Improvement Of Thermal Shock Resistance Research Articles

Mechanical behavior and thermal shock resistance of MgO-C refractories: Influence of graphite content

The mechanical and thermo-mechanical properties of MgO-C refractories are of major importance in the industrial applications, and highly depend on the optimization of their microstructural design. In the present work, the influence of flaky graphite content on mechanical behavior and thermal shock resistance of such refractories was investigated with the aid of the wedge splitting test, fractal and microscopic fractographic analysis. The results showed that the increase of graphite content in the specimens led to an enhanced non-linear fracture behavior, a reduced nominal notch tensile strength (σNT), and a higher specific fracture energy (Gf), characteristic length (lch) and thermal shock resistance parameter (Rst). The fractal analysis of the crack propagation path of the specimens after the wedge splitting test indicated that increasing graphite content in the refractories can enhance their irregularity of the crack propagation path during fracture. Also, it was suggested from microscopic fractographic analysis that the improvement of thermal shock resistance of MgO-C refractories was positively correlated with the increase of interface crack propagation.

Fracture behavior of low carbon MgO–C refractories using the wedge splitting test

The fracture behavior of low carbon MgO–C refractories containing various carbon sources were investigated by means of the wedge splitting test and microscopic fractographic analysis to evaluate quantitatively their thermal shock resistance in the present work. The results showed that the addition of various nanocarbons in MgO–C specimens can lead to more tortuous crack propagation path during the wedge splitting test and much better thermal shock resistance compared to the specimen with flaky graphite as carbon source; particularly, the specimen containing carbon nanotubes had the most outstanding thermal shock resistance. Also, it was suggested from the correlation analysis that the increase of the specific fracture energy and interface crack propagation as well as the decrease of the modulus of elasticity, coefficient of thermal expansion and transgranular crack propagation can contribute to an improvement of thermal shock resistance of MgO–C refractories.

Thermo-mechanical properties of mullite—zirconia composites derived from reaction sintering of zircon and sillimanite beach sand: Effect of CaO

Mullite—zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sintering route utilizing Indian coastal zircon flour and sillimanite beach sand. 4%–12% of CaO (mole fraction) with respect to zirconia was used as additive. The effect of additive on densification, microstructure as well as various mechanical and thermo-mechanical properties was studied. Incorporation of CaO reduced the densification temperature of the composites to 1550 °C compared to 1600 °C (for CaO free samples). CaO formed small amount of liquid phase (calcium aluminosilicate), which facilitated sintering. Average grain size of the composites decreased up to 4% CaO addition, afterwards grain size increased with further addition of CaO. Samples with 4% CaO exhibited ∼225 MPa of flexural strength, ∼6 MP·m1/2 of fracture toughness and significant improvement in thermal shock resistance. CaO stabilized the tetragonal zirconia phase and thus improved the mechanical properties.

In-situ PIP-SiC NWs-toughened SiC–CrSi2–Cr3C2–MoSi2–Mo2C coating for oxidation protection of carbon/carbon composites

A novel precursor infiltration and pyrolysis (PIP) technique was used to in-situ synthesize SiC nanowires (NWs) on C/C composites, and then pack cementation was applied to prepare a silicide coating. The results show that the NWs could not only grow uniformly on the C/C surface but also be synthesized in some microholes through PIP technique. During thermogravimetry tests from 800 °C to 1400 °C in simulated air, the mass loss of the coated specimen decreased from 11.8% to 1.5% after incorporating PIP-SiC NWs. The introduction of PIP-SiC NWs also increased the thermal shock resistance of the coated specimens. After 24 times thermal cycles between room temperature and 1500 °C, the final mass loss reduced from 3.23% to 0.37%. The improvement of thermal shock resistance of the coated specimens was owing to the nanoscale toughening mechanism including pulling-out, debonding and bridging of PIP-SiC NWs. In addition, NWs at the coating/substrate interface could play as a barrier to restrain the propagation of microcrack and thus decreased the size of micocrack.

Mechanical Properties and Thermal Shock Resistance of Alumina/Hexagonal Boron Nitride Composite Refractories

Alumina/hexagonal boron nitride (Al2O3/h-BN) composite refractories were fabricated with various h-BN contents, to study the effects on the mechanical properties and thermal shock resistance (TSR) of the composite refractories. The results showed that the mechanical properties of the composite refractories, including fracture strength, Young’s modulus, fracture toughness, work of fracture, and decreased with the increase of h-BN amount. However, the residual flexural strength ratio after thermal shock test was increased with increasing the h-BN content, showing an improvement of TSR. The resistance to fracture propagation caused by the thermal stresses was increased, as supported by the RIV and RST parameters.

Thermal Shock Resistance of Laminated ZrB2–SiC Ceramic Evaluated by Indentation Technique

In this work, the thermal shock behavior of laminated ZrB2–SiC ceramic has been evaluated using indentation‐quench method based on propagation of Vickers cracks and compared with the monolithic ZrB2–SiC ceramic. The results showed that the laminated ZrB2–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔTc) of laminated ZrB2–SiC ceramic (ΔTc ≈ 590°C) was much higher than that of monolithic ceramic (ΔTc ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.

Effect of surface oxidation on thermal shock resistance of ZrB2–SiC–ZrC ceramic at temperature difference from 800 to 1900 °C

In the present work, the ZrB2–ZrC–SiC ceramic was fabricated at 1900°C through the reaction (5Zr+2B4C+Si→4ZrB2+ZrC+SiC). The effect of oxidation behavior on the thermal shock resistance of the ZrB2–ZrC–SiC ceramic was investigated in two kinds of heat conditions (air and vacuum) by measuring the residual strength after water quenching for the temperature difference ranging from 200 to 1900°C. The improvement of thermal shock resistance was attributed to the formation of oxide layers. The results of the present work indicate that the short-term oxidation is favorable to improve the thermal shock resistance of ZrB2-based ceramics.

Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia

Gadolinium zirconate (Gd2Zr2O7, GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3mol% yttria partially-stabilized zirconia (YSZ) incorporation. The fracture toughness of the composite of 90mol% GZ-10mol% YSZ (GZ–YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ–YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock lifetime of GZ–YSZ composite coating was improved, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ–YSZ composite coatings were discussed.

Characterization of the Mechanical Behavior of Magnesia Spinel Refractories Using Image Correlation

The improvement in thermal shock resistance of refractory materials is among the crucial properties that interest researchers and industrials. The “flexibility” in terms of large strain‐to‐rupture is, up to now, an important parameter to develop such characteristic. This can be obtained by generating a network of microcracks within the microstructure of the material under mechanical solicitations. As an outcome, the mechanical behavior can vary from fragile to a large nonlinear one depending on the degree of microcracking present inside the material. In fact, this nonlinear behavior of ceramics related to their microstructure is behind the possibility to enhance the level of strain‐to‐rupture for a better accommodation to the high level of strain induced by thermal shock solicitations. This study is devoted to apply digital imaging correlation method to support mechanical behavior analysis for the evaluation of the thermal shock ability of refractory materials. In this paper, mechanical characterization of magnesia spinel materials has been carried out using different experimental tests. These nonlinear materials are characterized by a mechanical behavior strongly dependent on their microstructure.

In-situ synthesis and thermal shock resistance of cordierite/silicon carbide composites used for solar absorber coating

Cordierite/silicon carbide (SiC) composite ceramics with 10, 20 and 30wt% cordierite were in-situ prepared via pressureless firing technique using silicon carbide, alumina, Kaolin and talc as starting materials. The results show that the synthesized composites all exhibited the good physical and mechanical properties. Among those, A2 composite with 20wt% in-situ cordierite fired at 1280°C exhibited the better bending strength and thermal shock resistance. The improvement of thermal shock resistance could be attributed to the proper cordierite wrapped around SiC, the homogeneous distribution of pores, the well-grown grains, as well as the formation of some glass phase. During the thermal shock test, a small amount of glass phases generated from the oxidation of SiC could react with Al2O3 and MgO, and then more cordierite was formed due to the incorporation of nucleating agent TiO2. With in-situ cordierite content rising, both absorptance and emittance of composite reduced, but A2 composite had the better absorbing ability for the solar energy. It is believed that these in-situ cordierite/SiC composites should be a promising candidate to apply in the field of solar absorber coating owing to the better thermal shock resistance and absorptance of solar energy.

Microstructure and thermal shock resistance of SiC/CNT–SiC double-layer coating for carbon/carbon composites

To improve the thermal shock resistance of SiC coated carbon/carbon (C/C) composites, a SiC/CNT–SiC double-layer coating has been prepared by depositing carbon nanotubes (CNTs) onto C/C substrate using electrophoretic deposition (EPD) and followed by chemical vapor deposition of SiC. The CNTs are randomly oriented on C/C substrate and form a porous layer. SiC is deposited in the porous CNT layer to form a dense CNT–SiC inner layer, and then is further deposited to form a SiC outer layer. SEM shows that the thickness of CNT–SiC inner layer increases along with the increasing of EPD time from 25 to 85s. In comparison to the SiC coating without CNTs, the SiC/CNT–SiC coating shows fewer and smaller cracks and better thermal shock resistance. After thermal cycling between 1200°C and room temperature 15 times, the SiC/CNT–SiC coating with CNTs EPD for 85s exhibits the best thermal shock resistance, whose weight loss decreases from 15.82% to 11.84%. The improvement of thermal shock resistance can be attributed to the role of CNT–SiC inner layer in alleviating the thermal stress and suppressing the cracks in the coating.

Densification behavior, mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1wt.%Nb–0.1wt.%Ni material sintered at 1600°C was obviously greater than 93.7% of theory density for W material sintered at 2000°C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1wt.%Nb–0.1wt.%Ni powder, and the powder milled for 10h (W10) could be sintered to near full density (99.4% of theory density) at 1600°C. The ball milling for 15h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.

Properties of Low-Carbon Al<sub>2</sub>O<sub>3</sub>-C Refractories with Different Critical Particle Size

Low-carbon Al2O3-C refractories were prepared using white fused corundum, α-Al2O3 powders and flake graphite as main raw materials. The critical particle sizes of corundum selected in this experiment were 0.5mm, 1mm and 2mm. The effects of corundum critical particle size on physical, mechanical and thermo-mechanical properties of low-carbon Al2O3-C refractories were investigated. The results show that the increase of critical particle size is conducive to the improvement of thermal shock resistance and fracture energy, but little effect on thermal expansion. The cold modulus of rupture after thermal shock test of samples using 0.5mm critical particle size corundum was 2.09MPa, while using 2mm critical particle size corundum was 2.98MPa. And the fracture energy increased from 265N/mm to 588N/mm when the critical particle size increased from 0.5mm to 2mm. The effects of critical particle size on apparent porosity, bulk density and modulus of rupture were insignificant.

Thermal shock behavior of nano-sized ZrN particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications owing to its excellent stability and mechanical properties such as high rigidity and good chemical stability. Thermal shock resistance is a major concern and an important performance index of refractories and high-temperature ceramics. While zirconium nitride (ZrN) particles have been proven to improve mechanical properties of AlON ceramic, the thermal shock behavior has not been evaluated yet. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of hot-pressed 2.7% ZrN–AlON composites by a water-quenching technique over a temperature range between 225°C and 275°C. The residual strength and Young's modulus after thermal shock decreased with increasing temperature range and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. The presence of nano-sized ZrN particles exhibited a positive effect on the improvement of both residual strength and critical temperature difference of AlON ceramic due to the toughening effects, the higher thermal conductivity of ZrN, the refined grain size and the reduction of porosity. Different toughening mechanisms including crack deflection, crack bridging and crack branching were observed during thermal shock experiments, thus effectively enhancing the crack initiation and propagation resistance and leading to a considerable improvement in thermal shock resistance in the ZrN–AlON composites.

Improvement of thermal shock resistance for thermal barrier coating on aluminum alloy with various electroless interlayers

NiCoCrAlY/8YSZ coating was firstly directly deposited on aluminum alloy 5A06 by atmospheric plasma spray to make it applicable to short-time high temperature condition. The failure after thermal shock test was mainly due to the stress caused by thermal expansion mismatch between the bond coat and the substrate as well as the galvanic corrosion of the aluminum alloy. Ni–P, Ni–W–P and Ni–Cu–P as interlayers were electrolessly deposited on the substrate in order to mitigate the thermal stress. The composition and thermal transformation of the interlayers were investigated. Thermal shock resistance and bonding strength of multilayer coatings (interlayer/NiCoCrAlY/8YSZ) were tested. Diffusion layers mainly composed of AlNi, Al3Ni2 and Al3Ni were observed between the interlayers and the substrate after thermal shock test. The oxidation of the substrate was effectively inhibited. Ni–P interlayer obtained at lower pH value was superior to the other two interlayers and enhanced the thermal shock life from 38 to more than 200 cycles. With the application of the Ni–P and Ni–Cu–P interlayers, the bonding strength examined by pull-off test was also largely improved from 11.7MPa to 18.8 and 19.0MPa, respectively.

Thermal Shock Resistance Behavior of Alumina Ceramics Incorporated with Boron Nitride Nanotubes

Alumina-matrix composite containing 2.0 wt% boron nitride nanotubes and monolithic alumina were fabricated by hot pressing, and their thermal and mechanical properties were investigated. Thermal shock resistance was evaluated by water quenching and subsequently by measuring the residual flexural strength by three-point bending test. Though the residual flexural strength of the composite is higher than that of the monolith at temperature differences lower than 280°C, the thermal shock of the composite is more sensitive to temperature change, exhibiting no significant improvement in thermal shock resistance. The variation in thermal shock resistance for the composite is resulted from the introduction of BNNTs.

The thermal shock resistance of the ZrB 2–SiC–ZrC ceramic

In the present work, the thermal shock resistance of the ZrB 2–SiC–ZrC ceramic was estimated by the water quenching method and the flexural strength of the quenched specimen was measured. The measured critical temperature difference of the ZrB 2–SiC–ZrC ceramic was significantly greater than that of the ZrB 2–15 vol.% SiC ceramic. The improvement in thermal shock resistance was attributed to its higher fracture toughness (6.7 MPa m 1/2) and lower flexural strength (526 MPa) relative to the ZrB 2–15 vol.% SiC ceramic (4.1 MPa m 1/2 and 795 MPa) based on Griffith fracture criterion. Furthermore, the temperature and thermal stress distributions in the specimen during instantaneous water quenching were simulated by Finite element analysis.

Thermal shock behavior of 30wt%BAS/Si3N4 self-reinforced composite

Abstract The thermal shock behavior of 30 wt%BAS/Si3N4 composite was evaluated by the water-quenching technique. The results show that the investigated composite possesses excellent thermal shock resistance. The critical temperature difference (ΔTc) of the composite is over 1100 °C and the retained strength at a quenching-temperature difference of 1100 °C is hardly affected by the quenching cycles. It is attributed to its high strength and low Young's modulus. The in situ growth of elongated β-Si3N4 grains are also beneficial to the improvement of thermal shock resistance due to the increased crack propagation resistance by crack deflection and grain pull-out mechanisms. In addition, the formation of an oxide layer on the sample surface before quenching may act as an additional thermal barrier and leads to a reduction in the intensity of thermal shock.

용존산소활량 측정용 지르코니아 갈바니 센서

Magnesia partially stabilized zirconia(Mg-PSZ) solid electrolytes for an improvement of thermal shock resistance, which is suitable for the measurement of oxygen activity in a molten steel, were prepared by post-thermal aging treatment. The steelmaking oxygen sensor elements were formed by an injection molding method, sintered at <TEX>$1650^{\circ}C$</TEX>, and then thermal aged ranged from 1250 to <TEX>$1400^{\circ}C$</TEX>. Sintered density and porosity were decreased as increasing the magnesia content in a zirconia-magnesia solid solution. Fractions of cubic phase to the synthesized Mg-PSZ solid electrolytes were ranged from 13.13 to 79.54.% after post-thermal aging treatment. Very dense microstructure without voids in the grains was obtained by the post-thermal aging process. Fine tetragonal phase crystallites precipitated on the cubic surface during post-thermal aging up to <TEX>$1300^{\circ}C$</TEX> improve a thermal shock resistance and reappearance of electro motive force(EMF) curve.

Thermal shock behavior of as-sintered and pre-oxidation Zr 2Al 4C 5–20SiC composites

A water-quenching technique was adopted to evaluate the thermal shock behavior of as-sintered and pre-oxidation Zr 2Al 4C 5–20 vol.%SiC composites in an air atmosphere. The strength retention of the two kinds of composites was measured after varying temperature (Δ Tc) up to 800 °C. As-sintered Zr 2Al 4C 5–20 vol.%SiC showed rapidly drop in flexural strength above Δ T of 400 °C. However, the pre-oxidation Zr 2Al 4C 5–20 vol.%SiC composite showed a higher retained strength up to 800 °C, and the critical thermal shock temperature difference (Δ Tc) is as high as 660 °C. The main reasons for a great improvement of thermal shock resistance (TSR) were also discussed and analyzed.