Ultra-high Temperature Ceramics Research Articles

Polymer precursor synthesis of novel ZrC–SiC ultrahigh-temperature ceramics and modulation of their molecular structure

In this study, ZrC–SiC composite ceramics were prepared with varying Zr/Si molar ratios using sol–gel method. Composites were characterized by Fourier-transform infrared spectroscopy (FT–IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). FT–IR analysis confirmed macromolecular network structure of composites, in which the precursor is composed of polyvinyl butyral (PVB) as main chain, silane molecules are interlinked via –OH moieties in PVB side chains, and Zr atoms are crosslinked with Si in corresponding proportion. Ceramic precursor begins to decompose at a temperature exceeding 1300 °C and is completely transformed into ZrC–SiC composite ceramics with corresponding Zr/Si molar ratio at 1600 °C. Raman spectroscopy and TEM results reveal that after annealing at 1600 °C, ZrC powder uniformly covers surface of SiC ceramics, and high-crystallinity graphite carbon covers ZrC powder.

Oxidation response of ZrB2–SiC–ZrC composites prepared by spark plasma sintering

Considering the importance and application of ultrahigh temperature ceramics in oxidizing environments, in this research, the effect of ZrC content and spark plasma sintering parameters (temperature, time and pressure) on the oxidation response of ZrB2–SiC composites has been investigated. After fabricating the ternary composite samples in different SPS conditions and with different amounts of ZrC, the post-sintering oxidation process was carried out in a box furnace at the temperature of 1400 °C. Increasing the time and temperature of the SPS process caused the decrease in the oxidation resistance of the samples. The reason for such observations was attributed to the extreme growth of grains with increasing the temperature and time of the sintering process despite the better densification of the samples. This research did not reach a clear result about the effect of SPS pressure on composites oxidation behavior. Increasing the amount of ZrC also did not have a positive effect on the oxidation resistance of the samples because this phase itself undergoes oxidation at low temperatures.

Anisotropic thermal expansion in high-entropy multicomponent AlB2-type diboride solid solutions

High-entropy (HE) ultra-high temperature ceramics have the chance to pave the way for future applications propelling technology advantages in the fields of energy conversion and extreme environmental shielding. Among others, HE diborides stand out owing to their intrinsic anisotropic layered structure and ability to withstand ultra-high temperatures. Herein, we employed in-situ high-resolution synchrotron diffraction over a plethora of multicomponent compositions, with four to seven transition metals, with the intent of understanding the thermal lattice expansion following different composition or synthesis process. As a result, we were able to control the average thermal expansion (TE) from 1.3 × 10−6 to 6.9 × 10−6 K−1 depending on the combination of metals, with a variation of in-plane to out-of-plane TE ratio ranging from 1.5 to 2.8.

(Hf0.5Ta0.5)C ultra-high temperature ceramic solid solution nanowires

Ultra-high temperature ceramic (UHTC) nanowires are potential reinforcement materials due to it combines the perfect properties of bulk materials and unique geometric properties of one-dimensional (1D) nanostructures. Thus, developing 1D nanomaterials that have excellent morphology and structure retention in ultra-high temperature environments is of prime importance to bring their outstanding performance into full play. Herein, we report the novel solid solution ((Hf0.5Ta0.5)C) ceramic nanowires, which could not only maintain morphological and structural stability at 1900 °C but also exhibit 1D nanostructures under oxyacetylene scouring and ablation at 2300 °C. The morphology evolution of nanowires obeys the Rayleigh instability mechanism, and the internal structure and element distribution of nanowires remain unchanged even if the surface atoms are rearranged. The fascinating nanowires are demonstrated to have great potential as ideal reinforcement materials of composite materials and toughening phases of ceramics that are applied in ultra-high temperature environments, as well as excellent performance enhancement phases of functional materials. Our work may provide new insights into the development of ceramic nanowires and widen their applications.

Effect of internal lattice structure on the flexural strength of 3D printed hierarchical porous ultra-high temperature ceramic (ZrB2)

3D printing of technical ceramics using direct ink writing (DIW) of multiphase colloidal inks has the unique ability to create structures with hierarchical features. To facilitate the application of 3D printed hierarchical porous ultra-high temperature ceramics (UHTCs), additional limiting factors such as strength and the effect of 3D printed internal lattice structure need to be better understood. This study reports on the strength dependence of common DIW print parameters including internal lattice structure shape, nozzle diameter and spacings between adjacent filaments. The present study applies Weibull statistics to the experimental array that considers macro features introduced through print parameters as flaw types, which shows strength of 3D printed hierarchical ZrB2 is highly dependent on the introduced 3D printed structure, size and the stressed volume. This work provides essential information that can be used in the initial stages of design when considering implementation of additively manufactured hierarchical porous UHTCs.

Optimal Process Parameters for a Thermal-Sprayed Molybdenum-Reinforced Zirconium Diboride Composite on a Dummy Substrate

Thermal spray is an effective process for the fabrication of a metal matrix composite (MMC), where a zirconium diboride reinforcement is embedded in a molybdenum matrix to enable the combining of favorable properties in a new composite. The combination of two leading materials in the category of ultra-high-temperature ceramics (UHTCs) is due to a very high melting point (Mo: 2623 °C and ZrB2: 3245 °C), high thermal conductivity (Mo: 139 W/m°C and ZrB2: 24 W/m°C), good thermal shock resistance, low coefficient of thermal expansion (Mo: 5.35 µm/m°C and ZrB2: 5.9 × 10−6 K−1), retention of strength at elevated temperatures and stability in extreme environments. Thermal spraying of the Mo/ZrB2 composite possesses a non-linear behavior that is influenced by many coating variables. This characteristic makes finding the optimal factor combination difficult. Therefore, an effective and strategic statistical approach incorporating systematic experimental data is needed to optimize the process. In this study, the L9 orthogonal array in the Taguchi approach was utilized to optimize the spraying distance (SD), number of passes (NP), pressure (P) and coat-face temperature (TCF) using a dummy fiberglass substrate. The performance was evaluated based on the coating density (Cd) of the surfaces. Based on confirmation tests, our Taguchi analysis determined the ideal process parameters, which considerably enhanced the coating process. From the output response of the ANOVA, the most influential parameters for achieving a high coating density (Cd) were determined to be SD = 20 cm, NP = 24, P = 4 bar and TCF = 330 °C ((SD.)1-(NP.)3-P2-(S.T.)3). These observations show that the coating density (Cd) was significantly influenced by the coat-face temperature, followed by the number of passes, spraying distance and pressure with the following contributions 6.29, 17.89, 17.42 and 3.35%, respectively.

Study on the mechanism of ultra-high temperature ablation of ZrB2–SiC–TaSi2 coatings by low-pressure plasma spraying on the C/C composites

To improve the high-temperature oxidation resistance of C/C composites, ZrB2–SiC (ZS) and ZrB2–SiC–TaSi2 (ZST) ultra-high temperature ceramics coatings were prepared on their surfaces using induction plasma spheroidization and low-pressure plasma spraying (LPPS). Ablation behaviors of the ZS and ZST coatings were evaluated at 1800°C, 2000°C, and 2200°C using the oxyacetylene flame, we found that the ZS coatings had good ablation resistance at 1800°C, but rapidly failed at 2000°C. Whereas, the ZST coating always performed good oxidation and ablation resistance at even 2000°C with a mass ablation rate of 4.68 × 10-5 g/s and a linear ablation rate of -1.63 × 10-5 mm s-1. The enhancement of the high temperature ablation resistance property was attributed to the introduction of the TaSi2, which improved the density of the coatings prepared by LPPS, reduced the volatilization of the glassy phase at high temperature, and enabled ZrO2 to form a dense oxide layer.

Reactive laser synthesis of ultra-high-temperature ceramics HfC, ZrC, TiC, HfN, ZrN, and TiN for additive manufacturing

Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp > 3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods.

Synthesis of ZrB2@SiC composite particle through sol-gel and carbothermal reduction

Ultrahigh-temperature ceramics have been investigated for their high melting points and outstanding mechanical properties. Among these ceramic materials, ZrB 2 exhibits good thermal properties and chemical resistance owing to its strong combination of ionic and covalent bonds, making it a promising candidate in various industrial fields. However, when ZrB 2 is oxidized, two phases of oxide are produced on the surface of ZrB 2 : ZrO 2 and B 2 O 3 , which degrade the thermal and mechanical properties of ZrB 2 . In this study, a novel method is developed for the fabrication of core-shell-structured ZrB 2 @SiC composite particles to enhance the anti-oxidation properties of ZrB 2 with a fully covered SiC layer through the sol-gel process and carbothermal reduction. Using the sol-gel process with tetraethyl orthosilicate, core-shell ZrB 2 @SiO 2 particles are fabricated, and an amorphous phased shell is seen in the X-ray diffraction pattern. Thereafter, ZrB 2 @SiO 2 is transformed into ZrB 2 @SiC by adding 7 wt% carbon. It has a distinct SiC crystal structure, and its shell thickness is measured as 5.2 nm. High-resolution transmission electron microscopy images and selected area electron diffraction patterns confirm that core-shell-structured ZrB 2 @SiC composite particles are successfully fabricated. • ZrB 2 @SiC nanoparticle had been fabricated through sol-gel and carbothermal reduction. • Pretreatment with acid solution removed oxygen impurity and made -OH bond for SiO 2 . • Carbothermal reduction was carried out to make SiC without the reaction with ZrB 2 • ZrB 2 @SiC nanoparticle can be applied for the refractory and thermal protection.

Reactive two-step additive manufacturing of ultra-high temperature carbide ceramics

Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperatures (>2000 °C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultra-high temperature ceramic TiCx. Readily available equipment including a polymer powder bed fusion AM machine and a traditional tube furnace were used to produce UHTC cubes and lattice structures with sub-millimeter resolution. This processing scheme incorporated, (1) selective laser sintering of a Ti precursor mixed with a phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid conversion of the green body in CH4 to form TiCx structures. Reactive post-processing in CH4 resulted in up to 98.2 wt. % TiC0.90 product yield and a reduction in net-shrinkage during consolidation due to the volume expansion associated with the conversion of Ti to TiC. Results indicated that reaction bonding associated with the Gibbs free energy release associated with TiC formation produced interparticle adhesion at low furnace processing temperatures. The ability to bond highly refractory materials through this type of process resulted in structures that were crack-free and resisted fracture during thermal shock testing. Broadly, the additive manufacturing approach presented could be useful for the production of many UHTC carbides that might otherwise be incompatible with prevailing AM techniques that do not include reaction synthesis.

High-aspect-ratio ZrC whiskers: Synthesis, growth mechanism and electromagnetic wave absorption properties

Stealth materials with high dependability at elevated temperatures and outstanding mechanical properties are urgently needed for practical applications. As one-dimensional ultrahigh temperature ceramic (UHTC) materials, zirconium carbide whiskers (ZrCw) have attracted a great deal of attention due to their desirable mechanical and ablation resistance performance in high-temperature environments. We have successfully synthesized ZrCw using a carbothermal reduction technique without the introduction of metal catalytic in this paper. ZrCw shows a typically prismatic structure with the diameter of 1–2 μm and the aspect ratio of up to 250. The growth of ZrCw is controlled by a solid-liquid-solid (SLS) and vapor-solid (VS) compound mechanism in conjunction with the auxiliary action of mesophase Na3ZrF7. The ZrCw/paraffin hybrids achieve the minimum reflection loss (RL(min)) of −25.77 dB at 13.28 GHz under the thickness of 1.25 mm, and reach an effective absorption bandwidth (EAB) of 3.04 GHz (14.96–18.00 GHz) with a thickness of only 1.0 mm. This work presents a promising approach for large-scale producing high-purity whiskers, and verifies that ZrCw has extensive application prospects in the field of stealth materials.

Mechanical, structural and oxidation behavior of ultra high-temperature ceramic Ti–B–Si hard composite

The synthesis of Ultra-High Temperature Ceramic (UHTC), Titanium Diboride–Si composite TiB2–Si (Ti–B–Si) composite was carried out by Self-propagating high-temperature synthesis SHS). The structural, mechanical, oxidation behavior is studied. A very dense Ti–B–Si composite with very fine grain structure (1–3 μm), high hardness (∼29 GPa) & (648.8 GPa) modulus and good oxidation resistance up to 900 °C was synthesised by SHS process.

Functionally graded ultra-high temperature ceramics: From thermo-elastic numerical analysis to damage tolerant composites

To maximize the toughening contributions due to fiber bridging and residual stresses upon layering, ultra-high temperature ceramics containing variable amounts of short carbon fiber in functionally graded stacking sequences were designed and characterized. Stress fields evaluated by finite element model on (AB)nA and more complex asymmetric architectures were compared to the experimental fracture toughness pointing to an effective toughness increment in those structures where the notch fell in zones of residual compression. For the best composite, toughness at room temperature achieved 7 MPa·m0.5 and further increased to 10 MPa·m0.5 when tested at 1500 °C within a light ZrB2-based composite with density below 4 g/cm3.According to the numerical simulations and the effective microstructural features of the composites, the main guidelines for the realization of ceramics with simultaneous failure tolerance and ablation resistance were established.

Optical properties of bulk high-entropy diborides for solar energy applications

So far, the studies regarding the innovative High-Entropy Borides (HEBs), which belong to the more general class of Ultra-high temperature ceramics (UHTCs), have been entirely confined to their fabrication or characterization from the microstructural, mechanical and oxidation resistance viewpoints.In this work, the optical properties of two members of HEBs, i.e. (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2 and (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2, are evaluated for the first time to assess their possible utilization in the thermal solar energy field. The bulk samples (96.5 % and 97.4 % dense, respectively) are obtained as single-phase products by Spark Plasma Sintering (1950 °C/20 min/20 MPa) starting from powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). The optical characterization, whose results are discussed by comparing HEBs to the individual borides, shows that they are characterized by intrinsic spectral selectivity and low thermal emittance, resulting therefore interesting for high-temperature solar absorbers applications.

Experimental characterization of fatigue life of ZrB2-SiC based ultra high-temperature ceramic matrix composites

The development process of new Ultra High-Temperature Ceramic Matrix Composites (UHTCMC) for use in severe environments typical of space applications, such as thermal protection or rocket components, still lacks useful data for what concerns the properties influencing the structural behavior under the different load conditions. In particular, the fatigue life of the material plays a fundamental role in the design of full systems determining their resistance to typical vibrational loads. However, this property has been rarely investigated for UHTCMCs and its experimental characterization presents uncertainties related to the material specific behavior, which does not allow the application of traditional methodology employed for isotropic materials. In the present work, two different methodologies are proposed and applied to new ZrB2-SiC-based UHTCMCs, with either short or long continuous fibers, yielding in combination with numerical analyses to the determination of the σ/N curve for the two classes of materials.

Temperature‐dependent elastic and thermodynamic properties of ZrC, HfC, and their solid solutions (Zr0.5Hf0.5)C

AbstractZirconium carbide (ZrC) and hafnium carbide (HfC) have been identified as ultrahigh temperature ceramics with excellent thermal conductivity performance. The temperature profiles of ZrC and HfC have been studied; however, the temperature‐dependent of solid solution of (Zr0.5Hf0.5)C is still lacking. Herein, we report the temperature‐dependent elastic and thermodynamic properties of (Zr0.5Hf0.5)C using first‐principles calculations. The covalent characters of ZrC, HfC, and (Zr0.5Hf0.5)C are weakened at high temperatures by analyzing their respective electronic structures. In addition, the equilibrium volumes at different temperatures can be determined from the energy–volume (E–V) curves under the quasi‐harmonic approximation. Throughout the temperature ranges studied, the HfC material shows the highest bulk modulus and lowest thermal expansion. When T &gt; 1000 K, (Zr0.5Hf0.5)C exhibits better shear and Young's modulus performance close to HfC and shows the highest anisotropy. The lattice thermal conductivity decreased as temperature increased for ZrC, HfC, and (Zr0.5Hf0.5)C, and (Zr0.5Hf0.5)C has the smallest lattice thermal conductivity. These results provide fundamental and useful information for the practical application of ZrC, HfC, and (Zr0.5Hf0.5)C.

Elucidating the role of preferential oxidation during ablation: Insights on the design and optimization of multicomponent ultra-high temperature ceramics

Multicomponent ultra-high temperature ceramics (UHTCs) are promising candidates for thermal protection materials (TPMs) used in aerospace field. However, finding out desirable compositions from an enormous number of possible compositions remains challenging. Here, through elucidating the role of preferential oxidation in ablation behavior of multicomponent UHTCs via the thermodynamic analysis and experimental verification, the correlation between the composition and ablation performance of multicomponent UHTCs was revealed from the aspect of thermodynamics. We found that the metal components in UHTCs can be thermodynamically divided into preferentially oxidized component (denoted as MP), which builds up a skeleton in oxide layer, and laggingly oxidized component (denoted as ML), which fills the oxide skeleton. Meanwhile, a thermodynamically driven gradient in the concentration of MP and ML forms in the oxide layer. Based on these findings, a strategy for pre-evaluating the ablation performance of multicomponent UHTCs was developed, which provides a preliminary basis for the composition design of multicomponent UHTCs.

Fractal Geometry and Convolutional Neural Networks for the Characterization of Thermal Shock Resistances of Ultra-High Temperature Ceramics

The accurate characterization of the surface microstructure of ultra-high temperature ceramics after thermal shocks is of great practical significance for evaluating their thermal resistance properties. In this paper, a fractal reconstruction method for the surface image of Ultra-high temperature ceramics after repeated thermal shocks is proposed. The nonlinearity and spatial distribution characteristics of the oxidized surfaces of ceramics were extracted. A fractal convolutional neural network model based on deep learning was established to realize automatic recognition of the classification of thermal shock cycles of ultra-high temperature ceramics, obtaining a recognition accuracy of 93.74%. It provides a novel quantitative method for evaluating the surface character of ultra-high temperature ceramics, which contributes to understanding the influence of oxidation after thermal shocks.

Size-controlled preparations of tungsten and molybdenum borides in calcium or aluminum melt

Tungsten and molybdenum borides are widely used in ultra-high temperature ceramics (UHTCs) and electrode material. In this research, ultra-fine and ultra-coarse binary borides (WB, W2B5, MoB, Mo2B5) and MoAlB powders were prepared with W/Mo and B4C as raw materials and Ca/Al as the decarbonization agent. The feasibility of using Ca and Al as decarburizing agents was assessed by thermodynamic calculations. Meanwhile, the effects of reaction temperature, decarbonization agent type and its additional amount on the phase, morphology and size distribution of the powder products were thoroughly investigated. The results revealed that single phase products with carbon content of less than 1 wt% could be prepared. Moreover, the morphology and particle size of products were significantly affected by the reaction temperature and type of decarburization agent. When using Ca as the decarburizing agent, ultra-fine WB and MoB powders were effectively prepared, probably owing to the fact that W/Mo are essentially insoluble in Ca melt. However, the great solubility of W/Mo in Al makes it easily to synthesize ultra-coarse borides at high temperatures via the dissolution-precipitation mechanism. Additionally, ultra-fine borides particle can also be prepared by lowering the temperature and amount of Al to weak the dissolution-precipitation mechanism.

Grain boundary segregation induced strong UHTCs at elevated temperatures: A universal mechanism from conventional UHTCs to high entropy UHTCs

Ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties, including ultra-high melting point, excellent chemical stability, and good oxidation resistance, which make them promising candidates for aerospace and nuclear applications. However, the degradation of high-temperature strength is one of the main limitations for their ultra-high temperature applications. Thus, searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed. To achieve this goal, grain boundary segregation of a series of carbides, including conventional, medium entropy, and high entropy transition metal carbides, i.e., Zr0.95W0.05C, TiZrHfC3, ZrHfNbTaC4, TiZrHfNbTaC5, were studied by atomistic simulations with a fitted Deep Potential (DP), and the effects of segregation on grain boundary strength were emphasized. For all the studied carbides, grain boundary segregations are realized, which are dominated by the atomic size effect. In addition, tensile simulations indicate that grain boundaries (GBs) will usually be strengthened due to segregation. Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs, since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.