Influence of nickel silicides presence on hardness, elastic modulus and fracture toughness of gas-borided layer produced on Nisil-alloy

The two-stage gas boriding in N2−H2−BCl3 atmosphere was applied to producing a two-zoned borided layer on Nisil-alloy. The process was carried out at 910 °C for 2 h. The microstructure consisted of two zones differing in their phase composition. The outer layer contained only a mixture of nickel borides (Ni2B, Ni3B) only. The inner zone contained additionally nickel silicides (Ni2Si, Ni3Si) occurring together with nickel borides. The aim of this study was to determine the presence of nickel silicides on the mechanical properties of the borided layer produced on Ni-based alloy. The hardness and elastic modulus were measured using the nanoindenter with a Berkovich diamond tip under a load of 50 mN. The average values of indentation hardness (HI) and indentation elastic modulus (EI) obtained in the outer zone were respectively (16.32±1.03) GPa and (232±16.15) GPa. The presence of nickel silicides in the inner zone reduced the indentation hardness (6.8−12.54 GPa) and elastic modulus (111.79−153.99 GPa). The fracture toughness of the boride layers was investigated using a Vickers microindentation under a load of 0.981 N. It was confirmed that the presence of nickel silicides caused an increase in brittleness (by about 40%) of the gas-borided layer.