Innovative galvanic coatings for high temperature applications

The research regards the production and the characterization of two different types of Ni based galvanic coatings, which should guarantee higher performances at high temperature applications in comparison to the pure Ni coatings. The work is divided in two parts, the first one on the production and characterization of Ni matrix composite coatings containing either micro- or nano-particles of Al and the second one on the production and characterization of Ni-B alloy coatings with low B content produced with dimethylamine borane (DMAB) as boron source in the plating bath. In both cases, after an initial optimization of the process parameters, different types of coatings have been produced and tested. The coatings microstructure, microhardness and chemical composition both prior and after heat-treatments at different temperatures have been studied and compared to pure Ni deposits. The coatings performances such as protective properties, wear and tribocorrosion resistance have been also evaluated and the results have been correlated to the microstructural and chemical modifications. As plated pure Ni coatings present a columnar structure, a hardness HV0.05 of about 170 and good corrosion resistance. After heat treatment at temperatures starting from 400°C the Ni deposits undergo recrystallization which lead to a noticeable decrease of the mechanical properties. Ni/Al micro-composite deposits heat treated at 600°C consists of a biphasic γ-Ni and γ′-Ni3Al system, while after heat treatment at 800°C of a solid substitutional solution of Al in the γNi. The diffusion of Al in the metal matrix hinders the recrystallization of the Ni matrix and changes the preferential orientation. The more uniform distribution of the Al nano-particles and the smaller dimensions lead to an advance of the diffusion at lower temperature (400°C). The heat treated Ni/Al nano-composite deposits consist of γNi and present a columnar structure with narrow and short columns as the fast diffusion of Al that blocks the recrystallization of the Ni matrix. Both micro- and nano-composite coatings present higher hardness in comparison to pure Ni deposits after heat treatments at high temperatures. The introduction of Al micro-particles decreases the protective properties while the use of nano-particles lead to a marked increase of the microhardness without penalizing the corrosion resistance of the matrix. Ni-B alloy coatings present a marked grain refinement and a linear increase of the hardness by increasing the B content. The B content must be kept below 0.12% wt. in order to avoid the formation of cracks. The presence of B hinders the recrystallization of the coatings up to 600°C. Crack-free Ni-B coatings present similar protective properties, higher wear and tibocorrosion resistance in comparison to pure Ni deposits.