Exploring The Structures And Hardness Of MoB₂and WB₃Synthesized By High Pressure And High Temperature

Superhard materials are one of the most important materials applying in industryas cutting, polishing, and machining parts. However, the tranditional superhardmaterials including diamond and cubic boron nitride (c-BN) cann’t satisfy the needin industry due to diamond is unstable under high temperature and disable to cut iron.Furthermore, c-BN is not as hard as diamond. Searching for new superhard materialsto instead tranditional superhard materials is necessary. Transition-metal borides(TMBs) have attracted much attention since their excellent mechanical properties.Directional covalent bonding of boron atoms and high electron concentrationintroduced by transition metals are considered as two essential parameters fordesigning new superhard materials in TMBs. Moreover, according to the structure ofdiamond and c-BN, one way to design superhard materials in TMBs is forming threedimension (3D) boron network in TMBs which may exhibit super-rigidity in thestructure. However the experimental results indicate three dimension (3D) boronnetwork in TMBs are not superhard materials. So, why three dimension (3D) boronnetwork in TMBs are not superhard materials? How to design superhard materials inTMBs? To understand these questions, exploring the hardness mechanism of TMBs issignificant for designing new superhard materials.To study the hardness mechanism in TMBs, three different compounds areselected in this work. One is α-MoB2, which has graphite-like boron layers (2D) in thestructure. Another is β-MoB2which has the puckeried boron layers in the structure (quasi-3D). The last one is reported “WB4” which formed3D boron skeleton in thestructure. It is very important to clarify the difference in the hardness of2D, quasi-3D,and3D boron structure which is useful to understand the hardness mechanism.In this work, α-MoB2, β-MoB2, and “WB4” were successfully synthesized at highpressure and high temperature. α-MoB2, which is predicted as an unstable phase, isclarified as a metastable phase. the most stable phase in MoB2is β-MoB2.Furthermore, the graphite-like boron layers in α-MoB2can transfer to puckeried boronlayers in β-MoB2with enhancing the Vickers hardness of7GPa. Based on the resultsof in situ high-pressure angle-dispersive synchrotron x-ray powder diffractionmeasurement, both of the MoB2structures are stable under high pressure more than30GPa. Besides the puckeried boron layers in β-MoB2has the B-B bonds anglelimitation which results in second order transition under pressure of26.6GPa.The reported WB4actually was reconfirmed as WB3with partial defect in Watom position. Vickers hardness of WB3which is lower than ReB2was obtained.Unsuficient electron in boron atoms result in the distorted sp2hybridization, whichcan weaker the direction of boron covalent bonds in WB3, and this is the intrinsicreason of B-B bonds weaker and break under indentation shear in WB3.According to the experimental results, a concept to design TMBs superhardmaterials is suggested: generating suitable boron atoms framework and selecting theTM atoms which can supply the sufficient valence electrons to B-B covalent bonds isthe challenge to design new hard or superhard TMBs.