Investigating the effects of micro arc oxidation (MAO) process on thermal and elevated temperature mechanical properties of AZ91 MG alloy

Abstract

Magnesium and its alloys are widely used in automotive, aerospace and communications industry due to their low density, high strength to weight ratio, castability, recyclability, magnetic shielding, and high vibration damping. However, the poor surface properties such as corrosion and wear resistance restrict their more common use. AZ91 is one of the mostly used Mg alloys due to its excellent castability, corrosion resistance and ductility combination. However, its creep resistance is low. It has been observed that the surface properties of Mg alloys are improved by the Micro Arc Oxidation (MAO) process. Its elevated temperature properties such as creep could also be improved thanks to high thermal insulating properties of the ceramic oxide layer produced by the MAO process. Moreover, when AZ91 alloy is exposed to the wear at an elevated temperature (~120 oC) in automobile components such as motor blocks, transmission box, its elevated temperature wear behaviour would need to be considered. Thus, it is proposed that elevated temperature wear resistance of the AZ91 alloy can be improved with the MAO process. A passive oxide layer is formed by the micro arc oxidation method at the surface of the material, similar to the anodization. However higher voltage or current density is applied in the MAO method than those in anodization. Thus, few hundreds μm coating thickness can be obtained. While light metal alloys (such as Ti, Al, Mg) are connected to the positive pole (anode) of the power supply, the cathode (stainless steel container) is connected to the negative pole and the substrate is immersed in the basic electrolyte. In the first stage of the process, passive oxide film is formed. Then, as a result of the dielectric breakdown of this passive oxide layer, micro arcs are discharged. The melting oxide layer results of the discharge is cooled by the electrolyte at room temperature. The ions in the electrolyte react with the melted oxide under the effect of the discharge and then, as a result of its contact with the electrolyte, the oxide layer solidifies and forms the oxide coating. Since the cooling rate can reach 108 K/s, cracks, and peeling off can be seen. Coating morphology is porous due to gas envelops and discharge channels formed due to discharge formation. There are two parameters, that effecting the coatings' morphology, composition and homogenity such as electrical parameters and electrolyte. Electrolyte which is consisted of silicate, aluminate and phosphate compounds as a major addition, is alkaline in the MAO process. Coating composition and properties can be controlled in a wide range by employing various electrolyte combinations in the MAO process. In the electrolyte, hydroxide ions react with Mg in the substrate and form MgO by the heat of the plasma. Ions in the solution such as SiO32-, PO43- and AlO21- react with the MgO film with the effect of electrostatic force and discharge. As a result, Mg2SiO4, Mg3(PO4)2, Mg(AlO2)2 compounds are formed in the coating. Fluoride salts form a MgF2 passive layer, forming a homogeneous and corrosion resistant coating. Wear, corrosion resistance and thermal barrier properties of MAO coatings have been improved by dissolving zirconium compounds in the electrolyte. In addition, with the increase of electrolyte concentration, the discharge caused by breakdown becomes stronger. Parameters such as current density, voltage, frequency, duty cycle, and processing time constitute electrical parameters of the MAO process. As voltages or current density increases, the energy of microarcs increases, which increases the rate of coating formation. However, increasing arc temperature and duration with increasing current density and voltage may cause coating spallation. The temperature of micro arcs varies between 1000-10000 K, and their duration can range from a few microseconds to hundreds of microseconds. When the coating, which reaches very high temperatures in a short time, is cooled rapidly with the electrolyte at room temperature, thermal stresses occur in the coating. As the frequency decreases or the duty cycle increases, the arc temperature and duration also increase due to the increase in voltage or current density. Therefore, decreasing the frequency or increasing the duty cycle increases the coating formation rate. However, due to increasing thermal stresses, roughness, cracks and spalling increase. In order to provide the best performance under all conditions, the coating obtained as a result of the MAO process is required to be homogeneous, high thickness and have optimum surface roughness. The aim of this study is to investigate the effect of single (silicate) or dual phase (aluminate and phosphate) electrolyte and different ZrO2 sources (K2ZrF6 and Na2ZrO3) on the thermal barrier properties, high temperature mechanical properties (creep and wear) and morphology of the coating. In this study, it is also aimed that high temperature properties of the AZ91 alloy such as elevated temperature wear resistance and creep resistance enchanced with the MAO coating due to the ceramic properties of the coating. Therefore, it is aimed that coating with the highest thickness and optimum roughness. Then, MAO process was applied on AZ91 substrate at different electrolyte and electrical parameters. Furthermore, coating with the excellent thickness, homogenity and roughness combination was selected for thermal barrier tests such as thermal conductivity and thermal shock. Finally, coating showing best thermal barrier properties was selected for high temperature mechanical tests such as elevated temperature wear test and creep test. In the first section of this thesis, magnesium alloys and the MAO process were introduced, and the objective of this thesis was presented. In the second, third and fourth chapters, the journal articles prepared from the studies carried out within the scope of this thesis are included. The final chapter is Conclusions, which summarizes the general results of the journal articles. In the following paragraphs, the contents of each article are briefly summarized. In the second chapter of the study, AZ91 Mg alloy was micro-arc oxidised using different voltages in silicate- and aluminate/phosphate-based (dual) electrolytes that included K2ZrF6 or Na2ZrO3 as the zirconium source for synthesising ZrO2 in the coatings. Structural characterisations were done using SEM examinations and XRD analysis. Structural and morphological characteristics of the MAO coatings of different samples were compared by measuring coating thickness, surface roughness, pore size, and pore fraction. Furthermore, both hardness and pull-off tests were applied to characterise mechanical and adhesion properties. Thermal conductivity measurements and thermal shock tests were also carried out to evaluate the effect of both the electrolyte composition and zirconium containing compound addition on the thermal properties of synthesized the MAO coatings. It was found in the present study that the equivalent thermal conductivity of the MAO'ed samples was reduced up to 30% compared to the bare AZ91 alloy. The decrease of the thermal conductivity was mainly attributed to both the formation of a thicker and denser MAO coating and the incorporation of ZrO2 phase within the generated MAO coating. Finally, increased thermal shock resistance was strongly correlated with the lower hardness and higher cohesive bonding strength of the MAO coating, which also leads to smaller crack formation and spallation-free characteristics. In the third chapter of the thesis, tribological properties of the coatings were investigated. Low wear resistance of AZ91 alloy is the main factor limiting its more common use in industrial applications. It is therefore the MAO process is mostly applied to the alloy to improve its wear resistance at room temperature (RT). However, the effect of the MAO coating on the wear behaviour at elevated temperatures is investigated in limited works. In order to investigate the tribological behavior of the coatings, the MAO process was performed on an AZ91 alloy in single-phase (silicatecontaining) and dual-phase (aluminate+phosphate) electrolytes, and its wear behaviour was investigated at both RT and 200 ℃ compared to the bare alloy. The results showed that the wear resistance of the alloy can be significantly improved both at RT and 200 ℃, and the silicate-based electrolyte provided a better wear resistance at both temperatures. The results also showed that the dominant wear mechanism of the bare alloy is oxidation and brittle fracture of the MAO-treated alloys at both temperatures. In the fourth chapter of the thesis, creep properties of the coated alloy were investigated. AZ91 Mg alloy has a wide range of applications in the automotive industry, although its use is restricted to powertrain applications due to its low creep resistance. In this study, the effect of the micro arc oxidation (MAO) coating on the creep resistance of an AZ91 Mg alloy was investigated to take advantage of the coating layer with high thermal insulation properties. In this context, the MAO process was applied to AZ91 Mg alloy using a bipolar pulsed DC power supply. The creep tests were conducted at different temperatures (150 – 200 oC) and stresses (25 – 90 MPa) for the bare and coated samples, and the minimum creep rates were determined. It has been shown that the MAO coating reduces the creep rate of the bare alloy by 35% – 84% depending on the temperature and the stress due to the stress-reducing effect and thermal barrier properties of the MAO coating. Based on calculations based on creep activation energy and stress exponents, creep mechanisms were proposed for the bare and coated alloys. Activation energy was also calculated and lattice diffusioncontrolled dislocation climb was determined to be the effective creep mechanism for both samples at lower stresses, while pipe diffusion-controlled dislocation climb was effective at higher stresses. As a result, among the MAO coatings produced in single (silicate main phase) and dual electrolytes (aluminate and phosphate main phase), the dual electrolyte provided better thermal properties, and therefore the sample, which were micro arc oxidized in the dual electrolyte was selected for the creep tests. Also, elevated and room temperature wear tests were conducted on single phase coating with the highest hardness and dual phase coating and then single phase coating showed the higher wear resistance than dual phase coating due to the highest hardness. Finally, according to the creep test, MAO coating showed the higher creep resistance than bare AZ91 alloy since dual phase coating showed the highest thermal barrier property. However, creep mechanism of the AZ91 alloy was not changed with the MAO process at 150-200oC temperatures and 25-90 MPa stress.