Thermal shock resistance study of stereolithographic additive-manufactured Al2O3 ceramics by in situ digital radiography

Thermal shock resistance is critical to ensure the service safety of ceramic hot-end components. The thermal shock performance of stereolithographic additive-manufactured ceramics has not yet been studied. In this study, a series of thermal shock experiments with various temperature differences was conducted on stereolithographic additive-manufactured Al₂O₃ ceramics. The surface cracks were analysed based on photographs captured before and after the thermal shock experiments. Three-point bending tests with in situ X-ray digital radiography were conducted to determine the thermal shock resistance. Crack initiation, propagation, and coalescence were observed under flexural loads. The critical temperature difference of the stereolithographic additive-manufactured Al₂O₃ ceramics was determined to be 267.22 °C. The crack length increased and residual strength decreased with increasing temperature differences. The layered structure of the stereolithographic additive-manufactured ceramics slowed crack propagation. We expect that this study will serve as a reference for the performance of stereolithographic additive-manufactured Al₂O₃ ceramics in extreme environments.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

纳米BN复合SiC材料的抗热震性研究

将分析纯H3BO3、CO(NH2)2按物质的量比为1∶2.5溶于无水乙醇中,搅拌过程中按质量分数为80%加入平均粒径0.2μm的β-SiC,在850℃氮气中(纯度99.99%,压力为0.92~0.93 MPa)反应15 h制得纳米BN复合SiC粉体,然后在0.92~0.93 MPa N2气氛中以30 MPa轴向压力于1 750~1 800℃保压0.5~1 h热压烧结上述粉体,制成纳米BN复合SiC试样,采用三点弯曲及透射电镜、扫描电镜等方法研究了纳米BN复合SiC材料的抗热震性。结果表明:在SiC材料中引入纳米BN,一方面可以降低材料的弹性模量,有利于抗热震性的提高。另一方面由于基体SiC与第二相六方氮化硼(h-BN)的热膨胀系数相差较大,热失配作用导致h-BN晶粒发生晶间脱层,在复相陶瓷材料内产生许多微孔,这些微孔的存在可以有效缓解由于高温引起的热膨胀作用,从而极大地改善材料的抗热震性。     

低碳MgO-C材料的抗热震性研究

以电熔大结晶镁砂、天然鳞片石墨、纳米炭黑、酚醛树脂、铝粉等为主要原料制备w(C)=3%的低碳MgO-C材料,以其抗热震性为考核指标,选取颗粒级配、复合抗氧化剂、石墨粒度和复合结合剂4个因素,进行了四因素三水平正交试验。结果表明:在本试验范围内,颗粒级配是影响低碳MgO-C材料抗热震性的主要因素,复合抗氧化剂次之,石墨粒度和复合结合剂的影响基本相当;通过极差分析确定,镁砂颗粒级配(3～1、1～0.088和≤0.088mm的镁砂的质量比)采用50:23:27,复合抗氧化剂采用Al2.5+Mg-Al0.5+B4C0.5,石墨粒度采用10μm的,复合结合剂采用炭黑N220+沥青+酚醛树脂,可制备出抗热震性最佳的低碳MgO-C材料。     

样板晶生长法制备Y-TZP/Al2O3复相陶瓷的力学和抗热震性能

以带有玻璃涂层的氧化铝微粉、小尺寸氧化铝板晶(样板晶)和Y-TZP微粉为原料,常压烧结得到板状氧化铝增强Y-TZP复相陶瓷.对该复相陶瓷的常温～800℃的力学性能和抗热震性进行研究后发现,氧化铝板晶对裂纹有反射和桥连作用,该复相陶瓷材料在800℃下的抗弯强度和断裂韧性仅比室温下的分别下降20%和40%.氧化铝板晶的加入能够有效地阻止热震引起的裂纹扩展.     

Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

Thermal shock resistance of additively manufactured gas-permeable SiO2 ceramic structures for HIP-quench applications

The increasing demand for hot isostatic pressing (HIP) means that a reliable and efficient operation of modern HIP units with fast cool capability is indispensable. A key factor for efficient operation is the ceramic crucible used as the load basket. Its task is to keep as much of the HIPed parts as possible effectively within the hot zone and to prevent them touching the furnace wall. This work focuses on designing a gas-permeable ceramic structure with a high thermal shock resistance that can be scaled up to a load basket for future HIP applications. Stereolithography (SL) 3D printing of a ceramic resin is employed to build various scalable framework structures inspired by nature and by existing engineering applications. Thermal shock tests with water quenching reveal that framework structures with evenly distributed triangular bracings offer the highest flexural strength, whereas auxetic structures are best at retaining their flexural strength after thermal shock.     

Effect of print path process on sintering behavior and thermal shock resistance of Al2O3 ceramics fabricated by 3D inkjet-printing

In this study, Al₂O₃ ceramics parts were printed by inkjet printing technology with different printed paths distributions, such as the spiral printed path, round trip straight printed path and ladder lap printed path. The influences of inkjet printed paths on sintering performance and thermal shock resistance of the Al₂O₃ green bodies were investigated. The sintering performance of the green sample with the ladder lap printed path is the highest among the three samples. Sintered at 1550?℃, its bulk density and porosity reached 3.73?g/cm³ and 10.80%, respectively. In addition, the thermal shock resistance of the sample with the step print path reached 11 times. The results suggest that the optimization of the printed path provides an effective way to print 3D ceramics with good performances through 3D inkjet-printing technology.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Enhanced thermal shock resistance of low-carbon Al2O3-C refractories with direct CVD synthesis of nano carbon decorated oxides

Novel low carbon Al₂O₃-C refractories were prepared through adopting chemical vapour deposition (CVD) synthesized nano carbon decorated Al₂O₃ powder. The phase compositions, microstructures, mechanical properties and thermal shock resistance of Al₂O₃-C refractories were characterized and evaluated. The results show that the morphologies of nano carbon composites are mainly dominated by the concentration of catalyst. Specifically, the growth of MWCNTs is preferred with a Ni²⁺ concentration at 0.1?mol/L, while higher concentrations e.g. 0.3?mol/L would stimulate the formation of nano-onion like carbon. With the introduction of nano carbon decorated Al₂O₃ additives, the residual strength after thermal shock can reach 12.4?MPa, which is much higher than the 2?wt% nano carbon black containing specimens (6.4?MPa). The enhanced thermal shock resistance should be attributed to that the nano onion-like carbon reduces the cohesion between the matrix and the Al₂O₃ particles and decreases the thermal expansion coefficient.     

Effect of CaF2 on sintering behavior and thermal shock resistance of Y2O3 materials

Y₂O₃ materials have become a popular candidate for preparing refractory crucibles for ultra-pure high-temperature alloy melting in recent years. However, its difficulty in sintering and poor thermal shock resistance limited its industrial application. The effect of CaF₂ on the densification microstructure, mechanical properties, and thermal shock resistance of Y₂O₃ materials was investigated in this paper. The main purpose of this study was to optimize the amount of CaF₂ added in the preparation of Y₂O₃ materials to improve its thermal shock resistance and get better mechanical properties. The mechanism of the densification process of CaF₂-doped Y₂O₃ materials was analyzed by phase analysis and microstructure. The results showed that successive doping of large Ca²⁺ ions caused more lattice distortion in the Y₂O₃ materials, and the diffusion rate of Y³⁺ was increased, thus enhanced grain boundary diffusion and promoted sintering densification in the Y₂O₃ materials. Meanwhile, the addition of CaF₂ also significantly reduced the apparent porosity and enhanced the mechanical properties of the materials. The improvement of these properties was attributed to the increased relative density of CaF₂-doped Y₂O₃ materials and the high sintering activity of CaF₂. In addition, crack deflections effectively improved the thermal shock resistance of the materials. The residual flexural strength ratio of Y₂O₃ materials doped with 1 wt % CaF₂ was increased by 21.2% after thermal shock test compared with undoped specimens.     

炭/陶复合材料的抗热震性能研究

对含石墨的炭／陶复合材料优良的抗热震性能进行了讨论。这种性质与石墨的导热系数大、断裂功高、热膨胀和弹性模量小密切相关。     

Thermal shock resistance and bonding strength of tri-layer Yb2SiO5/mullite/Si coating on SiCf/SiC composites

A Yb₂SiO₅/mullite/Si tri-layer environmental-barrier-coating (EBC) were coated on SiC_f/SiC substrates via Air Plasma Spraying (APS). The thermal cycle tests (TCT) were conducted under thermal corrosive condition of vapor-oxygen (50 vol% H₂O and 50 vol% O₂) with thermal shock from 1200 °C to 200 °C. Microstructures, weight loss and bonding strength of the samples were systemically investigated after 101, 396, 606 and 700 TCT cycles respectively. The results show that the corner of the tri-layer coating peel off from the sample with weight loss of 1.3% after 700 TCT cycles. The bonding strength between substrate and tri-layer coatings gradually decreases to 6.79 MPa (approximately 55.2% of virgin specimens) after 700 cycles due to thermal shock induced cracks distributed horizontally within Si layers and between Si layer and outer layers.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Thermal stability and oxidation resistance of C/Al2O3 composites fabricated from a sol with high solid content

To improve fracture toughness of monolithic Al₂O₃ ceramics, three-dimensional carbon fiber preform was used as reinforcement, and the C/Al₂O₃ composites without interfacial coating were fabricated through vacuum impregnation-drying-heat treatment route with an Al₂O₃ sol as starting material. Characteristics of the Al₂O₃ sol with high solid content were firstly analyzed. Then thermal stability and oxidation resistance of the C/Al₂O₃ composites were investigated. It is found that the Al₂O₃ sol is an appropriate raw material for the fabrication of C/Al₂O₃ composites. The C/Al₂O₃ composites with a total porosity of 15.5% show a flexural strength of 208.5 MPa and a fracture toughness of 8.1 MPa m^1/2. Strength loss is observed after the composites were annealed at 1400 °C and 1600 °C under inert atmosphere. Oxidation resistance of the C/Al₂O₃ composites is unsatisfactory because of the existence of open pores and microcracks. When Al₂O₃ matrix was modified with SiO₂, the oxidation resistance is remarkably improved due to the viscous flow effect of SiO₂.     

Pt-Sn/Al2O3 sol-gel catalysts: Metallic phase characterization

Highly dispersed platinum was found in Pt-Sn/Al₂O₃ catalysts prepared by the sol-gel method, i.e., cogelation of aluminum tri-sec-butoxide and tetrabutyltin and subsequent impregnation with hexachloroplatinic acid. X-ray diffraction studies showed that the cell parameter of pure platinum is not modified in bimetallic Pt-Sn samples. Moreover, energy dispersive X-ray analysis studies (EDX) showed that platinum, but not tin, was detected in the catalysts where tin was incorporated by the sol-gel coprecipitation of tin and aluminum. Incorporation of tin into the alumina network is suggested.     

Thermal conductivities of YSZ/Al2O3 composites

The thermal conductivities/diffusivities of YSZ/Al₂O₃ composites have been investigated by a laser flash technique. The thermal conductivity of the composite increases with an increase in the Al₂O₃ volume fraction, and it can be fitted well to the Maxwell theoretical model. The consistency of the thermal conductivities of the composites with the predicted values indicates the absence of obvious interfacial thermal resistances in the composites. The negligible thermal resistance effect from the YSZ and Al₂O₃ grain boundaries is due to the much lower phonon mean free path compared with the grain size in the composite. The low Kapitza resistance of the YSZ/Al₂O₃ interface is discussed in terms of the “clean” and coherent nature of the YSZ/Al₂O₃ interface, together with the small difference between the elastic properties of YSZ and Al₂O₃.     

A new sol-gel route to prepare dense Al2O3 thin films

Anew sol-gel route has been applied to synthetize dense Al₂O₃thin films from aluminum isopropoxide (Al(OPri)₃)as raw precursor material. The results show that, in the solution, acetylacetone (AcAc) and aluminum form a complex compound which effectively suppresses the growth of colloidal particles and makes the sol very stable. Al₂O₃thin films fabricated by spin-coating method and calcined at 500 °C for 3 h possess an amorphous structure and exhibit a highly homogeneous surface texture without evidence of holes or cracks throughout the film. Moreover, the prepared films display a low leakage current and a high transmittance. This new sol-gel route appears to be a highly promising method to synthetize dense Al₂O₃ thin films from Al(OPrⁱ)₃, and could provide a wide range of optical and electric applications.     

加入AlON或SiAlON对Al2O3-MgO钢包浇注料热震稳定性的影响

采用在不同热震温差下测残余抗折强度保持率和1100℃水淬冷两种方法,对分别加入阿隆(AlON)和塞隆(SiAlON)于Al2O3-MgO质钢包浇注料后,对其热震稳定性的影响作了考察.结果表明,加入AlON或SiAlON后,在较小的热震温差范围内(300～500℃),浇注料的抗热震性有所改善;而在更大的热震温差范围内,浇注料的热震稳定性则比不加AlON或SiAlON时有所降低.显微结构分析表明不加AlON或SiAlON的Al2O3-MgO浇注料的基质主要由MA尖晶石和CA6相组成,并形成交织的结构,有利于浇注料受到热冲击时抵抗热应力;而加入AlON和抗氧化剂SiC或SiAlON和抗氧化剂B4C后,浇注料的基质仅由刚玉、MA和少量杂质相组成,不再存在CA6相和交织的结构.     

Enhanced ionic conductivity and thermal shock resistance of MgO stabilized ZrO2 doped with Y2O3

The present work focused on the effect of Y₂O₃ co-doping on the phase composition, microstructure, ionic conductivity and thermal shock resistance of 8 mol% MgO stabilized ZrO₂ (Mg-PSZ) electrolyte ceramics for high temperature applications. The addition of Y₂O₃ could promote the process of monoclinic-to-cubic/tetragonal phase transformation and became the metastable phase at room temperature. Meanwhile, the grain size of Mg-PSZ decreased. It was demonstrated that an appreciable increase in the ionic conductivity and compressive strength occurred on substituting MgO with Y₂O₃ in the Mg-PSZ electrolyte ceramics across the measured temperature range. Moreover, the Y₂O₃ addition could restrain the adverse effect of the cyclic thermal shock on the ionic conductivity and compressive strength of Mg-PSZ. The main reason was that the increase of the amount of monoclinic phase caused by cubic/tetragonal-to-monoclinic phase transformation by the cyclic thermal shock was restrained after the Y₂O₃ addition.