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.     

The effect of the SiC content on the high duration erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating produced by shielding shrouded plasma spray method

In this research, the effect of the ZrB₂ middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB₂– SiC/ZrB₂ functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB₂–SiC layers containing 10, 20 and 30 wt% SiC and, ZrB₂ as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB₂–SiC layer between SiC and ZrB₂ coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, ?0.072 × 10^?4 g.cm^?2.s^?1, 0.0166 μm s^?1 respectively had the best erosion resistance under oxy propane flame.In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB₂-20% wt. SiC/ZrB₂ coating had the lowest erosion rate.     

Microstructure and oxidation behavior of a novel bilayer (c-AlPO4–SiCw–mullite)/SiC coating for carbon fiber reinforced CMCs

A novel cristobalite aluminum phosphate particle (c-AlPO₄) modified SiC whisker toughened mullite coating (c-AlPO₄-SiC_w-mullite) was prepared on SiC coated carbon fiber reinforced SiC composites (C/SiC) by a new sol-gel method combined with air spraying to improve the oxidation resistance of SiC_w-mullite coating. Results show that c-AlPO₄-SiC_w-mullite coatings with 10 and 20 wt.% of c-AlPO₄ exhibited obviously improved oxidation resistance at 1773 K in ambient air for 100 h than SiC_w-mullite coating. Moreover, the oxidation resistance of c-AlPO₄-SiC_w-mullite coatings were rapidly declined when the c-AlPO₄ in c-AlPO₄-SiC_w-mullite coating were set to 30 and 40 wt.%. The c-AlPO₄-SiC_w-mullite coating with 20 wt.% of c-AlPO₄ showed most pronounced oxidation resistance, the weight loss rate after the oxidation in ambient air for 210 h was merely 3.00 × 10^?5 g·cm^?2 h^?1. The failure of c-AlPO₄-SiC_w-mullite coating with 20 wt.% of c-AlPO₄ was due to the generation of penetrative micro-cracks and micro-holes in the coating, which cannot be self-healed by the silicate glass layer after long time oxidation at 1773 K.     

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB₂–SiC–TaSi₂–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm²·h^0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10⁻³ after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO₂ as skeleton phase with SiO₂ as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB₂–SiC–TaSi₂ major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB₂–SiC–TaSi₂–Si layer was a vital factor as well.     

A mullite/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a mullite coating was prepared on SiC precoated C/C composites by a hydrothermal electrophoretic deposition process. The phase composition, microstructure and oxidation resistance of the prepared mullite/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free mullite coatings. The mullite/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 322 h with a weight loss rate of only 4.89 × 10^?4 g/cm² h. The failure of the multi-layer coatings is considered to be caused by the volatilization of silicate glass layer, the formation of microholes and microcracks on the coating surface and the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 111.11 kJ/mol.     

Evaluation of Rare‐Earth Modified ZrB2–SiC Ablation Resistance Using an Oxyacetylene Torch

Rare‐earth modified ZrB₂–SiC coatings were prepared via mechanical mixing Sm₂O₃ or Tm₂O₃ powders with spray‐dried ZrB₂, or by chemically doping samarium ions into spray‐dried ZrB₂. In either approach, SiC powders were also added and coatings were fabricated via shrouded air plasma spray. An oxyacetylene torch was utilized to evaluate the coatings under high heat flux conditions for hold times of 30 and 60 s. The resulting phases and microstructures were evaluated as a function of rare‐earth type, modification approach, and ablation time. A brittle m‐ZrO₂ scale was observed in the ZrB₂/SiC‐only coating after ablative tests; during cooling this scale detached from the unreacted coating. In contrast, rare‐earth modified coatings formed a protective oxide scale consisting primarily of either Sm_0.2Zr_0.8O_1.9 or Tm_0.2Zr_0.8O_1.9, along with small amount of m‐ZrO₂. These rare‐earth oxide scales displayed high thermal stability and remained adhered to the unreacted coating during heating and cooling, offering additional oxidation protection.     

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

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

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB₂–SiC coatings, ZrB₂, silicon, and graphite powders were used as pack powders to prepare ZrB₂–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB₂–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.     

In situ studies of oxidation of ZrB2 and ZrB2–SiC composites at high temperatures

High temperature oxidation of ZrB₂ and the effect of SiC on controlling the oxidation of ZrB₂ in ZrB₂–SiC composites were studied in situ, in air, using X-ray diffraction. Oxidation was studied by quantitatively analyzing the crystalline phase changes in the samples, both non-isothermally, as a function of temperature, up to ～1650 °C, as well as isothermally, as a function of time, at ～1300 °C. During the non-isothermal studies, the formation and transformation of intermediate crystalline phases of ZrO₂ were also observed. The change in SiC content, during isothermal oxidation studies of ZrB₂–SiC composites, was similar in the examined temperature range, regardless of sample microstructure and composition. Higher SiC content, however, markedly retarded the oxidation rate of the ZrB₂ phase in the composites. A novel approach to quantify the extent of oxidation by estimating the thickness of the oxidation layer formed during oxidation of ZrB₂ and ZrB₂–SiC composites, based on fractional conversion of ZrB₂ to ZrO₂ in situ, is presented.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

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.     

Effect of tantalum addition on microstructure and oxidation of spark plasma sintered ZrB2-20vol% SiC composites

Almost full density (>99% theoretical density (ρ_th)) was achieved for ZrB₂-20vol% SiC-Xwt.% Ta (X = 2,5, 5 and 10) composites after Spark Plasma Sintering (SPS) (Temperature: 1900 °C, Pressure: 50 MPa; Time: 3 min). The microstructure of ZrB₂-based composites exhibited core-rim structure and it consists of major crystalline phases (ZrB₂ core, (Zr, Ta)B₂ rim, SiC), minor amounts of ZrO₂ and (Zr, Ta)C solid solution phases. Both the specific weight (from 22.91 to 18.77 mg/cm²) and oxide layer thickness (401–195 μm) of ZrB₂-20vol% SiC composites decreased with increasing addition of Ta after the isothermal oxidation at 1500 °C for 10 h in air. The cross-sectional microstructure of oxidized samples displayed presence of a stack of three distinctive layers, which includes thick dense SiO₂ top layer, SiC depleted intermediate layer and unreacted bulk. The present work clearly demonstrated the advantage of tantalum addition in improving the oxidation resistance of ZrB₂-20vol% SiC.     

Wet-oxygen corrosion resistance and mechanism of bi-layer Mullite/SiC coating for Cf/SiC composites

A bi-layer oxidation-resistant coating consisting of a mullite outer coating, and a SiC inner coating on the surface of C_f/SiC composites was prepared by the chemical vapour deposition and an air spray sol-gel process, and its corrosion behavior was evaluated in a wet-oxygen coupling environment. Results show that the formation of SiO₂ glass layer and its reaction with mullite particles to form aluminosilicate glass layer, leading to an increase in the density of the mullite outer coating, so that the weight loss of bi-layer Mullite/SiC coating coated C/SiC sample was only 1.11 × 10^?3 g·cm^?2 in the first 100 h of oxidation. However, the weight loss of the coated sample reached 26.82 × 10^?3 g·cm^?2 after 200 h of oxidation due to a part of the mullite outer coating was detached. The SiO₂ glass phase reacted with water vapour to generate gaseous Si(OH)_x, which created distinct holes on the surface of the SiO₂ glass layer or inside the molten aluminosilicate glass layer. Eventually, the mullite outer coating was blistered and detached from the surface of the sample due to the combination and growth of holes.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

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.     

Effect of processing parameters on microstructure and ablation behavior of SiC/ZrB2 coating for graphite

In this research, a SiC/ZrB₂ coating was produced on graphite by reactive melt infiltration and plasma spraying method. The coating characterization was performed using XRD analysis, electron microscopy equipped with energy dispersive spectrometer (EDS), and supersonic flame ablation test at 2073 K. The results indicated that the dense C/SiC coating with good ablation resistance can be obtained at 1873 K. The coating thickness decreased with increasing infiltration temperature. The results of ablation test showed that by increasing the infiltration temperature and holding time, weight loss and mass ablation rate decreased from 22.63% to 9.83% and 3.63 × 10⁻³ g cm⁻² s⁻¹ to 1.34 × 10⁻³ g cm⁻² s⁻¹, respectively. The results showed that by using the ZrB₂ as outer coating the ablation resistance improved remarkably. The weight loss and mass ablation rates for the SiC/ZrB₂ coating were 12.79% and 1.857 × 10⁻³ g cm⁻² s⁻¹, respectively.     

Erosion mechanism of ternary-phase SiC/ZrB2-MoSi2-SiC ultra-high temperature multilayer coating under supersonic flame at 90° angle with speed of 1400 m/s (Mach 4)

A ternary-phase SiC/ZrB₂-MoSi₂-SiC multilayer coating was prepared on graphite by two-step reactive melt infiltration (RMI) method. The formation mechanism of the coating was studied by HSC chemistry software 6.0. The erosion resistance of the coating was investigated by supersonic flame erosion test at 90° angle, temperature of 2173 K and speed of 1400 m/s (Mach 4) for 120 s. Erosion test results revealed that the SiC/ZrB₂-MoSi₂-SiC multilayer coating had very good erosion resistance. Weight change percentage, mass erosion rate and linear erosion rate of the coating were −0.18 %, −0.027 × 10⁻³g cm⁻² s⁻¹ and 0.33 μm s⁻¹, respectively. Microstructural characterization demonstrated that interesting structures such as rod-like, flake-like, spherical, worm-like and fibrous structures were formed during erosion test. The erosion mechanism of ZrB₂-MoSi₂-SiC coatings is controlled both chemically and mechanically. The reduction of chemical degradation can be attributed to the presence of MoSi₂ particles and the reduction of mechanical degradation can be related to the presence of ZrB₂ particles.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.