When first introduced in the late 1970s, cutting tool coatings—especially titanium nitride (TiN)—were embraced by tool manufacturers for their ability to extend tool life. As workforce materials have expanded from conventional ferrous and nonferrous metals to exotic alloys, composites, ceramics, and others, coatings have likewise progressed and, thanks to new formulations and deposition methods, are extending cutting tool capabilities as well as tool life.
The past five years have seen major developments in both coating composition and technology based on the response to individual customer needs. “Modern coatings are increasing tool life by addressing both abrasion and thermal resistance, thereby enabling higher metal-removal rates and greater production capabilities,” said Brian Hamil, vice president, product development, at Kyocera SGS Precision Tools Inc. (Munroe Falls, OH).
His associate, Jon Paggett, director of coating development, agreed: “Hard coatings increase resistance to oxidation and lower the coefficient of friction, reducing built-up edge. Multilayer coatings, for example, inhibit crack propagation by the deflection of cracks at layer interfaces.”
The considerable improvements in today’s coatings have been made possible by advances in nanotechnology and the ability to incorporate micro-thin layers of various separate coatings. Pat Miller, senior technical manager at Bohler Uddeholm (Elgin, IL), a specialist in the development of substrates, commented: “It is important that the substrates possess a greater uniformity of microstructure to ensure both strength and consistency throughout the process.”
Larry Williams, director of sales, medical/orthopedic for eifeler Coatings Technology Inc. (St. Charles, IL), Bohler Uddeholm’s sister company, explained: “Multilayered coatings build more than one benefit onto a tool. The nanostructure enables layers to be applied separately to deliver a combined package of benefits. TiN, used as a thicker base coating, creates a greater bonding to the substrate, while alternate layers of selected coatings address such characteristics as thermal resistance and smoothness. Coatings are engineered for specific applications, considering each of their individual properties.”
Mark Falkingham, process and applications engineer at eifeler, emphasized the growing importance of thermally-resistant coatings to major manufacturers. “For economic, environmental and manufacturing reasons, companies are moving toward dry machining to eliminate the costs and problems associated with coolant use and disposal,” he said. “A cutting tool with the correct geometry and a thermally-resistant coating can meet high-speed production goals with the added benefits of excellent chip evacuation and longer life without the need for coolant.”
Coating Process Evolution
Early coating processes were accomplished via chemical vapor deposition (CVD), a procedure in which tools are subject to high temperatures (900–1200º F). Gases introduced into the coating chamber then react and deposit a layer of coating on individual tools. A later method—physical vapor deposition (PVD)—combines exposing tools to a negative electrical charge and concurrently vaporizing the desired coating material with an electron beam that imparts a positive charge to the ions. When nitrogen gas is introduced, the coating process occurs.
There is general agreement among the sources interviewed for this article that comparing CVD and PVD is an “apples and oranges” situation, with each process useful for specific applications. More recent developments in the industry, however, favor PVD. For example, CemeCon Inc. (Horseheads, NY), which produces coating machines, coatings and coating services, recently developed the HiPIMS system (High-Power Impulse Magnetron Sputtering), according to Gary Lake, CemeCon president. The system, which can vaporize virtually any material, excites the plasma surrounding the tools, with the result that deposition rates are higher than conventional PVD, and coatings are thicker, smoother and less subject to stress.
PVD coatings are quite versatile, according to Mike Schultz, co-owner of Surface Solutions Inc. (Fridley, MN), a supplier of high-performance PVD coatings for cutting tool manufacturers. “Most of our customers use PVD due to temperature constraints. CVD coatings are applied at a high temperature, which requires re-heat treating HSS and other tool steels,” he said. “With PVD coatings, industrial tools made of tool steels can be coated, then put directly into service, eliminating the need for any post-coat heat treating and reducing the possibility of tool distortion or softening. On carbide inserts, CVD coatings require that the edges of the insert be radiused. PVD coatings can be applied to sharp edges, which work great on milling inserts.”
Williams of eifeler commented: “PVD is the method of choice for conventional cutting tools such as drills and end mills. CVD processes, using much greater temperatures, will anneal the substrate, but PVD, referred to as the ‘cold process,’ protects and maintains the integrity of the substrate and offers optimum tool life.”
Diamond Shines Brightly
Although PVD has developed more rapidly, the superiority of CVD in the application of diamond coatings has spurred renewed interest in view of diamond’s superior cutting capabilities, especially in newly developed work materials.
RobbJack Corp. (Lincoln, CA) specializes in diamond tools, including both solid-diamond and diamond coated. Mike MacArthur, vice president of engineering, noted some recent innovations in diamond coatings. “We see an influx of application-specific coatings, largely prompted by the increasing use of graphite and composite work materials,” he said. “RobbJack can vary the structure of the diamond coatings to meet customer requirements. In so doing, we take into account all aspects of the part and the material, including the orientation of the fibers and the resins. In addition to the coating, the tool needs a special geometry to be effective.”
Growth in the demand for diamond coatings is fueled by several factors, including new materials such as green ceramics (which are extremely abrasive), the need in certain industries such as moldmaking for tolerances that can extend to millions of an inch, and the fact that diamond-coated tools can extend tool life 10–25 times of that of carbide.
As impressive as diamond coatings are, they are most efficiently used for specific applications and are not suitable for working with steel alloys, noted Jeff Davis, vice president of engineering for Harvey Tool Co. (Rowley, MA), a supplier of specialty miniature tools under the Harvey Tool name and larger-diameter high-performance standard products under the Helical Solutions brand. “Different coatings address different concerns,” said Davis. “Some coatings, when used improperly, can cause problems, including stickiness and galling. Although diamond works well in graphite and composites, ferrous applications could result in excessive thermal build-up, coating breakdown, and damage for both the tool and the part.”
Mike Wochna, president of Melin Tool Co. (Cleveland), commented on the development of DLC—diamond-like coating: “DLC is really the ‘poor man’s diamond
coating.’ A PVD product is effective in cutting materials that normally respond to diamond, but it doesn’t offer the longer tool life of a true diamond coating. It’s important to remember that certain varieties of diamond, in terms of composition and crystalline structure, are specific to composite and some to graphite. It’s critical to match the tool to the coating.” He added that another factor in the mix is the end user’s equipment. If the machine isn’t fitted for high-precision operation and workholding, the benefits of the most precise coating will be lost.
The increased cost of coated tools, as well as the fact that coatings are typically applied to tools developed for specialty applications, has influenced the extent to which such tools are re-sharpened. “Generally speaking, re-sharpening is more practical for non-diamond tools,” said CemeCon’s Lake.
MacArthur of RobbJack agreed. “Diamond can’t be re-sharpened given the fact that the heat involved would be destructive to the tool. DLC, on the other hand, can be re-sharpened and recoated, as can most PVD tools. Some industries are extremely strict about resharpening. In fact, while some in the aircraft industry do resharpening, others choose not to resharpen tools due to the possibility of scraping an expensive, high-value part.” Falkingham of eifeler cautioned that although recoating is prevalent, PVD coating can spall if there are too many recoats. “There’s definitely a limit to the number of times a tool can be effectively reconditioned and recoated,” he said.
Coatings: Not Just for Cutting Tools…
Other factors are also involved in tool reconditioning. “The effectiveness of recoating depends on how worn the tool is and the types of coatings that have been applied,” said Kyocera SGS’ Hamil. Also, too many recoats can result in delamination of the coating due to increasing residual stress with increasing thickness, according to Paggett, his associate.
Another issue is that when tools will be recoated, that factor needs to be designed-in. “It’s common to recoat tools such as gearhobs and shaping tools, but the coating must be capable of being stripped,” said Miller of Bohler Uddeholm. “The greatest success is achieved when coatings and substrates are developed together.”
Harvey Tool’s Davis agreed. “Reconditioning a tool is most effective on high-cost tools, but these generally are developed with a proprietary geometry,” he said. “Therefore, we recommend a process that preserves the original configuration.”
Darex (Ashland, OR), a fourth-generation, family-owned business, has manufactured sharpening tools for the industrial and consumer markets for over 40 years. Jim Wiltrout, operations and engineering manager, stressed the resharpening of coated drills. “When it comes to regrinding a coated drill, the economics are obvious: a resharpened carbide drill will deliver 60–80% of the life of a new one,” he said. “During the resharpening process, the coating is removed from the conic surface but remains in the gullet, leaving the coating on the edge of the drill that contacts the workpiece. Depending on the alloy being drilled and the speeds and feeds involved, a recoat may not be necessary.”
However, when a manufacturing plant performs multiple, repetitive drilling applications, the ability to do recoating on-site would be a significant advantage. “That’s something we expect to see a good deal more of in the future,” said Wiltrout. “The manufacturing facility that can effectively re-sharpen and recoat their tooling retains much greater control of the process, lead time, and ultimately the results.”
Fast Forward to the Future
The multiple advantages derived from coating conventional and specialty tools and the progress in coating technology ensure a bright future. A growing number of cutting tool manufacturers are purchasing proprietary coating equipment and developing partnerships with the makers of their units, who in turn supply both coating materials and applications assistance. Lake of CemeCon sees an evolution in customer demand. “Originally, coatings were all about extending tool life. Now, we’re increasingly concerned with characteristics like finish.”
Among both OEMs of coating systems and the companies that provide coating services, there is a commonality in the increasing number of coating materials, both traditional and newly developed, that they source from suppliers or originate in-house. Development of a coating to meet a specific need is often an empirical process in which a number of samples might be tried before the desired effect is achieved. As Hamil explained, “Developing just the right coating is, to a great extent, like cooking: you start with the basic family recipe for spaghetti and meatballs and once you complete the basics, you add just the right spices to get the flavor you want.”