Properties and Growth Kinetics of the Boride Layer of a Boriding-Strengthened Fe-Based Powder Metallurgical Material

Abstract

The Fe-based powder metallurgical material was directly sintered by the packed powder boriding method to obtain specimens with the boride layer. These materials were treated at 1123, 1223, and 1323 K for 3, 5, 7, and 10 h, the morphology and thickness of the boride layer were examined with SEM. The phase composition of the boride layer, was analyzed with X-ray diffractometer, and the Rockwell-C indentation test was used to estimate the quality of the bonding strength between the boride layer and substrate. The growth kinetics of the layer was fitted to obtain the plot of thickness variation dynamics. The boride layer thickness is 35–183 μm. The biphase (Fe2B+FeB), and single-phase Fe2B boride layers are obtained at 1323 K and at 1123 and 1223 K, respectively. The quality of the bonding strength of the boride layer obtained at 1223 K for 5 h is approved by the HF3 grade according to the specifications. The diffusion activation energy of B atoms in this experiment is 164 kJ/mol. The sintering temperature and boriding time are closely related to the thickness of the boride layer, and the relationship between the boriding time and the thickness of the boride layer is parabolic. The squared thickness and boriding time are linearly related and consistent with the Arrhenius formula. The microhardness of the boride layer is significantly higher than that of the substrate. The number of defects in the layer is growing with time. The characteristic comb-tooth shape of the boride layer is gradually passivated, and the tip is not obvious until the front edge of the layer is relatively flat. This phenomenon is more pronounced at high temperatures.