Cutting benefits of ceramic and cermet tools

“Everyone’s image of ceramic is a ceramic coffee mug that if you drop it, it shatters,” said Brian Wilshire. “How in the world can you cut metal with it?”

But as technical center manager for Kyocera Precision Tools Inc., Wilshire is quite familiar with the metalcutting capabilities of ceramic composite, as well as cermet, tools. The Hendersonville, N.C.-based toolmaker’s offerings include ceramic and cermet inserts for milling and turning. The types of ceramic tool materials include Al₂O₃, silicon nitride and silicon aluminum oxynitride (SiAlON).

NTK Cutting Tools USA

Because SiN and SiAlON are relatively tough ceramics, especially at elevated temperatures, tools made of these materials are best-suited for roughing cast iron and heat-resistant superalloys (HRSAs), Wilshire said. “The Al₂O₃-based ceramics are still pretty brittle, so those are mainly for hard turning with no interruptions or for finishing cast iron.”

According to Steve Howard, marketing and engineering manager for NTK Cutting Tools USA, Wixom, Mich., SiN-based ceramic grades, such as NTK SX6 and SP9, have twice the fracture toughness of alumina-based ceramics. This level of toughness enables turning and milling cast iron at speeds up to 3,000 sfm.

In addition, NTK’s SiAlON grades blend the strength of silicon-nitride ceramic with the heat and wear resistance of alumina oxide, Howard said. “NTK’s five grades, with SX3 being the newest, feature excellent strength and thermal shock resistance under high temperatures, as well as notching resistance, which are ideal characteristics for high-speed machining of heat-resistant alloys common in aerospace, power generation and oil and gas industries.”

NTK’s five grades of black ceramics—alumina with hard carbide added to improve toughness and hardness—have a high-hot hardness and low plasticity to turn steel, chilled or ductile iron rolls and some powdered metals as hard as 62 HRC, Howard said. “Our finest grain structure—ZC4 grade—can perform finishing cuts on steels up to 70 HRC. These ceramics are cost-effective alternatives for applications previously limited to CBNs.”

Like the name implies, cermet combines ceramic elements (such as TiC, TiCN and molybdenum carbide) with metallic binders (such as nickel, as well as molybdenum and cobalt to a lesser degree), Wilshire noted.

Cermet is more brittle than tungsten carbide, so cermet cutting tools are susceptible to chipping and breaking when stressed while taking heavy cuts, he said. As a result, more users consider cermet tools suitable for finishing only at a 0.010″ to 0.020″ DOC. However, Kyocera has developed cermet tools that can take a 0.060″ to 0.080″ DOC in “clean” material.

“If you have interruptions or scale, carbide is still going to be the way to go there,” he said.

Technical Developments

Although cermet and ceramic cutting tools have a long history, toolmakers continue to make advancements. One such advancement at Kyocera creates a gradient in cermet material via a combination of techniques employed during the insert pressing and sintering processes, Wilshire said. “We call those ‘hybrid cermets’ because they offer a hard outer layer and more toughness in the center.”

Kyocera also combines tough and hard in ceramic inserts. Wilshire explained that the company is able to produce a honeycomb-shaped grain structure in which the hexagon shape that forms the honeycomb is made of tougher ceramic than the harder central core that it surrounds. If the center part develops a crack, the crack will not propagate beyond the shell and cause the insert to break.

The MC275 Walter Prototyp ceramic milling cutter has a solid-carbide shank. Image courtesy of Walter USA

“It competes with the whisker material,” he said.

Kyocera does not produce inserts made of whisker-reinforced ceramic but, according to Wilshire, offers a second technology to improve the fracture resistance of ceramic similar to one reinforced with SiC crystals. In the company’s newer SiAlON materials, the grain structure is manipulated to cause some of the grains to become needle-shaped, which functions like whiskers. “During the sintering process, the crystals actually grow,” he said. The structure also includes hard particles for wear resistance.

With Reinforcements

Greenleaf Corp., on the other hand, is well-known for its whisker-reinforced ceramics, having introduced the uncoated WG-300 grade in 1985, said Jan Andersson, global director of the TechTeam and marketing for the Saegertown, Pa., toolmaker. Greenleaf later introduced the WG-600 coated whisker-reinforced ceramic followed by the WG-700, which has a different substrate and a nanostructured coating.

“There are a lot of applications where whisker-reinforced ceramics are still—by far—the best solution,” Andersson said.

A more recent development from Greenleaf is its XYSTIN-1 phase-toughened ceramic. It is a thermally stable, whisker- and particulate-toughened SiN with almost twice the strength of any other commercially available ceramic tool from Greenleaf, the company reports.

In theory, all ceramic tools remove metal by raising the temperature in the cutting zone to a level that causes the material to plasticize, which makes it easier to machine, Andersson said. Without plasticization, tool edge chipping and top and flank slicing can occur.

“That’s when you see catastrophic failures,” he added. “But once we get to XYSTIN-1, the theory goes out the door.”

According to Andersson, the phase-toughened ceramic’s high strength enables an XYSTIN-1 tool to efficiently cut without the need for full plasticization to occur. This ability is beneficial when turning a large and heavy or imbalanced part that’s not practical to spin at high speed. For instance, when machining an imbalanced part made of Inconel 718 with a hardness of 46 to 48 HRC, the low-end cutting speed needed for ceramics to plasticize the material is about 550 to 600 sfm. In contrast, an XYSTIN-1 insert can run at 350 to 400 sfm whereas a carbide tool would reach only 150 to 180 sfm.

“I can take the middle road and gain the productivity that you get from ceramics without being forced to hit 600 sfm,” Andersson said.

According to NTK Cutting Tools, its BIDEMICS—JX1, JX2 and a new release at IMTS—represent the latest advanced composite material for effectively machining HRSAs since the introduction of whisker ceramics. The material’s unique physical characteristics provide exceptional tool life, impart fine surface finishes and enable cutting speeds up to 1,600 sfm.

Head Strong

Another type of “hybrid” tool is available from Walter USA LLC, Waukesha, Wis. The MC275/MC075 Walter Prototyp ceramic milling cutters have a ceramic head brazed onto a solid-carbide shank. The toolmaker also offers a design with interchangeable ceramic heads that can be screwed onto a shank. The target application is milling HRSAs.

The MC275 features a universal geometry for multiple purposes while the MC075 has a geometry for achieving high feeds in challenging materials. Both designs can achieve cutting speeds up to 3,300 sfm. “We are almost reaching values that are usually seen in aluminum machining, with a feed of up to 380 ipm,” said Stefan Benkóczy, component manager of aerospace and business application development for Tübingen, Germany-headquartered Walter AG.

Kyocera manipulates the grain structure in some of its newer SiAlON ceramic turning grades, such as KS6040, to cause some of the particles to become acicular, or needle-shaped, and function like fracture-resistant whiskers. Image courtesy of Kyocera Precision Tools