io r to

t h a t

o f

pure

nicke l

or

chromium

a l loys .

Amorphous

a l loys genera l ly

have

b e t t e r

res i s t ance to a t t ack

than equ iva len t po lycrys ta l l ine

mate r i a l s

because o f t h e i r

freedom from grain o r

phase

boundaries and because o f th e

glassy f i lms which form on and pass iva te t h e i r su r faces .


18/24

Effec t o f

Environment.

When p ro pe rly a pp lie d

Elec t ro les s

Nickel

i s

almost

t o t a l l y

r e s i s t a n t

to a lk a l i e s

to

s a l t

so lu t ions

and

br ines to chemical and

petroleum

e n - ~ n m e n t s , and to a l l types of hydrocarbons and

so lven t s .

d ep osits a ls o have

good

r es i s t ance

to

ammonia

so lu t ions

to organic ac ids and to reducing inorganic ac ids . The are

only

s ign i f i can t ly

at tacked

by

s tr on g ly o xid iz in g

media

2 .

Some examples of

the

cor ros ion exper ienced in d i f f e ren t

environments i s shown in Table 3

2,9,32

Effec t

o f

Composit ion. The cor ros ion r es i s t ance of an

e lec t ro l e s s

nicke l coat ing

i s

a

funct ion

of

i t s

composi t ion.

Most

deposi t s

are natura l ly pass ive and

very

r e s i s t a n t to

a t tack in most

environments .

Thei r degree

of

pass iv i ty

and cor ros ion

r e s i s t ance

however,

i s

qrea t ly a f fec t ed by

t he i r phosphorus

con ten t . Alloys

conta in ing more

than 10

percen t

phosphorus

are g en era lly

more

r e s i s t a n t

to

a t t ack

than

those with lowe r pho spho ru s

conten ts

9 15 .

This

i s


by

Figure 16,

which

compares th e c orro sio n

exper ienced

by e lec t ro l e s s

nicke l co at in g s cont ai ni ng

S

to

1 0 ~

percent

phosphorus

in ae ra ted 6 percent c i t r i c acid

a t

SOoC

120 F 33.

FIGURE 16

4

Effect of phosphorus

ont nt

on th

orrosion

of e lec t ro less nickel

coat ings.

2

2

;

- - - ; - ~ ; - - - ; - - ~ - - - - : - - ; - - 7 . ; - - - : - - ~

PHOSPHORUS CONTENT WT PERCENT


19/24

CORROSION OF

TABLE 3

ELECTROLESS NICKEL IN

VARIOUS

ENVIRONMENTS

EN \lI:ROtM ENT

TEMPERATURE

CORROSION RATE

c

Il

m

y

mpy

cet ic

cid

Glac i a l

20 0 .8

0.03

Acetone

20

0.08

0.003

Aluminum Sul fa te 27 20

5

0.2

Armnonia 25 20

16

0 .6

Ammonium

Ni tra te 20 20 15

0 .6

Ammonium

Su l f a t e Satura ted 20 3 0 .1

Benzene 20 Nil Nil

Brine

Sa l t

CO

2

Satura ted 95

5

0 .2

Brine 3 >

Sa l t

H

2

S

Satura ted

95 Nil

N il

Calcium

Chlor ide

42

20

0.2 0.01

Carbon Te t rach lor ide 20 Nil

N il

Ci t r i c cid

Satura ted

20

7 0 .3

Cupric Chlor ide

5

20

25

1 .0

Ethylene

Glycol

20 0 .6

0.02

Ferric

Chlor ide 20

200

8 .0

lOrmic cid 88

20

13

0 .5

Hydrochloric cid 5

20

24

0 .9

Hydrof luqr ic Acid 2

20

27

1 .1

Lact ic cid 85

20 1 0.04

Lead

Aceta te 36

20

0 .2

0.01

Ni t r i c cid 1

20

25

1 .0

Oxalic Acid

10

20 3 0 .1

Phenol 9 20

0.2

0.01

Phosphoric Acid

8

20 3

0 .1

Potassium

Hydroxide

50

20

Nil

Nil

Sodium

Carbonate Satura ted 20 1

0.04

Sodium

Hydroxide

45

20

Nil

Nil

Sodium Hydroxide

50 95

0.2

0.01

Sodium SUl fa t e 10 20 0.8

0.03

Sul fur i c Acid 65 20

9

0 .4

Water cid

Mine

3.3

pH

20

7

0 .3

Water Dis t i l l ed N

2

deaera ted

100 Nil

Nil

Water Dis t i l l ed

O

2

Satura ted 95 Nil Nil

Water Sea

(3 >

Sal t

95 Nil Nil


20/24

In t h e s e t e s t s

c o r r o s i o n

o f

t h e

d e p o s i t was o n l y about

one h a l f o f t h a t o f t h e lower

phosphorus

c o a t i n g s .

O f t e n t h e

t ramp

c o n s t i t u e n t s p r e s e n t i n

an

e l e c t r o l e s s n i c k e l

a r e even more i m p o r t a n t

t o

its

c o r r o s i o n

r e s i s t a n c e t h a n its

phosphorus c o n t e n t . Most c oa t i n gs a re

a p p l i e d

from b a t h s

i n h i b i t e d

w i t h

l e a d

t i n

cadmium

o r

s u l f u r .

Codeposi t ion

o f

t h e s e elements i n

more

t h a n t r a c e

amounts

c a u s e s

a

s e v e r e

r e d u c t i o n i n t h e c o at i n g s p a s si v it y and

c o r r o s i o n

r e s i s t a n c e .

This

i s

i l l u s t r a t e d

by Table

4 , which

shows t h e r e s u l t s

o f

c o r r o s i o n

t e s t s

w i t h

6

d i ff e re n t e le c tr o le s s n ic ke l d e p o s it s

i n

C02

s a t u r a t e d

p e r c e n t s a l t

b r i n e a t

9SoC

200

0F 9

TABLE 4

EFFECT OF COMPOSITION

CORROSION RESISTANCE

OF ELECTROLESS

NICKEL

IN

CO2

SATURATED,

PERCENT SALT BRINE AT

9SoC

DEPOSIT

PHOSPHOR LS OTHER

CORROSION

RATE

CONTENT,

ELEMENTS,

lJro/y

ropy

ELNIC

10.2 tr e

5

0 .2

11.8

0.04 Sn 7

0 .3

C

8.3

0.05 Cd 24

0.9

10.3

0.12 Pb

15 0.6

E

8 .0 0.13

S

15 0.6

F

10.4

0.05 Pb

and

11

0 .4

0.08

Cd

S i m i l a r t e s t s i n

10

p e r c e n t

RCl

a t ambient

t empera tu re

showed

l o s s e s

r a n g i n g from l S lJm/v

0.6 mpy

f o r to

660

umz y 26 mpy)

f o r

D e p o s i t D

The

pr imary d i f f e r e n c e between t h e s e d e p o s i t s was

n o t

t h e i r

phosphorus

c o n t e n t b u t t h e i r

b a t h s

i n h i b i t o r .

D e p o s i t s

B,

D.

and

F

a l l c o n t a i n e d more

than 10

p e r c e n t

phosphorus .

B

t o

F, were a p p l i e d

s u l f u r and c o n t a i n e d

D e p o s i t s

from b a t h s i n h i b i t e d w it h m et a ls o r

s i g n i f i c a n t

amounts

o f

t h e s e

elements

9.


21/24

Ef fec t of Heat Treatment. One of the

most

impor tan t var i ab le s

e f f ec t i ng

the

corrosion

o f e lec t ro l e s s nicke l

i s i t s hea t

t rea tment . As

e lec t ro l e s s

nicke l d ep os i ts are heated to

temperatures above

220 to

260C

420 to 500F n icke l phosphide

pa r t i c l e s begin to

form,

reducing the

phosphorus

co nten t o f

the remaining

mater ia l .

This reduces the coa t i ng s corros ion

r e s i s t ance . The

pa r t i c l e s

also

crea te

small

ac t ive /pass ive

c or ro si on c el l , fur the r

cont r ibut ing

to the

depos i t s

des t ruc t ion .

The e f f ec t

of

these changes i s


by

Table 5 ,

which shows the r e su l t s o f t e s t s with

depos i t s ,

h ea t t re ate d to represen t d i f f e r en t commercial t rea tments and

then exposed to 10 percent HCl a t ambient tempertaure. A

secondary

e f f ec t

of

hea t

t rea t ing

i s t h a t the depos i t shr inks

as hardens , which can crack the coat ing and

expose

the

subs t ra te

to

a t t ack .

A

cross - sec t iona l view

o f th e cracks

throuqh an

e lec t ro l e s s

n ic ke l c oa tin q a f t e r h ea t tre atm en t

a t

400C

7500F i s

shown

in Figure 7

9

FIGURE 7

ross

se t ion l

vi w

of

a 7 r

thick

_

e lec t ro

less nickel de-

pos i t

a f te r heat t rea t

ment

a t

400C

for

hour. Cracks formed

due

to shrinkage

of

the

deposit .

lOOX

magnification.

Etched in ni t a l .


22/24

TABLE 5

THE EFFECT OF HEAT TREATMENT

ON THE

CORROSION OF ELECTROLESS NICKEL

IN 10 HCl

HEAT

TREATMENT DEPOSIT CORROSION

RATE

HARDNESS,

z y

ropy

vllNIOO

on

480

15 0.6

190C (375 for

hours

500 20 0.8

290C (550F) for 6 hours 900

1900 75

290C

550E

for 10 hours

970 1400 55

340C (650E)

for

4

hours

970 900 35

400C (750 F) for 1 hour 1050 1200

47

Baking a t 190C 375F , l i k e t h a t used

f o r

hydrogen e m b r i t t l e -

ment r e l i e f

caused

no

s i g n i f i c a n t

i n c r e a s e i n c o r r o s i o n .

Harden ing ,

however ,

caused t h e c o a t i n g s c o r r o s i o n r a t e t o

i n c r e a s e

from 15 ).Im/y

0 .6

mpy

t o more t h a n

900 ).Im/y 35

mpy

T e s t s

i n

o t h e r

environments

showed a

s i m i l a r r e d u c t i o n i n

r e s i s t a n c e

a f t e r

h a r d e n i n g . Where

c o r r o s i o n

r e s i s t a n c e

i s r e q u i r e d hardened

c o a t i n g s

should n o t be used

9


23/24

SU RY

OF THE PROPERTIES OF

ELE TROLESS NI KEL

COMPOSITION

STRUCTURE

INTERNAL STRESS

DENSITY

MELTING POINT

ELECTRICAL RESISTIVITY

THERMAL

CONDUCTIVITY

MAGNETIC

SUSCEPTABILITY

TENSILE STRENGTH

DUCTILITY

MODULAS OF ELASTICITY

COEFFICIENT OF

THERMAL

EXPANSION

ADHESION STRENGTH

HARDNESS

AS DEPOSITED

IMRDNESS HEAT TREATED

400C/l hr)

COEFFICIENT

OF

FRICTION VS STEEL

TABER

WE R

RESISTANCE

AS DEPOSITED

TABER WE R RESISTANCE

HEAT TREATED 400C/l

hr)

CORROSION

RESISTANCE

RN l t

December 31, 1980

Revised

May 19,

1983

Alloy

of

10 to 11

percent phosphorus

dissolved

in nicke l ,

containing

l e s s

than 0 0 5 p er ce nt other impuri t ies

Completely amorphous without

an y

crysta l

or phase s tructure , lamination

or

internal

segregat ion

During heat

treatment part ic les of Ni3P

precipi tate

and

th e coating

crystal l izes

2 to 3 MFa

2 .5

to 4 ks i

compressive

7.75

g/cm

3

9 ~ Q c m as

deposited

0.08 W/cm oK

4.6

Btu/ft-hr-OF), as deposited

-

pproximately 10 mks

>700 MPa >100 ksi as

deposited

to

percent elongation

as

deposited

6

200 GPa 28 x 10 psi)

300

to

400

MPa

40

to

60

ksi)

480

to

500 VHN

10 0

1000 to 1100 VHN

10 0

0.13

lubricated)

to

0.4 non-lubricated)

18

to

20 mg/1000

cycles

10

to

12 mg/1000

cycles

A barrier

coating with

e xce ll en t r es is tan ce

to attack

by a l l but th e most severely

oxidizing

environments


24/24

engineering properties of en coatings

Documents