Oxidation inhibition behaviors of HfB2-MoSi2-SiC oxygen blocking coating prepared by spark plasma sintering

The HfB₂-MoSi₂-SiC oxygen blocking coatings were prepared by the spark plasma sintering (SPS) technique, whose oxidation inhibition ability was further strengthened by the pre-oxidation treatment. The effect of MoSi₂ content and pre-oxidation treatment process on the oxygen blocking ability of the HfB₂-MoSi₂-SiC coating at 1973 K were conducted. After SPS, for the HfB₂-MoSi₂-SiC coatings with 20 wt%, 40 wt%, and 60 wt% MoSi₂, the relative density of the coatings are 92.6%, 93.9%, and 85.6%, respectively. Owing to the enhanced compactness of the coatings, increasing MoSi₂ content can significantly improve the protection efficiency of the coatings during the activation oxidation stage. However, due to the increased formation of gaseous by-products during the inerting oxidation stage, excessive MoSi₂ weaken the oxidation inhibition ability of the coatings. The sufficient dispersion of Hf-oxides nanocrystals in the glass layer conduces to enhance the oxygen blocking ability of the glass layer, making the 40HfB₂-40MoSi₂-20SiC coating present the best oxidation protective ability. The pre-oxidation treatment at 1773 K conduces to form the steady glass layer with fewer defects at the cost of a lower oxidation consumption of the coating, which enhanced the protection efficiency of the coating from 96.9% to 99.8% and reduced the oxygen permeability from 0.13% to 0.028%.     

The improvement of the self-healing ability of MoSi2 coatings at 900–1200 °C by introducing SiB6

The brittleness of MoSi₂ ceramic and the thermal mismatch between MoSi₂ coating and C / C composite lead to brittle cracking of the coating at 900−1200 °C. This problem has been overcome in this studyby introducing submicron-SiB₆ into the coating. The pre-fabricated cracks and a kinetics model of hot-pressed SiB₆-MoSi₂ ceramic could quantitatively predict the glass growth and crack healing. As expected, enhancing temperature and SiB₆ content increased the growth rate of the borosilicate glass and the crack healing ability of MoSi₂ ceramic, which was ascribed to the lower oxidation activation energy and larger specific surface area of submicron-SiB₆. For the plasma sprayed coating, SiB₆ with submicron structure was benefit for cracking inhibition and formation of borosilicate glass during oxidation, reducing the oxygen permeability and the consumption of inner coating. Hence, the 15 % SiB₆-MoSi₂ coatings raised the protection times to 84 and 120 h at 900 and 1200 °C respectively, presenting favorable oxidation protective performance.     

Oxidation of TaB2-SiC coatings prepared by spark plasma sintering and effect of pre-oxidation treatments

To suppress the oxidation of TaB₂-SiC coatings, the effects of pre-oxidation temperature on the oxygen hindering properties of TaB₂-SiC coatings were investigated to prepare TaB₂-SiC coatings with enhanced oxidation behavior. The addition of 40 wt% TaB₂ made the oxygen permeability of the coating decrease by 62.16%. However, excessive TaB₂ weakened the oxygen hindering ability of the coating due to the large ion complex ability of Ta⁵⁺. The pre-oxidation temperature at 1500 °C led to a homogeneous dispersion of Ta-oxide nanocrystal particles in the Ta-B-Si-O complex-phase glass layer. In contrast with the untreated samples, the active factor and inert factor values of the TaB₂-SiC coating after pre-oxidation treatment at 1500 °C decreased by 43.12% and 17.33%, respectively, which improved the dynamic stability of the coating during oxidation.     

High-temperature oxidation resistance of spent MoSi2 modified ZrB2–SiC–MoSi2 coatings prepared by spark plasma sintering

The spent MoSi₂ modified ZrB₂–SiC–MoSi₂ coatings were prepared on carbon matrixes by spark plasma sintering. A continuous metallurgical bonding was formed at the interface between the coating and matrix, and no obvious defects such as pores and cracks were observed inside. The effects of spent MoSi₂ content and trace doping in the spent powder on the oxidation behavior of the coatings in air at 1700 °C were investigated. During the active oxidation stage, the spent MoSi₂ promoted the densification of the coating and enhanced the structural oxygen barrier properties. With the increase of service time, during the inert oxidation stage, doping an appropriate amount of spent MoSi₂ helped to increase the fluidity of the rich-SiO₂ protective layer so that the Zr oxides fully dispersed in the generated Zr–B–Si–O–Al multiphase glass layer, which could impede the penetration of oxygen and enhance the oxidation protection efficiency. However, excessive spent MoSi₂ exacerbated the volatilization of gas by-products, forming pores and cracks in the glass layer and rising the oxidation loss. When the content of spent MoSi₂ was 20 vol%, the glass layer is dense and uniform, with few defects and the best oxygen resistance property. Moreover, compared with commercial powders, spent MoSi₂ contained Al₂O₃ and SiO₂. Al₂O₃ had an excellent modification effect, while SiO₂ glass can promote liquid phase sintering and seal the defects in the coatings. By adding spent MoSi₂, the modified ZrB₂–SiC–MoSi₂ composite coatings could inhibit the formation of defects and improve the dynamic stability of the coatings effectively.     

Preparation and high-temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2-based materials

Spent MoSi₂ and MoB were used as raw materials to prepare multilayer MoSi₂/MoB coating on molybdenum by the two-step method of slurry deposition and spark plasma sintering. The results showed dense MoSi₂/MoB coating after sintering while penetrated cracks appeared in MoSi₂ coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi₂/MoB coatings, MoB and Mo₂B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high-temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm²) was obtained for MoSi₂/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi₂ coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi₂/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti-oxidation properties at high temperature.     

A novel ultra-high-temperature oxidation protective MoSi2-TaSi2 ceramic coating for tantalum substrate

To protect refractory metal against oxidation at ultra-high temperatures, a MoSi₂-TaSi₂ ceramic coating was prepared on a pure tantalum (Ta) substrate using a novel three-step process, which included dip-coating with a molybdenum slurry, vacuum sintering, and halide-activated pack cementation (HAPC). The original coating had a MoSi₂-TaSi₂ double-layer structure from the surface to the substrate. After oxidation at 1700°C for 8 h in air, the coating exhibited a complex multi-layer structure composed of SiO₂-Mo₅Si₃-MoSi₂-(Mo,Ta)₅Si₃-TaSi₂-Ta₅Si₃ from the outer layer to the inner layer, due to the high-temperature phase transition and diffusion of Si and O. The coating effectively protected the Ta substrate at 1700°C for 12 h without failure, thereby demonstrating great improvement to its service life in an ultra-high-temperature aerobic environment. The protective effect was attributed to the integrity of the ceramic coating and the formation of a dense, uniform SiO₂ film that effectively lowered the inward oxygen diffusion rate.     

Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

Effects of SiC or MoSi2 second phase on the oxide layers structure of HfB2-based composites

Monolithic HfB₂, HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂ composites were prepared by SPS and oxidized in stagnant air at 1500 °C for 70 min. The microstructure of the oxide layer cross-sections showed that the oxidation extents were as follow: monolithic HfB₂ > HfB₂-30 vol%SiC > HfB₂-10 vol% MoSi₂.According to the EDS Line-scan, only one porous oxide layer containing a minor amount of B₂O₃was found on the HfB₂ oxidized surface whereas a thick silicate glass layer and a porous oxide layer below that existed on the surface of HfB₂-30 vol% SiC. After oxidation, the surface of HfB₂-10 vol% MoSi₂ had a narrow silicate-oxide compact layer covered by a very thin glass layer. X-ray diffraction patterns of the oxidized surfaces showed the monolithic HfB₂,the HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂composites contain, upon oxidation, only m-HfO₂ phase, mainly m-HfO₂ with a minor amount of HfSiO₄ and mainly HfSiO₄ with a minor amount of m-HfO₂ phases, respectively. Based on the observations in this study, it is suggested that the elimination of the porous layer and subsequent increase of the HfSiO₄ phase are the main reasons for the better oxidation resistance of HfB₂-10 vol% MoSi₂.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi₂–Al₂O₃ composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ～2 μm thick Mo₅Si₃ diffusion layer, a ～65 μm thick MoSi₂ inner layer and a ～36 μm thick outer layer of mixture of MoSi₂ and Al₂O₃. After oxidation at 1600 °C for 5 h, all MoSi₂ phases were completely converted to intermediate silicide Mo₅Si₃ by solid-state diffusion, and the formed Mo₅Si₃ phase would be transformed into Mo₃Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO₂-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi₂–Al₂O₃ coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi₂–Al₂O₃ coating is due to the formation of multi-layer structure containing a SiO₂-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.     

Ultra‐High‐Temperature Ceramic HfB2‐SiC Coating for Oxidation Protection of SiC‐Coated Carbon/Carbon Composites

To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra‐high‐temperature ceramics (UHTCs) HfB₂‐SiC coating was prepared on SiC‐coated C/C composites by in situ reaction method. The outer HfB₂‐SiC coating consists of HfB₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB₂‐SiC coating. The HfB₂‐SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10^?2 g/cm² weight loss at 1773 K in air. During the oxidation process, SiO₂ glass and HfO₂ are generated. SiO₂ glass has a self‐sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO₂ glass can react with the formed HfO₂, thus forming the HfSiO₄ phase. Owing to the “pinning effect” of HfSiO₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.     

Low-temperature oxidation resistance of the silica-coated MoSi2 powders prepared by sol-gel preoxidation method

MoSi₂ has been regarded as one of the most promising structural materials, due to its excellent high-temperature oxidation resistance and high emissivity. However, as the most commonly used emittance agent in high emissivity coatings, it can be easily oxidized when the operating temperature is below 600 °C. To address this issue, the silica-coated MoSi₂ powders with good low-temperature (400–600 °C) oxidation resistance were fabricated by a sol-gel method followed by a preoxidation process. Here in, by the combination of the non-isothermal and cyclic-isothermal oxidation analysis, we demonstrate that the silica gel film coated on MoSi₂ can prevent oxidation to some extent, but the preoxidation treatment plays a more critical role in improving its oxidation resistance at low temperature. Both the sol-gel and preoxidation processes enable the integrated and compact encapsulation of the silica coating. The synthesized silica-coated MoSi₂ exhibits excellent oxidation resistance with slightly weight gains (<1 wt%) after cyclic-isothermal oxidation at 400–600 °C for 12 h.     

A novel combination of precursor pyrolysis assisted sintering and rapid sintering for construction of multi-composition coatings to improve ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites

A double-layer coating composed of MoSi₂–SiO₂–SiC/ZrB₂–MoSi₂–SiC was designed and successfully constructed by a novel combination of precursor pyrolysis assisted sintering and rapid sintering to improve the ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites (CSs). The ZrB₂–MoSi₂–SiC inner layer coating was in relatively uniform distribution in the zone of 0–3 mm from the surface of CSs through the slurry/precursor infiltration in vacuum and SiOC precursor pyrolysis assisted sintering, which played a predominant role in improving oxidation and ablation resistance and maintaining the morphology of CSs. The MoSi₂–SiO₂–SiC outer layer coating was prepared by the spray and rapid sintering to further protect CSs from high-temperature oxidation. The ablation resistance of CSs coated with double-layer coating was evaluated by an oxygen-acetylene ablation test under the temperature of 1600–1800 °C with different ablation time of 1000 and 1500 s. The results revealed that the mass recession rates increased with the rise of ablation temperature and extension of ablation time, ranging from 0.47 g/(m²·s) to 0.98 g/(m²·s) at 1600–1800 °C for 1000 s and from 0.72 g/(m²·s) to 0.86 g/(m²·s) for 1000–1500 s at 1700 °C, while the linear recession rates showed negative values at 1700 °C due to the formation of oxides, such as SiO₂ and ZrO₂. The ablation mechanism of the double-layer coating was analyzed and found that a SiO₂–ZrO₂–Mo_4.8Si₃C_0.6 oxidation protection barrier would be formed during the ablation process to prevent the oxygen diffusion into the interior CSs, and this study provided a novel and effective way to fabricate high-temperature oxidation protective and ablation resistant coating.     

MoSi2-based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying

To protect carbon/carbon (C/C) composites against oxidation, MoSi₂-based oxidation protective coatings for SiC-coated carbon/carbon composites were prepared on them by supersonic plasma spraying. The MoSi₂-based coatings primarily consist of MoSi₂, Mo₅Si₃ and glassy SiO₂. Only a few pinholes and some microcracks are observed on the surface and no through-thickness cracks penetrate the cross-section. Weight loss of the MoSi₂-based coated specimens is only 1.14% after 400 h oxidation in air at 1773 K and the coated C/C composites remain intact after 11 thermal cycles between 1773 K and room temperature. The outstanding anti-oxidation ability is mainly attributable to the formation of SiO₂-based layer on the surface of MoSi₂-based coatings.     

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.     

Oxidation protection of graphite materials by single-phase ultra-high temperature boride modified monolayer Si-SiC coating

To improve the oxidation resistance of Si-SiC coating, single-phase ultra-high temperature boride (ZrB₂ or TaB₂) modified Si-SiC coating was designed and established on graphite substrates by combination of dipping and reactive infiltration process. ZrB₂ or TaB₂ phase was introduced in Si-SiC coating by directly mixing raw materials and phenol formaldehyde resin in the slurry, and then the ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were fabricated on the graphite samples by dipping-curing, pyrolysis, and siliconizing. The crystalline phases and microstructure of the as-obtained multiphase coatings were investigated by X-ray diffraction analysis and scanning electron microscopy. The interrupted oxidation tests from room-temperature to 1500?°C were conducted to assess the anti-oxidation property of the prepared coatings. After 1200?h of oxidation at 1500?°C in air (30 times thermal cycles), the mass losses of the graphite substrates coated with ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were 0.086% and 0.537%, respectively, and the high-temperature stability of the modified coatings was greatly improved compared to the Si-SiC coating. The excellent anti-oxidation performances of the compound coatings were attributed to the compact structure of the coatings and the formation of compound oxide layers covering on the surfaces. The compound Zr-Si-O and Ta-Si-O films possessed low oxygen diffusion rate and appropriate viscosity, which can provide appreciable oxidation protection for the internal coatings, thus obtaining the excellent oxidation and spallation resistance property.     

A different chemical route to prepare hafnium diboride-based nanofibers: Effect of chemical composition

This study reports on the synthesis of hafnium diboride (HfB₂)-based nanofibers via electrospinning of polyhafnoxanesal (PHO)-based solution followed by pyrolyzing hafnium-boron containing polyvinylpyrrolidone precursor fibers by a moderate heat treatment at 1500°C under argon atmosphere. The influence of the molar ratios of C/Hf and B/Hf in preceramic polymer method is investigated on the final phase of HfB₂-based nanofibers. Structural, thermal, microstructural, and physical properties of the hafnium-based fibers are evaluated using Fourier transform infrared spectra (FTIR), thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffractometer (XRD), high-temperature X-ray diffraction (HT-XRD), field-emission scanning electron microscope/energy-dispersive spectrometer (FE-SEM/EDS), and Brunauer-Emmett-Teller (BET). The results unveiled that the acidic pH was the optimal condition needed for obtaining the single phase of HfB₂ nanofibers. The precursor fibers with the stoichiometric ratio of 1:4:5 of Hf:B:C prepared under the acidic conditions converted into pure HfB₂ nanofibers having rough and porous surface after pyrolysis at 1500°C for 2 hour in argon, whereas HfB₂-HfC composite nanofibers with smooth surface were produced in the neutral conditions. The HfB₂ nanofibers with a mean diameter of ~100 nm prepared under the acidic conditions showed a higher specific surface area compared to HfB₂-HfC composite nanofibers with a diameter of ~121 nm derived in the neutral conditions.     

Oxidation protection of B4C modified HfB2-SiC coating for C/C composites at 1073–1473 K

B₄C modified HfB₂-SiC coating for C/C substrate was designed to expand the application of HfB₂-SiC based coating in low-medium temperature environment. The oxidation protection behavior of HfB₂-SiC based ceramic coatings with and without B₄C at 1073, 1273 and 1473 K was tested and analyzed. The experimental results reveal that the oxidative damage of HfB₂-SiC coated C/C reduces by over 20% after introducing B₄C, which may be due to the protection of borosilicate glass with more suitable viscosity during oxidation. Meanwhile, B₄C can improve the oxidation protection ability of HfB₂-SiC coating best at 1473 K. And the introduction of B₄C can reduce the mass loss of HfB₂-SiC coated C/C sample by 77.6% after oxidation for 58 h at 1473 K. The fluidity of glass film becoming better with temperature-rising, and the fluid borosilicate glass layer makes the coated samples have the best anti-oxidation properties at 1473 K among these three temperatures.     

TaB2–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites

To improve the oxidation resistance of the carbon/carbon (C/C) composites, a TaB₂–SiC–Si multiphase oxidation protective ceramic coating was prepared on the surface of SiC coated C/C composites by pack cementation. Results showed that the outer multiphase coating was mainly composed of TaB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The coating could protect C/C from oxidation for 300 h with only 0.26 × 10^?2 g²/cm² mass loss at 1773 K in air. The formed silicate glass layer containing SiO₂ and tantalum oxides can not only seal the defects in the coating, but also reduce oxygen diffusion rates, thus improving the oxidation resistance.