Synthesis of MoSi2–Al2O3 nanocomposite by mechanical alloying

MoSi₂–Al₂O₃ nanocomposite was synthesized by mechanical alloying (MA) of MoO₃, SiO₂ and Al powder mixture. The structural evolution of the powders was studied by X-ray diffraction (XRD). Both β-MoSi₂ and -MoSi₂ were obtained after 3 h of milling. The spontaneous formation of β-MoSi₂ during milling proceeded by a mechanically induced self propagating reaction (MSR), analogous to that of the self propagating high temperature synthesis (SHS). After 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. Residual Mo and Si in the 3 and 70 h milled samples formed β-MoSi₂ and Mo₅Si₃ during heating at 1000 °C, respectively.     

Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.     

Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles

TiB₂–Al₂O₃ composites with Ni–Mo as sintering aid have been fabricated by a hot-press technique at a lower temperature of 1530 °C for 1 h, and the mechanical properties and microstructure were investigated. The microstructure consists of dispersed Al₂O₃ particles in a fine-grained TiB₂ matrix. The addition of Al₂O₃ increases the fracture toughness up to 6.02 MPa m^1/2 at an amount of 40 vol.% Al₂O₃ and the flexural strength up to 913.86 MPa at an amount of 10 vol.% Al₂O₃. The improved flexural strength of the composites is a result of higher density than that of monolithic TiB₂. The increase of fracture toughness is a result of crack bridging by the metal grains on the boundaries, and crack deflection by weak grain boundaries due to the bad wetting characters between Ni–Mo and Al₂O₃.     

原位自生纳米Al2O3/Al-Zn-Cu复合材料的力学性能

将纳米ZnO粉末和Al粉球磨后冷压成Al-ZnO预制块,然后将其加到Al-Zn-Cu熔体中进行Al-ZnO原位反应,制备出纳米Al₂O₃颗粒增强Al-Zn-Cu基复合材料。能谱面扫描分析和透射电镜观察结果表明,复合材料由纳米Al₂O₃颗粒和Al₂Cu析出相两种颗粒/析出相组成。纳米Al₂O₃颗粒通过异质形核和晶界钉扎,细化了Al-Zn-Cu合金晶粒组织和Al₂Cu析出相。原位纳米Al₂O₃颗粒的生成提高了基体合金的拉伸性能,轧制+热处理使Al₂O₃/Al-Zn-Cu复合材料的拉伸强度比相同处理的基体合金提高约100%,总伸长率提高约98%。     

High-temperature tensile properties of Mg/Al2O3 nanocomposite

Mg/1.1Al₂O₃ nanocomposite was synthesized using solidification process called disintegrated melt deposition technique followed by hot extrusion. Microstructural characterization showed that reasonably uniform distribution of reinforcement leads to significant grain refinement of commercially pure magnesium matrix and effectively restricted the grain growth during high-temperature tensile test. Physical properties characterization revealed that addition of nano-Al₂O₃ particulates as reinforcement improves the dimensional stability of pure magnesium. Mechanical properties characterization revealed that the presence of thermally stable nano-Al₂O₃ particulates as reinforcement leads to a significant increase in room temperature microhardness, dynamic elastic modulus, 0.2% yield strength, UTS and ductility of pure magnesium and efficiently maintained the strengthening effect up to 150 °C. Fractography revealed that fracture behavior of magnesium matrix change from brittle to mixed ductile mode with activation of non-basal slip system in room temperature to complete ductile mode at high temperature due to the presence of nano-Al₂O₃ particulates.     

High performance nanocomposites for tribological applications: Preparation and characterization

High performance nanocomposites were prepared by incorporating 0–12 vol.% nano-sized (39 nm) Al₂O₃ particles into PEEK matrix using compression molding. The microhardness and dynamic mechanical properties of the nanocomposites increase with increasing Al₂O₃ content. The wear resistance of the nanocomposites evaluated at a sliding speed of 1.0 m/s and nominal pressure from 0.5 MPa to 1.25 MPa under dry sliding conditions was improved more than threefold at 0.8 vol.% Al₂O₃ content. However, the wear resistance of the nanocomposites containing above 1.67 vol.% Al₂O₃ was deteriorated, despite their higher hardness and stiffness as compared to that of nanocomposites containing lower Al₂O₃ content. The surface roughness of the wear track formed over the countersurface increases with increasing Al₂O₃ content. The coefficient of friction of nanocomposites was higher than that of pure PEEK. SEM and optical microscopy have shown that wear of pure PEEK occurs by the mechanism of adhesion mainly, whereas of nanocomposites by microploughing and abrasion. Energy dispersive spectrometry (EDS) shows that Fe and alloying elements of countersurface transfer to the wear debris at higher Al₂O₃ content.     

Comparative study on chemical stability of dielectric oxide films under HF wet and vapor etching for radiofrequency microelectromechanical system application

HF wet and vapor etching of dielectric oxide films, which were prepared by thermal atomic layer deposition (ALD) and plasma-enhanced ALD (PEALD), are examined for radiofrequency microelectromechanical system (RF MEMS) application. The chemical stability of oxide films was increased in the order of ALD–Al₂O₃ < PEALD–ZrO₂ < PEALD–TiO₂ ≈ ALD–Ta₂O₅ under wet etching in 6:1 buffered HF aqueous solution, but in a different order of Ta₂O₅ < ZrO₂ < TiO₂ ≈ Al₂O₃ under anhydrous HF/CH₃OH vapor etching at 4 kPa. The unstable films were uniformly and completely etched under the wet etching, while transformed to have increased thickness or non-uniformly etched with thicker residue under the vapor etching. Al₂O₃ and TiO₂ (Ta₂O₅ and TiO₂) can be used for RF MEMS capacitive switch fabricated by using HF vapor (wet) etching of sacrificial SiO₂.     

Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

Modeling and optimizing of factors affecting erosion–corrosion of AA6063–(TiC/Al2O3) hybrid composites by experimental design method

In this article, modeling and optimizing of factors affecting erosion–corrosion wear of aluminum alloy A6063 reinforced with (Al₂O₃/TiC) particles have been determined by experimental design method. The erosion–corrosion wear characteristics and mechanism of AA6063–(TiC/Al₂O₃) with experimental parameters namely; type and concentration of corrosive media in the slurry, erosion speed and time have been investigated. Two models for reinforced and unreinforced alloys were applied to describe the influences of these factors on the erosion behavior of alloys. The erosion–corrosion mechanisms of the AA6063–(TiC/Al₂O₃) were dominated by particles erosion wear in alkaline slurry, and by the interaction of particle erosion wear and medium corrosion in acidic slurry. The results of experimental work are coinciding with that of calculated ones confirming the successful modelization.     

Processing, microstructure and properties of C40 Mo(Si,Al)2/Al2O3 composites

The C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composites were prepared by spark plasma sintering (SPS) of mechanically alloyed (MA) powders. The Mo(Si_0.75Al_0.25)₂/0–20 vol.% Al₂O₃ materials, showing micron and submicron composite structure, possess a hardness of 13.9–14.6 GPa but a poor toughness of 1.78–1.80 MPa m^1/2. The addition of 30 vol.% Al₂O₃ leads to the formation of the micron C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composite with an intergranular distribution of Al₂O₃, that results in a drop of the hardness to 10.2 GPa and an improvement of the toughness to 3.67 MPa m^1/2. The transition of the cleavage facets to the intergranular fracture with the addition of Al₂O₃ is assumed as the main toughening mechanism.     

Improvement of the tensile strength of Al18B4O33w/Al composite at elevated temperatures by change of interfacial state

In general, it is very difficult to obtain obviously reinforced effect in discontinuously reinforced aluminum matrix composites at the temperature above 400 °C. In the present study, we report an effective method to improve the high-temperature tensile strength of Al₁₈B₄O₃₃w/Al composite by change of interfacial state. The pure aluminum matrix composites reinforced by Al₁₈B₄O₃₃w with different ZnAl₂O₄ coating contents were fabricated by squeeze casting. The results indicate that ZnAl₂O₄ coating of the whiskers can effectively improve the high-temperature tensile strength of Al₁₈B₄O₃₃w/Al composite, although the tensile strength of the composite decreases with increasing the tensile temperature. On the basis of fractograph analysis, the fracture mechanism of the composites at elevated temperatures was investigated.     

Structure and hardness of surface of Al18B4O33w/Al composite by laser surface melting

As a new material, aluminum borate whisker reinforced aluminum composites have attracted interest and have been considered for a wide range application because of their high specific strength, high modulus and low cost. The study included a detailed characterization of the laser melting surface in terms of microstructures, phase analysis and a ratio of Al₂O₃ (the decomposition products of whiskers during laser process) and Al in intensity with tests parameters as an indication of Al₂O₃ distribution. Microhardness of the laser layer was also studied in detail. The results indicated that the most of γ-Al₂O₃ exists at the bottom of the laser pool, which led to a maximum value in the hardness be obtained. Microhardness of the laser layer was improved to 294 Hv as compared to 178 Hv of the as-cast composite, because of the existence of Al₂O₃ particles, solid solution hardness and the grain refinement of the laser layer following rapid quenching associated with the process.     

Al2O3纤维对SiO2基陶瓷型芯的烧缩阻滞

向SiO₂基体粉料中添加Al₂O₃纤维,采用热压注法制备Al₂O₃/SiO₂陶瓷型芯。分析Al₂O₃纤维含量对陶瓷型芯性能的影响。研究结果表明：Al₂O₃纤维含量对Al₂O₃/SiO₂陶瓷型芯的线收缩率、体积密度和抗弯强度均有较大的影响。当Al₂O₃纤维含量大于1wt%时,Al₂O₃/SiO₂陶瓷型芯的线收缩率大幅度降低,稳定在0.335%左右,体积密度随之降低,稳定在1.790 g · cm^-3左右;当Al₂O₃纤维含量为1wt%时,陶瓷型芯抗弯强度达最大值20.48 MPa。分析了Al₂O₃纤维对Al₂O₃/SiO₂陶瓷型芯烧结收缩的阻滞作用机制。     

Nanocrystallization of SrBi2Nb2O9 from glasses in the system Li2B4O7---SrO---Bi2O3---Nb2O5

Transparent glasses in the system (100−x)Li₂B₄O₇–x(SrO---Bi₂O₃---Nb₂O₅) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi₂Nb₂O₉ (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (_r) and the loss factor (D) for the lithium borate, Li₂B₄O₇ (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.     

Synthesis and mechanical properties of niobium aluminide-based composites

Niobium aluminide-based composites reinforced with in situ and externally added Al₂O₃ and TiC particulates were fabricated by hot-pressed sintering at 1400 °C. In particular, Nb₂Al–Al₂O₃–TiC in situ composites were successfully obtained from the raw powder mixtures of Nb60Al40 (in at.%)–TiO₂20C8 (in wt.%) by means of this process. The influences of ceramic particulates on the microstructures, flexural strength and fracture toughness were examined. The experimental results indicate that the presence of ceramic particulates yielded a remarkable improvement in both the strength and fracture toughness in comparison with previous results for monolithic niobium aluminide compounds.     

Formation of MgAl2O4 in Al2O3-(Al-4wt.%Mg) composites

The Al₂O₃ particles are introduced into the Al-4wt.%Mg melt by the “vortex” method. After being cast, Al₂O₃-(Al-4wt.%Mg) composites are remelted at 700, 750, 800 and 850°C for different residence times to investigate the formation of MgAl₂O₄ (spinel).

The results show that MgAl₂O₄ is the unique interface of the Al₂O₃-(Al---Mg) composites held at 700–850°C. Fine MgAl₂O₄ crystals grow on the surface of the Al₂O₃ particle but, as the holding temperature and the residence time increase, some spinels will form themselves into pyramidal shape. The MgAl₂O₄ grows not only at the matrix-particle interface but also on the surface of the composite specimens. The formation reactions of interfacial MgAl₂O₄ are as follows: Mg(1) + 2Al(1) + 2O₂(g) = MgAl₂O₄(s)3Mg(1) + 4Al₂O₃(s) = 3MgAl₂O₄(s) + 2Al(1) Both of them are equally important.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Characterization of the Reaction Products That Develop in the Processing of Al2O3/BaZrO3 Laminated Composites

The absence of a chemical reaction at an interface is conventionally thought to be an important criterion in producing a tough ceramic matrix composite (CMC). As a result of this criterion, interphases in CMCs were chosen on the basis of their chemical reactivity. A weak interface results in crack deflection, crack bridging, and, in fiber-reinforced ceramics, fiber pullout, resulting in an increased fracture toughness. In this paper, we present microstructural observations on alumina (Al₂O₃)–barium zirconate (BaZrO₃) laminated composites wherein the reaction products that develop during processing resulted in sharp interfaces and appear to be weak enough to deflect cracks. These in situ reaction products in Al₂O₃–BaZrO₃ laminated composites were characterized with the use of a scanning electron microscope, an electron microprobe, and a transmission electron microscope. The phases that develop, ZrO₂, BaO·Al₂O₃, and BaO·6 Al₂O₃, produced sharp interfaces and are arranged in a sequence that could be predicted by using information from the phase diagram.     

Bi2O3助熔剂对CaO-B2O3-SiO2/Al2O3玻璃陶瓷复合材料性能的影响

以CaO-B₂O₃-SiO₂(CBS)玻璃粉体和Al₂O₃陶瓷粉体为原料,通过在CBS与Al₂O₃的质量比固定为50:50的玻璃-陶瓷复合材料中添加适量的Bi₂O₃作为烧结助熔剂,探讨了Bi₂O₃助熔剂对CBS/Al₂O₃复合材料的烧结性能、介电性能、抗弯强度和热膨胀系数的影响规律.研究表明:Bi₂O₃助熔剂能通过降低CBS玻璃的转变温度和黏度促进CBS/Al₂O₃复合材料的致密化进程,于880 ℃下烧结即能获得结构较致密、气孔较少的CBS/Al₂O₃复合材料.然而,过量添加Bi₂O₃将使玻璃的黏度过低,从而恶化CBS/Al₂O₃复合材料的烧结性能、介电性能及抗弯强度.当Bi₂O₃的添加量为CBS/Al₂O₃复合材料的1.5wt%时,于880 ℃下烧结即能获得最为致密的CBS/Al₂O₃复合材料,密度为2.82 g·cm^-3,这一材料具有良好的介电性能(介电常数为7.21,介电损耗为1.06×10^-3),抗弯强度为190.34 MPa,0~300 ℃的热膨胀系数为3.52×10^-6 K^-1.     

Kinetic analysis on Al2O3/Cu composite prepared by mechanical activation and internal oxidation

The Al₂O₃/Cu composite was prepared by mechanical activation and internal oxidation process. Kinetic factors, which influenced the internal oxidation process, were also discussed in the present paper. The results showed that the duration of the internal oxidation was highly shortened after the powders were treated by mechanical activation. The typical internal oxidation duration was only 1 h. It is unnecessary for further prolonging internal oxidation time. Besides, in order to get a complete internal oxidation heating rate and green density of the compact are another two control factors. Their parameters should be controlled from Al contents in the powders. In this research, the optimum internal oxidation duration, heating rate and green density are 1 h, 20 °C/min and 80%, respectively, for Cu–0.8 wt% Al/Cu₂O powders. After the internal oxidation, uniform spherical -Al₂O₃ particles with an average size 0.5–0.8 μm in diameter were observed in the Cu matrix.