Effect of BN nanoparticle content in SiC matrix on microstructure and mechanical properties of SiC/SiC composites

BN-nanoparticle-containing SiC-matrix-based composites comprising SiC fibers and lacking a fiber/matrix interface (SiC/BN + SiC composites) were fabricated by spark plasma sintering (SPS) at 1800°C for 10 min under 50 MPa in Ar. The content of added BN nanoparticles was varied from 0 to 50 vol.%. The mechanical properties of the SiC/BN + SiC composites were investigated thoroughly. The SiC/BN + SiC composites with a BN nanoparticle content of 50 vol.%, which had a bulk density of 2.73 g/cm³ and an open porosity of 5.8%, exhibited quasiductile fracture behavior, as indicated by a short nonlinear region and significantly shorter fiber pullouts owing to the relatively high modulus. The composites also exhibited high strength as well as bending, proportional limit stress, and ultimate tensile strength values of 496 ± 13, 251 ± 30, and 301 MPa ± 56 MPa, respectively, under ambient conditions. The SiC fibers with contents of BN nanoparticles above 30 vol.% were not severely damaged during SPS and adhered to the matrix to form a relatively weak fiber/matrix interface.     

Stepwise-Graded Si3N4–SiC Ceramics with Improved Wear Properties

The processing of stepwise graded Si₃N₄/SiC ceramics by pressureless co-sintering is described. Here, SiC (high elastic modulus, high thermal expansion coefficient) forms the substrate and Si₃N₄ (low elastic modulus, low thermal expansion coefficient) forms the top contact surface, with a stepwise gradient in composition existing between the two over a depth of ∼1.7 mm. The resulting Si₃N₄ contact surface is fine-grained and dense, and it contains only 2 vol% yttrium aluminum garnet (YAG) additive. This graded ceramic shows resistance to cone-crack formation under Hertzian indentation, which is attributed to a combined effect of the elastic-modulus gradient and the compressive thermal-expansion-mismatch residual stress present at the contact surface. The presence of the residual stress is corroborated and quantified using Vickers indentation tests. The graded ceramic also possesses wear properties that are significantly improved compared with dense, monolithic Si₃N₄ containing 2 vol% YAG additive. The improved wear resistance is attributed solely to the large compressive stress present at the contact surface. A modification of the simple wear model by Lawn and co-workers is used to rationalize the wear results. Results from this work clearly show that the introduction of surface compressive residual stresses can significantly improve the wear resistance of polycrystalline ceramics, which may have important implications for the design of contact-damage-resistant ceramics.     

High temperature anti‐oxidation behavior of in situ and ex situ nanostructured C/SiC/ZrB2‐SiC gradient coatings: Thermodynamical evolution,microstructural characterization,and residual stress analysis

Nanostructured C/SiC/ZrB₂–SiC oxidation protective gradient coating was prepared by a two‐step reactive melt infiltration method. In order to reduce production cost, ZrB₂ phase was synthesized by the in situ reactive that included low‐cost ZrO₂ and B₂O₃ powders as raw materials. High‐temperature oxidation behavior of coatings was evaluated by isothermal oxidation test at 1773 K in air for 10 hours. Thermodynamical behavior of the coatings at various temperatures during oxidation test and coating process was predicted by HSC Chemistry 6.0 software. Compressive residual stresses of 36.9 MPa and 41 MPa were calculated for in situ and ex situ coatings by Williamson‐Hall method. After 10 hours of isothermal oxidation at 1773K, in situ and ex situ coatings showed 12.84% and 15.69% of weight losses with oxidation rates of 1.87 × 10^?2 g cm^?3 h^?1 and 0.91 × 10^?2 g cm^?3 h^?1, respectively. These results indicated that the oxidation protection ability of the coating produced by the in situ method was very close to ex situ coating.     

Improved antioxidative and mechanical properties of SiC coated C/C composites via a SiO2-SiC reticulated layer

To improve the oxidation resistances of SiC coated C/C composites by a pack cementation (PC) method at high temperature and alleviate the siliconization erosion of molten silicon on C/C substrate during the preparation of SiC coating, a SiO₂-SiC reticulated layer with SiC nanowires was pre-prepared on C/C composites through combined slurry painting and thermal treatment before the fabrication of SiC coating. The presence of porous SiO₂-SiC layer with SiC nanowires was beneficial to fabricate a compact and homogeneous SiC coating resulting from synergistic effect of further reaction between SiO₂ and pack powders and the reinforcement of SiC nanowires. Therefore, the results of thermal shock and isothermal oxidation tests showed that the mass loss of modified SiC coating was only 0.02 % after suffering 50-time thermal cycles between room temperature and 1773 K and decreased from 5.95 % to 1.08 % after static oxidation for 49.5 h in air at 1773 K. Moreover, due to the blocking effect of SiO₂-SiC reticulated layer on siliconization erosion during PC, the flexural strength of SiC coated C/C composites with SiO₂-SiC reticulated layer increased by 64.8 % compared with the untreated specimen.     

Effect of additives on slip casting rheology,microstructure and mechanical properties of Si3N4/SiC composites

The SiC/Si₃N₄ composites were fabricated with sintering process. To produce SiC/Si₃N₄ composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO₂ phase.     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Formation of Ti3SiC2 interphase coating on SiCf/SiC composite by electrophoretic deposition

To improve the oxidation resistance of SiC composites at high temperature, the feasibility of using Ti₃SiC₂ coated via electrophoretic deposition (EPD) as a SiC fiber reinforced SiC composite interphase material was studied. Through fiber pullout, Ti₃SiC₂, due to its lamellar structure, has the possibility of improving the fracture toughness of SiC_f/SiC composites. In this study, Ti₃SiC₂ coating was produced by EPD on SiC fiber; using Ti₃SiC₂‐coated SiC fabric, SiC_f/SiC composite was fabricated by hot pressing. Platelet Ti₃SiC₂ powder pulverized into nanoparticles through high‐energy wet ball milling was uniformly coated on the SiC fiber in a direction in which the basal plane of the particles was parallel to the fiber. In a 3‐point bending test of the SiC_f/SiC composite using Ti₃SiC₂‐coated SiC fabric, the SiC_f/SiC composite exhibited brittle fracture behavior, but an abrupt slope change in the strength‐displacement curve was observed during loading due to the Ti₃SiC₂ interphase. On the fracture surface, delamination between each layer of SiC fabric was observed.     

Ablation resistance of SiC‐modified ZrC coating prepared by SAPS for SiC‐coated carbon/carbon composites

This study evaluated the ablation resistance of ZrC/SiC coating for carbon/carbon (C/C) composites at different temperatures and heat fluxes, which improved the researches on ultra‐high temperature oxidation of ZrC/SiC system. Results showed that the protection of coating depended on temperature and heat flux. Ablation test for 120 seconds under heat flux of 2.4 MW/m² at 2270°C revealed a good ablation resistance, with the linear ablation rate reduced by 96.4% and the mass gain rate increased by 383.3% compared with those of pure ZrC coating. The good ablation resistance was attributed to the formation of dense oxide scale surface. SiC could improve the compactness of the oxide scale at this temperature by forming SiO₂. A dense scale could not form at 2105°C after ablation for 120 seconds, resulting in a dissatisfactory ablation resistance of the coating. After ablation for 120 seconds at 1738°C, the coating was integrated due to the protection of glassy SiO₂ encapsulated ZrO₂. The coating could not resist the strong shear force from the flame at heat flux of 4.2 MW/m² and was severely damaged after ablation for 60 seconds.     

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).     

Micro Four‐Layer SiC Coating on Matrix Graphite Spheres of HTR Fuel Elements by Two‐Step Pack Cementation

A micro four‐layer SiC coating, which includes inner transition layer, fine‐grained layer, dense bulk layer, and outer loose layer, was fabricated on the matrix graphite spheres of high‐temperature gas‐cooled reactor fuel elements to improve the oxidation‐resistant property by a two‐step pack cementation process. According to the experiment results, the micro four‐layer can be differentiated by SiC grain size and microstructure variation. The oxidation tests at 1773 K for 200 h reveal that the coating structure could effectively improve the oxidation resistance of matrix graphite spheres with a weight gain of 0.52 wt%, and the fine‐grained and dense bulk layers are evidenced as two main antioxidation layers. Although the thermal expansion coefficients of SiC and matrix graphite do not match each other so well, no obvious stress cracking was observed after thermal shocking tests from 1773 K to room temperature for 100 times.     

Effect of processing parameters on microstructure and ablation behavior of SiC/ZrB2 coating for graphite

In this research, a SiC/ZrB₂ coating was produced on graphite by reactive melt infiltration and plasma spraying method. The coating characterization was performed using XRD analysis, electron microscopy equipped with energy dispersive spectrometer (EDS), and supersonic flame ablation test at 2073 K. The results indicated that the dense C/SiC coating with good ablation resistance can be obtained at 1873 K. The coating thickness decreased with increasing infiltration temperature. The results of ablation test showed that by increasing the infiltration temperature and holding time, weight loss and mass ablation rate decreased from 22.63% to 9.83% and 3.63 × 10⁻³ g cm⁻² s⁻¹ to 1.34 × 10⁻³ g cm⁻² s⁻¹, respectively. The results showed that by using the ZrB₂ as outer coating the ablation resistance improved remarkably. The weight loss and mass ablation rates for the SiC/ZrB₂ coating were 12.79% and 1.857 × 10⁻³ g cm⁻² s⁻¹, respectively.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Thermal shock resistance of tri‐layer Yb2SiO5/Yb2Si2O7/Si coating for SiC and SiC‐matrix composites

A new tri‐layer Yb₂SiO₅/Yb₂Si₂O₇/Si coating was fabricated on SiC, C/SiC, and SiC/SiC substrates, respectively, using atmospheric plasma spray (APS) technique. All coated samples were subjected to thermal shock test at 1350°C. The evolution of phase composition and microstructure and thermo‐mechanical properties of those samples before and after thermal shock test were characterized. Results showed that adhesion between all the 3 layers and substrates appeared good. After thermal shock tests, through microcracks which penetrated the Yb₂SiO₅ top layer were mostly halted at the Yb₂SiO₅‐Yb₂Si₂O₇ interface and no thermal growth oxide (TGO) was formed after 40‐50 quenching cycles, implying the excellent crack propagation resistance of the environmental barrier coating (EBC) system. Transmission electron microscopy analysis confirmed that twinnings and dislocations were the main mechanisms of plastic deformation of the Yb₂Si₂O₇ coating, which might have positive effects on crack propagation resistance. The thermal shock behaviors were clarified based on thermal stresses combined with thermal expansion behaviors and elastic modulus analysis. This study provides a strategy for designing EBC systems with excellent crack propagation resistance.     

结合方式对碳化硅浇注料氮化后抗热震性和显微结构的影响

研究了铝酸钙水泥(CAC)+二氧化硅微粉(MS)结合、水硬性氧化铝(HA)+二氧化硅微粉(MS)结合、二氧化硅溶胶结合的3种结合方式对氮化处理后碳化硅浇注料抗热震性和显微结构的影响。结果显示:3种结合方式制备的试样经600、800和1 100℃水冷1次热震后的残余抗折强度和强度保持率都随着温度的升高而降低,但在1 100℃热震后,3种结合方式的试样的残余抗折强度相当,二氧化硅溶胶结合的浇注料具有较好的抗热震性。分析表明,本试验的3种结合体系中,无水泥结合方式的试样中原位生成的SiAlON具有更大的长径比,特别是纤维状的SiAlON,对高温下结构的保持和抗热震性的提高更为有利。     

Effect of the incorporation of SiC nanowire with double protective layers on SiC coating for C/C composites

To maintain the thermal stability of SiC nanowires during SiC coating fabrication process, carbon and SiC double protective layers were covered on the surface of nanowires. And SiC nanowires with double protective layers toughened SiC coating were prepared by pack cementation. The results showed that after introducing the SiC nanowires with double protective layers, the fracture toughness of the SiC coating was increased by 88.4 %. The coating protected C/C for 175 h with a mass loss of 3.67 %, and after 51 thermal shock cycles, the mass losses of the oxidized coating were 3.96 %. The double protective layers are beneficial to improve the thermal stability of nanowires, leading to good fracture toughness and thermal shock resistance of SiC coating. SiC nanowires consume the energy of crack propagation by fracture, pullout and bridging, leading to an increase in fracture toughness.     

Siliconization elimination for SiC coated C/C composites by a pyrolytic carbon coating and the consequent improvement of the mechanical property and oxidation resistances

Carbon/carbon (C/C) composites have a wide application as the thermal structure materials because of their excellent properties at high temperatures. However, C/C composites are easily oxidized in oxygen-containing environment, which limits their potential applications to a great degree. Silicon carbide (SiC) ceramic coating fabricated via pack cementation (PC) was considered as an effective way to protect C/C composites against oxidation. But the mechanical properties of C/C composites were severely damaged due to chemical reaction between the molten silicon and C/C substrate during the preparation of SiC coating by PC. In order to eliminate the siliconization erosion, a pyrolytic carbon (PyC) coating was pre-prepared on C/C composites by the chemical vapor infiltration (CVI) prior to the fabrication of SiC coating. Due to the retardation effect of PyC coating on siliconization erosion, the flexural strength retention of the SiC coated C/C composites with PyC coating increased from 46.27 % to 107.95 % compared with the specimen without PyC coating. Furthermore, the presence of homogeneous and defect-free PyC coating was beneficial to fabricate a compact SiC coating without silicon phase by sufficiently reacting with molten silicon during PC. Therefore, the SiC coated C/C composites with PyC coating had better oxidation resistances under dynamic (between room temperature and 1773 K) and static conditions in air at different temperatures (1773?1973 K).     

Effect of SiC coating and heat treatment on the thermal radiation properties of C/SiC composites

The effects of SiC coating and heat treatment on the emissivity were investigated for 2D C/SiC composites prepared by CVI in the 6–16 μm range. SiC coating had an obvious effect on the spectral emissivity of the composites but caused just 5% difference in the total emissivity. A radiation transport model was applied to explain those changes caused by SiC coating. Heat treatment affected the thermal radiation properties of the composites through the microstructure evolution. Base on the complementary analytical techniques, the changes in the emissivity were attributed to a good graphitization degree of carbon phases, large β-SiC grain sizes and high α-SiC content resulting in high emissivity.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.