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The study is devoted to pack-powder boriding of ASTM A709 steel in a powder mixture containing boron carbide, potassium fluoroborate and silicon carbide. This surface hardening process is conducted between 1123 and 1273 K for from 2 to 8 h. The formed diiron boride layers are studied by different techniques (scanning electron microscopy, x-ray diffraction (XRD) analysis, glow discharge optical emission spectroscopy (GDOES), Vickers microhardness testing, surface profilometry, Rockwell-C indentation cohesion and pin-on-disc tests). Kinetically, an integral-method-based approach is applied to calculate the boron diffusivities in Fe2B. The activation energy of the process is deduced and compared to reported data. The same model is ultimately verified by comparing the empirical values of the thickness of the Fe2B layers obtained at 1223 K and 1273 K for 9 h and the predicted values. [ABSTRACT FROM AUTHOR]

Byline: I. Campos-Silva, M. Ortiz-Dominguez This study evaluated the growth kinetics of Fe2B hard coatings obtained by the paste boriding process at the surface of AISI 1018 steels. The treatment was carried out at temperatures of 1123, 1173, 1223 and 1273 K with different exposure times for each temperature using a 4 mm thick layer of boron carbide paste over the material surface. The boron diffusion coefficient was determined by the mass balance equation at the Fe2B/substrate interface, assuming that the growth of Fe2B layers obey the parabolic growth law. Likewise, the growth constant was related as a function of the boride incubation time, the boron surface content and the boron diffusion coefficient, in order to estimate the weight gain in the borided samples and the instantaneous velocity of the Fe2B/substrate interface.

In the current study, the powder-pack boriding was applied to the AISI 12L14 steel in the temperature range 1123 K to 1273 K for an exposure time between 2 and 8 hours. The produced boride layer was composed of Fe2B with a sawtooth morphology. A diffusion model based on the integral method was applied to investigate the growth kinetics of Fe2B layers. As a main result, the boron diffusion coefficients in Fe2B were estimated by considering the principle of mass balance at the (Fe2B/substrate) interface with an inclusion of boride incubation times. The value of activation energy for boron diffusion in AISI 12L14 steel was estimated as 165 kJ mol−1 and compared with other values of activation energy found in the literature. An experimental validation of the present model was made by using four different boriding conditions. Furthermore, the Rockwell-C adhesion test was employed to assess the cohesion of boride layers to the base metal. The scratch and pin-on-disc tests were also carried out to analyze the effect of boriding on wear behavior of AISI 12L14 steel. [ABSTRACT FROM AUTHOR]

In this study, the growth kinetics of nitride layers that develop during the powder-pack nitriding process on the surface of ARMCO pure iron was estimated. The powder-pack nitriding of pure iron was performed according to the Pulnieren (H.E.F. Durferrit) method using a 'Pulnier' powder and an activator, at 798-848 K with different exposure times (2-12 h) for each temperature. In addition, for the entire set of nitriding conditions, three different activator/'Pulnier' powder ratios (0.20, 0.25, and 0.35) were used to evaluate the activation level during the growth of nitride layers. The kinetics of the nitride layers over the surface of ARMCO pure iron were estimated by two mathematical approaches, that consider the mass balance equations at the growth interphases. The resulting expressions for the effective diffusion coefficients in the nitride layers were evaluated as a function of nitriding temperatures and activator/'Pulnier' powder ratio. Finally, based on the experimental parameters ascribed to the powder-pack nitriding process, two expressions were proposed to estimate the nitride layer thicknesses at 798 and 823 K after 9 h of exposure for each temperature, to validate the diffusion models used in this work. [ABSTRACT FROM AUTHOR]

Abstract: In this study, new data about the growth kinetics and indentation properties of cobalt boride layers formed on the surface of the ASTM F-75 biomedical alloy were estimated. The boron diffusion at the surface of the biomedical alloy was conducted using a powder-pack boriding process at temperatures of 1223–1273K with different exposure times for each temperature. Two mathematical approaches were proposed to determine the boron diffusion coefficients of the CoB and Co2B layers in the range of boriding temperatures, in which the experimental results of the kinetics of the cobalt boride layers were compared with those estimated using diffusion models. Finally, the mechanical characterization of the cobalt boride layers was evaluated by indentation techniques with applied loads of 0.98N and 50mN, respectively. According to the nanoindentation tests, a maximum hardness value (30GPa) and a maximum Young's modulus (380GPa) were obtained near the surface region (5μm) of the borided cobalt alloy. [Copyright &y& Elsevier]