Behaviour towards the corrosion of the system of copper-aluminium thin films

摘要

Thin films Al-Cu were deposited on glass substrates from composite targets by cathodic magnetron radio frequency (13,56 MHz) sputtering. The microstructure was studied by transmission electronic microscopy (TEM) and X ray diffraction(DRX) consisted of either only one phase cfcCu for the films with compositions equal or higher than 86,17 at. %Cu or the two phases cfcAl + the predicted theta (Al2Cu) phase for the compositions equal or lower than 45,99 at. %Cu. For the intermediate compositions, it was a mixture of the cfcAl phase + an unexpected phase which was identified as the Cu3Al phase with L1(2) type, Cu3Au structure and a measured lattice parameter of about 0,306 nm. A phase separation was observed for the first time in the binary Al-Cu alloys system, this one consisted of a mixture of the three distinct phases : cfcAl + cfcCu + Cu3Al in the deposits of nominal composition Al-66.64 at. %Cu. The co-sputtered Al-Cu thin films are much harder than conventional corresponding alloys, the Vickers micro hardness and Young modulus increase regularly with copper concentration reaching respectively a maximum of about 8000 MPa and 200 GPa for the films with compositions close to equi-atomic compositions. The corrosion potential evolution of the co-sputtered Al-Cu thin films have been performed in an aerated neutral 37 g/l NaCl with a classical apparatus with three electrodes conducted by a microcomputer. The working area was about 0.5 cm(2). Two electrochemical test types are usually used for the study of the corrosion potential. The first consists in measuring the corrosion potential according to the time. The general interest of this one is the stabilization of the corrosion potential. The working time, after stabilization of the corrosion potential according to time was about one hour. The second test consists in measuring the corrosion potential E-corr of the samples according to the corrosion current I-corr. The working time after formation of the voltametric curve was also about one hour. The corrosion-potentials for all films are directly given by the microcomputer and are in agree with that extrapolated from stabilization and voltametric-curves. The corrosion potential was about -275 mV for the films with nominal 66.64 at. %Cu compositions and the general tendency is the ennoblement of the corrosion potential of the co-sputtered Al-Cu films with copper contents enrichment. Compared to corrosion potential of steel XC18 (about of -500 mV in the same conditions), the Al-Cu films containing less than 20 at. %Cu are anodic, for higher contents they are cathodic. In the first case, they can be used as sacrificial coatings for the cathodic protection of steel. But, some precautions are to be taken where the surfaces to be coated might be very well polished and cleaned, the deposits did not have any discontinuities which cause localized corrosion pitting.