Effect of oxidation and residual stress on mechanical properties of SiC seal coated C/SiC composite

The effect of oxidation and thermal residual stress on mechanical properties of SiC seal coated C/SiC composite at ambient temperature and high temperature were studied. The oxidation of SiC seal coated C/SiC composite at 1300 and 1500 °C resulted in carbon fibres burn area near through thickness micro cracks in the SiC seal coating. With the increase in exposure time, the formation of SiO₂ layer in SiC matrix near carbon fibres burns area was found. Residual mechanical properties of SiC seal coated C/SiC composite after exposure in air show significant degradation. First time, a continuous measurement of Young's modulus with temperature of C/SiC composite was carried out using an impulse excitation technique. The effect of relaxation of thermal residual stress on mechanical properties was observed with the help of continuous measurement of Young's modulus as a function of temperature in an inert atmosphere.     

Synthesis of a silicon carbide coating on carbon fibers by deposition of a layer of pyrolytic carbon and reacting it with silicon monoxide

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

Molten salt synthesis of a SiC coating on graphite flakes for application in refractory castables

A silicon carbide coating was formed on the surface of graphite flakes by reaction of molten Si with carbon at 1100–1300 °C in a 95%KCl-5%NaF molten salt under Ar atmosphere. The effect of temperature and Si/graphite ratio in the initial mixture on the quality and the amount of SiC were investigated by XRD and SEM/EDS analyses. Also, the water wettability, oxidation resistance and zeta potential of un-coated and coated graphite were examined by TGA analysis and sedimentation test. The results show the amount of coating to increase in the coated flakes with increasing temperature and Si/graphite ratio. The SiC coating improves water wettability of graphite and acts as a protective layer to enhance oxidation resistance. The zeta potential of coated graphite was also increased which indicated a better dispersion in water based systems. These improvements in both the water dispersivity and oxidation resistance of SiC coated graphite would make it as promising candidate raw materials for application in C-containing refractory castables.     

Microstructural evolution of multi-walled carbon nanotubes in the presence of mixture of silicon and silica powders at high temperatures

Microstructural evolution of multi-walled carbon nanotubes (MWCNTs) in the presence of mixture of silicon and silica powders in a coke bed is studied in the temperature range of 1000–1500 °C by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and thermogravimetry–differential scanning calorimetry (TG–DSC). The results showed that a thin amorphous SiO₂ coating was formed on the surface of MWCNTs at the temperature below 1300 °C. With the increase of the treated temperature, the coating became thicker, 3–7 nm in thickness at 1400 °C and a maximum of 10 nm at 1500 °C. Meanwhile, SiC nanowires and SiC nanocrystals around Ni catalyst at the tip of MWCNTs were formed at 1400 °C and 1500 °C, which were related to the vapor–vapor (V–V) and vapor–liquid–solid (V–L–S) reactions between SiO (g) and CO (g) or C (s), respectively. The oxidation resistance of all the treated MWCNTs was better than that of as-received ones. The oxidation peak temperature reached 804.2 °C for the treated MWCNTs, much higher than 652.2 °C for as-received ones.     

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

Due to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.     

Effect of Temperature on Oxidation Behavior of 3‐Dimensional C/SiC Composites Coated by Different Structural SiC and a‐BCx Coatings in Static Wet Oxygen Atmosphere

The effect of temperature on oxidation behavior of 3‐dimensional C/SiC composites coated with SiC and a‐BC_x coatings was investigated. Below 1000°C, C/SiC composites with SiC coating (sample SSS) showed low residual strength, while the composites with a‐BC_x coating (sample BBB) and the composites with a‐BC_x/a‐BC_x/SiC coatings (sample BBS) showed high residual strength. Above 1000°C, samples SSS and BBS showed high residual strength, while sample BBB showed the opposite results. Sample BBS presented better oxidation resistance during 700–1200°C. The oxidation temperature made an effect on self‐sealing mode of sample BBS.     

Scheelite coatings on SiC fiber: Effect of coating temperature and atmosphere

Scheelite coating was deposited on SiC fiber tows from various liquid-phase precursors followed by heat treatments between 900 °C and 1100 °C in different atmospheres. The tensile strength was fully retained for the coated fibers treated at 900 °C in vacuum. Subsequent heat treatment at 1100 °C in Ar had little effect on the fiber strength, which is explained by the excepted good thermal stability between the scheelite coating and SiC fiber. However, larger strength degradation and poor spool ability of coated fibers prepared in Ar/air were found. Assisted oxidation of SiC fiber by calcium salts is suggested to be responsible for the much larger strength degradation of fibers prepared in Ar/air.     

A Si–SiC Oxidation‐resistant Coating for Carbon/Carbon Composites by Hot‐pressing Reaction Technique

A Si–SiC coating was prepared by hot‐pressing reactive sintering (HPRS) technique for protecting carbon/carbon (C/C) composites against oxidation. The Si–SiC coating has a dense and crack‐free structure with a thickness of 70–90 μm. The Si–SiC coating by HPRS has a higher SiC content and lower Si content than the coating by pressure‐less reactive sintering (PRS). It also exhibits better oxidation‐protective ability than that prepared by PRS. With hot‐pressing, the flexural strength of the Si–SiC coated C/C composites decreases from 121 MPa to 99 MPa, and the interface bonding strength increases from 6 MPa to 10 MPa.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Oxidation of Polymer‐Derived HfSiCNO up to 1600°C

Oxidation behavior of HfSiCNO ceramics for Hf/Si ratio of 0.09 at 1400°C–1600°C in ambient air is reported. Quantitative X‐ray analysis of oxidized powders shows crystalli‐zation of the amorphous phase into tetragonal hafnia, hafnon, and cristobalite (carbides, seen in inert atmosphere heat treatments are absent). Cross‐sectional SEM shows the oxide overgrowth on the particles to contain precipitates of hafnia/hafnon, while the interior of the particles is decorated with nanoscale grains of hafnia in a necklace‐like formation. The oxidation kinetics of these materials, determined both from weight‐change measurements and from direct observation of oxide overgrowth, are shown to be comparable to the oxidation of SiC single crystals. Oxidation of SiC–SiC minicomposites (straight fiber bundles infiltrated with a SiC matrix), coated with thin films of HfSiCNO prepared by dip‐coating was studied. The overgrowth thicknesses for oxidation time of 1000 h at 1600°C are compared for uncoated, SiCN(O)‐coated, and HfSiCNO‐coated minicomposites.     

Silicon carbide fiber oxidation behavior in the presence of boron nitride

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O₂ at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO₂ due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.     

Spherical indentations on hafnium carbide- and silicon-carbide-coated carbon–carbon composites after thermal shock test in air

In this study, high-temperature oxidations and indentations on silicon carbide (SiC)- and hafnium carbide (HfC)-coated carbon–carbon (C/C) composites were investigated to prevent the oxidation of C/C composites in air. The SiC and HfC layers were coated to protect the composite from oxidizing in air at a high temperature. High-temperature oxidation tests were performed at 500 °C, 1,000 °C, 1,350 °C, and 1,500 °C for 1 h in air and then cooled to room temperature by thermal shock. This cycle was repeated until the coating layer was damaged. As a result of measuring the weight change according to the thermal cycle and observing the fiber damage in the C/C composites, the oxidation resistance of HfC was evaluated to be superior to that of SiC. The change of the mechanical behavior was investigated using an indentation test with a tungsten carbide ball with a radius of either 3.18 or 7.93 mm before and after the high-temperature oxidation test by thermal shock. The relative elastic modulus was predicted based on the slope of the indentation load–displacement curve during loading or that of the curve during unloading. The relative hardness was also analyzed based on residual displacement after indentation. The hardness and elastic modulus of the HfC and SiC coating were higher than those of C/C composites and the mechanical properties of the HfC-coated C/C composite were relatively good compared with those of the SiC-coated composites prior to the thermal shock test. In particular, in this study, it was found that the mechanical behaviors of HfC-coated C/C composites exposed to temperatures >1,000 °C in air were transferred from elastic to quasi-plastic. The quasi-plasticity of the HfC coating layer was evaluated to be excellent regarding its resistance to mechanical damage as the coating layer was not easily delaminated or damaged even by oxidation.     

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.     

Effect of precoated carbon layer on microstructure and anti‐erosion properties of SiC coating for 2D‐C/C composites

To improve the erosion resistant of carbon‐carbon composites, an SiC coating was synthesized on carbon‐carbon composites by the in situ reaction method. They are firstly coated with carbon layer by slurry, and then SiC coatings are obtained by chemical vapor reaction. The effects of precoated carbon layer on the microstructure and anti‐erosion properties of SiC‐coated C‐C composites were studied and characterized. The thickness of the SiC coating increased with the increase in the precoated carbon layer thickness. The different thickness of carbon layer affects hardness of the SiC coatings, resulting in diverse erosion resistance of the coatings. The SiC coating prepared with moderate thickness of precoated carbon layer exhibits the best erosion resistance, and show better resistance at an impact angle of 30° than 90°. The eroded surface revealed that coating cracking and brittle fracture, fiber‐matrix debonding, fiber breakage, and material removal, and the additional microcutting and microploughing at oblique impact angle are the major erosion mechanism of SiC coating for C/C composites.     

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB₂–SiC coatings, ZrB₂, silicon, and graphite powders were used as pack powders to prepare ZrB₂–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB₂–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.     

Improving thermal stability of SiC whiskers via nanoscale Ti3SiC2 coatings

Effects of SiC whiskers (SiC_w) on the mechanical properties of composites largely depend on their thermal stability at high temperature. In this study, pure SiC_w and Ti₃SiC₂ coated SiC_w were thermal treated at 1600–1800°C for 1 h. Their phase assemblage, morphology, and structural evolution were investigated. Oxygen partial pressures in the graphite furnace resulted in the breakdown of SiC_w into particles at 1600°C, and the degradation became more pronounced with temperature increasing. The thermal stability of SiC whiskers at 1600–1700°C was significantly improved by a thin Ti₃SiC₂ coating on them, as both thermodynamic calculations and experimental observations suggest Ti₃SiC₂ coating could be preferentially oxidized/decomposed, prior to the active oxidation of SiC. At 1800°C, the protective role of the coating on the whiskers became weakened. SiC was converted into gaseous SiO and CO, with the remaining of interconnected TiC micro-rods and amorphous carbon.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

SiC Depletion in ZrB2–30 vol% SiC at Ultrahigh Temperatures

The formation of a porous SiC‐depleted region in ZrB₂–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO₂ + C/borosilicate oxidation product layer sequence was observed above the ZrB₂–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB₂ matrix below the ZrO₂ and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO₂ (T < 1627°C) and SiC depletion in ZrB₂ (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas‐phase diffusion‐controlled process.     

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.