Papers by Keyword: Thermal Shock Resistance

Abstract: The article examines the physical-chemical features of the formation of the garnish on the lining of rotary kilns for firing cement clinker, the functional necessity of the garnish and the material science difficulties for its creation when developing new types of refractories, which solves the problem of the formation of Cr(VI) compounds in Portland cement. Thus, the analyzed problem of the formation of Cr(VI) compounds in Portland cements that are dangerous for the biocenosis and humans is the result of scientific and technological progress, the pursuit of leading manufacturers for the recycling of secondary raw materials and energy resources. At the same time, the timeliness of identifying the problem and its proactive, comprehensive solution with state support give a new impetus to the scientific and technical development of not only cement, but also related enterprises and scientific organizations. In Ukraine, the problem has been solved only in relation to the development and experimental testing of a new type of refractory material to replace magnesia-chromite for lining high-temperature zones of rotary kilns for firing cement clinker.

Abstract: Composite mold samples used in squeeze casting was fabricated by plasma spraying with 5CrMnMo and 3YSZ, which was shown high bonding strength and good thermal shock resistance. The best parameters with transition coat were explored by mechanical analyses. As results, the transition layer structure made of 75 percent NiCoCrAlY powders showed bonding strength was higher to 34.35MPa and that thermal cycles were up to 46.8 times. The effect of the transition layer was analyzed by microstructures and the failure mechanism of the coating material with a transition layer was discussed. The conclusion was that the physical mismatch and thermodynamic mismatch between the matrix and ceramic layers were the main cause of the failure.

Abstract: Si₃N₄/hBN composites were fabricated by hot isostatic pressing at 1700°C/3h with 1, 3 and 5 wt. % micro-sized or nano-sized hexagonal boron nitride particles added to silicon nitride matrix. An indentation quench test method was used for estimation of thermal shock resistance of monolithic Si₃N₄ and Si₃N₄/hBN composites. Thermal shock resistance of the composites increased with the increase of size and volume of hBN particles. The critical temperature difference for the composites with micro-sized hBN was significantly higher (over 900°C) compared to the monolithic silicon nitride (580°C).

Abstract: ZTA/cordierite composites were prepared using ZrO₂, Micro-Al₂O₃/Nano-Al₂O₃ and cordierite as raw materials by pressureless sintering. The influence of Nano-Al₂O₃ particles content on phase composition, microstructure and thermal shock resistance of ZTA/cordierite were investigated. The results show that m-ZrO₂, t-ZrO₂ and Al₂O₃ existed in the matrix and t-ZrO₂ content increased with the increase of Nano-Al₂O₃ powders content. By adding Nano-Al₂O₃ powders and ultrasonic dispersion, which can refine grain and promote sintering, the grain size is uniform, the porosity is less and some grains were pulled out, the fracture mode changes from intergranular fracture to intergranular fracture and transgranular fracture. The thermal shock test was carried out at 300°C-1000°C, the critical thermal shock temperature of ZTA ceramic without adding cordierite and Nano-Al₂O₃ particles is 800°C and the residual strength retention rate of the material is only 32.4%. But by adding Nano-Al₂O₃ powers and ultrasonic dispersion 30 min,the residual strength retention rate, relative density, flexural strength and fracture toughness of ZTA/cordierite sintered at 1550°C for 4 h increased greatly and were up to 73.6%, 97.2%, 436 MPa and 4.6 MPa.m^1/2, respectively.

Abstract: Rare-earth silicates, especially ytterbium silicate (Yb₂SiO₅ and Yb₂Si₂O₇), have been developed for promising environmental barrier coatings (EBCs) for SiC-matrix composites. In this study, double-layer Yb₂SiO₅/Si and Yb₂Si₂O₇/Si EBC systems were deposited on C/SiC composites by air plasma spray (APS) technique, respectively. Both systems were subjected to thermal shock tests at 1400 °C. The evolution of phase composition and microstructure of those samples before and after thermal shock test were characterized. Results showed that there were penetrating microcracks in the top Yb₂SiO₅ layer and horizontal microcracks at the Yb₂SiO₅-Si interface after thermal shock test. While extremely few microcracks and no horizontal microcracks were presented in the Yb₂Si₂O₇/Si sample. The EDS analysis also showed that the Si bond layer of the Yb₂SiO₅/Si sample was oxidized more serious than that of the Yb₂Si₂O₇/Si sample. The different thermal shock behaviors of both systems were clarified based on the thermal expansion behavior, phase composition and microstructure analysis.

Abstract: When a sudden temperature difference is applied to a brittle material such as ceramics, some cracks will occur in the material and it may fracture in some case. The generated cracks as a fracture origin may cause the strength reduction, so the evaluation of thermal shock resistance is very important for ceramic materials. In the conventional evaluation of the thermal shock resistance (thermal shock fracture temperature, ΔT_C), the fracture stress is measured after thermal shock test as a function of temperature difference. For this method, however, many specimens are required to estimate fracture stress by bending test and the variation of the stress is large. In the present study, we tried to specify the temperature of crack initiation by measuring Young's modulus and fracture stress before and after a thermal shock test with different temperature difference. Polycrystalline alumina with high purity was used for evaluation of thermal shock resistance. The Young's modulus of all specimens was measured by resonance method. The specimen at the prescribed temperature between 200°C and 600°C, it was quickly put into water to apply thermal shock. The Young's modulus of specimens after the test was measured and the change in Young's modulus before and after thermal shock test was evaluated. Further, the specimen after the evaluation was measured the fracture strength. As a result, it was found that Young's modulus is possible to estimate thermal shock resistance. Further it is also possible to evaluate thermal shock behaviors using only one specimen.

Abstract: The impact of impurities on the microstructure and thermal shock resistance of YSZ TBCs was investigated. ZrO₂-7.5wt%Y₂O₃ (7.5YSZ) coatings of 5 spices of compositions (4 of them were doped with one type of impurity, namely, Al₂O₃, Fe₂O₃, SiO₂ and TiO₂, and the other was not doped) were manufactured by APS. Thermal shock tests of these TBCs were carried out at 1100 °C, and the samples were quenched in cold water in the temperature range from 20 to 25°C. The microstructure evolution and phase analysis were performed before and after the thermal shock tests by scanning electron microscope (SEM) and X-ray diffractometer (XRD). The results showed that the thermal shock lifetime of YSZ TBCs doped with impurities was obviously reduced compared to that of no-impurity TBCs. Additionally, the differences in the microstructure and phases of YSZ TBCs were ascertained and the elements distribution was tested by Energy Dispersive Spectrometer (EDS) and Energy Dispersive X-Ray Spectroscopy (EDX).

Abstract: Deposition of NiCr-CrC(20NiCr) metal matrix composite (MMCs) coating have been applied on the substrate of boiler tubes material with High Velocity Oxygen Fuel (HVOF) thermal spray method and constant parameter. Variation of particle size and composition on MMCs was conducted to determined the optimum conditions for boiler applications. Microvickers hardness, metallography and thermal shock resistance testing were investigated. The best performance for boiler tubes application is MMC NiCr-CrC(20NiCr) with 270 mesh of NiCr particles size and 60:40 of composition as evidence by the highest of hardness value (410 Hv) and slightly of discoloration after thermal shock resistance with two variation cooling medium. While at MMC NiCr-CrC(20NiCr) with 70:30 variation composition, coating hardness value will decrease in line with the smaller of particle size of NiCr.

Abstract: Spinel-C material is important for monolithic stopper and submerged nozzle, which usually determine the application effects. Spinel-C specimens with 8% flake graphite is characterized by good corrosion resistance but poor thermal shock resistance. The effects of additives (Al-Si alloy, silicon powder, B₄C and SiC), flake graphite (+199/-199), ultrafine flake graphite, expanded graphite and nickel nitrate + pitch powder on thermal shock resistance of spinel-C material were investigated by using water-quenching method. The results show that: (1) The addition of Al-Si alloy and B₄C can significantly decrease the residual strength ratios (RSR) of the specimens. The addition of SiC and silicon powder can improve the RSR obviously. (2) The flake graphite of different particle sizes has little effect on the RSR of the specimens. With the increasing of the expanded graphite content, the RSR of the specimens increases. (3) The addition of mesophase pitch and Carboresâ P can receive the RSR rise of 74% and 59% respectively.

Abstract: A kind of ceramic composite consisted of Mg-Y co-doped ZrO₂ and MgAl₂O₄ was prepared as candidate material to replace the high destiny graphite crucible which was often used in nuclear industry. When the addition of MgAl₂O₄ was 7 wt%, samples with good thermal shock resistance were obtained. The obtained samples were tested up to 12 h in molten uranium, and no apparent cracks were found on the corrosion surface. Meanwhile, the corrosion rate decreased significantly from 30 mg·cm^-2·h^-1 to 3 mg·cm^-2·h^-1 as the corrosion time prolonged from 2 h to 12 h due to the formation of a protection layer consisted of UO₂, Al₃Zr₂ and ZrC during the uranium corrosion test. These results suggested that the ceramic composite prepared in this study owned both good corrosion resistance to the molten uranium and thermal shock resistance at high temperatures,so it’s a high potential material to replace the high density graphite crucible in nuclear industry.