SiCn/SiC oxidation protective coating for carbon/carbon composites

Nano-SiC (SiC_n) coating was deposited on SiC pre-coated C/C composites by a hydrothermal electrophoretic deposition. The phase compositions, surface and cross-section microstructures, and anti-oxidation properties of the multilayer coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to prepare smooth and homogeneous SiC_n coating on SiC-C/C composites. The as-prepared SiC_n/SiC multilayer coatings can effectively protect C/C composites from oxidation in air at 1773 K for 202 h with a weight loss of 0.79% and at 1873 K for 64 h with a weight loss of 1.3%.     

ZrC ablation protective coating for carbon/carbon composites

A zirconium carbide (ZrC) protective coating was deposited on carbon/carbon (C/C) composites by atmospheric pressure chemical vapor deposition. The phase compositions, surface and cross-section microstructures, and anti-ablative properties of the coatings were investigated. Results show that the method is an effective route to prepare a dense and thick ZrC coating on C/C composites. The coating can effectively protect C/C composites from ablation for 240 s in an oxy-acetylene torch system with a mass ablation rate of 1.1 × 10⁻⁴ g/cm² s and a linear ablation rate of 0.3 × 10⁻³ mm/s.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

Mullite-Al2O3-SiC oxidation protective coating for carbon/carbon composites

In order to exploit the unique high temperature mechanical properties of carbon/carbon (C/C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al₂O₃-mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C/C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C/C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface.     

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.     

SiC coating with high crack resistance property for carbon/carbon composites

A novel SiC coating with a relatively high crack resistance property (crack extension force (G_C): 12.0 J·m^?2) and outstanding thermal shock resistance was achieved merely by pack cementation. Compared with the conventional SiC coating with Al₂O₃ addition (AOSC2), SiC coating with Al–B–C additions (ABSC2) possesses refined and denser microstructure owing to different effects in promoting SiC densification under different additions. Therefore, the improvement in microstructures results in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (between 1500 °C and room temperature).     

A multilayer coating of dense SiC alternated with porous Si-Mo for the oxidation protection of carbon/carbon silicon carbide composites

In order to improve the oxidation behavior of carbon/carbon silicide carbide composites prepared by liquid silicon infiltration of carbon/carbon porous preforms, a multilayer coating of dense SiC alternated with porous Si-Mo was prepared by chemical vapor deposition combined with slurry painting. Oxidation test showed that weight loss of the coated sample was only 0.25% after 150 h oxidation in air at 1673 K. And the coated sample gained weight in the course of 46 cycles of thermal shock test between 1673 K and 373 K. The coating remained intact during the two kinds of tests and no obvious failure was found. The excellent oxidation protective ability and thermal shock resistance of the SiC/Si-Mo coating can be attributed to the alternated structure.     

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.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

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.     

SiC coating toughened by SiC nanowires to protect C/C composites against oxidation

A dense SiC coating toughened by SiC nanowires was prepared on carbon/carbon (C/C) composites using a two-step technique of chemical vapor deposition (CVD) to protect them against oxidation. The morphologies and crystalline structures of the coatings were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. SiC nanowires played a role in decreasing the size of the cracks and improving the thermal shock resistance of the coating. The result of thermal shock between 1773 K and room temperature for 21 times indicates that, compared with the SiC coating without SiC nanowires, the average size of the cracks in the SiC coating toughened with SiC nanowires reduced from 5 ± 0.5 to 3 ± 0.5 μm. The weight loss of the SiC coated C/C composites decreased from 9.32 to 4.45% by the introduction of SiC nanowires.