Reactive hot pressing of Al2O3-Ni composites

Reactive hot pressing has been used to form Al₂O₃-Ni composites from Al and NiO. The effect of attrition milling on the precursor powder and subsequent composite formation was examined. The surface area, phase assemblage, reaction temperature, and morphology of precursor powders were characterized as a function of milling time, which ranged from 0 (unmilled) to 480 min (8 hrs). During milling, particle surface area increased from less than 1 to more than 11 m²/g as the size of the Al and NiO particles decreased. At the same time, the temperature at which Al and NiO reacted to form Al₂O₃ and Ni decreased from more than 1000°C to around 600°C. Formation of Al₂O₃ or Ni during milling was not detected, regardless of time. Precursor milling time also affected the morphology and phase assemblage of composites produced by reactive hot pressing. Composites formed from unmilled powders contained a small amount of unreacted NiO and had a Ni ligament size greater than 10 m. The composite forming reaction went to completion when powders milled for one hour or more were hot pressed. Based on microstructural evidence and analogy to similar reactions, it appears that the composite forming reaction proceeds by Al diffusing into and reacting with NiO.     

Eutectic solidification in the system Al2O3/Y3Al5O12

The eutectic solidification in the system Al₂O₃/Y₂Al₅O₁₂ has been investigated. A Bridgman-type crystal-growing furnace was used in this investigation. A temperature gradient of 190° C/cm and growth rates which were varied between 2 and 12 cm/h were employed in the directional solidification studies. Three types of microstructure were observed depending upon the composition and the growth rate. At a growth rate of 4 cm/h and at compositions removed from the eutectic composition, a mixture of primary phase and fine eutectic dispersion was found. At growth rates between 2 and 12 cm/h at the eutectic composition, a colony type microstructure was most commonly observed. At growth rates above 4 cm/h at the eutectic composition, regions in the solidified ingot were found to have a highly oriented eutectic microstructure consisting of both rods and platelets. These eutectic microstructures indicate that coupled growth can occur in this system. The method of Sunquist and Mondolfo [15] was used to determine whether Y₃Al₅O₁₂ was the first phase to nucleate at the eutectic.     

Plasma spraying of Al2O3 and Al2O3Y2O3

A microstructural study has been carried out of plasma-sprayed Al₂O₃ and mixed and sintered Al₂O₃Y₂O₃. In order to ascertain the degree of metastability achieved by plasma spraying, these results are compared with a similar experiment utilizing a CO₂ laser for melting and the hammer-and-anvil technique for quenching of the same materials. X-ray diffraction methods were used to determine the obtained phases and crystal structures. In addition, transmission electron microscopy was used to confirm the phases present and to study their morpology. The porosity was studied with both mercury intrusion porosimetry and small angle neutron scattering. The addition of Y₂O₃ is shown to decrease the porosity from 15% to 7.5%. Adhesion is likewise related to the addition of Y₂O₃ and it is seen that adhesion of the mixture is measurably improved over that of pure Al₂O₃. The implication of these results is discussed.     

Effect of Al2O3 addition on phase reaction of the AIN-Y2O3 system

It is well known that the existence of oxygen in the sintering of aluminium nitride (AIN) causes the degradation of thermal conductivity. To clarify the role of the oxygen, the effect of Al₂O₃ addition on the phase reaction of the AIN-Y₂O₃ system at high temperatures of 1900 and 1950°C was investigated. In the AIN-Al₂O₃ system, -AION and 27R polytypoid were formed at 1900 and 1950°C, respectively, and the ratio of each crystal increased with increasing Al₂O₃ content. The resultant phase was found to be in agreement with the phase diagram given by McCauley and Corbin. In the AIN-Y₂O₃ system, YAM phase or YAM plus unreacted Y₂O₃ were identified. Phase reaction in the AIN-Al₂O₃-2Y₂O₃ system was quite different from that of the AIN-Al₂O₃ system: namely, -AION and 27R coexisted even at 1950°C, which is inconsistent with McCauley and Corbin's diagram. It was confirmed by the dilatometric method and phase reaction data using some couples, that Al₂O₃ strongly affected the phase reaction in the AIN-Y₂O₃ system, indicating that the shrinkage of the Al₂O₃-doped specimen was initiated at somewhat lower temperatures. The grain morphology depended on the phase changes shown above, in which AIN and -AION were granular, and 27R polytypoid was plate-like.     

Al2O3/Al2O3 Joint Brazed with Al2O3-particulate-contained Composite Ag-Cu-Ti Filler Material

1.IntroductionTo improve the mechanical properties and relieve mis-matches between the filler metals and base materials,the particulates of superalloys,ceramic or carbon fiberswere added into the conventional brazing filler metal toform composite filler material.The method has beenused in aero-engine component repairing[1,2],fine castcomponent joining[3],wide clearance butt jointing[4],ce-ramic brazing[5,6]and electronic package[7].However,the method was used mostly in metal brazing.The mi-cro…     

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al₂O₃.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

熔体反应渗透形成Al2O3基复合材料的组织结构

运用光学显微镜和扫描电子显微镜对铝熔体分别反应渗透SiO2陶瓷和SiO2 SiC陶瓷预制体形成的Al2O3/Al Si和SiC/Al2O3/Al-Si复合材料的组织结构进行了研究,用X-射线衍射分析了材料的组成.     

Multilayer composites in Al2O3/MoSi2 system

A new multilayer composite with a super-plastic layer, a hard layer and a weak interface was proposed. The hard layer can provide the strength of the multilayer composites at high temperature, the plastic layer can deform plastically at high temperature and disperse the applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Such multilayer composites were prepared by tape casting in the Al₂O₃/TiC/MoSi₂–Mo₂B₅ system. It was found that such design is effective on the increase of fracture energy both at room temperature and at high temperature, and the strength at high temperature could be remained in a relatively high revel.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

Al2O3—Coated Nano—SiC Particles Reinforced Al2O3 Matrix Composites

Properties of Al2O3-coated nano-SiC have been compared with those of as-received SiC.The isoelectric point(IEP) of SiC changed from pH3.4 to pH7.3 after coating with the alumina precursor,which is close to that of alumina.Because both surfaces of coated SiC and Al2O3 possess higher positive charge at pH=4.5-5.0 ,they are uniformly dispersed in the two-phase aqueous suspensions.Then a mixed powder containing nano-SiC dispersed homogeneously into the Al2O3 matrix was achieved from flocculating the two-phase suspension.Finally,Al2O3/SiC nanocomposited were obtained by coating nano-SiC with Al2O3 ,in which the majority of SiC particles were located within the Al2O3 grains.The observation by transmission electron microscopy(TEM) and the analysis by the X-ray photoelectron spectroscopy(XPS) showed that cracks propagated towards the intragranular SiC rather than along prain boundaries.     

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al₂O₃:Ti crystal in comparison with those for undoped Al₂O₃ crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25 eV. Apart from the main emission band peaked at 725 nm related to the ²E → ²T₂ radiative transitions of Ti³⁺ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290 nm and the luminescence of F⁺–Ti and F–Ti centers in the bands peaked at 325 and 434 nm are also found in the emission spectra of Al₂O₃:Ti crystal. We show also that the luminescence of Ti³⁺ ions in Al₂O₃:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.     

The eutectic and liquidus in the Al2O3-ZrO2 system

A high-temperature X-ray diffraction method was employed to establish,in situ, the eutectic temperature (1910±20° C) and the liquidus line in the Al₂O₃-ZrO₃ binary system. The eutectic composition was determined by optical microscopy to be 42.5±1 wt% ZrO₂. No evidence was found for the existence of a reported-Al₂O₂ high temperature phase.     

Oriented eutectic microstructures in the system Al2O3/ZrO2

Oriented eutectic microstructures have been produced in the system Al₂O₃/ZrO₂ using a Bridgman-type crystal-growing furnace. Ingots consisted of elongated columnar grains or colonies. Inside the colonies a rod-type eutectic microstructure consisting of rods of ZrO₂ surrounded by an Al₂O₃ matrix was observed. The eutectic point was re-established at 63 mol % Al₂O₃/37.0 mol % ZrO₂ and 1870±5° C. Al₂O₃ is the first phase to nucleate when eutectic growth occurs.     

Surface,grain-boundary and interfacial energies in Al2O3 and Al2O3-Sn,Al2O3-Co systems

Using the multiphase equilibrium method for the measurement of contact angles, the surface and grain-boundary energies of polycrystalline Al₂O₃ in the temperature range of 1473 to 1923 K were determined. Linear temperature functions were obtained by extrapolation for both quantities between absolute zero and the melting point of Al₂O₃. The temperature dependence of the surface and grain boundary energies can be expressed as $$\gamma _{{\rm A}l_2 O_3 } = 2.559 - 0.784 \times 10^{ - 3} T(J m^{ - 2} )$$ and $$\gamma _{{\rm A}l_2 O_3 - Al_2 O_3 = } 1.913 - 0.611 \times 10^{ - 3} T(J m^{ - 2} )$$ respectively. The interfacial energies of Al₂O₃ in contact with the molten metals tin and cobalt revealed a linear dependence on temperature.     

Microstructure and Strength of Al2O3/Al2O3 Joints Bonded with ZnO-Al2O3-B2O3-SiO2 Glass-Ceramic

ZnO-Al2O3-B2O3-SiO2 （ZABS） glass powder was used as interlayer to join alumina ceramics. The effect of joining temperature on the microstructure and strength of joints was investigated. The results showed that the ZABS glass can react with alumina substrate to form a layer of ZnAl2O4 at Al2O3/glass interface. Bending test exhibited that low joining temperature （1150℃） led to low joint strength due to the formation of pores in the interlayer, originated by high viscosity of the glass. High joining temperature （1250 ℃） also resulted in low joint strength, because of large CTE （coefficient of thermal expansion） mismatch between amorphous interlayer and alumina substrate. Therefore, only when the joining temperature was appropriate （1200℃）, defect-free interface and high joint strength can be obtained. The optimum joint strength reached 285 MPa, which was the same as the base material strength.