Boriding of Ferrous and Non-Ferrous Metals and Alloys in Fluidised Bed Reactor

Heat treatment of alloys in fluidised bed reactors has been carried out for more than twenty five years. Recently this technology has been used for surface engineering applications in the deposition of hard and/or corrosion resistant layers. In the present study fluidised bed technology (FBT) has been used to deposit boride coatings on ferrous and non-ferrous metals and alloys. The coatings were examined by means of optical microscopy, X-ray diffraction, and Vickers microhardness testing. These techniques were used to characterise coating thickness and morphology, phase formation, and hardness. The as produced coatings were generally characterised by very good adherence and uniformity. On 0·5 wt-%C steel it was found that only one phase, Fe₂B, was formed during the treatment. Fluidised bed boriding of various steel substrates produced boride layers similar to those obtained by the pack cementation boriding method. On pure Ti, a TiB layer was found, while on Ti-6Al-4V alloy both TiB and TiB₂ phases were detected. Boriding pure Ni gave mainly a Ni₃B coating, while boriding pure Co produced both Co₄B and Co₃B layers. The borided Ni showed improved tribological properties under dry wear conditions.