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.     

Oxidation of zirconium diboride with niobium additions

Oxidation of ZrB₂ ceramics containing Nb additions at 1500 °C resulted in the formation of a two-layer oxide scale. The outer surface was partially covered by a glassy layer containing B₂O₃ with smaller amounts of Nb and Zr oxides dissolved into it. With increasing exposure time, evaporation of B₂O₃ from the outer layer resulted in precipitation of oxide particles in the receding glassy phase. Between the outer layer and the unoxidized (Zr,Nb)B₂ was a porous layer that consisted of particles containing Zr, Nb, and O. The formation of Nb₂Zr₆O₁₇ was observed in the porous oxide layer. Since this compound is solid at the oxidation temperature, liquid phase sintering of the ZrO₂ scale was not possible. However, dissolution of Nb into B₂O₃ increased the stability of the liquid/glassy layer, which acted as a barrier to the transport of oxygen at higher temperatures compared to the scale formed on nominally pure ZrB₂.     

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m^1/2, respectively, compared to the pure Ti(C_0.5,N_0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m^1/2. The wear scar depth of the (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic sample was 57.36% to that of the Ti(C_0.5,N_0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.     

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

Early transition metal carbides are considered to be superior candidate materials for oxidizing environments at temperatures exceeding 2000°C. Generally, the remarkable oxidation resistance is largely attributed to a carbonaceous oxide interlayer (eg, Hf–O–C, Zr–O–C, and Ta–O–C), located at the interface between the external oxide layer and internal carbide (eg, HfC, ZrC, and TaC), acting as the primary oxygen barrier. However, the oxygen barrier mechanism of the carbonaceous oxide interlayer remains unclear. Herein, through studying the oxidation behavior of a novel multicomponent carbide Hf_0.5Zr_0.3Ti_0.2C in oxidizing environments up to 2500°C, the oxygen barrier mechanism of the carbonaceous oxide was recently revealed. We found that the oxygen barrier resulted from the slow oxygen diffusion through the inner grains of Hf-Zr–Ti–O due to the presence of carbon formed at the grain boundaries because of the existence of compact external oxide layer, beneath which the Hf–Zr–Ti–O–C interlayer possesses much lower oxygen activity and temperature that allow carbon to exist stably. This as-formed carbon strongly retarded the fast diffusion of oxygen along the grain boundaries of oxides. Additionally, desirable synergisms of the designed multicomponent system, particularly, the outward short-circuit diffusion of Ti, lead to the self-healing of the external oxide layer, evidently enhancing integral protection performance against oxidizing environments.     

Multiphase composite Hf0.8Ti0·2B2–SiC–Si coating providing oxidation and ablation protection for graphite under different high temperature oxygen-containing environments

A novel multiphase composite coating composed of Hf_0.8Ti_0·2B₂ solid solution, SiC and Si was prepared by a joint procedure of slurry method and silicon reactive infiltration (SRI). The oxidation and ablation experiments were conducted to investigate oxidation and ablation resistance of the Hf_0.8Ti_0·2B₂–SiC–Si coated graphite samples, respectively. The results revealed that the coated sample was oxidized at 1823 K for 108 h with a mass gain of 1.49%, which was ascribed to the high viscosity oxide layer improved by HfSiO₄ and TiO₂ in conjunction with dense structure of the coating, thereby presenting excellent high temperature stability. Furthermore, after 90 s ablation at 3273 K under a heat flux of 5.62 MW/m², the composite coating was not peeled off, which had mass ablation rate (MAR) and linear ablation rate (LAR) of 3.1 mg/s and 1.5 μm/s, respectively. The refractory oxide layer comprising oxides of Hf and Ti on the surface acted as an oxygen barrier, which can weaken the mechanical erosion force of oxyacetylene flame, finally protecting the inner coating and graphite matrix from further consumption.     

Bioactive silicon nitride by surface thermal treatment

Silicon nitride-based ceramics with SiO₂, CaO and Ca₃(PO₄)₂ as sintering additives, have been prepared in order to study the bioactivity. Dense ceramic bodies were oxidized by an oxy-acetylene flame at approx. 1475 °C for 60 s, in order to modify the surface in terms of bioactivity enhancement and the formation of optimal porosity for cell viability. During oxidation two concurrent processes occurred on the ceramic body surface: (i) formation of thin glassy layer with a composition close to that of grain boundary phase in ceramic body, and (ii) partial decomposition of silicon nitride matrix. The latter one resulted in the formation of gases (N₂ and SiO), which formed bubbles in the viscous surface glassy phase, resulting in porosity required for cell adhesion (small pores) and tissue ingrowth (large pores). The best bioactivity was obtained for oxy-acetylene flame treated Si₃N₄ ceramics with Ca₃(PO₄)₂ sintering additive.     

Ohmic potential drop during alkaline water electrolysis

Advanced alkaline water electrolysis may be an attractive method of producing hydrogen in the future. Large energy losses in water electrolysis are caused by evolution of gas bubbles. The main aim of this investigation is to obtain a dimensionless correlation for the increase of ohmic resistance between working electrode and diaphragm due to the presence of gas bubbles. The ohmic potential drop across the solution layer between diaphragm and working electrode, distance from the rear of the working electrode to the back wall of the compartment of the working electrode, the height in the electrolytic cell, the electrode type, nature of the electrode surface and of the gas evolved. From the experimental results the dimensionless correlation has been deduced as

where ΔR_wm^* denotes the increase in resistance of solution layer between diaphragm and working electrode due to bubbles, divided by the resistance of this layer in absence of bubbles, Re Reynolds number, h^* reduced height in the electrolytic cell, d_wm^* reduced distance between the working electrode and the diaphragm, n empirical experimental constant depending on nature of electrode surface, nature of gas evolved and whether or not the bubbles can enter a chamber behind the working electrode, K empirical constant depending on nature of both electrode surface and gas evolved. A remarkable result is that ΔR_wm^* is independent of current density in the current density range from 1–10 kA m^?2.     

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.     

Effects of Ti ion deposition on corrosion resistance of WE43 alloy

Ti films with different thicknesses were successfully deposited on the surface of WE43 alloy by filtered cathode vacuum arc technology, and the microscopic morphology, structural composition, and corrosion resistance of the films were studied by means of X-ray diffractometer, X-ray photoelectron spectroscopy and scanning electron microscope. The results show that when the deposition time of Ti ions is 800 s, the thickness of the Ti film is 2.35 μm, the surface of the film is dense, and there are few defects. Meanwhile, Ti800 alloy has the best corrosion resistance among the four modified alloys. It has a corrosion current density (I_corr) of 2.9 μA·cm⁻², which is about 50 times lower than that of unmodified alloy. This conclusion is also confirmed by the complete film layer of Ti800 alloy and the tight bonding with the substrate after immersion experiments. Good corrosion resistance is attributed to a dense and relatively chemically stable TiO₂/Ti structure in simulated body fluid corrosive media.     

A facile route for the synthesis of high-entropy transition carbides/borides at low temperatures

As the main candidates in the field of ultra-high temperature ceramics, high entropy carbides/borides (HECs/HEBs) have good oxidation resistance properties, high hardness, as well as excellent thermal and electrical conductivities, which are the focused points of research nowadays. In the current study, (Hf,Ta,Zr,Nb,Mo,Ti)C powders were successfully synthesized by a three-step process, including the mixing process of raw oxides and carbon black with spaying Fe(NO₃)₃ solution, carbothermal reduction and subsequent calcium posttreatment. For the preparation of (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders, during the calcium posttreatment process, equal stoichiometric ratio of B₄C was added for the purpose of boriding reaction. The relevant X-ray diffraction and SEM characterizations indicate the successful preparations of face-centered cubic HECs and hexagonal HEBs. However, slight Mo local segregation was found in the prepared (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders. The iron generated from Fe(NO₃)₃ promotes the solid solution process between monocarbides during the carbothermal reduction process via the dissolution-diffusion-precipitation mechanism. In the calcium posttreatment process, the liquid calcium ensures the boriding reaction take place at a low temperature. In addition, the residual carbon could be combined with calcium to generate CaC₂ which is easy to be removed by acid leaching, and meanwhile, the added Fe could also be finally eliminated to produce pure HEC/HEB powders. The current method does not require the long-time high energy ball milling of raw materials, but only simple and mild mixing is enough. Therefore, such a facile route has a great potential application prospect for industrially preparing high entropy phase powders in a large scale.     

Oxidation resistance and strength retention of ZrB2–SiC ceramics

Oxidation behavior and effect of oxidation on the room-temperature flexural strength were investigated for ZrB₂–10 vol% SiC (ZB10S) and ZrB₂–30 vol% SiC (ZB30S) in air at 1500 °C with times ranging from 0.5 h to 10 h. The oxide scale of both ZB10S and ZB30S was composed of an outer glassy layer and an inner extended SiC-depleted layer. The changes in weight gain, glass layer thickness, and extended SiC-depleted layer thickness with oxidation were measured. Analysis suggested that the extended SiC-depleted layer was most indicative for evaluating the oxidation resistance. Compared to the ZB10S, the improved oxidation resistance in ZB30S was attributed to the viscosity increase of glassy layer and the lower number of ZrO₂ inclusions in the glassy layer. Because of the healing of surface flaws by the glassy layer, the strength increased significantly by ～110% for ZB10S and by ～130% for ZB30S after oxidation for 0.5 h.     

Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Bubble phenomenon of ZrB2 based composites at high temperatures

Bubble phenomenon is common for ultra-high temperature ceramics (UHTCs) during oxidation or ablation processes, which will impair the oxidation/ablation resistant properties. This work is aiming to illuminate the formation and rupture processes of bubbles. In this work, ZrB₂-SiC-WB composite coatings were prepared via vacuum plasma spray technique and oxidized at 1500?°C for different durations. Obvious bubble phenomenon was observed. The morphology and distribution of bubbles were characterized. The formation mechanism of bubbles was calculated and analyzed based on thermal dynamics. The results showed that B₂O₃ gas played a key role in affecting the bubble behaviors. Bubbles tended to nucleate near the interface between the solid and liquid oxide layers. Small bubbles aggregated to large bubbles near the outmost liquid layer. Large bubbles near the surface were easy to rupture. The calculated results were consistent with the observed results.     

Formation of stable passive film on stainless steel by electrochemical deposition of polypyrrole

Stable passive film has been formed on 304 stainless steel during the electrochemical deposition of polypyrrole (PPy) from sulfuric acid solution. The stability of passive film under PPy increases with aging in H₂SO₄ and this film has much higher resistance to pitting than that formed by anodic polarization under the same condition of aging. XPS studies indicate that the content of chromium components in the oxides under PPy layer is about twice that of the anodically formed passive film with larger value of the ratio Cr₂O₃/Cr(OH)₃. Higher content of iron with a ratio of Fe²⁺/Fe³⁺ more than unity is also observed with a considerable lower hydration and sulfate content in the oxides under PPy layer. It is suggested that sulfate ion is consumed as a dopant in the formation of PPy film and the oxidation of stainless steel is achieved under the environment of lower concentration of water and sulfate molecules. This may result in the enhancement of formation of corrosion-resisting oxides rather than hydroxides and sulfates.     

Effects of hydrogen on the transition layers between diamond films and Zr, Hf substrates

Highly purified polycrystalline wafers of Zr and Hf were chosen as substrates for diamond deposition by means of the d.c. arc discharge plasma CVD method. Polycrystalline diamond films were formed on both substrates. Scanning electron microscopy and high resolution X-ray diffraction were used to investigate the surface morphology of the diamond films and the composition of the interface layers between the diamond films and substrates. The experiments showed that the transition layer between the diamond films and Zr substrates was ZrC, ZrH and ZrC_0.32H_1.2. For Hf substrates, in addition to HfC, it is supposed that there were also some hydrides or carbohydrides in the transition layer, because several X-ray reflections appeared which could not be assigned to Hf, HfC or diamond.     

Carbon membranes from cellulose and metal loaded cellulose

The focus of this work was to find a low-cost precursor for carbon molecular sieve (CMS) membranes, and a simple way of producing them. In addition, several ways of modifying a carbon material are described. The modification method used in this study was metal doping of carbon. CMS membranes were formed by vacuum carbonization of cellulose and metal loaded cellulose. Metal additives include oxides of Ca, Mg, Fe(III) and Si, and nitrates of Ag, Cu and Fe(III).The carbon membrane containing Fe-nitrate has promising separation performance for the gas pairs O₂/N₂ and CO₂/CH₄. Carbon containing nitrates of Cu or Ag show high selectivity, but reduced O₂ and CO₂ permeability compared to carbon with Fe-nitrate. Element analysis indicates that Cu migrates to the carbon surface, creating an extra layer resistance to gas transport. A silver mirror is also seen on the surface of Ag-nitrate-containing carbon. However, the Ag- and Cu-containing membranes show a high H₂ permeability. Adding metal oxides makes the carbon membranes retard the transport of easily condensable gases (e.g. CO₂). This can be exploited for enhanced H₂/CO₂ separation efficiency.     

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.     

Oxidation resistance and microstructure evolution of ZrB2–SiC–La2O3/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures

The oxidation behaviors of a ZrB₂–SiC–La₂O₃/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures (50, 5 and 0.5 kPa) were investigated. The results showed that with the decrease of air pressure, the oxidation kinetics of the coated samples changed from parabolic weight gain to linear weight loss. A protective oxide scale consisted of ZrO₂ and SiO₂ with La dispersed was formed on the coating surface after oxidation in 50 kPa air. The oxide scale formed in 5 kPa air was full of bubbles. Only porous ZrO₂ layer was left on the coating surface after oxidation in 0.5 kPa air. At 1800 °C, the active oxidation of SiC occurred and gaseous SiO formed at the coating/oxide interface. The surface volatilization of SiO became severe with the decrease of air pressure, resulting in the presence of non-protective oxide scale.     

Densification,solid solution formation,and microstructural investigation of reactive pressureless sintered HfB2-TiB2-SiC-MoSi2 quadruplet composite

Dense HfB₂-TiB₂-SiC-MoSi₂ quadruplet composite was produced by a reactive pressureless sintering method at 2050 °C for 5 h. The relative density was improved and reached 98% by in situ formation of SiC and MoSi₂ phases. Microstructural studies proved that SiC and MoSi₂ second phases were mostly formed during the sintering process. Moreover, the Sintering mechanism of the composite was investigated by HSC software. TiB₂ co-matrix was improved the sinterability of the composite by the formation of (Hf,Ti,Mo)–B and (Hf,Ti,Mo)–C solid solutions.Mechanical properties such as Vickers hardness (23.2 GPa), fracture toughness (5.4 MPa m^1/2), and elastic modulus (430 GPa) were effectively enhanced by tailoring the composite.     

Bubbles growth and their stability in reactive flotation process

Reactive flotation is a process originally used to separate dolomite from phosphate. It depends mainly on the formation of CO₂ bubbles as a product of acid reaction with dolomite surface in presence of a polyvinyl alcohol (PVA) polymeric membrane. Elasticity of PVA membrane is one of the critical factors that affect the bubble stability at the interface. The dynamic surface tension (DST) at different CO₂ rates was used as a measure for the membrane elasticity. The DST, at different CO₂ rates, was used to simulate the bubbles formation at the interface due to the surface reaction of dolomite with acidic media. The results indicated that the high evolution of CO₂, due to vigorous reaction, negatively affects the membrane elasticity and leads to fast rupture of the formed bubbles.