Advances and challenges in the development of UHTCMCs - A review of the C3harme project

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Study of SiC fiber-matrix interaction in ultra-high temperature ceramics by transmission electron microscopy

In the last few years ISTEC activity has been focused on the development of toughened ultra-high temperature ceramics, in order to fill the gap of the low intrinsic fracture toughness of this class of materials. Toughness improvement from 3.5 to around 6 MPa.m1/2 has been achieved through the addition of SiC short fibers, by varying processing methods, sintering additives, thermal treatments and type of fiber. In turn, all these parameters affect fiber integrity and the matrix/fiber interface chemistry, ultimately affecting the mechanical properties. The aim of this study was to understand how the sintering additive or specific treatments on the fiber can control the interface fiber/matrix. Transmission electron microscopy was mainly used as fundamental technique for the characterization at nanoscale level of the fiber morphology and interface with the matrix. A comparison between the evolution of the Hi Nicalon fiber versus the Tyranno type in ZrB2 matrices is presented, evidencing that different thermal stability and chemical inertness may affect the local and global microstructure. Relevant mechanical properties are also reported and related to the microstructural feature

ZrB2 and HfB2 toughened with Hi Nicalon SiC chopped fibers

ZrB2 and HfB2 are candidate materials for use in aggressive environment, owing a unique combination of favourable properties of high temperature stability and excellent engineeristic properties. This class of materials is raising always more interest for future generation space vehicles, as wing leading edges and nose tips, as well as propulsion system elements. The most investigated system is based on ZrB2-SiC, owing to a high strength, up to 1 GPa, high hardness, around 20 GPa and oxidation resistance at least up to 1600?C. The major weak point remains the low fracture toughness, 3 to 5 MPam1/2. It has been recently demonstrated that the introduction of elongated secondary phases and the choice of the proper sintering additive, can lead to almost twofold increase of the fracture toughness. This work presents the last developments of ZrB2 and HfB2 ceramics toughened with HI Nicalon? SiC chopped fibers. The effect of various sintering additives, MoSi2, Si3N4, ZrSi2 and TaSi2, is investigated in relationship to the microstructure evolution upon sintering, the fiber interaction, the resulting interface with the matrix and to the high temperature behaviour. Scanning and transmission electron microscopy was used to investigate the microstructure modification at nanoscale level. The fiber morphology resulted notably modified depending on the sintering temperature and the sintering additive; in particular, silicides had a more aggressive behavior toward the fiber, which developed a multilayered aspect. Nanoindentation was employed to characterize hardness and Young\u27s modulus of each scale. As for the mechanical properties, flexural strength, with the 4-point method, and toughness, measured by the CNB technique, were compared to values of reference unreinforced materials to assess the effective improvement. Oxidation tests in a bottom-up loading furnace box were also performed on selected composites in the temperature range 1200-1700?

Influence of SiC on the oxidation resistance of carbon fibre reinforced UHTCMCsAntonio Vinci

Ultra-High-Temperature-Ceramics (UHTCs) are a novel class of refractory materials characterized by melting points exceeding 3000°C and very good thermo-mechanical properties [1]. In particular, ZrB2 composites have been extensively investigated as potential candidates for the fabrication of reusable Thermal Protection Systems (TPS) for aerospace applications due to their relatively low density and high thermal conductivity. The main drawbacks are the low oxidation resistance of ZrB2 above 1000°C, due to the formation of a porous ZrO2 scale and evaporation of B2O3, and low fracture toughness. Silicon carbide has been found to increase its oxidation resistance up to 1650 owing to the formation of a protective, viscous borosilicate scale [2][3]. However, their low fracture toughness and thermal shock resistance remain major obstacles for their application [4][5]. For this purpose, continuous carbon fibres (~45 vol.%) were used as reinforcement in order to increase their fracture toughness and thermal shock resistance. The resulting materials were labeled “UHTCMCs” (Ultra High Temperature Ceramic Matrix Composites). In the present work, the oxidation resistance of carbon fibre reinforced ZrB2/SiC composites was studied. Composites with SiC amounts ranging from 5-20 vol.% were fabricated by slurry infiltration and hot pressing at 1900°C and 40 MPa. Oxidation tests were carried out on cut specimen (2 x 2.5 x 12 mm3) in a bottom-up loading furnace at 1500°C and 1650°C. The resulting microstructures were analysed by SEM-EDS and X-ray diffraction analysis. Weight loss per surface area was recorded after each test. Results show that the formation of a viscous borosilicate glass phase is essential for the protection of carbon fibres from oxidation; low amounts of SiC do not provide enough protection against fibre degradation, but with increasing the SiC amount there is an increase in the thickness of the protective layer and a decrease in weight loss. References [1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. doi:10.1111/j.1551-2916.2007.01583.x. [2] L. Zhang, K. Kurokawa, Effect of SiC Addition on Oxidation Behavior of ZrB2 at 1273??K and 1473??K, Oxid. Met. 85 (2016) 311–320. doi:10.1007/s11085-015-9585-9. [3] J. He, Y. Wang, L. Luo, L. An, Oxidation behaviour of ZrB2–SiC (Al/Y) ceramics at 1700°C, J. Eur. Ceram. Soc. (2016). doi:10.1016/j.jeurceramsoc.2016.02.037. [4] R. Zhang, X. Cheng, D. Fang, L. Ke, Y. Wang, Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500??C, Mater. Des. 52 (2013) 17–22. doi:10.1016/j.matdes.2013.05.045. [5] E. Zapata-Solvas, D.D. Jayaseelan, H.T. Lin, P. Brown, W.E. Lee, Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (2013) 1373–1386. doi:10.1016/j.jeurceramsoc.2012.12.009

Impact of residual stress on thermal damage accumulation, and Young's modulus of fiber-reinforced ultra-high temperature ceramics

Ultra-high temperature ceramic matrix composites (UHTCMCs) based on ZrB2-matrix reinforced with 45 vol% of unidirectional continuous carbon fibers are studied through the thermal mechanical hysteresis in order to investigate the thermal damage accumulation. The analysis carried out allowed to extrapolate the Young's modulus of the matrix from thermal expansion measures. It was found that the initial matrix Young's modulus of 195 GPa steadily decreases by thermal cycling the samples between RT and 1300 °C as a consequence of matrix cracking. On the other hand, the analysis suggested that carbon fibers keep their Young's modulus constant at 780 GPa. Finally, the residual stresses due to the different coefficient of thermal expansion between matrix and carbon fibers are discussed and let to justify the Young's modulus of 230 GPa, which cannot be explained with the so-called "rule of mixtures" generally valid and widely used in the composite science. Keywords: Boride, Ceramic matrix composite (CMC), Pitch-derived carbon fiber, Thermal expansion coefficient, Thermomechanical hysteresis loops, Linear elasticit

Synthesis and characterization of group IV and V metal diboride nanocrystals via borothermal reduction of metal oxide with NaBH4

Group IV and V metal diborides (MB2) have a unique combination of properties such as a very high melting point (\u3e3000°C), high hardness, good solid-state phase stability, high thermal and electrical conductivity. Metal diboride-based ceramics are expected to be potential candidate materials for ultra-high-temperature applications in the aerospace industry [1]. Due to the poor sinterability of commercial powders, the availability of nanometric boride particles has indeed the potential to improve several stages of ceramic processing [2], or for instance to facilitate the sintering of bulk ceramics due to enhanced particle reactivity [3]. Several synthesis have been developed to achieve nanoborides: chemical route from inorganic precursors, mechanical alloying and self-propagating high-temperature synthesis [4–6]. In this work we proposed the synthesis of group IV and V metal diboride (MB2, M= Ti, Zr, Hf, Nb, Ta) nanocrystals by a thermal treatment of the metal oxide and sodium borohydride (NaBH4) at 700°C under atmospheric pressure [7]. The reaction occurs first via decomposition of NaBH4, followed by the formation of amorphous boron and crystalline ternary species with general formula NaxMyOz and NaxByOz. Finally all of the intermediary species yield metal diboride (MB2) and sodium meta-borate (NaBO2). Synthesized TiB2 nanocrystals have an average size of 11 nm and the powder has a specific surface area (s.s.a) of 33.45 m2/g. ZrB2 grains have a platelet morphology with an aspect ratio of 10, average size of 22.5 nm and s.s.a of 24.97 m2/g; HfB2 has a similar morphology with a crystals size of 28 nm, while the s.s.a is even higher, 36.36 m2/g. As far as we know, the latter is the finest powder obtained via borothermal reduction of metal oxides ever reported. Synthesized NbB2 powder consists of crystallites around 12 nm and has a s.s.a of 21.09 m2/g. TaB2 powder has a s.s.a of 11.38 m2/g and consists of 200 nm agglomerates of spherical and needle-shaped nanocrystals with average size of 11 nm

TaB2-based ceramics: microstructure, mechanical properties and oxidation resistance

Among ultra-high temperature ceramics, major attention has been devoted to zirconium and hafnium borides and carbides. Tantalum composites remain a less explored class of ceramics. In this contribution, TaB2-based ceramics were hot pressed with addition of 5-10 vol% MoSi2. Temperatures in the range of 1680-1780?C led to relative density around 90-95%. The microstructure was studied through X-ray diffraction, scanning and transmission electron microscopy and the results enabled the rebuilding of the densification mechanisms occurring upon sintering. The hardness was about 18 GPa, the fracture toughness 4.6 MPam1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased to 220 MPa at 1200?C. The composite containing 10 vol% of MoSi2 was tested in a bottom-up furnace in the temperature range 1200-1700?C for 30 minutes. The microstructure appeared covered by a SiO2 layer, but the bulk remained unaltered up to 1600?C. At 1700?C the specimen vaporized. Nanoindentation was employed on the oxidized cross section to detect eventual mechanical properties modification associated to chemical/microstructural chang

Flash spark plasma sintering of pure TiB2

Abstract Flash Spark Plasma Sintering (FSPS) was used to rapidly sinter pure titanium diboride (TiB2). A pre-sintered sample (O = 20 mm with relative density 60%) was crossed under a current of 2–2.5 kA flowing entirely across the sample. The samples were locally densified up to 98% of relative density in a very short time of 20 s. The rapid heating (≈6000 °C/min) prevented the complete evaporation of B2O3, leading to the formation of rarely seen segregation of boron at the grain boundaries. Compared to SPS or hot press, the rapid FSPS processing promoted the formation boron rich grain boundaries during sintering, thus enhancing consolidation. The FSPS approach might be suitable to consolidate other refractory borides