Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

Experimental and numerical study of grain growth in alumina fibres heat treated at 1700°C

The studied material is an alumina fibre acting as a reinforcement in a NiAl/Al₂O₃ composite. The processing of this composite involves a 1700°C heat treatment. Grain growth phenomena taking place within the fibre during that step are investigated. Above 1600°C, the size of the grains reach the fibre diameter for a 1 h heat treatment. Further morphological evolution is discussed using thermodynamic calculations. Taking into account the presence of the liquid alloy, a deepened grain boundary groove is predicted and experimentally observed. The second part of this work deals with grain growth modelling during the heating and cooling steps. A cubic grain growth kinetics law is found and is used to predict final grain sizes as a function of heating and cooling rates.     

Densification and ablation behavior of ZrB2 ceramic with SiC and/or Fe additives fabricated at 1600 and 1800 °C

The effects of Fe and SiC additions on the densification, microstructure, and ablation properties of ZrB₂-based ceramics were investigated in this study. The sample powders were conventionally mixed by cemented carbide ball then sintered by spark plasma sintering. The ablation rates and behavior of the ceramics were investigated under an oxyacetylene torch environment at about 3000 °C. A sample with high relative density (96.3%), high flexural strength (415.6 MPa), and low linear ablation rate (−0.4 µm/s) was obtained via SPS at 1600 °C. Adding 4 vol% Fe was more beneficial to the density of ZrB₂ sintered at 1600 °C as compared to ZrB₂ sintered at 1800 °C. The ablation behavior and rates were similar among samples sintered at 1600 °C and 1800 °C.     

Oxidation behavior of 3D SiCf/SiBCN composites at 800–1200 °C

3D SiC_f/BN–SiC/SiBCN composites were fabricated via precursor impregnation and polymer infiltration pyrolysis (PIP). Oxidation behavior of the composites heated in air at 800 °C, 1000 °C and 1200 °C for 50 h was investigated. Following the oxidation treatment, it was found that the bending strength of the composites at different oxidation temperatures was degraded. The weight loss of the composites decreased gradually over the range of oxidation times of 1–50 h. In order to clarify the oxidation mechanism of the composites, reconstructed images, microstructures, phase compositions, the oxide layer formed on the composites and main chemical reactions were all analyzed. It was revealed that the degradation in the fracture strength of the composites was closely related to the oxidation of SiBCN matrix and BN-SiC interphase, whereas there was no signs of oxidation products about SiC fiber, which indicated that SiC fiber could be protected from oxygen by SiBCN matrix at 800?1200 °C in air.     

Flexural strength and fracture behavior of ZrB2–SiC ultra-high temperature ceramic composites at 1800 °C

ZrB₂–15 vol.%SiC and ZrB₂–30 vol.%SiC composites with smaller starting particle sizes in which the particle sizes of ZrB₂ and SiC are 2 μm and 0.5 μm, respectively, demonstrated marked plasticity and significant reduction in the flexural strength at 1800 °C. The flexural strengths of these two composites are 112 ± 12 MPa and 48 ± 10 MPa, respectively, and their corresponding strength retentions are 13% and 7%, respectively. Large ZrB₂ grains were commonly observed in the samples containing 15 vol.%SiC, which are always the sites for the crack initiation. Cavities were found in the samples containing 30 vol.%SiC and the grain boundaries are the main sites for the crack and cavity nucleation. To improve ultra-high temperature strength, larger starting particle sizes (ZrB₂ and SiC are 5 μm and 2 μm, respectively) were used for the preparation of ZrB₂–15 vol.%SiC. This sample fractured in an elastic manner up to 1800 °C and showed a very high strength with a value of 217 ± 16 MPa.     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Ablation mechanisms of Ti3SiC2 ceramic at 1600 °C in nitrogen plasma flame

Present work showed that Ti₃SiC₂ ceramic has good ablation resistance at 1600 °C. The mass ablation rate and linear ablation rate after exposure to plasma flame for 120 s are ?0.23 mg/s and 5.58 μm/s, respectively. After ablation for 120 s, the micro-morphologies of cross section of the ablation fringe show that the ablation layer was divided into four-layers: an oxide outer layer, a porous layer, a decomposition layer, and the matrix layer. The formation of porous layer is mainly related to the Ti vacancies caused by the outward diffusion of Ti ions, and outward diffusion of gaseous SiO and CO. The dense and large grain-sized TiO₂ oxide layer is dismembered by the generated SiO₂ to form a fine-structured TiO₂ and SiO₂ mixture. The solid-liquid mixed oxide layer is quickly blown away under the shearing force of plasma flow, resulting in a significant increase of linear ablation rate.     

Creep behavior and failure mechanisms of CVI and PIP SiC/SiC composites at temperatures to 1650 °C in air

Creep properties of 2D woven CVI and PIP SiC/SiC composites with Sylramic™-iBN SiC fibers were measured at temperatures to 1650 °C in air and the data was compared with the literature. Batch-to-batch variations in the tensile and creep properties, and thermal treatment effects on creep, creep parameters, damage mechanisms, and failure modes for these composites were studied. Under the test conditions, the CVI SiC/SiC composites exhibited both matrix and fiber-dominated creep depending on stress, whereas the PIP SiC/SiC composites displayed only fiber-dominated creep. Creep durability in both composite systems is controlled by the most creep resistant phase as well as oxidation of the fibers via cracking matrix. Specimen-to- specimen variations in porosity and stress raisers caused significant differences in creep behavior and durability. The Larson-Miller parameter and Monkman-Grant relationship were used wherever applicable for analyzing and predicting creep durability.     

Cyclic oxidation performances of new environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 coated SiC at 1375 °C and 1475 °C in the air environment

The objective of this work is to study the cyclic oxidation performances of the environmental barrier coatings (EBCs) containing the novel HfO₂-SiO₂ bond coats in the air environment. Bi-layer HfO₂-SiO₂/Yb₂Si₂O₇ (50HfO₂-50SiO₂, 70HfO₂-30SiO₂ bond coats) and conventional Si/Yb₂Si₂O₇ EBCs were deposited on SiC substrate using atmospheric plasma spray. The effect of the pre-mixing ratios of HfO₂/SiO₂ on the cyclic oxidation behavior of HfO₂-SiO₂/Yb₂Si₂O₇ EBCs was examined. The results showed that the higher content of the HfSiO₄ formed from the 50HfO₂-50SiO₂ bond coats, and it remained intact. A thermally grown oxide (TGO) SiO₂ layer was formed at the bond coat/SiC interface. The parabolic oxidation rate constant (k_p, μm²/h) of the TGO has been reduced 2 orders of magnitude in 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs coated SiC compared to the bare SiC at 1475 °C, indicating that the 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs effectively protected the SiC substrate at 1475 °C.     

Oxidation resistance and microstructure evolution of ZrB2–SiC–La2O3/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures

The oxidation behaviors of a ZrB₂–SiC–La₂O₃/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures (50, 5 and 0.5 kPa) were investigated. The results showed that with the decrease of air pressure, the oxidation kinetics of the coated samples changed from parabolic weight gain to linear weight loss. A protective oxide scale consisted of ZrO₂ and SiO₂ with La dispersed was formed on the coating surface after oxidation in 50 kPa air. The oxide scale formed in 5 kPa air was full of bubbles. Only porous ZrO₂ layer was left on the coating surface after oxidation in 0.5 kPa air. At 1800 °C, the active oxidation of SiC occurred and gaseous SiO formed at the coating/oxide interface. The surface volatilization of SiO became severe with the decrease of air pressure, resulting in the presence of non-protective oxide scale.     

Long-term oxidation and ablation behavior of Cf/ZrB2–SiC composites at 1500, 2000, and 2200°C

Although C_f/ZrB₂–SiC composites prepared via direct ink writing combined with low-temperature hot-pressing were shown to exhibit high relative density, high preparation efficiency, and excellent flexural strength and fracture toughness in our previous work, their oxidation and ablation resistance at high and ultrahigh temperatures had not been investigated. In this work, the oxidation and ablation resistance of C_f/ZrB₂–SiC composites were evaluated via static oxidation at high temperature (1500°C) and oxyacetylene ablation at ultrahigh temperatures (2080 and 2270°C), respectively. The thickness of the oxide layer of the C_f/ZrB₂–SiC composites is <40 μm after oxidizing at 1500°C for 1 h. The C_f/ZrB₂–SiC composites exhibit non-ablative properties after oxyacetylene ablation at 2080 and 2270°C for >600 s, with mass ablation rates of 3.77 × 10⁻³ and 5.53 × 10⁻³ mg/(cm² s), and linear ablation rates of −4.5 × 10⁻⁴ and −5.8 × 10⁻⁴ mm/s, respectively. Upon an increase in the ablation temperature from 2080 to 2270°C, the thickness of the total oxide layer increases from 360 to 570 μm, and the carbon fibers remain intact in the unaffected region. Moreover, the oxidation and ablation process of C_f/ZrB₂–SiC at various temperatures was analyzed and discussed.     

Thermal corrosion behavior of Yb4Hf3O12 ceramics exposed to calcium-ferrum-alumina-silicate (CFAS) at 1400 °C

In current work, the interaction between representative CFAS deposit (33CaO-10FeO_1.5-13AlO_1.5-44SiO₂) and Yb₄Hf₃O₁₂ ceramics at 1400 °C was investigated. Results indicated that the Yb₄Hf₃O₁₂ ceramics are of high resistance to infiltration of CFAS melt. Microstructure characterization revealed that Yb₄Hf₃O₁₂ reacted with CFAS to form a continuous reaction layer mainly composed of Yb-Ca-Si apatite, which inhibits CFAS further infiltration. Before the formation of the reaction layer, CFAS melts underwent a crystallization process at high temperatures, precipitating CaYbFeAlSi-garnet, which raised the viscosity of CFAS and thus inhibited the fluidity of CFAS.     

Morphology and microstructure of SiC/SiC composites ablated by oxyacetylene torch at 1800°C

An oxyacetylene torch tested the ablation of SiC/SiC composites at 1800℃. According to the distribution of ablation product silica, the morphology could be divided into three regions. The fibers in the oval central region were ablated and broken, and the fracture surface is the conical tip. The silica liquid film in the transition region plays a role in resisting the ablation of the material. However, the generation of airflow channels destroys the liquid film's continuity and reduces the material's ablation resistance. Bean sprouts-like nano-sized silica was grown on the surface of the dome-top SiC matrix in the marginal region.     

Experimental and theoretical study on the resistive properties of LaCo0·4Ni0·6O3 ceramic with Mg-ion substitution

To explore an air-fireable and economically functional phase for thick-film resistors, the crystal structure, microstructure, sintering, and resistive properties of LaCo_0·4Ni_0.6−xMg_xO₃ (LCNM, x = 0.00–0.08) ceramics are studied in this paper, based on solid-state reaction experiments and theoretical calculations. With Mg-ion increase, the impurity phases of NiO and La₃Ni₄O₁₀ increased, the maximum shrinkage rate occurred at a higher temperature, and the microstructures became worse. The addition of the Mg ion increased the resistivity and decreased the temperature coefficient of resistivity (TCρ). The calculation results, including the bond population, electron density distribution, and density of states of the system agreed with the change in experimental resistivity. Relatively low resistivity and TCρ with a small absolute value could be achieved for specimens with x ≤ 0.02 sintered at 1200 °C and 1250 °C. Hence, the LCNM (x ≤ 0.02) ceramics are possible functional material for thick-film resistors.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Joining of Cf/SiC composites and Si3N4 ceramic with Y2O3–Al2O3–SiO2 glass filler for high-temperature applications

C_f/SiC composites and Si₃N₄ ceramics are candidate materials for applications in thermal protection system. This paper investigated the joining of C_f/SiC and Si₃N₄ using Y₂O₃–Al₂O₃–SiO₂ glass. The reliability of joints was evaluated by thermal shock tests. In this present work, the typical joint structure was C_f/SiC-glass-Si₃N₄. The results demonstrate that Direct bonding has been identified as the interfacial bonding mechanism at the SiC/glass and glass/Si₃N₄ interfaces. The maximum shear strength of the C_f/SiC–Si₃N₄ joint was ~34 MPa, which delivered an effective method to achieve strong, reliable bonding between the dissimilar materials. In addition, after thermal shock for 10 cycles, the residual strength remained ~13 MPa. Bubbles instead of microcracks formed in the glass filler, which was the main factor causing the degradation of the joint performance. It is suggested that improving the high temperature resistance of joining materials is the key to realize the application of this joint structure.     

Crack-healing behavior of Si3N4/SiC ceramics under stress and fatigue strength at the temperature of healing (1000 °C)

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending specimens made according to the appropriate JIS standard. A semi-circular surface crack of 100 μm in diameter was made on each specimen. We systematically studied crack-healing behavior, and cyclic and static fatigue strengths at the service temperature (1000 °C) by using three kinds of specimens (smooth, cracked and crack-healed). The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have the excellent ability to heal a crack at 1000 °C; (2) this sample could heal a crack even under cyclic stress at 1000 °C; (3) a new crack-healing process was proposed. The sample crack-healed at 1000 °C by the process exhibited a sufficient static and cyclic fatigue strength at 1000 °C.     

Microstructure and corrosion behavior of in-situ grown Y3Si2C2 coated SiC fibers exposed to air and wet-oxygen at 1400 ℃

Y₃Si₂C₂ ternary ceramics were in-situ grown on the third-generation Chinese commercial SiC fiber (KD-SA SiC fiber) surface via molten salt method. Microstructures and oxidation/corrosion behavior of in-situ grown Y₃Si₂C₂ coated SiC fibers exposed to air and wet-oxygen at 1400 ℃ were investigated. Results indicated that the layered Y₃Si₂C₂ slices with thickness of approximately 15 nm can be successfully in-situ grown on SiC fibers. The product on the fibers surface after oxidation/corrosion at 1400 ℃ for 1 h in both ambient air and wet-oxygen are Y₂Si₂O₇ and SiO₂. Moreover, microstructural characterization indicates that the immigration and expansion of gaseous bubbles induced by oxidation product, mainly CO, result in microstructural differences of SiC fiber specimens, and finally oxidation mechanism based on the microstructural difference were proposed.     

Improving the Cyclic-Oxidation Resistance of Ti3AlC2 at 550°C and 650°C by Preoxidation at 1100°C

Preoxidation of Ti₃AlC₂ at 1100°C for 2 h was conducted to improve its cyclic-oxidation resistance at the testing temperature of 550°C and 650°C in air. The cyclic oxidation of the preoxidized Ti₃AlC₂ was found to follow a parabolic rate law rather than the linear oxidation rate for that without preoxidation. Through the X-ray diffraction and SEM analysis, the remarkable improvement of the cyclic-oxidation resistance of preoxidation Ti₃AlC₂ is suggested due to the existence of protective α-Al₂O₃ layers formed during the preoxidation treatment, which inhibits the formation of amorphous Al₂O₃, which can result in larger thermal stress and stress-induced microcracks.