Microstructural aspects of the protective ceramic coatings applied on the surfaces of refractory alloys produced by additive manufacturing

The possibility of depositing multi layers made of metals/alloys and high temperature ceramics by electron beam physical evaporation process under high vacuum (EB-PVD) on the surface of a parallelepiped sample made by selective laser melting (SLM) from a Ni base refractory super alloy was experimentally tested. The SEM-EDAX micro structural analysis revealed the morphology and thickness of the coatings consisting of a NiCrAlY base alloy as bonding layer and three successive YSZ, LZO and GZO ceramic layers on the Ni-based super alloy substrate obtained by additive manufacturing. The adhesion of the layer deposited was evaluated by the scratch test method. The analysis highlighted the importance of pre-preparing the surface of the additive manufactured substrates, in order to control the adhesion and uniformity of the deposited layers

Design, fabrication, and characterization of new materials based on zirconia doped with mixed rare earth oxides: review and first experimental results

Monazite is one of the most valuable natural resources for rare earth oxides (REOs) used as dopants with high added value in ceramic materials for extreme environments applications. The complexity of the separation process in individual REOs, due to their similar electronic configuration and physical–chemical properties, is reflected in products with high price and high environmental footprint. During last years, there was an increasing interest for using different mixtures of REOs as dopants for high temperature ceramics, in particular for ZrO2-based thermal barrier coatings (TBCs) used in aeronautics and energy co-generation. The use of mixed REOs may increase the working temperature of the TBCs due to the formation of tetragonal and cubic solid solutions with higher melting temperatures, avoiding grain size coarsening due to interface segregation, enhancing its ionic conductivity and sinterability. The thermal stability of the coatings may be further improved by using rare earth zirconates with perovskite or pyrochlore structures having no phase transitions before melting. Within this research framework, firstly we present a review analysis about results reported in the literature so far about the use of ZrO2 ceramics doped with mixed REOs for high temperature applications. Then, preliminary results about TBCs fabricated by electron beam evaporation starting from mixed REOs simulating the real composition as occurring in monazite source minerals are reported. This novel recipe for ZrO2-based TBCs, if optimized, may lead to better materials with lower costs and lower environmental impact, as a result of the elimination of REOs extraction and separation in individual lanthanides. Preliminary results on the compositional, microstructure, morphological, and thermal properties of the tested materials are reported