13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Though from magnified surface morphology results the region between two UNSMed marks was smoother the process of more strike number, the UNSMed marks mainly lead to the worse of the surface roughness. For the nitriding samples, the UNSMed marks show unobvious as the reason of nitrides within top surface region reducing the plastic deformation of sample surface. Compared with the soft un-nitriding sample, better surface roughness can be obtained for the sample surface with higher hardness. It also means fewer surface defects produced on the nitriding sample surface. Figure 2. The surface morphology of specimens before and after UNSM. Figure 3. The surface roughness of specimens before and after UNSM. The microstructures of the samples before and after UNSM treatment were shown in Figure 4. Through the other studies [3, 6] it can be seen that a gradual change microstructure from nano-sized grain in the sample top surface to the grain size of base material can be produced by the S2PD treatment. With picric acid solution the grain boundaries can be etched easily. For the un-UNSMed samples the grain can be distinguished from top surface to interior and no difference between each
RkJQdWJsaXNoZXIy MjM0NDE=