Tool-Coating Advances Continue
New coatings are going the boutique route, using multiple layers and new materials to optimize for a particular application
By Bruce Morey
"Any place you have wear is a candidate for a thin-film ceramic coating," says Fred Teeter, principal at Teeter Consulting (Sanborn, NY) and managing director of the Surface Engineering Coating Association (SECA; Amherst, NY). While the use of coatings on parts such as turbine engine components is increasing, cutting tools use the bulk of coatings. Coatings have certainly proved their worth, increasing tool life from 2 to 10 times when compared to uncoated tools, according to SECA. Newer coatings are achieving ever-higher maximum operating temperatures and hardness, and are increasingly tuned for specific properties, such as abrasive wear or toughness.
The first choice in thin-film coatings decades ago was TiN, the familiar yellow-gold coating still often used today. Dozens more have been developed in the meantime. Combined in multiple layers, coatings achieve a balance of properties not possible with a single type. As new thin-film ceramic coatings proliferate, the problem facing many engineers now may be one of too many choices, rather than too few.
"In the last five to 10 years, I've seen a marked reduction in the costs of coatings due to shorter processing times. Also, the move to more multi-layered coatings takes advantage of the strengths of different coatings, reducing the need for lubricants," says Mahesh Sukumaran, General Manager for Eifeler Coating Services (Santa Fe Springs, CA). Commenting on general trends in coatings for cutting tools, he explains that some customers still ask him about TiN, unaware of the many choices now available. Eifeler offers nine different PVD coatings that include the standard TiN, but also include ZrN, TiCN, and even AlTiSiN. One of the most popular is Exxtral Plus, a layered coating of AlTiN and AlTiCrN for machining hard steels at high speed. He recommends it for semi-dry and dry high-speed machining with carbide end mills. The company claims a thermal threshold of 800°C and a Vickers hardness of 3500 ±500 for Exxtral Plus. "The main challenges I see [in coating technology] are in increasing adhesion properties in some coatings, and reducing coating thicknesses," says Sukumaran.
"Conventional coatings are not inherently lubricious. Our multilayered coatings combine lubricity and resistance to wear. This combination reduces or eliminates the need for spray lubricants by reducing friction between the tool and workpiece, and produces better metal flow," he explains.
A noticeable trend in tool coatings is that they are becoming application specific. Christopher Halter, a manager with Oerlikon Balzers (Amherst, NY), a supplier of coatings and surface technologies aimed at improving the performance of tools and precision components, certainly sees this. Oerlikon Balzers now offers 24 different coatings. Halter describes the Oerlikon Balzers Balinit Alcrona, an AlCrN coating, as the last grade they developed as a general-purpose coating. He points to new application-specific coatings such as the Balinit Aldura for milling hardened tool steels with hardness exceeding RC 50 as the trend for the future. Another is Balinit Helica, an AlCr-based coating designed specifically for twist drills.
Balinit Aldura uses an aluminum-chromium-nitride (AlCrN) based functional layer deposited onto a TiAlN support layer, all on a carbide substrate. The TiAlN ensures good adhesion and mechanical strength, while the AlCrN layer features excellent hothardness and oxidation resistance (up to 1100°C) and insulates the tool from the heat of cutting.
Oerlikon Balzers has developed a new coating technology that should garner interest from tooling engineers: P3e (pulse-enhanced electron emission). For the first time, aluminum-oxide-based coatings with high hardness, as well as thermal and chemical stability, can be created by a PVD process, according to Halter. The first application of P3e technology is a coating Oerlikon Balzers calls X3turn. It has tested well in turning applications. Under dry cutting conditions, he states, X3turn already matches the performance of conventional CVD-coated insert grades. Interrupted cutting conditions and cooled machining can employ this new technology to advantage, according to Oerlikon. With a deposition temperature below 600°C, all fine and very-fine cemented carbide substrates are now coated with thermodynamically stable, alpha-aluminum oxide-based coatings without distorting the tool. "We expect that the new P3e technology opens up new applications and improvements of productivity for users of turning and milling inserts," says Halter.
Platit (Grenchen, Switzerland), a coatings company that sells machines to tooling companies, has a lot to say about layers. Platit develops and produces coating equipment based on PVD, employing patented technology that allows not only layered, nanolayer, and gradient coatings, but also nanocomposite structures. Nanolayers are simply a finer version of multilayers, where each layer is less than 20-nm thick. Hardness depends on the thickness of alternating layers. There may be hundreds of such layers.
Nanocomposites are composed of typical coatings, such as TiAlN, embedded in an amorphous matrix of Si3N4. The resulting nanocomposite improves hardness, lubricity, and maximum operating temperatures. Platit also offers a patented triple coating composed of an adhesion layer, an intermediate layer (mono or gradient), and a nanocomposite top layer for maximum wear resistance and toughness: for example, their nACo3 triple coating, which has a bottom layer of TiN, a middle layer of AlTiN, and a nanocomposite top layer nACo. nACo is itself a nanocomposite of AlTiN embedded in an amorphous matrix of Si3N4M. This coating boasts a hardness of 38–45 GPa and a max operating temperature up to 1200°C, according to data provided by Platit.
The company's coating units work on the conventional cathodic ARC principle and on patented LARC (LAteral Rotating Cathodes) and CERC (Central Rotating Cathodes) technologies. Platit offers 29 separate coatings, and machines come standard with three "base" coatings—TiN, TiCN, and TiAlN—which the company says currently make up more than 80% of the world's coating market. The other coatings Platit offers are formulated for particular applications, such as 17 optional coatings, six nanocoatings, and three of the specialized triple coatings.
The three pillars of tool design are substrate, coating, and geometry, says Sherwood Bollier, president and CEO of Niagara Cutter (Amherst, NY). While new coatings enable performance improvements, he contends that you will not necessarily get such improvements without optimizing the whole tool.
"It's easy to have a new coating with a new color and higher hardness and maximum temperature. However, what matters is the performance [of the tool]. For new coatings, we test the tool's performance and make adjustments as needed," says Bollier. "Once a coating has been selected for the tool material and the cutting geometry, we are discovering that edge preparation is very important." Niagara Cutter's expertise is in round cutting tools such as end mills and—through its microend mill division—in micro milling tools used in medical and electronics manufacturing. The appropriate edge preparation depends upon both the coating used and the material cut. One that works well for aluminum may not work for cast iron. He also describes secondary processes, like polishing the coating, as being as important as proper tool design and edge preparation.
Bollier explains that while coatings keep the edge sharp, it can be too sharp. Uncoated edges wear a small amount after the first few cuts, 'settling' in and eliminating chatter. Preconditioning the tool with the right edge preparation simulates an uncoated tool 'settling' in during its first cuts.
"We have several new coatings that are substantially better, but they have been tuned to that application," says Bollier. As an example, Balzers recommended a newer AlCrN coating called Alcrona. Alcrona had proven itself in gear hobs. It seemed natural to extend it to end mills. "We had to work hard to make Alcrona work for us. It really came down to a proper edge preparation. Once we had run enough experiments and testing, we could extend Alcrona into our uses." For a given test case, Niagara reports that it doubled a user's tool life at speeds and feeds that were increased by 35%.
Sandvik Coromant (Fair Lawn, NJ) has an extensive coating-engineering team. Although Coromant could outsource the coating of its tools, the company believes it's vital to have the expertise and capability in-house. A total cutting solution comes from matching coatings with the right substrate, geometry, and edge preparation, according to Margareta Palsson, global manager—grade management for Sandvik AB.
"Every application has small differences in needs that translate into significant differences in throughput," says Palsson. "We work to fine tune the product process to fit the application."
Sandvik develops six to eight new grades each year, some for new applications, others that are improvements over existing coatings. Coromant believes in a thorough testing program when matching coatings to tools and tests 25,000 edges per year, generating as much as 45 tons (40.8 t) of chips, according to Palsson.
For example, this year Sandvik introduced its GC4205 grade for steel turning that boasts a new MT-CVD coating of TiCN/Al203 designed for high metal removal in stable steel machining at high edge temperatures (1200°C plus).
With hundreds of cutting inserts to choose from, Sandvik Coromant recognized the problem of overloading users with too much information. CoroKey is Sandvik's way to reduce the choices available to an engineer based on his or her specific needs. The user selects a category of workpiece material, the type of application, machining conditions, and cutting data, and the CoroKey helps them choose the types of cutting inserts that would be applicable. Although coatings are a significant element of each tool, CoroKey helps the engineer select the right tool by matching materials and applications to a recommended cutting depth, feed, and speed.
Future applications Palsson believes will require extensive tool-performance engineering include developing tools—and their coatings—for the newer aerospace titanium alloys, composites, powder metals, and compacted graphitic iron for automotive engine blocks. "In each of these cases, small differences in atomic structure, proportions of metals, smoothness, edge preparation, and crystal orientation can make a big difference in throughput," says Palsson.
Exploiting crystal structure is what Seco Tools (Warren, MI) did to create its new process and coating it calls DurAtomic. Tilting the crystal orientation of its CVD-applied Al2O3 layer brings a harder and tougher direction into the cut, according to Don Graham, product manager—turning. "We orient the crystals at an atomic scale," he explains. "This increases both the toughness and wear resistance of a cutting insert—two features that usually go in the opposite direction. Tilting each crystal grain even just a few degrees increases its toughness substantially." The DurAtomic coating has a TiCN lower layer topped by the new Al2O3. Earlier grades such as Seco's TP000 series used an additional topcoat of titanium nitride, now no longer needed due to the strength and smoothness of the aluminum oxide.
Seco's first DurAtomic-developed coating, turning grade TP2500, offers increased wear resistance and toughness in a wide variety of steel applications. Additionally, TP2500 optimizes stainless turning and cast-iron applications, effectively machining materials from K10 to M40. Field-tested in more than 80 applications, TP2500 is used in roughing, facing, and finishing in a variety of materials. "The TP2500 coating also shows reduced edge buildup," says Graham. "Field tests have shown up to 25% improved productivity in combination with a 40% increase in tool life over general-purpose coated tools. We measure an 11% increase in hardness. The tool has been observed running 3–4% cooler with the new coating."
Iscar Metals Inc. (Arlington, TX) has developed a new coating technology for its new Sumo Tec line of tungstencarbide grade milling, holemaking, turning, and parting tools. Using both CVD and PVD processes, the Sumo Tec coating technology builds on the company's existing ALTec PVD and Alpha-Tec CVD coating technologies. "The most common failure mechanism that people experience is chipping on the edges," says Mike Gadzinski, national training manager, Iscar Metals. "We typically use a three layer coating—a TiCN bottom layer, a middle layer of Al2O3 or AlTiN, and a thin finish layer of TiN. The TiCN improves resistance to chipping, while the middle layer provides a heat barrier." To improve on this basic coating, Iscar uses a proprietary post processing step to produce a smooth surface that increases resistance to both heat and chipping. "In tests against previous products, we see anywhere from 15 to 80% improvement in tool life," says Gadzinski.
This article was first published in the April 2008 edition of Manufacturing Engineering magazine.