Co-dépôt électrolytique de particules de SiC dans une matrice NiP

Abstract

This work deals with the study of elaboration and characterization of electrodeposited NiP/SiC composite coatings. We observe a strong dependence between phosphorus acid content of the electrolyte and the surfacic quantity of codeposited particles. We show that the presence of particles in the electrolyte and their moving in the diffusion layer disturbs the process of phosphorus codeposition with nickel. In consequence of that phenomenon, a fall of the phosphorus content of the matrix of the composites appears, especially for those elaborated with electrolyte of high phosphorus acid concentration. The strong dependence of the mechanism of phosphorus and nickel codeposition with hydrogen let us think that hydrogen reduction is favoured by the presence of SiC particles. So the volume of occluded hydrogen of composites coatings should be higher than that of particle-free ones. We confirm that hypothetis by measuring an increase of the volume of desorbed hydrogen with the increase of particles concentration of the electrolyte. The evolution of the mechanical properties with increasing phosphorus content is in good agreement with previous works. The increase of microhardness after a heat treatment at 190°C, probably due to a stress relaxation linked with hydrogen desorption, is more surprising. During high temperature heat treatments (400 °C) a 'composite effect' appears between phosphides dispersion and particles dispersion witch lead to high hardness values of NiP/SiC coatings. The SiC particles divide by 4 the mass loss of the coatings measured after the abrasion test. The abrasion resistance of heat treated composites show that the SiC particles hide the effect of the structural hardening.