A jelly-like ceramic fiber at 1193 K

A high-strength ceramic Al₃₁Gd₉O₆₀ continuous fiber with a fiber diameter of about 20 μm and an amorphous structure could be made successfully by using the melt extraction method. This fiber can be freely shaped by viscous flow deformation in the supercooled liquid state (about 1193 K). The fiber strength is about 2 GPa and this strength is maintained up to around 973 K. A high-strength ceramic continuous fiber with a uniform GdAlO₃ nanocrystalline in an amorphous matrix can also be obtained with a suitable crystallization from the amorphous state by heat treatment. The heat resistance, Young’s modulus, and other properties are therefore improved. The amorphous ceramic fiber is promising as a ceramic that can be easily shaped at a relatively low temperatures (around 1193 K) in agreement with temperature range of superplastic processing of Ti alloys. Received: 25 August 1999 / Reviewed and Accepted: 28 October 1999     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Effect of the microstructure on the mechanical properties of a directionally solidified Y3Al5O12/Al2O3 eutectic fiber

The strength and fracture behaviors of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth (EFG) technique. The microstructure was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The room temperature tensile strength and Weibull's modulus of the eutectic fiber before and after heat treatment at 1460°C were measured. The fracture toughness and crack propagation behaviors were investigated using an indentation technique. Significant coarsening of the lamellar microstructure was observed after heat treatment at 1460°C in air. The degradation of the room temperature tensile strength in the Y₃Al₅O₁₂/Al₂O₃ eutectic fiber after heat treatment was attributed to the development of surface grooves at the surface of the fiber. Also, the Y₃Al₅O₁₂/Al₂O₃ eutectic fiber showed a radial (Palmqvist) crack type and exhibited an anisotropic crack propagation behavior during the indentation tests.     

Development of a new continuous Si-Ti-C-O fibre using an organometallic polymer precursor

A polytitanocarbosilane, which is useful as the precursor polymer for ceramic fibre, was synthesized using polydimethylsilane, polyborodiphenylsiloxane and titanium tetraisopropoxide. The polytitanocarbosilane was melt-spun and using the continuous heat-treatment process from the polymer fibre to ceramic fibre, flexible Si-Ti-C-O fibre was produced. The density, tensile strength and Young's modulus of this amorphous ceramic fibre were found to be 2.35 g cm^–3, 3.0±0.2 and 220±10 GPa, respectively. The Si-Ti-C-O fibre retained its high tensile strength to higher temperatures (about 1200° C). The specific resistance of this ceramic fibre covered a wide range of 10⁷ to 10^–1cm. This ceramic fibre is considered to be useful as reinforcement fibre for composites.     

Compatibility between alumina fibres and aluminium

An investigation is carried out on the interfacial wetting behaviour and reactions between aluminium and alumina fibres (85mass% Al₂O₃ and 15mass% SiO₂). Aluminium is coated onto alumina fibres by a vacuum evaporation technique and the surface of the fully coated fibres and the edge of the partially coated fibres are examined by scanning electron microscope after heat treatments at various temperatures. Within a temperature regime between 943 and 1273 K, occurrence of such interfacial reactions as 4Al(I) + Al₂O₃(s) 3Al₂O₃(g) and 4Al(I) + 3SiO₂(s) 2Al₂O₃(s) + 3Si(s) are detected. It is found that molten aluminium can cover the alumina fibre surface but it peels off near the edge of the coating film on a partially coated fibre, showing the very weak interface cohesion. This is ascribed to the lack of a stable compound formation at the interface. Results of tensile test show that the strength of the coated fibres is degraded after heat-treating at above the melting point of aluminium. The culprits for the tensile failure of alumina fibres are evaluated by the Weibull distribution theory.     

Tensile testing of ceramic fibres by video extensometry

A computer-assisted video extensometer was used to measure the Young's modulus and tensile strength of commercially available alumina fibres (11.5 m in diameter). The results showed excellent agreement with manufacturers reported values (384 ± 12 GPa and 3132 ± 296 MPa for Young's modulus and tensile strength, respectively). The machine was initially tested with 100 m diameter SiC monofilaments to identify optimum experimental conditions. The mechanical properties of these fibres were independent of the crosshead speed and fibre length. This non-contacting extension measuring system allowed testing of fragile materials and a submicron resolution could be achieved with a high-precision CCD camera. The results in term of precision and resolution therefore meet the requirements for strain measurements in mechanical ceramic materials testing.     

Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites

New Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ ternary Melt Growth Composites (MGCs) with a novel microstructure have been fabricated by unidirectional solidification. These MGCs displayed superior high-temperature strength characteristics. The flexural strength increases progressively in the range 650–800 MPa with a rise in temperature from room temperature up to 1873 K. These excellent high-temperature characteristics are closely linked to such factors as: a microstructure consisting of three-dimensionally continuous and complexly entangled single-crystal Al₂O₃ with a hexagonal structure, single-crystal YAG with a garnet structure and fine ZrO₂ with a cubic structure; characteristic dimensions of the microstructure of Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics of around 2–3 m for YAG, around 2–3 m for Al₂O₃ and around 0.4–0.8 m for ZrO₂; and the fact that no amorphous phase is formed at interfaces between any of the phases.     

A novel oxide composite reinforced with a ductile phase for very high temperature structural materials

The strength of nearly all metallic and ceramic polycrystalline materials drops off rapidly with increases in temperature. Here we have developed a strong and tough oxide composite reinforced with a ductile phase at 1873 K with a different kind of microstructure, made by unidirectional solidification of an Al₂O₃-Y₃Al₅O₁₂(YAG)-ZrO₂ ternary eutectic mixture. This composite has a microstructure in which continuous networks of single-crystal Al₂O₃, single-crystal YAG and single-crystal c-ZrO₂ interpenetrate without grain boundaries. No amorphous phases are observed at the interfaces between these phases and relatively compatible interfaces are formed. This material displays plastic deformation at 1873 K indicating high yielding flexural strength. This composite’s average flexural strength at 1873 K of around 850 MPa – approximately 57 times higher than that of the sintered composite with the same composition, and its ductile fracture at 1873 K have never been found up to now. Consequently several useful applications can be considered in mechanical engineering. Received: 15 November 2000 / Reviewed and accepted: 11 April 2001     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Structure and properties of Si-Ti-C-O fibre-bonded ceramic material

Si-Ti-C-O fibre-bonded ceramic material was synthesized from pre-oxidized Si-Ti-C-O fibre with an oxide layer 400–600 nm thick, by hot-pressing at 2023 K under 50–70 MPa. The interstices in the Si-Ti-C-O fibre-bonded ceramic material were packed with an oxide material which existed on the surface of the pre-oxidized Si-Ti-C-O fibre, and the oxide material formed a small amount of the matrix phase (10 vol%). At the fibre-matrix interface, aligned turbostratic carbon, which was oriented around the fibre, was formed during hot-pressing. The existence of the interfacial carbon layer indicated the Si-Ti-C-O fibre-bonded ceramic material to have a fibrous fracture pattern with high fracture energy. The Si-Ti-C-O fibre-bonded ceramic material showed excellent durability even at 1773 K in air, because a protective oxide layer is formed on the surface at a high temperature (above 1273 K) in air. Moreover, the Si-Ti-C-O fibre-bonded ceramic material almost maintained its initial strength in the bending and tensile tests, even at 1773 K in air.     

R-curve effect on strength and reliability of toughened ceramic materials

An improved theoretical analysis is presented for the strength and mechanical reliability of ceramic materials with an R-curve characteristic. There is good agreement between the predicted flexural strength distribution of an Al₂O₃/ZrO₂ ceramic and experimental datA. Unlike the conventional two-parameter Weibull approach, this new analysis is able to predict the non-linearity observed in the lnln 1/(1–P _f) versus In _f strength distribution curve. Compared to the untoughened Griffith material, the R-curve material has higher strength and better strength reliability.     

Microchemistry and microstructure of a multiphase aluminosilicate ceramic

The microstructure and microchemistry of a sintered (≈ 1700° C) aluminosilicate ceramic (60 wt % Al₂O₃-40 wt% SiO₂) was investigated by optical, scanning (SEM and EDAX), and analytical electron microscopy (TEM and STEM). The microstructural features of the fired ceramic consisted of unreacted Al₂O₃, glass, porosity, and equilibrium and metastable mullite phases. Residual Al₂O₃ agglomerates (≈ 15 to 30 μm in size) were surrounded by a ≈ 6 μm layer of equilibrium mullite (≈ 71.3 to 73.5 wt% Al₂O₃). The unreacted Al₂O₃-equilibrium mullite assembly formed islands embedded in a silica rich glass (≈ 4.5 to 14wt% Al₂O₃) which also contained 2 to 3 μm thick metastable mullite needles (≈ 70 to 77 wt% Al₂O₃). Phase separation and alumina rich glass compositions (≈ 57 to 59 wt% Al₂O₃) were also observed in some areas of the microstructure.     

Crack-healing and mechanical behaviour of Al2O3/SiC composites at elevated temperature

Alumina/silicon carbide (Al₂O₃/SiC) composite ceramics with large self‐crack‐healing ability, high strength and high heat‐resistance limit temperature for strength were developed and subjected to three‐point bending. A semicircular surface crack 100 μm in diameter was made on each sample. Crack‐healing behaviour was systematically studied, as functions of crack‐healing temperature and healing time, and the fatigue strengths of the crack‐healed sample at room temperature and 1373 K were investigated. Four main conclusions were drawn from the present study. (1) Al₂O₃/SiC composite ceramics have the ability to heal after cracking from 1273to 1673 K in air. (2) The heat‐resistance limit temperature for strength of the crack‐healed sample is ?1573 K, and ?68% of the samples fractured from outside the crack‐healed zone in the testing‐temperature range 873–1573 K. (3) The crack‐healed sample exhibited very high fatigue limit at room temperature and also 1373 K. (4) The large self‐crack‐healing ability is a desirable technique for the high structural integrity of ceramic component.     

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) or Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10^–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al₂O₃/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.     

New transparent Er-doped oxyfluoride tellurite glass ceramic with improved near infrared and up-conversion fluorescence properties

Transparent glass ceramics have been obtained by nucleation and growth of Y₂Te₆O₁₅ or Er₂Te₅O₁₃ cubic phase in a new Er³⁺-doped oxyfluoride tellurite glass. Effect of heat treatment on absorption spectra, luminescence and up-conversion properties in the oxyfluoride tellurite glass has been investigated. With heat treatment the ultraviolet absorption edge red shifted evidently for the oxyfluoride telluride glass. The near infrared emission that corresponds to Er³⁺:⁴I_13/2 → ⁴I_15/2 can be significantly enhanced after heat treatment. Under 980 nm LD pumping, red and green up-conversion intensity of Er³⁺ in the glass ceramic can be observed much stronger than that in the base glass.     

Shock-compaction features and shock-induced chemical reaction in some ceramic powders

Shock compaction features are experimentally investigated for some selected ceramic materials and ceramic composite systems, i.e. SiC, AIN, SiC/AIN, and AIN/Al₂O₃. A typical microstructure to support the skin model proposed in a previous paper is obtained by using SiC powder with a particle diameter of several micrometres. AIN transforms into an unknown phase. The transformation needs a small amount of oxygen. The amount of the unknown new phase decreases with the increase of shock temperature. Chemical reaction of AIN/Al₂O₃ mixture into ALON (cubic aluminium oxynitride spinel) occurs under shock loading and proceeds along with increasing shock temperature. Crystallite size and microstrain of the samples are determined by X-ray line broadening analysis. The microstrain for mixture sample and fine powder is larger than that for single-component sample and coarse powder, respectively. As suggested by the skin model, the requirement that the initial particle size is less than 1 m is essential for ceramic powder to be consolidated by a shock compaction technique in order to yield good compacts of optimum strength and to achieve chemical reaction accompanying mass transport.     

High-temperature oxidation process of intermetallic compound Ti-42 at% Al

The oxidation process of two-phase (Ti₃Al and TiAl) intermetallic compound, Ti-42 at% Al, in air at 1073 and 1273 K has been investigated. The oxidation at 1273 K is much faster than that at 1073 K; however, the oxidation kinetics are similar. During heating up, TiO₂ scale forms initially on the compound surface at about 973 K, and then Al₂O₃ scale forms at about 1273 K. For the isothermal heating, TiO₂ scale slowly grows up at 1073 K, while at 1273 K both TiO₂ and Al₂O₃ scales grow up drastically. The outer oxide scale consists of TiO₂ and the inner one consists of a mixture of TiO₂ and Al₂O₃. The volume of Al₂O₃, which forms after TiO₂ formation at the initial stage of oxidation, is larger at an area adjacent to the oxide-compound interface.     

Ultrasonic determination of single crystal sapphire fiber modulus

Single crystal -Al₂O₃, (sapphire) fibers of 100 m diameter have recently emerged as candidates for stiffening and strengthening high temperature composites. The Young's modulus of these fibers depends upon their crystallographic orientation, ranging from a high of 461 GPa for thec-orientation to a low of 373 GPa for orientations 45° off thec-axis. A deviation of the fibers' axial orientation from the c-axis and thus a reduction in the fibers' axial modulus can sometimes occur during the fiber growth process, and so a simple reliable method is needed to characterize the modulus and/or orientation of the fibers. A laser generated-piezoelectric transducer detected ultrasonic method has been used for this purpose. It has been found that a clear correlation exists between the velocity of the first arriving ultrasonic signal and the deviation of the fiber's axis from thec-axis. The measured velocity is found to be in reasonably good agreement with the calculated bar velocity, _b =E/, for the fiber, providing an estimate of the fiber's orientation dependent Young's modulus. The small differences between the measured and the calculated velocities are believed to be caused by a combination of measurement errors, uncertainties in the reference elastic compliance constants of -Al₂O₃ and the presence of small volume fractions of pores and other (low modulus) aluminum oxide phases in the fibers.     

陶瓷纤维增强氧化硅气凝胶复合材料力学性能试验

氧化硅气凝胶具有极低的热导率和密度,可作为很好的隔热材料,而脆弱的力学性能限制了其在隔热领域的应用。在不影响隔热效果的前提下,通过复合陶瓷纤维可增加氧化硅气凝胶的强度及韧性。试验探索了陶瓷纤维增强氧化硅气凝胶在室温下的拉伸、压缩和剪切等基本力学性能,分别研究了300℃、600℃和900℃下复合材料纤维铺层面方向的压缩性能,并采用扫描电子显微镜对高温试样微观结构进行了观察分析。结果表明：陶瓷纤维增强氧化硅气凝胶的性能表现出方向性,弹性模量在铺层面内方向与厚度方向的数值最大相差约28倍,强度极限亦然;在室温条件下,复合材料的拉伸和压缩弹性模量不同,X 、Y 和 Z 方向拉伸模量与对应的压缩模量之比分别为1.60、1.83和0.56;高温下复合材料沿厚度方向收缩,收缩量随温度升高而增大,900℃下的最大收缩量可达10.8%;高温下复合材料铺层面内方向压缩性能随温度升高而增强。     

Strengthening of porous alumina bodies using carboxylate-alumoxane nanoparticles

Aqueous solutions of acetate-functionalized alumina nanoparticles (A-alumoxane), with an average particle size of 28 nm, have been used as alumina precursors for the infiltration of porous -alumina bodies in order to produce composite structures with homo-interfaces between substrate and infiltrate. Alternatively, if metal doped-methoxy(ethoxyethoxy)acetic acid-functionalized alumina nanoparticles (M-doped MEEA-alumoxane; M = Ca, Er, La, Ti, and Y), with an average particle size of 67 nm, are used in combination with A-alumoxane, a hetero-interface is formed between substrate and infiltrate. Samples were characterized by SEM, BJH, hardness and bend strength measurements. The bulk hardness of the -alumina substrates increases with sintering temperature, but this increase is significantly smaller than the effect of infiltration. The composite hardness generally increases with decreased average pore size although the exceptions to this trend suggest that the identity of the infiltrate is of equal or greater importance. Overall the hetero-interfaces show higher strength than the homo-interface; the latter showing only slightly better performance than high temperature sintering. For the samples fired at 1000°C, the MgAl₂O₄/Al₂O₃ and CaAl₁₂O₁₉/Al₂O₃ combinations appear to provide the greatest enhancement, with both the LaAl₁₁O₁₈/Al₂O₃ and Y₃Al₅O₁₂/Al₂O₃ hetero-interface samples show marked increase in hardness between 1000 and 1400°C. The elastic modulus and bend strength of the -alumina substrate increases significantly for the Er₆Al₁₀O₂₄/Al₂O₃ and LaAl₁₁O₁₈/Al₂O₃ infiltrates. The identity of the hetero-interface has a significant effect on the bulk properties of the composite.