Date of Award

4-6-2021

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Ceramic coatings, non-valve metals, plasma electrolysis

Supervisor

Xueyuan X. Nie

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

A modified plasma electrolytic oxidation (PEO) treatment has been successfully developed for the non-valve metals of Fe and Cu in electrolyte containing sodium aluminate and sodium phosphate. This process could also be termed as plasma electrolytic aluminating (PEA) since the formation of passive films mainly relies on the aluminate ions. The passive film will hinder the current flow and cause charge build-up. When a critical voltage is reached, dielectric breakdown of the passive film will ignite the sparks. X-ray photoelectron spectroscopy (XPS) analyses indicate the passive film formed on the Fe consist of FeAl2O4, which means iron substrate participated in the reaction. On the other hand, the copper substrate was not involved in the passive film formed on the Cu, which consists of Al(OH)3. The different mechanisms could be attributed to the different reduction potentials of Fe and Cu. Taguchi analyses were used to investigate the influence of selected process parameters, including the concentration of NaAlO2 in the electrolyte (C), the frequency (f) and duty cycle (δ) of the power supply. ANOVA analysis revealed that C has the most significant contribution to hardness, corrosion resistance and thickness. While f has significant influence on hardness and corrosion resistance, δ contributes significantly to the thickness. Higher frequency means shorter duration of a single discharge which leads to denser coating with higher hardness and corrosion resistance. Higher duty cycle represents the higher power input during the PEA treatment. Therefore, the coating’s thickness increased with higher duty cycle. The coating prepared on iron substrate mainly consists of Al2O3 and FeAl2O4. The hardness, polarization resistance and thermal conductivity of the coating were 822 HV, 296 kΩ·cm2 and ~0.5 W/(m·K), respectively. The low thermal conductivity comes from the mesopores, nano-grains and amorphous materials. After cyclic thermal shock tests, the coating retained its porous structure without spallation. Post-treatments like electroless nickel plating (EP) and sol-gel silica coating were applied to seal the open pores and cracks. Both the PEA-EP and PEA-SiO2 hybrid coatings could retain good corrosion resistance after immersed in sodium chloride solution for five days, while the PEA coating degraded due to pitting corrosion at these open pores and cracks. The coating deposited on pure copper consists of ceramic matrix (Al2O3 and Cu2O) embedded with Cu particles. The amount of Cu particles increased with increased coating thickness, which could be attributed to intensified plasma discharges. The hardness, polarization resistance and thermal conductivity of the coating were 1050 HV, 141.7 kΩ·cm2 and ~5.1 W/(m·K), respectively. The increased thermal conductivity could be attributed to the presence of metallic Cu. The coating has excellent wear and corrosion resistance, which might be used for wear-corrosion protection of copper alloys.

Download

Share

COinS