ABRASION RESISTANT COATINGS FOR STEELS SUBJECT TO SEVERE WEAR CONDITIONS ARE FREQUENTLY APPLIED USING CVD TECHNIQUES OR TROUGH CONVENTIONAL PROCESSES OF NITRIDING, CARBURIZATION OR BORONISING. THESE SOMETIMES HAVE LIMITATIONS DUE TO THE HIGH TEMPERATURES REQUIRED TO ACHIEVE DEPOSITION, OR BECAUSE THE LAYERS PRODUCED ARE THIN. IN MANY CASES IT IS ALSO NECESSARY TO PROVIDE A HIGH DEGREE OF INITIAL SURFACE PREPARATION.
Electrodeposition techniques have been developed, on a laboratory scale, for the aluminide or silicide coating of steel components. Complex shapes can be treated and the coating thickness can be readily controlled. Benefits of such coatings have been demonstrated in different corrosion and wear conditions.
Silicide coatings are obtained by diffusion of silicon in the steel and formation of an adherent layer of iron silicide (Fe3Si). The microhardness of 42CD4 silicided steel is similar to that of nitrided 42CD4 but similar results can be obtained for silicided low price steels. The friction wear on alumina is similar to nitrided steel as is the behaviour in wet corrosion, except in hydrochloric acid. The silicide coating also protects the steel up to 650 C in pure oxygen and up to 500 C in hydrogen sulphide containing 1% hydrogen.
The aluminide coating technique produces on adherent layer of iron aluminide (Al5Fe2). Aluminided 42CD4 steel shows an increase in microhardness of 40% in comparison to the nitrided steel. However, friction wear tests on alumina were unsatisfactory unless postnitridation was carried out. Wet corrosion behaviour is similar to nitrided steel, except for hydrochloric acid, whereas in dry corrosion the aluminided steel performs as well as, or better than, refactory steel in both oxygen and hydrogen sulphide at 800 C.
Combined aluminium and silicon treatments were studied with further nitriding, which did not enhance hardness values, but wear values were increased by 50%.
Aluminium coatings appear exploitable, although the market for the main potential applications - corrosion resistance - is not currently very active. A demonstrator exists for the production of small treated components.
On a laboratory scale, coatings were deposited on a 42CD4 steel using a molten salt bath process. By controlling the temperature and the electrical conditions of the reaction, a range of deposited coatings was achieved. An electrolytical technique allows coating of complex geometries, control of coating thickness and diffusion at the interface.
Many industrial steel tools are subject to heavy wear are coated by abrasion resistant coatings. The properties of these coatings are often limited by the high temperatures that are necessary to achieve deposition, and by the thinness of the layers produced. Aluminium and silicon rich coatings deposited by electrochemical means could have the advantage of increasing hardness and layer thickness without necessitating high temperatures. Electrochemical methods also have the advantage of providing a high degree of independent control of the depositing species, and can provide coatings on complex geometrical parts with comparative ease.
ABRASION RESISTANT COATINGS FOR STEELS SUBJECT TO SEVERE WEAR CONDITIONS ARE FREQUENTLY APPLIED THROUGH CONVENTIONAL PROCESSES WHICH SOMETIMES HAVE LIMITATIONS DUE TO THE HIGH TEMPERATURES REQUIRED TO ACHIEVE DEPOSITION, OR BECAUSE THE LAYERS PRODUCED ARE THIN.
IN MANY CASES IT IS ALSO NECESSARY TO PROVIDE A HIGH DEGREE OF INITIAL SURFACE PREPARATION.
THIS PROJECT AIMS TO STUDY AN ELECTROCHEMICAL METHOD OF DEPOSITING AL AND SI-RICH COATINGS OF BETWEEN 1 AND 100 MICROMETRES ON STEEL WHICH CAN HAVE THE ADVANTAGE OF PROVIDING A HIGH DEGREE OF INDEPENDENT CONTROL OF THE DEPOSITING SPECIES AND CAN ALSO PROVIDE COATINGS ON COMPLEX GEOMETRICAL PARTS WITH COMPARATIVE EASE.
Funding SchemeCSC - Cost-sharing contracts