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THE PRODUCTION AND PROPERTIES OF COMPOSITION MODULATED ALLOY (CMA) COATINGS BY ELECTRODEPOSITION

Objective


The results of the present project confirmed that electroplating is a suitable technique for manufacturing CMA materials. The two main principles for electrodeposition of CMA coatings - the dual bath and the single bath techniques - exist in a number of variations which can be used with advantage depending on the specific CMA coating in question. The dual bath technique is simpler to use than the single bath technique, but the latter is in general more practical for production of CMA coatings with bi-layer thickness in the nanometer range.

In single bath electrodeposition of CMA materials, very thin layers with alternating composition can be produced by fast and precise time controlled current. The lower limit of the layer thickness depends on the sharpness of the interfaces, which again depends on the nature of the electrolyte, but layers in the nanometer range have been produced. The concept of computer controlled pulse plating was developing further to be specific suitable for electrodeposition of CMA coatings by single bath technique.

The Cu-Ni alloy system was used as a model system for electrodeposition of CMA coatings by single bath technique. The results verified the theory for CMA formation by transport limitation of the most precious of the two involved metals. Well-defined multilayered structures with bi-layeres in the nanometer scale were deposited. Cu-Ni CMA foils produced by the dual bat technique were exposed to tensile testing. A relation between tensile strength and fracture mechanism was formulated. Three types of fratures were observed:
a) dimple rupture,
b) crack propagation forming terraces, and
c) mix mode fractures.
The highest strength (up to 70% higher than a pure electrodeposited nickel foil) was related to the crack propagation fracture mechanism. A similar result was observed for Cr-Cr structurally modulated alloys. The increased strength is explained in that way that much more energy is needed to form terraces with very large exposed surface.

A large number of electrolytes were examined to identify processes suitable for deposition of corrosion and wear protective CMA coatings of the Ni-P type or the Co-Mo type alloys. The examination included electrolytes for depositing the following alloys: Ni-P, Ni-P-Cr, Ni-P-Sn, Ni-P-W, Co-Mo and Co-W. Plating processes useful for production as well-defined CMA coatings were identified for the Ni-P, Ni-P-W and Co-W alloy systems. These CMA coatings exhibited good corrosion protection of steel, due to high resistance against crack formation and pitting corrosion. The combination of layers with small difference in electrochemical potential seems to play a keyrole in preventing pittings to develop. Structural investigations of heat treated coatings showed that the tungsten containing CMA coatings have potential applications as protective coatings at elevated temperature. Improved adhesive wear resistance were observed for coatings with bi-layer thickness less than approximately 150 nm. No improvement was observed in abrasive wear resistance when the load was exposed perpendicular to the multi layeres structure.

Both Zn-Ni and Zn-Fe CMA coatings was electrodeposited by both single bath and dual bath techniques. The CMA coatings performed better cathodic protection of steel than both pure zinc and homogenous zinc alloy coatings. The reason is again believed to be a more uniform corrosion of the coating due to the layered structure leaving no weak spots in the coating.

Conclusively it can be stated that both the scientific and industrial objectives have been met. Several potential industrial applications of electrodeposited CMA coatings have been identified. The next step will be to select industrial demonstrators to verify the beneficial use of CMA electrodeposits in industrial service.
Composition modulated alloy (CMA) coatings may be defined as multi-layered structures whose thicknesses may be defined as multi-layered structures whose thicknessdes may vary from 1 nm to 1000 um and which may be produced by a single coating technique or stage through varying simple process parameters. Such multi-layers yield enhanced properties, from artificial superlattice development to engineering wear and corrosion performance.

In this programme computer-assisted electrochemical techniques will be used to develop such multilayer coatings, the technique enabling a range of thickness and thickness combinations to be obtained at relativety low cost and with considerable processing speed. Although this technique is not new in concept it has not yet been developed to industrial scale usage in Europe and requires basic research to be completed to understand process parameter behaviour adequately.

The parameters to be studied are current variations with time - "pulse plating" - and variable agitation rates. The product properties to be enhanced include corrosion and wear resistance for alloy electrodeposition systems already being used commercially, e.g. Ni-P, Zn alloys. Other applications known to have validity include X-ray mirrors, magnetic moment design, thin films of high tensile strength, amorphous alloys.

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Coordinator

TU DENMARK
Address
Dth 425
2800 Lyngby
Denmark
 

Participants (3)

LOUGHBOROUGH UNIVERSITY
United Kingdom
Address
Ashby Road
Loughborough
 
NTU ATHENS
Greece
Address
Zografou Campus
15780 Athens
 
UNIV LINKOPING
Sweden
Address

581 83 Linkoping