Crystalline γ‐Alumina Deposited in an Industrial Coating Unit for Demanding Turning Operations

Abstract

Cutting of Ni alloys such as Inconel 718 or austenitic stainless steel is one of the greatest challenges in high-speed machining because of low thermal conductivity, the tendency to strain hardening, and adhesion on the cutting tools, resulting in high tool wear and high thermal load on the cutting tool. However, these difficult-to-machine materials are increasingly employed. Therefore, cutting tools with a high oxidation resistance, a low tendency to adhesion-related sticking, high hot hardness, and high abrasion resistance are necessary. One of the promising candidates for these types of cutting operations is the crystalline g-alumina phase. It offers high hot hardness, high resistance against adhesion-related sticking, and high chemical and thermal stability at elevated temperatures. [1,2] The excellent performance of g-Al 2 O 3 for cutting operations has already been reported. [3] However, the unexpected thermal stability of the metastable g system as thin film has not been completely investigated and understood. Alumina is a polymorphous material and a-alumina is the only stable phase. In industrial processes, a-alumina is deposited by means of CVD (chemical vapor deposition at temperatures of 1000 8C. [4,5] These high temperatures reduce the choice of substrates. The deposition of g-Al 2 O 3 by means of PVD (physical vapor deposition) allows lower substrate temperatures (500-650 8C) and is therefore interesting for many applications. Besides the lower deposition temperature, the g-phase generated by PVD has some advantages over the a-phase generated by CVD: PVD-alumina coatings are smoother and have a smaller grain size. They therefore have a higher fracture toughness and strength. [6] Nevertheless, the working temperature of g-Al 2 O 3 has to be considered because of the transformation into the stable a-phase. We analyzed (Ti,Al)N/g-Al 2 O 3 thin films with common thin film test equipment. Furthermore, in situ annealing tests were performed to determine the thermal stability of the coating. To test the coating's performance for cutting operations, we compared (Ti,Al)N/g-Al 2 O 3 and a state of the art nanocomposite (Ti,Al)N as described in Reference [7] by pin-on-disk tests and turning tests. Experimental Details Deposition SetupNanocomposite (Ti,Al)N and alumina coatings were deposited on WC-Co cutting inserts. Two different inserts were chosen: THM 12, SNUN 120412 (Kennametal Holding GmbH) for characterization of the coatings and for tribological Crystalline PVD g-Al 2 O 3 -coatings offer great potential for their use in high-speed cutting operations. They promise high hot hardness and high oxidation resistance at elevated temperatures. This is important for coatings that are used for machining of materials with low thermal conductivity such as stainless steel or Inconel 718 because heat generated during cutting can barely be dissipated by the chip. Because of the prevailing bonding forces of alumina, adhesion-related sticking can be reduced even for dry cutting. Furthermore, the high formation enthalpy ...

Entwicklung von HPPMS‐Al2O3‐Beschichtungen für die Zerspanung von schwer zerspanbaren Werkstoffen

“…Aluminiumoxid (Al 2 O 3 )-Beschichtungen sind vielversprechend für Werkzeuge aufgrund ihrer Oxidationsund Diffusionsbeständigkeit, ihrer chemischen Stabilität sowie ihrer hohen Warmhärte. Sie bieten ein hohes Potenzial für die Anwendung in der Zerspanung von schwer zerspanbaren Werkstoffen wie Gusswerkstoffen oder Titanlegierungen [1][2][3][4][5].…”

“…In bisherigen Forschungsarbeiten konnten bereits grundlegende Erkenntnisse zur Abscheidung von kristallinen g-Al 2 O 3 -Dünnschichten mittels medium frequency Magnetron Sputtering (mfMS) bei T S = 650 8C Substrattemperatur, deren Charakterisierung und Einsatzverhalten in der Zerspanung erlangt werden [2,[6][7][8][9]. Eine Weiterentwicklung dieser Technologie stellt die High Power Pulsed Magnetron Sputtering (HPPMS)-Technologie dar, die aufgrund der hohen Ionisation des abgesputterten Materials eine Verbesserung der Schichtmorphologie und -eigenschaften verspricht [9][10][11].…”

Investigation of Reactive HPPMS Process and Influence of Bias Voltage during Deposition of Alumina Coatings

“…[1][2][3][4] Alumina exists in several crystallographic phases with different properties. [1][2][3][4] Alumina exists in several crystallographic phases with different properties.…”

“…Their studies showed that lower temperature around T ¼ 650 C can be used to produce a-Al 2 O 3 coatings. [11] HPPMS technology offers the possibility to increase the ionization degree in the deposition flux, [1,[12][13][14] which leads to a dense coating microstructure. [3,[7][8] One of the important aspects of reactive sputtering is the hysteresis effect.…”

“…Crystalline alumina coatings deposited by means of PVD gained a growing interest over the last decade due to their high thermal stability, high hot hardness, and chemical stability. [1][2][3][4] Alumina exists in several crystallographic phases with different properties. Many of them are metastable, e.g., g, k, h, u besides the thermodynamically stable a-phase (corundum).…”