Surface Enhancements for Automotive Applications
Low Friction, Hard, and
Corrosion Resistant
JÖRG VETTER
GÉRARD BARBEZAT
SULZER METCO
The surface treatments used in the manufacturing
of parts for the automotive industry have to meet
both functional and decorative requirements. The
functional demands on modern automotive systems
include increased load and corrosion resistance, a
longer life span, and the reduction of weight and
friction. New and improved deposition techniques
have been developed over the last decade. These
new treatments are becoming more and more
common in power train and engine applications. In
the case of the surface treatments required for
decorative use, environment-friendly processes and
materials are increasingly replacing traditional
chrome plating. Sulzer Metco offers a broad range
of solutions that fulfill virtually all of the surface
treatment needs of the automotive industry.
In automotive coating applications, base materials, or
substrates, include aluminum alloys, various steel grades, such as
heat treatable or ball bearing
steels, and—increasingly—nonmetallic materials, e.g., electro-
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SULZER TECHNICAL REVIEW 1/2007
plated plastics. In addition to external surfaces, internal geometries such as cylinder or bearing
bores are being coated. Apart from
client expectations regarding reliability, functionality, comfort, and
safety, car manufacturers also have
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to consider aspects such as production, material consumption,
and environmental impact. The
search is therefore continuing for
flexible manufacturing solutions,
new design concepts, and vehicles
that are easier to assemble. Surface
enhancement engineering solutions are growing more and more
popular in the automotive industry where the goal is to reduce
wear, friction, and the corrosion of
power-train parts and engines.
New surface solutions are also being applied for interior and exterior decoration (Fig. 1).
Future-Oriented Processes
Surface enhancement engineering
alters the surface of a material
through additive processes, such
as thermal spray, physical vapor
deposition (PVD), or plasma enhanced chemical vapor deposition
(CVD), and thermochemical heat
treatments like nitriding or nitrocarburizing. Sulzer Metaplas, a
German subsidiary of Sulzer Metco, offers Ionit Ox®, a combination
of plasma nitrocarburizing and oxidizing. These treatments create a
new surface material that is superior to the original. Hard carbon
overlays produced by Sulzer Euroflamm are a solution for exacting applications in transmission
parts such as synchronization
rings or torque converters (see
STR 1/2006, p. 19).
Innovative Coatings Reduce
Size and Weight
In engine blocks, the conventional
aluminum cast alloys do not have
the necessary tribological properties for the piston group. Today,
the most frequently used solution
is the insertion of cast iron sleeves.
However, this has several disad-
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3
2
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1 A variety of car
components is treated using different
surface technologies. Power train and
engine parts as well
as interior or exterior
components are
finished using Sulzer
Metco surface technologies.
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16
5
6
15
7
9
Ionit Ox
Thermal spray
PVD
1
Differential gear shaft
5
Synchronizer rings
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2
Brake parts
6
Clutch discs
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Fuel injection
3
Ball pivots
7
Shifter forks
15
Sign (decoration)
4
Gear selector shafts
8
Piston rings
16
Gears (in development)
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Brake discs (in development)
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Valve seats and springs
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Cylinder bores
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Connecting rods
vantages: the pitch distance is still
relatively high compared to the
bore diameter; the heat flow from
cylinder bore to the cooling system
is not regular; and the oxides that
form between the cast iron sleeves
and the aluminum cast material
are not distributed homogeneously and distort the bores, thus increasing the tendency for blow-by
and reducing the level of power
generated. Internal plasma spray
coatings are now used in the production of a variety of gasoline
and diesel engines (Fig. 2). The deposited plasma sprayed coatings
offer significant advantages over
cast iron sleeves or monolithic cast
iron with lamellar graphite (see
STR 2/2001, p. 8). The plasma
sprayed coatings have the potential to reduce the friction of the pis-
Piston rings
2 Plasma
sprayed coatings
in cylinder bores
replace conventional cast iron
sleeves. This
solution reduces
friction, wear,
and fuel consumption.
SULZER TECHNICAL REVIEW 1/2007
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problems relating to biofuels containing ethanol or methanol. In
heavy-duty diesel engines, extremely durable metal matrix composite coatings solve abrasion
problems linked to the exhaust-gas
recirculation. A cylinder bore with
a coating thickness of 150 µm has a
service life of over 1 million km.
OIL
Corrosion Protection without
Harmful Chromium
3 Surface topography after diamond honing of
plasma-sprayed cylinder bores. The open residual
porosity has an important function for the
hydrodynamic lubrication (oil pockets).
ton groups by around 30%, leading to a drop in fuel consumption
of around 3%. Oil consumption
can be significantly reduced—usually by a factor of 2. With only a
few nanometers per service hour,
the wear rate is extremely low.
During the coating, solid lubricants are built and deposited in the
functional material.
Further corrosion resistant coatings were developed to tackle
4 Nitriding using
the Ionit Ox process
developed by Sulzer
Metaplas improves
the surface properties of automotive
parts working in
tough conditions,
such as these ball
pivots.
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The 3-step Ionit Ox process—
patented by Sulzer Metaplas—
consists of gas nitrocarburizing
(GNC), plasma activation (PNC),
and oxidation (OX). The sensorcontrolled GNC process creates a
diffusion and a compound layer.
The diffusion layer serves as a
structural base for the subsequent
layers. Controlled growth of a
pure ε-compound layer produces
a defined pore structure on this
iron-nitride base. PNC involves
the modification of the compoundlayer surface to grow an extra fine,
dense oxide layer. The oxidation
step produces a strongly adhering
oxide layer of 2–3 µm thickness
that provides corrosion stability
on the surface of the modified
compound layer. Various parts,
such as gear selector shafts, pump
cases, and ball pivots, are treated
on an industrial scale (Fig. 3).
Wear Resistant and
Environment-friendly
Significantly increased corrosion resistance
Improved wear resistance
Low friction values
Low counter-body wear
No contact corrosion with
aluminum
Attractive visual appearance
SULZER TECHNICAL REVIEW 1/2007
Piston rings have to seal the combustion gas and control the lubrication oil. Surface treatments are
used to reduce wear and to prevent seizures. A wide variety of
substrate materials and surface
treatments exist in view of the diverse range of piston rings for different applications. Today, a variety of surface treatments, includ-
ing PVD, high-velocity oxy-fuel
(HVOF) processes, and the combination of nitriding and PVD coating, are used in industrial applications.
Coating of Bearing Bores
New developments in surface
technologies—driven mainly by
material aspects—provide opportunities to improve systems and
their components. The development of internal plasma spraying
has given rise to a new range of applications. Connecting rod bores
with an internal diameter of
40–60 mm can be coated using this
technology, which is especially
useful in the case of cracked connecting rods. The parts can be
coated before or after cracking.
The coating deposition is performed in stacks, which increases
productivity.
Primarily lead-free copper base alloys are deposited under good
metallurgical conditions using atmospheric air plasma spraying.
On the heat-treatable steels for
connecting rods, acceptable bond
strengths were measured to a
thickness of 550 µm. Bearing materials present different requirements in terms of mechanical
properties, seizure resistance, and
the possibility of embedding foreign particles. The plasma spray
technology using powder as feedstock offers excellent flexibility in
the choice of materials.
Combined Advantages
Thermal barrier coatings are normally used for turbine blades. Initial studies have been carried out
for the use on engine components.
In the case of high-performance
combustion engines, the deposition of a thermal barrier of partial-
PVD
also gears are being coated to minimize the required lubrication and
to increase the specific loads
(Fig. 6). This procedure is not used
yet in the industrial mass production of car power trains .
Plasma nitriding
and PVD
Hardness
Plasma nitriding
Durable and Decorative
0–20
20–800
Compound zone
Depth (µm)
1–50
Diffusion zone
Core material
CrN
5 Hardness over distance from the surface for the separate treatments
and the combination of plasma nitriding and PVD. Nitriding before the
deposition of the hard coating increases the load-bearing capacity of the
coating substrate system.
ly stabilized zirconia reduces the
creep tendency of the aluminum
piston head significantly. The coating also allows the use of a cheaper aluminum material and reduces
the sensitivity to material defects.
The plasma nitriding of metallic
components of various materials is
a well-established treatment for
parts such as crankshafts, springs,
or synchronizers. It improves performance due to an increase in
hardness and fatigue strength, as
well as the creation of residual
compressive stresses. In contrast,
PVD coatings are more wear resistant and have a low coefficient of
adhesion, but require a stable base
material. The combination of nitriding and PVD results in surfaces
superior to those treated with either process (Fig. 5). This procedure is used for applications in the
power train and engine, e. g., for
highly loaded piston rings.
The interiors and exteriors of modern cars are designed in accordance with current tastes. It is becoming increasingly common for
PVD coatings to be deposited by
sputtering or vacuum arc evaporation and applied to different materials including galvanized plastics
(ABS). The wide range of available
metallic colors—e.g., anthracite, titanium, steel, gold, or chromium
—guarantees wear-resistant decorative surface finishes, especially
for interior applications (Fig. 7).
Growing Application Range
DLC—the Low Friction Coating
Diamond-like coatings (DLC) are
one solution if low friction is the
goal. Metallic DLC deposited by
reactive PVD processes (W-C:H)
and pure carbon coatings deposited by plasma-enhanced CVD
(a-C:H) have been applied in injection systems for almost 10 years.
In addition to injection systems,
6 DLC (W-C:H) coating of gears reduces micro pitting and allows
increased loads.
The variety of plasma-assisted surface treatments applied in the automotive industry will increase
further in the near future due to
higher loads in engines, injection
systems, and power trains. Reducing manufacturing cost, extending
life cycles of parts, and decreasing
emission levels are challenges that
will be met using surface treatments and systems chosen from
the broad range offered by Sulzer
Metco.
7 Decorative PVD
coatings, which
are available in
many colors, also
improve the physical surface properties of the substrate.
Contact
Pinion uncoated
Gear uncoated
Load: 1500 N/mm2
1.35×106 cycles
20% micropitting
Pinion coated
with Maxit® W-C:H
Gear uncoated
Load: 2000 N/mm2
5.4×107 cycles
No micropitting
Sulzer Metco AG
Gérard Barbezat
Rigackerstrasse 16
5610 Wohlen
Switzerland
Phone +41 56 618 81 79
Fax +41 56 618 81 00
gerard.barbezat@sulzer.com
SULZER TECHNICAL REVIEW 1/2007
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