Coating for stamping and forming tools

Three options for improved wear

STAMPING JOURNAL® MARCH 2005

March 8, 2005

By: ,

Editor's Note: This article is adapted from Yury Madorsky's and Matthew Thompson's workshop "New Developments in Wear-resistance Tool Coatings for Stamping Applications" presented at the 2nd annual STAMPING Journal® Forum: Competing to Win With Less, May 11-12, 2004, Detroit.

A technician mixes the TRD bath in preparation for coating. This high-temperature bath will be used to generate vanadium carbide coatings.

A lot of confusion exists in the stamping and forming industries about tool coating processes.

The most widely used coating processes are physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal reactive diffusion (TRD). The primary goal of each process is to protect the surface of the tool or component against abrasive, adhesive, and corrosive wear while maintaining dimensions and tolerances.

The coatings' high hardness and low coefficient of friction on the tool surface help the tool to run longer. The high hardness restricts abrasive wear, and a low coefficient of friction provides additional lubricity, which can be particularly beneficial if the lubricant flow to the tool surface is interrupted because of tight clearances or other reasons. Both coating attributes also reduce adhesive wear, which causes galling that can lead to tool failure.

It is extremely important for stampers to choose the correct coating process and composition based on the application, tool substrate, and tooling tolerances.

Figure 1
Vacuum heat treating, performed in units such as the one shown here, is recommended for all tools regardless of the coating process. The oxidation caused by non-vacuum-heat-treating processes will inhibit coating adhesion.

Tool Design Decisions

Of course, coatings are not necessarily a cure for every tooling problem. Many good decisions need to be made about the tooling before the optimal coating can be chosen. Major decisions that will impact a coating's ability to perform are material, heat-treating procedures, and surface preparation. Only after these questions have been properly addressed should a coating be chosen.

Material. The full benefits of surface coatings can be realized only if the coatings are supported by a material with a microstructure that provides a good foundation. Therefore, tool material selection is the first step in the successful design of a forming tool.

In addition to tungsten carbides and conventional tool steels, powder steels are available with many different combinations of properties that are suitable for various applications. Powdered metals have a unique microstructural characteristic: small metal carbide particles that are uniformly distributed in the steel matrix. These steels have a finer grain size and are tougher than most conventional steels.

Heat Treating. Heat treating should be done in a vacuum to prevent surface damage from oxidation or decarburization. Even heat-treating scale that has been removed by sandblasting may affect the surface coating quality. The tools should be properly tempered to equalize the thermal stresses inside the metal structure and prepare it for the subsequent coating (seeFigure 1).

All coatings have very high intrinsic stresses that may overlap with the thermal stresses from the heat treating. Most coating processes use elevated temperatures that may cause a problem with dimensional stability after coating. For this reason it is imperative that any discussion of surface coatings with a coating supplier touch on the tolerances of the specific tooling.

Surface Preparation. Surface preparation is critical in any tooling application, and especially in stamping applications. Any marks on the surface of a stamping tool will work as a nucleation site for adhesive wear. The workpiece material will flow into the microscopic imperfections on the tool surface and stay there. During further strokes, more material will build up on these areas, causing galling.

Figure 2
The TiN PVD coating on this soda tab punch helps to prevent material pickup and premature tool wear.

Galling is a common problem in metal forming and stamping operations. It results in increased roughness on the part and premature tool wear. Therefore, improving the finish of the working surfaces is a very important part of the tool preparation.

The recommended surface finish is 8 microinches or more; a high polish, if possible, often is best. After polishing, the tool must be inspected for surface quality. If machining or grinding marks are still visible, then the die needs to be repolished. Stoning before polishing will produce a uniform surface. It is extremely important that the last polishing steps be performed in the direction of the metal flow.

PVD Coatings

Physical vapor deposition (PVD) coatings refer to a family of low-temperature coating processes. Among the different technologies used for PVD, cathodic arc deposition is the most common.

The cathodic arc process is performed in a vacuum chamber in which tools are heated to temperatures generally less than 900 degrees F. The heating of the substrate enhances the adhesion characteristics of the coatings. This temperature is below the tempering temperatures of most steels used for forming and stamping tools, so there is no loss of hardness or dimensional instability.

PVD coatings such as titanium nitride (TiN), titanium carbonitride (TiCN), chromium nitride (CrN), and aluminum titanium nitride (AlTiN) work well for many applications. PVD forms a mechanical bond between the tool and coating and is suitable for tools that are closely toleranced, which are common in the stamping industry (see Figure 2).

The minimum coating thickness for metal stamping and forming applications should be no less than 4 to 5 micrometers. The coating parameters can be adjusted to create an average combined coating thickness in the range of 0.0002 to 0.0003 inch. These results must be verified before shipping the coated die or punch to the customer, and if the coating is thinner, it should be reapplied to the proper thickness.

But this begs the question: How thick can the PVD coatings be, and is thicker always better? Because of their very high hardness and formation mechanisms, PVD coatings also have very high residual stresses. These stresses are compressive, which is favorable for many applications because compressive stresses are resistant to fatigue failure.

On the other hand, stresses that are too high in the coating layer may cause the coating to delaminate from the substrate if the stresses exceed the adhesion strength. In this case, the coating "recipe" has to be modified to find the proper balance between the desired coating thickness and stability.

To solve this problem, many coating manufacturers have created multilayer coatings. The different layers and different hardness characteristics help to prevent the propagation of microfractures from layer to layer, thereby creating an overall more durable coating.

The first layer (starting from the substrate) usually is softer, and the subsequent outer layers are harder. The typical hardness of the more common PVD coatings are as follows: chromium nitride (CrN): 1,800 Vickers hardness; chromium carbide (CrC): 2,200 HV; titanium nitride (TiN): 2,400 HV; titanium carbonitride (TiCN): 3,200 HV; titanium aluminum nitride (TiAlN): 3,200 HV; and aluminum titanium nitride (AlTiN): 3,400 HV.

Figure 3
These extrusion punches are used to form AA battery casings. Because tight tolerances are specified, the TiCN PVD coating is a suitable choice for this application.

CrN coating has the lowest hardness of all the commonly used PVD coatings. Its benefit is that it can be applied more thickly because of its lower range of internal stresses. The combination of two coating compositions, such as CrN and CrC, produces both the ductility of CrN and the higher hardness of the CrC layer (seeFigure 3).

TiCN usually is applied as a multilayer TiN/TiCN coating. The TiCN layer has high hardness and commonly is used as a step up when TiN monolayer coatings are not working.

An alternative to TiN/TiCN is TiAlN and AlTiN coatings. Both TiAlN and AlTiN have a higher hardness, which gives them an advantage for resisting abrasive wear. TiAlN and AlTiN coatings have different percentages of titanium and aluminum in their chemical compositions. AlTiN has a higher hardness than TiAlN (3,200 HV versus 3,400 HV), so it provides better absorption resistance, but the better ductility of TiAlN may work better in forming some materials.

In addition to these standard PVD coating technologies and compositions, new coatings are emerging with extremely low coefficients of friction. These include a family of diamondlike carbon (DLC) coatings (such as ta-C and aC:H films); a family of metal DCL coatings (such as tungsten carbon carbide [WCC]), and dry lubricant sulfide-containing coatings (such as molybdenum disulfide [WS2]).

The dry lubricant films applied by PVD and other methods have a coefficient of friction in the 0.1 to 0.15 range— much lower than conventional PVD films. Sulfide-containing films usually are soft and thin and work best if applied on top of other PVD coatings. This combination leads to good adhesion, a decreased coefficient of friction, and increased durability of the combined coating layer.

As an example, a bearing manufacturer was using a compound punch made out of CPM M4 steel to produce custom bearings assemblies in three operations: forming the race, extruding the teeth profile, and blanking the center of the part. The workpiece material was 0.028-in.-thick annealed 1074 spring steel.

Without using a coating, the bearing manufacturer could produce about 10,000 parts before replacing the compound punch. With a CrN/CrC multilayer coating, the company produced 55,000 parts from one punch. With an AlTiN coating, productivity went up to 100,000 parts per punch. Finally, by adding a sulfide-containing dry lubricant film on top of the AlTiN coating, the company produced up to 250,000 parts before replacing the punch.

CVD Coatings

Figure 4
These extrusion punches are used to form AA battery casings. Because tight tolerances are specified, the TiCN PVD coating is a suitable choice for this application.

The chemical vapor deposition (CVD) coating process is widely used for improving the life of heavy forming and stamping tools (see Figure 4). It involves a chemical reaction between a gaseous phase and the heated surface of a substrate, carried out at about 1,900 degrees F. Since CVD coating is a gaseous process, all surfaces, including deep blind holes, may be uniformly coated. This can be helpful for coating complex die shapes.

CVD coatings usually are deposited in multilayer compositions. A TiC/TiN multilayer, for instance, provides the lubricity of TiN and the abrasion resistance of TiC. Coating thickness is in the range of 0.0002 to 0.0004 in. per surface.

Tools to be coated with the CVD process are prepared first by polishing and cleaning. Because the coatings are deposited at high temperatures, tools made out of steel are annealed during coating and then heat-treated after coating to restore core hardness. The heat treatment, which is done in a vacuum furnace, may cause some dimensional changes in the tools.

Compensation for this dimensional movement must be incorporated into the tool's manufacturing process. Communication between the tool builder and the coater is critical. It is helpful to have a print for a part and to have that print marked with precoating sizes; the coating company should be able to help with this process. In addition, heat treatment in a vacuum furnace before coating will make the dimensional changes after coating more predictable.

CVD coating nucleates and grows on the metal carbides present in the material. Excellent candidate materials for CVD coating include all cemented carbide materials, as well as AISI D, H, M, and T steels.

Steels such as A2 and S7 should not be coated with this process if the tool will undergo impact loads, because the coating temperature is higher than the austenizing temperature for these steels. Overheating will degrade the material structure, and high temperatures will cause grain growth, which in turn will negatively affect the impact strength. This may even cause tool breakage when repeated dynamic loads are applied.

TRD Coatings

Thermal reactive diffusion (TRD) is a high-temperature coating process for producing metal carbides (typically vanadium carbide) on the surface of a carbon-containing substrate (see Figure 5).

It is similar to CVD in nature, but the TRD process is carried out in a high-temperature salt bath filled with reactive chemicals in precise concentrations. This multistage coating process includes a preheating segment and a soaking segment. During the soak, parts are suspended in the salt bath for a specific time at a specific temperature to build up a coating layer of the desired thickness.

The time and temperature of the coating process can be adjusted according to the substrate material and coating application. Furthermore, the addition of some carbide-forming elements other than vanadium can potentially increase the hardness of the coating layer up to 4,000 HV.

Steels such as A2, D2, S7, and other air-hardened tool steels with hardening temperatures in the 1,650- to 1,900- degree-F range will obtain a high hardness when the tools are pulled out from the coating bath into the air. Posthardening in the vacuum furnace may be necessary for larger tools and high-speed steel substrates.

TRD coatings typically are used for the same applications as CVD coatings, and they are especially well-suited for tools that form and stamp stainless steel. The TRD process also has shown good results on tools used for shear cutting stainless steel tubes at high cycle rates.

It coats at low temperatures and can coat lower tool steel grades with a smooth coating surface. Sometimes the postcoating hardening operations can be eliminated with TRD.

The TRD process may provide better dimensional stability than CVD because of the possibility of eliminating postcoating hardening operations. Also, TRD can be done at temperatures lower than standard CVD process temperatures.

Potential Results

Of course, none of these processes alone is a one-size-fits-all solution for every stamping and forming application. Coating suppliers can help ensure that stampers select the appropriate process and composition that fit their individual application requirements.

Yury Madorsky is general manager and Matthew Thompson is sales manager with Richter Precision Inc., 1021 Commercial Ave., P.O. Box 159, East Petersburg, PA 17520, 717-560-9990, fax 717-560-8741, info@richterprecision.com, www.richterprecision.com.



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STAMPING Journal® is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping. Print subscriptions are free to qualified stamping professionals in North America.

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