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.

针对一枚经受过重载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）开展了表征分析。处于适宜角度位置的一片石墨片，以纳米级分辨率展现了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进而生成磁铁矿。被迫嵌入基体的剥离少层石墨烯（exfoliated few-layer graphene）团簇发生了严重形变。将该圆盘表面的拉曼光谱（Raman spectra，其呈现出典型的含碳特征谱带），与不同球磨时长下振动磨（shaker mill）中经变形处理的石墨的拉曼光谱进行对比后发现：相较于额外承受压缩与冲击力的球磨样品，制动表面的剪切应力在诱导结构无序化方面的效果更为显著。在剪切载荷作用下，石墨晶体结构的弹性模量具备高度各向异性，且石墨基面间的粘附力较弱，这使得褶皱少层石墨烯团簇发生剥离，由此产生的缺陷相关拉曼谱带数量多于高能球磨过程中机械应力所引发的谱带。