In-situ deformation mechanism and orientation effects in sintered 2D boron nitride nanosheets

Two-dimensional hexagonal Boron nitride nanosheets (BNNS) are consolidated as a monolithic pellet by spark plasma sintering at 1650 °C with a pressure of 50 MPa without any sintering aid. X-ray diffraction study confirmed the h-BN structure after sintering of BNNS along with B2O3 and B4C as trace impurities. The preferred orientation of the basal plane (0002) perpendicular to the direction of pressure is observed on the top surface of the pellet after consolidation. High load (several hundred N) in-situ indentation studies inside SEM are carried out in the cross-section and on the top surface of the sintered BNNS pellet to understand the orientation effect on the deformation behavior. Total energy dissipation during indentation along the top surface was 50% greater than the cross-section. Predominant deformation mechanisms observed on the top surface indentation are compression of the layered sheets, delamination and BNNS pile-up. For the cross-section direction, the key deformation mechanisms are cracking and fracturing with an insignificant delamination. The dominant deformation mechanisms in sintered BNNS pellet are directly related to the preferred orientation of the crystallographic planes which was validated with TEM and crystallographic texture studies. Along the top surface, there is no active slip system. However, twinning was observed. In the cross-section, active pyramidal slip system resulted in a plastic deformation. Additionally, the weak van der Waals forces between the layers caused crack propagation along the cross-section.