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Limites d'utilisation des revêtements d'étain en connectique automobile

Abstract : Tin coatings are widely used in automotive connectors for applications where temperature are not too severe. Currently, there is no consensus concerning the limit temperature to use tin coatings. The aim of this study is to study the thermal ageing of the copper/tin systems in the connector field, and to estimate the lifetime performance, i.e. under a medium current (producing Joule effect), and in a vibratory state (fretting-corrosion). The copper/tin interdiffusion leads to the formation of intermetallic phases (Cu6Sn5 et Cu3Sn). The growth kinetics of intermetallics is studied as a function of the nature of the coating, of the use of a nickel underlayer or not, (leading to the formation of a Ni3Sn4 intermetallic phase), and of the nature of the copper substrate (alloying elements). The measure of the parabolic growth coefficient allows to rank the substrate/coating systems in terms of the growth rate of the intermetallics. Intermetallics are signifificantly harder than pure tin. Joule effect tests (leading to a temperature rise) and fretting-corrosion tests are undertaken on different specimens, aged and non aged. The temperature rise due to Joule heating is directly a function of the contact area and the coating hardness is a decisive parameter in the control of the temperature rise at the contact spot. But the conductive contact area also depends on the oxydability of the coating, because the presence of oxide can reduce it. The phenomenon can be increased by vibrations : oxide production is accelerated, and and formation of oxide debris can lead to microcuts and disruption of the contacts. Fretting-corrosion tests are also performed to predict reliability, with two types of experiments: a variable displacement test and a lifetime test. The variable displacement test are done to identify the transition between partial slip (low displacement values, for which the lifetime is considered infinite), and gross slip (higher displacement values, for which lifetime is limited). Lifetime tests allow to compare the durability of the different specimens tested. Observations of the fretting scars are made to understand the failure mechanism. The degradation of electrical properties of tinned contacts are due to oxidation and wear, leading to microcuts emergence. The degradation of intermetallic contacts is due to an embrittlement of the coating, leading to the spalling of the intermetallics and to a change in the failure mode with the production of copper oxide debris, leading rapidly to an electrical insulation at the contact spot.
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  • HAL Id : tel-00460544, version 1

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Mélanie Bordignon. Limites d'utilisation des revêtements d'étain en connectique automobile. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2009. Français. ⟨NNT : 2009ENMP1667⟩. ⟨tel-00460544⟩

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