Two-stage gas boriding of Nisil in N2–H2–BCl3 atmosphere

Boriding can be applied to a wide range of steels, Ni-based alloys, Co-based alloys, molybdenum or sintered carbides. If Ni-based alloys have to be applied under conditions of appreciable mechanical wear (adhesive or abrasive), these materials will require suitable wear protection. Boronizing offers a suitable alternative for protecting these materials from wear. In this paper, two-stage gas boronizing in N2–H2–BCl3 atmosphere was proposed for producing the boride layer on Ni-based alloy called Nisil. This process consisted in two stages: saturation by boron and diffusion annealing. The stages were alternately repeated. The process was applied in order to accelerate the saturation by boron and its diffusion. It turned out to be more effective because of the elimination of the excess boron from the atmosphere and because of the boron diffusion into the substrate during the second stage. The microstructure consisted of nickel borides (Ni2B, Ni3B) and nickel silicides (Ni2Si, Ni3Si) formed as the result of the movement of silicon towards the core by following the boron diffusion front. The formed boride layer was 1.5 times thicker than that obtained by pack-boronizing using powder without SiC at comparable temperature and time.