Coatings Technology Handbook Episode 1 Part 8 ppsx

Electroless Plating

27

-5

good electrical conductivity and ductility and, therefore, are used in the production of printed circuit
boards by additive processes. The entire circuit pattern is obtained by electroless techniques.
Coatings of cobalt and its alloys may be used to take advantage of their specific magnetic properties;
silver and gold coatings are used because of their good electrical conductivity, optical properties, and
inertness.
Electroless plating may be performed by using the plating solution once (until a greater part of any
component in the solution is consumed and the reaction rate has sharply decreased) or by replenishing
the substances that have been consumed in the course of plating. Long-term exploitation of solutions
reduces the amount of plating wastes and ensures a higher labor productivity, but at the same time, it
imposes more stringent requirements on plating solutions: they must be stable, and their parameters
should not vary significantly with time. Besides, special equipment is required for monitoring and
controlling the composition of such solutions. For this reason, long-term exploitation of solutions is
applied only in large-scale production processes.
Single-use solutions are more versatile, but they are less economical and less efficient. A single-use
method may be applied rather efficiently, however, when the solution has a simple composition and the
basic components (first of all, metal ions) are fully consumed in the plating process, while the remaining
components (such as ligands) are inexpensive and do not pollute the environment. In this case, single-
use processes may be practically acceptable even in mass production.
An extreme case of single use of plating solutions is aerosol spray plating,

5

in which droplets of two

solutions begin sprayed by a special gun collide on, or close to, the surface being plated. One solution
usually contains metal ions, while the other contains the reducing agent. Metal ion reduction in this case
should be rapid enough to permit a greater part of the metal to precipitate on the surface before the
solution film runs off it. This method is practical for deposition of such easily reducible metals as silver
and gold, though such aerosol solutions are known for deposition of copper and nickel as well. The
aerosol spray method is highly suitable for deposition of thin coatings on large, flat surfaces: this process
is similar to spray painting.
Since the components of electroless plating solutions, first of all metal ions, may be toxic and pollute
the environment, techniques have been developed for recovery of metals from spent plating solutions and
rinse water. Other valuable solution components, such as ligands (EDTA, tartrate), may also be recovered.
Electroless plating usually does not require sophisticated equipment. The tank for keeping plating
solutions must exhibit sufficient chemical inertness, and its lining should not catalyze deposition of metals.
Such tanks are usually made of chemically stable plastics; metal tanks may be used as well — they can be
made of stainless steel or titanium. To prevent possible deposition of metals on the walls, a sufficiently
positive potential is applied to them using a special current source (anodic protection). Parts for plating
may be mounted on racks; small parts may be placed in barrels immersed in the plating bath. Heating
and filtration of solutions are carried out in the same way as in electroplating processes. Special automatic
devices have been developed for monitoring and controlling the composition of plating solutions.

27.5 Mechanisms of Autocatalytic Metal Ion Reduction

Autocatalytic metal ion reduction processes are highly complex: they contain many stages, and their
mechanism is not understood in detail. At present, it is possible to give an accurate description only of
the basic stages of the catalytic process. Localization of the reduction reaction on the metal–catalyst
surface (the cause of catalysis) is usually attributed to the requirement for a catalytic surface for one or
more stages of the process to proceed. In accordance with one of the earlier explanations, only on a
catalytic surface is an active intermediate product obtained, which then reduces metal ions. First, atomic
hydrogen and, later, a negative hydrogen ionhydride were considered to be such products. A reaction
scheme with an intermediate hydride gives a good explanation of the relationships observed in nickel
and copper plating processes.

5

However, there is no direct proof that hydride ions are really formed
during these processes. Moreover, the hydride theory explains only the reactions with strong hydrogen-
containing reducers, which really may be H

–

donors.

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Coatings Technology Handbook, Third Edition

A more versatile explanation of the causes of catalysis in these processes is based on electrochemical
reactions. It is suggested that reducing agents are anodically oxidized on the catalyst surface and the
electrons obtained are transferred to metal ions, which are cathodically reduced. The catalytic process
comprises two simultaneous and mutually compensating electrochemical reactions. In this explanation
of the catalytic process, electrons are the active intermediate product. However, electrons are fundamen-
tally different from the conversational intermediate products of reactions. They may be easily transferred
along the catalyst without transfer of the mass, and for this reason, the catalyst reaction, contrary to all
other possible mechanisms (which are conventionally called “chemical mechanisms”), occurs not as a
result of direct contact between the reactants, or the reactants, or the reactant and an intermediate
substance, but because of the exchange of “anonymous” electrons via metal.

On the metal surface, when anodic oxidation of the reducer
(27.2)
and cathodic reduction of metal ions
(27.3)
proceed simultaneously, a steady state in the catalytic system of electroless plating is obtained, in which
the rates of both electrochemical reactions are equal, while the metal catalyst acquires a mixed potential

E

m

. The magnitude of this potential is between the equilibrium potentials

E

c

of the reducer and of the
metal. The specific value

E

m

depends on the kinetic parameters of these two electrochemical reactions.
Electrochemical studies of catalytic metal deposition reactions have shown that the electrochemical
mechanism is realized practically in all the systems of electroless plating.

4,6,7

At the same time, it has become clear that the process is often not so simple. It appears that anodic
and cathodic reactions occurring simultaneously often do not remain kinetically independent but affect
each other. For example, copper ion reduction increases along with anodic oxidation of formaldehyde.

8

The cathodic reduction of nickel ions and the anodic oxidation of hypophosphite in electroless nickel
plating solutions are faster than in the case in which these electrochemical reactions occur separately.
This interaction of electrochemical reactions probably is related to the changes in the state of the
metal–catalyst surface.
Electrochemical reactions may also hinder each other: for example, in reducing silver ions by hydrazine
from cyanide solutions, their rate is lower than is separate Ag–Ag(1) and redox systems.
The electrochemical nature of most of the autocatalytic processes discussed enables us to apply
electrochemical methods to their investigation. But, they must be applied to the entire system of electroless
plating, without separating the anodic and cathodic processes in space. One suitable method is based on
the measurement of polarization resistance. It can provide information on the mechanism of the process
and may be used for measuring the metal deposition rate (both in laboratory and in industry).

9

The
polarization resistance

R

p is inversely proportional to the process rate

i

:

(27.4)
(27.5)
where

b

a

and

b

c

are Tafel equation coefficients (b

≈

1/

α

nf

),

α

is the transfer coefficient,

n

is the number
of electrons taking part in the reaction for one molecule of reactant, and

f



=



F

/

RT

(

F



=

Faraday number).
Red →+Ox ne
Me

n+
+ ne
i
bb
Rb b
=
+
ac
pa c
()
R
dE
di
i
p
=






=0

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© 2006 by Taylor & Francis Group, LLC

Electroless Plating

27

-7

Autocatalytic metal reduction reactions also may not proceed in an electrochemical manner. Two
courses of such reactions have been shown: (a) an intermediate metal hydride is formed, which decom-
poses to meta and hydrogen (reduction of copper ions by borohydride); and (b) the metal complex is
hydrolyzed, resulting in precipitation of metal oxide on the surface, which then is reduced to metal by
the reducer present in the solution (reduction of silver ions by tartrate).

27.6 Stability of Plating Solutions

Electroless plating solutions containing metal ions and reducing agents are thermodynamically unstable
systems. Metal ion reduction must proceed in the bulk of the solution.
The difference in the rate of metal ion reduction on the required surface (controlled catalytic reaction)
and that of a reduction reaction in the bulk of a solution shows the effect of catalysis, and it determines,
to a substantial degree, the practical usefulness of plating solutions. In an ideal case, the reaction in the
bulk of a solution should not occur at all.
Formation of metal in the bulk of a solution is hindered by energy barriers: the activation barrier of
homogenous reactions between metal ions and reducer and the barrier of the formation of a new phase
(metal). The magnitude of the second barrier may be evaluated on the basis of thermodynamic principles.

10

It was established empirically that the stability of plating solutions decreases with an increase in the
concentration of reactants and temperature, with a decrease in the stability of metal ion complexes, and
with the presence of solid foreign particles in the solution. Besides, it was found that stability decreases as
the catalytic process rate and load increase. This may be attributed to the transfer of intermediate catalytic
reaction products from the catalytic surface to the solution, where they may initiate a reduction reaction.
To enhance the stability of solutions, it is recommended that lower concentration solutions and more stable
metal complexes be used and that solid particles in the solution be removed by filtration. The most effective

solution stabilization method is the introduction of special addition agents — that is, stabilizers.

4,11

Stabi-
lizers, the number of which is very great, may be divided into two large groups: (a) catalytic poisons, such
as S(II), Se(II) compounds, cyanides, heterocyclic compounds with nitrogen and sulfur, and some metal
ions, and (b) oxidizers. It is assumed that stabilizers hinder the growth of fine metal particles, close to
critical ones, by absorbing on them (catalytic poison) or passivating them (oxidizers).
Modern electroless plating solutions always contain stabilizers. Their concentration may be within the
range of 1 to 100 mg/l. Stabilizers, by hindering deposition of metal on fine particles, usually slow the
rate of the catalytic process on the surface being plated. This process may stop completely at a sufficiently
high concentration of the stabilizers. In some cases, however, small amounts of stabilizers increase the
deposition rate.

27.7 Electroless Plating

27.7.1 Copper Deposition

Though copper coatings may be deposited using various reducers, only formaldehyde copper plating
solutions are of practical importance. Autocatalytic reduction of copper ions by formaldehyde proceeds
at room temperature in alkaline solutions (pH

=

11–14); here, copper ions must be bound into a complex.
Suitable Cu

2+

ligands for electroless copper plating solutions are polyhydroxy compounds (polyhydroxy
alcohols, hydroxyacid anions) and compounds having a tertiary amine group and hydroxy groups
(hydroxyamines, EDTA, and others). In practice, tartrate, EDTA, and tetraoxypropylethyl ethylenedi-
amine (Quadrol) are used most often.
In the course of copper plating, along with the main reduction reaction,
(27.6)
Cu CH O H Cu HCOO H H O
2
222
240 2 2++→+ ++
−−

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27

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Coatings Technology Handbook, Third Edition

formaldehyde is consumed in the Cannizzaro reaction, and a total of 3 to 6 moles of CH

2

O is consumed
for the deposition of 1 mole of copper. During copper plating, much alkali is used including the
Cannizzaro reaction. Consumption of OH

–

may be determined according to the following equation
(amounts of substances in moles):
(27.7)
Var ious formulations of copper plating solutions, which are totally stable and suitable for long exploi-
tation (e.g., solution B in Table 27.2), have been developed. Three types of electroless copper plating
solution have been distinguished in the literature: (a) low deposition rate solutions (0.5 to 1.0

µ

m/h),
suitable for deposition of a copper underlayer; (b) solutions giving deposition rates of 4 to 5

µ

m/h (i.e.,
exhibiting a higher autocatalytic effect); and (c) solutions for deposition of highly ductile and strong
copper coats (e.g., solution C in Table 27.2). All these solutions, essentially, have the same composition:
they differ mostly by their additives. Besides, highly ductile coatings, which are used in the production
of printed circuit boards by additives processes, are obtained at higher temperatures (

>

40

°

C) and at a
relatively low copper deposition rate.

27.7.2 Nickel Plating

Electroless nickel plating, in which hypophosphite is used as a reducer, is the most popular process.

12,13

Autocatalytic nickel ion reduction by hypophosphite occurs both in acid and in alkaline solutions. In a
stable solution with a high coating quality, the deposition rate may be as high as 20 to 25

µ

m/h. This
requires, however, a relatively high temperature, about 90

°

C. Because hydrogen ions are formed in the
reduction reaction,
(27.8)
a high buffering capacity of the solution is necessary to ensure a steady-state process. For this reason,
acetate, citrate, propionate, glycolate, lactate, or aminoacetate is added to the solutions; these substances,
along with buffering, may form complexes with nickel ions. Binding Ni

2+

ions into a complex is required
in alkaline solutions (here, besides citrate and aminoacetate, ammonia and pyrophosphate may be added);
moreover, such binding is desirable in acid solutions, because free nickel ions form a compound with
the reaction product (i.e., phosphate), which precipitates and hinders further use of the solution.

TA BLE 27.2

Examples of Electroless Copper Plating

Solutions

Components (g/l) and
Parameters

Solutions
ABC

CuSO

4



⋅

5H

2

O71515
K-Na tartrate 25
Na

2

EDTA 30 45
NaOH 4.5 10 10
Formaldehyde (40%) ml/l 25 20 10
Additives

a

2 1–0.005 1–0.03
2–0.03 2–0.05
pH 12.2–12.5 12.7 12.6
Te m p e r ature,

°

C202070
Deposition rate

µ

m/h 0.4–0.5 2 3

a

Solution A: NiCl

2



⋅

6H

2

O; solution B: sodium diethyldithio-
carbamate, K

4

Fe(CN)

6
; solution C: 2.2′-dipyridyl, polyethylene
glycol (MW = 600).
∆∆ ∆OH Cu II CH O
−
=+312
2
() /
Ni H PO H O Ni H PO H H
2
22 2 23 2
22 2 2
+− − +
++→+++
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© 2006 by Taylor & Francis Group, LLC
Electroless Plating 27-9
Stabilizing additions for nickel plating solutions are less necessary than for copper solutions; never-

theless, they are added to ensure the stability of long-lived solutions.
Phosphorus is always present in the coatings when reduction is performed by hypophosphite. Its
amount (in the range of 2 to 15 mass percent) depends on pH, buffering capacity, ligands, and other
parameters of electroless solutions.
Borohydride and its derivatives may also be used as reducers for electroless nickel plating solutions.
While temperatures of 60 to 90°C are required for the reduction of nickel ions by borohydride, dimeth-
ylaminoborane (DMAB) enables the deposition of Ni–B coatings with a small amount of boron (0.5 to
1.0 mass percent) at temperatures in the range of 30 to 40°C. Neutral and alkaline solutions may be used,
and their compositions are similar to those of hypophosphite solutions (Table 27.3).
27.7.3 Cobalt, Iron, and Tin Plating
Deposition of cobalt is similar to that of nickel — the same reducers (hypophosphite, borohydride, and
its derivatives) are used, and reduction relationships are similar.
14
Reduction of cobalt is more difficult,
however, and cobalt deposition rates are lower than those of nickel; it should be noted that it is difficult
to deposit cobalt from acid solutions. The Co–P and Co–B coatings obtained are of particular interest
due to their magnetic properties.
Electroless iron plating is more difficult, and only one sufficiently effective iron plating solution is
known, in which Fe ions form a complex with tartrate and NaBH
4
is used as a reducer. Fe–B coatings
(about 6% B) are obtained in an alkaline solution (pH 12) at a temperature of 40°C and a deposition
rate of about 2 µm/h.
It is rather difficult to realize an autocatalytic tin deposition process. A sufficiently effective tin
deposition method is based on the tin (II) disproportionation reaction in an alkaline medium.
15
In 1 to
5M NaOH solutions at 80 to 90°C, it is possible to obtain a deposition rate of a few micrometers per hour.
27.7.4 Deposition of Precious Metals
Electroless silver plating is the oldest electroless metallization process; its present performance however,

lags behind nickel or copper plating.
1
Unstable single-use ammonia silver plating solutions (with glucose,
tartrate, formaldehyde, etc., as reducers) are usually employed. The thickness of coatings from such
solutions is not great (<1 µm). Such unstable solutions are more suitable for aerosol spray.
More effective electroless silver plating solutions have been developed using cyanide Ag(I) complex
and aminoboranes or hydrazine as reducers: at temperatures of 40 to 50°C, the deposition rate is 3 to 4
µm/h, and in the presence of stabilizers, these solutions are quite stable. Sufficiently stable electroless
silver plating solutions may be obtained using metal ions such as Co(II) compounds as reducers.
TA BLE 27.3 Examples of Electroless Nickel Plating
Solutions
Components (g/l) and
Parameters
Solutions
AB C
NiCl
2
⋅6H
2
O303025
NaH
2
PO
2
⋅H
2
O1020 30–40
Sodium acetate 8
NH
4

Cl 30
NH
4
OH (25%), ml/l 30–35
Glycine 20
NaNO
2
0.02–0.1
pH 5 6 9
Te m p e r ature, °C9080–90 30
Nickel deposition rate µm/h 15 7–15 1.8
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© 2006 by Taylor & Francis Group, LLC
27-10 Coatings Technology Handbook, Third Edition
Gold coatings may be deposited employing various reducers: however, the solutions are usually unsta-
ble. Solutions of sufficient stability have been developed with borohydride or DMAB as reducers using
a stable gold cyanide complex.
16
At temperatures of 70 to 80°C, the fold deposition rate reaches 5 Am/
h, and gold coatings of sufficient purity are obtained.
Thin gold coatings may be deposited on plastics by an aerosol spray method: gold complexes with
amines are employed with hydrazine as a reducer, and a relatively thick coat (deposition rate as high as
0.4 µm/min) may be obtained.
Palladium coatings are easily deposited with hypophosphite as a reducer in alkaline solutions, in which
Pd
2+
ions are bound in a complex with ammonia, EDTA, or ethylenediamine. Palladium plating is
performed at 40 to 50°C, the deposition rate of the Pd–P (4 to 8 P) coat being in the range of 2 to 5 µm/h.
Coatings of platinum, ruthenium, and rhodium may be deposited using borohydride or hydrazine as
a reducing agent. The process rate in a stable solution is low (0.5 to 2 µm/h).

27.7.5 Deposition of Metal Alloys
About 60 coatings of a different qualitative composition containing two or more metals may be deposited.
Such metals as copper, iron, zinc, tin, rhenium, tungsten, molybdenum, manganese, thallium, and
platinum group metals may be introduced into nickel and cobalt coats, and nickel, cobalt, tin, zinc,
cadmium, antimony, bismuth, lead, and gold into copper coats.
In the electroless deposition of metal alloys, the same thermodynamic relationships as those of alloy
deposition by electroplating techniques are valid; it is clear that it is difficult to introduce into coatings
metals that are difficult to reduce, such as chromium and manganese. Besides, in the case of chemical
reduction, an additional factor — catalytic properties of metals — becomes apparent. Great amounts of
additional metal may be introduced into a coat of nickel, copper, and so on, only when that metal is
catalytic or, at least, inert with respect to oxidation of the reducer. The amount of metals–catalysts in
the alloy may be as high as 100%, that of catalytically inert metals may be up to 50%, and that of
metals–inhibitors may be only 10 to 20%. When a less catalytically active metal is introduced, the
deposition rate decreased.
27.8 Properties of Chemically Deposited Metal Coatings
Only in rare cases are chemically deposited metal coatings so pure, and have so regular a structure, that
their properties are the same as those of the corresponding chemically pure substance. Very different
properties may be exhibited by coatings containing a nonmetallic component — phosphorus or boron.
The density of coatings is a little lower than that of bulk metal. This is related to a rather irregular
coatings structure: they contain more defects (pores and inclusions of foreign matter). For example,
chemically deposited copper usually has a great number of microscopic voids 20 to 300 Å in diameter,
formed by the hydrogen occluded in the coating. Ni–P and Ni–B coatings usually have a layered structure,
which results from the nonuniform distribution of phosphorus and boron in the coatings.
Mechanical properties of the coatings may vary within a wide range depending on the electroless
plating conditions, plating solutions composition, and deposition rate.
For chemically deposited finish copper coatings, such as those on printed circuit boards, sufficient
resistance and ductility are of great importance. Coatings that have a tensile strength of about 40 to 50
kg/mm
2
may be obtained at a temperature of 50 to 70°C. Their ultimate elongation, which characterizes

ductility, may be as high as 6 to 8%. Copper coatings obtained at room temperature are more brittle.
Highly ductile coatings may be obtained only from solutions containing special additives. Ductility
increases when deposited coatings are heated in an inert atmosphere at temperatures of 300 to 500°C.
Ni–P and Ni–B coatings are relatively hard; after deposition, their hardness, which depends on the
amount of P and B, is 350 to 600 kg/mm
2
(3400 to 5900 MPa) of Ni–P coatings and 500 to 750 kg/mm
2
(4900 to 7400 MPa) for Ni–B coatings, while after heating at about 400°C, it is 800 to 1000 kg/mm
2
for
DK4036_book.fm Page 10 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
Electroless Plating 27-11
Ni–P and 1000 to 1250 kg/mm
2
for Ni–B. Hence, such coatings have the same hardness as that of
chromium coatings. The tensile strength of Ni–P coatings is in the range of 40 to 80 kg/mm
2
.
The ductility of nickel coatings, if their hardness is taken into account, is rather high: their ultimate
elongation is less than 2%. Such a combination of hardness, wear resistance, and ductility is unique.
The electrical conductivity of chemically deposited coatings is usually lower than of the respective
pure metals. Resistivity of thin copper coatings (0.5 to 1.0 µm) deposited at room temperature is 3 to 4
× 1.0
–8
Ω⋅m — twice as great as that of pure bulky copper. The surface resistance of such coatings is 0.03
to 0.07 Ω/m. However, ductile copper coatings that are obtained at temperatures of 50 to 70°C have a
resistivity of 2 × 10
–8

Ω⋅m, close to that of pure copper.
The resistivity of Ni–P and Ni–B coatings depends on the amount of nonmetallic component, and it
is usually in the range from 3 to 9 × 10
–7
Ω⋅m; that is much higher than that of pure bulky nickel (0.69
× 10
–7
Ω⋅m). Heating causes reduction in resistivity.
Magnetic properties of the coatings of such ferromagnetic materials as nickel and cobalt may vary
within a very wide range. With an increase in the amount of phosphorus in nickel coatings, their
ferromagnetism decreases, and coatings containing more than 8 mass percent of phosphorus or 6.5 mass
percent of boron are nonmagnetic.
Coatings of Co–P, Co–B, and cobalt alloys with other metals have highly different magnetic properties.
These depend on the composition of the coating, their structures, and their thicknesses, and they may
be controlled by changing the composition, pH, and temperature of electroless plating solutions. Usually,
cobalt coatings exhibit a high coercivity (15 to 80 kA/m); however, soft magnetic coatings (0.1 to 1.0 kA/
m) may be deposited as well.
Optical properties of coatings are less varied and do not differ so much from those of pure metals.
Chemically deposited coatings are usually dull; when special additives are introduced, bright coatings
are obtained. Since they are not used as finish decorative coatings, properties of appearance and brightness
usually are not essential.
Silver and gold coatings are often used as mirrors, but the light-reflecting surface is usually the inner
surface, which is adjacent to the smooth glass surface. Chemically deposited thin gold films are employed
as optical filters; they pass visible light but reflect infrared rays and radio waves.
Chemically deposited coatings are usually less porous than the respective electroplates; therefore, they
provide better protection of the basis metal against corrosion. Corrosion resistance of the coatings
themselves may be different depending on structure and composition. Ni–P and Ni–B coatings are more
resistant to corrosion than nickel electroplates; this may be due to their fine crystalline structure.
References
1. A. Brenner and G. Riddell, J. Res. Natl. Bur. Stand., 37, 31 (1946).

2. W. Goldie, Metallic Coating of Plastics. Hatch End, Middlesex, England: Electrochemical Publica-
tions Ltd., Vol. 1, 1968; Vol. 2, 1969.
3. F. Pearlstein, “Electroless plating,” in Modern Electroplating, 3rd ed., F. A. Lowenheim, Ed. New
Yo rk: Wiley, 1974, p. 710.
4. M. ≥alkauskas and A. Vakelis, Khimicheskaya Metallizatsiya Plastmass. Leningrad: Khimiya, 1984.
5. R. M. Lukes, Plating, 51, 969, 1066 (1964).
6. M. Paunovic, Plating, 55, 1161 (1968).
7. F. M. Donahue, Oberfläche-Surface, 13, 301 (1972).
8. A. Vakelis and J. Jaciauskiene, Elektrokhimiya, 17, 816 (1981).
9. I. Ohno and S. Haruyama, Surface Technol., 13, 1 (1981).
10. A. Vakelis, Elektrokhimiya, 14, 1970 (1978).
11. E. B. Saubestre, Plating, 59, 563 (1972).
12. K. M. Gorbunova and A. A. Nikiforova, Physicochemical Principles of Nickel Plating, Translated
from Russian, 1963, TT 63–11003.
DK4036_book.fm Page 11 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
27-12 Coatings Technology Handbook, Third Edition
13. G. Gawrilov, ChemischelStromläse/Vernickelung. Saulgau, Wurt.: Eugen Leuze Verlag, 1974.
14. K. M. Gorbunova et al., Fiziko-Khimichesklye Osnovy Processa Khimicheskogo Kobaltirovaniya.
Moscow: Nauka, 1974.
15. A. Molenaar and J. J. C. Coumans, Surface Technol., 16, 265 (1982).
16. Y. Okinaka, in Gold Plating Technology, H. Reid and W. Goldie, Eds. Hatch End, Middlesex, England:
Electrochemical Publications, Ltd., 1974, p. 82.
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28

-1

28

The Electrolizing
Thin, Dense,

Chromium Process

28.1 General Definition

28-

1
28.2 Applications

28-

2

28.3

28.4 Solution

28-

5
28.5 Properties

28-

5

28.1 General Definition

The Electrolizing process uniformly deposits a dense, high chromium, nonmagnetic alloy on the surface
of the basic metal being treated. The alloy used in Electrolizing provides an unusual combination of
bearing properties: remarkable wear resistance, an extremely low coefficient of friction, smooth sliding
properties, excellent antiseizure characteristics, and beneficial corrosion resistance. Electrolized parts
perform better and last up to 10 times longer than untreated ones.
The solution and application processes are carefully monitored at all Electrolizing facilities. The result
is a fine-grained chromium coating that is very hard, thin, and dense and has absolute adhesive qualities.
The Electrolizing process deposits a 99% chromium coating on the basis metallic surfaces, whereas normal
conventional chromium plating processes tend to deposit 82 to 88% chromium in most applications.
Electrolizing calls for the cleaning and removal of the matrix on the basis metal’s surface by multi-
cleaning process, using a modified electrocoating process that causes the chromium metallic elements of
the solution to bond to the surface porosity of the basis metal. It is during this process that the absolute
adhesive characteristics and qualities of Electrolizing are generated. The Electrolizing coating will not
flake, chip, or peel off the basis metal substrate when conventional ASTM bend tests and impact tests
are performed. Three basic factors are always present after applying Electrolizing to metal surfaces:
•Increased wear (Rockwell surface hardness of 70 to 72 R

c

)
•Added lubricity characteristics
•Excellent corrosion resistance

Michael O’Mary

The Armoloy Corporation

DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
General • Specific
Thickness • Adhesion • Corrosion • Wear Resistance (Surface
Hardness) • Lubricity • Conformity • Heat Resistance •
Surface Preparation 28-4
Brightness • Hydrogen Embrittlement

The Electrolizing Thin, Dense, Chromium Process

28

-3

•Automotive
•Business machines
•Cameras and projectors
•Computers
•Cryogenics
• Data processing
• Electronics
•Food processing
• Gauges and measuring equipment
•Medical instruments
•Metalworking
•Molds (plastic and rubber)
•Motor industry
•Nuclear energy
• Pharmaceutical
• Photography (motion and still)

•Refrigeration
•Textile industry
•Transportation
Specifically, Electrolizing is approved and meets the following aerospace, nuclear, and commercial
specifications:
•Air Research Company, Garrett, CO
•American Can Company
• AMS 2406
• AMS 2438
•AVCO Lycoming — AMS 2406
•Bell Helicopter
•Bendix Company
Utica, NY, division
Te terboro, NJ, division
Kansas City, MO, division
South Bend, IN, division
•Boeing
BAC 5709 Class II, Class IV
QQC 320
•Cleveland Pneumatic Tool-CPC Specs (Chromium), QQC320
•Colt Industries
Menasco, TX, division
•DuPont
•Fairchild Camera
•Fairchild Republic
•General Dynamics
•General Electric
Lynn, MA
Cincinnati, OH (aircraft)
Wilmington, MA

Wilmington, NC (nuclear)
Fitchburg, MA
• Gillette Company, Boston
•Grumman Aircraft

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© 2006 by Taylor & Francis Group, LLC

29

-1

29

The Armoloy

Chromium Process

29.1 General Definition

29-

1
29.2 Applications

29-

1

29.3 Surface Preparation

29-

2
29.4 Properties

29-

2

29.1 General Definition

The Armoloy process is a low temperature, multistate, chromium alloy process of electrocoating based
on a modified chromium plating technology. However, instead of the customary chromium plating
solutions, the Armoloy process uses a proprietary chemical solution. The solution and application process
are carefully monitored at all Armoloy facilities. The result is a satin finish chromium coating that is very
hard, thin, and dense and has absolute adhesive qualities. Armoloy deposits a 99% chromium coating o
the basis metallic surfaces, whereas conventional chromium plating processes tend to deposit 82 to 88%
chromium in most applications.
The Armoloy process involves cleaning and removing the matrix on the basis metal’s surface by special
proprietary means and using a modified electrocoating process that causes the chromium metallic
elements of the solution to permeate the surface porosity of the basis metal. It is during this process that
the absolute adhesive characteristics and qualities of Armoloy are generated. The Armoloy coating actually
becomes part of the basis metal itself, and the result is a lasting bond and a continuous, smooth, hard
surface. The surface will not chip, flake, crack, peel, or separate from the basis metal under conditions
of extreme heat or cold, or when standard ASTM bend tests are involved.
Three basic factors are always present after applying Armoloy to metal surfaces:
•Increased wear (70 to 72 R

c

surface hardness)
•Added lubricity characteristics (including the ability to utilize Armoloy against Armoloy)
•Excellent corrosion resistance

29.2 Applications

29.2.1 General Applications

All ferrous and most nonferrous materials are suitable for Armoloy application. Service life of parts has
been increased to 10 times normal life and even higher in certain applications. However, basis metals of
aluminum, magnesium, and titanium are not good candidates for the Armoloy process.

Michael O’Mary

The Armoloy Corporation

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© 2006 by Taylor & Francis Group, LLC
General Applications • Specific Applications
Thickness • Adhesion • Corrosion • Wear Resistance • Lubricity •
Embrittlement
Conformity • Heat Resistance • Brightness • Hydrogen

The Armoloy Chromium Process

29

-5

The plating cycle times are very short, and the Armoloy chrome is deposited so rapidly that Armoloy
seals the surface porosity of the basis metal before hydrogen ions can invade the surface of the basis
metal. However, if required, Armoloy can be and will be postplate heat treated to specification.

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© 2006 by Taylor & Francis Group, LLC

30

-1

30

Sputtered Thin

Film Coatings

30.1 History

30-

1
30.2 General Principles of Sputtering

30-

1
30.3 Sputter Deposition Sources

30-

3

30.4 Other Process Considerations

30-

8
30.5 Properties of Sputtered Thin Film Coatings

30-

8
30.6 Thin Film Materials

30-

9
30.7 Applications for Sputtered Thin Films

30-

9

30.8 Additional Resources

30-

10
Bibliography

30-

10

30.1 History

Sputtering was discovered in 1852 when Grove observed metal deposits at the cathodes of a cold cathode
glow discharge. Until 1908 it was generally believed that the deposits resulted from evaporation at hot
spots on the cathodes. However, between 1908 and 1960, experiments with obliquely incident ions and
sputtering of single crystals by ion beams tended to support a momentum transfer mechanism rather
than evaporation. Sputtering was used to coat mirrors as early as 1887, finding other applications such
as coating fabrics and phonograph wax masters in the 1920s and 1930s. The subsequent important process
improvements of radio frequency (rf) sputtering, allowing the direct deposition of insulators, and mag-
netron sputtering, which enables much higher deposition rates with less substrate damage, have evolved
more recently. These two developments have allowed sputtering to compete effectively with other physical
vapor deposition processes such as electron beam and thermal evaporation for the deposition of high
quality metal, alloy, and simple organic compound coatings, and to establish its position as one of the
more important thin film deposition techniques.

30.2 General Principles of Sputtering

Sputtering is a momentum transfer process. When a particle strikes a surface, the processes that follows
impact depend on the energy of the incident particle, the angle of incidence, the binding energy of surface
In sputtering, the incident particles are usually ions, because they can be accelerated by an applied
electrical potential. If the kinetic energy with which they strike the surface is less than about 5 eV, they

Brian E. Aufderheide

W. H. Brady Company

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© 2006 by Taylor & Francis Group, LLC
Direct Current Diode Sputtering • Triode Sputtering • Radio
Electrical • Magnetic • Optical • Mechanical • Chemical •
Decorative
Frequency Sputtering • Magnetron Sputtering • Beam
Sputtering • Reactive Sputtering
atoms, and the mass of the colliding particles (Figure 30.1).

Sputtered Thin Film Coatings

30

-5

“racetrack” effectively increases the number of ionizing collisions per electron in the plasma. The magnetic
confinement near the target results in higher achievable current densities at lower pressures (10

–3

to 10

–2

torr), nearly independent of voltage. This manner of cathode operation is described as the magnetron
mode and is capable of providing much higher deposition rates (10 times dc diode) with less electron
bombardment of the substrate and therefore less heating. Factors affecting deposition rate are power
density on the target, erosion area, distance to the substrate, target material, sputter yield, and gas pressure.
Dc is usually used for magnetron sputtering, but rf can be used for insulators or semiconductors. When

magnetic materials are sputtered, a thinner target is often necessary to maintain sufficient magnetic field
strength above the target surface. The three most common magnetron cathode designs, described below,
are illustrated in Figure 30.5.

30.3.4.1 Planar Magnetron

An array of permanent magnets is placed behind a flat, circular or rectangular target. The magnets are
arranged such that areas in which the magnetic field lines are parallel to the target surface form a closed
loop on the surface. Surrounding this loop, the magnetic field lines generally enter the target, perpen-
dicular to its surface. This produces an elongated electron racetrack and erosion pattern on the target
surface. Because of the nonuniformity in target erosion, utilization of target material is poor, typically
26 to 45%. This also results in nonuniform deposition on a stationary target. Uniformity is provided by
substrate motion, usually linear or planetary, combined with uniformity aperture shielding. Planar
magnetron cathodes are usually operated at 300 to 700 V providing a current density of 4 to 60mA/cm

2

or a power density of 1 to 36 W/cm

2

.

FIGURE 30.5

Clockwise from upper left: schematic representations of planar magnetron, gun-type magnetron, and
cylindrical post magnetron sputtering sources. (Adapted from J. A. Thornton, in

Deposition Technologies for Films
and Coatings

, R. F. Bunshah, Ed. Park Ridge NJ: Noyes Publications, 1982, pp. 194–195.)
?
?
?
Anode
Anode
Anode
Magnetic
Field
Magnetic
Field
Cathode
Cathode
Cathode
E × B
Electron
Motion
E × B
Electron
Motion
Plasma
Ring
Plasma
Ring
Plasma
Magnetic
Field Line
Primary
Electrons

Ultimate
Electrons

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© 2006 by Taylor & Francis Group, LLC