Ultra-high Temperature Ceramic Matrix Research Articles

Novel ceramic fibre - Zirconium diboride composites for solar receivers in concentrating solar power systems

Ultra-high temperature ceramic matrix composites (UHTCMCs) are a new class of composites which combine boride phases with carbon fibres to achieve new structural and functional properties and were originally designed for aerospace applications. For the first time, this work investigates the optical response of UHTCMCs in view of their possible application as high temperature solar absorbers in Concentrating Solar Power systems. Carbon fibre-reinforced ZrB2 pellets with different fibre type and arrangement were manufactured and characterized from the point of view of optical properties and surface microstructure. The composites had high fractions of ZrB2 (∼40 vol%) and carbon fibres (up to 60 vol%). Spectral hemispherical reflectance in the range 0.3–16 μm wavelength was assessed and compared to pure boride and graphite phases. Irrespective of the fibre architecture, the solar absorptance significantly increased with respect to the pure boride, reaching or even overpassing the solar absorptance of silicon carbide. Thermal emittance, calculated from 800 K to 1500 K temperature, increased as well in comparison to the pure boride, but always remained lower than that of SiC. These optical properties, combined with structural damage tolerance and light weight, suggest that UHTCMCs are of great interest as high temperature solar absorbers, overpassing the weaknesses of current solar receivers, as well as those of the most promising alternative materials to date.

Characterization of carbon fiber-reinforced ultra-high temperature ceramic matrix composites fabricated via Zr-Ti alloy melt infiltration

Carbon fiber-reinforced ultra-high temperature ceramic matrix composites (C/UHTCMCs) were fabricated via Zr-Ti alloy melt infiltration (Zr-Ti MI) using carbon-carbon composite (C/C) preforms and alloys with three different compositions. Alloys were successfully infiltrated into C/C to form solid solutions of TiC and ZrC, with melting temperatures > 2900 °C. Notably, residual alloys were not observed after MI occurred at 1750 °C. Bending strength and fracture toughness of the C/UHTCMCs at room temperature and 1500 °C in air/Ar revealed that mechanical properties of the composites were similar to those of the C/C preform. During arc wind tunnel tests at 2000 °C, a recession of C/UHTCMCs fabricated using Ti-rich alloys was observed; however, this behavior was not observed for the composites prepared using Zr-rich alloys owing to the formation of a ZrO2 solid solution. Accordingly, Zr-Ti MI is a viable method for preparing C/UHTCMCs without degrading the mechanical properties of the C/C preform, while increasing the ablation resistance.

Ablation behaviour of Cf–ZrC-SiC with and without rare earth metal oxide dopants

Continuous carbon fibre/carbon composites are candidates for ultra-high temperature applications due to their ability to retain their strength at elevated temperatures, however they easily oxidize and ablate in high temperature oxygen-based environments, a fact that seriously limits their applications in the aerospace industry. The introduction of ultra-high temperature ceramics (UHTCs) as a matrix in carbon fibre preforms effectively improves their oxyablation resistance at ultra-high temperatures in air atmospheres. In the present study, zirconium carbide-based ultra-high temperature ceramic matrix composites (UHTCMCs) simultaneously doped with silicon carbide and rare earth (RE) metal oxides were prepared by a slurry impregnation and pyrolysis method using phenolic resin as the bonding agent. The combination of injection and vacuum impregnation achieved a maximum density of up to 60% of theoretical. These composites were subsequently evaluated in terms of their ablation properties using an oxyacetylene (OAT) torch at temperatures of ∼2500 °C for 60 s. No significant surface damage was observed on any of the ablated samples. When ceria was used as the RE metal oxide dopant, the evidence suggests that cerium silicate was formed by the ceria dissolving in the molten silica; though this didn't appear to offer very much additional oxyablative protection. In contrast, when yttria was used, it improved the performance via stabilization of the cubic zirconia that formed on oxidation; this provided the greatest protection of all the samples tested. The combination of both ceria and yttria yielded the combined mechanism.

A systematic approach for horizontal and vertical scale up of sintered Ultra-High Temperature Ceramic Matrix Composites for aerospace – Advances and perspectives

Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) are the next generation composites developed for application in the harsh environment of aerospace. Qualification of these materials requires the manufacturing of demonstrators for testing in relevant environments, which in turn, requires to scale them up. This paper illustrates the systematic approach adopted for the scale-up of UHTCMCs based on carbon fibre reinforced zirconium diboride plus silicon carbide from laboratory to industrial scale dimensions. The scale-up process covered a period of three years and concerned an increase in diameter of ∼10 times (from 40 to 400 mm), and in thickness of ∼30 times (from 5 to 160 mm). Small-scale products were consolidated by hot pressing at ISTEC whilst larger samples were consolidated by an industrial spark plasma sintering facility (NanokerResearch, Spain). After each scale-up step, reproducibility of composition, microstructure and properties was carefully checked. Thanks to the homogenous fibre distribution and sapient dosing of secondary phases, composites with different diameters and thickness were successfully consolidated by both techniques with little adjustment of sintering parameters up to the maximum size of available industrial furnaces . The large discs produced allowed production of 170 mm long bars for tensile testing and large tiles for hypersonic wind tunnel tests . Thick samples were useful to machine complex shapes such as screws and nuts, vertical bars for characterization of properties along the composite thickness and nozzle demonstrators. • ZrB 2 -Cf composites were scaled-up 10 times in diameter, 30 times in thickness. • A manufact of UHTCMC material weighting 11 kg was produced for the first time. • The same mechanisms were efficient for scale-up in diameter and thickness. • Reproducible microstructure-properties were assessed from lab to industrial scale. • Large components such as plates, nozzle inserts, screw and nuts were fabricated.

Thermal Qualification of the UHTCMCs Produced Using RF-CVI Technique with VMK Facility at DLR

Ultra high-temperature ceramic matrix composites (UHTCMCs) based on carbon fibre (Cf) have been shown to offer excellent temperature stability exceeding 2000 °C in highly corrosive environments, which are prime requirements for various aerospace applications. In C3Harme, a recent European Union-funded Horizon 2020 project, an experimental campaign has been carried out to assess and screen a range of UHTCMC materials for near-zero ablation rocket nozzle and thermal protection systems. Samples with ZrB2-impregnated pyrolytic carbon matrices and 2.5D woven continuous carbon fibre preforms, produced by slurry impregnation and radio frequency aided chemical vapour infiltration (RF-CVI), were tested using the vertical free jet facility at DLR, Cologne using solid propellants. When compared to standard CVI, RFCVI accelerates pyrolytic carbon densification, resulting in a much shorter manufacturing time. The samples survived the initial thermal shock and subsequent surface temperatures of >2000 °C with a minimal ablation rate. Post-test characterisation revealed a correlation between surface temperature and an accelerated catalytic activity, which lead to an understanding of the crucial role of preserving the bulk of the sample.

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 Cf/ZrB2-SiC composites. This work presents an effective method for UHTCMC manufacturing in a shorter time than traditional PIP process.

Qualification and reusability of long and short fibre-reinforced ultra-refractory composites for aerospace thermal protection systems

Qualification and reusability campaigns were performed on ultra-high temperature ceramic matrix composites (UHTCMCs) made of a ZrB2-SiC matrix with short/long carbon fibre to assess their performance as thermal protection systems. Heat fluxes and stagnation pressures were set following those of reference re-entry missions. An interdisciplinary analysis followed by combining infrared measurements of the thermal response, estimation of surface properties and microstructural characterization. Compared to conventional C/SiC composites, the increased erosion resistance of the new UHTCMCs was assessed in ultra-hot regime: although these experienced much higher thermal loads, they underwent minimal erosion and preserved their structural stability even after multiple exposures.

Thermo-elastic properties in short fibre reinforced ultra-high temperature ceramic matrix composites: Characterisation and numerical assessment

The thermo-elastic properties of a novel class of ceramic composites based on an ultra-refractory matrix and containing short carbon fibre to provide failure tolerance and thermal shock resistance were experimentally characterised and numerically simulated. The composites samples were fabricated with fibre volume fractions ranging from 5 to 60 vol% and were featured by homogeneous dispersion of fibre along the basal plane due to the processing route. A random sequential adsorption algorithm was implemented to generate Representative Volume Elements (RVEs), following which finite element (FE) analyses were conducted to determine the elastic modulus and the coefficient of thermal expansion of the composites . Comparison of the experimental and modelling results showed good agreement for the modulus at low fibre volume fractions, and for CTE at higher volume fractions. Discrepancies are attributed to significant evolution of the composite microstructure upon processing at high temperatures, as revealed by microscopic analysis. The observations emphasise the need to account for the effects of microstructural changes on the constituent properties in the modelling of the thermo-mechanical properties in sintered ceramic composites.

Processing and characterization of carbon fibres reinforced ZrB2 ultra high temperature ceramic matrix composite

A process technology for the processing of the continuous carbon fibre reinforced ZrB2 (Cf – ZrB2) ultra high temperature ceramic matrix composite (UHTCMC) has been developed. The processing steps are slurry infiltration followed by curing and pyrolysis. The composite was then subjected to high temperature pressureless sintering. The microstrcutural and mechanical properties of Cf – ZrB2 UHTCMC were characterized at the completion of each processing step. The de-lamination between the carbon fibre and the ZrB2 matrix layer in 2D Cf – ZrB2 composite after pressureless sintering was found. However, the problem of de-lamination after sintering was not detected in the 2.5D Cf – ZrB2 composite by stitching the infiltrated layers in the Z-direction. Significant improvements in the sintered density, Young's modulus and flexural strength were achieved through 2.5D Cf – ZrB2 composite.

Use of Zr–Ti Alloy Melt Infiltration for Fabricating Carbon-Fiber-Reinforced Ultrahigh-Temperature Ceramic Matrix Composites

Carbon-fiber-reinforced ultrahigh-temperature ceramic (C/UHTC) matrix composites are an attractive candidate for fabricating various hot structures. The present study aimed to establish a Si-free Zr–Ti melt-infiltration method for fabricating C/UHTC matrix composites. To achieve this, the wettability of Zr–Ti alloys on carbon and their reactivity to carbon were examined. The alloys were melted on graphite plates and infiltrated into model preforms, which were made of porous carbon and had median pore diameters of 3 μm. The results showed that the apparent contact angle between Zr–Ti and C measured from melted alloys on carbon in room temperature was ~20–42° and that the alloys infiltrated into the preforms regardless of the Zr or Ti content. However, with an increase in the Zr content in the alloys, carbon disappeared and was absorbed into the alloys since the reactivity of Zr was higher than that of Ti and the specific surface area of the porous preform was higher than that of carbon-fiber-reinforced carbon composites, which are a typical preform of C/UHTC matrix composites. These results clearly indicate that not only the capillary flow during infiltration but also the reactivity of alloys to preforms should be considered in the process design for fabricating high-density composites via Zr–Ti infiltration.

Significant improvement of the self-protection capability of ultra-high temperature ceramic matrix composites

The oxidation behaviour of Cf/ZrB2-SiC with different fibre architectures, manufactured by slurry impregnation, polymer infiltration and mild pyrolysis, was investigated. Short term oxidation tests in air were performed for 1 min and 5 min at 1500 °C and 1650 °C in a bottom loading furnace. Microstructure, oxide scale thickness and composition were analysed by SEM/EDS/XRD. Results indicated that a good dispersion of ZrB2 particles in the polymer derived SiC(O) matrix promoted the formation of compact scales filling surface holes left by fibre oxidation. 20−30 vol% of ZrB2 in the material was found a good compromise between lightness and oxidation resistance.

Laser ablation behavior and mechanism of Cf/SiC–ZrC ultra-high temperature ceramic matrix composite prepared by PIP method

Cf/SiC–ZrC ultra-high temperature ceramic matrix composites prepared by precursor impregnation and pyrolysis (PIP) method were exposed to laser with energy density of kW level and the ablation behavior and mechanism were investigated. The results show that under different ablation times, the composites exhibited similar ablation region divisions, namely, central ablation pit, ablation transition zone, oxidation zone and non-ablated zone. Sample subjected to laser ablation of 5 s has the largest linear ablation rate of 0.0946 mm/s, which decreased to 0.0748 mm/s for 10 s ablation sample, and after that increased to 0.0876 mm/s for 15 s ablation sample. Under high-energy laser ablation, ZrO2 was the main anti-ablative component among the oxides formed by the oxidation of the matrix, which underwent morphology changes from sheets, fragments to micro and nano particles. Dense structure and continuous anti-ablation layer with high melting point components are the key factors to improve the anti-laser ablation performance of materials.

Direct ink writing of ZrB2-SiC chopped fiber ceramic composites

Ultra-high temperature ceramics (UHTCs) are of interest for thermally- and/or mechanically- extreme environments because of their high melting temperatures (> 3000 °C) and ablation resistance. More widespread use is limited by low fracture toughness and inability to be processed into complex-shaped components. Here, we report the production of fiber-reinforced UHTC matrix composites (UHTCMCs) formed via the additive manufacturing technique of direct ink writing (DIW). Slurry 'inks' were developed containing up to 47.5 vol% of the UHTC zirconium diboride (ZrB 2 ), up to 10 vol% chopped silicon carbide fiber (SiC f ), and a silicon carbide (SiC) precursor polymer. Lattice structures and flexural specimens were printed and pyrolyzed to form UHTCMCs with aligned (relative to the print direction) SiC f in the ZrB 2 – SiC matrix. Flexural strength of fiber-containing parts is presented, and fiber alignment due to deposition is analyzed with X-ray computed tomography. Defects that occurred during the DIW process, and their probable causes and mitigation strategies are also discussed.

Properties of large scale ultra-high temperature ceramic matrix composites made by filament winding and spark plasma sintering

In this paper, for the first time, we report the manufacturing and characterization of large UHTCMCs discs, made of a ZrB2/SiC matrix reinforced with PyC-coated PAN-based carbon fibres. This work was the result of a long term collaboration between different institutions and shows how it is possible to scale-up the production process of UHTCMCs for the fabrication of large components. 150 mm large discs were produced by filament winding and consolidated by spark plasma sintering and specimens were machined to test a large set of material properties at room and elevated temperature (up to 1800 °C). The extensive characterization revealed a new material with mechanical behaviour similar to CMCs, but with intrinsic higher thermal stability. Furthermore, the scale-up demonstrated in this work increases the appeal of UHTCMCs in sectors such as aerospace, where severe operating conditions limit the application of conventional materials.

Theoretical characterization of the temperature dependence of the contact mechanical properties of the particulate-reinforced ultra-high temperature ceramic matrix composites in Hertzian contact

The novel theoretical models for the temperature dependence of the contact mechanical properties of the particulate composites in Hertzian contact are proposed in this paper, based on an assumption of the temperature-independent constant energy storage capacity for a specific brittle particulate composite, associating with the material yielding, and the theory of Hertzian contact. The yield stress, critical loads for the onset of the brittle and quasi-plastic damage modes, indentation strength, and the more commonly used fracture strength of the composites are expressed in terms of the basic material properties—the temperature-dependent Young’s modulus and specific heat capacity, and the critical flaw size. The models enable the priori and quantitative predictions of the contact damage behavior of the composites during the whole process of the Hertzian contact. The models are validated by comparing against the experimental measurements of the temperature-dependent contact mechanical properties of the particulate-reinforced ultra-high temperature ceramic matrix composites in the temperature range of 25 ∼ 800 °C, which were found probably to be the only reported experimental studies of the particulate composites. The temperature dependence of the contact damage behavior of the composites is revealed to be mainly governed by the temperature-dependent Young’s modulus and specific heat capacity, and the change of the flaw size above a certain temperature point. The models provide additional adequate methods for the determination of the nature and the underlying physical control mechanisms of the damage behavior of the composites at high temperatures.

Selection, processing, properties and applications of ultra-high temperature ceramic matrix composites, UHTCMCs – a review

ABSTRACT Composites are, in general, a rapidly evolving and growing technical field with a very wide range of applications across the aerospace, defence, energy, medical and transport sectors as a result of their superior mechanical and physical properties. Ultra-high temperature ceramic matrix composites, UHTCMCs, are a new subfield within the wider grouping of CMCs that offer applications in rocket and hypersonic vehicle components, particularly nozzles, leading edges and engine components. The design and development of structural materials for use in oxidising and rapid heating environments at temperatures above 1600°C is therefore of both great scientific and engineering importance. UHTC materials are typically considered to be the carbides, nitrides, and borides of the transition metals, but the Group IV compounds (Zr, Hf & Ti) plus TaC are generally considered to be the main focus of research due to the superior melting temperatures and stable high-melting temperature oxide that forms in situ. This review presents the selection, processing, properties, applications, outlook and future directions of UHTCMCs.

Scale characterisation of an oxidised (Hf,Ti)C-SiC ultra-high temperature ceramic matrix composite

A hybrid carbide ultra-high temperature ceramics matrix [(Hf,Ti)C-SiC] reinforced with BN-coated carbon fibres was fabricated and tested for surface oxidation resistance. The UHTC composite showed an average mass ablation rate of 0.0014 g/s after exposure to a high heat flux (∼17 MW/cm2) oxyacetylene flame test for 30 s above 2500 °C. The cross-sectional profile of the oxides scale formed was characterised and analysed. The scale was multicomponent; consisting of oxides of Hf, Ti and Si, as well as HfTiO4 and HfSiO4, which underwent phase separation and immiscibility. Multiple glassy bubbles formed on the scale surface due to the impediment of escaping gases by the glassy layer on the outer scale. The largest pores in the scale and surface bubbles that resisted rupture were the dominant features of the outermost phase-separated layer. Phase separation in the scale top layer improves the resistance to scale rupture.

Co-reinforcing of ZrB2–SiC ceramics with optimized ZrC to Cf ratio

Characteristics of ZrB2–SiC ultrahigh temperature ceramic matrix composites (UHTCMCs) reinforced with ZrC and carbon fiber (Cf) were investigated in this article. Spark plasma sintering (SPS) process was utilized to fabricate the samples at 1800 °C for 5 min under 30 MPa punch pressure and vacuumed atmosphere. In all samples, the volume ratio of ZrB2: SiC was equal to 4:1, and the summation of ZrC and Cf reinforcements was 7.5 vol% with different ZrC: Cf ratios. Field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), densitometry, flexural strength, and hardness measurements were employed for characterization of the prepared samples. Microstructural inspection revealed the formation of SiC sheath around the carbon fibers due to several reactions in the surface SiO2 layers existed on the SiC particles. Optimal flexural strength (628.4 MPa) and hardness (20.8 GPa) values were achieved for the sample co-reinforced with 6.5 vol% ZrC and 1 vol% Cf, with a relative density of 97.7%.

Design of Lu2O3-reinforced Cf/SiC-ZrB2-ZrC ultra-high temperature ceramic matrix composites: Wetting and interfacial reactivity by ZrSi2 based alloys

The wettability and infiltration of molten ZrSi2 and ZrSi2-Lu2O3 alloys into Cf/SiC and B4C-infiltrated Cf/SiC composites were investigated to understand the interfacial interactions that occur during the development of Cf/SiC-ZrC and Cf/SiC-ZrB2-ZrC-Lu2O3 materials. A significant evaporation of Si from the liquid affected the wetting behaviour of the alloy when tested in a vacuum at 1670 °C. The better wetting and spreading of the alloy over the surface was observed for the composites with lower overall porosity (12 %). On the other hand, the formation of an outer dense layer, followed up by the uniform infiltrated region up to ∼ 1 mm was observed for the Cf/SiC with higher porosity (21 %). The infiltrated alloy reacted with SiC matrix to form ZrC or with B4C-infiltrated SiC matrix to form ZrB2-ZrC-SiC. The Lu2O3 particles were not wetted by the melt, and were pushed away of the reaction zone by the solidification front.

Ultra-high temperature ceramics developments for hypersonic applications

Ultra-High Temperature Ceramics are good candidates to fulfil the harsh requirements of hypersonic applications. For more than a decade, the Materials and Structures Department (DMAS) of ONERA has been actively involved in several programmes to develop such materials for different applications (hypersonic flights, propulsion systems...). In our laboratories, monolithic and composite materials have been investigated as well as several processing methods. In this paper, we present for example the ZrB2-SiC and HfB2-SiC compositions with TaSi2 or Y2O3 additions which have been especially studied in the European Projects ATLLAS and ATLLAS II. Assessments of several prototypes in realistic environment are also described. Furthermore, based on these material developments, a specific study on the oxidation behaviour of such monoliths from 1200 °C to 2400 °C with a dedicated test bench using a 2 kW CO2 laser has been carried out (oxidation under air and water vapour atmospheres). Recently, some work on the manufacturing of Ultra-High Temperature Ceramic Matrix Composites has been initiated using slurry infiltration and pyrolysis. The behaviour and properties of these materials are encouraging.