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.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

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 temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70 µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600 °C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44 h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO₂ layer, porous SiO₂ layer and clusters of honeycomb-like SiO₂ grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO₂ declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.     

Microstructure and ablation behavior of SiC coated C/C–SiC–ZrC composites prepared by a hybrid infiltration process

In this study, C/C–SiC–ZrC composites coated with SiC were prepared by precursor infiltration pyrolysis combined with reactive melt infiltration. The pyrolysis behavior of the hybrid precursor was investigated using thermal gravimetric analysis-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy techniques. The microstructure and ablation behavior of the composites were also investigated. The results indicate that the composites exhibit an interesting structure, wherein a ceramic coating composed of SiC and a small quantity of ZrC covers the exterior of the composites, and the SiC–ZrC hybrid ceramics are partially embedded in the matrix pores and distributed around the carbon fibers as well. The composites exhibit good ablation resistance with a surface temperature of over 2300 °C during ablation. After ablation for 120 s, the mass and linear ablation rates of the composites are 0.0026 g/s and 0.0037 mm/s, respectively. The great ablation resistance of the composites is attributed to the formation of a continuous phase of molten SiO₂ containing SiC and ZrO₂, which seals the pores of the composites during ablation.     

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.     

High-temperature erosion resistance and aerodynamic oxidation mechanism of multi-layer MoSi2–CrSi2–Si/SiC coated carbon/carbon composites in a wind tunnel at 1873 K

The high-temperature erosion resistance of multi-layer MoSi₂–CrSi₂–Si/SiC coated carbon/carbon (C/C) composites was investigated in a wind tunnel. To study the aerodynamic oxidation mechanism and analyze the failure of the coated C/C composites, the shear force and bending moment distribution of the tested specimens in a wind tunnel were calculated. Flexural strengths and thermogravimetric analysis of the coated specimens were measured. These results show that the multi-layer MoSi₂–CrSi₂–Si/SiC antioxidation coating can protect the C/C composites from high-temperature erosion in a wind tunnel at 1873 K for more than 86 h. Due to the high viscosity of SiO₂, the multi-layer coating lacked effective oxidation resistance from 900 to 1500 K, resulting in extensive mechanical damage and the fracture of the tested specimens.     

Ablation resistance of HfC–SiC coating prepared by supersonic atmospheric plasma spraying for SiC-coated C/C composites

In order to improve the ablation properties of carbon/carbon composites, HfC–SiC coating was deposited on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC–SiC coating were characterized by SEM and XRD. The ablation resistance test was carried out by oxyacetylene torch. The results show that the structure of coating is dense and the as-prepared HfC–SiC coating can protect the C/C composites against ablation. After ablation for 30 s, the linear ablation rate and mass ablation rate of the coating are −0.44 μm/s and 0.18 mg/s, respectively. In the ablation center region, a Hf–Si–O compound oxide layer is generated on the surface of HfC–SiC coating, which is conducive to protecting the C/C composites from ablation. With the ablation time increasing to 60 s, the linear ablation rate and mass ablation rate are changed to −0.38 μm/s and 0.26 mg/s, respectively. Meanwhile, the thickness of the outer Hf–Si–O compound layer also increases.     

Ablation behaviour of a TaC coating on SiC coated C/C composites at different temperatures

To improve the ablation resistance of carbon/carbon (C/C) composites, a TaC coating was prepared by supersonic plasma spraying on SiC coated C/C composites. The microstructure and morphology of the coatings were characterised by Scanning Electron Microscopy and X-ray diffraction. The ablation properties were studied at different temperatures under oxyacetylene torch. At 2100 °C, the oxides were blown away and resulted in high ablation rates: 1.2×10^?2 mm/s and 3.9×10^?3 g/s. However, most oxides can remain in ablation centre and serve as a coating at low temperature (1900 and 1800 °C). Therefore, the TaC/SiC coated samples exhibited zero linear ablation rate and lower mass ablation rate.     

Ablation resistance of HfC coating reinforced by HfC nanowires in cyclic ablation environment

To improve the ablation resistance of carbon/carbon composites in cyclic ablation environment, SiC/HfC ceramic coating reinforced by HfC nanowires was prepared. The microstructure, bonding strength, coefficient of thermal expansion and cyclic ablation resistance of the as-prepared coating were investigated. After incorporating HfC nanowires, the bonding strength between inner SiC coating and outer HfC coating was increased. HfC nanowires could relieve the coefficient of thermal expansion mismatch between inner and outer coating and improve the toughness of the outer coating. By introducing HfC nanowires, the coated sample’s cyclic ablation resistance was improved. After cyclic ablation under oxyacetylene flame for 60 s × 3, the mass and linear ablation rates of the coated sample with HfC nanowires were only 0.444 mg/s and −0.767 μm/s, respectively.     

Effects of porous C/C density on the densification behavior and ablation property of C/C–ZrC–SiC composites

C/C–ZrC–SiC composites were prepared by precursor infiltration and pyrolysis process using a mixture solution of organic zirconium-containing polymer and polycarbosilane as precursors. Porous carbon/carbon (C/C) composites with density of 0.92, 1.21 and 1.40 g/cm³ were used as preforms, and the effects of porous C/C density on the densification behavior and ablation resistance of C/C–ZrC–SiC composites were investigated. The results show that the C/C preforms with a lower density have a faster weight gain, and the obtained C/C–ZrC–SiC composites own higher bulk density and open porosity. The composites fabricated from the C/C preforms with a density of 1.21 g/cm³ exhibit better ablation resistance with a surface temperature of over 2400 °C during ablation. After ablation for 120 s, the linear and mass ablation rates of the composites are as low as 1.02 × 10⁻³ mm/s and −4.01 × 10⁻⁴ g/s, respectively, and the formation of a dense and continuous coating of molten ZrO₂ solid solution is the reason for their great ablation resistance.     

A SiC–Si–ZrB2 multiphase oxidation protective ceramic coating for SiC-coated carbon/carbon composites

In order to improve the oxidation protective ability of SiC-coated carbon/carbon (C/C) composites, a SiC–Si–ZrB₂ multiphase ceramic coating was prepared on the surface of SiC-coated C/C composite by the process of pack cementation. The microstructures of the coating were characterized using X-ray diffraction and scanning electron microscopy. The coating was found to be composed of SiC, Si and ZrB₂. The oxidation resistance of the coated specimens was investigated at 1773 K. The results show that the SiC–Si–ZrB₂ can protect C/C against oxidation at 1773 K for more than 386 h. The excellent oxidation protective performance is attributed to the integrity and stability of SiO₂ glass improved by the formation of ZrSiO₄ phase during oxidation. The coated specimens were given thermal shocks between 1773 K and room temperature for 20 times. After thermal shocks, the residual flexural strength of the coated C/C composites was decreased by 16.3%.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

A MoSi2-SiOC-Si3N4/SiC anti-oxidation coating for C/C composites prepared at relatively low temperature

In this study, SiC coating for C/C composites was prepared by pack cementation method at 1773 K, and MoSi₂-SiOC-Si₃N₄ as an outer coating was successfully fabricated on the SiC coated samples by slurry method at 1273 K. The microstructure and phase composition of the coatings were analyzed. Results showed that a porous β-SiC inner coating and a crack-free MoSi₂-SiOC-Si₃N₄ coating are formed. Effect of Si₃N₄ content on the oxidation resistance of the coated C/C composites at 1773 K in air was also investigated. The weight loss curves revealed that introducing the appropriate proportion of Si₃N₄ could improve the oxidation resistance of coating. The MoSi₂-SiOC/SiC coated C/C sample had an accelerated weight loss after oxidation in air for 20 h. However, the coating containing 45% Si₃N₄ could protect C/C composition from oxidation for 100 h with a minute weight loss of 0.63%.     

Design of an inlaid interface structure to improve the oxidation protective ability of SiC–MoSi2–ZrB2 coating for C/C composites

To improve the oxidation protective ability of SiC–MoSi₂–ZrB₂ coating for carbon/carbon (C/C) composites, pre-oxidation treatment and pack cementation were applied to construct a buffer interface layer between C/C substrate and SiC–MoSi₂–ZrB₂ coating. The tensile strength increased from 2.29 to 3.35 MPa after pre-oxidation treatment, and the mass loss was only 1.91% after oxidation at 1500 °C for 30 h. Compared with the coated C/C composites without pre-oxidation treatment, after 18 thermal cycles from 1500 °C and room temperature, the mass loss was decreased by 30.6%. The improvements of oxidation resistance and mechanical property are primarily attributed to the formation of inlaid interface between the C/C substrate and SiC–MoSi₂–ZrB₂ coating.     

Fabrication of cylindrical SiCf/Si/SiC-based composite by electrophoretic deposition and liquid silicon infiltration

Cylindrical SiC-based composites composed of inner Si/SiC reticulated foam and outer Si-infiltrated SiC fiber-reinforced SiC (SiC_f/Si/SiC) skin were fabricated by the electrophoretic deposition of matrix particles into SiC fabrics followed by Si-infiltration for high temperature heat exchanger applications. An electrophoretic deposition combined with ultrasonication was used to fabricate a tubular SiC_f/SiC skin layer, which infiltrated SiC and carbon particles effectively into the voids of SiC fabrics by minimizing the surface sealing effect. After liquid silicon infiltration at 1550 °C, the composite revealed a density of 2.75 g/cm³ along with a well-joined interface between the inside Si/SiC foam and outer SiC_f/Si/SiC skin layer. The results also showed that the skin layer, which was composed of 81.4 wt% β-SiC, 17.2 wt% Si and 1.4 wt% SiO₂, exhibited a gastight dense microstructure and the flexural strength of 192.3 MPa.     

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.     

Microstructure and ablation mechanism of C/C–SiC composites

C/C–SiC composites were prepared by molten infiltration of silicon powders, using porous C/C composites as frameworks. The porosities of the C/C–SiC composites were about 0.89–2.8 vol%, which is denser than traditional C/C composites. The ablation properties were tested using an oxyacetylene torch. Three annular regions were present on the ablation surface. With increasing pyrocarbon fraction, a white ceramic oxide layer formed from the boundary to the center of the surface. The ablation experimental results also showed that the linear and mass ablation rates of the composites decreased with increasing carbon fraction. Linear SiO₂ whiskers of diameter 800 nm and length approximately 3 μm were formed near the boundaries of the ablation surfaces of the C/C–SiC composites produced with low-porosity C/C frameworks. The ablation mechanism of the C/C–SiC composites is discussed, based on a heterogeneous ablation reaction model and a supersaturation assumption.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

The ablation behavior and mechanical property of C/C-SiC-ZrB2 composites fabricated by reactive melt infiltration

In order to improve the ablation resistance of carbon/carbon (C/C) composites, SiC-ZrB₂ di-phase ceramic were introduced by reactive melt infiltration. The ablation properties of these composites were evaluated by oxyacetylene torch with a heat flux of 2.38 MW/m² for 60 s. Compared with the pure C/C composites, the C/C-SiC-ZrB₂ composites show a significant improvement in the ablation resistance, and the linear and mass ablation rates decreased from 10.28×10⁻³ mm/s to 6.72×10⁻³ mm/s and from 3.08×10⁻³ g/s to 0.61×10⁻³ g/s, respectively. After ablation test, the flexural strength retentions of the C/C and C/C-SiC-ZrB₂ composites near the ablated center region are 39.7% and 81.6%, respectively. The higher strength retention rate of C/C-SiC-ZrB₂ composites was attributed to the introduction of SiC-ZrB₂ ceramic phases, which have excellent ablation resistant property. During ablation test, an ‘embedding structure’ of Zr-O-Si glass layer was formed, which could act as an effective barrier for oxygen and heat. The oxide ceramic coating could protect the C/C-SiC-ZrB₂ composites from further ablation, and thus contribute to retaining the mechanical property of C/C-SiC-ZrB₂ composites after ablation.