Al2O3 Whiskers Research Articles

In-situ synthesis and characterization of Al2O3 nanostructured whiskers in Ti–Al intermetallic matrix composites

Abstract In-situ Al2O3 rod-like whiskers were synthesized by pressure-less sintering at 1 493 K for 30 min. The whiskers were characterized by X-ray diffraction and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. The results indicate that the diameter distribution of the Al2O3 rod-like whiskers ranges from 20 to 90 nm and the lengths are up to 20 μm. The growth mechanism of the Al2O3 nanostructured whiskers is considered to be a vapor–liquid–solid process. Al liquid acts as both a catalyst and a key component of forming Al2O3 whiskers. Nb2O5 additive results in a lot of small diameter whiskers distributed more evenly in the substrate.

Synthesis and formation of alumina whiskers from hydrothermal solution

Al2O3 whiskers with an average length of 5 μm have been synthesized from hydrothermal solution. Al(NO3)3 and urea were mixed and put into a Teflon-lined stainless steel autoclave and then treated at 120 °C to fabricate precursor whiskers NH4Al(OH)2CO3 (AACH). AACH whiskers were heated at 1200 °C in a furnace to obtain Al2O3 whiskers. The time-dependent examinations revealed that the formation process of AACH whiskers involves two sequential processes: a short liquid–solid deposition process in the initial stage and a long Ostwald ripening process. During calcinations, AACH transforms to amorphism then to α-Al2O3. The escape of NH3 and CO2 induced the distortion and toothlike morphology on the surface of the Al2O3 whiskers.

Hybrid Effect on Whisker Orientation Dependence of Composite Strength of Aluminum Cast Alloy Reinforced by Al2O3 Whiskers and SiC Particles

The hybrid effect on the orientation dependence of the composite strength of an aluminum cast alloy reinforced by Al2O3 whiskers and SiC particles is studied experimentally and numerically. Two types of specimens are prepared for monotonic bending tests. The longitudinal specimen orientation (maximum stress direction) is parallel to or normal to randomly oriented whiskers in plane. The monotonic strength is 18% higher when the hybrid metal matrix composite (MMC) is subjected to an external load parallel to the random whisker orientation in plane than when the load is perpendicular to the whisker orientation. The whisker orientation dependence of composite strength in hybrid composite is weaker than that in whisker-reinforced composite. On the fracture surface of the specimen loaded along the direction parallel to the random whisker orientation in plane, most whiskers are broken while many de-bonded interfaces between the whiskers and matrix are observed on the fracture surface of the specimen loaded along the direction perpendicular to the whisker orientation. To characterize the hybrid effect on the whisker orientation dependence of composite strength, a three-dimensional hybrid composite unit cell model including one whisker and a few particles under a periodic boundary condition is developed using the finite element method. The hybrid composites have higher whisker stress than whisker-reinforced composite when subjected to an external load parallel to the whisker orientation if these composites have the same total volume fraction of reinforcement and the particles are distributed randomly. Under an external load perpendicular to the whisker orientation, the interface stress of hybrid composites is lower than that of whisker-reinforced composite. As a result, the strength difference for parallel and perpendicular loading conditions of the hybrid composites is smaller than that of whisker-reinforced composite. Thus, the weak whisker orientation effect in the hybrid composites is due to a change in microscopic stress distribution induced by interaction between whiskers and particles.

Fracture mechanisms of aluminium cast alloy locally reinforced by SiC particles and Al2O3 whiskers under monotonic and cyclic load

In the present paper, fracture mechanisms and corresponding stress distributions in aluminium cast alloy locally reinforced by SiC particles and Al2O3 whiskers under monotonic and cyclic load are investigated experimentally and numerically. The material is monotonically and cyclically deformed to failure at room temperature. The fracture origin and the fracture path are investigated on the fracture surfaces. The fracture occurs in the reinforced part under both monotonic and cyclic loads. Scanning electron microscope (SEM) analysis of fracture surface shows that the fatigue fracture is controlled by the fracture of coarse Al2O3 whiskers. The static fracture (monotonic loading) shows that the fracture mechanism is the combination of reinforcing particle fracture and interfacial debonding between reinforcing ceramics and matrix metal. The stress distributions around the boundary between the reinforced part and the unreinforced part are calculated based on an inclusion array model considering the microscopic inhomogeneous effects. Both the experimental results and the finite element simulation show that the critical location for fracture is changed by the external stress level which controls the local stress distribution through plastic constraint between reinforcing particle and matrix alloy.

Effect of Whisker Orientation on Monotonic and Fatigue Strength of Aluminium Cast Alloy Locally Reinforced by SiC Particles and Al2O3 Whiskers Under Monotonic and Cyclic load

In this study, the effect of whisker orientation on monotonic and fatigue strength of aluminium cast alloy locally reinforced by SiC particles and Al2O3 whiskers are investigated. The material is monotonically and cyclically deformed to failure at room temperature. A significant effect of reinforcement orientation on the monotonic strength and fatigue strength are observed experimentally and numerically. With respect to the stress direction the whisker orientation gives significant difference in strength of this material. SEM analysis shows almost all whiskers are transversely debonded when whisker direction is perpendicular to the stress direction and almost all whiskers are broken when the whisker direction is parallel to the stress direction. A three-dimensional single whisker unit cell model of cylindrical shape whisker in the periodic boundary condition is conducted using finite element method (FEM) to describe the overall behavior of the composite. The prediction results based on FEM analysis is found to be in reasonable agreement with the experimental observations.

Strength Evaluation of Aluminium Cast Alloy Locally Reinforced by SiC Particles and Al2O3 Whiskers and Its Fractography

In this paper, fracture mechanisms under monotonic and cyclic load and its stress distribution of an aluminum cast alloy locally reinforced by SiC particles and Al2O3 whiskers are investigated experimentally and numerically. The material is monotonically and cyclically deformed to failure at room temperature. The fracture origin and the fracture path are investigated on the fracture surfaces. The fracture occurs in the reinforced part under both monotonic and cyclic loads. SEM analysis of the fracture surface shows that the fatigue fracture is controlled by the fracture of coarse Al2O3 whiskers. On the other hand the static fracture (monotonic loading) shows that the fracture mechanism is the combination of reinforcing particle fracture and interfacial debonding between reinforcing ceramics and matrix metal. The stress distributions around the boundary between the reinforced part and unreinforced part are calculated based on an inclusion array model considering the microscopic inhomogeneous effects. Both the experimental results and the finite element simulation results show that the critical location for fracture is changed by the external stress level which controls the local stress distribution through plastic constraint between reinforcing particle and matrix alloy.

In situ processing of Al 2O 3 whiskers reinforced Ti-Al intermetallic composites

In situ Al2O3 whiskers reinforced Ti-Al intermetallic composites were fabricated at ∼1200°C by reaction sintering of cold-consolidated fillets consisting mainly of Ti, Al, and different additives. The phases and microstructures of the sintered composites were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The process of synthesis was investigated using differential thermal analysis (DTA). The effects of processing parameters and additives on the microstructures of the composites and the development of whisker were examined. It is found that the morphology of the whisker is strongly influenced by the additives, the exothermal reaction process, and the processing parameters.

Production of alumina borate filaments

Reinforcing materials by incorporation of inorganic fibers and whiskers [1] provides the possibilty to obtain composite materials of high mechanical strength. Besides structural strength chemical inertness at high temperatures in different environments is the main demand towards reinforcing additives. Metal oxide ceramics fulfill this requirement, making them one of the most prominent materials for fiber and whisker production. Endless fibers of various ceramics with diameters in the μm range can be spun while whiskers with smaller diameters and lenghts of several μm can be synthesized by a variety of methods. Al2O3 whiskers were produced by vapor-phase reactions [2] and vapor-liquidsolid deposition [3]. Mixtures of Al2O3 and B2O3 form alumina borate whiskers at high temperatures [4, 5]. Alumina borate needles can also be obtained in a potassium sulphide flux from aluminum salts and boric acid at 1100 ◦C [6]. Here we report a simple and efficient method to produce alumina borate filaments by a vapor solid reaction between fumed alumina and boron trifluoride, BF3. The reaction was carried out in a resistively heated flow reactor. In the center of the quartz flow tube (inner diameter 22 mm) a quartz boat with typically 100 mg fumed alumina (Degussa) was placed. The system was evacuated to 10−5 Torr to remove air and checked for leaks. The temperature of the furnace was ramped to 1200 ◦C before a flow of 5 sccm BF3 at a pressure of 50 Torr was established. After 1 h the gas flow was stopped and the reactor evacuated before the system was allowed to cool down to room temperature. The reaction product was collected from the quartz boat and weighed. Optical characterization of the material was performed by scanning electron and transmission electron microscopy (SEM/TEM). Energy dispersive X-ray fluorescence (EDX) spectra were measured in order to determine the elemental composition of the product. The fumed alumina powder which was used as the solid reactant is shown in the inset in the upper part of Fig. 1. It consists of ultrafine particles with diameters around 17 nm. The appearance of the fumed alumina is not altered by the heating process in vacuum or when it is exposed to N2 or H2 at 1200 ◦C. The reaction with BF3 at 1200 ◦C and 50 Torr, however, results in a complete transformation of the alumina powder into filamenteous material. The filaments which are displayed in Fig. 1 have diameters around 200–300 nm and are

Morphology and growth mechanism of alumina whiskers in aluminum-based metal matrix composites

A process has been invented for growing alumina (Al2O3) whiskers in the matrix of the aluminum-based metal matrix composite by the internal oxidation reaction between aluminum (Al) and molybdenum oxide (MoO3) at 850 °C. The whiskers were formed during the firing stage and distributed uniformly throughout the metal matrix. The fractional volume of the whiskers was about 30%, and their average size was 1 μm in diameter and 10 μm in length. The growth mechanism of the Al2O3 whiskers was studied. It was found that the nutrient for the growth of the whiskers was supplied from the molten Al at the base of the whiskers where they reacted with MoO3 forming the Al2O3 whiskers and the Al–Mo intermetallic phases.

Novel Ribbon-Shaped α-Al2O3 Fibers

The use of Al2O3 polycrystalline fibers and whiskers as strengtheners in high-temperature composites is of great interest owing to their high elastic modulus and their thermal and chemical stability. The authors describe in this communication a novel type of crystalline α-Al2O3 fiber. These new fibers—termed ribbons by the authors—have a high specific surface area, which, together with their optical properties and low cost, makes them suitable for use as protective films in composites and as crystalline surfaces for other α-Al2O3 fiber production methods.

Evaluation of Modified Weibull Distribution for Describing the Strength of Ceramic Fibers and Whiskers with Varying Diameters

Abstract A modified Weibull distribution is evaluated for characterizing the statistical strength of ceramic fibers and whiskers with varying diameters from filament to filament. Many commercial ceramic fibers and whiskers have a significant range of diameters. A single-modal Weibull distribution is found inadequate to describe the statistical strength of these fibers and whiskers because of the effect of fiber diameter variation on strength. Procedures for extracting distribution parameters for the modified Weibull distribution from experimental data are presented. Comparison of the modified Weibull distribution with the single-modal Weibull distribution is made for the strength data from Nicalon fibers, Nextel (Al2O3) fibers, hydridopolysilzazlane (HPZ) Si-N-C-O fibers, Al2O3 whiskers, Si3N4 whiskers, and SiC whiskers. Due to its ability to account for the diameter effect on strength, the modified Weibull distribution can yield a more accurate β value than the single-modal Weibull distribution. The Modified Weibull distribution is shown to fit experimental data well and is recommended for characterizing the strength of ceramic fibers and whiskers, the diameters of which vary from filament to filament.

等価介在物法によるアルミニウム合金基複合材料の熱伝導率の解析

Eleven kinds of aluminum-base composites containing SiC, K2O·6TiO2, 9Al2O3·2B2O3 or Al2O3 whiskers were prepared using the liquid metal forging process and their thermal conductivities were determined by the laser-flash method at room temperature. Anisotropic features in thermal conductivity have been detected: samples in which the axes of dispersed whiskers are oriented, exhibit smaller conductivities in a perpendicular direction than in parallel one. The estimations of thermal conductivities were made using the equivalent inclusion method as well as Maxwell and Landau equations and compared with experimental data.

Carbothermal reduction of sillimanite

Preliminary studies on the carbothermal reduction of sillimanite (Al2O3SiO2) powders are reported. The effects of various reaction parameters, e.g. reaction temperature (1550–1700 °C), C: Si molar ratio (4.5–9.0) in the reactant mixtures, the nature and source of carbon (carbon black or activated charcoal) and the reaction atmosphere (argon or nitrogen under a constant positive gas pressure of 0.15 MPa), on the carbothermal reduction have been investigated. It has been observed that Al2O3 and β-silicon carbide whiskers [β-SiC(w)] are the ultimate products formed in an argon atmosphere. Under a nitrogen atmosphere, β′-sialons are also formed. In all cases, mullite forms as the first intermediate phase at lower temperatures, which later converts progressively to the higher-temperature products.

Mechanical properties and recrystallization of alumina ceramics reinforced by silicon carbide whisker

The mechanical properties and recrystallization of a material consisting of Al2O3 and SiC whisker are considered for different hot pressing regimes. The best properties are obtained at an SiC whisker content of 20% in hot pressing in an argon medium at 1600°C (the ultimate bending strength is 600 MPa, the critical coefficient of stress intensityKIc=5.5 MPa · m1/2). The bending strength measured at 1000°C is at least 400 MPa for the material pressed at 1700°C in air. It is established that the presence of SiC whisker on grain boundaries of Al2O3 noticeably impedes the growth of alumina grains in recrystallization for all chosen regimes of hot pressing (1650 – 1900°C, 15 – 45 min). The dominant grain size is 1 –5 µm. A mathematical model is suggested for evaluating the maximum grain size in uniform grain growth, which isD*=0.571 [1++(1+16.151/f)1/2]d, whered, f are the diameter and the volume fraction of SiC whisker. The material can be recommended for use under conditions of thermomechanical stress and abrasive wear.

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface‐Induced Coating

Al2O3–ZrO2–SiC whisker composites were prepared by surface‐induced coating of the precursor for the ZrO2 phase on the kinetically stable colloid particles of Al2O3 and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO2 and SiC whisker phases throughout the Al2O3 matrix. The fracture toughness values of the Al2O3–15 vol% ZrO2–20 vol% SiC whisker composites (∼12 MPa.m1/2) are substantially greater than those of comparable Al2O3–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot‐pressed composites.

Combustion synthesis of Ni3Al and Ni3Al-matrix composites

The self-propagating mode of combustion synthesis (SHS) of Ni3Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni3Al matrix composites with up to 2 wt pct A12O3 whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al2O3, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al2O3 whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratioD Al/D Ni ≥ 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesisD Al/D Ni ≃ 0.3.

1100 to 1300 K slow plastic compression properties of Ni38.5Al composites

Compressive properties of Ni-38.5Al (in at. pct) composites containing 4, 7.5, and 15 vol pct Al2O3 whiskers or 20 vol pct TiB2 particles were investigated in samples compacted to full density by vacuum hot pressing at 20.7 MPa and 1725 K, followed by HIP at 207 MPa and 1725 K. Constant-velocity compression tests were conducted in air at 1100, 1200, and 1300 K to 8 percent strain. It was found that the elevated-temperature compressive properties of Ni-38.5Al+Al2O3 whisker composites were only marginally better than those of the matrix, and that the composite containing 20 vol pct TiB2 particles was only slightly stronger than unreinforced Ni-38.5Al.

Solid Particle Erosion of Brittle Materials (Part 7)

Ten kinds of commercial ceramic tools were eroded by SiC and Al2O3 abrasives with an average diameter of 500μm at 250-300m/s. These ten kinds of commercial ceramic tools were classified into four groups from matrices (Al2O3 or Si3N4) and additives present. The hardness of ceramic tool consisting of Al2O3 as the matrix and TiC as the main additive is 20-22GPa (at a 20kg load), and the fracture toughness is 5.3-6.4MPa·m1/2. The erosive wear resistance of this type of Al2O3/TiC composites was better than that of other three groups of ceramic tools, hot-pressed Si3N4, high purity Al2O3 and SiC whisker reinforced Al2O3, for both SiC and Al2O3 abrasives. The erosion rate of the ceramic tools was in the order of 10-3cm3/cm3 for SiC abrasive and 10-4cm3/cm3 for Al2O3 abrasive. The difference in erosion rate was caused by different erosion mechanisms depending on whether the Vickers hardness ratio of the ceramic tool to the abrasive is larger or smaller than unity.

Al<SUB>2</SUB>O<SUB>3</SUB>繊維およびSiCウイスカ強化した6061Al合金の機械的性質および摩耗特性

The mechanical properties and wear resistance of Al2O3 fiber and hybrid of Al2O3 and SiC whisker reinforced AA6061 were investigated.The elastic modulus of T6 heat treated Al2O3/6061 and Al2O3-SiC/6061 composites can be expressed as(This article is not displayable. Please see full text pdf.) \ oindentrespectively. The fiber arrangement coefficient α is found to be 0.7 for Al2O3 and 0.45 for SiC whisker.The wear resistance of Al2O3/6061 composites is inferior to that of SiC/6061 composites. The addition of small volume fractions of SiC whisker to Al2O3/6061 composites gives rise to remarkable improvement of the wear resistance which is, on the contrary, superior to that of SiC/6061 composistes.

Growth of Al2O3 whiskers by vapour-phase reactions

Needle-like or wool-like Al2O3 whiskers can be grown on a substrate by the hydrolysis of aluminium fluoride vapour. Effects of the growth conditions on the morphology of the crystals and growth processes of the whisker were investigated, and the optimum growth condition of whiskers was examined. Adequate vapour pressure conditions of the reactant gases for the growth of the wool-like whisker were found to be\(P_{AIF_3 } \times P_{H_2 O}^{3/2}< 3.0 \times 10^{ - 3} (torr^2 )\). Aluminium oxyfluoride was assumed to be the most probable intermediate species for the growth of Al2O3 whiskers. A variety of crystal morphologies are also shown and described with respect to the experimental conditions.