MMC Magazine

Artificial materials that are nearly as hard as diamond for cast iron machining

Machining tools have evolved through the development of new materials and looking at a history of machining tools wouldn’t be complete without talking about CBN (cubic Boron Nitride). CBN is the second hardest material on earth after diamond and having lower affinity to iron (Fe) than diamond makes it highly suitable for machining difficult-to-cut materials used in the automotive and other industries. For this reason, tool manufacturers around the world have competed with one another to be the first and best. How has Mitsubishi Materials maintained its position at the forefront of the industry? Let’s look at the early history of CBN and the search for next-generation solutions.

What is CBN (cubic Boron Nitride)?

CBN does not exist in nature. It is artificially synthesized under high temperatures and pressure in the same way artificial diamond is manufactured. The crystal structure of CBN is similar to diamond, making it extremely hard and heat resistant.

The development of CBN tools was closely tied to artificial diamond. As is well known, diamond is the hardest material in nature. Beyond its value as a precious stone, its characteristic properties make it an excellent material for tools such as grindstones, dies and drills.

Natural diamond powder was already being used as a polishing agent as far back as 700 B.C. in ancient India. Polishers using diamond powder were being used in Belgium in the 1400s, and diamond dies were developed in England in the 1800s for use in the manufacture of piano wire.

Full-scale research on the syntheses of diamond using ultra-high-pressure sintering began in the 1950s. It was thought that sintering artificial diamond powder to produce a sintered body would make it possible to develop tools in a wide range of forms. In 1955, the world’s first artificial synthesis of diamond was accomplished by an American company founded by Thomas Edison. In 1957, using ultra-high-temperature and pressure technology established for the artificial synthesis of diamond, the first CBN was synthesized. In 1969, CBN sintered body was commercialized.

Taking advantage of this cutting-edge technology, the Mitsubishi Material Central Research Centre (present-day Innovation Centre) started research and development of CBN tools in 1979. In 1982, Mitsubishi Materials succeeded in the development of a CBN sintered body and transferred the production line to its Gifu Plant. In 1983, MB10 and MB20 were launched. In the 1990s, MB810, MB820 and MB825 were launched to good market reviews. Compared to the standard inserts at that time however, these were expensive, which meant limited sales.

In the late 1990s, when competitors were launching general-purpose CBN tools, the Gifu Plant initiated the development of new materials that would be superior to others. The development team focused on the interfacial reaction layer between ceramic binder and CBN particles used for a sintered body. The challenge they faced was an uneven reaction layer that prevented them from increasing the strength of the CBN body. The unevenness caused wear and other damage to the tools.

In 2000 after much trial and error, Mitsubishi materials developed a unique technology called particle-activated sintering. With this they succeeded in producing an even interfacial reaction layer and achieved both reduced wear and damage. Until then, improvement of one feature was usually at the sacrifice of the other.

On November 5, 2003, Mitsubishi Materials launched a tool made using the MB8025 type of CBN, a general-purpose model based on the particle-activated sintering method. This was the beginning of second-generation CBN tools, which became the technological foundation for CBN tools that has lasted until today.

At the same time, in the early 2000s, PVD (physical vaper deposition) coating technology was applied to CBN tools by both Mitsubishi Materials and competitor companies. The Gifu Plant applied Miracle Coating technology based on titanium aluminium nitride. Coating with ceramic, which has high thermal stability, achieved significantly longer tool life compared with non-coated products. In 2005, Mitsubishi Materials launched two coated CBN tools, the MBC010, suitable for continuous machining, and the MBC020, suitable for general-purpose machining.

To enhance the superiority of CBN tools used for finishing, in addition to CBN sintered body and PVD coating, honing shapes (edge treatment) must continue to evolve. There were only three types of conventional honing, F (continuous cutting), G (light interrupted cutting), and T (interrupted cutting). However, Mitsubishi Materials combined these with A (conventional type), S (restraining chatter and burrs type;), and N (restraining wear on the cutting face) types to produce nine variations.

The Gifu Plant also hosted seminars such as the CBN College for distributers to popularize CBN tools made by Mitsubishi Materials. Combining technical innovation and sales expansion projects, Mitsubishi Materials strived to prevail over its competitors in product development.

4 2020 ~
Knowledge and Technology from Development Passed Down to the Next-Generation

CBN tool development at the Gifu Plant is constantly being passed down to the next generation. In April 2019, a new material development project was initiated with a focus on involving newer employees. They attempted a wide range of approaches from different perspectives not only to improve tool life, which is a constantly ongoing theme, but also to expand the areas of machining that will meet the demands of the modern metal cutting market.

Such approaches helped to develop new technology, such as ultra-micro-particle and heat-resistant binders. Maintaining the strength of the ultra-micro-particle binder to reduce cracks improved heat resistance to reduce crater wear. Application of ultra-multi-layer coatings also led to improved chipping and fracture resistance. In 2020, Mitsubishi Materials launched the BC8200 series for high-speed machining while achieving improved efficiency. Tool life of the BC8210 continuous cutting tool, improved to 1.4 times that of conventional products at 200m/min or greater. The tool life of the general purpose BC8220 improved to more than 1.6 times that of conventional products under high-speed machining, including interrupted cutting.