Environmentally friendly FeP based coating as alternative to hard chromium

In this thesis work Fe-P deposits are produced using two different electrolytes: namely Iron (II) chloride and Iron (III) ammonium sulphate. The composition of the baths was continuously changed to refine it, in order to obtain a coating with the desired properties. During the deposition, pH variation inducing variation in the iron ions complexation is investigated. Furthermore, the effects induced by the pH variation in terms of film appearance have been evaluated. The effect of process temperature, which going to change coating appearance and P content, with subsequent variation in crystalline structure and deposit performances, has been studied for both optimized solutions performing the electrodeposition process at room temperature and 60 °C. During these studies, coating properties have been evaluated by means of XRD, SEM, Vickers diamond indenter and Taber test. The two electrolytes allow to obtaining coating with 7-8% wt in case of Fe(III)P deposit and 9-10% wt in case of Fe(II)P one. These two different amount of P in the alloy correspond to different crystallographic structures: low P, i.e. obtaining from Fe(III)-based electrolyte, corresponds to a crystalline deposit, whereas high P content, obtained from Fe(II)- based electrolyte, lead to a deposition of amorphous coating. To understand the effect of a thermal treatment, inducing crystalline structure variations, on morphological, mechanical and tribological properties, samples have been treated for one hour at different temperatures, i.e. 250°C and 450°C. The morphological analysis of the samples has been done with the use of XRD, optical images and SEM micrographs before and after heat treatment. It has been discovered that the structure of the two coatings was different. After a proper study of the two solutions, bright coating has been obtained from both electrolytes, opening the possibility to be used for different kind of applications: decorative in case of coating obtained from Fe(III)-based electrolyte and functional in case of Fe(II) one. For this reason, different thicknesses have been chosen: 7-10 microns for the Fe(III)P deposit and 20-23 microns for Fe(II)P one. Tribological and microhardness characterizations reveal that Fe (III) P shows better wearing coefficient than Fe(II)P, comparable to that of hard chromium. However, the friction coefficient in both case is lower than that of hard chromium. From hardness measures it was possible to evince that in case of Fe(III)P coatings, the harness value after the plating process is comparable to the chromium one and higher with respect to Fe(II)P.