Hvof-Sprayed Materials Replace Hard-Chromium Plating

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.