Papers by Keyword: Thermal Shock Resistance

Abstract: The interface properties of multi-layered functionally graded Cr₃C₂-NiCr coatings deposited by plasma spraying technique were experimentally studied in this paper. The microstructure and phase structure of coatings were studied with scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The thermal shock resistance was investigated by cyclic heating and cooling tests using an electro-calefaction furnace. The crack appearances of the coatings were observed carefully. Results show that the plasma sprayed multi-layered functionally graded coatings are compact and the adhesion between the layers and the substrate is good. The coatings have better macro-hardness than the substrate, and the 6-layers coating has the highest macro-hardness and the best wear resistance. Besides, the micro-hardness of coatings increases with increasing content of Cr₂C₃ in coating materials. Results of cyclic thermal shock show that the main failure styles of the coatings are crack and desquamation and the thermal shock resistance of the coatings is improved obviously by increasing the number of coating layers.

Abstract: Alumina ceramics are the most widely used structural ceramics, especially in aviation, spaceflight and war industry area. However, alumina is of covalent bond, with low fracture toughness and poor thermal shock resistance, which baffle the application of the alumina ceramics in engineering. In this paper, mullite fiber was used to improve its toughness and thermal shock resistance. Alumina-mullite composite ceramics were prepared by hot press sintering. The effect of mullite fiber on thermal shock resistance of ceramics was investigated through indention quenching method. Meanwhile, the mechanism of its reinforcing and toughening and the relationship between mechanical properties and addition of mullite fiber were also discussed.

Abstract: In this paper the new process of arc spraying was researched on CuCo2Be alloy surface,the high strength and high wear resistance Cr3C2/Fe coating were prepared on CuCo2Be alloy surface by adopting a series of different arc spraying process parameters. Based on the test the arc spraying process parameters was optimized, the optimized result was: spraying voltage was 38V, the current is 180A, spraying distance was 150mm, spraying air pressure was 0.7MPa, the bonding strength of the coating and the substrate was 30.5MPa; and the thermal shock resistance of coating Cr3C2/Fe was studied in detail by different spraying process parameters, it was showed: thermal shock resistance of the coating was best by spray low voltage and low current process. By using microhardness tester, super depth of field microscope, SEM, EDS and XRD, mechanical properties, microstructure, phase composition and its content distribution of Cr3C2 / Fe coating were studied, the study results were showed: the micro-hardness of coating was 992.75HV, coating was a typical lamellar structure, and a small amount of pore and oxides in the coating; the mainly phase of coating included: Fe, Fe-Cr, C0.14 Fe1.86 and Fe7C3.

Abstract: So far, thermal shock theories have not been well verified in refractories. This paper compared hypothesis and failure mechanism of two typical thermal shock theories, analyzed the applications of two typical theories in refractories from working environment, failure process and microstructure differences. And thinks that the critical stress fracture theory is suitable for nearly ideal brittle refractory with ceramic bond and used in thermal shock extremely harsh environment; thermal shock damage theory suites to evaluate multiple loops thermal shock. Analyzed the factors influencing the thermal shock parameters, and discussed Ways to improve the thermal shock resistance of refractories from destructive force and resistive force of refractories. Conclusions are that choosing small swelling expansion rate and high thermal conductivity material, reducing the micro stress concentration and adding flexible phase, these three methods can be used to make refractories with good resistance to thermal shock.

Abstract: Novel 4-phase ceramic abradable seal coatings have been prepared by plasma spray. The as-prepared coatings were characterized by SEM, EDS and XRD. The results showed that the 4-phase ceramic abradable coating exhibited porous structures and had good adhesion with the bond coat. The ZrO₂ and Al₂O₃ in the coating existed in the form of tetragonal-ZrO₂ and α-Al₂O₃, respectively. The result of thermal shock test revealed that compared with the 3-phase abradable coating, the thermal shock life of the 4-phase abradable coating had increased tenfold to 1900~2000 cycles from 100~200 cycles. The reason is owing to the reduction of oxygen partial pressure and thermal stresses at the top/bond coat interface due to the addition of the fourth nanoAl₂O₃ phase.

Abstract: Al₂O₃ composite ceramics materials used for heat transmission pipeline were prepared by pressureless sintering in air. Calcined bauxite, talc and commercial alumina powder were used as the main raw materials, partially stabilized zirconia was also added to improve the thermal shock resistance of the samples. The effects of composition and sintering temperature on density, phase composition, microstructure and thermal shock behavior of samples were evaluated. It shows that the B2 sample which was sintered at 1400 °C for 2 h has optimal properties with porosity 0.4%, bulk density 3.2 g•cm^-3.The bending strength increases 19.46 MPa after 30 times thermal shock cycles from 1100 °C to room temperature. The main phases of B2 are corundum and spinel, meanwhile, a small amount of monoclinic zirconia and α-quartz are also detected according to XRD pattern. Microstructure analysis reveals that spinel crystals are interlocked by prismatic corundum crystals, bright white beaded monoclinic zirconia particles are distributed uniformly, and it is beneficial to improve the thermal shock resistance of sample.

Abstract: The influences of hexagonal BN content on the porous Si₃N₄ material was investigated. SEM micrographs showed that the columnar crystal overlapping structure of the porous Si₃N₄ material with a small BN addition was almost invariant. BN uniformly dispersed in the spaces of crystal overlapping. The experiment showed that the apparent densities of the porous Si₃N₄ material with a small BN addition marginally increased, the porosity, the strength and the dielectric constant marginally decreased, but The thermal shock resistance was greatly improved.

Abstract: Factors determining the durability of a material include among others its thermal shock resistance, as a non-homogenous temperature field may trigger the formation and propagation of cracks. To evaluate thermal shock resistance by means of one of the resistance criteria: the R_st or R₄ parameter, it is necessary to know the work of fracture (γ_WOF), which is determined in a cracking test. Technical testing conditions influence the recording of crack propagation in a material sample. Ordinary and refractory castables are a group of materials in which the effect of temperature on the course of their cracking is among others related to the presence of deformations, internal stresses and cracks. Tests were performed for mullite, andalusite and boxite doped chromia castables, having known phase compositions, for which the temperature of liquid phase appearance in the matrix was calculated. A considerable increase in the value of castables γ_WOF at 900°C was noted compared to the values at 300°C and 600°C. In the group of the examined castables the highest γ_WOF value was obtained for andalusite castable. The applied methods of testing enabled simultaneous presentation of crack propagation in terms of energy (load vs. deformation) and kinetics (load vs. time).

Abstract: We chose a purging plug with good performance to analyze its insert plate. By X ray fluorescence analysis method, the main chemical composition is alumina and the small amount is silica. XRD analysis shows its phase compositions, main crystal phase is corundum, the secondary phase is mullite. SEM analysis is for the microstructure, grains are packed closely. They are sintered tabular corundums with smooth shape, which means there is liquid phase in sintering process to help mass transfer. As a conclusion, this product is designed from the point of material selection and structure control to improve its thermal shock resistance. Compacted microstructure combining with good thermal shock-resistance mineral phases improve the service life of purging plug.

Abstract: The mechanical properties and thermal shock behavior of Mg-PSZ/LaPO₄ ceramics was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three-point bending test to determine the flexural strength degradation. Mg-PSZ/15LaPO₄ composite showed a higher thermal shock resistance and behaved as a typical refractory. The calculation of thermal shock resistance parameters for the composites and the monolith had indicated possible explanations for the differences in thermal shock behavior.