http://www.123seminarsonly.com/Seminar-Reports/2013-04/80827267-Coatings-tribology-contact.pdf

Coatings tribology—contact mechanisms and surface design

Significant progress has been made in the study of contact mechanics of engineering surfaces, particularly layered surfaces, in the past decade. This mainly involves the removal of the assumptions in the Hertztheory. The application of numerical techniques including the integral transform and finite element methods has successfully solved many contact problems which are difficult with analytical solutions. The present paper reviews the recent developments in contactmechanics of engineering surfaces, with emphasis on the layered surfaces where a coating or several coatings with varying mechanical properties are produced on top of the substrate by means of surface engineering. The developments in non-layered contact theories are first briefly discussed, followed by detailed discussion of the layered surface contact theories which have been developed on this basis. Finally, further work which is needed in the near future concerning layered surface contacts is proposed.

Contact mechanics of engineering surfaces: state of the art

The paper employs a rough-surface numerical elastic contact method designed to analyze Hertzian elastic contact effects of surface coatings. In particular the paper explores the differences in the surface contact mechanics and the resulting sub-surface stresses experienced over a range of differing coating material-properties, thickness, and machined roughness levels in a quantitative manner. The effect of a range of surface roughness properties and in particular root mean square roughness (σ) and correlation length (β*), on the magnitude and depth of maximum shear stresses in the layer under individual asperities is investigated. This is done for a hard and stiff, and also for a soft and compliant coating, and for two coating thicknesses in each case. The results suggest that the magnitude of the local shear stress increases with increasing ratio σ/β* approximately linearly. The depth of the maximum local shear stress is found to correlate best with β*, however a further clear trend is observed between this depth and the number of profile peaks. The depth also shows a relation to the ratio σ/β* but the correlation in this case is weaker with significant deviations. Neither the magnitude nor the depth of shear stresses shows any significant trend in relation to the roughness (a) alone. The tensile stresses at the interface, and the subsequent potential for delamination, are also investigated and found to be significant. Approximate correlation between the magnitude of interface tensile stress and root mean square roughness is achieved, but no clear trend in relation to correlation length is evident.

A Numerical Study of the Contact Mechanics and Sub-Surface Stress Effects Experienced Over a Range of Machined Surface Coatings in Rough Surface Contacts

A model has been developed to investigate the effects of a rough elastic indenter in sliding contact with a viscoelastic layer bonded to an elastic semi-infinite plane. The viscoelastic layer is modeled as a one-dimensional Maxwell solid (small strain) while the substrate of the semi-infinite plane is modeled as a two-dimensional elastic half plane. The Fredholm integral equation of the second kind was obtained to determine the contact stresses. The results indicate that the viscoelastic properties of the layer and the velocity of the indenter significantly affect the contact pressure and internal stress distributions. The viscoelastic layer causes the pressure and internal stresses to become nonsymmetrical. High asperity densities reduce the amplitude of maximum shear stress at a fixed depth below the surface. Results are also presented for various coefficients of friction.

Internal Stresses in Contact of a Rough Body and a Viscoelastic Layered Semi-Infinite Plane

Tribological Properties of Metallic and Ceramic Coatings

A 2D simulation of a rough elastic cylinder normally loaded on a plane coated elastic medium is presented. The effect of soft and hard coatings on the rough contact sure field is significantly altered by the surface roughness. The presence of a soft coating greatly reduces these perturbations and Von Mises stresses under the surface. Hard coatings also reduce these stresses, but in a less order, and for thicknesses less than 10μm the maximum Von Mises stress is located at the coating/substrate interface.

J.M. Leroy

Papers

Paper VI (i) Effect of surface coatings in a rough normally loaded contact