Advances in ultra-high temperature ceramics, composites, and coatings

Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, highentropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.

Fabrication of C/C–SiC–ZrB2 Ultra-High Temperature Composites through Liquid–Solid Chemical Reaction

In this paper, we aimed to improve the oxidation and ablation resistance of carbon fiber-reinforced carbon (CFC) composites at temperatures above 2000 °C. C/C–SiC–ZrB2 ultra-high temperature ceramic composites were fabricated through a complicated liquid–solid reactive process combining slurry infiltration (SI) and reactive melt infiltration (RMI). A liquid Si–Zr10 eutectic alloy was introduced, at 1600 °C, into porous CFC composites containing two kinds of boride particles (B4C and ZrB2, respectively) to form a SiC–ZrB2 matrix. The effects and mechanism of the introduced B4C and ZrB2 particles on the formation reaction and microstructure of the final C/C–SiC–ZrB2 composites were investigated in detail. It was found that the composite obtained from a C/C–B4C preform displayed a porous and loose structure, and the formed SiC–ZrB2 matrix distributed heterogeneously in the composite due to the asynchronous generation of the SiC and ZrB2 ceramics. However, the C/C–SiC–ZrB2 composite, prepared from a C/C–ZrB2 preform, showed a very dense matrix between the fiber bundles, and elongated plate-like ZrB2 ceramics appeared in the matrix, which were derived from the dissolution–diffusion–precipitation mechanism of the ZrB2 clusters. The latter composite exhibited a relatively higher ZrB2 content (9.51%) and bulk density (2.82 g/cm3), along with lower open porosity (3.43%), which endowed this novel composite with good mechanical properties, including pseudo-plastic fracture behavior.

Experimental Study on the Properties of Copper Base Alloy by Laser Cladding on the Surface of Pure Copper Contact

Arc ablation is the main reason for the failure of pure copper switches. In order to improve the electric ablation resistance of pure copper contacts, Cu-W-Ni alloy cladding layers with different tungsten contents were prepared on the surface of pure copper matrix by laser cladding technology. The microstructure, composition distribution, hardness and corrosion resistance of the cladding layer were analyzed. The results show that the surface of the cladding layer and the pure copper matrix is metallurgical, without holes, cracks and other defects, and the dilution rate is low, and the average hardness is significantly increased. On the whole, the comprehensive properties of CuW10Ni3 cladding layer are slightly better than the other two Cu-W-Ni cladding layers. Compared with the pure copper matrix, the contact hardness is significantly improved under the premise of ensuring the electric conductivity of the inner pure copper matrix.

AERO-THERMO-DYNAMIC STUDY OF ULTRA-HIGH- TEMPERATURE CERAMIC COMPOSITES FOR THERMAL PROTECTION SYSTEMS AND ROCKET NOZZLES

The most extreme aero-thermo-dynamic conditions encountered in aerospace applications include those of atmospheric re-entry, characterized by hypersonic Mach numbers, high temperatures and a chemically reacting environment, and of rocket propulsion, in which a combusting, high-pressure, supersonic flow can severely attack the surfaces of the motor internal components (particularly nozzle throats), leading to thermo-chemical erosion and consequent thrust decrease. For these applications, Ultra-High-Temperature Ceramics (UHTC), namely transition metal borides and carbides, are regarded as promising candidates, due to their excellent high-temperature properties, including oxidation and ablation resistance, which are boosted by the introduction of secondary phases, such as silicon carbide and carbon fibers reinforcement (in the so-called Ultra-High- Temperature Ceramic Matrix Composites, UHTCMC). The recent European H2020 C3HARME research project was devoted to development and characterization of new-class UHTCMCs for near-zero ablation thermal protection systems for re-entry vehicles and near-zero erosion rocket nozzles. Within the frame of the project and in collaboration with several research institutions and private companies, research activities at the University of Naples “Federico II” (UNINA) focused on requirements definition, prototypes design and test conditions identification, with the aim to increase the Technology Readiness Level (TRL) of UHTCMC up to 6. Experimental tests were performed with two facilities: an arc-jet plasma wind tunnel, where small specimens were characterized in a relevant atmospheric re-entry environment (Fig.1a), and a lab-scale hybrid rocket engine, where material testing was performed with different setups, up to complete nozzle tests, in conditions representative of real propulsive applications (Fig.1b). The characterization of the aero-thermo-chemical response and ablation resistance of different UHTCMC formulations was supported by numerical computations of fluiddynamic flowfields and materials thermal behavior. The UNINA activities provided a large database supporting the achievement of the project objectives, with development and testing of full-scale TPS assemblies and a large-size solid rocket nozzle.