Thermophysical and radiative properties of pressureless sintered SiC and ZrB2-SiC laminates

The evaluation of thermal and radiative properties of materials to be used as a hot part of thermal protection systems is a key issue for the design process of the HTC and UHTC components. Ceramic laminates with composition 100?vol%SiC and 80?vol%ZrB₂-20?vol%SiC were prepared by the tape casting technique and pressureless sintered. Thermal properties such as the thermal expansion coefficient, specific heat, thermal diffusivity and conductivity were measured; in addition the total emissivity was evaluated. A comparison of the thermal behavior of these two kinds of laminates is made. Moreover their possible integration in a unique structure is discussed.     

Unveiling enhanced oxidation resistance and mechanical integrity of multicomponent ultra-high temperature carbides

The development of a new class of multicomponent ultra-high temperature ceramics (MC-UHTCs), often referred to as high-entropy UHTCs, has gained increased interest due to the possibility of improved thermomechanical and oxidation properties. In this study, a systematic approach by gradual addition in the UHTC components ranging from a binary to a dense quaternary (Ta,Nb,Hf,Ti)C is synthesized using spark plasma sintering (SPS). The solid solutioning was the critical factor in homogenizing the composition in the multicomponent system. The segregation of NbC and HfC was seen in binary and ternary UHTC systems, while a single-phase homogeneity was observed in the quaternary UHTC improving its hardness up to 34.8 GPa. The presence of closely spaced slip lines in the MC-UHTCs enhances resistance to indentation damage up to 72% at an applied load of 200 N. The formation of complex mixed oxide phase of Hf₆Ta₂O₁₇ ensued in the lower to negligible oxidation even up to 3 min of plasma exposure with temperature exceeding 2800°C. In sum, though the entropy remains medium (0.96R) for the selected system, the quaternary UHTC system undoubtedly has significantly better thermomechanical performance when compared to established baseline UHTCs. This raises the debate on the justification for calling a multicomponent system a “high entropy” to be seen in a new light. The developed MC-UHTCs elicits the paradigm of this new class of UHTCs expanding their potential in thermal protection systems for hypersonic applications.     

航天飞行器热防护系统技术综述

综述表明,C/C和C/SiC复合材料是宇宙输送系统飞行器前端部位热防护系统的最佳材料选择,多层抗氧化涂层、超高温陶瓷(UHTC)涂层、UHTC基体改性是提高其高温长期使用的有效途径。指出多层UHTC涂层、纳米级UHTC颗粒、火花等离子浇结(SPS)及碳气凝胶填充碳泡沫新型热防护结构等在高温热防护材料方面已显现出实际应用方向。     

Additive manufacturing of ZrB2–ZrSi2 ultra-high temperature ceramic composites using an electron beam melting process

Owing to their high melting points and ability to resist extreme thermal stresses, ultra-high temperature ceramics (UHTCs) are important materials for critical applications such as hypersonic flights, space re-entry vehicles, and rocket engines. Traditional manufacturing processes restrict the freedom to manufacture UHTCs with complex geometries due to the limitations of die and mold designs. Electron beam melting (EBM) is an established powder-bed layer-by-layer additive manufacturing (AM) process for metal parts. In this research, an effort was made to evaluate the feasibility of EBM for the AM fabrication of UHTC-based materials, and to investigate the microstructures of the fabricated materials under different processing conditions. A mathematical model was developed to simulate and optimize the processing parameters for the fabrication of ZrB₂-30 vol% ZrSi₂ UHTC using EBM. The simulation results were compared with experimental observations. For EBM fabrication of ZrB₂-30 vol% ZrSi₂ composites, the optimal processing parameters are beam power of 500 W with scanning speeds of 500, 750, and 1000 mm/s, and beam power of 1000 W with scanning speed of 1000 mm/s. This study demonstrates the potential for additive manufacturing of UHTCs with complex geometries by the EBM technique.     

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB₂ ultra‐high temperature ceramic (UHTC) particulate filled C_f/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc‐jet testing of the UHTC composites highlighted the excellent ultra‐high temperature oxidation performance of these materials.     

Thermo-chemical surface instabilities of SiC-ZrB2 ceramics in high enthalpy dissociated supersonic airflows

The response of three different SiC-ZrB₂ ceramics obtained by hot-pressing was studied at typical conditions of thermal protection systems of a re-entry spacecraft. Button-like lab-scale demonstrators were manufactured and tested in high enthalpy dissociated supersonic airflows using an arc-jet ground facility. Under severe aero-heating of up to 21 MJ/kg of specific total enthalpy and 3.5 MW/m² of (cold-wall) heat flux the SiC-ZrB₂ UHTC buttons endured rather well, though thermo-chemical surface instabilities started taking place for side wall surface temperatures of some buttons above 2050 K. The experimental determinations of the surface temperature, correlated to the microstructure changes occurred during testing, allowed to interpret the observed phenomena. Potentials and limits of the oxidation-resistant SiC-ZrB₂ system to withstand such extreme conditions were outlined.     

Carbon fiber reinforced ultra-high temperature ceramic matrix composites: A review

Recent studies on carbon fiber-reinforced ultra-high temperature ceramic matrix (C/UHTC) composites fabricated by hot-pressing, chemical vapor infiltration, polymer impregnation and pyrolysis, and melt infiltration (MI) are reviewed. Various efforts have been made to improve these preparation processes and to combine two or more of these because they have both the advantages and disadvantages in terms of the processing time, operating temperature, and the porosity of the resulting C/UHTC composites. In addition, the parameters governing the fracture toughness, thermal conductivity, and recession behavior (in oxidizing environments) of these composites have been discussed. This review demonstrates that C/UHTC composites with Zr- or Hf-based UHTC matrices fabricated via MI are potential candidates for advanced heat-resistant structural materials.     

Evaluation of ultra-high temperature ceramics foraeropropulsion use

Among the ultra-high temperature ceramics (UHTC) are a group of materials consisting of zirconium diboride or hafnium diboride plus silicon carbide, and in some instances, carbon. These materials offer a good combination of properties that make them candidates for airframe leading edges on sharp-bodied reentry vehicles. These UHTC perform well in the environment for such applications, i.e. air at low pressure. The purpose of this study was to examine three of these materials under conditions more representative of a propulsion environment, i.e. higher oxygen partial pressure and total pressure. Results of strength and fracture toughness measurements, furnace oxidation, and high velocity thermal shock exposures are presented for ZrB₂ plus 20 vol.% SiC, ZrB₂ plus 14 vol.% SiC plus 30 vol.% C, and SCS-9a SiC fiber reinforced ZrB₂ plus 20 vol.% SiC. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion applications.     

Self passivating behavior of multilayer SiC under simulated atmospheric re-entry conditions

In view of possible application of multilayer SiC as oxidation resistant and self-passivating component of re-usable thermal protection systems for space re-entry vehicles, this material was tested in a re-entry simulation chamber. A multilayer SiC laminate was processed by tape casting and pressure less sintering. Both the as-processed multilayer SiC and a similar passivated material, with a surface silica layer obtained by high temperature oxidation, were investigated. The microstructure and the mechanical features of these two materials were compared before and after the re-entry test. Microstructure was investigated by XRD, SEM-EDS, XPS and density measurements. Flexural strength, Young modulus and hardness were measured as well. Oxidation, occurring during re-entry test or resulting from the pre-test oxidation treatment, only affected the external part of the multilayer, while the material core was practically unchanged after oxidation. Every kind of specimen retained almost completely the original mechanical features after 100 re-entry simulations.     

Preparation of UHTCMCs by hybrid processes coupling Polymer Infiltration and Pyrolysis with Hot Pressing and vice versa

Carbon fibre reinforced ultra-high temperature ceramic (UHTC) matrix composites were fabricated coupling water-based powder slurry infiltration, Polymer Infiltration and Pyrolysis (PIP) and Hot Pressing (HP) techniques. This study aims to identify the best sequence of consolidation techniques to better integrate the carbon fibre cloths into an ultra-refractory sintered ceramic matrix of ZrB₂-SiC. Infiltrated preforms with UHTC powder slurry were densified via: a) a pre-sintering step by HP followed by two PIP cycles with polycarbosilane, and vice versa, b) two PIP cycles followed by a cycle of HP. Flexural strengths at room temperature and at 1500 °C (167 MPa and 592 MPa, respectively) were found to be significantly higher for composites obtained by the second route, suggesting that sintering of polymer-derived SiC during HP improves the structural properties of C_f/ZrB₂-SiC composites. This work presents an effective method for UHTCMC manufacturing in a shorter time than traditional PIP process.