Behavior And Mechanism For Boron Atom Diffusing Across Tungsten Grain Boundary In The Preparation Of Wb Coating: A First-Principles Calculation

For improving the thickness and property of WB coating prepared using atomic diffusion theory, the behavior and mechanism for B atom diffusing across W grain boundary (GB) were investigated. The result indicates that, no matter which W GB for B atom diffusing across, stage I (O-bulk -> S1) decides the diffusion rate, and the energy barrier is not depended on the boundary itself, but controlled by the atomic arrangement structure near GB. W GBs have neighboring octahedral interstices connected by their edges, such as Sigma 5(2 1 0)[001] GB, Sigma 7(213) [1-20] GB and Sigma 11(323)[1 -31] GB, are the fast diffusion channels for B atom diffusion because of the smaller energy barriers at stage I (<= 2.476 eV). While W GBs have neighboring octahedral interstices connected by their faces, including Sigma 3(111)[1-10] GB, Sigma 3(112)[1-10] GB, Sigma 5(210)[001] GB and Sigma 9(114)[1-10] GB, are the two-dimensional barriers due to the larger energy barriers (>2.476 eV). Both ionic bonding and covalent bonding between B atom at saddle point and the surrounding W atoms at stage I on two-dimensional barriers are stronger than those on fast diffusion channels, which is the reason two-dimensional barriers show the larger energy barriers and the smaller diffusion coefficient for B atom diffusing across.