Amorphization of Graphite Flakes in Gray Cast Iron Under Tribological Load

A gray cast iron disc, which had been submitted to a heavy duty automotive brake test, was examined with energy filtered transmission electron microscopy. A graphite flake in a convenient angular position showed the shear interaction of graphite layers with the iron matrix in nano-scale resolution. Atomic layers of graphite were wedged into the ferritic bulk, allowing the entrance of oxygen and the subsequent formation of magnetite. The exfoliated few-layer graphene batches deformed heavily when forced into the matrix. When Raman spectra from the disc surface, which show distinctive carbonaceous bands, were compared with Raman spectra from graphite subjected to deformation in a shaker mill with different milling times, it could be seen that the shear stress on the brake surface was much more effective to induce disorder than the milling, where compressive and impact forces had been additionally exerted on the sample. During shear load the high anisotropy of elastic modulus in the graphite crystalline structure and the low adhesion between graphite basal planes allowed the exfoliation of wrinkled few-layer grapheme batches, causing the formation of more defect related Raman bands than the mechanical stress during high-energy milling.本研究对经受过重型汽车制动试验的灰铸铁圆盘，采用能量过滤透射电子显微镜（Energy Filtered Transmission Electron Microscopy）开展表征分析。处于适宜观测角度的石墨片清晰展现出纳米级分辨率下石墨层与铁基体之间的剪切相互作用。石墨原子层嵌入铁素体基体内部，使得氧气得以侵入并后续生成磁铁矿。被挤入基体的剥离少层石墨烯团聚体发生了严重变形。将该圆盘表面的拉曼光谱（其呈现出典型的含碳特征峰），与不同研磨时长下振动磨中形变石墨的拉曼光谱进行对比后可发现：制动表面的剪切应力，相较于额外施加了压应力与冲击力的研磨过程，更能有效诱导石墨产生晶格畸变。在剪切载荷作用下，石墨晶体结构中弹性模量的高度各向异性，以及石墨基面间较弱的界面粘附力，使得褶皱少层石墨烯团聚体发生剥离，由此产生的缺陷相关拉曼特征峰数量远多于高能研磨过程中机械应力所引发的缺陷。