A critical analysis of the parameters affecting the oxidation behavior of ultra-high-temperature diboride ceramics

In the last few decades, research on the processing and properties of ultra-high-temperature ceramics (UHTCs) has generated a substantial base of knowledge and left several unanswered questions. There is a large scatter in the literature data associated with the processing of UHTC borides prompting the non-reproducibility and non-uniformity in the microstructure and, thus, desired properties. Herein, the data on the oxidation behavior of UHTC borides ubiquitous in the entire literature are analyzed to understand the effect of composition, sintering parameters, densification, grain size, and oxidation conditions. A conjunction of graphical methods has been utilized to converge the scattered data and correlate the effect of variables and testing environment on the oxidation behavior. It was concluded that high densification (>95%), large grain size (∼8 μm), and 20–30 vol.% of Si-containing additives (SiC, Ta₅Si₃, and TaSi₂) could augment the oxidation resistance. The study elucidates that oxide scale thickness should be preferred over mass gain in future studies as a metric for measuring oxidation. The analysis presented here will allow the UHTC community to optimize diboride materials' design for hypersonic applications. The developed database on the oxidation performance of diborides will also transfer knowledge beyond the memory banks of the experts in the field. Furthermore, well-structured databases such as the one developed herein could be employed in data-driven approaches to optimize the design and manufacturing of ultra-high-temperature materials in an efficient scheme.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

UHTC-based matrix as protection for Cf/C composites: Original manufacturing,microstructural characterisation and oxidation behaviour at temperature above 2000 °C

In space propulsion applications, the development of new ceramic matrix composites with improved resistance to oxidation and ablation at high temperature is needed and ultra-high temperature ceramics-based ones appear the most suitable. Combination of both powder impregnation (ZrB₂, C) and liquid silicon infiltration enabled manufacturing of UHTC based matrices in C_f/C preforms with less than 10 vol% open porosity and various proportions and homogeneous distribution of C, ZrB₂, SiC and Si. Oxidation behaviour was evaluated on composite structures using an oxyacetylene torch at temperatures higher than 2000 °C. Chemical analyses and microstructural observations before and after oxidation testing evidenced the protection ability of ZrB₂-SiC-Si matrices thanks to the formation of multi-oxide scales which resisted even tested durations of 6 min and pointed the unharmful presence of residual 12 vol% silicon on the composite for use at high temperature under high gas flows.     

In-situ oxide scale investigation of Sm-doped ZrB2/SiC Billets

Samarium-doped zirconium diboride/silicon carbide (Sm-ZBS) ceramics possess an emittance of 0.9 at 1600 °C and develop oxide scales that have excellent ablation performance. This study investigates the oxide scale development of 3 mol% doped Sm-ZBS which contains 80 vol% ZrB₂ and 20 vol% SiC when exposed to temperatures in excess of 1800 °C in an oxidizing atmosphere. Samples were prepared via chemical infiltration of samarium nitrate into spray-dried powders of 80 vol.% ZrB₂/20 vol.% SiC; powders were then pressed into billets and sintered without pressure. Samples cut from these billets were then oxidized for 10, 60, and 300 s, respectively, using an oxyacetylene torch. A Sm-depletion region was observed and believed to form due to glass transport to the surface. X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS, beginning with the formation of ZrO₂ and Sm₂O₃. The final oxide scale was determined to be c₁-Sm_0.2Zr_0.8O_1.9, with a melting temperature exceeding 2500 °C. SEM and EDS were also used to investigate microstructural formation due to the bursting of convection cells.     

Oxidation behavior of ZrB2-MoSi2-SiC composites in air at 1500 °C

We investigated the oxidation behavior and the effect of the amount of SiC added on oxidation resistance in both hot-pressed ZrB₂-MoSi₂-SiC composites, 55ZrB₂-40MoSi₂-5SiC and 40ZrB₂-40MoSi₂-20SiC (vol.%), exposed to dry air at 1500 °C for up to 10 h. Quantitative electron microprobe analysis characterizations of the chemical compounds of post-oxidized composites were carried out. Parabolic oxidation behavior was observed for both composites. The addition of SiC improved the oxidation resistance of ZrB₂-MoSi₂-SiC composites, and the improvement enhanced with amount of SiC added. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. The oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of oxidation resistance with SiC addition is associated with the presence of a thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Effects of Si3N4 and WC on the oxidation resistance of ZrB2/SiC ceramic tool materials

ZrB₂/SiC, ZrB₂/SiC/Si₃N₄ and ZrB₂/SiC/WC ceramic tool materials were prepared by spark plasma sintering technology, and their oxidation resistance was tested at different oxidation temperatures. When the oxidation temperature is 1300 °C, the oxide layer thickness, oxidation weight gain and flexural strength of ZrB₂/SiC/Si₃N₄ ceramic tool material after oxidation are 8.476 μm, 1.436 mg cm^?2 and 891.0 MPa, respectively. Compared with ZrB₂/SiC ceramic tool materials, the oxide layer thickness and oxidation weight gain are reduced by 8.2% and 11.8%, respectively, and the flexural strength after oxidation is increased by 116.1%. However, the addition of WC significantly reduces the oxidation resistance of the ceramic tool material. A dense oxide film is formed on the surface of ZrB₂/SiC/Si₃N₄ ceramic tool material during oxidation, which effectively prevents oxygen from entering the inside of the material, thereby improving the oxidation resistance of the ceramic tool material.     

Influence of chromium diboride on the oxidation resistance of ZrB2-MoSi2 and ZrB2-SiC ceramics

The effect of chromium diboride addition on the densification process and oxidation behavior of two ZrB₂-MoSi₂ and ZrB₂-SiC baseline systems was studied. CrB₂ was beneficial in lowering the sintering temperature owing to the tendency of its oxide to react with MoSi₂ and SiC forming low-melting phases that helped the powder consolidation. Oxidation at 1500 °C induced the formation of further boron oxide as first consequence. In one case, when CrB₂ was combined with MoSi₂, an improved oxidation resistance was observed due to the stabilization of Cr-borides in the subscales saturated with B₂O₃. In the other case, when it was combined with SiC, the excessive low viscosity of the borosilicate glass facilitated the consumption of a thicker portion of materials as compared to the ZrB₂-SiC reference.     

Multiscale toughening of ZrB2-SiC-Graphene@ZrB2-SiC dual composite ceramics

The design of bioinspired architectures is effective for increasing the toughness of ceramic materials. Particularly, a dual composite equiaxial architecture is ideal for fabricating weak interface-toughened ZrB₂-SiC ceramics with isotropic performance. In this paper, ZrB₂-SiC-Graphene@ZrB₂-SiC dual composite ceramics were synthesized via an innovative processing technique of granulating-coating method. ZrB₂-20 vol.% SiC containing 30 vol.% Graphene was selected as weak interface to realize multiscale toughening and improve the thermal shock resistance of ZrB₂-SiC ceramic materials. The incorporation of ZrB₂-SiC-Graphene weak interface into the ZrB₂-SiC matrix improved the damage tolerance and critical thermal shock temperature difference. The design of equiaxial structures moderated the anisotropy of performance in different planes. The graphene sheets incorporated in the ZrB₂-SiC-Graphene interface phase played a key role in multiscale toughening, including macroscopic toughening of crack deflection and microcracks, and microscopic toughening of graphene bridging and pull-out.     

Physical characterization and arcjet oxidation of hafnium-based ultra high temperature ceramics fabricated by hot pressing and field-assisted sintering

For this study, HfB₂-based ultra high temperature ceramic (UHTC) samples were prepared by hot pressing and field-assisted sintering (FAS) with 10–20 vol.% SiC (baseline), 5 vol.% TaSi₂, and 5 vol.% iridium. Dense billets were tested for hardness and mechanical strength. When compared, the FAS method consistently yielded materials with a grain size 1.5–2 times finer than samples processed via hot pressing. In general, room temperature flexural strengths of these materials were found to be lower (～400 MPa) than similar fully dense HfB₂–SiC materials, with strengths between 500 and 700 MPa. Oxidation resistance testing of flat-face models was conducted in a simulated re-entry environment, at Q_{Cold Wall} ～250 W/cm² for 5 min. Samples processed by FAS had reduced oxide thickness and SiC depletion zones compared to the baseline HfB₂–20SiC material. In all cases oxide thickness was reduced by ～3× and SiC depletion zone thickness was reduced ～3× over the baseline.     

Oxidation behavior of ZrB2-SiC-ZrC in oxygen-hydrogen torch environment

The oxidation behavior of four ZrB₂-SiC-ZrC compositions with varying ZrC contents (20, 34, 50, and 64 vol.%) was compared to that of ZrB₂-SiC. The ceramics were oxidized at 1700 °C in an oxygen-hydrogen torch environment. The liquid oxide on the ZrB₂-SiC sample came off from the surface under such an environment. In contrast, the all ZrB₂-SiC-ZrC samples maintained the convex oxide on the surface, which consisted of ZrO₂ and SiO₂. The convex oxide of ZSZ with higher ZrC content was thicker, with the exception of ZrB₂-SiC-64vol.%ZrC sample. The ZrB₂-SiC-64vol.%ZrC sample formed a ZrO₂-rich layer, which was clearly denser than the ZrO₂-SiO₂. This densification was caused by ZrO₂-sintering, and it was specific behavior under the dynamic pressure.     

Pressureless sintering of ZrB2–SiC composite laminates using boron and carbon as sintering aids

Abstract

A processing method common to composite ceramics with very different ZrB₂/SiC ratios was developed in order to exploit ZrB₂–SiC laminates comprising alternate layers with different compositions for thermal protection systems of re-entry vehicles. Ceramic laminates were made using SiC, ZrB₂ and composites with a SiC/ZrB₂ ratio ranging from 100 vol.-%SiC to 100 vol.-%ZrB_2. The preparation was performed by tape casting of a slurry, layer stacking, debinding and pressureless sintering. Boron and carbon proved to be suitable sintering aids for SiC laminates as well as for composite laminates containing SiC. In the case of composites with a ZrB₂ matrix, the second phase of SiC acted as a sintering aid. This process obtains very similar densification for specimens with very different compositions (ranging from 100%SiC to 80ZrB₂–20SiC). The stiffness of ZrB₂–SiC laminates increased with the ZrB₂ content increase, while the bending strength was not affected by the ZrB₂/SiC ratio.     

Zirconium-diboride silicon-carbide composites: A review

Zirconium diboride (ZrB₂) and silicon carbide (SiC) composites have long been of interest since it was observed that ZrB₂ improved the thermal shock resistance of SiC. However, processing of these materials can be difficult due to high and different sintering temperatures and differences in the thermodynamic stability of each material. ZrB₂–SiC composites have been processed in a variety of ways including hot-pressing, spark-plasma sintering, reactive melt infiltration, pack cementation, chemical vapor deposition, chemical vapor infiltration, stereolithography, direct ink writing, selective laser sintering, electron beam melting, and binder jet additive manufacturing. Each manufacturing method has its own pros and cons. This review serves to summarize more than 60 years of research and provide a coherent resource for the variety of methods and advancements in development of ZrB₂–SiC composites.     

Influence of SiC content on the oxidation of carbon fibre reinforced ZrB2/SiC composites at 1500 and 1650 °C in air

The microstructure and the oxidation resistance in air of continuous carbon fibre reinforced ZrB₂–SiC ceramic composites were investigated. SiC content was varied between 5–20?vol.%, while maintaining fibre content at ～40?vol.%. Short term oxidation tests in air were carried out at 1500 and 1650?°C in a bottom-up loading furnace. The thickness, composition and microstructure of the resulting oxide scale were analysed by SEM-EDS and X-Ray diffraction. The results show that contents above 15?vol.% SiC ensure the formation of a homogeneous protective borosilicate glass that covers the entire sample and minimizes fibre burnout. The scale thickness is ～90?μm for the sample containing 5?vol.% SiC and decreases with increasing SiC content.     

Influence of silicon carbide particle size on the microstructure and mechanical properties of zirconium diboride–silicon carbide ceramics

The influence of silicon carbide (SiC) particle size on the microstructure and mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics was investigated. ZrB₂-based ceramics containing 30 vol.% SiC particles were prepared from four different α-SiC precursor powders with average particle sizes ranging from 0.45 to 10 μm. Examination of the dense ceramics showed that smaller starting SiC particle sizes led to improved densification, finer grain sizes, and higher strength. For example, ceramics prepared from SiC with the particle size of 10 μm had a strength of 389 MPa, but the strength increased to 909 MPa for ceramics prepared from SiC with a starting particle size of 0.45 μm. Analysis indicates that SiC particle size controls the strength of ZrB₂–SiC.     

Thermal stability under laser heating of hot-pressed (Hf1-X ZrX)B2/SiC powder mixtures obtained by mechano-synthesis

The paper studied ultra-refractory (Hf_1-X Zr_X)B₂/SiC ceramics fabricated by hot-pressing (HP) of mechano-chemically assisted precursors synthesis, and tested their thermal stability under laser heating. Fully dense materials were obtained after HP. Microstructures of the sintered materials were analyzed by XRD and SEM-EDS, while 4-pt flexure strength in air at room temperature up to 1773 K was measured. The thermal stability was tested using a diode laser source. A SiC-free fully dense ZrB₂ ceramic was used as benchmark to identify the potential of the laser heating technique and separate the effects related to the addition of SiC into a diboride ceramic matrix. Infrared thermo-camera and 2-color pyrometer provided the real-time variation of the surface temperature vs time. For surface temperatures below 1900 K reached by the SiC-containing samples, SiC acted as a key player and provided excellent protection to the diboride matrices against oxidation: extensive post-test microstructural analyses documented this response.     

Synthesis of ZrB2/SiC composite powders by single-step solution process from organic–inorganic hybrid precursor

A precursor of a zirconium diboride/silicon carbide (ZrB₂/SiC) composite was synthesised via an organic–inorganic hybrid derived from gum karaya, tetraethyl orthosilicate, boric acid and zirconyl chloride starting materials. Fourier transform infrared spectroscopy of the as-synthesised dried hybrid revealed the formation of Si–O, Zr–O–C and B–O–B. X-ray diffraction revealed that the powder consists of only ZrB₂ and β-SiC. Scanning electron microscopy and TEM of the composite powders showed that SiC and ZrB₂ occurred in intimately mixed aggregates of spheroidal submicron sized particles for low (3M) boric acid concentration, while at high (5M) boric acid concentration, the two phases are larger with the ZrB₂ adopting a blocky, angular morphology (～10–30?μm long by 5?μm wide and thick), while the SiC remains spheroidal with ～1?μm diameter particles in 10–20?μm diameter aggregates. Thermogravimetry–differential thermal analysis with the help of X-ray diffraction analysis revealed that the formation temperature was low at 1275°C for ZrB₂ and 1350°C for the SiC with 40?wt-% yield.     

Enhanced oxidation resistance of ZrB2/SiC composite through in situ reaction of gadolinium oxide in patterned surface cavities

Micro-cavities on the surface of dense ZrB₂/20 vol.% SiC composites, machined by ultra-fast laser ablation, were filled with Gd₂O₃ nanopowder and oxidized in static air at 1600 °C. Optimized rectangular pattern of cavities, 10 μm diameter and deep, 20 μm apart conferred improved oxidation resistance compared to the untreated ZrB₂/20 vol.% SiC due to the formation of glasses of higher viscosity with lower oxygen diffusivities. Reduction of the oxidized depth was revealed by a significant decrease of 10 μm (60%) in the extent of the protective layer. The filled-cavity strategy leads to better protection against oxygen diffusivity into the composite without altering the bulk properties.     

Microstructure and indentation damage resistance of ZrB2‐20 vol.%SiC ipo‐eutectic composites

Zirconium diboride with 20 vol.% silicon carbide bulk composites were fabricated using directionally solidification (DS) and also by spark plasma sintering (SPS) of crushed DS ingots. During the DS the cooling front aligned the c‐axis of ZrB₂ grains and its Lotgering factor of f_(00l) was high as 0.98. The Vickers hardness was anisotropic and it was high as 17.6 GPa along the c‐axis and 15.3 GPa when measured in an orthogonal direction. On both surfaces, even when using 100 N indentation load, no cracks were observed, suggesting a very high resistance to crack propagation. Such anomalous behavior was attributed to the hierarchical structure of DS sample where the ZrB₂ phase was under strong compression and the SiC phase was in tension. In the SPSed sample, the microstructure was isotropic respect to the direction of applied pressure. Indentation cracks appeared around the indent corners but not emanated from the diagonals, confirming high damage resistance.     

On the crystallite size refinement of ZrB2 by high-energy ball-milling in the presence of SiC

The effect of SiC addition (5, 17.5, and 30 vol.%) on the high-energy ball-milling (HEBM) behaviour of ZrB₂ is investigated. It was found that the presence of SiC during HEBM did not alter ZrB₂ refinement mechanism of repeated brittle fracture followed by cold-welding, thereby leading to the formation of agglomerates consisting of primary nano-particles. SiC did, however, slow down the kinetics of crystallite size refinement and promoted the formation of finer agglomerates. Both of these phenomena became more pronounced with increasing SiC content in the ZrB₂ + SiC powder mixtures, and they were attributed to the energy dissipation effect of the nanocrystalline SiC particles during HEBM of the ZrB₂ + SiC powder mixture. This study offers the first evidence that the addition of harder materials to softer materials can slow down the refinement of crystallite sizes, and thus provides a new mechanism to control crystallite sizes during HEBM. The simultaneous attainment of nano-particles of ZrB₂ and SiC, reduced agglomerate sizes, and homogeneous SiC dispersion at the nanometre scale may have important implications for the ultra-high-temperature ceramic community, as it simplifies the processing route and is likely to facilitate the sintering of ZrB₂-SiC composites.