An investigation of the plasma electrolytic oxidation mechanism for coating of alumina-zirconia nanocomposite

Abstract

Plasma Electrolytic Oxidation (PEO) is an electrochemical surface modification technique which is able to coat ceramic coatings on valve metals from either cationic or anionic species in an aqueous electrolyte. Applying high voltage in the range of 300-600 V is required to achieve crystalline phases. PEO is a clean technology which offers several benefits including high deposition rates, excellent adhesion strength and mechanical properties. One of the interesting capabilities of the PEO process is its ability to form composite coatings. The following work aims to study the growth mechanism of nanocomposite layers coated by PEO. Two experimental routs have been explored to develop Alumina-zirconia composites on Al 7075 alloy through PEO method. In one route, Zr containing salt has been added to the electrolyte to form ZrO2 portion of the composite. In another way, ZrO2 portion of the composite is incorporated from zirconia nanoparticles added to the electrolyte. Potentiostatic and galvanostatic modes have been applied for all the samples to study the best condition for composite coating formation and surface sensitive properties improvement. Effect of various coating parameters such as PEO voltage, current density, growth time, and electrolyte composition on coatings characteristics has been studied. In this dissertation, the best coating conditions to achieve desirable properties (tribological and corrosion properties) have been reported. Potentiostatic mode by introducing higher energy to the surface, resulted in high hardness and crystalline phases and therefore, better tribological properties. While, in galvanostatic mode by formation of more compact layers, corrosion protection could be achieved. The dissertation is composed of three main chapters in which the microstructural evolution studies are correlated to the surface sensitive properties. The first part of this work focuses on formation of Alumina-zirconia nanostructured coatings on 7075 Al alloy in a DC potentiostatic mode. The composite coatings were produced in the range of 425-500V in an alkaline electrolyte containing 4g/L K2ZrF6. Tribological properties of coatings were investigated using dry sliding wear test against WC balls with a pin-on-disc tribometer. Wear rates were evaluated using optical profilometer. It was shown that the nanostructured alumina-zirconia composite coatings can be formed at voltages >450V. The coating thickness and roughness were in the range of 15.2-24.2 μm and 0.68-2.35 μm, respectively. The distribution of Al, Zr and O in the coatings was uniform. Increasing the PEO voltage led to porosity increment and formation of the high temperature tetragonal zirconia phase (t-ZrO2). Significant enhancement in tribological properties for coated samples was achieved: under optimum conditions, corresponding to the PEO treatment at 500 V for 200 s, the coating wear rate of 2.62x10-6 mm3N-1m-1 and friction coefficient of 0.22 were recorded that are about 120 and 3 times lower than those for the substrate. In the second part of this work formation mechanism of compact alumina-zirconia nanocomposite coatings on Al alloy through the PEO method in DC galvanostatic mode has been studied. The layers were coated at constant current density of 0.2 A/cm2 and 100-350 s growth time in an alkaline K2ZrF6 containing electrolyte. The characteristics of the coatings were investigated as a function of PEO processing time. Electrochemical properties of the layers were studied by conducting potentiodynamic polarization experiments in 3.5% NaCl solution. The results showed that under the present PEO experimental conditions, alumina-zirconia nanostructured coatings can be produced with 10-30 μm thickness and 0.4-2.35 μm roughness depending on the processing time. Phase analysis showed that the nanostructured coatings contained alumina and zirconia high temperature phases (tetragonal zirconia and a-alumina). Processing for 300 s was found to produce the most compact layer with low surface porosity (0.69%) and 26 mm thickness. This particular PEO treatment was found to reduce the corrosion rate by 2.5 orders of magnitude compared to the uncoated substrate. This significant improvement in corrosion resistance is attributed to the barrier effect of the dense layer and the presence of tetragonal zirconia. The composite coating formation mechanism in this case is oxidation of Al substrate to form alumina and oxidation of Zr+4 ions in the electrolyte to from zirconia. Finally, the last part of the research is dedicated to understand the mechanism of composite formation through PEO method. Alumina-zirconia nanostructured layers were coated on an aluminum alloy by PEO technique in a direct circuit galvanostatic mode at 0.1- 0.4 A/cm2 current density. The coatings were formed in an electrolyte containing monoclinic nano-ZrO2 powder as a zirconia source. The microstructure of the produced coatings was studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) to develop an understanding of the growth mechanism. The investigation of the coating process was complemented with voltage-time response measurements and in-situ optical spectroscopy observations. The results showed formation of various alumina-zirconia composite microstructures as a function of the current density during processing. At the higher current density, the composite layer consists of high temperature phases (tetragonal zirconia and -alumina) in addition to monoclinic zirconia. High current density introduced larger amounts of zirconia to the coated layer due to the high energy applied to the nanoparticles in the electrolyte. TEM analysis showed formation of four sub-layers across the coating. The coating-substrate interface (sub-layer 1) contained higher amounts of alumina (both  and ) while the amount of zirconia nanoparticles increased by moving toward the outer surface. In sub-layer 2, formation of tetragonal zirconia was observed resulting from a phase transformation of monoclinic to tetragonal zirconia. In sub-layer 3, the existence of untransformed monoclinic zirconia hints to inadequate energy for the phase transformation due to lower temperatures. At the top region of the coating, sub-layer 4, a shallow amorphous layer was formed due to quenching from the direct contact with the electrolyte. The PE process was found to be responsible for the monoclinic to tetragonal zirconia transformation, while the electrophoretic process facilitated the deposition of the original monoclinic zirconia from the electrolyte. The results showed that the coating mechanism involves a hybrid PE-Electrophoretic process.