Is hardness constant in covalent materials?

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 hard-ness 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 influ-ence 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.