Facile preparation of nanocrystalline Fe2B coating by direct electro-spark deposition of coarse-grained Fe2B electrode material

Nanocrystallization is a promising way to increase the toughness of brittle materials. In this work, to obtain toughened Fe2B coating and overcome various limitations of boriding, electro-spark deposition (ESD) has been used to prepare nanocrystalline Fe2B coating. A coarse-grained Fe2B electrode was used as anode. The microstructure, microhardness, nanomechanical properties and fracture toughness of the coating and the electrode material were comparatively investigated. Results showed that a metallurgically fused nanocrystalline Fe2B coating with few defects is obtained by ESD. Compared to the anode, the coating contains more Fe3B phase and a new phase of martensite. Moreover, the coating consists of planar crystals, cellular crystals, cellular dendrites, columnar dendrites and equiaxed dendrites due to the solidification conditions, and the grain size of the cellular crystals and the dendrites ranges from few tens of nanometers to several hundred nanometers. The transition zone between the coating and the substrate is the molten zone of the substrate, and the microstructure of this zone is greatly refined. The nanocrystalline Fe2B coating has improved fracture toughness while it has reduced hardness and Young's modulus, and some structures of the coating are remarkably toughened.