Surface Science Corporation

Publications

Paper Number 1999 – 01 - 3230
Hard Coatings for Heavy Duty Stamping Tools
Lee Segal, Rosen Tovbin

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:

reduced maintenance of tools. Elimination or reduction of pick-up results in 2 – 5 times less time for tools cleaning and repolishing. This is particularly applicable to coated stock, that requires more frequent cleaning
reduced scrap, due to increased time between tool failures. In numerous situation, quantities of defective parts are produced before the problem is noticed, the press is stopped and corrective action is taken
less downtime for presses, to correct or replace defective tools.

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 416/569-4574, E-mail lsegal@sursci.com or Website www.sursci.com