Maximum tool life in CFRP thanks to diamond coating

It’s the world’s hardest material: the diamond. That’s why it’s the cutting material of choice, for example when machining aluminium or CFRP (carbon fibre reinforced plastic). In this regard, we make a distinction between PCD (polycrystalline diamond) and diamond coatings. Binders are used for PCD blanks to hold the diamond crystals together. This binder is not necessary for CVD diamond coating (chemical vapor deposition). “With CVD we have 100 percent diamond on carbide”, says Dr Wolfgang Baumann, who is responsible for coating and cutting material technology at MAPAL. Another difference between the two types of diamond is the sharpness of the cutting edge. Whenever an extremely sharp cutting edge is required, nothing holds a candle to PCD. In many other cases, CVD diamond coating is a real alternative.

In the field of PCD tools, MAPAL has the largest production worldwide and for decades has been finding the best strategy for the respective machining task together with its customers. MAPAL is now building up the same expertise with its new strategic partner SP3 in the field of diamond coatings. “Diamond-coated tools or indexable inserts have the best application values and tool lives, especially when machining CFRP, plastics and ceramics”, explains Dr Wolfgang Baumann. Diamond-coated carbides are also suitable for machining graphite electrodes, for example in the die & mould sector.

SP3 is a coating company based in Silicon Valley in the USA. The company, which gets its name from the chemical structure of diamond, has developed its own processes for applying CVD diamond coatings to various substrate materials. Bob DeFeo, Managing Director of SP3, says: “Working closely with tool manufacturers, carbide suppliers and tool users is our recipe for success”. This close cooperation has resulted in various coatings, which SP3 today offers its customers as a service. 

“Depending on the application and our customer’s requirements, we find the optimum solution together”, promises DeFeo and goes on to describe diamond coating as “a complex process. Numerous factors must be taken into account”. The most important points he mentions are the carbide, the tool geometry, the preparation of the cutting edge or substrate for the coating as well as the coating thickness and roughness.

For the actual coating, SP3 uses the so-called “hot filament CVD” process. Dr Wolfgang Baumann explains the process: “We use tungsten wires to heat up hydrogen and methane to 2,550 degrees Celsius. This produces very reactive methane radicals. These gradually deposit their C-atoms on diamond nuclei on the carbide surface, which causes the diamond to grow. The size of the crystals varies depending on pressure, gas flow and temperature. It ranges from nanocrystalline to microcrystalline. The surface of the coating depends on the size of the crystal produced. It ranges from extremely smooth to rough”.

In order to carry out the coating process in the best possible way, SP3 has developed its own CVD diamond reactors. The company has designed a special arrangement of the heating wires for uniform energy distribution and has had this patented. In particular, it’s the maximum coating thickness of 50 µm that makes the process developed by SP3 so special. “Our system is very impressive due to the excellent uniformity of the diamond thickness. As well as this, the process variables can be controlled in real time”, says DeFeo. This makes the coatings precise and repeatable, which is extremely important, particularly in sectors such as the aerospace industry. “Only tools that are qualified are used for this. They must consistently deliver the same performance every time to ensure the reliability of the process”, emphasises Bob DeFeo.

Coating adhesion is also important for the later use of the coated tools or indexable inserts. “The substrate as well as how it’s pre-treated are crucial for this”, reveals Bob DeFeo. The pre-treatment, in which the majority of the expertise is also invested, is divided into etching, cleaning and sterilisation.

Als erstes gemeinsames Projekt von SP3 und MAPAL stand die Beschichtung des OptiMill-Composite-Speed-Plus an. „Gemeinsam haben wir intensiv daran gearbeitet, die optimale Beschichtung zu entwickeln“, sagt Baumann. Der Fräser, der vor allem in der Luftfahrtindustrie zum Einsatz kommt, ist auf die Bearbeitung von CFK ausgelegt. Durch ein neues Hochleistungssubstrat in Kombination mit einem verstärkten Kerndurchmesser steigerten die Entwickler bei MAPAL die Bruchfestigkeit des achtschneidigen Fräsers im Vergleich zum Vorgängermodell um 50 Prozent. Das optimierte Nutprofil sorgt für eine schnelle und sichere Abfuhr von Stäuben und Prozesswärme auch bei extrem hohen Zerspanungsvolumen. Der Schneidkeil wurde eigens auf die Anforderungen spröder Werkstoffe optimiert. „Das i-Tüpfelchen war dann die Beschichtung, die für höchste Standwege sorgt“, freut sich DeFeo. Dank der Diamantbeschichtung kann zudem mit höchsten Einsatzwerten gefahren werden. 

In der CFK-Bearbeitung bei einem Nutzer des Fräsers wird beispielsweise mit einer Drehzahl von 5.968 min-1, einer Schnittgeschwindigkeit von 150 m/min und einem Vorschub von 955 mm/min gearbeitet. Dabei beträgt sowohl die Schnitttiefe als auch die Schnittbreite 8 mm. „Wir erzielen mit der neuen Beschichtung beste Ergebnisse und erreichen eine Standzeit, die 20 Prozent über der des Vorgängermodells liegt“, sagt DeFeo.

Die Beschichtung des OptiMill-Composite-Speed-Plus ist der erste Erfolg der strategischen Partnerschaft, dem sicherlich zahlreiche weitere folgen werden, sind sich DeFeo und Baumann einig. Denn: „Um die bestmögliche Leistung eines CVD-diamantbeschichteten Werkzeugs zu erreichen, müssen Werkzeughersteller und Beschichter sehr eng zusammenarbeiten. Nur so sind höchste Standzeiten, prozesssichere Ergebnisse und beste Oberflächen für den Anwender realisierbar.“