MMC Magazine

Contributing significantly to stable machining of difficult-to-cut materials

Cutting tools are like people working behind the scenes. They work out of the spotlight to steadily support the advancement of industrial products. This advancement has passed a few major milestones. Cutting tool material has progressed from high-speed steel, called Haisu in Japanese; introduced at the end of the 19th century, to cemented carbide. Then much later the coating method was developed and this represented significant progress by coating the surface of cemented carbide with a very hard, thin ceramic film layer. We interviewed staff of the Innovation Center (former Central Research Institute) about the technological development that created cemented carbide tools using the CVD method.

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What is CVD coating technology?

Chemical vapor deposition (CVD) coa­ting is a process that heats mixed gases to 800 to 1100 degrees C to deposit hard ceramics on the surface of the cemented carbide. First, titanium carbon (TiC) was developed as a coating material, followed by titanium nitride (TiN), titanium carbon nitride (TiCN), aluminium oxide (Al₂O₃) and others. Currently, multi-layering tech­no­logy using bonded layers is deve­loping rapidly and has become the main trend.

Characteristics of CVD Coating
• Enhancing adhesion and crystal orientation controlling technology and drastically improving stability and wear resistance
• Significantly improving thermal stability and wear resistance for high-speed cutting
• Achieving reliable cutting for a broader range of machining

Part 1 : 1970~

Establishing mass production technology using decompression coating

In 1969, the world’s first coated cemented carbide (CVD) tool was introduced. CVD tools are made by coating the surface of cemented carbide tools with a very hard, thin ceramic film layer. The first CVD tool was introduced to the market by WIDIA, an old-established cemented carbide manu­facturer in West Germany. A few months later, the Swedish manufacturer Sandvik, also started selling CVD coated tools.

In the 1970s, CVD tools with an Al₂O₃ layer on top of a TiC coating were released by several manufacturers. These are the ori­ginal type of the present-day CVD multi-layer coating.

Mitsubishi Materials also began research on coatings in the late 1960s, and worked on the CVD technology development at both the former Tokyo Plant in Shinagawa and the former Central Research Institute in Omiya. Based on the results of this research and development, Mitsubishi Materials released CVD coated tools in 1971.

Mitsubishi Materials first conducted research on coating technology for TiC and Al₂O₃ separately, and then expanded the research to adhesion technology to bond these two different films. This adhesion technology led to TOUGH-Grip and Super TOUGH-Grip development.

Meanwhile, the Company also began the development of mass production technology. Coating was initially performed under increased atmospheric pressure. Theore­tically, diffusion of gas components is accelerated under decompression condi­tions, which makes it possible to process a large amount of high-quality film. The Company established mass production techno­logy through the development of outstanding equipment and decompression processing technology.

The Company worked on multi-layer coating using adhesion technology in the mid-1970s. Ti compounds with high wear resistance and a multi-layer structure of chemically stable Al₂O₃ satisfied the conditions required for machining in a well-balanced manner. Research on high-intensity adhesion of these two layers confirmed that using TiCO layer as the middle layer maximized the adhesion. Using this technology, the Company released U77 in 1977.

Part 2 : 1980~

Development of Cobalt diffusion prevention technology to further improve wear resistance

The next issue was preventing the diffusion of cobalt contained in substrates. CVD processing at 1000 degrees C causes cobalt diffusion. Diffused cobalt from the substrates enters into the TiCN layer above it, and the ceramic layer becomes a composite material composed of ceramic and metal materials (cermet), resulting in reduced wear resistance.

To address this, we established a barrier technology to prevent cobalt diffusion. Specifically, it’s a new method of using a highly activated gas, acetonitrile (CH₃CN). Being highly activated, CH₃CN can produce coatings at about 100 degrees C lower than conventional CH₄ gases. The low temperature significantly reduces cobalt diffusion from substrates, which made it possible to create a TiCN layer with high crystallinity and a columnar structure. This has remained the standard technology even 30 years after its development.

The major products using this technology are the UC6010 and UC6025 series that were launched in 1992. However, because of the outstanding technology, both products continued to enjoy great success even after 2000.

Part 3 : 1990~

Development of a new manufacturing method responding to the needs of high-speed and high-efficiency machining equivalent to a patented competitor's technology

In the 1990s, Mitsubishi Materials focused on the development of technology to create thick Al₂O₃ coating. The Al₂O₃ layer is produced by the reaction of AlCl₃ and H₂O, which is generated by a gas phase reaction between H₂ and CO₂. However, the speed of Al₂O₃ production is extremely fast, which makes it extremely difficult to produce coating with uniform quality.

Meanwhile, as the need for high-speed and high-efficiency machining increased in the market, the need for coatings with thick Al₂O₃ increased as well. At that time, there was a way to add an extremely small amount of H₂S to reactant gases to form thick coating while maintaining uniform quality. However, because this technology was patented by an overseas competitor. Mitsubishi Materials needed to develop a new method.

To do this, we performed repeated testing using a wide variety of gases while clarifying the mechanism of thick coating formation. We ultimately succeeded in ensuring uniformity in quality equivalent to that achieved by adding H₂S (patented by a competitor) using NO as the source of oxygen in the inert gas atmosphere.

Part 4 : 2000~

Aiming to develop harder, more stable, more wear-resistant cutting tools

Entering the 21st century, Mitsubishi Materials started improving the wear resistance of the Al₂O₃ layer. When thermally transforming k-Al₂O₃ in a metastable phase at 1050 degrees C, α-Al₂O₃ in a stable phase is formed. We found that this α-Al₂O₃ has excellent wear resistance. Using this discovery, we established a technology to form a coating by controlling the growth of α-Al₂O₃ with a c-axis orientation. We worked on the development of the technology that naturally orients crystals to improve the hardness. This became Nano Texture Technology and has been further expanded into Super Nano Texture Technology.

life by increasing the thickness of Al₂O₃, which led to an increase in the thickness of coatings. The patent we obtained for the series of such Al₂O₃ coating forming technology led to the establishment of the strong presence that the Mitsubishi Corporation has throughout the industry today.

We started developing TOUGH-Grip techno­logy, which increased the adhesion of TiCN and Al₂O₃, around the year 2010. Before then, we had already conducted research on adhesion technology for these two coatings while working separately on the coating technology for TiC and Al₂O₃. Different materials are layered in CVD coating, therefore what needs paying the closest attention to during CVD coating is making fine layers from the base material. An important point is the thermal expansion coefficients that change according to the differences in materials. TOUGH-Grip technology expands the adhesion area of the lower TiCN layer and the upper Al₂O₃ layer through microstructure refinement, leading to improved adhesion and the prevention of peeling.

Part 5 : Future Vision~

Technological development aiming for four to five years from now is key for commercialization in 2030

We are looking at developing an alternative technology for TiCN. The standard TiCN coating has been maturing for over 30 years, and it will be difficult to develop something that exceeds its performance. On the other hand, if we develop new technology, we will be able to establish a firm position in front of other competitors in the industry. We’ve already started research for the development of new technology.

However, it is extremely difficult to predict what the situation will be 10 years from now because it is impossible to imagine what kind of machining will be used in the future. After specifying the parts needed for a finished product, such as an automobile, the tools required to manufacture those parts will be determined.

As the shift from internal combustion engine vehicles to EV proceeds, we’ll see significant change in the cutting tool market.

We also need to consider the potential of new difficult-to-cut materials. Semiconductor devices may also shift from SiC to diamond; therefore, we also need to consider how to manage the shift. If flying automobiles are commercialized, we need to minimize the weight of components. There are so many things to consider for future potential, inclu­ding future components and cutting tools.

We continue taking steps forward every day to achieve breakthroughs for the difficulties encountered.

Looking back over the history of CVD coating technology

Oshika:　When I look at aluminium image data, it’s from the viewpoint of the aluminium ion. When I think about my skin, I see it from the viewpoint of carbon dioxide. Let’s think about the origin of the countless carbon atoms contained in our skin. They may be from 100's of years ago, or passed down from dinosaurs that lived millions of years ago. Considering the potential age of the carbon that composes our body while looking at the data, the images come alive in my mind.

Okude:　 The most important in my work at the Central Research Institute is ensuring that I observe the data very well. When looking at photos of samples, different people notice different things. Under­standing data really well and discovering the differences makes it possible for us to identify the direction we should move in our research. As someone in a leadership position, I feel that passing down such ways of thinking and looking at things to the next generation is an important part of passing down technology. Recent improvements in analytical devices allow us to notice changes that could not be seen before. I would like to continue emphasizing the importance of observing and reviewing things deeply.

Tatsuoka:　What I always keep in mind while conducting basic research is the attempt to discover unknown fields. Originality leads to the development of new technology and products. In regard to CVD technology, whose history goes back more than half a century, I use accumulated knowledge to find new things from my own viewpoint as well as a brand-new viewpoint. The supportive climate of the Central Research Institute makes it possible for me to continue moving forward to develop new technology.