Investigation of surface properties of micro arc oxidized ZA-8 alloy

Zinc-aluminium alloys are an alloy family which constitutes of zinc and aluminium as the main alloying element. Copper and magnesium are also added in small amounts to achieve desired mechanical properties. Zinc-aluminium alloys are mainly developed for die casting processes in the early 1900s. They possess high as-cast strength, high dimensional stability, excellent load bearing capabilities, good sliding and wear resistance. They were developed to replace traditional cast aluminium, bronze and brass due to their relatively low melting temperatures and overall better mechanical properties. Zinc-aluminium alloys are used in various applications such as automotive, marine and aerospace industry, as well as making of household materials, high resistance coatings, tools, engine parts and load bearing materials. Zinc-aluminium alloys are first developed as Zamak alloys. This trademark name comes from the initials of the elements in the composition, zinc, aluminium, magnesium and kupfer, being copper in German. Numerous variations of Zamak alloys have been produced with different elemental compositions, with only constant alloying element being the aluminium with a fixed amount of 3.5-4.3% in wt. Recently, newer series of zinc-aluminium alloys called ZA alloys have been developed with more aluminium content than the Zamak series. They also have overall better mechanical properties and lower density than that of Zamak series. They are named according to their aluminium content such as ZA-8 (8% Al in wt.) and ZA-27 (27% in wt.). The main difference in mechanical and physical properties of these alloys are due to the changes in the amount of alloying elements. Aluminium for example, enhances the mechanical properties while reducing the density. Magnesium is added in small amounts to overcome impurities. However, copper addition has both advantages and disadvantages in terms of mechanical properties. It enhances mechanical properties and corrosion resistance of the material but causes dimensional instability, especially in elevated temperatures. Therefore the copper addition is critical depending in which environment the material would be used. With the recent advances in technology and the need for parts that lasts longer and better, enhanced corrosion and wear resistance is required for such materials. Zinc-aluminium alloys are known to possess quite good wear and corrosion resistance but these properties are being limited by their low temperature thresholds, which the mechanical properties deteriorate above that. To overcome these challenges, a cheap and environmentally friendly surface modification method called micro arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO) can be used to produce hard, porous and protective oxide coatings on the surface of the material. Micro arc oxidation is an electrochemical surface treatment which is very similar to traditional anodizing process, with only difference being much higher applied potential. The main purpose of this process is that the applied potential can exceed the dielectric breakdown voltage of the passive oxide layer of the metal, then form micro-dimensional discharge channels. Inside these channels, local plasma reactions occur with the addition of ions from the electrolyte which modifiy the structure of the oxide layer. Electrical breakdown is a term where the applied potential reaches beyond the dielectric breakdown voltage of the material which causes a sudden reduction in electrical resistance. This results in the insulator material becoming electrically conductive. Micro arc oxidation is generally used to create protective oxide coatings on valve metals such as magnesium, aluminium and titanium. The resulting coatings show increased hardness, increased wear and corrosion resistance. Micro arc oxidation process has numerous advantages compared to other surface treatment methods. The importance of the sample preparation is not crucial which enables mass production. Electrolytes used in the process are easy to prepare, cheap and eco-friendly. One major disadvantage of micro arc oxidation is the much higher applied potential than the traditional surface treatment methods, such as anodizing. Even though this enhances the coating formation rate which results in thicker coatings, it also increases the production costs and creates a dangerous environment due to the high potential values. Micro arc oxidation is a relatively new process which has many process parameters that influence the properties of the final product, such as the composition of the material and the electrolyte, temperature of the electrolyte, process time, applied current density and potential. Therefore, it's still being investigated to optimize process parameters in order to create oxide coatings with desired mechanical properties. Micro-arc oxidation of zinc and zinc based alloys has been rarely studied in the past. The focus of this study is to observe the feasibility of micro-arc oxidation on ZA-8 zinc-aluminium alloy using different electrolytes and different coating times, then to investigate wear and corrosion resistance of the resulting coatings. In this study, micro arc oxidation is applied on standard ZA-8 alloy using different electrolytes. All electrolytes contained KOH and NaF solutions. Then, 5 different electrolyte solutions are prepared with the addition of NaAlO2, NaAlO2 with Na3PO4, Na2SiO3, Na2SiO3 with Na3PO4 and Na3PO4 separately into each KOH, NaF solution. An abbreviation has been assigned to each electrolyte solution according to their main component; Al, AlP, Si, SiP and P respectively. A direct current power supply with bipolar mode has been used. The process has ben carried out with a maximum positive voltage of 350 V and a maximum negative voltage of 70 V. Samples are coated in each electrolyte for 3, 5 and 8 minutes. Resulting coatings have been characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Coating thickness, surface roughness, microhardness, and porosity values have been measured. After the characterization processes, pullout, wear and corrosion tests have been made to investigate coating resistance. XRD analysis has shown that all samples consist of ZnO, with Si and SiP groups having an amorphous structure, while Al, AlP and P groups having crystalline structure. This is attributed to silicon preventing the formation of the crystalline zinc oxide in electrolytes with sodium silicate addition. Crystalline ZnAl2O4/Al2O3 phases have also been found in Al and AlP groups. The SEM micrographs of the coatings show typical MAO coating characteristics. All samples were highly porous and included varying amounts of surface cracks. Surface cracks are most likely resulted from the thermal stress created during the micro arc oxidation process due to extremely high temperatures on discharge channels (up to 104 K) with contrast to electrolyte bath in room temperature. The pores in Al and AlP groups have found to be very small and uniform compared to Si, SiP and P groups. AlP group samples show high amount of surface cracks and coating degeneration compared to Al group. Sodium phosphate addition to sodium aluminate most likely reduced the thermal shock resistance of the coating, resulting wider surface cracks. During the investigation of cross sectional SEM images of the coatings, all samples except AlP group had porous outer layer and dense inner layer. However, AlP group samples have shown wide cracks even in the inner layers of the coating and poor adhesion the surface when compared to the other groups. The largest surface pores are found in samples from the SiP group but their surface was more uniform than that of Si and P groups which include wider pore distribution and more surface cracks. The porosity measurement has been done using Clemex Vision PE software by importing multiple surface SEM images of the coatings, then coloring the pores with the software in order to calculate porosity. Due to their significantly smaller pore size, Al and AlP groups can not be analyzed. The analysis show that the porosity of the coatings changes between 5-10%. Al and AlP group samples show the thinnest coating values ranging from 7-10 μm while Si, SiP and P group samples had 20-75 μm of coating thickness which increases significantly more than the Al and AlP group on longer process times. The thinnest coatings also have been found to have the smoothest surface. Microhardness tests show that amorphous structured Si and SiP group samples have harder coatings than the crystalline structured P group samples. This is attributed to the lack of slip planes and grain boundaries in the amorphous structure where dislocation movements are much harder which results in increased hardness. Wear tests reveleaed that the samples coated for 8 minutes have the lowest wear rate within their respective groups. The alumina ball used in the wear test could not get past through the coating in any of the samples from Si and SiP groups. On the other hand, samples coated for 3 and 5 minutes from Al, AlP and P groups, the substrate material was worn by alumina ball indicating the coatings could not achieve the desired wear resistance. Comparing their relative wear resistances, it is seen that the sample coated for 8 minutes from Si group is 52 times more and the samples coated for 8 minutes from Al and SiP groups are 30 times more wear resistant than the untreated sample. Corrosion tests conducted on samples coated for 8 minutes show that all of them have higher resistance to polarization than the untreated sample. More uniformly packed Al and SiP group samples had the lowest corrosion current density, therefore the highest resistance to polarization values, due to them having less surface cracks and more uniform, denser structures. The lowest corrosion resistant coating has found to be the sample from AlP group. This result was also parallel to the previous characterizations where the samples from AlP group had coating morphology with more defects and exhibited poorer performance in the wear tests.