Oxidation of copper alloys studied by analytical transmission electron microscopy cross-sectional specimens
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In this work, the oxide structures of three polycrystalline copper grades, unalloyed oxygen-free (OF) copper and alloyed CuAg and deoxidized high-phosphor (DHP) copper, were studied using cross-sectional analytical transmission electron microscopy (AEM) samples. The oxidation treatments were carried out in air at 200 and 350 °C for different exposure times. The detailed oxide layer structures were characterized by AEM. At 200 °C, a nano-sized Cu2O layer formed on the all copper grades. At 350 °C, a nano-sized Cu2O layer formed first on the all copper grades. After longer exposure time at 350 °C, a crystalline CuO layer grew on the Cu2O layer of the unalloyed OF-copper. In the case of the alloyed CuAg- and DHP-copper, a crystalline and columnar shaped layer, consisting of Cu2O and CuO grains, formed on the nanocrystalline Cu2O layer. At 350 °C, the unalloyed copper oxidized notably slower than the alloyed coppers, and its oxide structures were different than those of the alloyed coppers.
I. INTRODUCTION
Oxidation of copper and copper alloys is of great importance in industrial applications since copper materials are widely used in different oxidizing environments and applications. These are, for example, applications requiring resistance to atmospheric or water exposure, marine applications, and heat exchanger, condenser, and electrical applications. Although oxidation of copper has been studied extensively, many aspects are still unclear, for example, the effect of alloying elements on the oxidation phenomenon. In addition, most classical models of oxidation are based on measuring weight changes, and therefore they assume a uniform oxide film growth.1–5 Due to the development of the experimental capabilities and equipment, it is now known that the copper oxide film nucleates and grows, even at atmospheric pressure, as oxide islands and not as a uniform layer. The oxidation phenomenon of copper includes a complex chemisorption period with reconstruction, followed by interfacial oxide nucleation, initial growth involving islands, coalescence, and, finally, overlayer thickening. Depending on temperature and oxygen partial pressure, oxide layers of copper consist of only Cu2O or both Cu2O and CuO. At low temperatures (
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