Hot-corrosion of refractory high-entropy ceramic matrix composites synthesized by alloy melt-infiltration

Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O₂, (Nb, Ta)₂(Zr, Hf)₆O₁₇, (Zr, Hf)TiO₄, and Ti(Nb, Ta)₂O₇, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO_(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.     

Oxidation of Hafnium Carbide in the Temperature Range 1400° to 2060°C

After hafnium carbide has been oxidized at temperatures in the range of 1400° to 2060°C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense-appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving-boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X-ray microanalysis, X-ray diffraction, and resistance measurements, the interlayer is an oxygen-deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.     

Ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide and (Hf–Ta–Zr–Nb–Ti)C–xSiC composites

The ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide (HEC-0) was investigated using a plasma flame in air for different times (60, 90, and 120 s) at about 2100°C. The effect of SiC content on the ablation resistance of HEC–xSiC composites (x = 10 and 20 vol%) was also studied. The linear ablation rate of HEC-0 decreases with increasing ablation time, showing the positive role of the oxide layer with a complex composition. The linear ablation rate of HEC–10 vol% SiC (0.3 µm s⁻¹) is only a 10th of that of HEC-0, showing a significant improvement in ablation resistance, probably due to the formation of a protective oxide layer containing melted SiO₂ and refractory Hf–Zr–Si–O oxides.     

Synthesis of rare earth containing single-phase multicomponent metal carbides via liquid polymer precursor route

Novel high-entropy carbide ceramics (HEC) containing rare earth metals, namely (Ti, Zr, Hf, Ta, La, Y)C, (Ti, Zr, Hf, Ta, Nb, La, Y)C, and (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C were prepared with single-phase structure by polymer precursor method. Controlled co-hydrolysis and polycondensation of equiatomic metal-containing monomers were conducted successively, followed by blending allyl-functional novolac resin as carbon source, and the polymer precursors were obtained as clear viscous liquid solutions. The single-phase formation possibility was theoretically analyzed from the aspects of size-effect parameter δ of the designed compositions. All as-obtained ceramics possessed single face-centered-cubic structure of metal carbides and high-compositional uniformity from nanoscale to microscale. The (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C ceramic powder pyrolyzed at 1800°C exhibited low-oxygen impurity content of 1.2 wt%. Thus, multicomponent high-entropy carbide nanoceramics with over five metal elements containing even rare earth element were firstly synthesized and characterized.     

Mechanisms responsible for enhancing low-temperature oxidation resistance of nonstoichiometric (Zr,Ti)C

A series of (Zr,Ti)C_x (x = 0.7–1.0) samples were fabricated by a modified spark plasma sintering apparatus to investigate the effects of carbon concentration and Ti substitutions on the oxidation behavior. Crushed powders of (Zr,Ti)C_x were oxidized in lab air (N₂–20-vol.% O₂) from room temperature to 900°C. The results indicated that Zr_0.8Ti_0.2C_0.8, with a nominal carbon concentration x = 0.8, displayed good oxidation resistance, which was attributed to the formation of dense t-(Zr,Ti)O₂ oxide solid solution. During the oxidation of (Zr,Ti)C_x, Ti substitutions for Zr enhanced the outward diffusion of carbon, enabling a uniform carbon layer and a Zr–Ti–C–O layer on the surface of carbides. The formed carbon layer improved the oxidation resistance of (Zr,Ti)C_x below 550°C, where carbon is relatively oxidation resistant. Increasing the Ti concentration was found to enhance the oxidation resistance of (Zr,Ti)C_x with an increased oxidation onset temperature (672 ± 2°C for Zr_0.8Ti_0.2C_0.8).     

Influence of internal diffusion barriers on carbon diffusion in pure titanium and Ti–6Al–4V during diamond deposition

Diamond coatings were deposited on pure titanium and Ti–6Al–4V, at a temperature in the range of 600–750 °C, in a microwave plasma from CH₄/H₂ and CO/H₂ mixtures. The influence on carbon diffusion of different intermediate layers, especially tungsten, niobium, titanium nitride and pure titanium previously deposited on titanium alloys by physical vapor deposition (PVD) is reported. These intermediate layers are always composed of at least two sub-layers: (1) an internal diffusion barrier and (2) an external titanium layer that allows some carbon diffusion to be maintained. After diamond deposition, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) observations coupled with energy-dispersive X-ray (EDX) analysis of the final multilayer systems allow us to determine the diffracting phases, their lattice parameters and the efficiency of the different barriers. The carbon diffusion coefficients in the titanium carbide phase and in the α-titanium solid solution are deduced from an experimental study carried out on pure titanium with or without an underlying diffusion barrier. The results are compared to the carbon diffusion in Ti–6Al–4V alloy. This work permitted us to calculate the carbon concentration profiles in both pure titanium and Ti–6Al–4V substrates.     

Microstructure and ablation behaviour of a strong,dense, and thick interfacial ZrxHf1-xC/SiC multiphase bilayer coating prepared by a new simple one-step method

In order to solve the shortcomings of chemical vapour deposition (CVD), such as CVD-prepared coatings that are weakly bound to the carbon base, Zr_xHf_1-xC/SiC multiphase bilayer ceramic coatings were prepared on substrate surfaces by slurry brushing and the one-step in-situ thermal evaporation reaction method. The coating exhibits multiphase bilayer characteristics due to the self-diffusion of the matrix carbon source and the self-assembly of gaseous Zr and Si with the matrix. The 200-μm-thick Zr_xHf_1-xC solid-solution phase is distributed on the outer coating layer, while the 100-μm-thick SiC phase is distributed in the inner layer such that it contacts the substrate. The coating prepared by brushing with Hf and vapour-deposited with a masterbatch containing 7:3 (w/w) Zr:Si (H-ST) exhibits excellent ablation resistance, attributable to the presence of dense and spallation-free oxide scale and the low oxygen diffusion coefficient of (Zr, Hf)C_yO_z.     

High-strength medium-entropy (Ti,Zr,Hf)C ceramics up to 1800°C

Medium-entropy (Ti,Zr,Hf)C ceramics were prepared by hot pressing a dual-phase medium-entropy carbide powder with low oxygen content (0.45 wt%). The results demonstrate that the medium-entropy (Ti,Zr,Hf)C ceramics sintered at 2100°C had a relative density of 99.2% and an average grain size of 1.9 ± 0.6 μm. The flexural strength of (Ti,Zr,Hf)C carbide ceramics at room temperature was 579 ± 62 MPa. With an increase in temperature to 1600°C, the flexural strength showed an increase up to 619 ± 57 MPa, and had no significant degradation even up to 1800°C. The high-temperature flexural strengths of (Ti,Zr,Hf)C were obviously higher than those of the monocarbide ceramics (TiC, ZrC, and HfC). The primary strengthening mechanism in (Ti,Zr,Hf)C could be attributed to the high lattice parameter mismatch effects between TiC and ZrC, which not only inhibited the fast grain coarsening of (Ti,Zr,Hf)C ceramics, but also increased the grain-boundary strength of the obtained ceramics.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

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.     

Design,synthesis, structure,and stability of novel multi-principal element (Ti,Zr,Hf,W)C ceramic with a miscibility gap

Here we design a novel multi-principal element carbide system (Ti,Zr,Hf,W)C with a miscibility gap using computational tools and report on the formation of a single-phase (Ti,Zr,Hf,W)C after spark plasma sintering. The (Ti,Zr,Hf,W)C shows high nanohardness (32.7 GPa) and fracture toughness (5 MPa·m^1/2). Aging studies at 1350 °C for 100 h show that the single-phase carbide solid solution is quite stable even though this temperature is within the predicted miscibility gap of the system. Detailed electron microscopy characterization shows that phase separation has initiated with minor decomposition after aging by forming rock-salt (Ti,W)C- and (Zr,Hf)C-rich phases as well as hexagonal WC precipitates. We show that the (Ti,W)C- and (Zr,Hf)C-rich phases form a lamellar structure upon aging and the interlamellar spacing is considerably coarser than what has been previously found for the binary (Ti,Zr)C system. The decomposition kinetics, on the other hand, is sluggish due to the reduced driving force for phase decomposition.     

Possible Role of the Oxygen Potential Gradient in Enhancing Diffusion of Foreign Ions on α-Al2O3 Grain Boundaries

Thermally grown, alpha-Al₂O₃ external scales formed on alloys after oxidation in pure oxygen at temperatures between 1000° and 1500°C were analyzed using FEG-STEM/XEDS. Alloy dopants such as Y, Zr, La, Hf, and Ti were found to segregate to the alpha-Al₂O₃ grain boundaries and to the alloy-scale interface. With increasing oxidation time and temperature, the amount of segregant on the oxide grain boundaries near the gas interface increases until a critical level is reached and precipitates begin to nucleate and grow. These observations are a result of the outward transport of dopants from the alloy, through the external alumina scale, to the gas interface. The apparent driving force for the dopant diffusion is the oxygen potential gradient in the growing oxide scale.     

The use of a carbonized phenolic formaldehyde resin coated Ni foam as an interlayer to increase the high-temperature strength of C/C composite-Nb brazed joints

A novel carbonized phenolic formaldehyde resin (PF) resin-coated Ni foam was used as an interlayer for brazing carbon fiber reinforced carbon composites (C/C) and Nb using a Ti–Ni filler. At first, uniformly distributed carbonaceous laminae with different mass fractions on the Ni foam surface were acquired after the carbonization process by controlling the concentration of the PF solution. Afterwards, the obtained carbonaceous laminae covered Ni foam composite (C-Ni_f) was applied as an interlayer for brazing C/C and Nb via an assembly of C/C/Ti foil/Ni foil/C-Ni_f interlayer/Ti foil/Nb. The morphologies and microstructures of the carbonization product and the interfacial microstructures of the joints were investigated. The brazing mechanism has been elaborated in detail. With the help of the interconnected porous structure of the Ni foam, the distribution of the in-situ formed (Ti,Nb)₂Ni particles, (Ti,Nb)C ring reinforcements as well as the Nb solid solution were uniformly obtained throughout the brazing seam. As a result, the joint residual stress was effectively released and consequently, the joint shear strength at elevated temperature (1000 °C) reached up to 33 MPa, which is 4.5 times higher than the directly brazed joint without an interlayer.     

Reactive sintering behavior and enhanced densification of (Ti,Zr)B2–(Zr,Ti)C composites

Reactive hot pressing was used to prepare (Ti,Zr)B₂–(Zr,Ti)C composites from equimolar ZrB₂ and TiC powders. The reaction and solid-solution coupling effect and enhanced densification in ZrB₂-50 mol.% TiC were proposed as contrasted to conventional consolidation of TiB₂-50 mol.% ZrC. The (Ti,Zr)B₂–(Zr,Ti)C composite sintered at a temperature as low as 1750 °C exhibited negligible porosity and average grain sizes of 0.30 μm for (Ti,Zr)B₂ and 0.36 μm for (Zr,Ti)C. Complete reaction and rapid densification of ZrB₂-50 mol.% TiC was achieved at 1800 °C for only 10 min. The densification mechanism was mainly attributed to material transport through lattice diffusion of Ti and Zr atoms with an activation energy of 531 ± 16 kJ/mol. This study revealed for the first time novel insights into rapid densification of refractory fine-grained diboride–carbide composites by reactive hot pressing at relatively low temperatures.     

Microstructures and mechanical properties of Cf/LAS composite and Ti60 alloy joints brazed with TiZrNiCu + Cf mixed powders

In this paper, carbon fiber reinforced lithium aluminosilicate (LAS) glass-ceramics matrix composites (C_f/LAS composites) are joined to Ti60 alloy using TiZrNiCu + C_f mixed powders by proper process parameters. The carbon fibers distribute uniformly in the brazing interlayer and react with Ti, Zr elements in the brazing alloy to form (Ti, Zr)C thin reactive layers, which are between the carbon fibers and the Ti, Zr elements. The effect of C_f content on the mechanical properties and microstructure of brazed joints are investigated. The microstructure of brazed joints varied obviously with the increasing of C_f content. The thickness of reactive layer between interlayer and C_f/LAS composites and Ti solid solution (Ti (s.s)) decrease gradually, and the volume of eutectic structure (Ti(s,s) + (Ti,Zr)₂(Ni,Cu)) decrease gradually. The obtained brazed joints exhibit a maximum shear strength of 73.5 MPa at room temperature using TiZrNiCu + 0.3 wt% C_f mixed powders. The enhanced shear strength can be attributed to the reduction in thermal stress and the reinforcing effect originated from the carbon fiber addition.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

Thermochemical Studies in Selected Metal–Carbon–Oxygen Systems

Carbon–carbon composites have unique physical and chemical properties but are limited by poor oxidation resistance at elevated temperatures in an oxidizing environment. Coatings based on metal carbides and metal oxides are currently being used for limited oxidation protection at temperatures exceeding 1200°C. However, problems exist for all current coatings and potential future coatings because of thermochemical instability at elevated temperatures. Thermochemical equilibrium calculations for reactions between oxides and carbides of the same metal system were used to predict the stability of the associated interfaces. The systems examined were carbon and oxygen in combination with five metals (Si, Ti, Ta, Hf, and Zr). The SiO₂/SiC system was determined to be the best choice up to about 1700°C, because of optimization of properties, while at higher temperatures, HfO₂/HfC and ZrO₂/ZrC systems appear to be more stable thermodynamically, although high thermal expansion coefficient and oxidation rate, for each carbide, may make them less physically compatible.     

Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures

The combustion temperatures and rates of mechanically activated (MA) Ta–Zr–C mixtures depending on the initial temperature T₀ are determined. The self-heating phenomenon is observed in argon atmosphere at T₀>380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. Zirconium oxide is formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone, while the binary tantalum–zirconium carbide (Ta,Zr)C is formed closer to the post-combustion zone. In order to maintain the layer by layer stationary combustion mode of SHS, the initial temperature T₀ needs to be 298 K, while the duration of mechanical activation needs to be less than 5 min. After longer mechanical activation, the mixtures are prone to bulk combustion even at low initial temperatures. Single-phase (Ta,Zr)C carbide with the lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Medium-entropy (Ti,Zr, Hf)2SC MAX phase

Medium- and high-entropy alloys or ceramics for tuning the physicochemical properties of materials by the combination of multiple principal elements have received much interest. Herein, a medium-entropy (Ti, Zr, Hf)₂SC phase was synthesized attributing to the structural and chemical diversity of MAX phases. The crystal structure of (Ti, Zr, Hf)₂SC was determined by the Rietveld refinement of XRD, SEM, and atom-resolved TEM along with EDS elemental analysis. Phase evolution of X-ray diffraction patterns and TG/DSC curves were employed to reveal the synthesis mechanism of (Ti, Zr, Hf)₂SC from 2TiC–Zr–ZrC-2HfH₂-3.2FeS reactant system. The Vicker's hardness and the electrical resistivity of (Ti, Zr, Hf)₂SC were found higher than those of Ti₂SC, but the thermal conductivity of (Ti, Zr, Hf)₂SC was lower.