Date of Graduation

2014

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Member

Hal Zhang

Committee Member

Xueyan Song

Abstract

The major interests of developing Ni-Cr alloy plating lie in the possible formation of a passive film by chromium and its good chemical and mechanical properties. And nano scale multilayer coatings exhibit improved properties due to nano scale effects (e.g.: high hardness and strength) and they combine and enhance different properties from individual components. Thus, electroplating of Ni-Cr multilayer coatings could have high potential for numerous engineering applications. Electroplating in aqueous and in organic solution by direct current were studied for Ni-Cr alloy coatings, and Ni-Cr multilayer coatings were prepared by pulse current electroplating in single bath.

Traditionally, chromium or chromium alloy coatings were deposited from Cr(VI) bath. The main advantage of a Cr(III) plating bath in comparison with a Cr(VI) bath is that Cr3+ ions are non-toxic. However, it is almost impossible to deposit the Cr coating from a simple aqueous Cr(III) solution due to a very stable [Cr(H2O)6]3+complex. In this study, two complex agents were used to promote the discharge of Cr 3+ at the cathode side. A good ratio of main salts and complexes agents with a decent wide range of current densities for Ni-Cr alloy was determined by Hull cell test. The control of Cr content in Ni-Cr alloy coating was achieved in the optimized plating bath.

The effects of Dimethylformamide (DMF) content in the solvent and current density on the surface morphology, chemical state and hydrogen evolution were studied. Hydrogen evolution and crack density decrease with the increase of DMF content. Crack-free Ni-Cr alloy coatings with Cr content up to 58% were successfully obtained from 40% DMF mixture electrolyte with wetting agent.

Cr-Ni multilayer coatings were achieved by pulse-current electroplating from a Cr(III)-Ni(II) bath at room temperature without cracks. The thickness of each layer was controlled to be tens nanometers by varying the duration of each pulse cycle, as characterized by transmission electron microscopy (TEM). The Cr-Ni multilayers were composed of alternate amorphous Cr-rich and nanocrystalline Ni-rich layers.

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