The Formation and Frictional Behavior of Ni and Cu Aluminide Coatings on Copper

Abstract

The microstructure and frictional behavior of candidate protective coatings for steel continuous casting copper molds were studied. A number of conventional coatings, consisting of the hard chromium deposited by electroplating, nickel-phosphorus deposited by electroless plating, and WC-17Co deposited by high-velocity oxy-fuel spraying, were compared with copper aluminide and nickel aluminide coatings. The copper aluminide coating was formed via pack cementation diffusion and aluminized in the Al-AlCl3-Al2O3 powder mixture at 500 °C. The nickel aluminide coating was obtained using the same aluminizing treatment, yet after depositing an electroless nickel-phosphorus interlayer. The microstructure of the coated specimens was characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy and x-ray diffraction phase analysis. Cu9Al4 was identified as the main phase of the copper aluminide coating, whereas Ni2Al3 and NiAl phases were identified in the nickel aluminide coating. The evaluation of the coatings frictional behavior through a pin-on-disk machine applying a 12 N loading showed that the copper aluminide coating is worn away and that the nickel aluminide coating remains intact up to the end of the test. The reference coatings revealed negligible wear with the material transfer from the pin to their surface. The copper aluminide coating demonstrated the least friction coefficient among the entire evaluated coatings.