Surface treatment selections for automotive applications

Abstract

Surface treatments used in daily manufacturing of parts for the automotive industry are selected to serve functional and decorative requirements achieved by mass production. Increased loads (mechanical, thermal, etc.), longer lifetime, weight reduction, friction reduction, and corrosion resistance are demanded for modern automotive systems. Within the last decade, improved and new deposition techniques were developed in PVD, PECVD, thermochemical heat treatment and thermal spraying. These new treatments are becoming more and more common in powertrain and engine applications. Generating optimized surfaces for different types of substrate materials (e.g. Al-alloys, case hardened steels, etc.) and geometries (e.g. bores) also impacts the running costs. Due to the new developments within these competing surface treatments, it becomes more and more common to substitute traditional treatment-substrate-systems with advanced treatments. Both the application potential and selected examples of different surface treatments will be shown. Aspects of the internal coating of bores inside the engine by plasma spraying, features of the corrosion protection of parts for the powertrain by the IONITOX process and piston ring treatments are discussed in more detail.

Full text

Surface treatment selections for automotive applications
J. Vetter
a,
*, G. Barbezat
b
, J. Crummenauer
a
, J. Avissar
c
a
Metaplas Ionon, Am Bo¨ttcherberg 30-38, 51427 Bergisch Gladbach, Germany
b
Sulzer Metco AG, Rigackerstr. 16, 5610 Wohlen, Switzerland
c
Sulzer Metaplas (US) Inc., 222 Goldstein Drive, Woonsocket, RI 02895, USA
Available online 25 October 2005
Abstract
Surface treatments used in daily manufacturing of parts for the automotive industry are selected to serve functional and decorative
requirements achieved by mass production. Increased loads (mechanical, thermal, etc.), longer lifetime, weight reduction, friction reduction, and
corrosion resistance are demanded for modern automotive systems. Within the last decade, improved and new deposition techniques were
developed in PVD, PECVD, thermochemical heat treatment and thermal spraying. These new treatments are becoming more and more common
in powertrain and engine applications. Generating optimized surfaces for different types of substrate materials (e.g. Al-alloys, case hardened
steels, etc.) and geometries (e.g. bores) also impacts the running costs. Due to the new developments within these competing surface treatments, it
becomes more and more common to substitute traditional treatment-substrate-systems with advanced treatments. Both the application potential
and selected examples of different surface treatments will be shown. Aspects of the internalcoating of bores inside the engine by plasma spraying,
features of the corrosion protection of parts for the powertrain by the IONITOX process and piston ring treatments are discussed in more detail.
D2005 Elsevier B.V. All rights reserved.
Keywords: PVD; Nitriding; Thermal spraying; Automotive; Corrosion; Wear; Friction; Decoration
1. Introduction
Automobile manufacturers have to consider in addition to
the expectations and satisfaction of customers regarding the
reliability, functionality, comfort and safety, additional aspects
such as: production, consumption and environmental issues.
The search continues for flexible manufacturing, new design
concepts and vehicles that are easier to assemble. Regional
environmental legislation and shorter product life cycles
require higher quality and more stringent materials require-
ments. Even though these challenges affect all aspects of the
industry, they can be met and overcome with the correct set of
resources. In order to compete effectively in this fast paced
and increasingly complex competitive environment, enter-
prising automotive executives will need to constantly optimize
their business models and improve existing technical solutions
or create new ones. Surface enhancement engineering
solutions are becoming more and more the goal of the
automotive industry in reduction of wear, friction and
corrosion for powertrain parts and engine interiors [1].New
surface solutions are also applied for interior and exterior
decoration. Surface enhancement engineering basically
involves changes to the surface of a material by additive
processes such as thermal spray (named TS), PVD, plasma
enhanced CVD (named PECVD), thermochemical heat treat-
ment (named TCHT) like nitriding or nitrocarburizing (with
oxidation named IONITOX) [2– 6]. In effect, this surface
treatment creates a new material superior to the original. Also
hard carbon overlays become a solution for increased
demands in transmission parts (e.g. for synchronization rings).
2. Plasma assisted processes for surface treatments of
automotive parts
Plasma assisted surface treatments used for automotive
parts include different methods of plasma generation in
terms of the plasmas used, the material generated and the
interaction between the material flow and the substrate
0257-8972/$ - see front matter D2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2005.08.011
* Corresponding author. Tel.: +49 2204 299 261; fax: +49 2204 299 266.
E-mail address: joerg.vetter@salzer.com (J. Vetter).
Surface & Coatings Technology 200 (2005) 1962 – 1968
www.elsevier.com/locate/surfcoat

surface during the deposition of a coating or during the
growth of a diffusion (and compound) layer.
Two plasma types are applied: the local thermodynamic
equilibrium plasma (LTE) and the thermodynamic non-
equilibrium plasma (cold plasma).
Plasmas in local thermodynamic equilibrium in which all
temperatures (except the radiation temperature) are equal in
each small volume of the plasma are the basis for plasma
spraying at atmospheric pressures (thermal plasma). This
thermal plasma is used for the melting of the material
resulting in the coating. The plasma does not play a
dominant rule during coating growth. Cold plasmas are
always generated by plasma assisted PVD and CVD
processes and in plasma assisted thermochemical heat
treatment (e.g. plasma nitriding processes). The cold plasma
is characterized by hot electrons and cold ions. The ions are
normally used during the coating growth or during the
diffusion treatment. Table 1 shows a summary of the
treatment types using different plasma states, the material
used and typical substrate materials.
Selected properties of the different treatments and
typical applications are shown in Table 2. It should be
Table 1
Summary of the treatment types of different plasma states, the materials used and typical substrate materials (GNC gas nitrocarburizing)
Type of plasmas Material source Plasma – surface interaction Substrate material
Atmospheric
plasma spraying
LTE-plasma Atmospheric arc
discharge
Nearly no plasma activity
at the surface
– Metal alloys, e.g.
Al-alloys
– Composite
Plasma assisted
TCHT
Cold plasma Pulsed glow discharge
in gas mixtures, e.g.
N
2
,H
2
– Ion impact – Metals, e.g. heat treatable
steels Ti-alloys– Surface acts as the
cathode
– Species for diffusion
are generated
IONITOX
activation step
Cold plasma Pulsed glow discharge
in N gas mixtures
– Ion impact – Metals, e.g. heat
treatable steels– Surface acts as the
cathode
– Surface modification
after GNC; before
oxidation
PECVD Cold plasma Pulsed glow discharge
in precursors, e.g. C
2
H
2
– Ion impact – Metals, e.g. bearing
steels– Surface acts as the
cathode
– Species for coating
growth are generated
PVD sputtering Cold plasma Atomizing by sputtering
due to glow discharge
at a target
– Ion impact – Metal alloys
– Extracted Ar-ions – Galvanized plastics
– Sputtered atoms as
growth materials
PVD vacuum arc Cold plasma Evaporation by vacuum
arc discharge
– Ion impact – Metal alloys
– Extracted metal ions – Galvanized plastics
– Evaporated atoms
and ions as growth
materials
Table 2
Selected properties of the different treatments and typical applications
PVD PVD/PECVD Thermochemical heat treatment Spraying Bonding
MeNCO DLC PN/PNC PNC + OX Metal Metal + ceramics Carbon onlay
Thickness [Am] 2 – 50 1 – 5 DZ/CZ
50 – 500
DZ/CZ
+ oxide 1 – 3
100 – 400 200– 500 400 – 700
Hardness (micro) 1600 – 3500 1000– 3000 600 – 1200 500– 800 300 – 600 400– 1000 Not measured
Friction against
steel
Dry 0.5 – 0.8 Dry 0.1– 0.3 Dry 0.7 – 0.9 Dry 0.6– 0.8 Lubricant
0.05 – 0.15
Lubricant
0.05 – 0.10
Lubricant
0.11– 0.13
Deposition
temperature [-C]
50 (plastics) – 500 60 – 250 350 – 800 (Ti) 400 – 570 50 –150 50 – 150 160 – 280
Typical
application
Piston rings Injection
systems
Piston rings Ball pivots Piston rings Piston rings Shift forks
Pumps Tappest Syncroniz Gear selector
shafts
Synchroniz Bores Synchroniz
Decoration Gears Clutches Pumps Shift forks Break discs Clutches
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 1968 1963

mentioned that the plasma assisted surface treatments are
more or less in competition with the non-plasma surface
treatments. The term ‘‘Spraying’’ in the table does not
stand only for plasma spraying, but stands for the whole
group of thermal spraying. The newer material carbon
onlay which is bonded at selected functional surfaces is
included for comparison.
However, the main issue of the paper is the discussion of
plasma assisted surface treatments to point out the speciali-
ties to generate new surface conditions for a variety of
applications.
3. Functional plasma assisted surface treatments
Fig. 1 shows a variety of car parts treated by different
surface technologies. Parts of the powertrain, the engine and
components of the interior or exterior are finished by plasma
assisted surface technologies.
Different functional surface properties of various auto-
motive parts require a variety of surface treatments. A rough
classification of the four main required surface properties
and typical application of plasma assisted surface treatments
are shown in Fig. 2.
The surface requirements (low friction, low wear, etc.)
are often more or less overlapping. However it becomes
clear that the adjustable properties of the surfaces within the
above mentioned groups of surface treatments cover an
enormously wide range (mechanical, electrical, tribological,
corrosive properties).
4. Selected application examples
4.1. Cylinder bores: low wear rates by rotating plasma
torches
The conventional Al cast alloys with 7% to 9% Si do
not produce the necessary tribological properties for the
piston group. The hypereutectic alloys with 17% to 20% Si
present several disadvantages concerning manufacturing
Fig. 1. Selected parts to be treated by surface technologies.
e.g. Injection
DLC
PVD/PECVD
Decorative
appearance
Corrosion
protection
e.g. Ball pivot
IONITOX
Friction
reduction
Wear
reduction
e.g. Piston rings
Plasma spraying
e.g. Signs
PVD
Properties
Application
Treatment
Fig. 2. Main functional properties required in automotive application.
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 19681964

costs and inferior mechanical properties (low toughness).
Today, the most frequently used tribological solution for
engine blocks in aluminum cast alloys is the insert of cast
iron sleeves. However the insertion of cast iron sleeves
presents several disadvantages. The pitch distance is still
relatively high in comparison with the bore diameter. The
heat flow from cylinder bore to the cooling system is not
regular. The oxides between the cast iron sleeves and the
Al cast material have a negative influence, because they
are not homogeneously distributed and some distortion of
the bores results. The distortion of the bores increases the
tendency for blow-by and has a negative influence on the
power generated.
There are some galvanic solutions but these also present
several disadvantages in terms of environmental impact and
costs. For these reasons galvanic solutions are used only for
few special applications.
The internal plasma spray coatings are now used in the
production of a variety of gasoline and diesel engines [7,8].
The deposited plasma sprayed coatings show three signifi-
cant advantages in comparison with cast iron sleeves or
monolithic material cast iron with lamellar graphite [9,10].
– The plasma sprayed coatings have the potential to reduce
the friction of the piston groups by roughly 30%. The
consequent fuel consumption reduction is around 3%.
– The oil consumption can also be significantly reduced in
comparison with the cast iron (usually by a factor of two).
– The wear rate is extremely low, only some nanometers
per service hour.
During the coating deposition of the low alloyed carbon
steel XPT 512 wustite FeO and magnetite Fe
3
O
4
are
produced. Both types of these crystals are working as solid
lubricants. The amount of iron oxide is controlled by the
choice of the spray parameters, primarily plasma gas
composition, dwell time of the particle in the plasma, and
plasma enthalpy. Table 3 shows typical deposited plasma
coatings and their characteristics.
The manufacturing steps of the cylinder bores finished
by plasma spraying are shown in Fig. 3.
4.2. Ball pivot: high corrosion resistance by IONITOX
The IONITOX process was developed as a combination
of gas nitriding and plasma activation processes to generate
surfaces with high corrosion resistance on carbon steels and
heat treatable steels. The plasma activation step is carried
out before the oxidation. Corrosion stability is reached due
to a dense oxide layer (Fe
3
O
4
) of a thickness of about 2 um.
Table 3
Typical plasma coatings for cylinder bore and their characteristics
Coating type Hardness
[HV
0.3
]
Microstructure
A: Carbon steel with
solid lubricant
wustite and
magnetite (XPT 512)
400 Ferrite with fine carbides,
FeO (wustite) and Fe
3
O
4
(magnetite)
B: Composite of tool
steel and molybdenum
400 Ferrite with fine iron
carbides and isolated
phase of molybdenum.
Low level in iron oxides
C: Stainless steels
(Cr and Mo alloyed)
350 Iron awith fine carbides
and some oxides
D: Metal matrix
composite MMC
(type 1)
450 Material A with addition
of about 20% discrete
particle of ceramic
E: Metal matrix
composite MMC
(type 2)
400 Material C with addition
of 20% discrete particle
of ceramic
Fig. 3. Steps of bore coating solution: 1. Casting and machining, 2. Surface activation, 3. Coated cylinder bore, 4. Finish machining (diamond honing).
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 1968 1965

Different parts are treated in industrial scale: gear selector
shafts, pump cases, etc. Fig. 4 shows a typical application –
ball pivots–with the improvements.
4.3. Piston rings: reduction of wear and seizure due to PVD
coatings
Piston rings have to seal the combustion gas and to control
the lubrication oil. The surface treatment is used to reduce
wear and to prevent seizure. A variety of piston rings exists
for different applications. Besides the different substrate
materials, also different surface treatments are used. Fig. 5
shows surface treatments developed and/or applied over the
past 20 years. Today nearly all these treatments are in the
industrial praxis. Depending on the application and the
manufacturing costs, spraying, galvanizing, nitriding, PVD
coatings and the combination nitriding plus PVD coating are
used. The most expensive method is the PVD-method. About
10–50 um thick Cr-based coatings are deposited mostly by
vacuum arc evaporation.
4.4. Decoration
Customers prefer attractive cars. The car’s interior and
exterior need to be styled according to modern trends. More
and more PVD coatings are deposited by sputtering and or
vacuum arc evaporation, and are applied to different
materials including galvanized plastics (ABS). The colours
can be adjusted over a wide range.
5. New developments
New developments in surface technologies, driven mainly
by material aspects, provide opportunities for improving
systems and their components. The new solutions have to be
able to compete with the classical solutions. Even if there are
apparent advantages, additional aspects such as financial,
reliability, compatibility with existing manufacturing methods,
etc., have to be considered. Newer developments are: plasma
borizing of gears [11], coatings for gears, plasma sprayed
bearing coatings, nitriding of powder metallurgical steels.
Four examples of surface treatments from the –more or less –
industrial development state will be discussed in more detail in
the following.
5.1. Deposition of coating for connecting rod bearing
The development of the internal plasma spraying
technology enables new horizons of application. Connecting
rod bores of 40 to 60 mm internal diameter can be coated
using the internal plasma spraying technology [12,13].
Especially interesting is the spraying technology for the
cracked connecting rod. The coating deposition can be
considered before or after the cracking of the parts. The
coating deposition is performed in stacks, so an interesting
productivity can be obtained. Primarily lead free copper
Fig. 4. Improvements of ball pivots by IONITOX treatment.
Cr
plated
Mo
sprayed
Gasoline
Diesel
High
performance
engine
Evolution of piston ring surfaces: wear / scuffing resistance
Mo
sprayed
Cr
plated
Cr plat.
+ diam.
PVD
Cr:N
wear resistance
Nitrided
Cr plated
+ Ceramic
HVOF
CrC/NiCr
HVOF
CrC/NiCr
+ Cericam
scuffing resistance
Cr plated
Nitrided
Cr plat.
+ diam. PVD
Cr:N
Fig. 5. Different piston ring treatments and the trend of the wear and scuffing resistance; HVOF: High Velocity Oxygen Fuel spray; Cr plat. +diam.: Cr
plated + diamond.
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 19681966

base alloys can be deposited under good metallurgical
conditions using the atmospheric spraying process APS.
On the different heat treated steels for connecting rods
reasonable bond strengths were measuredto a thickness of 550
um. Bearing materials present different requirements concer-
ning mechanical properties, seizure resistance and the possi-
bility of embedding foreign particles for example. The plasma
spray technology using powder as feedstock is characterized
by an excellent flexibility regarding the choice of the materials.
Fig. 6 shows an example of typical copper base material
deposited by internal plasma spraying in a connecting rod of
diameter 40 mm.
5.2. Thermal barrier coatings also for the engine?
Thermal barrier coatings are used normally for turbine
blades. First studies were made also for engine components.
In the case of high performance combustion engine the
creep tendency of the aluminium piston head can be
significantly decreased by the deposition of a thermal
barrier in PSZ (partial stabilized zirconia). The coating
system (MCrAlY bond coat and PSZ) is deposited by using
the atmospheric plasma spray process. The coating allows
also the use of cheaper aluminium material and decreases
the sensibility to the material defects.
5.3. Gear coatings: lubrication reduction by DLC coatings
DLC coatings in the form of a-C:H:Me coatings deposited
by reactive PVD processes and a-C:H coatings deposited by
PECVD have been applied in injection systems since about
the year 2000. Fig. 7 shows the excellent improvements of
the gear performance by a-C:H:Me coatings. These improve-
ments are partially industrially realized, e.g. gears for wind
Fig. 6. Typical microstructure of CuAlSnalloy deposited by internal APS on connecting rod (40 mm bore diameter): porosity: 2%, HV
0.3
=162, thickness 550 um.
Fig. 7. Reduction of micro pitting and load increase by a-C:H:Me (W-C:H) coatings gears.
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 1968 1967

power transmissions. The potential of the gear coating to
minimize the required lubrication and or to increase the
specific loads is not utilized in industrial practice for the
powertrain. One of the reasons might be the high cost factor
of production due to the small lot production of coated gears.
5.4. Combination coatings: new properties by nitriding and
PVD coating
Plasma nitriding of metallic components of different
materials is one of the well established treatments (for
crankshafts, springs, synchronizers, etc.) resulting in
improved performance due to an increase of the hardness,
fatigue strength and creation of residual compressive
stresses. However, PVD coatings are more wear resistant
(adhesive wear, micro abrasion, oxidation, etc.). The
combination of nitriding and PVD provides a product
superior to both. Fig. 8 shows the change in hardness as a
function of the distance from the surface for the separate
treatments and the combined treatment. The nitriding before
the deposition of the hard coating increases the load bearing
capacity of the coating-substrate-system. This combination
treatment is industrially used for high loaded piston rings
(see also Fig. 5). However, more applications might be
realized in the powertrain and engine.
6. Summary
1. The variety of plasma assisted surface treatments applied
in the automotive industry will increase in the upcoming
years due to higher loads in the engine, injection systems
and the power train.
2. Corrosion protection of different parts of the power train
can be produced by the IONITOX process.
3. The internal plasma spraying technology allows treating
different inside diameters (bores) in the engine.
4. PVD coatings become more and more interesting not
only for decoration, but also for high loaded parts like
piston rings.
5. DLC coatings still have a high potential for lubrication
reduction and or for the increase of the load, e.g. for
gears.
6. The various surface treatments available for selected
parts (e.g. synchronizers) are competing with each other,
in terms of costs and functionality.
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Hardness
Depth
CZ CrN Core CrN
PN PVD-coated PN plus PVD
(0-20) µm (20-800)µm (1-50) µm
CoreDZ CoreDZ
Fig. 8. Hardness versus distance from the surface for soft steels with a plasma nitriding treatment (PN), PVD coating, and combination nitriding plus PVD
coating; CZ: compound zone, DZ: diffusion zone.
J. Vetter et al. / Surface & Coatings Technology 200 (2005) 1962– 19681968