Effect of feb parti̇cle si̇ze on boroni̇zi̇ng parameters of AISI 4140

Boronising, has a spectacular place in thermochemical claddings because of higher surface hardness, lower friction coefficient, high corrosion and oxidation resistance and applicability to many metallic materials. A new boronising technique was developed for designing boronising coatings. Ferroboron was chosen as boron source for the coating process. Ferroboron powders were mixed with sodium silicate and the obtained jell was applied on the surface of AISI 4140 steel. The jell was mechanically alloyed before boronizing. The ferroboron and sodium silicate mixture was mechanically alloyed in a Spex type high energy atritor (1200 rev/min) for 1, 2 and 4 hours. The surface energy of the powders were determined by DTA analysis. Boronising parameters were chosen as 750-950 oC temperatures and 2-8 hour intervals. The microstructure and wear properties of the samples coated by iron boride plates were investigated. Optical microscobe, SEM and XRD were used for determination of the borides. Microhardness was used for determination of the hardness of the borides. It was aimed to decrease the temperature and time of the boronizing. The distribution of the hardness on the coated surface, coating thickness, and the change of the microstructure were investigated as the parameters of the boronizing temperature, boronizing time and the mechanicall alloying time. The kinetic of the diffusion was investigated by using GDOES analysis. Na2SO4, NaF, NH3Cl, Na2B4O7, NaCl were used as binder material, and the coating thickness were compared. Finally, the effect of the mechanical alloying on the activation energy, surface energy of the powders and the thickness of the boride plates were determined. Wear tests were performed by means of a tribometer in pin-on-disc configuration, without lubricant and in air, by using 20, 40 and 60 N coupling loads and 0.8 ms-1 sliding speeds. On samples coated for 8 hour and 900-950 oC conditions, wear is essentially oxidative until the failure of the coating, the fragments of which caused a third body abrasion. On the 4-6 hour and 800-850 oC coated samples the wear mechanism is mainly oxidative and the coating totally wears out without any cracks.