Phase composition and morphology as a function of depth for boride coatings grown on Fe-Ni alloys

Considerable R&D efforts are being carried out in order to produce materials with satisfactory resistance to wear, corrosion or both. In many applications, in fact, the in-service life of components is determined by the surface properties. In the important field of thermochemical treatments of steels, based on diffusion of species such as carbon, nitrogen or boron, boriding is in a peculiar position. Indeed, coatings constituted by iron borides generally display very high hardness, even in excess of 20 GPa, as well as good wear resistance under both sliding and abrasive conditions. Borided steel components display excellent performance in several tribological applications in the mechanical engineering and automotive industries. In spite of several researches performed since many years, some problems remain to be solved as, in particular, the role of alloying elements in the steel on growth and properties of the boride coating. In the present work the attention has been addressed to the role of Ni, investigated by boronizing binary alloys in order to avoid any interference by other alloying elements. Five Fe-Ni alloys with a Ni contents in the range from 11 to 80 wt.% were borided by a pack-cementation process, using a high boron potential powder mixture constituted by amorphous boron activated with 10 wt.% of potassium fluoborate. The thermochemical treatments were carried out at 850°C for exposure times up to 8 h. Phase composition and morphology of the boride coatings were characterized as a function of depth by the layer-by-layer removal technique as well as by observations carried out on metallographic cross-sections, using different and complementary techniques: X-ray diffraction (XRD), Mossbauer spectroscopy (MS), micro-Raman spectroscopy, optical (OM) and scanning electron microscopy (SEM), Vickers microhardness measurements (MHV).