FRETTING BEHAVIOR OF COATED AND TREATED TITANIUM ALLOY USING AN OPEN-LOOP SYSTEM Yantio Njankeu G.R., Paris J.-Y., Denape J. Laboratoire Génie de Production - École Nationale d’Ingénieurs de Tarbes – Tarbes, France Pichon L., Rivière J.-P. Laboratoire de Métallurgie Physique, UMR 6030 - Université de Poitiers - Chasseneuil Futuroscope, France ABSTRACT Titanium alloys are well known to present poor sliding behaviour and high wear values. Various coatings and treatments have been tested to prevent such an occurrence under fretting conditions at high frequency of displacement (100 Hz). An original test apparatus, using an open-loop system instead of a classical imposed displacement simulator, has been performed to directly display the phenomenon of seizure, defined as the stopping of the relative motion between the contacting elements. A classification of the tested coatings has been proposed on the basis of their capacity to maintain full or partial sliding conditions, to present low wear rates and to prevent seizure. INTRODUCTION The damage produced by oscillatory movements of weak amplitude (fretting) applied to two contacting surfaces is often considered as especially disastrous for industrial installations. The two main types of damage identified on such surfaces are crack pattern (failure risk) and particle detachment (material lost) [1]. The first type of damage occurs when surfaces are subjected to cyclic stresses under imposed amplitude of displacement. The nucleation and propagation of fatigue cracks are most favoured by partial sliding conditions where the central area of contact stays stuck. A way to prevent crack damage consists of favouring the full sliding conditions. These conditions can be reached with regard to only imposed displacements increasing in size the higher the applied load. However, these sliding conditions involve the production of wear particles with loss of material. Palliative materials and coatings like soft coatings often avoid fatigue crack propagation while hard ones rather decrease wear [2]. Also, solid lubricants in the form of bounded coatings usually reduce friction and wear. Titanium alloys are well known to present poor sliding behaviour and high wear values. The present study proposes a classification of the sliding ability of various coatings deposited on a titanium alloy under free displacement mode. The test apparatus has been developed to directly reveal the phenomenon of seizure (sticking condition) defined as the stopping of relative motion between the contacting surfaces as a result of interfacial adhesion. The determination of partial slip / full slip transitions, the mechanisms of surface accommodation and the wear behaviour, are also studied. MATERIALS AND EXPERIMENTAL WORK Soft thick coatings (copper-nickel alloys), thin hard coatings (amorphous carbon, silicon nitride referred to as SN1 and SN2) and surface treatments (plasma nitridations referred to as N1, N2 and N3) have been selected to reinforce a α+β titanium alloy substrate (Ti6Al4V). Thickness t, hardness H and average roughness Ra of each coating are shown in table 1. Table 1. Main properties of the studied materials. Values of H associated to an absolute uncertainty, proceed from nanoindentation measurements. Materials t (µm) Ra (µm) H (GPa) Ti6Al4V - 5.4 ± 0.3 Ti6Al4V/CuNiIn Ti6Al4V/CuNiIn+MoS 2 Ti6Al4V/CuNiSi 150 150+20 10 11.2 5 0.6 2.1 ± 0.1 0.42 ± 0.01 2.9 ± 0.5 Ti6Al4V/DLC Ti6Al4V/SN1 Ti6Al4V/SN2 0.3 0.3 0.3 0.05 0.04 0.05 13.6 ± 0.4 19 ± 3 14 ± 1 Ti6Al4V/N1 Ti6Al4V/N2 Ti6Al4V/N3 0.1 0.2 0.1 0.06 0.04 0.05 27 ± 3 12 ± 3 15 ± 2 Tests were carried out with a fretting device where oscillatory motion is controlled by a mechanical vibrating system driven in open-loop control (figure 1). The sliders were hemispherical in shape (curvature radius of 10 mm) and were Proceedings of WTC2005 World Tribology Congress III September 12-16, 2005, Washington, D.C., USA WTC2005-63531 1 Copyright © 2005 by ASME