# Effect of Surface Micro-Hardness Change in Multistep
## 3 Experimental Results and Analysis
### Wear Mechanism Analysis  
Hardness refers to the resistance of local area of the material to the hard object being pressed into its surface. It is an indicator of the degree of softness and hardness between various materials. Regarding the study of hardness on the friction properties of the material, generally speaking, the material with relatively high hardness had good wear resistance [23]. However, some studies had found that [9] the increase in hardness would make the deterioration of near-surface cracks more serious. This was mainly because the hardness increased, the material was more brittle, and the cracks and delamination wear were easily caused during friction, the wear resistance was worse and the wear rate was higher. At the same time, the hardness was lower, the plasticity was relatively larger, so the wear resistance was better. However, some studies had found that [24] reasonable improvement of the hardness of the core of the carburized steel could effectively improve the surface fatigue wear resistance. Because the carburized layer was too shallow, cracks would occur in the transition zone between the hardened layer and the core, causing the surface layer to peel off.  
Figure 14 was a cross-sectional schematic diagram showing the difference of the finishing subsurface layer micro-hardness, and the force was applied to the local area of the finishing surface along the feed direction. The blue area A was a relatively hard area, and the red area B was a relatively soft area (The degree of softness and hardness was relative  
Figure 12: The effect of the change of roughing cutting parameters on wear loss of titanium alloy under different friction times. (**a**) \(f\) = 0.25 mm/r, \(a_{p}\) = 1 mm. (**b**) \(v\) = 50 m/min,\(f\) = 0.25 mm/r.  
Figure 13: The subsurface wear microtopography under different friction times (\(v\) = 50 m/min, \(f\) = 0.25 mm/r, and \(a_{p}\) = 0.5 mm). (**a**) 15 min. (**b**) 30 min.  
to the average micro-hardness), and the broken line C was an easy spalling area in theory. During the experiment, the conditions applied in the adjacent regions were identical, so in the areas A and B, the horizontal component F1 and the vertical component F2 of the force F were the same, respectively. Due to the high hardness, the plasticity deformation of the area A subjected to the F1 was relatively small along the feed direction, and on the contrary, the plasticity deformation was relatively large in the area B due to the low hardness. However, in the transition zone of A and B, the micro-hardness changed significantly, so the plasticity difference changed greatly. When subjected to the alternating contact stress during the friction process, it was easier to cause cracks under the cyclic reciprocating conditions. The crack expanded continuously and eventually peeled off to form a spalling pit, and even the wear fracture striations occurred due to spalling. Therefore, by selecting appropriate cutting steps and parameters, the standard deviation of micro-hardness could be reduced and the friction and wear resistance of titanium alloy could be improved.