The paper briefly reviews the development / implementation of soft / lubricating coatings and details published machinability data for dry / near dry machining applications. Following on from this, statistically designed experimental work is presented relating to the performance of PVD coated cutting tools (Uncoated, Graphit-iC, Dymon-iC and MoST, with and without a hard coating underlayer) when dry drilling BS L168 aluminium alloy. Suitable coatings / coating combinations were determined using a uni-directional single pass wear-testing procedure which previously has been successfully used to determine coatings suitable for forming applications. The results from the new testing procedure were correlated with the machinability data obtained from the experimental work and the validity of using such a procedure for identifying coatings for machining applications is discussed. Machinability was assessed in terms of tool life / wear, cutting force, chip morphology, hole diameter, cylindricity and out of roundness.

Despite the use of soft / lubricating coatings, there was a strong tendency for the aluminium to adhere to the drill which contributed to low tool life. Chip packing in the drill flutes was a highly significant feature and largely governed cessation of tests. Tool life was lower when cutting dry as opposed to wet. When compared with uncoated drills, the combination of a hard underlayer and soft / lubricating top coat (MoST), improved hole quality with lower dispersion of values recorded for hole diameter, roundness and cylindricity.

It is known from previous studies [1,2] that MoST^TM coatings can improve the performance and life of cutting tools.

This paper presents the results of a systematic study of the performance of HSS drills coated with hard nitride coating and drills with a top coating of MoST^TM. The lifetime of drills operating under accelerated conditions with flood coolant was measured and the drill wear, feed force and torque were recorded throughout the tests. It was found that MoST^TM coatings reduced the drill wear, the feed force and the torque significantly as well a extending the drill life. Some preliminary dry drilling experiments are also included. The mechanism of the improvements given by MoST^TM are discussed.

[1] N.M. Renevier, H. Oosterling, U. Koenig, H. Dautzenberg, B.J. Kim, L. Geppert, F. G.M. Koopmans, J. Leopold, presented at ICMCTF 2002, San Diego, Ca, USA, 22-26/04, in print in Surface and Coatings. Technology.

[2] N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer and J. Hampshire, Surface and Coatings. Technology, Volume 123 (2000) 84-91.

Large Area Filtered Arc Deposition (LAFAD) technology is a tool to deposit atomically smooth coating at high depositing rate over extended coating areas¹. While inheriting all the advantages of conventional filtered arc technology (superhardness, improved adhesion, low density of pin-holes) the LAFAD technology allow the formation of functionally graded, multilayer and super-lattice architectures of multi-elemental composite coatings by electro-magnetic mixing of two plasma flows consisting of two different metal vapor plasma compositions. Nano-indentation was used to evaluate the mechanical properties of the coatings. The outstanding sharpness of the cutting edges of filtered arc coatings compared to coatings deposited by conventional technology is shown by SEM observations. For this study carbide drills and carbide end mills coated with Ti_xAl_1-xN coatings deposited by dual arc large area filtered arc plasma source (LAFAS) were tested in dry cutting of pre-hardened D2 steel. For the chosen cutting parameters uncoated tools failed by high temperature oxidation. A comparison with tools coated by unfiltered arc deposition is presented.

¹Vladimir I. Gorokhovsky , Rabi Bhattacharya and Deepak G. Bhat, Surface and Coating Technology, 140 (2) 2001, pp. 82-92.