ICMCTF2004 Session E2: Friction and Wear of Coatings II: Design and Modeling

Friday, April 23, 2004 8:30 AM in Room California

Friday Morning

Time Period FrM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2004 Schedule

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8:30 AM E2-1 Optimising Tribological Performance of Coated Surfaces by Stress Modeling and Fracture Calculation
K. Holmberg, A. Laukkanen (VTT Technical Research Center, Finland)

The presentation discusses the friction and wear mechanisms of coated surfaces on different scale levels, at macrolevel, microlevel and nanolevel. A concept for analysing the friction and wear mechanisms on macrolevel has earlier been introduced by the author. In this presentation special attention is given to the microlevel mechanisms, and in particular new techniques for modelling the mechanical properties of coated surfaces influencing friction and wear by stress and strain analysis and finite element computer techniques.

A general approach to controlling and modelling friction and wear in sliding contacts with coated surfaces is presented. The tribological aspect of the main material parameters, elasticity, plasticity and fracture is discussed.

The tribological contact conditions with a sphere sliding over a plate coated with a very thin coating has been analysed. The dominant parameters for friction and wear performance were identified and the appropriate material parameters needed for controlling friction and wear is discussed. It is shown how a 3D Finite Element Model was developed for calculating the first principal stress distribution in a scratch tester contact with a 200 m diameter diamond spherical tip moving with increased load on a 2 m thick titanium nitride (TiN) coated steel surface. The model is comprehensive in that sense that it considers elastic, plastic and fracture behaviour of the contacting surfaces as well as strain hardening effects.

Three main regions of stress concentration during the sliding action were identified and a method for calculation of the fracture toughness of the coating/substrate system was developed. The results show the difference in the stress fields between a ball sliding on a bulk surface and on a coated surface, it shows the development and changes in the stress fields with increasing load, the influence of coating elastic modulus and coating thickness and the influence of residual stresses in the coating.

The paper outlines how the new 3D FEM surface analysis method offers a possibility to optimise the tribological performance of a coated surface and how this can be used to support surface design and systematic coating development in direction to tailor surface properties for different applications.

9:10 AM E2-3 Numerical Model of Contact Pressure and Contact Stresses Field in Coating.
D.A Djamai, Z.H Zaidi (Université de Poitiers, France)
We will present a three-dimensional numerical model to solve the quasi-static sliding contact pin / thin coating on substrate. The contact surface deformation and the contact pressure field are calculated by application of double gradient. The first gradient is applied on a given penetration rate of pin. The second minimizes the variation between the normal load fixed and the calculated load. The influence coefficients matrix is determined using the Papkovich-Neuber potentials with double Fourier transformation. The resulting stresses are discussed for different values of the layer stiffness relative to the substrate and also compared with Von Mises criterion to investigate the deformation mode and principal tensile stresses are computed accordingly. We will discuss the influence of Young modulus, Poisson ratio, and the layer stiffness relative to the substrate on the repartition of pressure, and principal stresses.
9:30 AM E2-4 Lifetime of a Polymer Bonded Solid Lubricant in Fretting: Definition of a Local Dissipated Energy Criterion
V. Fridrici, S. Fouvry, P. Kapsa (LTDS, 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.
9:50 AM E2-5 Tribological Property Improvement of EHA Pump Parts by Si-modified CrN Coating
S.Y. Lee (HanKuk Aviation University, South Korea)
The extended fly-by wire concept becomes very important in designing modern aircraft as much improved performance due to enhanced pay load could be achieved. The key technology for the extended fly-by-wire concept is EHA (Electro Hydrosatic Actuator), which is essentially dependent upon performance of hydraluic pump and motors. In the present study, the pump parts made of AISI 4340 steel were treated with gradient Si-modified CrN coating and the structural characterization as well as tribologocal properties of pump parts with thin film coatings were investigated in terms of high speed wear testing, X-ray diffraction (XRD), spectroscopy (GDOES), scanning electron microscopy (SEM), Vickers micro-hardness tester and atomic force microscopy (AFM). Description of high speed wear tester and detailed experimental results will be illuminated.
10:10 AM E2-6 Optimization and Data Analysis for the Micro-scale Abrasion Test for Thin Coatings
I. Hutchings (Univerity of Cambridge, United Kingdom)
The micro-scale abrasion or ball-cratering test is being increasingly used as a quality assurance method for thin hard coatings, and is being developed as an international (ISO) standard. In this method a ball is rotated against a specimen in the presence of a slurry of fine abrasive particles, forming a wear scar with well-defined geometry which can be measured in a simple way to yield a measurement of worn volume. Wear rates and wear mechanisms can vary significantly with changes in test conditions. The modes of wear can be changed from ‘three-body’ abrasion (with rolling particle motion) to ‘two-body’ abrasion (with grooving particle motion) by changing the load, the abrasive concentration in the slurry, the abrasive particles, the materials of ball and specimen, and the ball surface condition. These phenomena will be reviewed, and recent work summarized which allows the wear mode to be predicted. There are many different approaches which can be used to analyse the experimental data from a micro-scale abrasion test on a coated substrate, in order to derive values for the specific wear rate for the coating and substrate; they can differ markedly in accuracy and stability. These approaches will be reviewed, and an optimum method of data analysis proposed.
10:50 AM E2-8 Micro-Abrasion-Corrosion Interactions of WC/Co Based Coatings
M.T. Mathew, M.M. Stack (University of Strathclyde, United Kingdom)

Micro-abrasion, a process which occurs with particles typically less than 10 µm, occurs in many tribological conditions ranging from artificial joints in the healthcare sector to valves and seals in the off-shore industry. In such cases, the micro-abrasion process is observed to undergo various regime transitions, depending on the applied load, the characteristics of the surfaces in contact and the environment.

In recent years, there has been increasing interest in the interactions of micro-abrasion with corrosion. However, there has been little work to characterize the performance of surface coatings in such environments or to provide a basis for coating optimisation. In addition, a basis for various micro-abrasion-corrosion interactions has not been suggested to date.

In this study the micro-abrasion-corrosion performance of a WC/Co HVOF coating was assessed and compared to the performance of the steel substrate. The results were used to identify regimes of micro-abrasion as a function of applied load and electrochemical potential. In addition, micro-abrasion-corrosion maps were constructed based on the results, showing the variation between micro-abrasion-corrosion regimes, as a function of applied load and potential.

11:10 AM E2-9 Microstructure and Properties of Novel Wear and Corrosion Resistant CrON/NbON Nano-sale Superlattice Coatings
T. Savisalo, D.B. Lewis, P.Eh. Hovsepian (Sheffield Hallam University, United Kingdom)

CrN/NbN superlattice coatings have found use in number of commercial applications where wear and corrosion determine the service life of the components. To further improve their performance a novel CrON/NbON top-coat has been developed.

Coatings were deposited using industrial sized Hauzer HTC 1000/4 UBM-ABS coating machine utilising Arc Bond Sputtering method where cathodic arc metal ion etching is used to prepare the interface prior to coating deposition by UnBalanced Magnetron Sputtering. The oxynitride process was performed in mixture of dry air and argon at bias voltages varying from Ub = -75V to -120V at total pressures during deposition from 3.5*10-3 mbar to 4.9*10-3 mbar. The thickness of the oxynitride film varied between 1.6-2.3 µm, whilst the total coating thickness varied between 4.6-5.3 µm. The XTEM investigation revealed that the microstructure of the oxynitride layer was dense columnar with a pronounced superlattice architecture. On XRD, the coatings were identified as crystalline with mixed texture. As the pressure during the oxynitride deposition stage was increased the crystal structure of the top layer became increasingly amorphous. The increase in the bias voltage also caused a shift from {100} texture towards {111} texture.

The best performing oxynitride coatings had similar low sliding wear rates as the reference standard coating without affecting their corrosion resistance. However, the wear rate on the 100Cr6 counter body was reduced by a factor of 10 and the friction coefficient from 0.57 to 0.49. The wear rate of both the coating and the counter body was reduced as the bias voltage was increased, whilst increasing the deposition pressure had adverse effects on the tribological properties. The wear behaviour can be related to the special superlattice structure of the oxynitride layer as coating with best tribological properties exhibits the most pronounced superlattice structure.

11:30 AM E2-10 Impact Angle Effects on the Erosion-Corrosion of CrN/NbN Superlattice PVD Coatings
Y. Purandare, M.M. Stack (University of Strathclyde, United Kingdom); P.Eh. Hovsepian (Sheffield Hallam University, United Kingdom)

Wastage due to the combined effects of erosion and corrosion is a serious issue in a wide range of industrial environments. The extent of wastage is dependent on a range of parameters relating to properties of the particle, target and the environment. In such cases, surface engineering (through providing a protective coating) may be used to avoid the deleterious effects of both processes.

In recent years, there has been an increasing interest in the potential of CrN/NbN superlattice PVD coatings to provide resistance to erosion-corrosion in such conditions. Initial studies on the effects of velocity have shown that they significantly outperform conventional coatings, although the behaviour depends on the conditions. There is however, poor understanding on the effects of many important tribological variables, such as particle size and impact angle, on the performance of such coatings.

In this work, the effect of impact angle on the erosion-corrosion of a range of CrN/NbN superlattice coatings was assessed at a range of applied potentials in a carbonate/bicarbonate solution. AFM and SEM techniques were used to characterize the coating following exposure. Erosion-corrosion maps were constructed based on the results, showing erosion-corrosion regime transitions for such coatings as a function of impact angle and applied potential.

11:50 AM E2-11 Tribological Effects on Machining Performance of Coated Tools: Experimental Approach
S. Dechjarern (King Mongkut Institute of Technology, Thailand); R. Hibberd (Imperial College London, University of London, United Kingdom); V. Weerasinghe (University of East London, United Kingdom)
This work involved the investigation of the effect of a wide range of surface roughness on coated tools performances. Coated tool wear is greatly affected by the tribological conditions at the tool/chip interface. The examination of the tribology behaviour of coated surface with different roughness has been carried out in parallel with machining experiments. Friction tests of coated surfaces were conducted using a modified pin-on-cylinder technique. The friction test results allowed the range of surface roughness where friction might be minimized, to be identified. The mechanism of friction for coated rough surface contact were analyzed. The possible explanation of the minimum friction was presented via the effect of third-body abrasion using the ratio of the amount of wear and the valley volume. The results of the machining tests show that the effect of surface roughness on the tool wear resistance. Surface roughness was found to influence the tool/chip contact length, the coating damage, the built up of debris in terms of the ratio between wear and valley volume and tool wear. However, surface roughness was not found to influence the chip thickness and cutting forces. The results of the coated tools shows that the tool life might be improved by applying a certain range of surface roughness on the substrate prior to coating and allowing the coating layer to follow the substrate pattern. The improvement in tool life was due to the shorter tool/chip contact length, the minimum amount of coating being damage and the lowest built up of wear debris, which cause abrasion or total fracture.
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