Morphology and Frictional Characteristics Under Electrical Currents of Al_2O_3/Cu Composites Prepared by Internal Oxidation

Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and copper against brass are investigated and compared. The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope （SEM） and energy dispersive X-ray spectrum （EDS）. The results show that the wear resistance of the Al2O3/Cu composites is superior to that of copper under the same conditions, Under a given electrical current, the wear rate of Al2O3/Cu composites decreases as the Al2O3-content increases, However, the wear rates of the Al2O3/Cu composites and copper increase as the sliding speed and pressure increase under dry sliding condition. The main wear mechanisms for Al2O3/Cu composites are of abrasion and adhesion; for copper, it is adhesion, although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise.

Al2_3/Al2O3 Joint Brazed with Al2O3-particulate-contained Composite Ag-Cu-Ti Filler Material

Microstructure and interracial reactions of Al2O3 joints brazed with Al2O3-particulate-contained composite Ag-Cu-Ti filler material were researched by scanning electron microscopy （SEM）, electron probe microscopy analysis （EPMA）, energy dispersive spectroscopy （EDS） and X-ray diffraction （XRD）. The interracial reaction layer thickness of joints brazed with conventional active filler metal and active composite filler materials with different volume fraction of Al2O3 particulate was also studied. The experimental results indicated although there were Al2O3 particulates added into active filler metals, the time dependence of interracial layer growth of joints brazed with active composite filler material is t^1/2 as described by Fickian law as the joints brazed with conventional active filler metal.

Synergistically Toughening Effect of SiC Whiskers and Nanoparticles in Al_2O_3-based Composite Ceramic Cutting Tool Material

In recent decades, many additives with different characteristics have been applied to strengthen and toughen Al2O3-based ceramic cutting tool materials. Among them, SiC whiskers and SiC nanoparticles showed excellent performance in improving the material properties. While no attempts have been made to add SiC whiskers and SiC nanoparticles together into the ceramic matrix and the synergistically toughening effects of them have not been studied. An Al2O3-SiCw-SiC np advanced ceramic cutting tool material is fabricated by adding both one-dimensional SiC whiskers and zero-dimensional SiC nanoparticles into the Al2O3 matrix with an effective dispersing and mixing process. The composites with 25 vol% SiC whiskers and 25 vol% SiC nanoparticles alone are also investegated for comparison purposes. Results show that the Al2O3-SiCw-SiCnp composite with both 20 vo1% SiC whiskers and 5 vol% SiC nanoparticles additives have much improved mechanical properties. The flexural strength of Al2O3-SiCw-SiCnp is 730＋ 95 MPa and fracture toughness is 5.6 ± 0.6 MPa.m1/2. The toughening and strengthening mechanisms of SiC whiskers and nanoparticles are studied when they are added either individually or in combination. It is indicated that when SiC whiskers and nanoparticles are added together, the grains are further refined and homogenized, so that the microstructure and fracture mode ratio is modified. The SiC nanoparticles are found helpful to enhance the toughening effects of the SiC whiskers. The proposed research helps to enrich the types of ceramic cutting tool and is benefit to expand the application range of ceramic cutting tool.

Effect of MoSi_2 content on dielectric and mechanical properties of MoSi_2/Al_2O_3 composite coatings

Molybdenum disilicide （MoSi2） sheath and aluminum oxide （Al2O3） core blended powders were fabricated by spray drying. A derived coating material was produced for the application as microwave absorbers using the as prepared powders by atmospheric plasma spray （APS） technology. The effects of MoSi2/Al2O3 mass ratio on the dielectric and physical mechanical properties of the composite coatings were investigated. When the MoSi2 content of the composites increases from 0 to 45%, the flexure strength and fracture toughness improve from 198 to 324 MPa and 3.05 to 4.82 MPa-m1/2 then decline to 310 MPa and 4.67 MPa-m1/2, respectively. The dielectric loss tangent increases with increasing MoSi2 content, and the real part of permittivity decreases conversely over the frequency range of 8.2-12.4 GHz. These effects are due to the agglomeration of early molten MoSi2 particles and the increase of the electrical conductivity with increasing MoSi2 content.

Reaction thermodynamics and kinetics on in situ Al_2O_3/Cu composites

The preferred internal oxidation of aluminum in Cu Al alloy was used to obtain in situ Al 2O 3/Cu composites. The reinforcement particles were mainly γ Al 2O 3, some θ Al 2O 3 and a little α Al 2O 3. Thermodynamics analyses show that the chemical reactions are 3Cu 2O+2Al=6Cu+Al 2O 3 or 3CuO+2Al=3Cu+Al 2O 3. A related equilibrium diagram was drawn. The experiments and investigation show that the formation rate of Al 2O 3 was controlled by the diffusion of oxygen in matrix.

Microstructure and evolution of(TiB_2+Al_2O_3)/NiAl composites prepared by self-propagation high-temperature synthesis

（TiB2＋Al2O3）/NiAl composites were synthesized by self-propagation high-temperature synthesis, and their phase compositions, microstructures and evolution modes were studied. The microstructures and shapes vary with the TiB2＋Al2O3 content in the NiAl matrix. TiB2 particles take a great variety of elementary shapes such as white bars, plates, herringbones, regular cubes and cuboids. These results outline a strategy of self-assembly processes in real time to build diversified microstructures. Some TiB2 grains in sizes of 2-5μm are embeded in Al2O3 clusters, while a small number of TiB2 particles disperse in the NiAl matrix. It is believed that the higher the TiB2＋Al2O3 content is, the more the regular shapes and homogeneous distributions of TiB2 and Al2O3 will be present in the NiAl matrix.

Hot deformation and processing maps of Al_2O_3/Al composites fabricated by flake powder metallurgy

The deformation behaviors of Al2O3/Al composites were investigated by compressive tests conducted at temperature of 300-450 °C and strain rates of 0.001-1.0 s-1 with Gleeble-1500 D thermal simulator system. The results show that the flow stress increases with increasing strain rate and decreasing temperature. The hyperbolic sine constitutive equation can describe the flow stress behavior of Al2O3/Al composites, and the deformation activation energy and constitutive equations were calculated. The processing maps of Al2O3/Al-2 μm and Al2O3/Al-1 μm composites at strain of 0.6 were obtained and the optimum processing domains are in ranges of 300-330 °C, 0.007-0.03 s-1 and 335-360 °C, 0.015-0.06 s-1 for hot working, respectively. The instability zones of flow behavior can also be recognized by the maps.

Direct Synthesis of Al_2O_3-modified Li(Ni_(0.5)Co_(0.2)Mn_(0.3))O_2 Cathode Materials for Lithium Ion Batteries

To improve the cyclic stability at high temperature and thermal stability, the spherical Al2O3-modified Li（Ni0.5Co0.2Mn0.3）O2 was synthesized by a modified co-precipitation method, and the physical and electrochemical properties were studied. The TEM images showed that Li（Ni0.5Co0.2Mn0.3）O2 was modified successfully with nano-Al2O3. The discharge capacity retention of Al2O3-modified Li（Ni0.5Co0.2Mn0.3）O2 maintained about 99% after 200 cycles at high temperature（55 ℃）, while that of the bare one was only 86%. Also, unlike bare Li（Ni0.5Co0.2Mn0.3）O2, the Al2O3-modified material cathode exhibited good thermal stability.

Preparation of Al_2O_3/Au nano-laminated composite coatings and their oxidation resistance on stainless steel

Al2O3/Au nano-laminated composite coatings were prepared by means of magnetron sputtering. The coating was compact and comprised of nano-laminated Al2O3 and Au layers. High temperature cyclic oxidation test was employed to investigate the oxidation resistance of the composite coatings. The results revealed that the applied Al2O3/Au nano-laminated composite coatings improved the oxidation and spallation resistance of the stainless steel substrate significantly. The mechanism accounting for oxidation resistance was related with the suppression of inward oxygen diffusion and selective oxidation of Cr in the substrate. The mechanism accounting for spallation resistance was attributed to the relaxation of thermal stress by the nano-laminated structure.

Effect of CuO particle size on synthesis temperature and microstructure of Al_2O_(3p)-Al composites from Al-CuO system

Al2O3p-Al composites were synthesized using an in-situ reaction in the 80%Al-20%CuO （mass fraction） system. The effects of the CuO particle size on the synthesis temperature and microstructure of the composites were investigated by various methods. The results indicate that the CuO particle size has a significant effect on the temperature at which the complete reaction in the Al-CuO system occurs：the temperature is 200 ℃ lower in the Al-CuO system containing CuO particles with sizes less than 6μm than that containing CuO particles with sizes less than 100μm. The interfacial bonding between Al2O3 particles and Al is not complete when the temperature is below a critical value. The morphology of the Al2O3 particles varies from ribbon-like shape to near spherical shape when the temperature is above a critical value. These two critical temperatures are affected by the particle size of CuO, and the critical temperature of the sample containing CuO particles with sizes less than 6μm is 100 ℃ lower than that of the sample containing CuO particles with sizes less than 100μm.

The Influence of Mo Additive on the Microstructure and Property of Ti/Al_2O_3 Composite

The influence of Mo on the microstructure,bending strength and HV of Ti/Al 2O 3 composite was studied,and the influence mechanism was analyzed.The results indicate that after the addition of Mo,the composite organization is finer and phases distribution is better-proportioned which make the microstructure denser,the bending strength and HV of composite are also increased to a degree.But the bending strength increases first then decreases with the increasing of Mo content,so the appropriate Mo content for the Ti/Al 2O 3 composite is to be further confirmed.

Microstructures and Mechanism of Al_2O_3/Al Composites Fabricated by the Reaction between SiO_2 and Molten Aluminum

Al 2O 3/Al composite was fabricated by the reaction between SiO 2 and molten aluminum.The microstructures of the composite obtained under different reaction conditions were analyzed. The formation mechanism of the composite microstructure was discussed. Results show that the reaction kinetics is influenced remarkably by the reaction temperature, reaction time and the quantity of SiO 2.The morphologies of Al 2O 3 have different features,depending on the reaction temperature.The composite has equiaxed Al 2O 3 grains when materials reacted below 1200℃,and the composite is composed of a large number of fine Al 2O 3 grains and aluminum.The composite has a frame shaped Al 2O 3 microstructure at the reaction temperature of above 1250℃.

Thermodynamic Analysis of Combusion Synthesis and Reaction Hot Pressing of Al_2O_3/B_4C Composite

The equilibrium phases and adiabatic temperature for combustion synthesis and reaction hot pressing of Al 2O 3/B 4C employing ① Al, B 2O 3 and C ② C, B, Al 2O 3 as starting reactants were analyzed by the CALPHAD technique. It is shown that the equilibrium phases at the adiabatic temperature in the combusion system (1) are not the intended composite Al 2O 3/B 4C but other phases. Good agreement with the experimental data was achieved for the calculated adiabatic temperature. The results were discussed with respect to the elimination of the by product in the combustion synthesis. It also revealed that the reactant mixture (2) is a weak exothermic or endothermic reaction system, which can be employed in the reaction hot pressing.

Identification of Growth Promoter to Fabrication SiCp/Al₂O₃Ceramic Matrix Composites Prepared by Directed Metal Oxidation of An Al Alloy

SiC particulates reinforced alumina matrix composites were fabricated using Directed Metal Oxidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with different interlayers (dopents) as growth promoters, will encompasses the early heating of the alloy ingot, melting and continued heating to temperature in the narrow range of 950°C to 980°C in an atmosphere of oxygen. Varying interlayers (dopents) are incorporated to examine the growth conditions of the composite materials and to identification of suitable growth promoter. The process is extremely difficult because molten aluminum does not oxidize after prolonged duration at high temperatures due to the formation of a passivating oxide layer. It is known that the Lanxide Corporation had used a combination of dopents to cause the growth of alumina from molten metal. This growth was directed, i.e. the growth is allowed only in the required direction and restricted in the other directions. The react nature of the dopants was a trade secret. Though it is roughly known that Mg and Si in the Al melt can aid growth, additional dopents used, the temperatures at which the process was carried out, the experimental configurations that aided directed growth were not precisely known. In this paper we have evaluated the conditions in which composites can be grown in large enough sizes for evaluation application and have arrived at a procedure that enables the fabrication of large composite samples by determining the suitable growth promoter (dopant). Scanning electron microscopic, EDS analysis of the composite was found to contain a continuous network of Al2O3, which was predominantly free of grain-boundary phases, a continuous network of Al alloy. Fabrication of large enough samples was done only by the inventor company and the property measurements by the company were confirmed to those needed to enable immediate applications. Since there are a large number of variable affecting robust growth of the composite, fabrication large sized samples for measurements is a difficult task. In the present work, to identify a suitable window of parameters that enables robust growth of the composite has been attempted.

Thermal shock fatigue behavior of TiC/Al_2O_3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference （ΔT） and number of thermal cycles （Ⅳ） on fatigue crack growth （Δa）. The mechanical properties and thermal fatigue resistance of TiC/Al203 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a ＂true＂ cycling effect （thermal fatigue）. Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.

IN SITU PROCESSING OF Al_(2)O_(3) WHISKERS REINFORCED Ti-Al INTERMETALLIC COMPOSITES

In situ Al2O3 whiskers reinforced Ti-Al intermetallic composites were fabricated at ~1200℃ 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.

Kinetics of Oxidation for β-Sialon in Diphase β-Sialon/Al_2O_3 Composite

Non-isothermal kinetic research has been carried out on oxidation behavior of β-Sialon in diphaseβ-Sialon/Al_2O_3 composite at high temperatures. A kinetic formula is established for non-isothermal oxidation process of β-Sialon. The rate of oxidation process is controlled by chemicalreaction at the initial stage and then by diffusion. The apparent activation energies and appar-ent rate constants at different temperatures are determined by treating TG data of the overallprocess.

Effect of hot extrusion on microstructure and tribological behavior of Al_(2)O_(3p) reinforced 7075 aluminum-matrix composites

The effects of hot extrusion and addition of Al_(2)O_(3p) on both microstructure and tribological behavior of 7075 composites were investigated via optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectrometry(EDS),and transmission electron microscopy(TEM).The experimental consequences reveal that the optimal addition of Al_(2)O_(3p) was 2 wt%.After hot extrusion,the Mg(Zn,Cu,Al)2 phases partially dissolve into the matrix and generate many uniformly distributed aging precipitation particles,the Al_(7)Cu_(2)Fe phases are squeezed and broken,and the Al_(2)O_(3p) become uniform distribution.The microhardness of as-extruded 2 wt%Al_(2)O_(3p)/7075 composites reaches HV 170.34,increased by 41.5%than as-cast composites.The wear rate of as-extruded 2 wt%Al_(2)O_(3p)/7075 composites is further lower than that of as-cast composites under the same condition.SEM-EDS analyses reveal that the reinforced wear resistance of composites can put down to the protective effect of the Al_(2)O_(3p) reinforced transition layer.After hot extrusion,the transition layer becomes stable,which determines the reinforced wear resistance of the as-extruded composites.

Effects of Cerium on the Mechanical Properties of the ZA22／Al_2O_3 Composites

ZA22/Al2O3 composites were prepared by means of squeeze casting process. The effects of Ce on the ultimate tensile strength (UTS), impact toughness, and hardness of the composites were studied. The results show that both the UTS and the hardness are improved and the impact toughness is decreased with the increase of the volume fraction of fibers (Vf). After Ce is added ,UTS (Vf>15%) and the hardness are improved at room temperature because of the modification of Ce, but the impact toughness and UTS at elevated temperature are lowered.The filtered action of the fiber preform results in that the influence of the amount of Ce added from 0. 1 wt% to 0. 5 wt% on the mechanical properties of the composites can be ignored.

Aluminum matrix composites reinforced by in situ Al_2O_3 and Al_3Zr particles fabricated via magnetochemistry reaction

Aluminum matrix composites reinforced by in situ Al2O3 and Al3Zr particles are fabricated from A356-Zr(CO3)2 system via magnetochemistry reaction,and the morphologies,sizes and distributions of the in situ particles as well as the microstructures,mechanical mechanisms of the composites are investigated by XRD,SEM,TEM and in situ tensile tests.The results indicate that with the pulsed magnetic field assistance,the morphologies of the in situ particles are mainly with ball-shape,the sizes are in nanometer scale and the distributions in the matrix are uniform.The interfaces between the in situ particles and the aluminum matrix are net and no interfacial outgrowth is observed.These are due to the strong vibration induced by the applied magnetic field in the aluminum melt,which in turn,accelerates the melt reactions.The effects of the magnetic field on the above contributions are discussed in detail.