Papers by Keyword: Thermal Shock Resistance

Abstract: A kind of TiB₂/Mo₂FeB₂ composite powders for reactive thermal spraying (RTS) and atmospheric plasma spraying (APS) were prepared using Mo powder, Fe-B alloy powder, Fe powder and TiB₂ powder. The TiB₂/Mo₂FeB₂ composite ceramic coatings were synthesized and deposited by APS and RTS on Q235 steel. Microstructure and phases were investigated by X-ray diffraction (XRD) and scanning electrical microscopy (SEM). The thermal shock resistance, wear resistance and corrosion resistance were characterized with the relevant lab testing equipment. The result indicates that the coatings are composed of the major phases Mo₂FeB₂, TiB₂ and α-Fe, a little of Fe₂O₃, FeO, and B₂O₃.The thermal shock times of RTS-sprayed coatings is 44, the corrosion resistance is 6.07 times than those of the substrate, respectively, while those of APS-sprayed coatings are 55 and 15.5, respectively. The mass loss of Q235 steel is 138.6g/m², while the mass loss of the coatings with 30% TiB₂ which were prepared by RTS is 28.0952g/m², the mass loss of the coatings with 30% which were prepared by APS is 15.4028g/m², when the testing load was 400N. Therefore, the coatings with 30%TiB₂ prepared by APS possess better wear resistance than the Q235 steel substrate.

Abstract: Material of Sm₂Zr₂O₇ composition powder was synthesized by coprecipitation method using Sm₂O₃ and ZrOCl₂•8H₂O as raw materials, and applied as ceramic topcoats of thermal barrier coating (TBC) by atmospheric plasma spraying (APS). The ceramics powder was prepared with proper distribution of particle size about 30-70μm by spray-dried method. Phase analyses revealed that Sm₂Zr₂O₇ coatings keeping the single phase with pyrochlore structure before and after the thermal cycling test. The thermally grown oxide at the topcoat-bondcoat interface and the formation of transverse crack were the major factors that lead to the coating failure on thermal cycling.

Abstract: The thermal shock behavior of alumina ceramics tested by two different approaches, water and air quenching, using an automatic experimental set was investigated. The changes of the flexural strength before and after the thermal shock was measured and used as an indicator of thermal shock resistance. The study reveals that air quenching test has limited impact on the changes of flexural strength, whereas the water quenching yields considerable decreases of the strength. The alumina ceramics was quenched in water at various temperature differences for five cycles. It is shown that the retained strength of the quenched specimens decreases abruptly at the temperature difference of 300°C, which indicates a great severity of thermal shock in this point. The thermal shock behavior of the specimens is evaluated by quenching in water as three different temperature differences, ΔT, setting at 300°C, 600°C and 800°C, respectively. The results show, for three different ΔT quenches, the strength reductions caused by the quenching exhibit similar trends: After a sharp drop, the residual strength remains almost unchanged at a certain level for each given quenching temperature difference, and the turning points all fall in the very first five to ten thermal cycles range. And the rank of the damage severity of alumina ceramics among the three different temperature differences is: ΔT₈₀₀°C > ΔT₆₀₀°C > ΔT₃₀₀°_C

Abstract: The physical and mechanical properties of alumina (Al₂O₃) ceramics by introduction of zirconia (ZrO₂) fiber were studied. ZrO₂/Al₂O₃ ceramics at different sintering temperature was investigated by porosity and water absorption measurements, flexual strength and thermal shock resistance analysis. Results showed that Al₂O3 ceramics containing 15 wt% ZrO₂ fiber with sintering temperature of 1650°C exhibited good mechanical properties and thermal shock resistance. The porosity and water absorption were 7.4% and 0.69%, respectively. The flexual strength was 613 MPa and the thermal shock times reached 29 times. Scanning electron microscope (SEM) was used to analyze the microstructure of Al₂O3 ceramics.

Abstract: To provide the test method for thermal shock resistance of throat insert material in rocket motor design, an experimental method with a SRM of scale down was developed to simulate the thermal load of full-size rocket motor’s throat insert. The test guaranteed the consistency of propellant、chamber pressure 、rate of ignition pressurization and the contour of the surface of throat insert with full-size rocket motor, and make a compromise of condition of throat insert at the same time. This method had well reappearance of the fracture failure of throat insert of a full-size rocket motor, which proved the feasibility of the test method to a certain extent. A comparison test for thermal shock resistance of two different composite materials was carried out. The experimental results indicate that two reserve C/C composite materials had good thermal shock resistance than three directions woven C/C composite material which used in the motor of failure, and the performance of carbon fabric pierced C/C composite material was better than no woven fabric pierced C/C composite material in some sort.

Abstract: This paper deeply studied the limitation of evaluating the thermal shock resistance of Al₂O₃-SiO2 refractory by measuring Strength Loss Rate (SLR). By means of supersonic, X-ray diffraction (XRD), the results were drawn as followed.1)After the thermal shock experiment for mullite based A-S refractory, which were composed by mullite and corundum, the supersonic velocity slowed down as it going through the sample while the cold crushing strength abnormally increased at initial state.2) The strength increased remarkably with the ascending of mullite fraction.3)It is inadequate to evaluate the thermal shock resistance of mullite based A-S refractory by strength loss rate.

Abstract: The Al₂O₃-Cr₂O₃ slag as raw material was analyzed by scanning electron microscopy (SEM). The effects of different size distribution of MgO-Al₂O₃ spinel grain on physical properties of refractory bricks made from Al₂O₃-Cr₂O₃ slag was studied according to YB/T376.1-1995（water quenching）, GB/T 5072－1985, GB/T 2997-1982 criterion. The results show that the main crystal phase of the refractory bricks were chromium corundum, corundum. The refractory bricks containing 10 wt% of the maximum size of 3.0 mm of MgO-Al₂O₃ spinel grain can significantly improve the thermal shock resistance of this refractory bricks. At the meanwhile, This kind of refractory bricks can be absolutely satisfied with the requirements about cold crushing strength, bulk density and apparent porosity.

Abstract: An Al/Al₂O₃ gradient coating applied on a heat-resistant stainless steel was prepared through a slurry process by using aluminum and γ-Al₂O₃ powders as the transition filler material, alpha alumina as the surface coating material, aluminum dihydrogen phosphate as the binder, and sodium molybdate as the corrosion inhibitor. The effects of heat treatment temperature, adhesive/filler ratio and γ-Al₂O₃ content on the bonding strength, microhardness and thermal shock resistance were investigated. The crystal phase and microstructure were characterized by XRD and SEM. The results indicated that the composition of the coating changed continuously from surface to coating-substrate interface. The coating could offer high bonding strength and good thermal shock resistance when heat treated at 700°C.

Abstract: Based on SiC grains and powder, flake graphite, AlN powder, Silicon powder, sintered alumina ultra-fine powder as the starting materials, the sample of SiAlON-Graphite-SiC composites was prepared by firing under N₂ atmosphere at 1 550°C and then analyzed in terms of high temperature performances by XRD, SEM and EDAX etc. The interrelation between composition, structure and performance of the material was also investigated. It indicates that this material provides excellent thermal shock resistance and molten alkali resistance, also proper oxidation resistance and applicable as the inner lining of the blast furnace.

Abstract: Nitrides(Si₃N₄/Sialon) bonded alumina castables were fabricated by in situ nitridation reaction. Effects of the formation of nitrides on the physical properties, hot strength and thermal shock resistance were investigated. The results show that the phase composition of castables is alumina, a-Si₃N₄, b-Si₃N₄ and Si₂N₂O after fired at 1400^ºC and 1450^ºC, and is alumina, b-Sialon and O’-Sialon after fired at 1500^ºC. Hot modulus of rupture and thermal shock resistance of specimens are noticeably improved due to the formation of nitrides.