Application of the Heat Balance Integral Method to the growth kinetics of nickel boride layers on an Inconel 718 superalloy

New results about the growth kinetics of nickel boride layers on an Inconel 718 superalloy were obtained. The nickel boride layers were developed by means of the powder-pack boriding process at 1173–1223 K with different exposure times for each temperature. According to optical and scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and nanoindentation techniques, the microstructure of the boride layer consisted in three zones: a high content of Ni₄B₃ on the outer-zone of the borided surface, while Ni₂B and Ni₃B were revealed at the inner zone of the layer and diffusion zone, respectively. Moreover, the boriding growth kinetics of nickel boride layers was estimated using the Heat Balance Integral Method (HBIM) based on a numerical resolution of a system formed by differential algebraic equations, and considering that the evolution of the layers is governed by the parabolic growth law. For the range of boriding temperatures, the boron activation energies in the Ni₄B₃, Ni₂B and Ni₃B resulted in 233, 206 and 218 kJ mol⁻¹, respectively.