Rivestimenti protettivi per valvole di motori navali

Intake and exhaust valves operating in diesel engines for naval propulsion suffer from several problems due to the combined and synergistic effects of mechanical stresses and a chemically harsh environment. Degradation mechanisms like wear and corrosion act on different sections of the valve, with the result that each part undergoes a different kind of damage. The present work particularly focuses on degradation phenomena such as low temperature corrosion and erosive wear on the stem; hot corrosion on the bottom of the head of the valve; wear on the seat ring. In order to prevent the valves from degrading, a possible solution is the application of protective coatings that may extend the service life of the components by preserving the mechanical properties of substrate materials, while improving the oxidation and corrosion resistance of the surface. The objective of this study is the development, testing and optimization of a system of corrosion and wear resistant coatings for the stem, bottom and seat ring of the valves. The analysis of the degradation mechanisms occurring on each section of the valves led to the selection of materials that could be suitable for the protection of the substrate. Specifically, two different kinds of ceramic-metallic coatings were selected for the protection of the stem and the bottom of the valves, while a modification of the state-of-the art coating applied on the seat ring was performed.