Papers

ICMCTF2005 Session E2: Friction and Wear of Coatings II: Design and Modeling

Friday, May 6, 2005 8:30 AM in Room Sunrise

Friday Morning

Start	Invited?	Item
8:30 AM		E2-1 High Resolution Observations of Wear Mechanisms in Multilayer PVD Coatings M. Rainforth (The University of Sheffield, United Kingdom) Nanoscale multilayers, such as superlattices, have emerged in recent years as wear-protective coatings. The hardness of a superlattice coating is dependent on several factors including the superlattice periodicity and other structural factors such as the degree of chemical mixing between layers. This talk will look at the structure of the coating and how this relates to the wear behaviour. The chemical intermixing between individual layers within a TiAlN/VN multilayer structure was determined at the atomic scale using ultra-high resolution high angle annular dark field imaging (HAADF) in a scanning transmission electron microscope (STEM), coupled with electron energy loss spectroscopy (EELS). Complex intermixing was observed, the profile of which closely correlated with theoretical predictions. The wear mechanisms in TiAlN/VN and TiAlN/CrN and monolithic TiAlCrN were investigated using detailed cross-sectional TEM. Plastic deformation to an equivalent tensile strain of 2.5 was detected at the worn surface of a TiAlN/CrN multilayer following sliding against an M2 high speed steel, despite the high hardness of the coating. The extent of plastic deformation was less than that found for TiAlYCrN coatings worn under the same conditions. Plastic deformation led to microcracking, providing a mechanism for subsequent wear through surface delamination. A transfer layer formed, predominantly of Fe₃O₄, but also containing components of the coating. The presence of Fe₃O₄ without Fe₂O₃ or FeO suggests temperatures in the range ~500-600oC. The loss of the multilayer structure in the outer ~40nm is consistent with diffusion and deformation processes occurring at this temperature. For the TiAlN/VN coatings low friction coefficients are observed in high temperature wear tests. It has been proposed that these are associated with the formation of V₂O₅. Focused ion beam (FIB) microscopy was used to produce site specific TEM specimens to determine the oxides formed during high temperature wear tests of TiAlN/VN. The worn surface structure was then compared to static oxidation studies for the same temperature. The inter-play between oxidation and wear is discussed.
9:10 AM		E2-3 Lifetime of a Polymer Bonded Solid Lubricant in Fretting: Definition of a Local Dissipated Energy Criterion V. Fridrici, S. Fouvry, P. Kapsa (Ecole Centrale de Lyon, France); P. Perruchaut (Snecma Moteurs, France) The knowledge of wear kinetics of low friction coatings is of great interest for industrial applications. For instance, in aeronautics, some titanium parts surfaces are protected by solid lubricants to prevent fretting damage (fretting occurs between two surfaces in contact submitted to vibrations) and the lifetime of these coatings has to be known. In this study, we develop a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the contact force and the sliding amplitude. This approach is applied to a polymer bonded molybdenum disulphide solid lubricant film used in aeronautical applications. A cylinder on flat contact geometry is used to investigate the effects of contact force and displacement amplitude. The evolution of the friction coefficient is studied in order to determine the lifetime of the coating. Indeed, the friction coefficient increases slowly from a low incipient value, then reaches a constant intermediate value which is maintained during the lifetime of the solid lubricant in the contact. In a second stage, the friction coefficient increases rapidly to reach a higher constant value which corresponds to the friction coefficient observed without solid lubricant. The results show that the lifetime is related to the maximum local dissipated energy, whatever the normal contact force and the tangential displacement amplitude. Thus, the local variable used is representative of the wear kinetics and the "lifetime vs. maximum local dissipated energy" curve is characteristic of the wear resistance of the considered coating. This approach can be used to select coatings or to determine maintenance frequency in order to reapply the solid lubricant film.
9:30 AM		E2-4 Velocity Effects on Erosion-Corrosion of CrN/NbN Superlattice PVD Coatings Y.P. Purandare, M.M. Stack (Univerisity of Strathclyde, United Kingdom); P.Eh. Hovsepian (The Sheffield Hallam University, United Kingdom) Erosion-corrosion or tribo-corrosion pose a threat to materials which are subjected to conditions involving corrosive slurries. Industries suffering from erosion-corrosion include the automotive, aviation, pump, mining, petroleum industries and many more. Material removal due to erosion-corrosion can be different than the damage caused by the individual components, i.e. erosion and corrosion. Hard and corrosion resistant coatings have been produced to resist this combined attack. Coatings include thermally sprayed coatings, composite cermets, multilayer and monolithically grown PVD coatings. They have improved wear and corrosion resistance owing to their high hardness and use of noble metals. Previous work on the erosion-corrosion of CrN/NbN superlattice coatings have shown changing erosion-corrosion mechanisms and volume loss with the change in impact angle and applied potential. Yet the effect of velocity on the change in the erosion-corrosion mechanisms of superlattice CrN/NbN PVD coatings is unexplored. The present work investigates the velocity effects on erosion-corrosion behaviour of range of multilayer CrN/NbN "superlattice" PVD coatings against that of uncoated M2 tool steel. This work discusses the possible synergism between erosion-corrosion for CrN/NbN PVD coatings. Erosion-corrosion mechanism maps and wastage maps have been generated based on the results to depict the changing mechanisms of material degradation with impact angle and changing velocity. The erosion-corrosion mechanisms identified have been supported with Scanning Electron Microscopy and Atomic Force Microscopy.
9:50 AM		E2-5 Friction Components in a Coated Deformed Contact H. Ronkainen, K. Holmberg, A. Laukkanen, S. Varjus (Technical Research Centre of Finland, VTT Industrial Systems, Finland) Stress and strain modelling and stress field computer simulations are today an important tool for systematic approach and optimisation of tribologically loaded coated contacts. They illustrate and quantify the dominating parameters resulting in crack initiation, crack growth and failure of coated surfaces. Friction and its components, adhesive friction and ploughing friction, are crucial parameters in modelling. The paper presents how friction is divided in its two components for modelling. The total friction can be experimentally measured, but in modelling the ploughing component is integrated in the model while the adhesive component needs to be determined as input value for stress simulations. The effect of the two friction components on crack initiation is shown by a three dimensional finite element micromodel (3D FEM) that simulates the spherical tip sliding on a coated flat substrate with increasing load similar to the conventional scratch test contact. Experimental measurements of 2 µm thick TiN and 1 µm thick DLC coatings on high speed steel substrates show how the adhesive friction component can be separated in both cases from the total friction, which was increased from 0.1 to 0.14 for TiN and from 0.07 to 0.1 for DLC coated surfaces during scratch testing. The breaking down of the contaminant layers covering the coated surfaces in multi-pass scratch tests is also shown. The contaminant layer breaks down after 4-10 passes which decreases the friction coefficient by 30 %.
10:10 AM		E2-8 Surface Reconstruction Mechanisms for Smart Nanocomposite Tribological Chameleon Coatings A.A. Voevodin, J.S. Zabinski (Air Force Research Laboratory) Current research on new tribological materials is focused on long service life and friction reduction for mechanisms operating across multiple environments and at high contact loads. The demand for high performance and reliable materials is largely driven by the aerospace industry, where environments include atmospheric gases, vacuum, and temperatures from cryogenic to 500°C. Coating reliability requires a high mechanical toughness to withstand various types of contact loading. It is not possible to meet this broad range of requirements with any single material with a predefined surface chemistry and structure. New coating materials were designed to re-arrange their structure and chemistry on demand to adapt to variable surface conditions. These materials have been dubbed chameleon because of their ability to change their surface chemistry and structure to avoid wear. The chameleon coating concept involves a nanocrystalline/amorphous nanocomposite structure, where individual phases are arranged to provide a high degree of mechanical and thermal stability and, at the same time, serve as nano reservoirs for tribological surface self-reconstruction. The stored materials are released from nanophase reservoirs in the process of wear and tribological surface chemistry and structure change to continuously reduce friction and wear. This surface response is triggered by changes in the operating environment and/or temperature. Several mechanisms are employed, including self-induced crystallization of amorphous dichalogenide phases, nucleation of nanograins of low melting point metals, formation of low melting point glassy ceramics, and change in the electron hybridization of carbon. These mechanisms were explored in sliding wear tests performed in controlled humidity air, dry nitrogen, and vacuum, as well as at 500-600 °C in air. The unique friction and wear performance of chameleon coatings in environmental cycling is demonstrated.
10:50 AM		E2-10 The Effect of CeO₂ Abrasive Size on Dishing and Step Height Reduction of Silicon Oxide Film in STI-CMP D.S. Lim, J.W. Ahn (Korea University, South Korea); H.S. Park, J.H. Shin (Hynix Semiconductor Inc., South Korea) The effect of CeO₂ abrasive size during STI-CMP was investigated to minimize the oxide dishing and planarization efficiency. Slurry samples A, B, and C were prepared based on a 1.0 wt% abrasive concentration with different sizes of ceria particles. Step height and dishing reduction depending on polishing time were investigated with pattern wafers in prepared slurries. Thickness reduction as a function of polishing time varied with slurries. The step height reduction of patterned wafer dependence on polishing time showed non-linear behavior in all tested slurries. The amount of dishing was also varied with types of slurries. The changes of the cross-sectional profiles of the oxide as a function of polishing time were analyzed to establish dishing and step height reduction model depending on abrasive size. The results also showed that smaller size abrasive reduces micro-roughness and thickness of the damage layer. Research supported by Memory Research & Development Division, Hynix Semiconductor Inc.
11:10 AM		E2-11 Mapping Erosion-Corrosion of Composite WC/Co-Cr Based Composite Coatings M.M. Stack, T.M.A. Abd El-Badia (Univerisity of Strathclyde, United Kingdom) In studies of erosion-corrosion of composite based coatings, there have been few attempts to map the mechanisms of the erosion-corrosion process. This is despite the fact cermet based coatings are being increasingly used to combat erosion on pipelines and pumps where the degradation is caused by a combination of sand particles and sea water. In addition, the interactions between the processes of erosion and corrosion have only been evaluated for a limited range of conditions, despite the fact that erosion-corrosion occurs over a wide range of variables in practice. In this study, the effects of velocity, particle concentration and potential were evaluated for a WC/Co-Cr based coating. The extent of wastage was assessed using scanning electron microscopy, atomic force microscopy and weight loss techniques. Mechanisms of erosion-corrosion were identified from the results. The results were used to generate erosion-corrosion mechanism maps for the coated and uncoated specimens. Velocity/potential maps were constructed showing the transitions between the erosion-corrosion regimes as a function of these parameters. The extent of synergy was also superimposed on such maps, showing the conditions where synergistic effects were likely to be at a minimum for the main process parameters.
11:30 AM		E2-12 Impact of Mechanical Properties Measured at Room and Elevated Temperatues on Wear Resistance of Cutting Tools with TiAl_xN and AlCrN Coatings G.S. Fox-Rabinovich, S.C. Veldhuis (McMaster University, Canada); B. Beake (Micro-Materials, Ltd., United Kingdom); J.L. Endrino (Balzers Inc., Liechtenstein); R. Parkinson (Micro-Materials Ltd., United Kingdom); L.S. Shuster, M.S. Migranov (Ufa Avia Institute, Russia) A set of mechanical properties for TiAl@sub x@N (x=0.5 and x@0.66), and AlCrN coated cutting tools such as microhardness, elastic modulus, H/E ratio, plasticity index (microhardness dissipation parameter, MDP) as well as nano-impact fracture resistance have been investigated at room and elevated temperatures (up to 500°C) using Micro Materials NanoTest Platform System. Cutting tool life was studied under the end milling conditions of structural 1040 steel. It was shown that such mechanical characteristics as H/E ratio and MDP as well as nano-impact fracture resistance measured at room temperature could be used to assess resistance to the adhesive-fatigue wear that is typical for end milling conditions. Microhardness of the coatings as well as H/E ratio drops down and MDP values growth vs. temperature of testing. The data obtained during quick lab nano-hardness testing could be used to rank the coating studied and in some cases predict the coated tool life.