水树生长早期的XLPE晶区破坏现象及机制

Damage Phenomenon and Mechanism of XLPE Crystalline Region in the Early Propagation Period of Water Trees

为了研究水树生长对交联聚乙烯(cross linked polyethylene,XLPE)材料晶区的破坏作用,研究水树生长早期XLPE材料的晶区结构变化及晶区破坏机制。采用加速水树老化平台对A、B、C三组XLPE薄片样本分别进行10天、20天、30天的水树老化,老化结束后用化学侵蚀法侵蚀样本,并用扫描电镜(scanning electron microscope,SEM)观测水树区域晶区形貌,用X射线衍射(X ray diffraction,XRD)检测水树区域晶区结构变化。SEM观测结果表明,侵蚀样本中的缺陷形态在不同老化时期存在差异:老化10天时,样本中出现细长螺型位错滑移线及较短刃型位错;老化20天时,样本中沿XLPE晶面方向萌生裂缝;老化30天后,样本中裂缝发展为数十微米。同时,样本中出现尖底位错蚀坑,且在蚀坑底部存在微孔。XRD检测结果表明,水树老化将导致材料晶区衍射峰产生尖角状畸变及宽化,并且随着老化时间的增长,样本结晶度下降。分析认为,水树老化将导致XLPE晶区发生破坏。非晶区水树微孔发展至晶区时,将对晶区造成电机械应力并使晶区发生形变,进而在晶区萌生位错。位错运动增殖导致出现滑移线及位错台阶。随着位错密度增大,不同晶面位错交割并产生位错塞积,进而在晶区中萌生微裂纹并产生位错凹坑,晶区发生破坏。

To investigate the influence of the water tree propagation on the damage of the crystalline regions of cross linked polyethylene(XLPE), this paper investigated the changes of the structural characteristics and damage mechanism of XLPE crystalline region in the early period of water tree propagation. Three groups of XLPE sheet samples A-C were subjected to an accelerated water tree aging for 10 days, 20 days, and 30 days, respectively. Afterwards the crystalline region morphology in water tree regions was observed by scanning electron microscope(SEM) after erosion,and the crystalline structural changes were detected by X ray diffraction(XRD). SEM observation results showed that the defect morphologies in the crystalline regions change in different aging periods. After 10 days of aging, slender screw dislocation slipping lines and short edge dislocations were observed in the samples. After 20 days of aging, microcracks were initiated along the crystal face directions in the samples.After 30 days of aging, microcracks developed into several ten micrometers. In the meantime, sharp cornered dislocation etch pits existed in the samples, and there were microvoids at the bottom of the etch pits. XRD detection results showed that water tree aging could result in the emergence of the sharp cornered distortions in the diffraction peak and the increase of the half-peak widths. In addition, the crystalline degree of the samples decreased with aging. It was analyzed that the crystalline regions of XLPE can be damaged by water tree aging. When the microvoids in amorphous regions developed to the crystalline regions, they exerted electro-mechanical forces on the crystalline regions and they deformrf, further initiating dislocations. Slipping lines and dislocation steps were then formed by the movement and proliferation of the dislocations. With the increase in the dislocation densities, the dislocations in different crystal faced intersect and resulted in the stacking of dislocations. As a result, microcracks and dislocation etch pits were initiated, thus resulting in the damage of the crystalline regions.