Mechanical properties of FeB and Fe₂B layers estimated by Berkovich nanoindentation on tool borided steel

In this study the mechanical behavior of FeB and Fe₂B layers formed at the surface of AISI D2 steels was estimated by the Berkovich nanoindentation technique. The boriding of AISI D2 steels was developed by the powder-pack method at temperatures of 1223, 1273 and 1323K with exposure times of 3-7h for each temperature. The mechanical characterization was performed to determine the effect of the set of experimental parameters of the boriding process with three experimental procedures: first, nanoindentation was performed along the depth of the surface layers at a constant load of 250mN to determine the hardness gradient and the state of the thermal residual stresses in the boride layers; then, loads in the range of 10 to 300mN were applied to the "pure" zone of the FeB layer at a distance of 10μm from the surface, and in the "pure" zone of the Fe₂B layer (40μm), respectively. Finally, the fracture toughness, compressive residual stresses, and brittleness of the boride layers were evaluated using the length of the cracks that originated from the corners of the Berkovich nanoindentation site after the application of loads ranging from 300 to 500mN. For a constant load of 250mN, the results showed that the state of thermal residual stresses and the hardness of both the FeB and Fe₂B layers were a function of the temperature and exposure time of the process. Moreover, the presence of the indentation size effect (ISE) in the FeB and Fe₂B layers was verified in the range of applied loads (10-300mN), in which the apparent or real hardness was estimated by the elastic recovery (ER) model according to the boriding experimental parameters. Finally, the estimated values of the fracture resistance and brittleness of the boride layers fell within the range of 1.48-3.02MPam and 2.01-4.65MPam, with the presence of compressive stresses in the range of 428 to 1604MPa for the FeB and Fe₂B layers, respectively.