Is hardness constant in covalent materials?

doi:10.1016/j.jmst.2021.10.032

Abstract

Abstract:

It has long been commonly believed that the hardness of covalent materials is related only to chemical bonds, leading to a constant covalent material hardness. Here, we systematically investigated the hardness of Cubic-diamond (3C-diamond) and Hexagonal-diamond (2H-diamond) structures using the ordered structure of functional units (OSFU) strategy. We found that although chemical bonds are the decisive factor in determining the hardness of covalent materials, the effects of crystal lattice, dislocation density, and grain size and orientation are also very important. These are all internal factors that determine the hardness of a material. In addition, external factors such as temperature and strain rate can also influence the hardness of a material to some extent by affecting the critical resolved shear stresses (CRSSs) of dislocation motion. In this work, we argue that the hardness of covalent materials is determined by a combination of internal and external factors, where internal factors such as the chemical bonds, crystal lattice, defects, and grains intrinsically determine the hardness of a material; likewise, external factors such as temperature and strain extrinsically affect the hardness of a material. Therefore, the hardness of covalent materials is not constant.

Key words: Hardness, Covalent materials, Cubic-diamond, Hexagonal-diamond

Guangpeng Sun, Xing Feng, Xue Wu, Sitong Zhang, Bin Wen. Is hardness constant in covalent materials?[J]. J. Mater. Sci. Technol., 2022, 114: 215-221.

Figures/Tables 5

Fig. 1. Crystal structures and hardness of 3C-diamond and 2H-diamond. (A) Unit cell and projection of 3C-diamond onto the (11¯0) plane. (B) Unit cell and projection of 2H-diamond onto the (21¯1¯0) plane. Brown balls represent carbon atoms. Red and blue lines indicate the shuffle-set and glide-set on {111} slip planes, respectively. (C) and (D) Calculated hardness compared to experimental data for 3C-diamond and 2H-diamond as a function of temperature and grain size. (E) Calculated hardness for 3C-diamond and 2H-diamond as a function of dislocation density. (F) Calculated hardness for 3C-diamond and 2H-diamond as a function of the strain rate.

Fig. 2. Influence of crystal lattice on the hardness of 3C-diamond and 2H-diamond. (A) and (B) Distributions of the Schmid factor for 3C-diamond and 2H-diamond as functions of loading direction. (C) and (D) Slip dislocation types for 3C-diamond and 2H-diamond.

Fig. 3. Calculated CRSS and hardness as a function of grain size and grain orientation. (A) CRSS for slip dislocation in 3C-diamond as a function of grain size. The Hall-Petch effect obtained by decreasing the grain size is used to evaluate the CRSS for slip dislocation. The inset shows the CRSS for dislocation motion at 300 K. (B) CRSS for slip dislocation in 2H-diamond as a function of grain size. (C) and (D) Hardness for 3C-diamond and 2H-diamond as functions of grain orientation.

Fig. 4. Relative activities of slip dislocations as functions of temperature, dislocation density, and strain rate. (A) and (B) Variations in slip dislocation types with stress direction and temperature for 3C-diamond and 2H-diamond. Schmid's law is used for the slip dislocations, and α is limited to zero. (C) and (D) The variations in slip dislocation types with the stress direction, dislocation density, and strain rate for 2H-diamond.

Fig. 5. Multiple spatial scales and external factors influence the hardness of covalent materials. The hardness of covalent materials is determined by multiple spatial scale combinations of chemical bonds, crystal lattices, defects, grains, and external factors, leading to variable hardness.

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