Temperature effect on the material removal mechanism of soft-brittle crystals at nano/micron scale
Temperature effect on the material removal mechanism of soft-brittle crystals at nano/micron scale
复制标题
温度对纳米/微米尺度软脆晶体材料去除机制的影响
DOI：
10.1016/j.ijmachtools.2020.103620
复制
发表时间：
2020-12-15
影响因子：
14
通讯作者：
Axinte, Dragos
中科院分区：
文献类型：
--
作者：
Liu, Qi;Liao, Zhirong;Axinte, Dragos
软脆性晶体，如KH2PO4（KDP），由于其高脆性，在加工过程中容易产生裂纹，因此难以切割。加工此类材料的传统方法是通过超精密加工在室温下引入韧性切削机制，只能去除纳米级的材料，因此材料去除率很低。虽然一些热辅助工艺最近已尝试改善一些难切削材料，如陶瓷的可切削性，有没有系统的了解软脆KDP晶体的材料去除机制的温度效应。在这项工作中，温度对材料去除机制的影响进行了研究，首次使用纳米划痕技术。当硬度和弹性模量随温度升高而降低时，断裂韧性却呈相反的趋势增加，表明高温下塑性变形能力较高。与在室温（RT）下由裂纹扩展和边缘碎裂引起的几乎完全脆性的划痕相比，在170 ℃下的划痕可以获得具有更大的临界未变形切削深度（3.61 μ m）的更延性区域表面，例如与在RT下的临界未变形切削深度（0.42 μ m）相比显著增加8.60倍。亚表面微观结构的TEM分析表明，在室温下，由于裂纹的扩展和相互作用，亚表面产生了大量的纳米颗粒，而在高温下，由于热激活位错的形核和演化，亚表面产生了一些晶格失准结构（LMS）和纳米晶体，这解释了KDP在高温下具有较高的塑性。本文的研究结果对于理解特定温度对切削机理脆韧转变的影响，为今后设计加工软脆性材料的热加工工艺提供了重要的理论依据。
Soft-brittle crystals, e.g. KH2PO4 (KDP), are difficult-to-cut due to their high brittleness which can easily generate crack during the machining process. The conventional method to machine this kind of material is by inducing ductile cutting mechanism at room temperature with ultra-precision machining, which can only remove materials at nanoscale level and hence yields very low material removal rate. While some thermal-assisted processes have been recently attempted to improve the machinability of some difficult-to-cut materials, e.g. ceramics, there is no systematic understanding of the temperature effect on material removal mechanism of soft-brittle KDP crystals yet. In this work, the temperature effect on the material removal mechanism has been investigated for the first time using nano-scratch technique. While a decreased hardness and elastic modulus have been observed with the increase of temperature, an increase of fracture toughness has been revealed with a contradictory tendency, indicating a higher capacity of plastic deformation at elevated temperature. In contrast to the almost totally brittle scratch at room temperature (RT) caused by crack propagation and edge chipping, the scratch at 170 degrees C can achieve more ductile-regime surfaces with a larger critical undeformed cutting depth (3.61 mu m), e.g. a significant increase of 8.60 times compared with that at RT (0.42 mu m). Moreover, the TEM analysis on the subsurface microstructures shows that a great number of nano grits was generated in the subsurface at RT as the result of crack propagation and interaction, while at elevated temperature some crystallographic lattice misaligned structures (LMS) and nano crystals have been brought about due to the nucleation and evolution of thermal-activated dislocations, which explains the higher plasticity of KDP at elevated temperature. The results present in this paper are of great significance for understanding the specific temperature effect on the brittle-to-ductile transition of the cutting mechanism for future designing thermal-involved processes to machine soft-brittle materials.