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.     

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.     

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.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

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.     

HfC nanowires to improve the toughness and oxidation resistance of Si-Mo-Cr/SiC coating for C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites, a dense HfC nanowire-toughened Si-Mo-Cr/SiC multilayer coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, thermal shock and isothermal oxidation resistance of the coating were investigated. HfC nanowires could improve the toughness of the coating and suppress the coating cracking. After incorporating HfC nanowires in the coating, both of the thermal shock and isothermal oxidation resistance of the coating were obviously improved. The multilayer coating with HfC nanowires could effectively protect C/C composites at 1773 K for 270 h, whose weight loss is only 0.19%. The good oxidation resistance is mainly attributed to the formation of a compound glass layer containing SiO₂ and Cr₂O₃.     

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%.     

SiC nanowire-toughened MoSi2-WSi2-SiC-Si multiphase coating for improved oxidation resistance of C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperature, a SiC nanowire-toughened MoSi₂-WSi₂-SiC-Si multiphase coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, mechanical properties and oxidation resistance of the coating were investigated. After the introduction of SiC nanowires, the elastic modulus, hardness, and fracture toughness of the MoSi₂-WSi₂-SiC-Si coating were increased by 25.48%, 4.09% and 45.03%, respectively. The weight loss of the coated sample with SiC nanowires was deceased from 4.83–2.08% after thermal shock between 1773 K and room temperature for 30 cycles and the weight loss is only 3.24% after isothermal oxidation at 1773 K in air for 82 h. The good oxidation resistance of the coating is mainly attributed to that SiC nanowires can effectively inhibit the propagation of cracks in the coating by the toughening mechanisms including bridging and pull-out.     

Dynamic oxidation resistance and residual mechanical strength of ZrB2-CrSi2-SiC-Si/SiC coated C/C composites

To improve oxidation resistance of carbon/carbon (C/C) composites, a multiphase double-layer ZrB₂-CrSi₂-SiC-Si/SiC coating was prepared on the surface of C/C composites by pack cementation. Thermogravimetry analysis showed that the as-prepared coating could provide effective oxidative protection for C/C composites from room temperature to 1490 °C. After thermal cycling between 1500 °C and room temperature, the fracture behaviors of the as-prepared specimens changed and their residual flexural strengths decreased as thermal cycles increased. The specimen after 20 thermal cycles presented pseudo-plastic fracture characteristics and relatively high residual flexural strength (83.1%), while the specimen after 30 thermal cycles failed catastrophically without fiber pullout due to the severe oxidation damage of C/C substrate especially the brittleness of the reinforcement fibers.     

Oxidation protection of C/C composites with a multilayer coating of SiC and Si + SiC + SiC nanowires

An oxidation protective Si–SiC coating with randomly oriented SiC nanowires was prepared on the SiC-coated carbon/carbon (C/C) composites by a two-step technique. First, a porous network of SiC nanowires was produced using chemical vapor deposition. This material was subjected to pack cementation to infiltrate the porous layer with a mixture of Si and SiC. The nanowires in the coating could efficiently suppress the cracking of the coating by various toughening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. The results of thermogravimetric analysis and thermal shock showed that the coating had excellent oxidation protection for C/C composites between room temperature and 1500 °C. These results were confirmed by two additional oxidation experiments conducted at temperature of 900 and 1400 °C, which demonstrated that the coating could efficiently protect C/C composites from oxidation at 900 °C for more than 313 h or at 1400 °C for more than 112 h.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

Enhanced bonding strength and thermal cycling performance of MoSi2–CrSi2–SiC–Si coating for carbon/carbon composites by surface modification via blasting treatment

Before the preparation of MoSi₂–CrSi₂–SiC–Si coating, blasting treatment of carbon/carbon (C/C) composites, as a surface modification method, was conducted under oxyacetylene torch. MoSi₂–CrSi₂–SiC–Si coating was prepared on the treated C/C composites by pack cementation, where an interlock interface was formed between the coating and the C/C substrate. After blasting treatment, the thermal expansion coefficient mismatch between the coating and C/C substrate was alleviated efficiently, and the bonding strength of the coating was increased by 45.6% and reached 26.2 MPa. To simulate the real working condition, thermal cycling test was conducted under oxyacetylene torch from 1600 °C to room temperature to construct an environment of combustion gas erosion. Due to the improvement of bonding strength and the alleviation of thermal expansion coefficient mismatch between the coating and the C/C substrate, thermal cycling performance of MoSi₂–CrSi₂–SiC–Si coating was enhanced. After 25 thermal cycles, the mass loss of the coated C/C composites without blasting treatment was up to 2.4%, and the C/C substrate was partially exposed. In contrast, the mass loss of the coated C/C composites with blasting treatment was only 1.1%.     

A double layer nanostructure SiC coating for anti-oxidation protection of carbon/carbon composites prepared by chemical vapor reaction and chemical vapor deposition

A double layer nanostructure SiC coating was prepared by chemical vapor reaction and chemical vapor deposition to protect carbon/carbon composites from oxidation. The obtained dense coating reveals a typical crystalline structure and combines well with the substrate. The outer layer of the coating consists of SiC nanocrystals and nanowires, whereas the inner layer is mainly composed of SiC microcrystals, nanocrystals and nanowires. The oxidation and cyclic thermal shock test performed at 1400 °C in air demonstrates that the prepared dense nanostructure coating has excellent anti-oxidation behavior and thermal shock resistance at high temperature. After 400 h oxidation and 34 cycles of thermal shock from 1400 °C to room temperature, the weight loss of the coated sample is only 1.67%. In the oxidation process, the amorphous silica formed at the beginning of the oxidation crystallizes to cristobalite as oxidation time increased. The formation of cristobalite resulted in micro-cracks formed along grain boundaries in the cyclic thermal shock test. As only cracks are formed on the coating surface, it can be concluded that the formation of the penetration cracks may be the reason for the weight loss of the SiC coated composite.     

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.     

Oxidation treatments for SiC particles and its compatibility with glass

Different thermal treatments were performed to produce a protective coating on the surface of SiC particles in order to allow their incorporation in a glass matrix. These oxidation treatments were carried out in air at different temperatures ranging from 800 °C to 1500 °C and different times at 1200 °C (10 min–48 h). The oxidation kinetics followed the Deal–Grove model and the thickness of the protective coating increased with temperature and SiC particle size. Protected SiC particles with different particle sizes were incorporated in a borosilicate glass. With small particles sizes foam glasses were obtained, whereas particles with higher grain size, i.e., higher coating thickness, were stable in the glass matrix and a smooth glass was obtained.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

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.     

Mullite whisker toughened mullite coating to enhance the thermal shock resistance of SiC pre-coated carbon/carbon composites

In order to improve the thermal shock resistance of the coated carbon/carbon (C/C) composites, a mullite whisker toughened mullite coating was fabricated on the surface of SiC pre-coated C/C composites (SiC-C/C) by molten-salt method with a later hot dipping process. The phase compositions, surface and cross-section microstructures, high temperature thermal shock resistance of the as-prepared multi-layer coatings were investigated. Results show that the introduction of mullite whiskers can effectively improve the density of the mullite outer coating and decrease the cracking of the coating during the thermal shock cycle process. After 100 times thermal shock cycles between 1773 K and room temperature, only 1.87 × 10⁻³ g cm⁻² weight loss has been detected, indicating the achievement of the excellent thermal shock resistance.     

Oxidation behavior of SiC/glaze-precursor coating on carbon/carbon composites

In order to improve the oxidation behavior of carbon/carbon composites in a wide range of temperature, a new SiC/glaze-precursor coating was developed.The SiC layer was produced by slurry and sintering, while the glaze precursor layer was prepared by slurry and drying. The microstructures and phase compositions of the coating were analyzed by SEM and XRD, respectively. The oxidation resistance of the coated composites was investigated using both isothermal and temperature-programmed thermogravimetric analysis in the temperature range from room temperature to 1600 °C. The results showed that the oxidation behavior of the coating was mainly controlled by the diffusion of oxygen during the test.The coating showed excellent oxidation resistance and self-healing ability in a wide range of temperature.