Oxidation behaviour of three-dimensional woven C/SiC composites

Abstract

The oxidation behaviour of a three-dimensional woven C/SiC composite protected with an SiC seal coating and with an SiC coating combined with an SiO₂–B₂O₃ glassy coating have been respectively investigated through an experimental approach based on mass and flexural strength changes. Three main temperature domains exist for C/SiC composites protected with an SiC seal coating. At low temperatures (<700°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics. At an intermediate temperatures (between 700 and 1100°C), the oxidation kinetics are controlled by gas phase diffusion through a network of microcracks in the SiC matrix and coating. At high temperatures (>1100°C), the oxidation kinetics are controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating. Composites of C/SiC with an SiC/(SiO₂–B₂O₃) coating exhibit better oxidation resistance. The filling of the pores and the microcracks and the flow of the glassy coating at higher temperatures result in a global decrease of mass loss in the composites. By researching the relationship between the residual flexural strength and the mass variation in different temperature ranges, it is shown that the change in the residual flexural strength is dominated by the degradation of carbon phase.     

A multilayer MoSi2-SiC-B coating to protect SiC-coated carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a multilayer MoSi₂-SiC-B coating was prepared on the SiC-coated C/C composites by a simple and low-cost slurry method. The phase, microstructure and element distribution of the as-received coating were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The as-received coating could effectively protect C/C composites against oxidation at 850 °C in air for 100 h without mass loss, which exhibits better oxidation protective ability than the multilayer MoSi₂-SiC coating prepared by the same method. At intermediate temperature (850 °C), the excellent oxidation protective ability of the coating is mainly attributed to the formation of the molten B₂O₃ for sealing the microcracks and preventing oxygen from attacking the C/C substrate.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

Synthesis and characterization of Yb and Er based monosilicate powders and durability of plasma sprayed Yb2SiO5 coatings on C/C-SiC composites

Rare-earth silicates such as Yb₂SiO₅ and Er₂SiO₅ are promising environmental barrier coating materials for ceramic matrix composites. In this work, Yb₂SiO₅ and Er₂SiO₅ ceramic powders have been synthesized by solid-state reaction using Yb₂O₃, Er₂O₃ and SiO₂ as starting materials. The fabricated powders were subjected to spray drying treatment for subsequent synthesis of coatings by plasma spraying. The spray drying resulted in well-dispersed and spherical powder particles with good flowability. Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry and differential scanning calorimetry (TGA/DSC) and dilatometry were applied to study the microstructural and thermal characteristics of the powders. Ultra-high purity monosilicate powders formed as a result of heating treatments at 1400 °C in a box furnace for 20 h. TG/DSC revealed the genesis temperatures of the silicate formation (low temperature polymorphs) and also showed that the solid-state reactions to form Yb and Er based monosilicates proceeded without any weight-loss in the tested temperature range. The values of coefficients of thermal expansion (CTE) of the fabricated compounds are found to be 7.1 ppm/°C for Yb₂SiO₅ and 7.5 ppm/°C for Er₂SiO₅ by dilatometric measurements. Besides these studies, coating formation by plasma spraying of spray-dried Yb₂SiO₅ powders on the ceramic matrix composite specimen such as C/C-SiC has also been evaluated. Well-adhered and uniformed coatings result on composite specimens whose durability is tested by thermal cycling from ∼400 °C to 1500 °C in a gas burner rig.     

Blue photoluminescence from continuous freestanding β-SiC/SiOxCy/Cfree nanocomposite films with polycarbosilane (PCS) precursor

Novel continuous freestanding β-SiC/SiO_xC_y/C_free nanocomposite films, namely, β-SiC nano-crystals in amorphous SiO_xC_y and free C cluster matrix material, were fabricated by melt spinning the polycarbosilane (PCS) precursor. Effects of oxidation curing time and sintering temperatures on the photoluminescence (PL) properties of nanocomposite films were investigated. The PL spectra show two strong blue emissions at 416 nm and 435 nm, which are unchanged neither with oxygen content nor with β-SiC crystallite size. The PL intensity of the films is enhanced by increasing curing time when sintered at 1200 °C. However, a reversed trend is identified after the films were sintered at 1300 °C. Spectroscopy and microscopy studies indicate that the radiative recombination of carriers is ascribed to the oxygen mono- and di-vacancy from SiO_xC_y at the surfaces of β-SiC nano-crystals, whereas the photogeneration of carriers occurs in the β-SiC nano-crystals cores. The obtained results are expected to have important applications in advanced optoelectronic devices.     

Multi-layer CVD-SiC/MoSi2–CrSi2–Si/B-modified SiC oxidation protective coating for carbon/carbon composites

To further improve the oxidation resistance of coating for carbon/carbon (C/C) composites, a multi-layer CVD-SiC/MoSi₂–CrSi₂–Si/B-modified SiC coating was prepared on the surface of C/C composites by pack cementation and chemical vapour deposition method, respectively. The microstructures, oxidation and thermal shock resistance of the coating were studied. The influence of B content in pack powder on the microstructure and oxidation resistance of B-modified SiC coating was also investigated. The results show that the B-modified SiC coating prepared with 10 wt.% B exhibited the best oxidation protection ability for C/C composites at 1173 K. The multi-layer coatings could protect the C/C composites at 1173 K for 30 h and 1873 K for 200 h, and endure 30 thermal cycles between 1873 K and room temperatures. The oxidation resistance and thermal shock resistance is mainly attributed to their dense structure and self-sealing property.     

TiO2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain TiO₂ coatings on three dimensional (3-D) SiC fibre (Nicalon ®-type) and carbon fibre substrates. Colloidal suspensions of commercially available TiO₂ nanoparticles in acetylaceton with addition of iodine were used. The EPD parameters, i.e., deposition time and voltage, were optimised for each fibre type. Strongly adhered TiO₂ deposits with high particle packing density were obtained. Scanning electron microscopy observations revealed high penetration of the titania nanoparticles into the fibre preforms. The TiO₂ deposits were sintered at 800°C for 1 h in order to produce relatively dense and uniform TiO₂ coatings covering completely the SiC or carbon fibres. For the carbon fibre/TiO₂ system, an effort was made to produce a 3-D titania matrix composite by further infiltration of the porous fibrous preform with TiO₂ by slurry dipping and subsequent pressureless sintering. The 3-D carbon fibre reinforced TiO₂ matrix composites fabricated contained residual porosity, indicating further infiltration and densification steps are required to produce dense composites of adequate structural integrity. For SiC fibre fabrics, oxidation tests in air established the effectiveness of the TiO₂ coating as oxidation protective barrier at 1000°C. After 120 h the increase of weight due to oxidation of coated fibres was more than twice lower than that of the uncoated fibres. TiO₂ coated SiC fibre preforms are attractive materials for manufacturing hot gas filters and as reinforcing elements for ceramic matrix composites.     

Oxidation behaviour and protection of carbon/carbon composites

The oxidation behaviour of carbon/carbon composite materials and graphite (in cube form), in flowing air, has been studied in the temperature range 500 to 1100 °C. Gasification for unprotected samples occurred at temperatures around 500 °C. SiC coatings offered only limited protection below their intrinsic protection range due to the diffusion of oxygen along microcracks. Diffusional control was more significant for thicker coatings. However, the use of boron oxide applied on an underlayer of SiC, gave good protection for extended periods at temperatures up to 1000 °C, due to microcrack sealing. The use of borate coatings, both with and without an SiC underlayer, was limited by the volatility of the borate.     

Cyclic oxidation behavior of glass-ceramic composite coatings on superalloy K38G at 1100 °C

The SiO₂-Al₂O₃-ZrO₂-CaO-ZnO glass-ceramic composite coatings (GC), nanocrystalline NiCoCrAlY coating, and their combinations (bi-layer GC/NiCoCrAlY) were prepared on K38G specimens. The thicknesses of the glass-ceramic coatings and the NiCoCrAlY coatings were about 10 μm and 20 μm, respectively. Cyclic oxidation tests were carried out at 1100 °C for 120 cycles. Microstructures of the specimens before and after oxidation tests were characterized by SEM, EDS and XRD. The glass-ceramic coatings with or without a NiCoCrAlY intermediate layer improved the isothermal and cyclic oxidation resistance of the Ni-base superalloy K38G at 1100 °C, and performed better than the NiCoCrAlY coatings. An alumina layer formed at the glass/metal interfaces of the specimens coated by the glass-ceramic coatings with or without a NiCoCrAlY intermediate layer. The NiCoCrAlY intermediate layer was beneficial to the cyclic oxidation resistance of the glass-ceramic coatings.     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni-CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni-CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.     

Thermal stability of interfaces in Ti-6Al-4V reinforced by SiC Sigma fibres

Interfacial reactions in Ti-6Al-4V/SiC Sigma fibres (coated with carbon and TiB₂) were studied at different temperatures (600, 700 and 1000 °C). Interface microstructure was investigated by scanning electron microscopy and Auger electron spectroscopy. A simulation of the chemical phenomena occurring at the interfaces was carried out using powders of pure titanium, carbon and TiB₂; the reaction products were identified by X-ray diffraction. The double coating of Sigma fibres is effective in delaying detrimental reactions with the matrix. At the interfaces matrix/TiB₂ and TiB₂/C, the TiB and TiC_x phases form, respectively. The protective coating of fibres shows a lifetime greater than 1000 and 750 h at 600 and 700 °C, respectively.     

Degradation behaviour of sputtered Co–Al coatings on superalloy

Degradation behaviour of sputtered Co–Al coatings on Superni-718 substrate has been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 900 °C for up to 100 thermal cycles between the peak and room temperatures. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS, AFM, and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al₂O₃ and spinel phases such as CoCr₂O₄ and CoAl₂O₄ in scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compare to bare substrate in air at 900 °C temperature up to 100 cycles.     

Oxidation behaviour of SiC-platelets and particulates-reinforced Al2O3/ZrO2 matrix composites

The oxidation behavior of hot-pressed SiC-platelets and particulates-reinforced Al₂O₃/ZrO₂ composites has been studied in an electric furnace at atmospheric pressure at different temperatures. The mass gain as a result of transformation of SiC into SiO₂ is described as a function of oxidation temperature, time and type of SiC. The mass gain up to 1100°C was low, but increased strongly at 1350°C. The oxidation process follows a parabolic rate at all oxidation temperatures. Oxidation of composites containing SiC-particulates is higher than the corresponding one containing SiC-platelets. The activation energy, obtained in the present investigation, was 297–333 kj/mol. Diffusion of oxygen and carbon monoxide through the matrix and oxide products appeared to be the rate controlling process. The reaction products were aluminosilicate glass phase and mullite as indicated by SEM and EDX.     

High temperature oxidation behavior of Ti0.5Al0.5N coating and Ti0.5Al0.4Si0.1N coating

Good oxidation resistance of hard coatings is important for cutting tools. Ti_0.5Al_0.5N coating and Ti_0.5Al_0.4Si_0.1N coating were deposited by cathodic arc evaporation and their oxidation behavior at 850 °C, 900 °C and 1000 °C was compared. The effect of Si addition on the oxidation resistance of Ti_0.5Al_0.4Si_0.1N was investigated. Results show that the oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating at 850-1000 °C is superior to Ti_0.5Al_0.5N coating. The improved oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating can be ascribed to the combined action of Al₂O₃ and SiO₂ barrier layer, the reduction of columnar structure and the refinement of grains. In particular, Si addition increases the diffusion coefficient of Al and promotes the preferential formation of Al₂O₃ barrier layer.     

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part II Pyrolysis and characterisation

The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state ²⁹Si and ¹³C NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 °C in argon, generally produced silicon oxycarbide (SiO _x C _y ) as the second major phase with -SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 °C, the silicon oxycarbide phase decomposed to give -SiC. Silica (SiO₂) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 °C, possibly yielding SiO, CO and SiC. At 1500 °C, crystalline -cristobalite was found as a minor phase with -SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of -SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 ° in vacuum, the cured PCS converted to almost stoichiometric -SiC.     

ZrB2 coating for the oxidation protection of carbon fiber reinforced silicon carbide matrix composites

In order to improve the anti-oxidation performance of carbon fiber reinforced silicon carbide matrix (C/SiC) composites, ZrB₂ coating was prepared on the surface of C/SiC composites by a two-step technique of pack cementation method. The anti-oxidation properties of coated composites were investigated. The results showed that ZrB₂ coating was obtained by the method, which was homogenous and dense. The weight loss percentage of the coated composites was only 1.52 after oxidation in air at 1500 °C for 30 min, which exhibited excellent oxidation resistance.     

Effects of water vapor on corrosion behaviors of C/SiC in oxidizing atmosphere containing Na2SO4 vapor

Corrosion of a C/SiC composite has been investigated in the atmosphere containing oxygen, water vapor and sodium sulfate vapor at the temperatures range from 1000 to 1500 °C. The effect of water vapor on the corrosion mechanism of C/SiC were discussed based on the weight change, the residual strength change, the microstructure and calculated results from FactSage. The corrosion of C/SiC is attributed to (i) the permeation of gas through the SiO₂ film below 1300 °C, (ii) the diffusion of oxidant through pores caused by bubbles broken in the SiO₂ film above 1300 °C. The water vapor does not change the corrosion mechanism of C/SiC composite but the temperature range in which the corrosion mechanism works by accelerating the oxidation of SiC and the corrosion of SiO₂.     

Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate Part II Thermo-oxidative stability of Cs2MoOS3/Si3N4mixtures

Cesium oxythiomolybdate (Cs₂MoOS₃) may be an excellent high temperature lubricant, providing a friction coefficient below 0.2 at 650°C. However, oxidation products provide the lubrication above 400°C. Lubricant effectiveness depends strongly on the composition of the substrate materials in contact, such as Si₃N₄, suggesting that tribochemical and/or thermal reactions at the interface produce new compounds. The thermo-oxidative stability of Cs₂MoOS₃/Si₃N₄and Cs₂MoOS₃/SiO₂mixtures have been evaluated between room temperature and 1000°C in air. The transition temperatures and oxidation products were identified. The thermal chemistry of Cs₂MoOS₃/Si₃N₄mixtures was significantly different than that of Cs₂MoOS₃alone, largely due to the oxidation of Si₃N₄to glassy SiO₂. Cesium oxythiomolybdate formed cesium oxides, which melted below 600°C. As SiO₂is formed, the cesium oxides diffused into it, creating a cesium silicate glass. Also, Cs₂MoO₄was preferentially formed over complex cesium molybdates and molybdenum oxides. In a tribological application, Cs₂MoO₄, oxides, and cesium silicate glass may be formed at contacting interfaces from Cs₂MoOS₃films deposited on Si₃N₄substrates. Lubrication would be provided as the shear strength of these compounds decreases with increasing temperature.     

Effects of water vapor on corrosion behaviors of C/SiC in oxidizing atmosphere containing Na2SO4 vapor

Corrosion of a C/SiC composite has been investigated in the atmosphere containing oxygen, water vapor and sodium sulfate vapor at the temperatures range from 1000 to 1500 °C. The effect of water vapor on the corrosion mechanism of C/SiC were discussed based on the weight change, the residual strength change, the microstructure and calculated results from FactSage. The corrosion of C/SiC is attributed to (i) the permeation of gas through the SiO₂ film below 1300 °C, (ii) the diffusion of oxidant through pores caused by bubbles broken in the SiO₂ film above 1300 °C. The water vapor does not change the corrosion mechanism of C/SiC composite but the temperature range in which the corrosion mechanism works by accelerating the oxidation of SiC and the corrosion of SiO₂.     

Micro-structural and tensile strength analyses on the magnesium matrix composites reinforced with coated carbon fiber

Magnesium matrix composites reinforced with SiO₂ coated carbon fibers have been investigated, with an emphasis given on the relation between the material strength and interfacial microstructure. The composites were studied as a function of aluminium (Al) content that is varied between 0 and 9 wt%. The obtained results indicate that the reactivity at the C/Mg–Al interface of the composite can be controlled by varying the Al content. The low Al content in C/Mg–1Al has been completely dissolved in the matrix with no segregation even after solidification, leading to the best mechanical performance. If the Al content is increased to ≥3 wt% (composites such as C/AZ31 and C/AZ91), the SiO₂ coatings are fully depleted due to an extensive formation of carbides at the interface. The precipitates are further identified as Al₂MgC₂ phase that is similar to binary carbide Al₄C₃. SiO₂ coating on the fiber layer prior to fabrication of composite is found to be a promising way to suppress the carbide formation and enable the use of Mg–Al matrix with appropriate Al content.