Molecular dynamics simulation on friction and wear behavior of WC–Co cemented carbides
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摘要: 利用分子动力学模拟研究了WC–Co硬质合金在不同条件下的摩擦过程，分析了晶粒尺寸、摩擦载荷和滑动速率等因素对硬质合金摩擦磨损行为的影响，从原子尺度揭示了硬质合金发生摩擦磨损的微观机制。结果表明，随晶粒尺寸增大，相比于晶粒转动，Co相和WC中的位错滑移逐渐在摩擦引起的塑性变形机制中起主导作用。摩擦载荷增大会导致易变形的Co粘结相被挤出表面而首先去除，通过减小晶粒尺寸可以抑制Co相的挤出–磨损机制，进而提高硬质合金的抗滑动磨损性能。滑动速率升高会降低磨损速率，主要原因是在高速滑动过程中，亚表层各相中位错的形核扩展缺乏持续的驱动应力，位错密度较低，WC不易发生断裂，Co相被挤出表面造成的磨损程度明显减轻。Abstract: The friction process of the WC–Co cemented carbides in the different conditions was investigated by molecular dynamics simulation in this work. The effects of grain size, friction load, and sliding velocity on the friction and wear behavior of the cemented carbides were analyzed. The friction and wear microscopic mechanism of the cemented carbides in the atomic scale was revealed. The results show that, with the increase of grain size, the dislocation slip in the Co and WC phases gradually plays more important role in the friction-induced plastic deformation mechanism rather than the grain rotation. The increase of friction load may lead to the deformable Co bonding phase being extruded from the surface and removed first. Nonetheless, the extrusion-wear mechanism of the Co phase can be suppressed by reducing the WC grain size, and the sliding wear resistance of the cemented carbides can be improved. Besides, the increase of sliding rate may reduce the wear rate. The main reason is that, in the process of high-speed sliding, the nucleation and expansion of dislocation in each phase of the subsurface layer lacks the continuous driving stress, and the dislocation density is low. Therefore, WC is difficult to fracture, and the wear degree caused by Co phase being extruded from the surface is significantly reduced.
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Key words:
- cemented carbides /
- molecular dynamics simulation /
- plastic deformation /
- dislocation /
- friction /
- wear
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图 1 用于分子动力学模拟摩擦过程的多晶WC–12%Co模型建立过程
Figure 1. Modeling process of the polycrystalline WC–12%Co for the molecular dynamics simulation of friction process
图 2 平均晶粒尺寸为5 nm的WC–Co硬质合金摩擦初始（a）和结束（b）的模拟组织以及摩擦结束后的剪切应变分布（c）和摩擦过程中WC晶粒通过转动协调塑性变形（d），其中原子位移矢量显示其运动方向
Figure 2. Simulated microstructure of the WC–Co cemented carbides with the mean grain size of 5 nm at the beginning (a) and the end (b) of the friction process, the shear strain distribution at the end of friction (c), and the plasticity coordination by WC grain rotation during the friction process (d), where the displacement vector of atoms indicates the moving direction
图 3 平均晶粒尺寸为12 nm的WC–Co硬质合金摩擦初始（a）和结束（b）的模拟组织以及摩擦结束后的剪切应变分布（c）和摩擦过程中WC晶粒的局部转动（d）
Figure 3. Simulated microstructure of the WC–Co cemented carbides with the mean grain size of 12 nm at the beginning (a) and the end (b) of the friction process, the shear strain distribution at the end of friction (c), and the local rotation of WC grains during the friction process (d)
图 4 平均晶粒尺寸为12 nm的WC–Co硬质合金在摩擦时发生WC晶粒开裂（a）和Co粘结相挤出–去除的过程（b）
Figure 4. Fracture of WC grains (a) and the extrusion-induced removal of the Co binder phases (b) during the friction process of the WC–Co cemented carbides with the mean grain size of 12 nm
图 5 两种晶粒尺寸的WC–Co硬质合金在不同压力作用下磨损量随滑动距离的变化
Figure 5. Variation of the wear rate with the sliding distance for the WC–Co cemented carbides with two different grain sizes in the various pressure
图 6 两种晶粒尺寸的WC–Co硬质合金在600 nN载荷下滑动摩擦不同距离时的组织结构：（a）12 nm；（b）5 nm
Figure 6. Microstructure of the WC–Co cemented carbides with two different grain sizes after the sliding friction at 600 nN in the various distance: (a) 12 nm; (b) 5 nm
图 7 两种晶粒尺寸的WC–Co硬质合金在不同摩擦载荷作用下的剪切应变响应：（a）12 nm，200 nN；（b）12 nm，600 nN； （c）5 nm，200 nN；（d）5 nm，600 nN
Figure 7. Shear strain response of the WC–Co cemented carbides with two different grain sizes under the different frictional loads: (a) 12 nm, 200 nN; (b) 12 nm, 600 nN; (c) 5 nm, 200 nN; (d) 5 nm, 600 nN
图 8 两种晶粒尺寸的WC–Co硬质合金在不同滑动速率作用下的磨损率（a）和损伤深度（b）变化
Figure 8. Variation of wear rate (a) and damage depth (b) with the sliding velocity for the WC–Co cemented carbides with two different grain sizes
图 9 两种晶粒尺寸的WC–Co硬质合金在不同摩擦速率作用下的结构演变：（a）12 nm, 0.1 nm·ps‒1；（b）12 nm, 0.4 nm·ps‒1； （c）5 nm, 0.1 nm·ps‒1；（d）5 nm, 0.4 nm·ps‒1
Figure 9. Structural evolution of the WC–Co cemented carbides with two different grain sizes in the various sliding velocity: (a) 12 nm, 0.1 nm·ps‒1; (b) 12 nm, 0.4 nm·ps‒1; (c) 5 nm, 0.1 nm·ps‒1; (d) 5 nm, 0.4 nm·ps‒1
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