Kinetics and Mechanical Characterization of Hard Layers Obtained by Boron Diffusion in 80/20 Nickel–Chromium Alloy

Abstract

This study examines the formation of hard layers containing Ni-B and Cr-B on the surface of 80/20 nickel–chromium alloy. The work evaluates the mechanical properties of the boride layers using instrumented nanoindentation. In addition, the growth kinetics of the coatings were assessed by applying a kinetic model that relates the layer thickness with the experimental parameters of temperature and treatment time. First, the boride layers were achieved using the powder-pack boriding process in a conventional furnace. The treatment time was set at 2, 4, and 6 h at temperatures of 900, 950, and 975 °C, respectively. The microstructure of the layers was analyzed by X-ray diffraction. The thickness of the layers showed a closed correlation with the experimental parameters of time and temperature, and was established between 38.97 and 156.49 µm for 2 h to 900 °C and for 6 h to 975 °C, respectively. The hardness and Young’s modulus values agree with those presented in the literature for boriding nickel alloys, being in the range of 1.3 GPa on average and 240 to 270 GPa, respectively. The resulting layers exhibited a characteristic diffusion zone where the hardness values decrease gradually without the typical high hardness gradient observed on borided steels.