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Paper Number 1999 – 01 - 3230
Hard Coatings for Heavy Duty Stamping Tools
Lee Segal, Rosen Tovbin
Sputtek Inc. – Thin Film Hard Coatings
Copyright ©
1999 Society of Automotive Engineers, Inc.
ABSTRACT
This paper describes the
development and use of new types of Physical Vapor Deposition (PVD) equipment and
deposition technology for thin film hard coatings on a variety of large heavy duty
stamping tools. The life of the tools increased significantly, beyond the limits of the
presently available coating technologies (Chemical Vapor Deposition – CVD and Thermal
Diffusion – TD). The pick-up is eliminated, the surface finish of the parts is
improved and the process friction is reduced. Coated stock is handled better, as well as
hot rolled high strength heavy gage steel up to 10 mm. thickness. This technique allowed
parts up to 725 x 725 x 250 mm. and 450 kg. to be coated, while coating of parts twice the
size and mass is possible in the near future.
INTRODUCTION
Some of the newest methods to
increase the life of stamping tools and improve the finish of stamped parts are the thin
film hard coatings and the thermal diffusion processes. These methods find ever increasing
appli-cations and brought significant advantages to their users. The thin film hard
coatings are nitride or carbide-based ceramics, with a thickness of 3 – 10 µm
. They are applied by two types of techniques:
Chemical Vapor Deposition
(CVD), in which the components of the coating (e.g. titanium and nitrogen) are supplied in
gaseous form, and the thermochemical reaction to form the coating is produced on the
surface of the tool, heated to approx. 1,000 °C.
Known in shops as the “hot process”.
Physical Vapor Deposition
(PVD), in which the metal component of the coating is produced from solid, in a high
vacuum environment. The generation of the metal atoms is done by evaporation, sputtering
or ion bombardment methods, at temperatures of approx. 500°C.
Known in shops as the “cold process”.
The Thermal Diffusion (TD) is
applied in a molten borax bath, with addition of vanadium, at approx. 1,000 °C.
The resultant vanadium carbide coating has very good results in numerous applications.
Both CVD and TD techniques
benefit from the full immersion of the tool in a gaseous or molten salt environment –
the coating is uniformly applied on all tool surfaces, including deep recesses and holes.
The relatively high temperature results in very clean surfaces to start with, as well as
energetic and complete reactions of the coating components, with good adhesion and
performance. Relatively large parts can be handled by the available coating systems.
The drawback for both methods
is the high temperature of application. At 1,000 °C
significant dimensional and geometrical changes take place. The coating must be followed
by heat treatment to produce or restore the substrate hardness. Due to the inability to
preserve tight tolerances through the sequence of operations, these procedures are not
suitable for precision stamping tools. The high temperature produces also a high
reactivity of the carbon contained in the superficial layers of the tool steel. As a
result, this carbon reacts with the supplied metal (titanium or vanadium) and contributes
to the formation of the carbide coating, decarburizing at the same time the external
layers of the substrate steel. The hardness of these layers is reduced, weakening the
foundation under the coating and accelerating the development of fatigue cracks under
cycling loading during the production process. The failure mechanism involves the
separation of a 0.25 – 1.00 mm. layer from the tool surface, together with the
coating. Such deep damage requires machining - when acceptable for the tool tolerances, or
scrapping of higher precision pieces. Similar problems occur for carbide tools, where the
high temperature weakens the cobalt binder. Present paper describes the development and
application of PVD deposition equipment and coating technology to overcome these
limitations and to increase the performance of heavy duty stamping tools.
COATING
SYSTEM
All PVD methods used at the
present time require a high vacuum chamber, to allow a relatively free path for the atoms
and molecules of metal and gas required to be fed and mixed for the reaction on the
surface of the tool. The PVD systems use one of the following methods to generate the
metal atoms required for the thermochemical reaction forming the coating:
electron gun, directing a
stream of high energy electrons toward the deposition metal in a crucible and evaporating
it in the high vacuum of the deposition system.
sputtering, in which ionized
argon bombards the deposition metal target and extracts the atoms required for coating
forming reaction
arc, vaporizing the
deposition metal and accelerating it toward the tool surface, together with the reactive
gas required (nitrogen, or carbon from methane)
Prior to vacuum chamber
loading, a multistage thorough cleaning process removes all contaminants from the tool
surfaces. The tools shall be cleaned on all surfaces, operational or not, to avoid
contamination of the process with unwanted chemical species. In the initial stage of the
vacuum process, a stream of ionized gas (argon) is used for a final ion cleaning of the
tool surface. A bias high voltage (around 1,000 V) charges the tool negatively versus the
metal source and magnetic techniques are used to focus and accelerate the stream of
electrically charged metal vapor (plasma) toward the tool surface.
Due to the ionization of
species involved in the coating forming reaction, the need for thermal energy is reduced
and the reaction runs satisfactorily at approx. 500°C.
At this temperature the temper of tool steels is maintained, allowing coating of precision
parts through the normal process of heat treatment, final grinding and coating. The
dimensional and geometrical changes are reduced, as well as the decarburization of the
tool steel. In spite of these obvious advantages, PVD methods have limited use in stamping
applications at the present time. The adhesion of the coating is not as good as in TD and
CVD methods and this translates in early failures of the coatings in heavy duty
applications. The method is limited to small, lightly loaded tools in fine blanking and
similar environments.
Present implementation of the
PVD arc technique produces a very high ionization of the metal vapor stream (90 – 95
%), that is 2 to 8 times higher than in the conventional PVD deposition systems. Combined
with an acceleration energy 100 to 1,000 times higher than conventional systems, the
coating forming reaction is a very energetic process, requiring less heat input from other
sources. These conditions allow the process to proceed at 400 – 450 °C
vs. 500 °
in conventional PVD systems, while producing a higher density coating with superior
interface adhesion. The new process, named High Ionization Deposition (HID), was further
developed into a double technology process called High Ionization Deep Diffusion
Deposition (HI3D), to increase the depth of coating components diffusion into the tool
superficial layers. The coating adhesion is increased 2 – 3 times over the original
HID process and this is reflected in increased tools life. An example of tool failure
mechanism on blocks of the same die coated with TD and HID methods is presented in Figure
1. The TD – coated block shows deep damage, while the HID – coated can be
restored to service through stripping, polishing and recoating
Fig.1 – Failure modes for TD and HID coatings
TOOL
PREPARATION
All tool steels can be coated with this
technique – the coating cycle temperature of 300 to 450 °C
is well under the last draw temperature of the normal heat treatment. The hardness,
dimensions and geometry of the tool are maintained after the coating process.
A number of rules apply to surface
preparation:
all part’s surfaces shall be free of oxidation, paint marks or
any other potential contaminants
superficial changes produced by EDM cutting, ion nitriding
“white layer” or heat treatment oxidation shall be removed mechanically
all inserts, bolts, etc. shall be removed, to allow for a complete
cleaning of the part
welded repairs are acceptable, as long as the welding is continuous
and fills all spaces, to avoid pockets of contaminant
working surfaces shall be thoroughly polished to a mirror finish (
0.1 to 0.4 µm
or better). The life of the coating and the finish of the produced parts depend heavily on
the finish of the tool surface. It is highly recommended to follow the practices of the
plastics tooling industry regarding the surface finish quality
If the tool has a previous worn-out or
damaged thin film coating, it can be removed through chemical or mechanical (dry blasting)
methods. Mechanical removal requires also re-polishing of the working surfaces, with a
0.02 to 0.04 mm. dimensional change on each surface. Areas with deep surface damage or
fatigue cracks shall be removed and welded or bolted inserts shall be installed.
The cost of the coating process is 2 to 3
times higher than hard chromium, 2 times lower than CVD and 3 to 4 times lower than TD. The economical return varies with the
application, but the savings are always very significant.
CASE STUDIES
The capabilities of the
coating system and the parameters of the deposition process were tested in a variables
matrix supported by metallographic, wear machine and full production tooling tests. The following case studies document some of the
results obtained in long duration surveys of production runs:
Draw ring 200 mm diameter, 125 mm high. Using CVD
coating, the tool averaged 250,000 hits. With HI3D, the tool life increased to 550,000
hits and further improvement of the coating application process allows 7 to 800,000 hits.
Wiping blocks 140x 100 x 50 mm each, D2 steel 58
– 60 HRC. CVD type coating produced 30,000 hits. TD coating, at 4 – 500,000
hits, drastically enhanced the performance. The HI3D type of coating produced over 500,000
hits until now and is still running.
Piercing punches 5.65 mm diameter, M2 steel.
Fig.2 – Piercing punch after 300,000 hits
CVD coating
produced repeatedly 10,000 hits. HI3D produces now 250 – 300,000 hits. Figure 2 depicts the punch at the end of life.
The component blocks of a large stamping assembly,
producing full pick-up truck chassis lon-gitudinal beams, work under very difficult
conditions on a 4,000 t. press. The product is made of high strength hot rolled steel
with a very rough, hard surface. The HI3D coating increased this tool life 2.5 - 3 times
vs. CVD. A partial view of this tool assembly is presented in Figure 3
Fig.3 – Truck longitudinal frame stamping tool, with high wear blocks coated by HI3D
The wear mechanism of the
coating in the high stress areas is evident in Figures 4 and 5. Figure 4 depicts the
failure of the radius region through compressive and sheer loading, leading to abrasive
wear and fatigue.
Fig.4 – Coating failure mechanism on a high load radius
The longitudinal cracks in
Figure 5 result from fatigue due to cyclical compressive stresses in substrate, as result
of the friction component of vertical force applied by the press. The complexity of the
part in this area requires redistribution of the hot rolled material, generating very high
pressure on the tool surface and a high resultant frictional force in the direction of
tool movement. The hardness, strength and adhesion of the coating in such areas are of
vital importance for the life of the coating and of the tool material under it.
Fig.5 – Fatigue failure of the tool substrate and coating
Work is in progress to reduce
the coefficient of friction of the coating. This improvement will solve the problem of
lubricants being squeezed completely out of the areas of very high pressure, resulting in
metal to metal contact. In an uncoated die such contact produces cold welding, pick-up and
part or tool damage. Combined with the separating effect of the coating between tool and
the processed material, the low friction will increase tool life further.
Tooling for stamping of seat slides in
Figure 6 produced 15 – 18,000 parts with CVD coating. Using HID, the tool produces
approx. 200,000 parts consistently, in long production runs.
Fig.6 – Coated seat slide tooling and the finished parts
The capability of the
technology to coat large stamping tools is illustrated in Figure 7, on a die 700 x 700 x
150 mm.
The technology can increase
considerably the life of the large body panels tools, by coating the inserts used in
achitecturally difficult areas subjected to high loads.
Fig.7 – HID coating on a large die (700 x 700 x 150 mm)
The savings are not limited to the
increased tool life, but extend in a number of other areas:
CONCLUSION
The High Ionization Deposition
thin film hard coating technology developed into a full fledged solution for wear and
pick-up control in large stamping tools used in difficult applications. By increasing the
surface hardness to 85 - 95 HRC equivalent and especially by modifying coating/substrate
interface failure mechanism, the life of the tool is increased considerably (2-30 times
vs. uncoated), at improved parts’ finish and higher process speed.
CONTACT
Dr. Segal can be contacted at Sputtek
Inc, 1 Goodmark Place #4, Toronto, ON – Canada, tel 416/213-9833, fax 416/213-9834,
E-mail info@sputtek.com or
Website www.sputtek.com |
