Nanomechanical characterization and fracture toughness of FeB and Fe2B iron borides produced by gas boriding of Armco iron

FeB and Fe2B hard ceramic phases were produced in Armco iron using gas-boriding in N2-H2-BCl3 atmosphere. This process was carried out at 920 °C (1193 K) for 3 h and caused acceleration in the diffusion of boron into the surface of a base material in comparison with other acceptable methods of diffusion boriding. FeB and Fe2B layers were characterized by a strong zonation, obtaining the average thickness of 32 μm and 125 μm, respectively. Young's moduli and hardness of iron borides were measured using the nanoindenter with a Berkovich diamond tip under load of 50 mN. The higher average indentation hardness (HIT = 20.95 ± 0.93 GPa) and Young's modulus (EIT = 308.86 ± 26.44 GPa) were characteristic of FeB phase. Fe2B boride was characterized by HIT = 17.42 ± 0.80 GPa, and EIT = 252.96 ± 15.57 GPa. The fracture toughness of iron borides was measured by microindentation technique using a Vickers diamond indenter under load of 100 gf (about 0.981 N). The average fracture toughness (Kc), measured in FeB zone, was equal to 1.79 ± 0.70 MPa·m1/2. Fe2B phase was characterized by higher fracture toughness, obtaining Kc = 2.42 ± 0.66 MPa·m1/2. However, at FeB/Fe2B interface the increase in brittleness was detected. Such a situation was caused by the differences in coefficients of thermal expansion of both iron borides and their mechanical properties. It could provide the preliminary cracks at this interface after cooling. During indentation, the value of shear stress probably exceeded the value of normal compressive stress. It could cause the failure at this interface, facilitating cracks' propagation.