Nanomechanical properties and fracture toughness of hard ceramic layer produced by gas boriding of Inconel 600 alloy

Abstract

Abstract Gas-boriding in N2−H2−BCl3 atmosphere resulted in the formation of a thick layer on Inconel 600 alloy. The microstructure of layer produced at 920 °C for 2 h consisted of a mixture of chromium borides and nickel borides. The objective of investigations was to determine the influence of the chemical and phase compositions of borided layer on its mechanical properties. The nanoindentation was carried out using Berkovich diamond tip under a load of 50 mN. The gas-borided layer was characterized by high indentation hardness HIT from 1542.6 HV to 2228.7 HV and high elastic modulus EIT from 226.9 to 296.4 GPa. It was found that the mixture with higher percentage of chromium borides was the reason for the increase in HIT and EIT values. The fracture toughness (KC) was measured using Vickers microindentation technique under a load of 0.98 N. The presence of high compressive stresses in normal direction to the top surface caused the strong anisotropy of the borided layer, in respect of fracture toughness. The high difference between the lowest (0.5763 MPa·m1/2) and the highest (4.5794 MPa·m1/2) fracture toughness was obtained. This situation was caused by the differences in chemical and phase compositions of tested areas, presence of porosity and residual stresses. Generally, the higher KC values were obtained in areas with lower chromium content.