The Coating Edge

Author Lisa Mitoraj
Published
February 01,2000 - 11:00am


SAC Internationa's multielement "rainbow" coating, Laser-Cut 964, was designed to increase productivity and tool life during wet or dry machining.

Tool coatings have come a long way since the introduction of TiN three decades ago. There’s no doubt that the coatings you’re using today are stronger and more cost-effective than ever.

But a new crop of coatings has emerged recently, promising better performance and higher profits for tough machining applications. Kind of makes it tempting to replace your entire coated-tool inventory, doesn’t it?

But wait! As with any new development, it’s important to weigh the potential advantages and disadvantages of the newcomers.

In the pages that follow, we’ll offer a brief update of several technologies, including multilayer, multielement and diamond coatings. We’ll also discuss how the recent push towards dry machining will affect coating R&D.

Multiple Advantages

Everyone’s talking about multilayer coatings these days. They allow users to take advantage of the strengths offered by several different materials.

Brett Wharton, sales manager at Multi-Arc Inc., St. Paul, Minn., is enthusiastic about the future of multicoatings.

"We’re working on some multilayer coatings and have had some phenomenal success with tool-life improvements," said Wharton. "One particular coating that we’re playing with now shows the ability to prevent built-up edge."

Mike Graham, a researcher at Northwestern University’s Advanced Coating Technology Group, Evanston, Ill., agreed that multilayer coatings will play an important role in the near future.

"Multilayer coatings, in general, are very promising in the near term because they are easy to make and properties that are needed for an application [can be selected]," he said.

ACTG is currently testing superhard coatings called "superlattice" coatings. They contain alternating layers of two hard materials, such as TiN/NbN, TiN/VN, NbN/VN, TiN/CrN or AlN/TiN. (Superlattice coatings are considered "nanocomposite" coatings because their ultrathin layers are measured on the nanometer scale.)

According to Graham, the coatings are twice as hard as traditional coatings such as TiN and TiCN. He has successfully used them on drills, endmills, inserts and gear-cutting hobs.

"Superlattice and other nanocomposite coatings offer the opportunity to design coating properties for specific application areas," said Graham. "[The user] has the ability to exert more control on the combinations of material properties—hardness, toughness, friction, electrical resistance—that will be advantageous for a range of applications."

Multielement Coatings

SAC International Inc., Dayton, Ohio, has come up with a unique twist to multilayer technology: multielement coatings. Its Laser-Cut 964 "rainbow" coating is similar to multilayer coatings in that it incorporates several different materials. However, Laser-Cut 964 does not consist of layers. Instead, eight different elements are combined into one superthin coating.

This coating is produced during a process called ion sputtering. First, gaseous elements are injected into a vacuum chamber at 850° F. An electrical charge causes atoms in the gases to explode, then a thin rainbow-colored film is deposited onto the tools.

According to Daryl Blessing, SAC’s CEO, Laser-Cut 964 greatly extends tool life for endmills, drills, taps and other cutting tools. "It offers three, five or even seven times longer tool life than other coatings," said Blessing. "This coating is ideal for any tool where friction and wear are a concern."

Laser-Cut 964 is extremely hard, having a Rockwell hardness of Rc 90 to 92. In addition, the high-lubricity coating has a coefficient of friction of 0.027. Although the cost of the Laser-Cut coating is approximately 25 to 30 percent higher than TiN, users often increase productivity by 25 to 50 percent, according to Blessing.

Northwestern’s Graham feels that multilayer- and multielement-coated tools are powerful additions to any toolcrib. And, he expects these "multi" technologies to continue to improve metalcutting in the future.

"Multilayer and multielement coatings, as a class, have much promise because of the variety of materials that can be chosen and the different properties that can be designed in," said Graham.

Diamond Coatings

Much has been written about the advantages of CVD diamond coatings: improved workpiece finish, lower cutting forces, less buildup and longer tool life. However, users have often complained that diamond coatings don’t adhere well to the tool substrate.

One response to this problem comes from Norton Diamond Film, Northboro, Mass. According to Brian Cline, a product engineer at Norton, its CVD diamond coatings provide excellent results without adhesion problems.

 


Shown are tungsten-carbide inserts coated with Norton Diamond Film’s DF500 CVD diamond coating for maximum wear resistance and toughness.

Cline said this is because of Norton’s proprietary coating technology, which creates a strong chemical bond and a tough, crack-deflection mechanism at the film-substrate interface. He also said that the strength and high wear-resistance of diamond coatings make them ideal for a wide variety of applications.

"[Diamond-coated tools] can definitely be used in graphite machining, ceramic and tungsten-carbide machining, abrasive or corrosive polymer machining and structural nonmetallic composites," said Cline. "In addition, they should be considered for metal-matrix-composite machining, silicon-aluminum machining, machining nonferrous metals, bimetallic machining and applications where PCD is routinely used but a superior finish is not necessary."

Thin-film diamond coatings are produced in a high-temperature vacuum chamber containing carbon. Hydrogen and methane molecules in the carbon are activated, causing pure diamond to "grow" on the substrate.

"There are several advantages offered by diamond coatings," said Cline. "These include the capability to coat complex shapes, longer tool life in a disposable tool and high-speed operation. Most importantly, it’s harder than other coatings—and the harder you make the coating, the better it performs."

Cline cautioned that it’s important to carefully study the diamond coatings available and that users should match them to their specific application. He also tells users not to expect diamond coatings to work for every application.

"Coating performance in metalcutting still varies widely depending upon the vendor," said Cline. "The most dramatic performance variations observed in the marketplace tend to be related to adhesion. If the cutting forces exceed the adhesion strength between the diamond coating and the underlying substrate, the unique properties of the coating are lost."

Norton tends to design its coatings for specific applications. For instance, its DF500 coating was designed specifically for roughing applications. The company guarantees that DF500-coated tools will not experience adhesion problems.

"It’s a very rough coating—one of the roughest hard coatings you’ll find," said Cline. "That will limit its ability to produce a fine finish, but it’s ideal for roughing and intermediate finishing."

In addition to its diamond coatings, Norton offers another alternative: freestanding CVD diamond tips that are brazed to the cutting tool like a PCD tip. Cline said that the technology permits the use of vapor-deposited diamond in applications where diamond coatings do not perform well due to adhesion, tolerance or surface-roughness limitations.

According to Cline, diamond coatings will undergo even more improvements in the near future and will gain a bigger share of the market. "A higher level of attention is likely to be given to diamond-coating surface finish and lubricity," said Cline. "Since the most wear-resistant diamond coatings tend to be the roughest coatings, multilayer diamond coatings are likely to evolve."

"The Holy Grail would be the first CBN coating. That’s going to be a significant advancement, because we would be able to run very fast in very hard materials."

Cutting Dry

One of the driving forces behind coating R&D is dry machining. Jim Maassel, president of Macro Specialty Industries Inc., Napoleon, Ohio, feels that this trend will have a big impact on the demand for coatings in the years to come.

  PVD CVD
Full name physical vapor deposition chemical vapor deposition
Process Temperature Low-200° C to 500° C High-1,000° C+
Coating Thickness 2µm to 7µm 2µm to 14µm
Materials Used TiN TiCN TiAlN TiC TiCN TiN Al2O3
Multiple Layers? No Yes
Applications
  • Drilling
  • Endmilling
  • Tools with sharp edges
  • Turning
  • Milling
  • Threading
  • Grooving
When choosing a coating process, it’s important to match the process to your specific application’s requirements. Shown are specifications for PVD and CVD, the most common coating processes. Several companies are currently testing low- and medium-temperature CVD processes, as well as alternative surface coating processes.

"The trend to run tools dry while optimizing tool life will greatly expand the influence of coatings in both the metalcutting and manufacturing industries," he said.

Graham agreed, adding that the pressure for ‘green’ engineering will continue. "Coatings that enable dry cutting or the reduced use of fluids will be more important," he said. "Cutting tool suppliers and users need to team with coating providers and independent R&D groups to provide the synergy for effective development of coatings for high-performance and cost-effective manufacturing."

Many coating manufacturers offer tool coatings that promise superior performance when machining dry. For instance, Balzers Tool Coating Inc., North Tonawanda, N.Y., offers a multilayer TiAlN and tungsten-carbide/carbon (WC/C) coating designed to facilitate dry machining. TiAlN resists wear, while WC/C has a low coefficient of friction and prevents the chip from welding to the tool.

SAC’s Blessing said that his company’s Laser-Cut 964 coating also is ideal for machining dry. "It works better with dry machining than other coatings because of its lubricity. It’s made for that," said Blessing. "You might have to lower your speed a little bit or test it through trial and error, but it works great."

The Future of Tool Coatings

With all of these developments currently on the table, it’s difficult to say what, exactly, lies ahead in the coating industry. However, our sources offered a few predictions.

"More choices will be available, along with much better tool performances," said Graham. "Users will [need] to have some coatings and process expertise so that communication with providers is effective."

Wharton agreed, adding that CBN coatings will be an important addition to the coating lineup. "I think that we’re going to see some significant advancements," he said.

Doug Bahun, an advanced manufacturing engineer at Delphi Harrison Thermal Systems, Moraine, Ohio, also believes that coatings will become increasingly important in the metalcutting industry. He envisions a more comprehensive array of coatings that will be usable with more materials and in more applications.

"I’m sure it will include a broader distribution of coated tools across the spectrum of workpiece materials," said Bahun. He projected, too, that coatings will play a vital role in solving many of metalcutting’s toughest problems.

"I think the future of metalcutting and manufacturing is extremely bright," said Bahun. "Engineers have such a wide array of possible solutions that the opportunities are almost endless. And I’m sure, too, that the creativity of chemists and materials experts will be moving forward at an ever-expanding rate."

Related Glossary Terms

  • abrasive

    abrasive

    Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

  • built-up edge ( BUE)

    built-up edge ( BUE)

    1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.

  • chemical vapor deposition ( CVD)

    chemical vapor deposition ( CVD)

    High-temperature (1,000° C or higher), atmosphere-controlled process in which a chemical reaction is induced for the purpose of depositing a coating 2µm to 12µm thick on a tool’s surface. See coated tools; PVD, physical vapor deposition.

  • chemical vapor deposition ( CVD)2

    chemical vapor deposition ( CVD)

    High-temperature (1,000° C or higher), atmosphere-controlled process in which a chemical reaction is induced for the purpose of depositing a coating 2µm to 12µm thick on a tool’s surface. See coated tools; PVD, physical vapor deposition.

  • coated tools

    coated tools

    Carbide and high-speed-steel tools coated with thin layers of aluminum oxide, titanium carbide, titanium nitride, hafnium nitride or other compounds. Coating improves a tool’s resistance to wear, allows higher machining speeds and imparts better finishes. See CVD, chemical vapor deposition; PVD, physical vapor deposition.

  • composites

    composites

    Materials composed of different elements, with one element normally embedded in another, held together by a compatible binder.

  • cubic boron nitride ( CBN)

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • endmilling

    endmilling

    Operation in which the cutter is mounted on the machine’s spindle rather than on an arbor. Commonly associated with facing operations on a milling machine.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • grooving

    grooving

    Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

  • hardness

    hardness

    Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

  • lubricity

    lubricity

    Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.

  • metalcutting ( material cutting)

    metalcutting ( material cutting)

    Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • physical vapor deposition ( PVD)

    physical vapor deposition ( PVD)

    Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

  • physical vapor deposition ( PVD)2

    physical vapor deposition ( PVD)

    Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

  • polycrystalline diamond ( PCD)

    polycrystalline diamond ( PCD)

    Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.

  • threading

    threading

    Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.

  • titanium aluminum nitride ( TiAlN)

    titanium aluminum nitride ( TiAlN)

    Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.

  • titanium carbide ( TiC)

    titanium carbide ( TiC)

    Extremely hard material added to tungsten carbide to reduce cratering and built-up edge. Also used as a tool coating. See coated tools.

  • titanium carbonitride ( TiCN)

    titanium carbonitride ( TiCN)

    Often used as a tool coating. See coated tools.

  • titanium nitride ( TiN)

    titanium nitride ( TiN)

    Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.

  • tolerance

    tolerance

    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

  • turning

    turning

    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

  • wear resistance

    wear resistance

    Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.

Author

Lisa Mitoraj is associate editor of Cutting Tool Engineering.