SiC/MoSi2 based coatings for Cf/C composites by two step pack cementation

In order to improve the oxidation resistance of C_f/Cs produced by chemical vapour infiltration, a multilayer coating based on silicon carbide and molybdenum disilicide was produced by two-step pack cementation technique. The inner SiC layer with a thickness up to 25 μm was obtained without promoted reaction additives by varying the composition, and thermal treatment conditions. The SiC/SiC-MoSi₂ coating was produced with a thickness up to 80 μm by two step pack cementation, considering the effect of the inner layer characteristic. The enhancement of the oxidation resistance, observed in SiC/SiC-MoSi₂ coated C_f/Cs by means of thermal analysis in flowing air up to 1500 °C, was due to the formation of SiO₂ promoted by the passive oxidation of silicon carbide and molybdenum disilicide.     

多孔碳化硅陶瓷的抗热震性研究

本文考察了多了孔碳化硅陶瓷的抗热震性,并探讨了不同制造工艺对多孔碳化硅陶瓷抗热震性的影响。同时研究了ＳｉＣ陶瓷在热处理过程中ＳｉＣ颗粒表面氧化形成的ＳｉＯ２在不同热处理温度的状态变化及其对试样抗热震性的影响。     

Effect of Oxidation on the Room-Temperature Flexural Strength of Reaction-Bonded Silicon Carbides

The oxidation behavior of reaction-bonded silicon carbide (RBSC) and the effect of oxidation on the room-temperature flexural strength of RBSC were investigated. Four different types of RBSC, each having various SiC particle-size distributions and free silicon contents, were exposed to air at 1300°C for up to 200 h. Parabolic weight gains, with respect to oxidation time, were observed in all the specimens. The strength of the RBSC increased after oxidation for up to 50 h, because of the blunting of cracks by the silica layer that was formed on the surface. However, with further oxidation, the beneficial role of the oxide layer was negated by the cracks that were newly generated on the surface because of the thermal mismatch between the substrate and the silica layer. The amount of free silicon had a negligible effect on the strength retention of the specimens after the oxidation processes.     

Investigations of SiC aggregates oxidation: Influence on SiC castables refractories life time at high temperature

This paper reports results concerning the oxidation of silicon carbide of pure SiC powder (98% of SiC) and the oxidation of two silicon carbide castables, used in waste-to-energy plants (WTE), and containing 60% and 85% of SiC, respectively. The investigated temperature range (800-1200 °C) corresponds to typical service conditions in WTE. Thanks to thermogravimetric and thermal expansion tests, kinetics of oxidation of the powder of SiC and of the castables has been investigated. According to these tests, several important points have been underlined. Firstly, the oxidation of SiC aggregates has a high influence on the thermal expansion and on the weight gain of SiC castables. Secondly, the grain size distribution of SiC aggregates within castables plays a dominating role (especially enhanced for the fine particles) in castable expansion behaviour induced by oxidation. This is a key point according to the evolution of thermal expansion of such materials.     

Oxidation Resistance,Compressive Strength and Thermal Shock Resistance of SiC Ceramics Prepared by Two Processing Routes

In the present paper, direct foaming technique and sacrificial template method were used to prepare porous alumino-silicate bonded SiC ceramics from mixtures of silicon carbide, kaolin and calcined alumina. The direct foaming process depends on generating bubbles inside the slurry of SiC mixtures through a reaction between CaC2 and Al powder in presence of H2O to form C2H2 and H2 gases in two steps of the reaction. In the sacrificial template method, the pores were created in the prepared SiC specimens after burning out the used sacrificial template, corn and potato starch. Some fired specimens were selected to study the effect of their composition and preparation conditions on their oxidation resistivity, compressive strength and thermal shock resistivity. Based on purity and homogenous microstructure, the present study offers SiC specimens with good oxidation resistance where they exhibit (0.1 to 2 mg/cm2) increase in mass after oxidation in air at a temperature of 1100 ̊C for 65 h, despite their high open porosity values. The present SiC specimens have high thermal shock resistivity. However, the foamed specimens showed better resistivity than those prepared by the sacrificial template method. Using the extruder for shaping the latter specimens improves their resistivity to thermal shock. The compressive strength of both groups of specimens is affected by their composition and preparation conditions.     

Surface charges and optical characteristic of colloidal cubic SiC nanocrystals

Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CH_x groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.     

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.     

Synthesis of monolithic SiC aerogels with high mechanical strength and low thermal conductivity

The monolithic silicon carbide (SiC) aerogels were converted from catechol-formaldehyde/silicon composite (CF/SiO₂) aerogels through carbothermal reduction and calcination. In the process of preparing the CF/SiO₂ aerogel, a new method was proposed to produce more silicon carbide and enhanced the mechanical properties of the SiC aerogel. This method was realized by adding an alkaline silica sol as supplemental silicon source. The principle process of CF/SiO₂ aerogels converting to SiC aerogels was discussed based on experiment and results analysis, while the microstructure, mechanical properties, and thermal properties of the prepared SiC aerogels were investigated. The results show that the as-synthesized SiC aerogels consist of β-SiC and a small amount of α-SiC nanocrystalline. It possessed a mesoporous structure and a low thermal conductivity 0.049 W/(m∙K), a relatively high compressive strength 1.32 MPa, and a relatively high specific surface area 162 m²/g. Due to their outstanding thermal and mechanical properties, the prepared SiC aerogels present potential applications in thermal insulation field, such as space shuttles and aerospace carrier thermal protection materials.     

Oxidation bonding of porous silicon carbide ceramics with synergistic performance

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process, in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. It has been shown that a high porosity can be obtained by adding a large amount of graphite into the SiC powder compacts and that the pore diameter can be controlled by the size of graphite particles and/or SiC powders. When a 0.3-μm SiC powder was used, a high strength up to 133 MPa was achieved at a porosity of 31.5%. Moreover, oxidation-bonded SiC (OBSC) ceramics were observed to exhibit an excellent resistance to oxidation and thermal shock.     

Carbon Interlayer Between CVD SiC and SiO2 in High‐Temperature Passive Oxidation

The oxidation behavior of high‐purity silicon carbide (SiC) prepared by chemical vapor deposition was investigated by thermogravimetry, transmission electron microscopy, and Raman spectroscopy in the temperature range 1534–1902 K in pure O₂. The carbon layer formed at the SiC/SiO₂ interface upon oxidation above 1784 K. Raman peaks corresponding to D‐ and G‐bands could be identified from the carbon layer. Bubbles were observed in the SiO₂ scale after the oxidation at 1873 K. This could be attributed to the accumulation of CO gas at the SiC/SiO₂ interface, resulting in the formation of the carbon layer and bubbles. These suggest that the oxidation rate of SiC is limited by the outward diffusion of CO in the SiO₂ scale in this temperature range.     

High-temperature mechanical properties and oxidation resistance of SiCf/SiC ceramic matrix composites with multi-layer environmental barrier coatings for turbine applications

Continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) ceramic matrix composites are considered promising materials as high-temperature components of advanced aero-engines. However, due to their susceptibility to oxidation and corrosion at high temperature, environmental barrier coatings (EBCs) must be applied on the surface of SiC_f/SiC. In this study, Si/Y₂SiO₅/LaMgAl₁₁O₁₉ (LMA) multi-layer EBCs were fabricated to protect SiC_f/SiC by using atmospheric plasma spraying (APS). The high-temperature tensile fatigue performance of SiC_f/SiC with and without EBCs was evaluated. The results indicated that EBCs significantly improved the tensile fatigue properties of SiC_f/SiC at high temperature in air atmosphere. Meanwhile the bending strength of specimens after isothermal aging or not was also tested. The multi-layer EBCs in this study may be a promising EBCs system for SiC_f/SiC after some improvements.     

Water vapor thermal treatment effects on spark plasma sintered nanostructured ferritic alloy‐silicon carbide systems

Spark plasma sintered pure silicon carbide (SiC) and nanostructured ferritic alloy‐silicon carbide (NFA‐SiC) systems are investigated in a water vapor containing air atmosphere at elevated temperatures up to 1000°C. Both of them exhibit excellent corrosion resistance with a dense amorphous SiO₂ layer as the main oxidation barrier. Crystalline α‐quartz and α‐cristobalite from the oxidation of silicides and SiC, respectively, further benefit the corrosion resistance. For the new NFA‐SiC system, the original graphite and silicide phases can be desirably sustained. The NFA‐SiC materials have promising applications in high temperature moist environments and are especially important for nuclear reactor cladding.     

A microstructural study of carbon–carbon composites impregnated with SiC filaments

Porous multidirectional carbon/carbon composite obtained by pulse chemical vapour infiltration (PCVI) was impregnated with silicon carbide (SiC) derived from pyrolysis of polymethylsiloxane resin (PMS). The impregnation process was made to improve oxidation resistance and mechanical properties of MD C/C composite. The resin was used as a source of silicon carbide component of the composite forming after heat treatment above 1000 °C. During this process SiC thin filaments were formed inside the porous carbon phase. The aim of this work was to investigate the structure and microstructure of the constituents of carbon composite obtained after pyrolysis of SiC PMS precursor. Microscopic observations revealed that during careful heat treatment of crosslinked polymethylsiloxane resin up to 1700 °C, the filaments (diameter 200–400 nm) crystallized within porous carbon phase. The filaments were randomly oriented on the composite surface and inside the pores. FTIR spectra and XRD analysis of the modified C/C composite showed that filaments had silicon carbide structure with the crystallite size of silicon carbide phase of about 45 nm. The Raman spectra revealed that the composite contains two carbon components distinctly differing in their structural order, and SiC filaments present nanocrystalline structure.     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.     

Thermal conductivity mapping of oxidized SiC/SiC composites by time-domain thermoreflectance with heterodyne detection

Silicon carbide/silicon carbide (SiC/SiC) composites are often used in oxidizing environments at high temperatures. Measurements of the thermal conductance of the oxide layer provide a way to better understand the oxidation process with high spatial resolution. We use time-domain thermoreflectance (TDTR) to map the thermal conductance of the oxide layer and the thermal conductivity of the SiC/SiC composite with a spatial resolution of 3 μm. Heterodyne detection using a 50-kHz-modulated probe beam and a 10-MHz-modulated pump suppresses the coherent pick-up and enables faster data acquisition than what has previously been possible using sequential demodulation. By analyzing the noise of the measured signals, we find that in the limit of small integration time constants or low laser powers, the dominant source of noise is the input noise of the preamplifier. The thermal conductance of the oxide that forms on the fiber region is lower than the oxide on the matrix due to small differences in thickness and thermal conductivity.     

Revealing thermal ablation mechanisms of C/SiC with in situ optical observation and numerical simulation

Structural ceramics such as C/SiC (carbon fiber reinforced silicon carbide) are of great importance for high temperature applications, where thermal ablation could occur and impair the structural integrity. Here we adopt in situ and real time optical visualization technique and numerical simulation to study the thermal ablation mechanisms of C/SiC subjected to oxyacetylene flame. The in situ optical setup captures the real time surface evolution at temperatures up to 1800℃, demonstrating clearly the formation of surface gas bubble, the flowing and merging process of liquid SiO₂, which are later well reproduced and explained by the numerical simulation. Four different surface ablation regions on the sample subjected to thermal ablation have been identified by the captured images and flow field streamlines, which reveal the distribution and flow mechanism of SiO₂ droplets on the C/SiC surface.     

Measuring the effects of heat treatment on SiC/SiC ceramic matrix composites using Raman spectroscopy

The evolution of residual stresses found within a silicon carbide/silicon carbide (SiC/SiC) ceramic matrix composite through thermal treatments was investigated using Raman microspectroscopy. Constituent stress states were measured before, during, and after exposures ranging from 900 to 1300°C for varying times between 1 and 60 minutes. Silicon carbide particles in the as-received condition exhibited average hydrostatic tensile stresses of approximately 300 MPa when measured at room temperature before and after heat treatment. The room temperature Raman profile of the silicon matrix was altered in both shape and location with heat treatment cycles due to increasing activation of boron within the silicon lattice as heat treatment temperatures increased. By accounting for boron activation in the silicon–boron system, little to no permanent change of any constituent stresses were observed, and the silicon matrix subsequently exhibited a complimentary average hydrostatic compressive stress of approximately 300 MPa at room temperature, measured before and after heat treatment. This result builds upon previous literature and offers increased insight into boron activation phenomena measured through Raman spectroscopy methods.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

提高氮化硅结合碳化硅窑具使用性能的研究

氮化硅结合碳化硅窑具的抗热震性能被破坏、不合理的支撑方式、承重变形因素导致其使用性能降低,影响了氮化硅结合碳化硅窑具的使用寿命。本研究制备了氮化硅结合碳化硅窑具,并结合生产实际主要研究了如何提高材料的抗氧化性、抗热震性以及其他高温理化性能,使氮化硅结合碳化硅窑具使用寿命延长。     

The cause of foaming in polishing porcelain stoneware tile residues during sintering: Interface corrosion of Silicate-SiC

The SiO₂ layer preferentially generated on the surface of silicon carbide abrasives in Polishing Porcelain stoneware tile Residues (PPR) is corroded by silicate liquid during firing, resulting in continuous oxidation and foaming of silicon carbide, which makes the PPR difficult to be reused. The PPR is fired to form a silicate liquid that wraps micron silicon carbide in it. In this work, it is possible to study the corrosion of silicate to silicon carbide at high temperature by using two (Ceramic-SiC) and three (Ceramic-SiO₂-SiC) layer structure model to simulate the corrosion of silicate to silicon carbide. The corrosion mechanism of SiC in PPR by silicate liquid during firing is revealed that the corrosion rate at its two sides (interfaces) determines the ultimate change in the thickness of the corrosion layer between the ceramic and the SiO₂ layer. Importantly, the corrosion layer thickness with time on the silicate-SiO₂ cross-section periodically presents wave-like changes, which can be attributed to the ionic migration between layers and the rate at which cations cross the corrosion layer when studying the three-layer model.