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... The detailed EDS analyses of the cauliflower-like particles reveal that they are rich in Si and O, but there is still some C, as shown in Figure 11a-d. According to recent studies [50][51][52], these particles could be the oxides of SiC, possibly a mixture of SiC, SiO2, and SiO phases. SiC acts as the core of small particles, and its outer layer consists of SiO2 and SiO phases. ...
... The EDS analyses of the peeled SiC fiber reveal that it is mainly composed of Si The detailed EDS analyses of the cauliflower-like particles reveal that they are rich in Si and O, but there is still some C, as shown in Figure 11a-d. According to recent studies [50][51][52], these particles could be the oxides of SiC, possibly a mixture of SiC, SiO 2 , and SiO phases. SiC acts as the core of small particles, and its outer layer consists of SiO 2 and SiO phases. ...
SiCf/SiC ceramic matrix composite (CMC), a hard and brittle material, faces significant challenges in efficient and high-quality processing of small-sized shapes. To address these challenges, the nanosecond laser was used to process micro-holes in the SiCf/SiC CMC using a two-step drilling method, including laser pre-drilling in air and laser final-drilling with a water jet. The results of the single-parameter variation and optimized orthogonal experiments reveal that the optimal parameters for laser pre-drilling in air to process micro-holes are as follows: 1000 processing cycles, 0.7 mJ single-pulse energy, −4 mm defocus, 15 kHz pulse-repetition frequency, and 85% overlap rate. With these settings, a micro-hole with an entrance diameter of 343 μm and a taper angle of 1.19° can be processed in 100 s, demonstrating high processing efficiency. However, the entrance region exhibits spattering slags with oxidation, while the sidewall is covered by the recast layer with a wrinkled morphology and attached oxides. These effects are primarily attributed to the presence of oxygen, which enhances processing efficiency but promotes oxidation. For the laser final-drilling with a water jet, the balanced parameters for micro-hole processing are as follows: 2000 processing cycles, 0.6 mJ single-pulse energy, −4 mm defocus, 10 kHz pulse-repetition frequency, 85% overlap rate, and a 4.03 m/s water jet velocity. Using these parameters, the pre-drilled micro-hole can be finally processed in 96 s, yielding an entrance diameter of 423 μm and a taper angle of 0.36°. Due to the effective elimination of spattering slags and oxides by the water jet, the final micro-hole exhibits a clean sidewall with microgrooves, indicating high-quality micro-hole processing. The sidewall morphology could be ascribed to the different physical properties of SiC fiber and matrix, with steam explosion and cavitation erosion. This two-step laser drilling may provide new insights into the high-quality and efficient processing of SiCf/SiC CMC with small-sized holes.
... This drastic decrease in the flexural strength is attributed the oxidation of the composite. 40 In contrast, all oxide composite Al 2 O 3 fibers and an Al 2 O 3 matrix made by the PIP process led to flexural strengths of approximately 405 and 369 MPa after 0 and 100 h at 1200 • C, respectively, with limited degradation due to the presence of an oxidized matrix. 41 The aim of this work was to develop a GMCs with high temperature resistance (above 1150 • C). ...
... The next fold is simply superposed to the already preparated fold in order to match 0/90 • textile orientation. Once all folds were superposed, the whole composite is consolidated in a heat chamber (1 atm) or a heating textile press (6 MPa) for different times (0.083, 0.3, 2, 6, 24, and 168 h) and different temperatures (20,40,60,80, 120, and 200 • C). The pressure of 6 MPa is performed by a heating textile press which could heat the composites by both its surface plates from 20 to 200 • C. Once consolidated, the weight and thickness of the composite geopolymer were measured. ...
Geopolymer matrix composites (GMCs) are actively being developed for high‐temperature applications. In this work, we investigated different composite preparation parameters, such as the elaboration conditions (time, temperature and pressure) and composite dimensions (number of folds and surface area). In addition, we studied the effect of alkali cations (NaK or K), on metakaolin based geopolymer matrix composition, and textile type (N312, N610) of the composites on fiber–matrix interactions in high‐temperature geopolymer composites and their flexural behavior. The process we ultimately selected was as follows: an 8.5‐fold composite of surface area 100 cm² was pressed for 2 h at 120°C. After thermal treatment at 1150°C, composites containing the N312 textile with B2O3 showed greater heterogeneity, viscous flow and lower flexural strength (7 MPa) due to fiber–matrix interactions. In contrast, above 1150°C, composites with the N610 textile had no fiber–matrix interactions and showed greater flexural strength (51 MPa). The specific flexural strength correlated well with the overmodifying element ratio based on the composite composition. In fact, the presence of viscous flow suggested that boron and some aluminum atoms could act as network modifiers. Thus, the two ratios were nSigeo+nAlgeonMgeo or nSigeo+N312+nAlgeo×0.5+nAlN312nMgeo+nBN312+nAlgeo×0.5 depending on the presence of viscous flow.
... This hybrid technique fully exploits the merits of both CVI and PIP techniques, and its produced composites have distinctive features such as high density, brilliant antioxidation property, good microstructural homogeneity, small mechanical property deviations as well as dramatically reduced fabrication durations, and so on. 17 It is worth pointing out that the processing temperature of both CVI and PIP processes is rather low, with the former ranging from 1000 to 1100 • C whilst the latter ranging from 1000 to 1200 • C. 18,19 Apparently, this manufacturing temperature is fiber friendly, but is much lower than the target application temperature of SiC f /SiC composites, which is 1300-1400 • C or even higher. A higher application than manufacturing temperature suggests that the composition, microstructure and thus properties of as-made composites might change during service, which perhaps could bring in some unexpected problems or even compromise the durability and reliability of SiC f /SiC composite parts, particularly for those PIP-derived matrix. ...
The mechanical and thermophysical properties of KD‐SA SiCf/SiC composites manufactured by the hybrid chemical vapor infiltration (CVI)–precursor impregnation and pyrolysis (PIP) technique were systematically studied at 1400, 1600, and 1800°C, respectively, under argon atmosphere for 1 h. The results show that Si element in SiC fiber escapes and diffuses to pyrolytic carbon (PyC) interphase after 1400°C heat treatment, forming a Si‐rich line in PyC interphase. With a further increase of heat treatment temperature, the Si sublimation in the Si‐rich line leads to the cracking of the PyC interphase as well as the increase of the order degree of the PyC interphase, resulting in a significant decrease in the interfacial shear strength of the composite. Due to the decomposition of PIP SiC matrix and Si sublimation of SiC fiber, the flexural strength of the composite decreases after heat treatment at 1400°C. When the heat treatment temperature increases to 1600 and 1800°C, the flexural strength of the composite is slightly increased compared with 1400°C. Moreover, the thermal conductivity of the composites has been improved significantly by raising temperatures due to an increase of the crystallinity of the PIP SiC matrix.
Ceramic matrix composites have broad application prospects in the aerospace field due to their high temperature resistance and oxidation resistance. The effect of temperature and environment atmosphere on the fracture toughness and failure mechanisms of two-dimensional plain-woven SiCf/SiC composites was investigated. The results show that they exhibit pseudo-plastic deformation behavior at different temperatures. The fracture toughness is as high as 48 MPa m1/2 at room temperature, and gradually decreases with rising temperature. The difference in fracture toughness between argon and air initially increases and then decreases with rising temperature. Furthermore, the high-temperature failure mechanisms of these composites were analyzed through macro and micro analysis. Based on this, a physic-based temperature-dependent fracture toughness model considering matrix toughness, plastic power, fiber pull-out, and residual thermal stress was developed for fiber-reinforced ceramic matrix composites. The model has been well validated by experimental results. An analysis of influencing factors regarding the evolution of fracture toughness was conducted by the proposed model. This work contributes to a better understanding of the mechanical performance evolution and failure mechanisms of ceramic matrix composites under multi-field coupling conditions, thereby promoting their applications.
The protection effects of an environmental barrier coating (EBC) consisting of silicon bond coat and mixed ytterbium monosilicate and ytterbium disilicate composite topcoat are directly evaluated by measuring the strength retention rate of SiCf/SiC composites completely wrapped up by the previous EBCs after soaking in a mixed oxygen and water vapor environment at 1300°C for up to 200 h. The results show that the mixed topcoat exhibits not only extremely excellent phase stability but also fantastic protection effects toward composites. After 200 h of corrosion, the fully protected composites are unveiled to present not only dramatically reduced weight gain ratios, less than .6%, compared to ∼10% for those unprotected ones, but also extremely higher strength retention rates, more than 90%, compared to only 10%–15% for those unprotected ones. Further, the fully protected composites show a quasi‐ductile load versus displacement curve, suggesting the retention of the oxidation‐prone pyrolytic carbon interphase of current composites.
In order to improve the degree of matrix densification of SiCf/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.
Comparing the microstructures of SiCf/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiCf/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiCf/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.
In addition, among the SiCf/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiCf/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.
Environmental barrier coatings (EBCs) are a commercially proven means of protecting SiC‐based materials in gas turbine environments. However, there are little specific data in the literature on the impact of coatings like Yb2Si2O7 on preventing accelerated SiO2 growth in the presence of H2O. Quantification of reduced rates are necessary for evaluating and comparing EBC effectiveness and incorporation of silica growth rates into future EBC lifetime models. In this study, baseline kinetics of silica formation on bare Si and chemically vapor deposited (CVD) SiC in the 1250–1425℃ range were obtained via 100 h isothermal exposures in dry air and steam environments utilizing a SiC reaction tube to mitigate specimen volatility. An Arrhenius plot of the resulting rates was constructed, representing baseline minimum and maximum rates for Si and SiC oxidation at ambient pressure. Various EBC systems on CVD SiC substrates including air plasma sprayed (APS) EBCs with and without a Si bond coating and with surface roughening to enhance Yb2Si2O7 adhesion were subjected to 1‐h furnace cycle testing in air with 90vol%H2O at 1250–1350℃ for up to 500 cycles. After exposure, silica formation rates were measured and compared to the baseline rates to assess EBC effectiveness, where EBC effectiveness is gauged as the propensity to reduce underlying rates of silica formation. With a Si bond coating, a ~180 µm Yb2Si2O7 (YbDS) top coating reduced rates over the entire 1250°‐1350℃ range. Without a Si bond coating, ~60 µm (YbDS) coatings deposited directly onto CVD SiC exhibited poor adhesion, and had to be deposited onto substrates with enhanced roughness at 1350℃. While exhibiting good adhesion at 1350℃, overall the single layer YbDS coating exhibited a decreasing effectiveness from 1250° to 1350℃.
The SiCf/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t0, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t0, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiCf/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiCf/SiC composites for industrial applications.
The increasing demand for more efficient and environmental-friendly gas turbines has driven the development of new strategies for material development. SiC/SiC ceramic matrix composites (CMCs) can fulfil the stringent requirements; however, they require protection from the operating environment and debris ingested during operation. Environmental barrier coatings (EBCs) are a protective measure to enable the CMCs to operate under harsh conditions. EBC-coated CMCs will enable an increased efficiency and reduced pollutant and CO2 emissions. In this review, the fundamentals of SiC/SiC ceramic matrix composites degradation in steam environments and under the presence of corrosive species, namely CaO-MgO-Al2O3-SiO2 (CMAS), are first presented. Then, a summary of EBCs along with a comprehensive summary of the current compositions and their interactions with steam and molten corrosive species is presented. Finally, an overview of the latest research directions for the potential next generation of EBCs are outlined.
High-temperature engine applications have been limited by the performance of metal alloys and carbide fiber composites at
elevated temperatures. Random inorganic networks composed of silicon, boron, nitrogen, and carbon represent a novel class
of ceramics with outstanding durability at elevated temperatures. SiBN3C was synthesized by pyrolysis of a preceramicN-methylpolyborosilazane made from the single-source precursor Cl3Si-NH-BCl2. The polymer can be processed to a green fiber by melt-spinning, which then undergoes an intermediate curing step and successive
pyrolysis. The ceramic fibers, which are presently produced on a semitechnical scale, combine several desired properties relevant
for an application in fiber-reinforced ceramic composites: thermal stability, mechanical strength, high-temperature creep
resistivity, low density, and stability against oxidation or molten silicon.
Environmental barrier coatings (EBCs) are crucial for SiCf/SiC composites to resist water vapor-oxygen corrosion and maintain the stability of their structure and properties in a complex combustion environment at high temperatures. In this work, an EBC system, composing Yb2Si2O7 topcoats and Si bond coats, has been prepared by an air plasma spraying (APS) route to completely wrap up SiCf/SiC composites. Then the corrosion and failure mechanisms of the EBC system in a water vapor-oxygen environment are systematically studied. The results indicate that the SiCf/SiC composites exhibit excellent stability under the full protection of the current EBCs. With the progression of corrosion, due to the difference of the thermal expansion coefficients (TECs) between the topcoat and bond coat materials, the vertical mud cracks generated by the thermal cycles become the intrusion passages for the corrosion species and then form transverse cracks and pores, resulting in the peeling off of the Yb2Si2O7 topcoat. Moreover, in the later stage of thermal cycles, the residue of the Si bond coat is shown to play an important protective role against the rapid corrosion of the substrates.
The corrosion behavior and corresponding mechanisms of SiCf/SiC composites have been studied in a mixed water vapor and oxygen environment at 1300 ℃ for up to 120 h.It has been unveiled that, the formation of liquid amorphous silica fulfilling gaps left by the oxidation of pyrolytic carbon interphase is extremely detrimental due to that: 1) the precipitation of liquid amorphous silica generates a very strong bonding with fibers; and that 2) the stress accumulation within silica layers owing to phase transformation and growth of silica drives them crack. These cracks would inevitably propagate to break fibers, thereby almost completely degrading composites.
Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.
A detailed gas-phase kinetic model consisting of 37 species and 318 reactions is developed for methane pyrolysis involved in the CVD/CVI processes. The model was used to perform the simulations of CH4 pyrolysis in a hot-wall CVD reactor over a wide range of operating conditions and validated against the experimental data. The overall trend of the concentration profiles of notable species is in good agreement with experimental results. The sensitivity analysis and reaction path flux diagram revealed that CH4 pyrolysis directly proceeds without any induction period. C2 species are the primary intermediates for the formation of species like acetylene, ethylene, and benzene. Besides, the accuracy of the present model is better than the models available in the literature. The detailed kinetic model was then systematically reduced to a small mechanism comprising 13 species and 29 reactions. The reduced mechanism's predictions also agreed well with the detailed mechanism at low inlet partial pressure (<10 kPa). Although the error in the mole fraction of light species was minimal, the relatively large error for C6H6 species was observed at high inlet partial pressure of CH4.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for components in the hot gas section of jet engines, as they exhibit high temperature resistance and low density compared to their metal alloy counterparts. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). This work describes a new approach of a combined PIP and RMI process. A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. Generally, the processing of SiC/SiC, produced solely by PIP route, is rather time-consuming and the composites show a certain residual porosity. In order to obtain a dense matrix and to reduce the processing time, an additional RMI with silicon alloy is carried out after a reduced number of PIP cycles. To protect the fibers during the siliconization, a CVD fiber coating was applied. Microstructure was examined via microCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60%. The overall processing time was reduced by 55% compared to standard PIP route. The hybrid material contained 70% less unreacted carbon than material produced by LSI process alone.
Three kinds of four-step three-dimensional braided silicon carbide fiber-reinforced silicon carbide matrix composites with pyrocarbon interface were prepared through precursor infiltration and pyrolysis under 1100 °C, 1300 °C, or 1500 °C, and they were oxidized at 1300 °C with simulated air. The effects of pyrolysis temperature on the microstructure and oxidation behavior of the composites were investigated. Results indicated that the SiC crystallite size and crystallinity of the as-fabricated composites increased with increasing pyrolysis temperature. During oxidation, the three composites initially underwent similar near-linear mass-loss stages associated with the consumption of carbon component. Subsequently, they exhibited different mass-gain stages associated with the oxidation of SiC component. The composites prepared at 1100 °C followed a linear mass-gain rule, whereas the composites prepared at 1300 °C and 1500 °C followed a two-step parabolic mass-gain rule. The mechanism of mass-gain behavior change was proposed, which might be associated with the pyrolysis-temperature-dependent precursor-derived ceramic SiC microstructure and its evolution during oxidation.
A joint chemical vapor infiltration-precursor infiltration and pyrolysis (CVI-PIP) process was used to fabricate C/C-SiC-ZrC composites with different volume ratio of CVI-derived C-SiC. The composite with a C-SiC volume ratio of 0.91 has a higher tensile and flexural strength of 181.7 MPa and 352.8 MPa, respectively, and also exhibits a better anti-oxidation property at 800∼1400 °C for 30 h. After an oxyacetylene ablation for 600 s at ∼2100 °C, the composite with a C-SiC volume ratio of 0.12 possesses a better anti-ablation performance with a linear and mass ablation rates of 0.94 um × s⁻¹ and 0.13 mg × cm⁻²×s⁻¹, respectively.
Environmental barrier coatings (EBCs) have widely been employed as protective coatings for SiC-based ceramic matrix composites (CMC-SiCs) against combustion corrosion in advanced gas turbines. During service, a layer of oxides forms beneath EBCs, which plays key role in lifetime of coated CMC-SiC parts. In this study, the kinetics of thermally grown silica oxide (silica-TGO) were examined through rare earth disilicates coated CMC-SiC composites subjected to corrosion under 50%H2O-50%O2 gas flow environment. Results revealed that growth of silica-TGO followed parabolic behavior, indicating diffusion controlled process. Calculated growth rates and activation energies of silica-TGO suggested that oxygen ion was the main active oxidant involved at relatively low partial pressure of water vapor. Corrosion environments containing different H2O/O2 ratios were also investigated and the data indicated that water vapor was also involved in growth of silica-TGO. The role of EBCs in corrosion process was also analyzed and discussed.
We report the microhardness of SiC fiber-reinforced SiC matrix (SiC/SiC) composites produced by a transient eutectic phase (TEP) hot-press consolidation process. The internal microstructure of the composites was evaluated by scanning electron microscopy and microhardness variations were determined, showing that both matrices and fibers have hardness values that are independent of geometrical features. This verification is of special interest for the production of SiC/SiC composite structures with more complicated geometries for prospective nuclear applications.
Delamination toughness of a mullite/Si/(SiC/SiC) environmental barrier coating (EBC) system has been measured using a shear-loading test. The EBC system is initially heated to a temperature of 1470 °C (above the melting point of Si (1414 °C)), and then repeated thermal exposure is carried out at a temperature 1320 °C (below the melting point of Si) for 2 h/cycle. Change of the delamination toughness with heat exposure is successfully obtained using the shear-loading test. Formation of cristobalite and transformation from α ↔ β during thermal cycling cause an increase of crack density in the Si bond coat layer and a resulting reduction in delamination toughness.
A composite with SiC matrix and 40 vol.% of SiC fibres with a β-SiC coating (SiCf/SiC) was prepared by the isothermal chemical vapour infiltration process. Oxidation of the SiCf/SiC composite was investigated at 1200 °C, 1300 °C and 1400 °C, for up to 100 h in static dry air, and for up to 24 h in combustion environment. In static dry air, the composite showed mass gain with nearly parabolic oxidation kinetics. The uniform oxide scale developed cracks as oxidation progressed. Oxidation in combustion environment was performed using an oxy-acetylene flame containing CO2 and H2O gases. The chemical erosion by the reaction of H2O with oxide scale was the likely ablation mechanism. At 1200 °C and 1300 °C, the oxidation rate was higher than that of ablation and the composite showed a net mass gain. However, at 1400 °C, significant ablation was observed. Localised attack by the combustion gases caused pitting of the β-SiC seal coating. The present study shows that β-SiC seal coating remains intact at relatively lower temperatures (below 1300 °C) for a shorter period of time (~24 h) and is lost it at higher temperatures (~1400°C) by ablation.
2D-Cf/SiC composite was manufactured by chemical vapor inflation (CVI) combined with polymer impregnation and pyrolysis (PIP) with SiC particle as inert fillers. The effects of CVI processes on SiC morphologies and the properties of composite were investigated. The composites were characterized by XRD, flexural strength test and SEM. The results revealed that uniform SiC coatings and nanowires were prepared when MTS/H2 ratio of 1:8 was employed, while gradient thick coatings were fabricated as MTS/H2 ratio of 1:1 was employed. The flexural strength of composites varied from 156 MPa at MTS/H2 ratio of 1:1 to 233 MPa at MTS/H2 ratio of 1:8. All of composites exhibited toughness due to significant debonding and pullout of fibers. The laminated structure of coatings on the fibers and nanowires were manufactured by combination of above different CVI process, and the obtained composites showed flexural strength of as high as 248 MPa and impressive toughness.
Because of renewed focus on ceramic-matrix composite research and development, prospects for successful and expanded insertion of these materials in gas turbines appear promising.
The cristobalite, transformed from amorphous silica basically, is ubiquitous in the thermodynamic stable region of quartz or tridymite. This formation mechanism of cristobalite metastability may be explained by the Ostwald's stage rule which is an experiential dynamic principle. The key factor which results in that the rate constant is higher in the process of the amorphous silica to transform to cristobalite than that in cristobalite to quartz or tridymite greatly is the similarity between the local intermediate range order structure for amorphous silica and the dynamic disorder structure of β-cristobalite. This statistical similarity leads to the increase of β-cristobalite valid nucleation rate greatly during the amorphous silica crystallization, and results in the formation of the metastabe cristobalite ultimately. The impure components in amorphous silica are propitious to decreasing the formation temperature of cristobalite and accelerating the reaction of amorphous silica to cristobalite. However, they are useless to formation of metastable cristobalite unless these impure components are in favor of stability of cristobalite structure.
Three-dimensional carbon fiber zirconium carbide–silicon carbide composite (3D Cf/ZrC–SiC) was prepared by precursor infiltration and pyrolysis (PIP) with reactive melt infiltration (RMI) process. The 3D Cf/C–SiC substrate with open porosity of 20% and density of 1.35 g/cm3 was designed to be infiltrated by Zr0.912Si0.088 intermetallic compound. Thermodynamics calculations revealed that C could react with liquid Zr0.912Si0.088 to form ZrC at 1600 °C. Composites were prepared by heating the two materials to 1600 °C for 1 h in vacuum. The micro-structural, mechanical and ablative properties of the 3D Cf/ZrC–SiC composite were studied. ZrC and SiC were identified to be the main constituents. The flexural strength of the composite was 101.5±8.16 MPa and the elastic modulus was 35.18±9.58 GPa. The mass loss rate and linear recession rate of the 3D Cf/ZrC–SiC composite during oxyacetylene torch test were 0.013 g/s and 0.022 mm/s, respectively. The formation of ZrO2 melt on the surface of the composite contributed mainly to the excellent ablative property.
The thermal oxidation of Si using H(2)zO=O(2) and H(2)O=N(2) ambients has been studied with an automated ellipsometer which can observe the oxidation in situ. The oxidations were carried out in the temperature range of 780 degrees 980 degrees C on < 100 > oriented single crystal Si. The resulting SiO(2) film growth data was analyzed according to a linear-parabolic oxidation model. The parabolic rate constant was found to increase abruptly with small additions of H(2)O to O(2) while the linear rate constant increased gradually over the range of added H(2)O (0- 2000 ppm). The over-all increase in the rate of oxidation due to H(2)O in O(2) was found to be greater than predicted based on the independent diffusion and reaction of O(2) and H(2)O related oxidant species. These effects of H(2)oO were found to be reversible. Therefore, the kinetic role of HO H(2)O on the oxidation of silicon is essentially twofold. The H(2)O acts both as an additional source of oxidaht and as an accelerator for the oxidation process involving O(2). It is postulated that this latter effect occurs because the H(2)O modifies the SiO(2) network thereby enhancing diffusion of the primary oxidant through the SiO(2) film.
C/SiC and SiC/SiC composites are tough ceramics when the fiber–matrix bonding is properly optimized, usually through a thin layer of an interfacial material referred to as the interphase. These composites can be fabricated by a variety of techniques that are briefly described and compared. The design of the interphase, matrix, and coating at the nanometer scale, in order to promote microcrack deflection and to enhance the oxidation resistance is discussed. Selected properties of the composites are presented and discussed. Examples of application in engines, heat shields, braking systems, and high-temperature nuclear reactors are shown to illustrate the potential of these materials and the key points that still require research and development.
SiC-based ceramic matrix composites, consisting of carbon or SiC fibers embedded in a SiC-matrix, are tough ceramics when the fiber/matrix bonding is properly optimized through the use of a thin interphase. They are fabricated according to different processing routes (chemical vapor infiltration, polymer impregnation/pyrolysis, liquid silicon infiltration or slurry impregnation/hot pressing) each of them displaying advantages and drawbacks which are briefly discussed. SiC-matrix composites are highly tailorable materials in terms of fiber-type (carbon fibers of SiC-based fibers such as Si–C–O, SiC+C or quasi-stoichiometric SiC reinforcements), interphase (pyrocarbon or hexagonal BN, as well as (PyC–SiC)n or (BN–SiC)n multilayered interphases), matrix (simple SiC or matrices with improved oxidation resistance, such as self-healing matrices) and coatings (SiC or engineered multilayered coatings). The potential of SiC-matrix composites for application in advanced aerojet engines (after-burner hot section), gas turbine of electrical power/steam cogeneration (combustion chamber) and inner wall of the plasma chamber of nuclear fusion reaction, all of them corresponding to very severe conditions is discussed.
The oxidation behaviour of one-dimensional and two-dimensional SiC fibre reinforced SiC was investigated up to 1520°C in air, as well as in water vapour saturated air and argon by calorimetry, DTA and TGA. The oxidation process takes place in three steps: (1) oxidation of free carbon in the carbon coated composites between 530 and 690°C with mass losses up to 7.5%; (2) a fast exothermal process associated with mass gain starting at 800°C and terminated below 1500°C within 1 h; (3) the diffusion controlled oxidation of bulk SiC. The processes follow a logarithmic rate law between 875 and 985°C with an effective activation energy Q = 84 kJ/mol. The amorphous reaction product Si02 transforms to cristobalite at (930 ± 50)°C. The oxidation kinetics follows a quadratic rate law above 1000°C with rate constants k = 3.5 × 10−7g2/cm4h for 1D SiC/SiC and k = 5 × 10−8g2/cm4h for 2D SiC/SiC at 1520°C in air. The rate constants are up to three orders of magnitude higher than that for high density monolithic SiC which is explained by the high porosity of the SiC matrix.
Preparation of cristobalite and its thermal characteristics
- Shen
The study of crystallization process of high－purity silica at high temperature
- Mo