The microstructure, phase, and mechanical properties of three aged porcelain insulators which were manufactured in different years (1973, 1995 and 2008) and which were used in the field for different amounts of time, were investigated. With X-ray 3D computed tomography (CT), defects with ~mm size can be detected without destroying the aged insulators. Defects of small specimens, which are cut from the aged insulators and polished, are analyzed with optical and scanning electron microscopy (OM and SEM), and defects of um size are detected by OM and SEM. The number and size of defects in all the aged insulators are similar. Porcelain insulators manufactured in 1973 contain more SiO2 (quartz and cristobalite) than those manufactured in 2008. Those manufactured in 2008 contain more Al2O3 than those manufactured earlier. The Vickers hardness of the insulator manufactured in 1973 has the lowest value. The formation of the cristobalite (SiO2) in the insulator manufactured in 1973 which can come from the phase transformation of quartz can cause stress in the insulator by formation of microcracks, which can lead to the low hardness of the insulator.

Abstract
 1. Introduction
 2. Experimental Procedure
 3. Results and Discussion
 4. Conclusion
 References

• Jun-Young Cho(Department of Materials Science and Engineering, Seoul National University)
• Woo-Chan Jin(Department of Materials Science and Engineering, Seoul National University)
• Sung-Hwan Bae(Nano Materials Science and Engineering, Kyungnam University)
• Chan Park(Department of Materials Science and Engineering, Seoul National University) Corresponding author