Caractérisation d'un placage de cobalt-phosphore nanocristallin

The adhesion of Co-P plating on various substrates (Inconel 718, Ti-6AI-4V and AISI 4340, AISI 9310 and HCM3 steels) used in aerospace industry is evaluated by means of a bend test realized according to the ASTM B571 standard and by a Micro Scratch Test. The hardness is measured by micro-indentation and by nano-indentation wich also gives the Young's modulus for each coating.

The corrosion resistance of the Co-P plating is compared to those of chromium, cadmium and an aluminum based (Alseal) coatings after immersion tests in fuel and engine oil. The coatings also underwent a salt spray corrosion test (5 % NaCl) according to the ASTM B117 standard. Open circuit potential and impedance spectroscopy allowed us to collect electrochemical data, to observe and to compare the corrosion mechanisms of the Co-P with chromium plating when immersed in a solution containing 3,6 % of NaCl. Finally, the wear resistance of Co-P is compared to chromium plating in dry wear (pin-on-disk) and tribocorrosion conditions.

This study shows that Co-P plated by pulsed electrodeposition is a nanocristallin coating constituted of equiaxed grains of cobalt supersaturated with phosphorus. The grain size lies between 5 nm to 15 nm. The Co-P plating exhibits a close-packed hexagonal structure and develops a (002) texture during its growth. The coating is dense and homogeneous with a nodular morphology at the surface The Co-P shows a good adhesion with many tested substrates. The micro-hardness of as deposited Co-P is equal to 600 HV and its Young's modulus is 165 Gap. These values increase after heat treatment to 750HV and 195 Gap respectively. The increase of the grain size after heat treatment is the main hardening mechanism of Co-P according to an "inverse Hall-Petch" law. The precipitation hardening mechanism stays hypothetical since no precipitate is observed in this study. The Co-P coating, even heat treated, presents lower hardness values than chromium plating (850 HV), but high enough to fulfill the desired tribo-mechanics functions (the required hardness for chromium is 600 HV).

The low wear rate of the coating and its low friction make the nano Co-P coating a potential candidate to replace chromium plating for wear resistance, in addition, the Co-P provides excellent corrosion resistance in every corrosive environment tested in this work. Therefore, it can also replace the cadmium and Alseal coatings since it was in compliance with all P&WC specification requirements of these two coatings. Finally, this work has found that the Co-P plating can be used to protect the surfaces which are subject to the degradation by the simultaneous effect of corrosion and wear. It exhibited a remarkable chemical stability by forming a passivation layer which slows down cobalt dissolution and also acts as a solid lubricant.

The Co-P salt spray corrosion resistance is diminished when the coating thickness gets thinner. It is also found that studied heat treatment increases hardness and Young's modulus of the nano Co-P but had no significant impact on the wear resistance of the coating. The corrosion resistance of the heat treated nano Co-P is slightly inferior to that of the as deposited plating.

The originality of this work comes from the fact that it tries to correlate for the first time the wear and corrosion resistance of a cobalt-phosphorous nanocristalline plating with its mechanical properties and its microstructures characteristics.