Effect of MoSi2 addition on densification and properties of ultra high temperature ceramics produced by pressureless sintering

Ultra high temperature ceramics are a class of material of great interest thanks to their combination of engineering properties such as high melting point, good thermochemical and thermomechanical properties in aggressive environment. Potential applications of these materials are forecast in aerospace industry and in industrial sectors like metallurgy, energy and power production.. To ensure that good properties are attained, control of densification and microstructure is necessary. Because of the high melting point of these compounds (ZrB2: 3245?C, HfB2: 3250?C, ZrC: 3540, HfC: 3890?C), these materials need pressure-assisted sintering procedure and temperature above 2000?C. It has been proved that the addition of ceramics like Si3N4, HfN and SiC can help the densification and allows less hard sintering conditions. This work aims to evaluate the influence of the addition of 5-20 vol% of MoSi2 on sinterability and properties of ultra high temperature ceramics, namely HfB2 and HfC. Sintering behaviour, microstructure and mechanical properties are examined and discussed. Dense composites were obtained by pressureless sintering at 1950?C. The final microstructures were fine and uniform (mean grain size: 1.5- 3 m) and a small amount of residual porosity was observed. By means of scanning electron microscope and energy dispersive spectroscopy secondary phases were detected: in the HfB2-system traces of HfC and HfO2; in the HfC-system a mixed product containing Hf, Si, Mo, indicating mutual solubility between the starting phases. The following mechanical properties were measured: Vickers hardness, Young\u27s modulus, fracture toughness by chevron notched beam method in flexure and 4-pt bending strength at room temperature, 1200 and 1500?C. Hardness ranged from 15 to 18 MPa; Young\u27s modulus values were in the range of 400-480 GPa thanks to the stiffness of the present phases; values of toughness were in agreement with those found in literature. Flexural strength values at room temperature were in the range of 380-570 MPa. The boride composites retained excellent strength values (~580 MPa) up to 1500?C. High temperature behaviour in aggressive environment was evaluated by plasma torch tests at temperature up to 2000?C on 5vol% MoSi2 containing sampl

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?

Arc-jet testing of UHTC demonstrators with a sharp profile

The ultra-high-temperature ceramics (UHTCs) are currently the most studied key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth\u27s atmosphere. They are characterized by sharp profiles to increase performance and maneuvrability. Wedges with a very sharp profile (0.2 mm radius of curvature) and blunt hemispheric articles (5 mm radius of curvature) were produced in the system ZrB2-SiC. The dynamic response to oxidation of such UHTC demonstrators was studied under aero-thermal heating using a high enthalpy supersonic flow of a N2/O2 gas mixture in a plasma wind tunnel. Microstructural features of the reaction scale developed upon oxidation were analyzed and correlated to test conditions through Computational Fluid Dynamics (CFD) simulations. The outputs of CFD simulations matched the in-situ determinations and the materials evolution during arc-jet testin

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

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

Toughened ZrB2-based ceramics with addition of SiC whisker or short fiber

In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730?C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcemen

Recent achievements on fabrication, properties and arc-jet testing of sharp UHTC leading edges

The present contribution is addressed to offer an overview of recent achievements on some borides-based composites that are conventionally classified as ultra-high temperature ceramics (UHTCs) for their extremely high melting points.UHTCs are actively studied as key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth\u27s atmosphere: increased performances and better maneuvrability are gained only through the design of very sharp profile. Sharp wedges and test articles with blunter profiles were fabricated in the ZrB2 base system, using SiC particulate or SiC short fibers as second phase: microstruture and fundamental thermo-mechanical characteristics were determined. The dynamic response to oxidation of such UHTCs was studied under aero-thermal heating using high enthalpy supersonic flows in arc-jet plasma wind tunnel. Microstructural changes were analyzed and correlated to specimen\u27s size and shape and test conditions through Computational Fluid Dynamics (CFD) simulation