Today, anhydrous hydrazine is commonly used as monergol propellant in orbit control for satellites. However, it will be prohibited by European Regulations because of its high toxicity. In this regard, the need for new “green” propellant has become imperative for Europe to stay in the course of satellite propulsion. There by, the CNES has engaged the development of new low-toxicity and high performance green propellants. However these energetic compounds have the common points to develop harsh conditions which include a high temperature flame (> 2700 K) and very oxidizing gases (particularly H2O(g)). Thus, new materials are needed to fulfil these requirements and the developments were oriented towards a Functionally Graded Material (FGM). Indeed, this kind of materials and especially ceramic/metal FGM demonstrated appropriate behaviour in an ultra-high temperature oxidizing environment, associated with good mechanical resistance. The gradual evolution of the composition allows to smooth the interfaces and thus, to reduce the stress concentrations. In this study, the FGM is constituted of a structural layer (refractory metal), an intermediate layer (metal/ceramic) and a ceramic oxide layer as a thermal and environmental barrier.
This work is focused on the development of a 2700 K oxide resistant ceramic layer having a low ionic conductivity to protect the metal part from oxidation and a thermal expansion coefficient fitting with that of the refractory material support. In our previous studies, hafnia-based samples with different doping strategies in terms of compositions have shown a high potential for their use as the ceramic part of the FGM. In particular, high amounts of rare earth oxides (RE₂O₃) (>33 mol. %) seem to be very promising.
In this study, hafnia based materials with different additives as Lu₂O₃, Y₂O₃ and Gd₂O₃ are sintered using natural sintering. Very high amounts of stabilisers are investigated ranging from 14 to 50 mol. %. The influence of temperature and dwell-time on the sintering of the samples is studied. After that, the microstructure of the samples is investigated by Scanning Electron Microscopy in terms of grain size, homogeneity, porosity (rate and shape) and composition (by Electron Dispersive Spectroscopy) as those parameters are determinant for further characterisations. Lattice parameters have been determined with X-Ray diffraction and an abacus has been proposed as a function of the amount of RE₂O₃ and the ionic radius of the Rare Earth cation (RE³⁺). Then hardness, toughness, Young’s modulus and mechanical strength are measured for each composition to establish the relationship between the microstructure and the mechanical properties. Thermal expansion coefficients (TEC) were measured between 473 and 1673 K and a decrease from 14 to 40 mol. % of RE₂O₃ is observed due to the creation of oxygen vacancies. Finally, ionic conductivity is measured up to 1273 K and few changes are detected above 33 mol. % of RE₂O₃. Results were useful to complete our understanding on the influence of the additive on the stabilisation of hafnia and to select the most appropriate composition. Finally, addition of Lu₂O₃ allows to reach the lowest TEC and ionic conductivity.