ICMCTF2013 Session G4-1+E: Coatings for Machining Advanced Materials and for use in Advanced Manufacturing Methods

Today without gear wheels almost nothing is turning. Manufacturers produce these components by the million for use in automotive gearboxes or in the gears of large wind turbines. Gear hobs have proven themselves for more than a hundred years now in the manufacture of gear wheels and other tooth-cutting tasks.

The ever increasing demand for higher productivity in manufacturing gears requires advanced hard coatings and new substrate materials. Up to now in this field of gear hobbing different substrate materials are used for single-piece hobs: powder metallurgy high-speed steel (PM-HSS) and cemented carbide. Today PM-HSS has a market share around 70% offering limited cutting speeds for wet and dry conditions. On the other hand cemented carbide offers strong performance related features like high cutting speeds up to 400 m/s. But due to the fact, that hobs have a typical life cycle time of 10-15 recondition cycles– hobs are often demounted – packed and shipped – decoated- regrinded and coated again- could cause small handling or production damages that result in a shorter tool life time and less reliability of the production process.

On the substrate side – there is a new generation of intermetallic phase substrate on the market offering compared to conventional PM-HSS higher hot hardness and as result from these higher cutting speeds. On the coating side the development focus is on introducing new coatings allowing new cutting features. In fact today hobs are coated and e.g. dry gear cutting is only possible with coated tools due to the prevention of chip welding. Today hob coatings that are available on the commercial market mainly based on TiAlN system. Now there is a significant switch in the market towards the system Cr-Al-N offering higher oxidation resistance and higher wear resistance. As a result of these developments it is possible to increase cutting speeds up to 50% compared to conventional coatings

For large modules hobs with inserts are offering an economic way of producing gears. There are gear milling cutters (single tooth method) and ICI hobs available. The selection of the best type of tool depends on the lot size to be manufactured and the corresponding number of teeth. Gear hobbing is the most productive method for cutting large-module gears with a high number of teeth. Gear milling cutters are especially to be preferred for low numbers of teeth or small lot sizes. In the last years the performance of these tools are driven by new cutting material grades like new ultra-fine grain carbide combined with new thick PVD coatings and special micro-surface preparation.

Hard coatings are widely used in surface engineering for wear protection of tools and mechanical components. Nowadays, dry machining technologies are emerging techniques due to both saving costs and environmental issues. Such type of coatings to be applied in tools for dry machining must combine not only high hardness and structure stability but also tribological aspects like ultra-low friction. Si₃N₄ thin films show outstanding performance, in terms of wear and corrosion resistance, in cutting tools at higher temperatures than 1000 ^oC. However, the incorporation of MoS₂ in Si₃N₄ thin films could reduce the friction coefficient maintaining high values of hardness in these thin films.

In this work, Si₃N₄-MoS₂ thin films were deposited on Si(100) substrates by dual rf and dc reactive magnetron sputtering from Si and MoS₂ targets in a Ar/N₂ plasma with different MoS₂ amounts. The composite thin films were characterized by glancing angle X-ray diffraction, Rutherford backscattering spectrometry, glow-discharge optical emission spectroscopy, nanohardness at different temperatures (23^oC to 400^oC) and nano-scratch and friction measurements at room temperature. In the whole layer both Si₃N₄ and MoS₂ compounds are stoichiometric and the deposition rate is 0.12 nm.s^-1. Moreover, the structure is amorphous and homogenous. The MoS₂ content in the composite thin film goes from 0.2 at. % to 4 at. %. In contrast to previous results where the hardness of the TiN-MoS₂ nanocomposite system decreases monotonously following the mixture rule when MoS₂ is incorporated, our Si₃N₄-MoS₂ thin films show a maximum hardness of 28.5 ± 1.5 GPa at a MoS₂ content of 1.6 at. %. The hardness of these Si₃N₄-MoS₂ thin films decrease slightly as a function of temperature. Finally, the friction coefficient tendency of the Si₃N₄-MoS₂ thin films will be discussed and compared to the TiN-MoS₂ nanocomposite system where a minimum friction coefficient is reached at intermediate MoS₂ contents.

In an attempt to explore the application of self-lubricant coatings on tribological components and to understand its influence in dry machining operations during tool-work sliding interaction, a laboratory based tribological simulation was conducted. To realize this, a wear test between Ti-6Al-4V alloy (counterface) and uncoated and coated cemented tungsten carbide-cobalt (WC-Co) alloy (pin specimen) at different sliding conditions was carried out using a pin-on-disc wear testing machine. The characteristics of wear rate, coefficient of friction and surface roughness were investigated with and without self-lubricant coatings on the carbide cutting tool specimen. In this work, electrostatic spray coating process was employed in deposition of molybdenum disulphide (MoS₂) solid lubricant powder particles on specimens. Microstructure of specimen and counter surface was analyzed using scanning electron microscopy and optical microscopy. The results showed that the presence of solid lubricant film on specimen will greatly influence the sliding performance and improves the wear resistance through reduction in the mechanical energy given in the sliding contact due to presence of MoS₂ lamellar structure as a transfer film.

Keywords: cemented carbide, solid lubricant, electrostatic spray coating, wear rate, friction, sliding performance

From the point of view of oxidation and high-temperature wear resistance, Al₂O₃-based coatings are among the best available. This is especially useful for dry high-speed milling or turning of materials from the groups M 10 – 25 and K 10 – 20. We will report about the preparation and cutting performance of (Al_1-xCr_x)₂O₃ coatings deposited by vacuum arc technology in the coating system PLATIT π311, which uses rotating cylindrical cathodes made of pure Al and Cr targets in an oxygen-containing reactive gas atmosphere, where poisoning of the Al cathode was controlled by the deposition of metallic Cr onto the Al cathode surface . This procedure significantly improved the uniformity of the erosion of the Al cathode and reduced the roughness of the deposited coatings. The deposited coatings have been evaluated in terms of their surface roughness, hardness, and structure. The life time of tools coated wi th the new Al₂O₃-based coating during machining has been compared with other coatings.