Design, Fabrication, and Characterization of New Materials Based on Zirconia Doped with Mixed Rare Earth Oxides: Review and First Experimental Results

Adrian Mihail Motoc,Bogdan St Vasile,Mythili Prakasam,Daniele Valerini,Roxana Trusca,Valentin Dragut,Anca Elena Slobozeanu,Sorina Valsan,Robert Radu Piticescu,Maria Luisa Grilli,Mihail Botan,Adrian Vasile Surdu,Mircea Corban

doi:10.3390/met10060746

Abstract

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.

Highlights

Necessity for better performance and efficiency in applications across stationary power plants, aerospace and automotive industries has led to the development of thermal barrier coatings (TBCs) and multi-layer coating systems over the last 50 years [1]
From the critical analysis of the studies reported in the literature it is evident how the thermal properties of zirconia may be improved by using different rare earth oxides
The aim of the present work is to show the potential of using mixed rare earth oxides (REOs) as naturally occurring from monazite concentrates for obtaining stabilized ZrO2 ceramic coatings for their potential used as TBCs

Summary

Introduction

Necessity for better performance and efficiency in applications across stationary power plants, aerospace and automotive industries has led to the development of thermal barrier coatings (TBCs) and multi-layer coating systems over the last 50 years [1]. The co-doping of zirconia with different REOs was reported to improve the thermal properties of thermal barrier coatings and oxidation properties due to reduction of mechanical stresses and porosity in the oxide layers [4,5]. These improvements play a crucial role in the segments mentioned above because they are effective means for protecting the hot parts against the effects of high temperature in corrosive/erosive environments, extending the life of the metallic parts and reducing the maintenance costs [6,7,8,9]. High melting point, phase stability, good adhesion to the substrate, chemical inertia, thermodynamic compatibility with the metallic substrate and TGO, high thermal cycling resistance are the major requirements to be achieved [6,11]

Objectives

Results

Conclusion