Growth kinetics of boride coatings formed at the surface AISI M2 during dehydrated paste pack boriding

The growth kinetics of the boride coatings (FeB and Fe2B) at the surface of AISI M2high speed steels were studied in this work. Boriding thermochemical treatment was carried out by dehydrated paste pack at three different temperatures 1173, 1223, and 1273 K and four exposure times 1, 3, 5, and 7 h, respectively. The presence of FeB and Fe2B phases was identified by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction method. In order to obtain the boron diffusion coefficients at the FeB/Fe2B boride coatings, a mathematical model based on the mass balance at the growing interfaces was proposed under certain assumptions. Likewise the parabolic growth constants and the boride incubation time were established as a function of the parameters eta(T) and epsilon(T). The activation energy values estimated for the FeB and Fe2B layers were 233.42 and 211.89 kJ mol(-1) respectively. A good agreement was obtained between the simulated values of boride layer thicknesses and the experimental results. Finally, empirical relationships of boride coating thickness as a function of boriding temperature and time are presented. (C) 2015 Published by Elsevier B.V.