Boride-based nano-laminates with MAX-phase-like behaviour

MAX-phases being usually composed of transition metals, group A elements and carbon/nitrogen are considered interesting materials for many applications because of their tremendous bulk modulus, “reversible” plasticity, and machinability. This is mainly due to their unique kind of bonding comprising covalent, ionic as well as metallic bonds providing “easy” planes of rupture and deformability due to the layered crystal structures.In transition metal boride systems, similar types of bonding are available. In particular the W2B5-structure type and its stacking variations allow the synthesis of strongly layered crystal structures exhibiting unique delamination phenomena.The paper presents ab initio calculations showing the similarities of bonding between the ternary carbides and the corresponding ternary or quaternary borides. Formation of boride-based nano-laminates from auxiliary liquid phases, from the melt as well as during sintering and precipitation from supersaturated solid solutions will be discussed by means of SEM and TEM studies. The role of impurities weakening the interlayer bonding will be addressed in particular. The pronounced cleavage parallel to the basal plane gives rise for crack deflection and pull-out mechanisms if the laminates are dispersed in brittle matrices such as boron carbide, silicon carbide or other transition metal borides.

Graphical abstractSome transition metal borides crystallise in a layered structure of alternating stacks of metal and boron atoms giving rise for strongly anisotropic properties. Their preferred cleavage parallel and the deformability perpendicular to the basal plan are similar to the peculiar mechanical behaviour recently described for MAX-phases. Ab initio calculations of the crystal structure prove the weak bonds between the layers for a variety of borides which can be used to reinforce ceramic materials on a nano-scale level.Figure optionsDownload full-size imageDownload as PowerPoint slide