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.     

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.     

Effect of La2O3 content on the oxygen barrier ability of the HfB2-SiC coating at 1973 K

To inhibit the destructive evolution of the HfB₂-SiC coating during oxidation, La₂O₃ was added to modify the oxygen-blocking capability of the HfB₂-SiC coating. The effect of La₂O₃ content on the oxygen barrier capacity of HfB₂-SiC was investigated. The addition of 5 vol.% La₂O₃ lowered the oxidation activity and strengthened the inert oxygen barrier capacity of the HfB₂-SiC coating, which made the oxygen permeability and maximum weight change rate of the coating decrease by 30.32% and 73.97%, respectively. Due to the solid solution reaction of the dispersed La₂O₃, HfO₂, and SiO₂ nanoparticles, a stable Hf-B-La-Si-O multiphase glass was formed on the surface of the coating. Besides, the high-melting-point particles, such as HfO₂, La₂Hf₂O₇, and HfSiO₄, dispersed in the glass layer as hard particles and formed a mosaic structure based on the Hf-B-La-Si-O compound, which increased the cumulative protection efficiency of the HfB₂-SiC coating to 98.5% and reduced the inert factor value by 84.57%. However, the excessive consumption of SiO₂ glass by La₂O₃ results in dendritization of the generated glass when the La₂O₃ content is above 10 vol.%, which reduced the self-healing capability of the generated glass and instead weakened the oxygen barrier properties of the HfB₂-SiC-La₂O₃ coatings.     

Gradient HfB2-SiC multilayer oxidation resistant coating for C/C composites

The gradient HfB₂ modified SiC coating was prepared on the surface of SiC-coated C/C composites by in-situ synthesis. Anti-oxidation behaviors of the coated C/C samples at 1773, 1873 and 1973?K were investigated. The results show that the gradient HfB₂ modified SiC coatings possess excellent oxidation resistance, which can protect C/C substrates from oxidation for 800, 305 and 100?h at 1773, 1873 and 1973?K, respectively. In addition, with the oxidation temperature increasing, the evaporation of the Hf-Si-O glass layer and the active oxidation of SiC were accelerated, which is the reason for the worst oxidation resistance of the sample at 1973?K among the three temperatures.     

Effect of TaSi2 addition on long-term ablation behavior of HfB2-SiC coating

In order to improve the thermal protective ability of HfB₂-SiC coating, TaSi₂ was introduced into the coating and its function on long-term ablation resistance was investigated by plasma ablation tests under two heat fluxes. The variations in ablation behavior and structural evolution of the coatings under low and high heat fluxes were elaborated. The results indicated that TaSi₂ significantly improved the ablation resistance of HfB₂-SiC coating due to the alleviation of volume expansion when the ablation temperature was above the melting point of Ta₂O₅ (∼1872 ℃). However, the results are reversed when the temperature was below the fusing point. The reasons for the variation were explained by the effect of temperature on the active oxidation of SiC and the oxidation behavior of TaSi₂. The improvement of the ablation resistance also can be attributed to the avoidance of the SiC-depleted region and the formation of Hf-Ta-Si-B-O compound glassy layer with low oxygen permeability and high viscosity.     

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%.     

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.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

Effect of SiC whiskers on the anti-oxidation properties of Yb2Si2O7/mullite/SiC tri-layer environmental barrier coating for CMCs

The mullite and ytterbium disilicate (β-Yb₂Si₂O₇) powders as starting materials for the Yb₂Si₂O₇/mullite/SiC tri-layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti-oxidation properties of Yb₂Si₂O₇/mullite/SiC tri-layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb₂SiO₅, α-Yb₂Si₂O₇, and β-Yb₂Si₂O₇ were gradually formed between 800 and 1000°C, and Yb₂SiO₅ and α-Yb₂Si₂O₇ were completely transformed into β-Yb₂Si₂O₇ at a temperature above 1200°C. The weight loss of Yb₂Si₂O₇/(SiC_w–mullite)/SiC tri-layer coating coated specimens was 0.15 × 10⁻³ g cm⁻² after 200 h oxidation at 1400°C, which is lower than that of Yb₂Si₂O₇/mullite/SiC tri-layer coating (2.84 × 10⁻³ g cm⁻²). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β-Yb₂Si₂O₇ outer coating, but also improve the self-healing performance of the mullite middle coating owing to the self-healing aluminosilicate glass phase formed by the reaction between SiO₂ (oxidation of SiC whiskers) and mullite particles.     

Effect of phase composition on the oxidation resistance of ZrB2-SiC coatings

ZrB₂-SiC-Al₂O₃ (ZSA) and ZrB₂-SiC-Si (ZSS) coatings were prepared on the surface of C/C composites by atmospheric plasma spraying. During oxidation at 1200 ℃ for 10 h, the formation of B₂O₃ in the ZSA coating volatilizes gradually, no glass phase is able to seal cracks and holes, resulting in poor oxidation resistance. On the other hand, the ZSS coating produces a large amount of borosilicate glass phase and still gains 0.8% weight after 10 h. During oxidation at 1500 ℃, a low viscosity SiO₂ glass phase forms on the ZSA coating, due to the presence of Al₂O₃ and this glassy phase has a good ability to seal defects. However, the rapid volatilization of B₂O₃ in the ZSS coating leads to the formation of large holes in the coating, and to progressively increased viscosity of SiO₂ glass phase, resulting in decreased oxidation resistance.     

Design and optimization of oxidation resistant coating for C/C aircraft brake materials

A SiCN/borosilicate glass anti-oxidation coating with double-layer structure was designed for C/C aircraft brake materials. The SiCN layer was introduced as transition layer to improve the wettability between borosilicate glass and C/C composites, and the microstructure results indicated that the coating with SiCN inner layer was dense and uniform. The oxidation resistance evaluation of the coated samples was conducted at 800 °C in air for 10 h. The weight loss of SiCN/borosilicate glass coated samples valued ~ 5.66% indicated that the oxidation resistant property of the simple SiCN/borosilicate glass coating was not good, which was mainly due to the relative large viscosity of borosilicate glass at 800 °C. B₄C was introduced to add into the outer glass coating to improve the self-healing ability of the coating. After oxidized at 800 °C in air for 10 h, the weight loss of the SiCN/borosilicate glass-B₄C coated samples was ~ 2.48%. B₄C could consume the oxygen diffused into the coating and the reacted product B₂O₃ with a better fluidity at 800 °C could effectively heal cracks and pores in the coating to improve the oxidation resistance property. The reaction of B₄C oxidized to B₂O₃ was accompanied with ~ 1.5 times volume expansion, which was also beneficial for the healing of defects.     

Determination of Retained B2O3 Content in ZrB2‐30 vol% SiC Oxide Scales

The composition of the borosilicate glass layer formed during oxidation of ZrB₂‐30 vol% SiC was determined to elucidate the extent of B₂O₃ retention in the oxide during high‐temperature oxidation. Oxidation was conducted in stagnant air at 1300°C, 1400°C, and 1500°C for times between 100 and 221 min. Specimens were characterized using mass change and scanning electron microscopy. After oxidation, the borosilicate glass layer was dissolved from the specimens sequentially with deionized H₂O and HF acid, to analyze the glass composition using inductively coupled plasma optical emission spectrometry. It was found that the average B₂O₃ content in the glass scale ranged from 23 to 47 mol%. Retained B₂O₃ content in the bulk of the glass decreased with increasing temperature, confirming increased volatility with temperature. Boron depth profiles were also obtained in the near surface region using X‐ray photoelectron spectroscopy and energy dispersive spectroscopy. The measured B concentrations were used to estimate the B₂O₃ concentration profile and B diffusion coefficients in the borosilicate glass. Implications for the ZrB₂‐SiC oxidation process are discussed.     

Microstructural evolution of HfB2 based ceramics during oxidation at 1600–2000°C

HfB₂–2?wt-%-La₂O₃, HfB₂–20?vol.-%SiC and HfB₂–20?vol.-%SiC–2?wt-%La₂O₃ were sintered by spark plasma sintering for 5?min between 1900 and 2000°C. Oxidation studies were carried out in static air on sintered ultra high temperature ceramics at 1600, 1800 and 2000°C, and the weight change and microstructure changes after oxidation were examined. LaBO₃ formed on the surface after oxidation for 1?h above 1600°C in all La₂O₃ containing materials, and the cross-section microstructure shows growth of LaBO₃ fibres from pores due to vapour phase reaction. Addition of La₂O₃ altered the oxidation kinetics of HfB₂, significantly increasing the oxidation layer thickness above 1600°C. However, above 1800°C reaction has occurred between the sample and the zirconia crucible.     

Oxidation resistance and thermal shock properties of self-healing SiCN/borosilicate glass-B4C-Al2O3 coatings for C/C aircraft brake materials

SiCN/borosilicate glass-B₄C-Al₂O₃ coating was deposited on carbon fiber-reinforced carbon matrix (C/C) brake materials to protect them from oxidation. Microstructural analysis revealed that the coating was dense and uniform. Fabricated coating showed excellent oxidation resistance and significantly low weight losses after oxidation in dry air for 10?h than SiCN/borosilicate glass-B₄C coated samples (ca. 0.12%, 0.51%, and 0.29% at 700, 800, and 900?°C, respectively). B₄C is believed to react with the oxygen diffused into the coating to produce B₂O₃, which could heal cracks of the coating and improve its self-sealing ability and oxidation resistance. The Al₂O₃ present in the outer glass layer is believed to inhibit volatilization of B₂O₃, thereby reducing weight losses in air. Fabricated coating also possessed excellent oxidation resistance under fresh and sea water conditions, with cracks and pores generated during oxidization process being effectively healed. Prepared coating materials showed excellent thermal shock resistances after 50 thermal shock cycles, with weight losses being as low as 0.23%.     

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.     

Role of WC additive on reaction,solid-solution and densification in HfB2–SiC ceramics

A comparative study of phase components and compositions was performed for the pressureless sintered HfB₂–SiC–WC composites by various analytical methods. The relative decrease of HfB₂ phase leads to a new reaction of HfO₂ removal by WC to create B₂O₃. By using SiC instead of Si₃N₄ as milling medium, the WB phase was suppressed to the trace level while the W solid-solution in HfB₂ phase was favored. The W solution in both the primary HfB₂ and resultant HfC phases indicates that the WC additive was involved throughout the sintering process by dissolving into sintering liquid, which remains at the intergranular regions to form amorphous oxides as well as trace W-rich phases. This is effectively a reactive liquid-phase sintering to realize the reaction, solid-solution and densification collectively to achieve a designable HfB₂–SiC–HfC composite by pressureless sintering, which may also be extended to other sintering methods.     

Effect of in situ SiC and MoSi2 phases on the oxidation behavior of HfB2-based composites

Monolithic HfB₂ and HfB₂-15vol%SiC-15vol%MoSi₂ composite samples were oxidized by a conventional electric furnace at 1700 °C for 5 h. Microstructural and phase analysis of the oxidized samples were performed by X-ray diffraction (XRD) analysis and field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Besides, for analyzing the oxidation mechanism of the samples, thermodynamic calculations were also accomplished by HSC software. The changes in weight and thickness of the oxide scale were measured and the oxide growth rate of the oxidized samples was subsequently calculated. The results showed that HfB₂-15vol%SiC-15vol%MoSi₂ composite was much more resisted than that monolithic HfB₂ due to the formation of a thin Si-rich glass layer on the surface of the composite sample. By acting to fill the porosities between HfO₂ grains, Si-based glass phase enhanced the oxidation resistance of HfB₂-15vol%SiC-15vol%MoSi₂ composite. Conversely, the oxidized monolithic HfB₂ had only a thick porous oxide layer (HfO₂) which led to considerably lower oxidation resistance. On the other side, three layers containing HfO₂ and Si-based glass phases were formed on the oxidized HfB₂-15vol%SiC-15vol%MoSi₂ composite. Moreover, no porosities and no porous layers were also detected on the oxidized composite sample. Consequently, HfB₂-15vol%SiC-15vol%MoSi₂ composite had a parabolic behavior owing to its diffusion-controlled oxidation under the isothermal oxidation process.     

Oxidation protection of carbon/carbon composites with SiC/indialite coating for intermediate temperatures

In order to improve the oxidation resistance of carbon/carbon composites at intermediate temperatures, a novel double-layer SiC/indialite coating was prepared by a simple and low-cost method. The internal SiC transition layer was prepared by pack cementation and the external indialite glass–ceramic coating was produced by in situ crystallization of ternary MgO–Al₂O₃–SiO₂ glass. The microstructures and morphologies of coating were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Oxidation resistance of the as-coated C/C composites was evaluated in ambient air at temperature from 800 °C to 1200 °C. Nearly neglectable mass loss was measured after 100 h isothermal oxidation test, indicating that SiC/indialite coating possesses excellent oxidation protection ability. The as-coated samples have a good thermal shock resistance and no obvious damage was found in the coating even after suffered more than 11 thermal cycles between test temperature and room temperature. The oxidation protection mechanism of this coating was also discussed.     

Preparation of TaB2-SiC oxidation protective coating for carbon materials by liquid phase sintering

To control the microstructure and amounts of TaB₂ phase in the TaB₂-SiC coating, a novel liquid phase sintering method was developed on the basis of in-situ reaction method to prepare the TaB₂-SiC coating, which includes synthesis of TaB₂ powders and further preparation of TaB₂-SiC coating. With Ta₂O₅, B₂O₃ and C employed as raw materials, hexagonal TaB₂ powders were prepared by carbothermal reduction method at 1500?°C, whose mean particle size is 491?nm. The TaB₂, SiC, C powders, and the low melting point phases Si and silica sol were used to prepare the TaB₂-SiC coating by liquid phase sintering at 2373?K. The thickness of the coating is about 350?µm. Compared with the SiC coating, the weight loss of the samples modified by TaB₂ decreased from 17.7% to 11.8%, and the average weight loss rate of the fastest weightloss zone reduced from ?6?×?10^?3 mg?cm^?2 s^?1 to ?5?×?10^?3 mg?cm^?2 s^?1. During oxidation, the Ta-oxides would gradually dissolve in the silicate glass to form Ta-Si-O glass ceramics with dendritic structure, which significantly improved the toughness and stability of the glass layer. The Ta-Si-O glass ceramics possesses the ability of sealing and arresting the microcracks, which can enhance the oxidation protective ability of the coating.     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.