Effect of deposition parameters on adhesion, hardness and wear resistance of Sn–Ni electrolytic coatings

The present investigation has been conducted in order to determine the influence of both the chemical composition of the bath and current density employed, on the adhesion, hardness and wear resistance of electrolytic Sn–Ni coatings deposited on a steel substrate. The micro-scratch test method was employed to evaluate the coating adhesion. Three scratches were performed at progressive loads. The subsequent observation of the surface damage by means of optical microscopy allowed the determination of the critical load, defined as the smallest load at which coating delamination will occur. The composite hardness of the system was determined by means of Vickers indentation and represented as a function of the relative indentation depth (R.I.D.). The absolute hardness of the coatings was computed by using a model published previously in the literature by one of the authors. The evaluation of the wear resistance was carried out through an accelerated wear test, without lubricant, under a ball-on-disc configuration, using a ball of AISI 52100 steel as a static counterpart. The wear scar morphology was studied by Scanning Electronic Microscopy (SEM) techniques coupled with the energy dispersive X-ray analysis (EDS). The results indicate that the Sn–Ni coatings obtained by means of the chloride–fluoride bath (CF) have apparently a higher adhesion to the steel substrate and wear resistance, in comparison with the coatings which were obtained by using the pyrophosphate bath (PF), irrespective to the current density value employed during deposition. However, the hardness results indicate that the coatings obtained from the pyrophosphate bath have a better endurance against indentation loads. Such a behavior has been partially explained in terms of the different microstructural features that characterized both coatings. The CF deposits were observed to present an elongated columnar grain structure free of cracks, whereas the PF coatings were observed to have an equiaxial structure with a high crack density. The present study has allowed the conclusion that the CF coatings have an overall better performance than the PF coatings.