Anisotropy of Boride Layers: Effect on the Mechanical Properties of AISI 4140 Borided Steels

The growth of iron borides over the surface of different steels is of high anisotropy. It was determined that the anisotropy of borided phases reveals a significant instability of properties in service. One of the techniques to determine the effect of anisotropy on the mechanical properties of boride layers is the induced-fracture by Vickers microindentation. During the present work, the fracture toughness (KC) of the Fe2B hard coatings has been estimated at the surface of AISI 4140 borided steels. The force criterion of fracture toughness was determined from the extent of brittle cracks originating at the tips of an indenter impression. The indentation loads were established between 1.9 to 9.8 N at three different distances from the borided surface. The KC values were expressed as a function of temperature, treatment time and the indentation distances from the surface. Likewise, the adherence of the coated system was evaluated by Rockwell-C indentation, where the borided steel showed sufficient adhesion.