Translating coating science into practical language
June 28, 2013
The ranks within the tool and die community have thinned with the shop closures and business consolidations that followed the Great Recession. As a result, the depth of knowledge about tooling coatings has decreased as well in recent years. This primer is intended to provide a quick explanation of the most common tooling coatings for those that need a quick education.
Can metal formers really make an informed decision about tool coatings for forming applications without answering all the questions on a supplier information form? (More on those questions later.)
In most instances, they can’t. They can’t know everything about all things involved in stamping because they have too many responsibilities and too few hours in the day. If they do have that knowledge, they might have retired or be looking to do so soon.
That’s why it’s good to learn a few things about tool coatings. This isn’t an in-depth study of all options available for tool protection, but it does provide a good starting point to begin the conversation.
Electroplating is one of the most common and cost-effective coating techniques for tooling. The process involves immersing a metal object with a negative charge attached to it into a solution that contains a metal salt. The metal salt contains positively charged metal ions that are attracted to the negatively charged object. As the positively charged metal ions reach the object, they encounter electrons that have been pumped from the power source. This reaction between the electrons and positively charged ions creates the metallic coating.
This approach, however, does not provide the hardness that more advanced techniques do, but it does an adequate job of protecting tooling used for basic forming operations. For instance, most large body panel dies and plastic molds are electroplated.
Nitriding, the process of binding nitrogen to a metal workpiece’s outer surface to harden it, is a step up the protection scale from electroplating, but again does not offer the advanced protection of more advanced coating techniques.
This type of coating typically is applied via three different diffusion processes:
Nitriding often is of interest to stampers that need some kind of protection for tools used in low-volume applications.
Thermal diffusion (TD) coating is another diffusion process, but this occurs at much higher temperatures—around
1,800 degrees F. Asian automakers used this type of coating widely both domestically and abroad in the 1990s because of its reliable protection.
During TD coating, tooling is placed in a salt pot and heated rapidly. The salt contains the element vanadium, and as the temperature approaches 1,800 degrees F, the vanadium reacts with the carbon at the tooling surface to create vanadium carbide, a very hard and tough layer of protection.
Of course, with this approach involving very high temperatures, the crystal structure of the tooling material changes. If engineers do not have experience working with subsequent dimensional changes in the tooling that come after TD coating application, they become frustrated very quickly. Unfortunately, a lot of engineers with TD coating experience have left the industry over the years, creating a vacuum for this expertise.
Additionally, the TD coating process sometimes can lead to die cracking in vulnerable areas—for example, sharp threads in a tapped hole. Residual stress from the thermal shock to the tool during the TD coating application reveals itself after regular use of the tool in its forming application.
Another trend working against further adoption of TD coating in the automotive industry has been the emergence of high-strength steels. TD does a good job of protecting tools in most forming applications, but when it comes to forming these advanced steels, stampers notice galling issues much more quickly than when using tools with more advanced coatings.
Chemical vapor deposition (CVD) is a process that delivers a coating similar to TD, but in a much different way. In CVD, the reactive element is not found in salt, but rather in a gas. As the interior of a controlled environment reaches 1,800 degrees F, the gases start to break down into the individual constituent parts. A chemical reaction takes place on the surface of the steel that has been placed in the chamber, and a subsequent coating is formed on the tooling.
CVD coatings, like titanium carbide, are very tough. Some stampers shy away from them, however, because, like TD coatings, the application process’s high temperatures lead to dimensional changes in the tooling. Many metal formers opt for an alternative advanced coating even though CVD coatings may prove to be the best long-term answer for tooling protection in high-volume or aggressive applications.
That often leads engineers to physical vapor deposition (PVD) coatings. Compared to other coating application processes, PVD is applied at much lower temperatures—generally less than 1,000 degrees F, but for the most part between 480 degrees F and 840 degrees F.
This coating application takes place in a vacuum, but no chemical reaction occurs. Energy, which might be a welder’s arc or some sort of electron beam bombardment, is directed at a solid material, which results in the creation of a metal vapor. The metal vapor is charged positively, while the tooling in the chamber is charged negatively. The vapor is attracted to the tool steel, but when it hits the tooling surface, it reacts with a gas such as nitrogen or acetylene that is present in the vacuum. That causes a coating to form on the tooling surface. Even though this process may seem like a chemical reaction, it is much more akin to the collection of condensation on a surface.
Certainly PVD is much more viable for engineers and tool- and diemakers worried about dimensional change in tooling. For example, most tooling materials being used today generally are heat-treated in a way that they can be PVD-coated without exceeding the tempering temperature. In this way, the tooling doesn’t change size and isn’t softened during the coating application process. Also, the PVD process can achieve a good coating on a very sharp cutting edge of a tool, such as one used for blanking or trimming; high-temperature processes struggle to accomplish that.
Duplex systems—coating proc-esses that combine two approaches, such as nitriding and PVD, have emerged in recent years. They provide the necessary level of protection—even if it is for a shorter duration when compared to TD or CVD coatings—and don’t result in dimensional changes of the tooling.
Of course, no one coating process is going to work for everyone. Each stamper must balance desired performance with protection levels and tool- and diemaking realities. That’s why they should ask the following questions when trying to choose a tool coating:
Tooling coatings can provide a much needed boost in today’s stamping environments, particularly as more challenging materials continue to hit the production floor. That’s why more companies are engaging coating suppliers before tools are even made and, in some cases, even designed.