Bimodal microstructure ZrB2-MoSi2 coating prepared by atmospheric plasma spraying for carbon/carbon composites against long-term ablation

To protect carbon/carbon composites against long-term ablation, a bimodal microstructure ZrB₂-MoSi₂ coating, consisting of an outer ZrB₂-MoSi₂ layer modified by Y₂O₃ and an inner basal ZrB₂-MoSi₂ layer, was prepared by atmospheric plasma spraying. The microstructure, phase composition and ablation resistance of the proposed coating were investigated in detail. Results showed that the bimodal coating maintained integrity in structure except for phase composition. There was no visible interlayer between the inner ZrB₂-MiSi₂ layer and the outer modified one. Mass ablation rate of the bimodal microstructure ZrB₂-MoSi₂ coated C/C composites was −2.02 × 10⁻³ g/s under an oxyacetylene flame ablation at 1873 K for 600 s, which exhibited better ablation resistance than a single ZrB₂-MoSi₂ coating. The excellent ablation resistance was ascribed to the positive effect of Y₂O₃, which not only pined in the glassy phase and alleviated the volatilization of SiO₂ glass phase by reacting with SiO₂ to form high viscosity of Y₂SiO₅, but also stabilized ZrO₂ and promoted its recrystallization and growth.     

Additively-manufactured multiphase coatings with laser protection ability for the repairing of MoSi2-SiC/SiC coated C/C composites

The Si-SiC-MoSi₂ and Si-SiC coatings were proposed to repair the damaged MoSi₂-SiC/SiC coated C/C composites by laser directed energy deposition. Laser ablation was used to assess the repair effect. Results showed that both the repaired coatings with dense structure could restore the geometric size of damaged area. Compared with the Si-SiC-MoSi₂ coating, the Si-SiC repaired coating with higher laser reflectivity and more free Si could reduce the heat generation and enhance the heat dissipation during ablation, which lowered the maximum temperature by 347.49 K and 810.77 K under 300 W and 500 W ablation for 7 s separately, beneficial to avoid the secondary laser damage of the repaired area.     

Effects of LaB6 on the microstructures and ablation properties of 3D C/C-SiC-ZrB2-LaB6 composites

C_f/LaB₆ preform was used to prepare LaB₆ doping C/C-ZrC-SiC composites by precursor infiltration and pyrolysis method, and effects of LaB₆ on the microstructure and ablation resistance of C/C-SiC-ZrB₂-LaB₆ composites were investigated. Results show that LaB₆ was reacted with Zr-precursor or its products to generate ZrB₂ and other compounds. Thanks to the reactive sintering effect of LaB₆, a compact ceramic skeleton was established in the substrate, which played a vital role in densifying and resisting ablation. The oxidation of the evenly-distributed La- and Zr-compounds produced the homogeneous oxide scale. The ternary phases of La₂Si₂O₇, La_0.71Zr_0.29O_1.65 and La₂Zr₂O₇ were participated in the formation of the oxide scale, evolving the layer from SiO₂ to SiO₂-La₂Si₂O₇ and then to SiO₂-La₂Si₂O₇-La_0.71Zr_0.29O_1.65-La₂Zr₂O₇-ZrO₂ layer. After plasma ablation for 360 s, the mass and linear ablation rates were 0.3848 mg/s and 0.3694 μm/s, respectively. The seamless multi-phase layer can effectively prevent the composites from long-time ablation.     

A new method to improve the laser-ablation resistance of Si-SiC coating on C/C composites: Laser cladding

New method to prepare Si-SiC coating on C/C composites by laser cladding (LC-Si-SiC) was established to improve the laser ablation resistance of the coating. Results showed that the LC-Si-SiC coating had lower roughness, better mechanical properties and superior laser-ablation resistance compared with the Si-SiC coating fabricated by the traditional coating preparation process: pack cementation (PC-Si-SiC). Due to the shorter heat treatment and less Si infiltration, the flexural strength of the LC-Si-SiC coated sample was 111.32 MPa, which was 144% higher than that of the PC-Si-SiC coated sample. Confirmed by finite element simulation, the LC-Si-SiC coating exhibited better laser-ablation resistance because of the “sweating cooling” mechanism. Under 23.89 MW?m^?2 ablation for 7 s, the surface temperature of the LC-Si-SiC coated sample was 3046 K, which was 157 K lower than that of the PC-Si-SiC, causing the mass loss rate of the LC-Si-SiC coating (0.10%) was only 45.45% of the PC-Si-SiC coating.     

Ablation resistance of HfB2-SiC coating prepared by in-situ reaction method for SiC coated C/C composites

To improve the ablation resistance of SiC coating, HfB₂-SiC coating was prepared on SiC-coated carbon/carbon (C/C) composites by in-situ reaction method. Owing to the penetration of coating powders, there is no clear boundary between SiC coating and HfB₂-SiC coating. After oxyacetylene ablation for 60 s at heat flux of 2400 kW/m², the mass ablation rate and linear ablation rate of the coated C/C composites were only 0.147 mg/s and 0.267 µm/s, reduced by 21.8% and 60.0%, respectively, compared with SiC coated C/C composites. The good ablation resistance was attributed to the formation of multiple Hf-Si-O glassy layer including SiO₂, HfO₂ and HfSiO₄.     

Comparing ablation properties of NbC and NbC-25 mol.% ZrC coating on SiC-coated C/C composites

In this work, ablation properties of NbC and NbC-25 mol.% ZrC coating, deposited on SiC-coated C/C composites by supersonic atmospheric plasma spraying, were tested by an oxyacetylene torch. Results showed that, for NbC coating, an unexpected smooth liquid film mostly composed of niobium suboxides (such as NbO₂ and NbO), rather than pure Nb₂O₅, generated during ablation for 45 s. Mechanical erosion resulted from the molten SiO₂, and the relatively low viscosity of the outer oxide layer owing to insufficiently high melting point of niobium suboxides were the key factors for the failure mechanism of NbC coating. While NbC–ZrC coating abated for 90 s has a 97.49 and 66.53% decrease of linear and mass ablation rate relative to NbC coating ablated for 45 s, since ZrO₂ hindering the evaporation of SiO₂ droplets, and more thermal-stable Nb–O–Zr liquid film endow (NbC–ZrC)/SiC/C/C composites with an outstanding anti-ablation property.     

New anti-ablation candidate for carbon/carbon composites: Preparation,composition and ablation behavior of Y2Hf2O7 coating under an oxyacetylene torch

Y₂Hf₂O₇ possesses low thermal conductivity and high melting point, which make it promising for a new anti-ablation material. For evaluating the thermal stability and the potential applications of Y₂Hf₂O₇ on anti-ablation protection of C/C composites, Y₂Hf₂O₇ ceramic powder was synthesized by solution combustion method and Y₂Hf₂O₇ coating was prepared on the surface of SiC coated C/C composites using SAPS. Results shown that the coating exhibits good ablation resistance under the heat flux of 2.4?MW/m² with the linear and mass ablation rates are 0.16?μm?s^?1 and ?0.028?mg?s^?1, respectively, after ablation for 40?s. With the prolonging of the ablation time, the increasing thermal stress causes the increase of cracks. Moreover, the chemical erosion from SiO₂ and the physical volatilization of low temperature molten products aggravate failure of the Y₂Hf₂O₇ coating.     

Ablation behavior of C/SiC with YSZ and ZrB2 coatings under high-energy laser irradiation

Carbon fiber-reinforced silicon carbide (C/SiC) composites are important candidates for laser protection materials. In this study, ablation mechanism of C/SiC coated with ZrO₂/Mo and ZrB₂–SiC/ZrO₂/Mo under laser irradiation was studied. ZrB₂–SiC multiphase ceramic and ZrO₂ ceramic were successfully coated on C/SiC composite by atmospheric plasma spraying technology with Mo as transition layer. Phase evolution and morphology of composite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Moreover, ablation behavior of the composite was investigated by laser confocal microscopy. Results showed that ablation mechanism of C/SiC composite was controlled by phase transformation, thermal reaction, and thermal diffusion, with solid–liquid transition of ZrB₂ and ZrO₂ being dominant factor. Endothermic reaction and good thermal diffusivity of coatings were also important factors affecting ablation performance. Reflectivity effect of ZrO₂ coating was limited under high-energy laser irradiation. Compared with ZrO₂/Mo single-phase-monolayer coating, designed ZrB₂–SiC/ZrO₂/Mo coating showed better ablation performance, and breakdown time of C/SiC increased from 10 to 40 s. The depletion of liquid phase in molten pool was identified as an important factor responsible for rapid failure of C/SiC. The coating failed when the entire liquid phase was consumed within molten pool, followed by rapid damage of C/SiC substrate. Results of this study can provide theoretical guidance and research ideas for design and application of laser protective materials.     

Intelligent cooling structure design of "Z-pins like" silicon rods to enhance the ablation resistance of C/C-ZrC-SiC composites above 2500 °C

In order to improve the ablation resistance of C/C-ZrC-SiC composites by reducing the damage of the protective oxide layer, novel "Z-pins like" silicon rods, which were designed and fabricated by liquid phase sintering, were utilised as a dissipative agent. The microstructure evolution and thermal dissipation behaviour were investigated after ablating above 2500 °C for 300 s. After the "Z-pins like" silicon rods were implanted, the anti-ablative property of the C/C-ZrC-SiC composites was drastically improved by the dissipative thermal protection mechanism. The linear ablation rate of the "Z-pins like" silicon rod-reinforced C/C-ZrC-SiC composite was -0.28 μm/s, which is 112.72% lower than the unmodified composite. Additionally, the actual ablative temperature dropped approximately 357 °C, which enabled abundant SiO₂ to remain in the ablation centre. Furthermore, a dense SiO₂-rich oxide layer with a low oxygen diffusion coefficient is formed that covers the entire ablative surface.     

Microstructure and ablation properties of La2O3 modified C/C-SiC composites prepared via precursor infiltration pyrolysis

Effects of La₂O₃ modification on the microstructure, mechanical and ablation properties of C/C-SiC composites were investigated. Experimental results show that a new La₁₀(SiO₄)₆O₃ phase was generated during heat treatment process. The presence of the La-compounds, namely La₂O₃ and La₁₀(SiO₄)₆O₃, had an important impact on the structure of reinforced skeleton and the molten oxide film, and thus strongly affected the mechanical and ablation properties of the composites. Excessive La addition induced the structural damage of the reinforced skeleton, resulting in weakened mechanical and ablation properties. The C/C-SiC composites with 25.65 wt.% La₂O₃ addition displayed better mechanical properties and the best ablation resistance. The La₁₀(SiO₄)₆O₃ phase could react with molten silica to form a viscous glass during ablation. The transformation of La-compounds into La₂Si₂O₇ can reduce the ablation of SiO₂ and enhance the glass film, so as to protect the composites from further ablation.     

Improved antioxidative and mechanical properties of SiC coated C/C composites via a SiO2-SiC reticulated layer

To improve the oxidation resistances of SiC coated C/C composites by a pack cementation (PC) method at high temperature and alleviate the siliconization erosion of molten silicon on C/C substrate during the preparation of SiC coating, a SiO₂-SiC reticulated layer with SiC nanowires was pre-prepared on C/C composites through combined slurry painting and thermal treatment before the fabrication of SiC coating. The presence of porous SiO₂-SiC layer with SiC nanowires was beneficial to fabricate a compact and homogeneous SiC coating resulting from synergistic effect of further reaction between SiO₂ and pack powders and the reinforcement of SiC nanowires. Therefore, the results of thermal shock and isothermal oxidation tests showed that the mass loss of modified SiC coating was only 0.02 % after suffering 50-time thermal cycles between room temperature and 1773 K and decreased from 5.95 % to 1.08 % after static oxidation for 49.5 h in air at 1773 K. Moreover, due to the blocking effect of SiO₂-SiC reticulated layer on siliconization erosion during PC, the flexural strength of SiC coated C/C composites with SiO₂-SiC reticulated layer increased by 64.8 % compared with the untreated specimen.     

Design and preparation of composite coatings with increased reflectivity under high-energy continuous wave laser ablation

High-energy continuous wave (CW) laser ablation can cause severe damage to structural materials in an extremely short time, which generates considerable concern in terms of material safety. For the purpose of reducing or even eliminating such laser-induced damage, a novel composite coating consisting of a boron-modified phenolic formaldehyde resin incorporating ZrC and SiC has been designed and prepared. The experimental results reveal that ZrC and SiC are rapidly oxidized to ZrO₂ and SiO₂ respectively, leading to the formation of a white ceramic layer consisting of ZrO₂ particles and melted SiO₂. After ablation at 1000 W/cm² for 50 s, elemental analysis indicates that no Si can be found in the central ablation zone because of gasification. A relatively compact ZrO₂ layer is formed through the sintering of adjacent ZrO₂ particles, which effectively improves the reflectivity of the coating from 7.3% (before ablation) to 63.5% (after ablation). The high reflectivity greatly reduces the absorption of laser energy. In addition, no obvious ablation defects are observed in the composite coating. The excellent anti-laser ablation performance of the coating makes it a promising system for protecting a material against the effects of long-term CW laser ablation.     

The mechanical properties of C/C-ZrC-SiC composites after laser ablation

Considering practical environment, the bending property of C/C-ZrC-SiC, C/C-SiC and C/C composites after ablation was worthily studied. Results revealed that C/C-ZrC-SiC composites had a better laser ablation resistance and higher bending strength retention compared with C/C-SiC and C/C composites. The mass loss rate and ablated depth of C/C-ZrC-SiC composites was − 0.09% and 190.377 μm, respectively. The retention of bending strength of C/C-ZrC-SiC composites was 217.67 ± 44.12 MPa, whose strength decreased by 3.57% compared with that of as-prepared C/C-ZrC-SiC composites. The excellent anti-ablation property and residual bending strength of C/C-ZrC-SiC composites were attributed to the lowest ablative temperature and the effective protection of the ZrO₂ grain and ZrO₂-SiO₂ layer, which were formed by oxidation of ZrC-SiC, evaporation of SiO₂, migration of liquid ZrO₂-SiO₂ and the infiltrated as well as grown ZrO₂. However, the fracture behavior transformation of composites from pseudo-plastic rupture to brittle rupture was induced by the ablation damage.     

Microstructure evolution of in-situ SiC-HfB2-Si ternary coating and its corrosion behaviors at ultra-high temperatures

An in-situ SiC-HfB₂-Si ternary coating was deposited on C/C composites (C/Cs) via slurry panting plus gaseous Si infiltration composite method, to improve the oxidation and ablation resistance of C/Cs above 1773 K. The coating formation mechanism was investigated by microstructural analyses and thermo-dynamic calculations. The oxidation behavior of the coated specimens subjected either to high-temperature testing at 1773 K and 1973 K in static air furnace or to ablation testing with oxyacetylene torch upon ultra-high temperature service were studied, base on thermo-dynamic computations, numerical simulations and microstructure evolution. The SiC-HfB₂-Si coating protected C/Cs against oxidation at 1773 K for more than 1507 h which is longer than that of the reported SiC-HfB₂-based coatings, due to the as-prepared compact mosaic coating filled with HfB₂-rich Si-based multiphase and the consequently formed dense Hf-Si-O oxide layer. Moreover, a good ablation resistance with relatively low linear and mass ablation rates of −0.72 μm/s and 0.07 mg/s, respectively, was achieved due to the stable oxide scale with high viscosity.     

Ablation property and oxide scale evolution of Y2O3 modified C/C-SiC composites fabricating via precursor infiltration and pyrolysis method

Employed precursors of Y(NO₃)₃·6H₂O and polycarbosilane, Y₂O₃ doped C/C-SiC composites were prepared by utilizing precursor infiltration and pyrolysis method. Results show that Y₂O₃ alters the composition of C/C-SiC composites with the generation of β-Y₂Si₂O₇ and Y₂O₃ together with slight Y₂SiO₅, Y_4.46(SiO₄)₃O and Y₂C₃. During ablation, β-Y₂Si₂O₇ is stable and directly liquefied to rapidly fill in the empties in the substrate; the other three are transformed to δ-Y₂Si₂O₇, evolving the oxide scale from SiO₂-β-Y₂Si₂O₇-Y₂O₃ (semi-fused) layer to SiO₂-β-Y₂Si₂O₇-δ-Y₂Si₂O₇ ones. The ablation rates after ablation for 120 s were respectively 0.210 mg/s and 14.58 µm/s, severally decreased by 57.32% and 8.59% compared with that for 60 s. Further evaporation of SiO₂ is suppressed and reconcilement of oxide layer is hence confirmed, implying decent anti-ablation resistance of oxide layer.     

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.     

Oxidation behavior and microstructure evolution of SiC-ZrB2-ZrC coating for C/C composites at 1673 K

To protect carbon/carbon (C/C) composites against oxidation, a SiC-ZrB₂-ZrC coating was prepared by the in-situ reaction between ZrC, B₄C and Si. The thermogravimetric and isothermal oxidation results indicated the as-synthesized coating to show superior oxidation resistance at elevated temperatures, so it could effectively protect C/C composites for more than 221 h at 1673 K in air. The crystalline structure and morphology evolution of the multiphase SiC-ZrB₂-ZrC coating were investigated. With the increase of oxidation time, the SiO₂ oxide layer transformed from amorphous to crystalline. Flower-like and flake-like SiO₂ structures were generated on the glass film during the oxidation process of SiC-ZrB₂-ZrC coating, which might be ascribed to the varying concentration of SiO. The oxide scale presented a two-layered structure ~130 µm thick after oxidation, consisting of a SiO₂-rich glass layer containing ZrO₂/ZrSiO₄ particles and a Si-O-Zr layer. The multiphase SiC-ZrB₂-ZrC ceramic coating exhibited much better oxidation resistance than monophase SiC, ZrB₂ or ZrC ceramic due to the synergistic effect among the different components.     

Preparation and ablation properties of SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C composites

A modification of the precursor infiltration pyrolysis (PIP) method was explored to prepare the integrated doped ceramic matrix and coating by the added SiC nanowires layer and shape-stabilization process. The epitaxial layer of SiC nanowires provided surficial attachments for the precursor. And the shape-stabilization process aggregated loose ceramic particles into a coating. Then the SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C (S/SZ-CZ/C) composite was simply prepared by the modified PIP method. The bonding strength between the coating and matrix of the S/SZ-CZ/C composite was improved. Through the ablation test, the mass and linear ablation rate of the S/SZ-CZ/C composite were 0.46 mg/s and 0.67 μm/s, which were 60.34 % and 74.91 % lower than those of the SiC nanowire-reinforced C/C–ZrC (S/CZ/C) composite, respectively. The integration of the coating and matrix enabled the formation of a continuous oxide layer of molten SiO₂ and ZrO₂ in the ablation process, which helped to block the oxygen and heat during the ablation test. Thus the ablation resistance of the materials was systematically and effectively improved.     

A gradient composite coating to protect SiC-coated C/C composites against oxidation at mid and high temperature for long-life service

To improve the oxidation resistance of carbon/carbon (C/C) composites at mid and high temperature, a gradient composite coating was designed and prepared on SiC-coated C/C composites by in situ formed-SiO₂ densifying the porous SiC-ZrSi₂ pre-coating. SiO₂ gradient distribution was conducive to inhibiting the cracking of the coating. A dual-layer structure with the outer dense layer and the inner microporous layer was formed in the coating during densifying. The dense layer had excellent oxygen diffusion resistance and the microporous layer alleviated CTE mismatch between SiC inner coating and dense layer. Moreover, ZrSiO₄ particles inhibited crack propagation and stabilized SiO₂ glass. Therefore, the coating can protect the C/C composites from oxidation at 1473 K, 1573 K and 1773 K for 810 h, 815 h and 901 h, respectively. The coated samples underwent 30 thermal cycles between room temperature and 1773 K without mass loss, exhibiting good thermal shock resistance.     

SiC/MoSi2 based coatings for Cf/C composites by two step pack cementation

In order to improve the oxidation resistance of C_f/Cs produced by chemical vapour infiltration, a multilayer coating based on silicon carbide and molybdenum disilicide was produced by two-step pack cementation technique. The inner SiC layer with a thickness up to 25 μm was obtained without promoted reaction additives by varying the composition, and thermal treatment conditions. The SiC/SiC-MoSi₂ coating was produced with a thickness up to 80 μm by two step pack cementation, considering the effect of the inner layer characteristic. The enhancement of the oxidation resistance, observed in SiC/SiC-MoSi₂ coated C_f/Cs by means of thermal analysis in flowing air up to 1500 °C, was due to the formation of SiO₂ promoted by the passive oxidation of silicon carbide and molybdenum disilicide.