Effect of zirconia content and particle size on the properties of 3D-printed alumina-based ceramic cores

Alumina-based ceramic cores are used to manufacture the internal structures of hollow alloy blades, requiring both high precision and moderate properties. In this work, zirconia is regarded as a promoter to improve the mechanical properties of sintered ceramic. The effect of zirconia content and particle size on the microstructure and mechanical properties of ceramics was evaluated. The results indicate that the flexural strength of sintered ceramics reached the maximum of 14.5 ± 0.5 MPa when 20 wt% micron-sized (10 μm) zirconia (agglomerate size, consistent with the alumina particle size) was added, and 26.5±2.5 MPa when 15 wt% 0.3 μm zirconia was added. Zirconia with submicron-sized (0.3 μm) particles effectively filled the pores between alumina particles, thus leading to the maximum flexural strength with a relatively low content. The corresponding sintered ceramics had a bulk density of 2.0 g/cm³ and open porosity of 59.6%.     

Oxidation behavior of TiAl coated with a fine-grain Co-30Cr-4Al film

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-m thickness has been studied at 1100–1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K. Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded. This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.     

Ceramic coating on ceramic with metallic bond coating

The change in structure and adhesion strength of the interface by heating in air has been investigated for a plasma- sprayed alumina coating on a ceramic substrate with a 50Ni- 50Cr alloy bond coating. A veined structure composed of NiO, NiCr ₂O₄, and NiAl₂O₄ oxides grew from the bond coating into cracks or pores in the top coating and the alumina substrate after heating at 1273 K for 20 h in air. The NiAl₂O₄ spinel may have formed by the oxidization of nickel, which subsequently reacted with the alumina coating or the substrate. The mechanism of the penetration of the spinel oxides into the cracks or pores is not clear. The adhesion strength of the coating is increased to about 15 MPa after heating at 1273 K for 20 h in air, compared to an as- sprayed coating strength of only 1.5 MPa.     

Ultrasonic technique for measuring porosity of plasma-sprayed alumina coatings

Porosity is an important factor in plasma-sprayed coatings, especially ceramic coatings. Excessive poros-ity can adversely affect the performance of the coated component in various ways. An ultrasonic nonde-structive measurement technique has been developed to measure porosity in plasma-sprayed alumina coatings. The technique is generic and can be extended to other ceramic coating systems. To test the tech-nique, freestanding alumina coatings with varying levels of porosity were fabricated via plasma spray. Samples with varying porosity, obtained through innovative fabrication techniques, were used to gener-ate a calibration curve. The ultrasonic velocity in the low-frequency range was found to be dependent on the density of freestanding coatings (measured via Archimedian techniques). This dependence is the basis of the development of a technique to measure the density of coatings.     

TiN，AlN／Al2O3复合陶瓷材料的研究

综述了TiN／Al2O3，AlN／Al2O3以及（TiN，AlN）／Al2O3复合材料的研究现状。并指出颗粒增韧是复相陶瓷材料增韧最简单的方式之一，其中纳米复合、纳微米复合、多相复合是实现颗粒增韧的有效途径。在复相陶瓷的制备中，原位反应烧结是很有希望的技术，可以直接在基体中生成弥散分布的超细第二相颗粒，而使复合材料的性能大幅度提高。     

Observations of nodule growth during the oxidation of pure binary iron-aluminum alloys

Oxidation studies were carried out in oxygen at 800°C, on a series of pure binary iron-based alloys with between 1.9 and 9.8 wt. % aluminum. The results are presented in conjunction with the existing literature and these permit the development of a classification of scale morphologies based on alloy composition. Alloys with less than about 2.4 wt. % aluminum form bulky stratified scales composed of Fe₂O₃ and Fe₃O₄ with FeAl₂O₄ and Al₂O₃ at the scale-metal interface. Alloys with between 2.4 and 6.9 wt. % form an external Al₂O₃ scale but this is interspersed with iron oxide nodules that penetrate the alloy substrate. Only alloys with greater than 6.9 wt. % aluminum form completely protective Al₂O₃ scales. Models based on oxide nucleation are presented for the growth of bulky scales and also the iron oxide nodules.     

A high-temperature oxidation-resistant Fe-Mn-Al-Si alloy

To develop satisfactory alloys without Cr or Ni for high-temperature application up to 1100C, three alloys based on Fe-10%Al-Si with differing fourth (or fifth) element additions were oxidized in air at 1100°Cfor 24 hr. A low carbon, Fe-30Mn-10Al-Si alloy exhibited excellent high-temperature oxidation resistance. The total weight gain for 24 hr oxidation in air at 1100°C was only 1.03 mg/cm ². After air oxidation for 6 days at 1100°C, no nodule formation or breakthrough oxidation occurred. Post-oxidation SEM and EDAX examination showed that a thin, compact, protective alumina scale formed on the alloy.Visiting Scientist (People's Republic of China).     

Al2O3/Al基复合材料与基体结合强度物理意义分析

测定了Al2 O3 /Al基复合材料与基体材料间的结合强度 ,探讨了结合强度的物理意义。其物理意义是 :界面抗拉强度与试样达到断裂应变时基体中的应力分别与纤维体积分数和基体体积分数的加权平均值。并根据测定的结合强度的值结合有限元分析得出几种Al2 O3 /Al基复合材料的界面抗拉强度值     

Effect of 20kHz ultrasound on alumina hydrate precipitation from seeded sodium aluminate solution

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纳米α-Al2O3超细磨料绿色制备工艺的研究

本文研究了以Al(NO3)3·9H2O和NH3·H2O溶液为原料,以聚乙二醇为分散剂,并通过在溶胶-凝胶过程中加入高纯氧化铝晶种和引入AlF3添加剂制备工艺在较低温度下合成了纳米α型氧化铝。通过X射线衍射分析和差热分析,研究了由凝胶至α-Al2O3粉末的转变温度、物相和晶粒度。结果表明:该工艺可显著降低过渡型氧化铝向α型氧化铝的相转变温度,所制氢氧化铝凝胶经950℃锻烧可获得平均粒径为45nm的α型纳米氧化铝。由于该工艺显著降低了相转变温度,可节约大量能源,是一种环境友好的绿色合成工业。