Hvof-Sprayed Materials Replace Hard-Chromium Plating

HVOF-Sprayed Materials
Replace Hard-Chromium Plating
4164
ANDREAS KIRSTEN
MANFRED OECHSLE
RICHARD MOLL
SULZER METCO
In industrial applications, hard-chromium
plating is increasingly being replaced with
HVOF-applied, carbide-containing materials.
While these coatings have good wear
protection properties, many hard-chromium
replacement applications also require
corrosion resistance. Sulzer Metco Woka, a
German subsidiary of Sulzer Metco,
develops corrosion-resistant coating
materials. During the development, a variety
of factors is taken into account, including
the coating chemistry, the spray process,
and the service environment of the coating.
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8
Hard chromium, applied
electrochemically as plating,
has a long and comprehensive
history as a surface solution used
to prevent wear and corrosion.
Industrial applications include aircraft
landing gear, turbine engines,
hydraulics, and propeller hubs
(Fig. 1). However, human health
and environmental concerns over
the presence of hexavalent chromium
associated with this process
have caused many industries to
look for alternative solutions. For
applications with larger surfaces,
thermally-spraying of carbidecontaining
materials using the
high-velocity oxy-fuel (HVOF)
process has proven to be a competitive
substitute (see box).
Lower Cost—
Better Performance
Experience shows that, in many
cases, the right choice of the coating
material can lower costs while
improving performance (Fig. 2). In
laboratory tests, however, some
HVOF-sprayed coatings have
demonstrated insufficient corrosion
resistance. Therefore, the development
of coating materials
SULZER TECHNICAL REVIEW 1/2006
that exhibit both wear and corrosion
resistance and can be used in
the HVOF process has high priority.
For carbide-containing materials,
the agglomeration and sintermanufacturing
technique is highly
promising, as many different
material combinations are possible
and because metallic matrices,
which influence the application
success, can be tailored precisely.
Cermet-type coatings, where the
carbide constituent is cemented
within a metallic matrix, are the
most suitable carbide-containing
materials with respect to wear and
corrosion resistance. In such a
coating system, the carbide provides
wear protection, and the
metallic matrix can be formulated
from a corrosion-resistant alloy.
Tungsten carbide and chromium
carbide are the most commonly
used carbides for thermal spray.
Specialized Alloys
for Difficult Applications
While tungsten carbide and
chromium carbide are highly corrosion-resistant,
they are not used
as HVOF coating material in pure
form but rather in metallic matri-
Sulzer Metco DiamondJet ®
HVOF: An Alternative to Chromium Plating
HVOF is a thermal spray technology where a powder is fed into a spray gun. There, a
fuel—usually ethylene, hydrogen, or kerosene—is burned with oxygen, and the heated
and softened powder is ejected as a spray with the supersonic gases (see STR 1/2004,
p. 4). HVOF is a flexible dry-coating technology with
low environmental impact. HVOF covers many applications
with a variety of possible coating materials.
The use of hard chromium, however, is so prevalent
that no single technology or material can replace
it generally. Among all choices, carbide-containing
materials are very promising due to their wide range
of application.
1 Apart from health concerns, legal
requirements also compel the elimination
of hard-chromium plating in
industrial use. Sulzer Metco develops
coating materials that can replace
hard chromium in applications that
require high corrosion resistance, like
the coating of aircraft landing gear.
ces. As in the cemented-carbide industry,
for thermal spray powders,
single metal matrixes of nickel and
cobalt are most often used as well.
For applications requiring more
specific properties, specialized alloys
can be used, such as cobaltchromium,
nickel-chromium, and
nickel-chromium/ -molybdenum.
These metals are added either as
prealloyed powders or, for costsaving
reasons, as finely powdered
elemental metals during the
homogenizing process of the spray
powder.
During sintering, they form
pseudoalloys with a corrosion resistance
that is nearly identical to
that of their prealloyed counterparts.
The matrix material, however, remains
the weak point of corrosionresistant
coatings. In addition, the
coatings must be dense to prevent
the worst-possible condition of
electrolyte penetration to the substrate.
This penetration causes galvanic
coupling and results in corrosion
of the substrate and delamination
of the coating.

Extensive Research
Sulzer Metco carried out a research
program to identify the
main parameters influencing the
hardness and corrosion resistance
of HVOF coating materials by
varying the fuel/oxygen ratio
lambda (λ) and the distance between
the HVOF spray gun and
the surface. The coatings were subjected
to standardized hardness
and wear test procedures and their
properties compared with those of
hard-chromium plating. In almost
every instance, the carbide coatings
exhibited higher coating
hardness and greater wear resistance
than galvanic hard-chromium
plating.
To study the corrosion resistance
of HVOF materials, the coatings
were electrochemically tested in
saline, acidic, and alkaline aqueous
solutions for two hours and
twenty-four hours immersion time
(Fig. 3). The corrosion rate of the
coatings was compared with that
of the uncoated, low-carbon steel
substrate and with that of galvanic
hard chromium. HVOF coatings
2 Coating landing gear—here a
Sulzer Metco DiamondJet spraying a
landing gear strut—is the major use
of hard chromium in aircraft. With its
improved performance, the HVOF
process lowers the expected overhaul
frequency, thereby reducing the lifetime
cost of ownership.
with matrices of nickel-chromium
alloys demonstrated superior resistance
in all solutions. Furthermore,
post-testing examination of
the interface of the coating to the
substrate revealed no signs of galvanic
corrosion.
Award-Winning Results
Carbide-containing materials certainly
rival hard-chromium plating
in terms of hardness and wear
resistance; tungsten carbide materials
also have potentially better
corrosion resistance than hardchromium
plating. Very good
durable corrosion resistance in different
corrosive environments can
be obtained through the choice of
matrix material and the careful
control of the HVOF spray
process. Coatings must be very
dense in terms of interparticle
bonding to prevent electrolytes
from penetrating the coating. This
shields the substrate from attack,
which is particularly important
with less noble substrate materials.
The HVOF spray parameters
show a strong influence on the corrosion
behavior of coatings in
service (Fig. 4). Provided that the
formation of brittle carbide phases
is avoided, a higher particle temperature
is helpful. Thanks to this
extensive research—which received
a certificate of merit by two
reputed materials technology associations—Sulzer
Metco now offers
carbide-containing coating
materials that can replace hardchromium
plating even in applications
where high corrosion resistance
is required.
3 Sulzer Metco Woka in Barchfeld (DE) carried out the
research. In the corrosion chamber (right), the polarization
resistance of the coatings was measured. Per definition, the
polarization resistance Rρ (kΩ ·cm –2 ) is inversely proportional
to the corrosion rate of the coatings.
4 The long-term corrosion tests revealed the strong influence
of the particle temperature, which is high for low
lambda values, or short stand-off distances. It was found
that the hotter the flame and, therefore, the hotter the
spray particles in the flame, the better the corrosion performance
of the coating—measured by the high polarization
resistance and high corrosion potential.
Polarization resistance Rρ (kΩ ·cm –2 )
100
90
80
70
60
50
40
30
20
10
0
–100
–150
–200
–250
–300
–350
–400
–450
–500
–550
–600
0 20 40 60 80 100 120 140
Time (hours)
Contact
Sulzer Metco Woka GmbH
Andreas Kirsten
Im Vorwerk 25
36456 Barchfeld
Germany
Phone +49 (0)36961 861 42
Fax +49 (0)36961 861 33
andreas.kirsten@sulzer.com
Distance
300 mm
300 mm
300 mm
350 mm
Lambda
1.25
1.15
1.05
1.05
Corrosion potential Ecorr (mV)
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