Investigation of Mixed-Type Craters and the Role of Bifluoride Additives to Produce Zirconia-Toughened Alumina-Based PEO
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JMEPEG https://doi.org/10.1007/s11665-018-3338-2
Investigation of Mixed-Type Craters and the Role of Bifluoride Additives to Produce Zirconia-Toughened Alumina-Based PEO Coating Zeeshan Ur Rehman, Seong Hun Shin, Tanveer Ahmad, and Bon Heun Koo (Submitted September 5, 2017; in revised form December 14, 2017) Al2O3-ZrO2 composite ceramic coatings were prepared on Al6061 aluminum alloy by plasma electrolytic oxidation in Na3PO4-K2ZrF6-Na2SiF6-based alkaline electrolyte. Optimum processing time for the coating formation was found to be 50 min. Cross section and surface morphology of the coatings were analyzed using scanning electron microscope. From the phase and elemental composition analysis, the presence of mZrO2 and t-ZrO2 phases was confirmed. It was further observed that the peak intensities of t-ZrO2 and aAl2O3 phases increased with processing time, which was attributed to the enhanced crystallinity caused by the efficient sintering conditions. Corrosion properties were investigated by potentiodynamic polarization test in 3.5 wt.% NaCl solution. The results showed high improvement in corrosion rate with minimum recorded value 0.25 mmy (mm/year) and corrosion current 0.15 3 1026 A/cm2. Keywords
corrosion, K2ZrF6, microstruture, PEO, zirconiatoughened alumina
1. Introduction Zirconia-toughened alumina (ZTA) ceramics have attracted significant scientific and technological attention during the last two decades due to their use in several paramount applications such as dental screws, cutting blades, electrosurgical insulators, valve seals, body armor, pump components, oxygen sensors, dies, cutting tools, and prosthesis components such as hip joints, etc. (Ref 1, 2). The high demand of ZTA is due its attractive properties, including high-temperature mechanical strength, excellent thermal shock resistance, wear, oxidation resistance, and low thermal conductivity (Ref 3, 4). ZTA ceramics have an important feature that the zirconia particles have restricted size and distributed homogeneously through the alumina matrix (Ref 5). To obtain ZTA with these two important features, in addition to the solid-state methods, electrochemical methods such a plasma electrolytic oxidation (PEO) can also be applied. PEO is an electrochemical technique by which hard ceramic coatings can be produced on the light metal alloys. The plasma process consists of several steps explained comprehensively in the previous reports (Ref 6-8). During the plasma process, a high voltage is applied across the electrodes in an appropriate solution having the substrate as an anode. The breakdown of the initial dielectric layer and the gas packets in the presence of ionic solution generates the sparks or micro-plasmas which further help to deposit species from the electrolyte solution on the substrate, together with micro-
sintering process as a result of high temperature. High coating growth rate, excellent adhesion strength (due to the conversion process), high hardness, and tribological properties are some excellent properties reported for the PEO c
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