Papers

ICMCTF2007 Session E4/G4: Tribological Studies of Coatings for Green Manufacturing and Dry Machining

Friday, April 27, 2007 8:00 AM in Room California

Friday Morning

Start	Invited?	Item
8:00 AM		E4/G4-1 Characterization of Nanocrystalline Diamond Coating Cutting Tools J. Hu, Y. Chou (The University of Alabama); R. Thompson (Vista Engineering, Inc.) Diamond coatings have been increasingly used in cutting tools applications, in particular, for machining lightweight high-strength materials. Chemical vapor deposition (CVD) is a major process for diamond coating tools aimed as an alternative to costly polycrystalline diamond (PCD) tools. However, most literature to date reported that, in addition to poorer part surface finish, CVD diamond tools mostly have a shorter tool life than PCD tools. Recently, microwave-plasma CVD technology has been developed to increase the diamond growth rate and with nitrogen gas, the grain sizes can be substantially reduced. The produced nanocrystalline diamond (NCD) coating consists of nano-crystals of diamond embedded into a hard amorphous diamond-like carbon matrix with high hardness and low surface roughness. In this research, NCD is characterized against conventional CVD diamond and PCD in several aspects including surface topography, edge radius, microhardness, and Raman spectra. Machining performance of the three types of diamond tools was evaluated by surface finish, tool wear, and cutting forces in turning A390 alloy and composites. The results are summarized as follows. (1) NCD has a smoother surface than CVD diamond, but slightly rougher than polished PCD. (2) NCD and CVD diamond tools have similar edge radii, both greater than PCD counterpart. (3) NCD has a much higher hardness than CVD diamond, but lower than PCD. (4) In Raman spectra, NCD has a disordered sp3 peak, but CVD diamond shows a noted disordered sp2 peak. (5) In machining, NCD and PCD tools generate better surface finish than CVD diamond tools. (6) NCD tools have a long tool life than CVD diamond tools, though both fail by the same mode, coating delamination. The metal deposit accumulated at the tool flank face strongly influences the onset of delamination. Cleaning the metal deposit may extend the tool life significantly.
8:20 AM		E4/G4-2 Synthesis and Characterization of Ultra-Thin Nanocrystalline Diamond Coatings for Micro End Mills P.J. Heaney (University of Wisconsin,Madison); C.D. Torres (University of Wisconsin, Madison); A.V. Sumant (Argonne National Laboratory); R.W. Carpick (University of Pennsylvania); F.E. Pfefferkorn (University of Wisconsin, Madison) There is growing interest in high precision machining to fabricate miniaturized parts using end mills with diameters ranging from 10 to 500 microns. This technology complements standard Si-MEMS fabrication processes with its ability to directly produce true 3D structures in many materials with high accuracy, low cost, and short cycle time. One of the important aspects of micro end milling is the choice of tool materials. Presently, tungsten carbide (WC) with cobalt binder is widely used. However, it suffers from a number of drawbacks such as limited operational life, difficulty in machining adhesive metals such as aluminum and copper, and poor surface finish. We have developed a new approach in coating existing tungsten carbide micro end mills with ultra-thin nanocrystalline diamond (NCD) coatings using hot filament chemical vapor deposition (HF-CVD). NCD coatings result in a dramatic increase in micro end mill performance. In this study we have altered the HF-CVD gas chemistry by introducing Ar to improve the NCD coating. The effect of gas chemistry on the resultant NCD growth is characterized using: SEM, AFM, and white light interferometry to evaluate coating thickness, growth morphology, and grain size; Raman and NEXAFS to characterize the composition and sp²/sp³ ratio; and micro end milling tests to evaluate machining performance (adhesion, wear, friction). We show that the evaluation and characterization of the NCD coatings is very important for the optimization of ultra-thin NCD coatings for better machining performance.
8:40 AM		E4/G4-3 Investigation on Thin Vanadium Oxide Films for Forming Tools Deposited by Pulsed PVD Process Technology P. Immich, K. Bobzin, R. Nickel, E. Lugscheider, M. Witt (RWTH Aachen University, Germany) The process of hot forging and extrusion with permanent moulds is a challenge with respect to very high thermal, mechanical and tribological loads of tools. Most hot working processes are using hot working steel tool material like 1.2343. But with increasing process temperature, the material loses its high hardness. To increase the tool life time PVD coatings can be used. The thin films offer many advantages compared to uncoated tool surfaces like for example high hot hardness against abrasive wear and high compressive residual stresses for reducing heat checkings. For reducing the friction in hot forging or extrusion of steel often bornitride powder is used for industrial applications. Another way is using self-lubrication oxide films to obtain low friction and wear. Magneli-phase systems like vanadium oxide or tungsten oxide offers this self-lubrication for elevated process temperatures. By using the pulsed PVD process technology it was possible to deposit vanadium oxide by varying different deposition parameters like pressure, oxygen content, bias voltage and substrate temperature. The properties of the films deposited by the reactive pulsed magnetron sputtering were analyzed by common thin film techniques revealing hardness, Young's modulus and coating adhesion. Additionally, application oriented tests were performed to characterize their behaviour for the forming process. These tests include high temperature ball on disc investigations at different sample tempera-tures. Structure and morphology of vanadium oxide strongly depends on deposition parameters. To obtain the crystallite size and the crystallographic structure for different temperatures, high temperature x-ray diffraction was carried out. By using the reactive pulsed PVD-process it was possible to deposit different vanadium oxide films. The self-lubrication vanadium oxide coatings were analyzed and showed promising test results in field tests as tool coatings for hot forming applications.
9:00 AM		E4/G4-5 Effect of Multi-Layer Coating Design on their Properties and Wear Mechanism in Dry Machining H. Gekonde (IonBond, LLC) The paper will focus on the effect of multi-layering on properties of coatings and the resulting wear mechanism in dry machining. The discussion includes the variation of the contact mechanics at the tool-chip interface with coating design and morphology. Analysis of the tribology of the tool chip interface will be carried out to identify prevalent wear and chip formation mechanisms. The influence of the dynamic behavior of the materials on chip formation and tool wear mechanisms will be analyzed and presented as a useful tool for coating design and selection.
9:40 AM		E4/G4-7 Conventional Drill Testing of State-of-the-Art PVD Coatings and Comparison to Microstructure Revealed by FEGSEM. Out of Place and Out of Time? T. Vom Braucke (Swinburne University of Technology, Australia); S.J. Dowey (Surface Technology Coatings, Australia); E.D. Doyle (Swinburne University of Technology, Australia) Current and state-of-the-art hard PVD coatings from various manufacturers were benchmarked by conventional lubricated drill testing in 'P20' and 'D2' materials. CNC ground, High speed steel (HSS) drills were used in the trials. Scatter is inherent in drill testing, however, statistical treatments and careful pre-selection of conditions afforded reasonable comparison of the results. FEGSEM showed that many different multilayered and nano-layered compositions are produced with corresponding variations in grain growth, all with the aim of enhancing 'performance'. Indeed, these different approaches are shown to lead to similar lifetimes. However, some coatings exhibit contrary results when materials are compared. Observations of fracture cross sections and the anachronism and anatopism in the test regime are discussed in regard to tool and coating applications in green manufacturing.
10:00 AM		E4/G4-8 Role of Original Surface State in Dry Cutting at Magnetic-Assisted Machining M. El Mansori (ENSAM, France); A. Mkaddem (LMPF- ENSAM, France) This paper covers the application of external electromotive force sources EMF (e.g. magnetic field) as an integral part of nearing no-wear conditions when cutting dry Particularly, the magnetic-assisted machining process (MAM) in which an external superimposed magnetic field reworks material as it is sheared by a single-point cutting tool with the objective of improving its machinability, is described The former usually involves short treatment times using low intensity, easy to produce and controls fields that are applied at room temperature Preliminary 3D cutting tests on ferromagnetic carbon steels have shown that the use of an external magnetic field increases the tool lifetime (2 to 3 times) in severe cutting conditions.¹Recently, results of orthogonal cutting tests carried out on dry cut of magnetized ferromagnetic AISI 1045 steel using non-magnetic carbide tools have shown similar effect.^2-3The present paper gives a review of recent research results describing the mechanisms causing observed effects, especially on the role of original state of machined surface Implications of theses results on the efficiency of the MAM process close the paper. ¹M. El Mansori et al, Reduction of tool wear in metal cutting using external electromotive sources, Surface and Coatings Technology, Volumes 163-164, 2003, 472-477. ²M. El Mansori et al, Improving surface wearing of tools by magnetization when cutting dry, Surface and Coatings Technology, Volumes 188-189, 2004, Pages 566-571 ³ M El Mansori and B. E. Klamecki, Magnetic Field Effects in Machining Processes and on Manufactured Part Mechanical Characteristics, Journal of Manufacturing Science and Engineering, Volume 128, Issue 1, 2006, pp. 136-145.
10:20 AM		E4/G4-9 Synthesis and Characterization of CrN, Mo₂N Multilayers and Different Phases of Molybdenum Nitride R. Koshy, M. Graham, L. Marks (Northwestern University) Deposition of the different phases of the oxides and nitrides of chromium and molybdenum were carried out using reactive gas sputtering in an unbalanced magnetron chamber. The phases synthesized were Cr₂O₃, MoO₂, Cr₂N, CrN, Mo-N_x (solid solution), β-Mo₂N, γ-Mo2_N. Hardness, friction and annealing products were characterized. Hardness of the Mo₂N phases were found to be in the 18-22 GPa regime while the hardness of the CrN phase was found to be in the 25-30GPa regime. Friction tests on the molybdenum nitride phases, using a 52100 steel ball, carried out at room temperature, showed an initial low friction regime in the Mo-N_x phase. The other Mo₂N phases seemed to stabilize in a relative high friction regime of 0.7. Wear of the ball resulted in metal transfer to the Mo₂N phases, and was studied using SEM and EDS imaging. We also noted transfer of the Mo-nitride to the ball. Similar tests on Mo0_x phases showed a relatively low friction coefficient of 0.12. Multilayer films of Mo₂N and CrN were synthesized with varying periodicities. Hardness of the multilayer films, of different superlattice periodicities, showed them to be within the rule of mixtures for its constituents (20-30GPa regime). Low angle XRD and cross-sectional SEM imaging of the coating shows that the multilayers showed little or no inter-diffusion when annealed in argon to 1000°C. Preliminary high temperature friction tests on these multilayers suggest that the expected in situ thermally activated self lubricating mechanism was functioning.