Effect of Y2O3 on the oxidation properties of ZrSi2/SiC coating prepared by SAPS on the carbon-carbon composites

In order to improve the oxidation resistance of carbon-carbon (C/C) composites at high temperature, different content of Y₂O₃ modified ZrSi₂/SiC coating for C/C composites were prepared by pack cementation and supersonic atmosphere plasma spraying (SAPS). Microstructure observation and phase identification of the coatings were analyzed by SEM, XRD, DSC/TG and EDS. Experimental results shown that the coating with 10?wt% Y₂O₃ effectively protected C/C composites from oxidation at 1500?°C in air for 301?h with a mass loss of 0.13% and experienced 18 thermal shock times from room temperature (RT) to 1500?°C. First, Y₂O₃ could restrain the phase transition of ZrO₂ to reduce the formation of thermal stresses of the coating; second, the random distribution of ZrO₂ ceramic particles and the formation of ZrSiO₄ enhanced the stability of the SiO₂; third, the formation of Y₂Si₂O₇ and Y₂SiO₅ could relieve the thermal mismatch between ZrSi₂-Y₂O₃ outer layer and the inner layer.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Mechanical properties and microstructural evolution of Cansas-III SiC fibers after thermal exposure in different atmospheres

Cansas-III SiC fibers were exposed in argon, air and wet oxygen (12%H₂O+8%O₂+80%Ar) atmospheres for 1 h at 1000–1500 °C. The pristine fiber consisted of β-SiC, free carbon and SiC_xO_y phases. After exposure in air and wet oxygen, an amorphous SiO₂ layer with embedding α-cristobalite crystals formed, while stacking faults were generated in the SiC core to release the residual stress. With the increasing oxidation temperature, lots of pores formed in the oxide layer, accompanied with the thickening, cracking and spallation of oxide layer. The average tensile strength decreased with the exposure temperature increasing and the exposure atmosphere deteriorating (argon→air→wet oxygen). After exposure at 1400 °C in argon and air, the fiber strength retention rates were 84% and 70%, respectively. However, after exposure at 1300 °C in wet oxygen, the strength retention rate was only 51%, indicating the accelerating oxidation and severe strength degradation of fibers.     

Nb2AlC-particle induced accelerated crack healing in ZrO2–matrix composites

The oxidation induced crack healing behavior of pre-cracked Nb₂AlC particle loaded ZrO₂-matrix composites was explored by annealing in air at 1200?°C for short periods of 10 and 20?min. Composites loaded with 0, 6.5, 13, and 19.5?vol% Nb₂AlC powder dispersed in 3Y-TZP matrix powder were manufactured by spark plasma sintering (SPS) at 1300?°C. Semi-elliptical artificial surface cracks with a length exceeding 220?µm were produced by Vickers indentation. The modulus of rupture of virgin, indented and annealed samples was measured in three-point bending mode. Compared to single phase 3Y-TZP strength recovery of the Nb₂AlC loaded composite upon annealing at 1200?°C in air is accelerated and reaches >?60% of the initial strength after a short healing period of 10?min only. A semi-empirical oxidation cohesive zone healing model was derived which describes the crack microstructure evolution as a combined effect of 3Y-TZP-matrix healing superimposed by Nb₂AlC particle oxidation induced healing.     

The improvement in thermal and mechanical properties of TiB2 modified adhesive through the polymer-derived-ceramic route

An air stable high temperature adhesive synthesized via the polymer-derived-ceramic route had received increased attention in the last two decades. To improve the thermal stability and adhesion strength of a polysilazane (PSNB) adhesive, TiB₂ was added as active filler to join SiC ceramic discs. The thermal stability, phase composition and microstructure were investigated by using TGA, XRD, FT-IR, BSE and SEM measurements. Effects of the pyrolysis temperature and active filler TiB₂ on the microstructure and adhesion strength have been investigated. After curing and heat-treating at 120?°C and 1000?°C in air for 2?h, respectively, the adhesion strength of the modified adhesive reached up to 10.07?MPa (3 times higher than that of pure PSNB) at room temperature, and, more importantly, retained a strength of 8.0?MPa at 800?°C in air. It should be noted that the formation of a glass comprised of SiO₂-B₂O₃-TiO₂ and the emergence of the hexagonal and granular TiB₂ in the adhesive layer are mainly responsible for the enhanced high temperature strength.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

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.     

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi₂, MoSi₂-20?vol% (ZrB₂-20?vol% SiC) and MoSi₂-40?vol% (ZrB₂-20?vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi₂-(ZrB₂-SiC) ceramics were investigated at 1600?°C for different soaking time of 60, 180 and 300?min, respectively. The oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics was conducted at a higher temperature of 1800?°C for 10?min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi₂ was improved by adding ZrB₂-SiC additives, and the MoSi₂-20?vol% (ZrB₂-20?vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi₂-based ceramics at high temperatures.     

Photoluminescence properties of SiC/SiO2 heterojunctions obtained by TiO2-assisted chemical vapor deposition

SiC/SiO₂ heterojunctions have been synthesized at 1400?°C by chemical vapor deposition, using Si and phenolic resin powder as starting materials and TiO₂ nanoparticles as catalyst. The morphology of the SiC/SiO₂ heterojunctions consists of chain-bead shaped and core-shell chains, and the heterojunctions are up to several hundred microns in length, with diameters of 0.3–1?µm in the chains and 2–8?µm in the beads. A vapor-solid tip-growth mechanism for the formation of the heterojunctions is proposed. Photoluminescence spectra of SiC/SiO₂ heterojunctions exhibit a significant blue-shift, which indicates that these materials are ideal for application in new optoelectronic devices. The TiO₂ nanoparticles play a key role in promoting the formation and growth of the heterojunction nanochains as well as in enhancing their luminescence properties.     

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.     

Densification of silicon dioxide formed by plasma-enhanced atomic layer deposition on 4H-silicon carbide using argon post-deposition annealing

Post-deposition annealing (PDA) was used to improve gate oxide physical and electrical properties. Deposition was accomplished by plasma-enhanced atomic layer deposition (PEALD). We investigated the densification silicon dioxide (SiO₂) formed by PEALD on 4H-silicon carbide (SiC) using PDA without oxidation and nitridation. PDA was conducted at 400–1200?°C in argon (Ar) ambient. The thickness of the SiO₂ was reduced by up to 13.5% after Ar PDA at 1000?°C. As the temperature of the Ar PDA increased, the etching rate of SiO₂ decreased. At temperatures greater than 1000?°C, the SiO₂ etching rate was low compared with that of thermal SiO₂. After PDA, the SiO₂/4H-SiC interface was smoother than that of thermal SiO₂/4H-SiC. The current density versus oxide field and capacitance versus voltage of the SiO₂/4H-SiC metal oxide semiconductor (MOS) capacitors were measured. Sufficient densification of SiO₂ formed by PEALD on 4H-SiC was obtained using Ar PDA at 1200?°C.     

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB₂–SiC–TaSi₂–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm²·h^0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10⁻³ after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO₂ as skeleton phase with SiO₂ as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB₂–SiC–TaSi₂ major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB₂–SiC–TaSi₂–Si layer was a vital factor as well.     

Degradation behavior of SiO2f/Al2O3-SiO2 composites: Anti-oxidation behavior and high-temperature flexural strength

2.5-Dimensional SiO₂ fiber-reinforced Al₂O₃-SiO₂ (SiO_2f/Al₂O₃-SiO₂) composites were prepared by the sol-gel method, using diphasic SiO₂ sol as the precursor into which Al₂O₃ powders were added. Their antioxidative behaviors and flexural strengths at high temperature were tested and compared. In an oxidation atmosphere, the composites showed high oxidation resistance, with a flexural strength retention ratio of over 90.00% at 1200?°C. After oxidation at 1500?°C, the mass retention ratio and flexural strength were 97.49% and 65.0?MPa respectively. The oxidation resistance of SiO_2f/Al₂O₃-SiO₂ composites was higher than that of SiO_2f/SiO₂ composites. After high-temperature test, the flexural strength retention ratios of SiO_2f/SiO₂ and SiO_2f/Al₂O₃-SiO₂ composites were 86.18% and 94.80% respectively, and the latter had a flexural strength of 134.9?MPa. SiO_2?f/Al₂O₃-SiO₂ composites worked better than SiO_2f/SiO₂ composites did in the flexural strength test at 1200?°C. The mechanical performance degradation and mass variations of the composites during tests were closely associated with their microstructural evolutions.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.     

Effect of temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70 µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600 °C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44 h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO₂ layer, porous SiO₂ layer and clusters of honeycomb-like SiO₂ grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO₂ declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.     

Water vapor corrosion behavior and failure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings

A new coating with mullite as the inner layer and Lu₂Si₂O₇-Lu₂SiO₅ (lutetium disilicate-lutetium monosilicate, LuDS-LuMS) composite as the top layer was designed and fabricated on the surface of porous SiC substrate by atmospheric plasma spraying (APS). The microstructure evolution, phase transformation, inter-diffusion and failure mechanism of the coated samples during steam cycling at 1450?°C were systemically investigated. The results indicated that the coated samples maintain weight gain and the average weight gain rate is 1.12?×?10^?1 mg/cm² h. The thermal expansion coefficient (CTE) mismatches between the coating and the substrate as well as between the two ceramic layers have produced a thermal mismatch stress. In addition, the chemical reaction induced the sintering of LuDS-LuMS layer, the amorphous-crystalline phase transformation and the transformation between LuDS and LuMS phases could result in aging stress. With the accumulation of thermal mismatch stress and aging stress, the through and horizontal cracks have appeared in the coating, leading to the coating failure.     

Effect of real working environment/formation of oxide phase on thermoelectric properties of flexible Sb2Te3 films

Flexible Sb₂Te₃ thin films, for thermoelectric generator applications, were deposited by DC magnetron sputtering. As-deposited films were annealed in air to simulated a realistic operating environment. The oxidation behavior of the films was studied by monitoring their phase change on exposure to air at different temperatures between 50 and 300 °C for annealing times from 1 to 15 h. Oxidation of Sb and Te formed Sb₂Te₄ and TeO₂ phases when annealing above 100 °C and Sb₂Te₃ decomposed into oxide phases at an annealing temperature of 250 °C for 15 h. The thermoelectric performance decreased as the content of Sb₂O₄ and TeO₂ phases increased. These findings show the limitations of Sb₂Te₃ films operating in air without vacuum or a protective environment. We propose that the kinetic growth of oxide formation on the Sb₂Te₃ thin films depend on chemical activation energy and oxygen diffusion through the oxide barrier by the variation of annealing temperature and annealing time, respectively.