Cutting Tool Coatings: The Demand to Increase Performance

tool coating

As a machinist, owner, or OEM who wants to increase the profitability of your cutting tools (or has experienced the increasing pressure to improve their performance), you should have a general knowledge on tool coatings and coating processes. Tool coatings are constantly evolving to provide even better performance in the most demanding applications. In this article, we examine how tool coating has evolved and what’s to come in the future. For the purpose of this article, “tools” will include both cutting and forming tools, including drills, mills, reamers, taps and dies, broaches, molds, punches and dies, drawing dies, and forging tools.

MANUFACTURERS’ HESITANCY TO APPLY TOOL COATING

A quick review of the history of PVD coatings for cutting tools is a good place to start. In 1980, ultra-thin film Titanium Nitride (TiN) was commercially released as a tool coating (even though it was an available technology since the early 1970s). Perhaps it wasn’t commercially implemented earlier because of the influence of business. The life cycle of a cutting tool can be significantly extended when this type of hard coating is applied to the cutting edge of a machine tool bit. However, extended tool life leads to less new tool sales, which isn’t in the best interest of tool manufacturers.

This scenario can be compared to when Eastman Kodak purposely delayed their entry into digital photography as a way to extend the life cycle and high profit margins of conventional photographic films. This was a deliberate decision to delay the introduction of the imaging sensors that had been developed in the Kodak Research labs so that the “cash cow” of film could be fully “milked”.  The rapid switch to digital photography eventually crushed photographic film and paper sales for all of the corporations that manufactured it. The difference with tool coatings is that they are a performance add-on that still allows for the sale of the tools; it is not a replacement technology.

TOOL COATING INCREASED LIFE CYCLE FROM 5 TO 125 HOLES TAPPED

One memorable  example of this increased tool life is an experience we had with a tap manufacturer. In the early 2000s, the manufacturer came to ACS for help with an application of deep hole tapping into cast iron. Their customer was getting only 4–5 holes tapped per tool. We suggested a coating and the manufacturer tested the tools in house before sharing the results with the customer. Tool life extended over 25X in the tapping application. The tap manufacturer informed us that they told the customer it was “an unsuccessful coating test,” because they didn’t want to offer the coating and sell 25X less taps.

EMBRACING TOOL COATING TECHNOLOGY

In the early 1980s, one of the tool manufacturers made the business decision to embrace the revolutionary and game changing technology by offering coatings as a premium product offering. This forced other manufacturers to follow their lead. Momentum took a few years to gain traction, but that initial start was enough to encourage process engineers to design new and improved coatings other than TiN.

TOOL COATING SOLVES MORE ISSUES (ADHESION, HEAT, FRICTION)

These initial coatings were designed to keep the sharpened tool edge in a sharp condition for a longer time. Then, as TiN was put into use, people began looking to PVD coatings to solve other tooling issues, including the sticking of the material being machined to the flutes and cutting tip, heat build-up at the tip, and friction. TiCN, TiAlN, TiAlCN, and AlTiN were all variations of the composition of TiN that helped solve these issues. Even today, many shops routinely request TiN as a universal coating for all of their tools, and don’t want to discuss other options. While we respect those decisions, we continually attempt to find pertinent ways to educate the community about how the technology has continued to evolve to provide improved options.

ADVANCING COATING STRUCTURES & GOING BEYOND TIN COATING

Since the late 1980s until recently, there has been a strategy focused on modifying the film structures from a simple single layer coating to multilayers, to gradient film structures, to nanolayers, to nanocomposites, and the modification of the compositional materials of the film to form new materials. But more importantly, there has been a recent focus on the synergy and interaction that exists between the tool design (geometry), the tool material and grade (substrate), pre and post treatment of the cutting edges, the coating, the material being machined, and the method of machining (application).

While TiN is still the workhorse in a number of machining applications in the industry by default (because of its relative low cost and familiarity to the end users), there are many preferred coatings that can offer improved performance, especially when the synergistic effects are factored into the equation.

“STATE OF THE ART” TOOL COATINGS RISING IN DEMAND

This new synergy focus is made evident by the media releases, custom solutions, and patent applications submitted by tool designers and manufacturers. Below are some examples of how manufacturers are going about this, as well as some other recent developments:

  • Manufacturers gather details from customers related to how the tool is being used, what material is being machined, if coolants during machining are a requirement or an option, speed or horsepower limitations of older machines, and a variety of other pertinent, but not always obvious, questions to investigate specific performance benefits.
  • Coating specifications (such a film structure, thickness, and composition) are being optimized for the tool type, design, and machining application.
  • Composite machining manufacturers are using CVD diamond coatings to produce chemically inert films that fill a niche (especially for the aerospace industry where the increased cost of the coating can be justified by the industry).
  • Pre-treatments (such as edge honing) are being used to microscopically round the cutting edge of tools. This solution addresses how PVD coatings don’t always grow in “perfect” structure on a highly sharpened edge. The minutely de-sharpened edge gains a disproportionately longer lifetime because the honed edge can be better protected by the hard coating.
  • Post-treatments (such as honing or polishing) are being used to optimize the surface finish of the tool, which improves its performance. However, the average tool regrind provider isn’t interested in a performance enhancement that requires them to pay for extra labor and processing equipment. Their main concern is to have fresh, functional coating applied that looks cosmetically pleasing on the surfaces that they have reground. Nevertheless, new tool manufacturers are increasingly being pressured to provide the newest enhancements to differentiate themselves from their competition.

HOW PVD COATING PROVIDERS ARE RESPONDING TO PERFORMANCE DEMANDS

The PVD coating industry is responding to this demand from the larger tool manufacturers by taking advantage of emerging technologies and processes. Cathodic arc deposition has been the workhorse of the tool coating industry for a number of years. This is due to the high deposition rates and highly ionized deposition plumes that allow for optimum film adhesion,  favorable film structures and short coating cycles.

E-Beam evaporation was one of the earliest deposition techniques for TiN, and is still used today for specific applications. Recent enhancements to the cathodic arc technology are the addition of DLC (Diamond Like Carbon) or amorphous carbon top coatings to the available film options, the incorporation of nanocomposite films into the film structures, and the development of oxi-nitride and oxide top coatings.

Each of these provide chemical and thermal isolation and reduced friction. Some manufacturers have tried to maximize the capital investment in the vacuum vessel by offering both cathodic arc and sputtering cathodes within the same chamber in an attempt to offer both technologies to customers.

As the next generation of sputtered films are being developed, designers of thin film  coatings  are finally able to see a commercial hard-coating process developed around the specialty power supplies for HPPMS (or HIPMS) sputtering. One proven benefit is the opportunity to provide films that are much smoother than the cathodic arc produced films. Once again, the business case needs to be evaluated for the battle between increased manufacturing costs versus improved film properties on the tools. At the end of the day, customers will validate the practical usefulness of this technology if they decide to demand the coatings made possible by HIPMS/HPPMS.

WHY TOOL COATINGS?

While not every coating application technique has been discussed, we hope that this article has helped you understand that coatings are only performance add-ons, providing financially beneficial upgrades in the right circumstances. No coating can resolve the selection of wrong tooling material, incorrect grinding geometry, or low quality tool blanks. If the uncoated tool is fully optimized, the coating can serve as a value added and cost effective performance boost that effectively reduces the cost of manufacturing for the end customer. Improperly optimized coatings can become nothing more than expensive window dressings that cause the machinist to question the usefulness of any coatings.

“NEW” TOOL COATING TAKES TIME TO BE ACCEPTED

What we have learned over time is that a “new” tool coating may not come into full acceptance for a number of years after its commercial introduction. Given ISO procedures and risk assessment in manufacturing, new coatings may not be trialed in production until manufacturing is in a real dilemma, when no other solution is on the horizon. Even then, you better have some promising hard data that shows a measurable improvement in a similar application if you hope to convince a manufacturer to test your “new and improved” coating.

PARTNERSHIP BENEFITS BETWEEN COATER & END USER

Whether dealing with the high base cost of some exotic tooling, or low cost jobber drills, any coating applied must be cost effective and provide a beneficial outcome on multiple levels. We have learned the many benefits of a true business partnership between the coater and end user. When both parties are committed to mutual success, a number of iterations can be tested in a specific application to find the optimized combination of tool design, construction and hard coating where true synergy exists. To see an example of how this works, read our article on how chip-breaker design and nanocomposite coating reduced machining cost and downtime.