Fracture Toughness of Thick Boride Layers Estimated by the Cross-Sectioned Scratch Test

New results about the fracture toughness (K_c) of thick boride layers estimated by the cross-sectioned scratch test are presented in this study. The FeB-Fe₂B layers developed at the surface of borided AISI 1018 and AISI 1045 steels and the Fe₂B layer formed on the borided AISI 1045 steel exposed to a diffusion annealing process (DAP) were used for this purpose. The cross-sectioned scratch tests were performed with a Vickers diamond stylus drawn across the thick boride layer under a constant load to produce a half-cone-shaped fracture near to the top surface of the borided steels. The height of the half-cone-shaped fracture as a function of the cross-sectioned scratch loads was used to determine the fracture toughness of the FeB and Fe₂B layers. The results showed a fracture resistance of ∼2.8MPam for the FeB layer formed at the surface of borided AISI 1045 steel. Likewise, the effect of the DAP on the surface of the borided AISI 1045 steel promoted the formation of an exclusively Fe₂B layer, with an increase in the fracture toughness of the whole boride layer around 5MPam. Finally, the principle of the technique can be used to minimize the influence of the anisotropic properties on the fracture toughness along the depth of boride layers.