Simulation of the growth kinetics of the (FeB/Fe2B) bilayer obtained on a borided stainless steel

The present work is an attempt to simulate the growth kinetics of the (FeB/Fe2B) bilayer grown on a substrate made of AISI 316 stainless steel by the application of the powder-pack boriding process, and using four different temperatures (1123, 1173, 1223 and 1273 K) and five exposure times (2, 4, 6, 8 and 10 h). The adopted diffusion model solves the mass balance equation at each growth front: (FeB/Fe2B or FeB/substrate) under certain assumptions and without considering the diffusion zone. To consider the effect of the incubation times for the borides formation, the temperature-dependent function ϕ(T) was incorporated in the model. To validate this model, a computer code written in Matlab (version 6.5), was developed with the purpose of simulating the kinetics of the boride layers. This computer code uses the following parameters as input data: (the boriding temperature, the treatment time, the upper and lower limits of boron concentration in each iron boride, the diffusion coefficients of boron in the FeB and Fe2B phases as well as the ϕ(T) parameter). The outputs of the computer code are the parabolic growth constant at each growth front and the thicknesses of the FeB and Fe2B layers. A good agreement was obtained between the experimental parabolic growth constants taken from a reference work [I. Campos-Silva et al., Formation and kinetics of FeB/Fe2B layers and diffusion zone at the surface of AISI 316 borided steels, Surf. Coat Technol., 205 (2010) 403–412] and the simulated values of the parabolic growth constants (kFeB and k1). The present model was also able to predict the thicknesses of the FeB and Fe2B layers at a temperature of 1243 K during 3 and 5 h. In addition, the mass gain at the material surface was also estimated as a function of the time and the upper boron content in each iron boride phase. It was shown that the simulated values of the generated mass gain are very sensitive to the increase of both temperature and the upper boron contents in the FeB and Fe2B phases.