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

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.

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.

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 U_b = -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.

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.