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New Generation Ultra-High Temperature Ceramic Matrix Composites for Aerospace Industry

Periodic Reporting for period 1 - CeramCom (New Generation Ultra-High Temperature Ceramic Matrix Composites for Aerospace Industry)

Reporting period: 2018-06-01 to 2020-05-31

The aim of the project was to develop new advanced Ultra High-Temperature Ceramic Matrix Composites (UHTCMCs) with significantly improved high temperature mechanical properties for aerospace applications, such as thermal protection system for hypersonic and atmospheric re-entry vehicles, and parts of propulsion systems. A successful operation of reusable hypersonic vehicles relies on Thermal Protection System (TPS), which is a barrier that shields the heat produced by the friction of atmospheric gasses against the outer surface of a space vehicle. Advanced space systems require materials capable of prolonged operations in oxidizing atmospheres above 2000°C, for example the maximum operating temperature for TPS of hypersonic vehicles is expected to exceed 2200°C, but the combustion temperature of a scramjet propulsion system reaches 2700°C. Therefore, it was the main objective of the project to offer a solution for the preparation of new advanced ceramic materials that would meet such strict requirements of aerospace industry.
To fulfil the aim of the project, the effect of various types (Eu2O3, Yb2O3, Lu2O3) and amounts (2, 5, 10 wt.%) of rare-earth (RE) oxides on the densification, microstructure and phase evolution, as well as mechanical properties of transition metal diboride ceramics (ZrB2-SiC) was investigated. The addition of 5 wt.% Lu2O3 was selected as the most promising additive, based on the excellent combination of room and high temperature properties of the materials. The project was then focused on the development of innovative processing way to incorporate ZrB2-SiC- Lu2O3 and HfB2-SiC-Lu2O3 into the carbon fibres-reinforced silicon carbide (Cf/SiC) composites. This relied on the incorporation of B4C powder into the Cf/SiC matrix, followed up by the deposition of the slurry containing either HfSi2+Lu2O3 or ZrSi2+Lu2O3 onto the Cf/SiC surface. Above the melting temperatures of disilicides, the melt infiltrated into the composites by capillary forces, where reacted with B4C and carbon fibres to form HfSi2/SiC/ZrC or ZrSi2/SiC/ZrC compositions. The desired composition was obtained in the subsurface layer (up to ~ 1 mm) of the Cf/SiC, but also in the form of a continuous outer layer on the surface. The Lu2O3 did not infiltrate into the composite matrix, but stayed homogenously distributed in the outer layer. The obtained knowledge was successfully transferred to the industrial partner Central R&T (CRT) Airbus Defence and Space.
The main results of the project can be summarized as follows:
• Understanding the effect of rare-earth oxides on the densification, microstructure and phase evolution. For the first time it was found out that the addition of sufficient amount of RE2O3 leads to the formation of refractory RE2Zr2O7 pyrochlore phase (where RE = Eu, Yb, or Lu) during the densification. Similarly, a direct relationship was found between the ionic size of rare-earth elements and both the room and high temperature mechanical properties of ZrB2-SiC. The results were reported in the form of invited lectures at international conferences, and some of them were published in a conference proceeding (http://www.sss.sav.sk/files/silikatnik19.pdf; p. 57-60, open access). The results can be exploited by the wide research community working on the development of new advanced ceramics for extreme applications.
• Understanding the high temperature interactions between molten disilicides (ZrSi2 and HfSi2) and the surface of Cf/SiC. The effect of different porosities of the substrates (Cf/SiC) as well as different environments (vacuum, argon) on the wetting behaviour of molten disilicides was clarified and understood. The results were published in the Journal of the European Ceramic Society (https://doi.org/10.1016/j.jeurceramsoc.2020.05.055; open access), and presented at international conferences. Since the melt infiltration process always depends on the wetting and the infiltration of the melt into the substrate, these results can be exploited by the researchers working on the development of UHTCMCs by melt infiltration process. Similarly, the results can also be exploited by the research community focused on joining of ceramics, as the understanding of the high temperature interactions between molten metals and ceramics substrates is crucial part of the joining process.
• Innovative approach for the development of RE-reinforced UHTCMCs. A new process, consisting of incorporating of molten Zr- and Hf- disilicides with Lu2O3 additives into the porous structure of B4C-infiltrated Cf/SiC was proposed and verified. This was the first time such a material composition was developed and characterized. This result may be exploited by the industrial partner or other partner from aerospace industry for the manufacturing of the prototypes of the final applications, such as thermal protection systems for hypersonic and atmospheric re-entry vehicles, but also propulsion systems.
• The project results were also disseminated via the project newsletter (http://www.uach.sav.sk/ceramcom-newsletter/) but also at various events, such as Researchers’ night, open day at the Institute of Inorganic Chemistry SAS, MSCA-IF seminars organized by the Slovak contact point, or via the interviews with journalists (Slovak national television, research portals).
The project generated new knowledge on the understanding of the effect of various types and amounts of rare-earth oxides on the densification, microstructure and mechanical properties of ultra-high temperature ceramics (ZrB2-SiC). This significantly improves the current state of the art, as such a systematic study has not yet been conducted on the transition metals diboride ceramics. The obtained results clearly confirmed that the addition of sufficient amount of RE oxides can lead to the in-situ formation of refractory pyrochlore phases in the materials during the sintering. Similarly, new UHTCMC material compositions were developed, in which a novel type of RE additive with the smallest cation size of RE3+ (Lu2O3) was used to reinforce the outer side of the Cf/SiC composites to form HfB2/SiC/ZrC/Lu2O3 and ZrB2/SiC/ZrC/Lu2O3. The project results clearly showed that among all of the investigated RE additives, the best combination of room and high temperature properties can be achieved when Lu2O3 is used. It is expected that the results of the projects will have a great impact on the needs of aerospace industry, but also some wider socio-economic impact. The newly developed UHTCMCs showed a great potential to be capable of prolonged operations in oxidizing atmospheres above 2000°C. The improved high temperature resistance and life-time of these materials may significantly increase reliability of re-usable space vehicles. This provides a cost-effective way for space exploration and other aerospace missions.
SEM microstructure of fhe final UHTCMC composition: Cf/SiC with HfB2/SiC/ZrC/Lu2O3
Ablated surfaces of ZrB2-SiC samples with different amount of Lu2O3 additives