Effect of the anisotropic growth on the fracture toughness measurements obtained in the Fe₂B layer

In this study, the fracture toughness of the Fe₂B layer was estimated using the Berkovich indentation technique. Boron diffusion at the surface of the AISI 1018 steel was conducted using a powder-pack boriding process at a temperature of 1273K with 6h of exposure. The mechanical characterization of the Fe₂B layer was performed on three different distances from the surface using a range of indentation loads (10 to 500mN) for each distance. The behavior of the hardness as a function of the indentation load showed the presence of the indentation size effect (ISE) at the different distances from the surface, in which the apparent or real hardness was estimated according to the concept of the Nix and Gao model. Finally, two indentation-cracking models were used to estimate the fracture toughness of the Fe₂B layer on the different distances from the surface; the results were ranged from 1.5 to 4.2 MPa m^1/2, which denoted the brittleness and the influence of the anisotropic nature of the boride layer.