Resumen de Desenvolupament i caracterització de nous recobriments de níquel químic: aplicacions multifuncionals de la micro a la nanoescala
This thesis covers the design and optimization of an electroless deposition process aiming to develop a novel range of Ni-P coatings exhibiting advanced features both at micro- and nanoscales using a proprietary low-P electroless nickel solution. At the microscale, the work focuses on the development of novel plating approaches enabling the production of multilayer coatings for functional applications, whereas at the nanoscale emphasis is laid on the production of ultrathin Ni-P coatings permeable to different wavelengths of the electromagnetic spectra.
In order to boost the performance of low-P electroless nickel coatings, the electrolyte underwent an optimization routine by the Taguchi method. First, it was possible to enhance individual coating properties via univariate optimization, such as P content, thickness, hardness, wear and corrosion resistance. Next, in order to achieve the best combination of wear and corrosion resistance, a multivariate optimization was carried out. The resulting electrolyte rendered coatings with three times lower corrosion rate in comparison with those derived from univariate optimization while maintaining the mechanical properties at the maximum values achieved in the univariate optimization.
The optimized solution was then employed for the production of Ni-P multilayer coatings. The multilayer architecture allowed, while keeping the mechanical properties at their best, increase the coating response towards corrosion thanks to the interphases generated during the plating process. In the as-deposited state, the presence of interphases delayed the corrosion attack. After annealing, the multilayer coatings are able to effectively protect the base material by modifying the path of the corrosive attack towards the substrate. Thus, when exposed to the corrosive environment, the interphases allow the lateral spreading of corrosion products which, when sufficiently accumulated, prompted the exfoliation of the outermost layer and exposed the fresh Ni-P layer underneath to the corrosive media. This behaviour was not observed in monolayered coatings. Tribocorrosion studies in open circuit potential conditions revealed that multilayer coatings exhibit 3.5 times less worn volume in comparison with monolayered coatings under the same testing conditions. These results highlighted the importance of the plating approach on the performance of low-P electroless nickel coatings.
Regarding the work performed at the nanoscale, a novel plating process leading to Ni-P coatings on polymeric materials has been developed, which allows producing ultrathin Ni-P coatings (75-150 nm) with permeability to radar waves in the 75-90 MHz band. Wave permeability was later enhanced by 50% after subjecting the coatings to cryogenic treatment. Interestingly, the same synthetic approach allowed the production of ultrathin Ni-P coatings featuring light permeability in the thickness range of 75-400 nm. The later makes it possible that the light passes through the coating, enabling an on/off aesthetic finishing depending on lighting conditions. This new process gives rise to a new range of Ni-P nanocoatings exhibiting unprecedented features, paving the way for novel applications in which electroless nickel has not been considered so far.
