ICMCTF2009 Session B3-1: CVD Coatings and Technologies

Monday, April 27, 2009 10:20 AM in Room Royal Palm 1-3

Monday Morning

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Start Invited? Item
10:20 AM B3-1-2 Microstructural Investigation of CVD Alumina Coatings Grown on Single Crystals of α-Al2O3
S. Canovic (Chalmers University of Technology, Sweden); B. Ljungberg, C. Björmander (Sandvik Tooling, Sweden); M. Halvarsson (Chalmers University of Technology, Sweden)
Multilayer coatings of TiC, TiN and Al2O3 are often formed on cemented carbide cutting tools by conventional chemical vapour deposition (CVD) in order to increase the wear resistance of the tools. In this work, CVD multilayer alumina coatings separated by thin layers of TiX (X=C or N) are investigated. There are several reports indicating that the interfacial structures are important for the characteristics of the TiX/alumina coatings. The multilayer coatings are deposited on α-Al2O3 single crystals with given top surfaces in order to avoid the influence of the diffusion of tungsten and cobalt from a cemented carbide substrate. Additionally, the use of these substrates makes it possible to study TiX/alumina interfaces where the nucleation surface is well defined and known beforehand. Previously, we have investigated ΤiX/alumina multilayer coatings deposited on single crystals of α-Al2O3 with 2 different top surfaces (c and r) and we observed some interesting differences. In this work the sample matrix is extended and includes CVD multilayer alumina coatings, separated by thin layers of TiC, deposited on α-Al2O3 single crystals with 4 given top surfaces (c, r, a, m). The main part of this work was carried out by transmission electron microscopy (TEM) using a Phillips CM 200 FEG working at 200 kV. Cross-section TEM thin foils were prepared using a combined FIB/SEM (focused ion beam/scanning electron microscope) instrument, whereby electron transparency was achieved throughout the coating thickness. The detailed microstructure of the different coatings will be described in this work.
10:40 AM B3-1-3 Residual Stress Evolution in CVD Multilayer Coatings on Cutting Tools at High Temperature
M. Klaus (Technische Universität Berlin, Germany); Ch. Genzel (Helmholtz-Centre Berlin for Materials and Energy, Germany); H. Holzschuh (Walter AG, Germany)
Cutting tools are usually coated by CVD methods to protect them from abrasive wear and to increase the lifetime of the products. To prevent crack propagation through the coatings, the films are not uniform but consist of stacks of alternating sublayers with different thickness, chemical structure and crystallographic texture. In case of Al2O3/TiCN mulilayer structures deposited on WC substrates, the as-grown coatings exhibit tensile residual stresses which is due to the differences in the thermal expansion coefficients. To generate beneficial compressive stresses in the coatings, which contribute significantly to the excellent mechanical properties, subsequent mechanical surface treatment such as grit blasting is applied. In previous work we showed by means of X-ray diffraction methods that the residual stresses introduced into the multilayer coatings by blasting are not uniform but occur in form of more or less steep intra- and interlayer stress gradients wit hin or between the individual sublayers and/or the substrate. In the present work we extend our investigations to residual stress analysis at high temperatures which occur under service conditions. The main goal of this study is to analyze the residual stress evolution in the individual sublayers and the substrate and to quantify the ratio between extrinsic (thermal) and intrinsic (blasting induced) stresses as a function of temperature.
11:00 AM B3-1-4 Surface Treatments for Enhancing the Adhesion of Nanocrystalline Diamond Coatings to Tungsten Carbide Micro End Mills
P.J. Heaney, C.D. Torres (University of Wisconsin - Madison); M.A. Hamilton (University of Pennsylvania); A.V. Sumant (Argonne National Laboratory); K. Sridharan (University of Wisconsin - Madison); R.W. Carpick (University of Pennsylvania); F.E. Pfefferkorn (University of Wisconsin - Madison)

Diamond has outstanding material properties such as high hardness, high thermal conductivity, chemical inertness, and extremely low friction and wear under particular conditions. Applying a diamond coating to cutting tool surfaces can significantly increase the tribological performance and hence the operational life-span of the tool. Coating micro end mills with diamond is a challenging task, due to the small size (5-300 µm diameter) and complicated shapes. We have been able to coat micro end mills with continuous diamond films as thin as 60 nm. Micro end milling tests under severe conditions (dry cutting of adhesive material) have demonstrated that the diamond coatings significantly increase the machining performance (reduced forces, adhesion, wear, friction) of the tools. The main failure mode of the tool is delamination of the coating, due to poor adhesion between the coating and the tungsten carbide (WC) tool. Coating adhesion must be improved to increase the ove rall tool life and performance of micro end milling.

Cobalt (Co) in the WC adversely effects diamond growth and coating adhesion, requiring a substrate preparation technique to prepare tools. The preparation must result in a surface that allows proper diamond nucleation, low Co content, prevent Co from diffusing to the surface during diamond deposition, and not weaken the tool or introduce residual stresses. The methods that we have studied include depositing a tungsten interlayer, treating the surface with low energy (< 5keV) carbon ion implantation, and chemical etching with a hydrofluoric acid solution. After surface preparation, the tools were characterized using scanning electron microscopy, atomic force microscopy, and white light interferometry to measure changes in morphology and roughness, x-ray photoelectron spectroscopy (XPS), Raman, and near-edge x-ray absorption fine structure spectroscopy (NEXAFS) to observe chemical and bonding changes of the substrat e and resulting diamond coatings, and machining tests to check the integrity of the tool. The tools were then coated with diamond and evaluated while dry machining aluminum. Coating adhesion while machining was measured by time before delamination, severity of delamination, and change in machining metrics.

Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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