Tensile and wear properties of rolled Al5Mg-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> or C particulate composites

Al5Mg alloy matrix composites reinforced with different percentages of Al₂O₃ (60 μm) or C (90 μm) particulates were prepared by the vortex method. The composites were then subjected to hot or cold rolling with different reduction ratios. The microstructures of the rolled composites revealed that the matrix grains moved around the particulate causing deformation. By continuing deformation, the particulates rearranged themselves in the matrix, leading to lensoid distortion. It was found that the addition of Al₂O₃ or C particulates increased the 0.2% proof stress and reduced both the tensile strength and ductility, compared with the monolithic alloy. Scanning electron microscopy (SEM) fractographic examinations showed that the composites reinforced with Al₂O₃ particulates failed through particulate fracture and matrix ligament rupture. However, the failure of the composites reinforced with C particulates was through particulate decohesion, followed by ductile failure of the matrix. Abrasive wear results showed that the wear rate of the Al5Mg alloy decreased with the addition of C particulates. However, increasing the volume fraction of C particulates did not have a prominent effect on the wear rate. The composites reinforced with Al₂O₃ particulates exhibited a higher wear rate than that of the unreinforced alloy. Furthermore, addition of both C and Al₂O₃ particulates into the Al5Mg matrix alloy did not significantly improve the wear resistance. For all composites studied in this work, hot or cold rolling had a marginal effect on the wear results.     

Cold spray of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with soft metal incorporation: A technical contribution

Al + SiC, Al + Al₂O₃ composites as well as pure Al, SiC, and Al₂O₃ coatings were prepared on Si substrates by the cold gas dynamic spray process (CGDS or cold spray). The powder composition of metal (Al) and ceramic (SiC, Al₂O₃) was varied into 1:1 and 10:1 wt.%, respectively. The propellant gas was air heated up to 330 °C and the gas pressure was fixed at 0.7 MPa. SiC and Al₂O₃ have been successfully sprayed producing coatings with more than 50 μm in thickness with the incorporation of Al as a binder. Also, hard ceramic particles showed peening effects on the coating surfaces. In the case of pure Al metal coating, there was no crater formation on hard Si substrates. However, when Al mixed with SiC and Al₂O₃, craters were observed and their quantities and sizes depended on the composition, aggregation and size of raw materials.     

Structure and electric property comparison between Ge nanoclusters embedded in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>

Metal-insulator-semiconductor (MIS) structures containing Ge nanocrystals embedded in both Al₂O₃ and ZrO₂/Al₂O₃ are fabricated by an ultra-high vacuum electron-beam evaporation method. Secondary ion mass spectroscopy (SIMS) results indicate that Ge embedded in Al₂O₃ diffuses towards the surface of the Al₂O₃ layer after annealing at 800°C in N₂ ambient for 30 min. Ge embedded in ZrO₂/Al₂O₃ is stable, thus inducing less leakage current. Capacitance voltage studies indicate that annealing can effectively passivate the negatively charged trapping centers. Memory effect of the Ge nanoclusters is verified by hysteresis in the C-V curves in the Al₂O₃/Ge+Al₂O₃/Al₂O₃ and ZrO₂/Ge+Al₂O₃/Al₂O₃ samples. This article is based on a presentation in “The 7th Korea-China Workshop On Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

Studies on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> high K gate dielectrics applied in a fully depleted SOI MOSFET

Al₂O₃/ZrO₂/Al₂O₃ gate stacks were prepared on ultrathin SOI (Silicon on insulator) substrates by ultrahigh vacuum electron beam evaporation and post-annealed in N₂ at 450°C for 30 min. Three clear nanolaminate layered structure of Al₂O₃(2.1 nm)/ZrO₂(3.5 nm)/Al₂O₃(2.3 nm) was observed with a high-resolution cross-sectional transmission electron microscope (HR-XTEM). High frequency capacitance voltage (C-V) characteristics of a fully depleted (FD) SOI MOS capacitor at 1 and 5 MHz were studied. The minority carriers determine the high frequency C-V properties, which is opposite to the case of bulk MOS capacitors. The series resistance of the SOI substrate is found to be the determinant factor of the high frequency characteristics of FD SOI MOS capacitors. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24≈27, 2003.     

Fabrication of A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties

Aluminum/alumina composites are used in automotive and aerospace industries due to their low density and good mechanical strength. In this study, compocasting was used to fabricate aluminum-matrix composite reinforced with micro and nano-alumina particles. Different weight fractions of micro (3, 5 and 7.5 wt.%) and nano (1, 2, 3 and 4 wt.%) alumina particles were injected by argon gas into the semi-solid state A356 aluminum alloy and stirred by a mechanical stirrer with different speeds of 200, 300 and 450 rpm. The microstructure of the composite samples was investigated by Optical and Scanning Electron Microscopy. Also, density and hardness variation of micro and nano composites were measured. The microstructure study results revealed that application of compocasting process led to a transformation of a dendritic to a nondendritic structure of the matrix alloy. The SEM micrographs revealed that Al₂O₃ nano particles were surrounded by silicon eutectic and inclined to move toward inter-dendritic regions. They were dispersed uniformly in the matrix when 1, 2 and 3 wt.% nano Al₂O₃ or 3 and 5 wt.% micro Al₂O₃ was added, while, further increase in Al₂O₃ (4 wt.% nano Al₂O₃ and 7.5 wt.% micro Al₂O₃) led to agglomeration. The density measurements showed that the amount of porosity in the composites increased with increasing weight fraction and speed of stirring and decreasing particle size. The hardness results indicated that the hardness of the composites increased with decreasing size and increasing weight fraction of particles.     

Effect of a surface treatment for LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> cathode material in lithium secondary batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode material was surface-treated to improve its electrochemical performance. Al₂O₃ nanoparticles were coated onto the surface of LiNi_1/3Co_1/3Mn_1/3O₂ powder using a sol-gel method. The as-prepared Al₂O₃ nano-particle was identified as the cubic structure of Al₂O₃. XRD showed that the LiNi_1/3Co_1/3Mn_1/3O₂ structure was not affected by the Al₂O₃ coating. With a coating of 3 wt.% Al₂O₃ on LiNi_1/3Co_1/3Mn_1/3O₂, the cyclic-life performance and rate capability were improved. However, heavier coatings (5 wt.%) on LiNi_1/3Co_1/3Mn_1/3O₂ resulted in a considerable decrease of the discharge capacity and rate capability. The thermal stability of LiNi_1/3Co_1/3Mn_1/3O₂ materials was greatly improved by the 3 wt.% Al₂O₃ coating.     

Corrosive wear behavior of 7075 aluminum alloy and its composite containing Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles

Corrosive wear behavior of 7075 aluminum alloy and a composite containing 0.10 volume fraction of alumina particles (VFAP) has been evaluated. Transient current (TC) generated as a result of impacting a rotating cylindrical electrode immersed in a 0.1M NaCl solution with a Vickers diamond hardness indenter has been used to measure the corrosive wear response. Age hardenable 7075 alloy shows TC values that are sensitive to prior solutionizing time. The effect of alumina particles in a 7075 aluminum alloy matrix has been studied by comparing the TC values of a monolith along with composites under almost identical experimental conditions. The role of microstructural features associated with composites, such as dislocations generated after solutionizing treatment and during the corrosive wear process, has been observed with the help of near surface microstructures through transmission electron microscopy (TEM). Deformation induced dislocations, as well as those that are due to differences in the coefficient of thermal expansion (CTE) values between the particles and the matrix during solutionizing, have been attributed to the experimentally observed TC values. They may also be affected by the aging response of the monolith and composites, depending on solutionizing time.     

HVOF-Sprayed Coatings Engineered from Mixtures of Nanostructured and Submicron Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Powders: An Enhanced Wear Performance

In previous studies, it has been demonstrated that nanostructured Al₂O₃-13 wt.%TiO₂ coatings deposited via air plasma spray (APS) exhibit higher wear resistance when compared to that of conventional coatings. This study aimed to verify if high-velocity oxy-fuel (HVOF)-sprayed Al₂O₃-13 wt.%TiO₂ coatings produced using hybrid (nano + submicron) powders could improve even further the already recognized good wear properties of the APS nanostructured coatings. According to the abrasion test results (ASTM G 64), there was an improvement in wear performance by a factor of 8 for the HVOF-sprayed hybrid coating as compared to the best performing APS conventional coating. When comparing both hybrid and conventional HVOF-sprayed coatings, there was an improvement in wear performance by a factor of 4 when using the hybrid material. The results show a significant antiwear improvement provided by the hybrid material. Scanning electron microscopy (SEM) at low/high magnifications showed the distinctive microstructure of the HVOF-sprayed hybrid coating, which helps to explain its excellent wear performance. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Preparation, microstructure and properties of molybdenum alloys reinforced by in-situ Al2O3 particles

The conventional molybdenum alloys, lacking of hard particles enhancing wear property, have relative poor wear resistance though they are widely used in wear parts. To resolve the above question, Mo alloys reinforced by in-situ Al₂O₃ particles are developed using powder metallurgy method. The in-situ α-Al₂O₃ particles in molybdenum matrix are obtained by the decomposition of aluminum nitrate after liquid-solid incorporation of MoO₂ and Al(NO₃)₃ aqueous solution. The α-Al₂O₃ particles well bonded with molybdenum distribute evenly in matrix of Mo alloys, which refine grains of alloys and increase hardness of alloys. The absolute density of alloy increases firstly and then decreases with the increase of Al₂O₃ content, while the relative density rises continuously. The friction coefficient of alloy, fluctuating around 0.5, is slightly influenced by Al₂O₃. However, the wear resistance of alloy obviously affected by the Al₂O₃ particles rises remarkably with the increasing of Al₂O₃ content. The Al₂O₃ particles can efficiently resist micro-cutting to protect molybdenum matrix, and therefore enhances the wear resistance of Mo alloy.     

Effect of Nano-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-8YSZ Ceramic Coatings

In this paper, the effect of nano-Si₃N₄ additives and plasma treatment on the wear behavior of Al₂O₃-8YSZ ceramic coatings was studied. Nano-Al₂O₃, nano-8YSZ (8 wt.% Y₂O₃-stabilized ZrO₂) and nano-Si₃N₄ powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al₂O₃-8YSZ coatings with and without Si₃N₄ additives was studied. Nano-Si₃N₄ additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al₂O₃-18 wt.% 8YSZ-10 wt.% Si₃N₄-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al₂O₃-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si₃N₄ and plasma treatment could improve the wear resistance of Al₂O₃-8YSZ coatings.     

High Wear-Resistant Aluminum Matrix Composite Layer Fabricated by MIG Welding with Lateral B<Subscript>4</Subscript>C Powder Injection

In this work, a low-cost technique combining MIG welding and lateral powder injection was developed to fabricate B₄C particles-reinforced aluminum matrix composite (AMC) layer on a T6 heat-treated 7075 aluminum alloy (AA7075-T6) substrate. The AMC layer was 6-7 mm thick and well bonded to the substrate. The B₄C particles were dispersed throughout the AMC layer with an average content of approximately 7 vol.%. No significant reaction products existed either at the particle–matrix interface or in the Al-matrix. In pin-on-disk dry sliding wear tests against Al₂O₃ grinding wheels, the AMC layer exhibited excellent wear resistance with volume wear rate approximately 1/10-3/10 that of the quenched AISI 1045 steel and only approximately 2-7% that of the AA7075-T6 alloy under the same wear conditions. A small addition of ceramic particles can greatly improve wear resistance, suggesting that this technique has good prospects for a wide variety of applications.     

Wear Resistance of Aluminum Matrix Composites Reinforced with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles After Multiple Remelting

Based on previous results, the commercial composites of A359 (AlSi9Mg) alloy reinforced with 22 vol.% Al₂O₃ particles were submitted to multiple remelting by means of gravity casting and squeeze-casting procedures. The studies were focused on tribological tests, x-ray phase analyses, and microstructural examinations. More promising results were obtained for squeeze-casting method mainly because of the reduction of the negative microstructural effects such as shrinkage porosity or other microstructural defects and discontinuities. The results showed that direct remelting may be treated as economically well-founded and alternative way compared to other recycling processes. It was underlined that the multiple remelting method must be analyzed for any material separately.     

Rapid synthesis and consolidation of nanostuctured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgSiO<Subscript>3</Subscript> composites by high frequency induction heated sintering

Nanopowders of MgO, Al₂O₃ and SiO₂ were made by high energy ball milling. The rapid sintering of nanostuctured Al₂O₃-MgSiO₃ composites was investigated by the high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibits grain growth. Highly dense nanostructured Al₂O₃-MgSiO₃ composites were produced with the simultaneous application of 80 MPa pressure and the induced output current of total power capacity (15 kW) within 2 min. The sintering behavior, grain size and mechanical properties of Al₂O₃-MgSiO₃ composites were investigated.     

Effect of Milling Time on Electrical Breakdown Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

In order to clarify the relationship between the microstructure and the arc erosion behavior of metal-matrix composite, Al₂O₃/Cu composites with different distributions of Al₂O₃ particles were prepared by high energy ball milling and powder metallurgy. The effect of milling time on microstructure, properties, and arc erosion behavior of Al₂O₃/Cu composite was investigated. The results show that the distribution of Al₂O₃ particles improves significantly with increase of milling time, but Al₂O₃ particles will be aggregated if milling time is too long. The optimal milling time is 24 h in the range of experiments. A uniform distribution of Al₂O₃ particles in copper matrix can improve the hardness, electrical conductivity, average breakdown strength, chopping level, and arc life. With improvement in the distribution of Al₂O₃ particles, the erosion area becomes larger, and the erosion pits become shallower and are dispersed more uniformly.     

Formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al Composites by Directed Melt Oxidation of Al-Si-Zn Alloy

Observations are presented on the initiation and growth of Al₂O₃/Al composites by the directed melt oxidation of Al-Si alloys containing metallic Zn or using external dopant ZnO. Thermal gravimetric analysis, optical microscopy, and x-ray diffraction analysis were employed to characterize the progress of oxidation and the nature of oxidation products. Both Zn and ZnO dopants were able to initiate the directed melt oxidation of Al-Si alloys without any Mg being present. Al₂O₃/Al composites were produced when the alloying Zn concentration exceeding 3 wt.%. The incubation period of the oxidation process for Al-Si-Zn alloys was shortened markedly and the amount of composite products increased with the increasing of Zn content in the alloy. In addition, doping with ZnO powder resulted in dense composite formation. A macroscopically planar surface and a fine microstructure promote oxidation growth in Al₂O₃/Al composites. Doping with ZnO powder offers a significant advantage over using metallic Zn for the directed melt oxidation of Al-Si alloy.     

Examination of wire electrical discharge machining of Al2O3p/6061Al composites

Alumina particle reinforced 6061 aluminum matrix composites (Al₂O₃p/6061Al) have excellent physical and chemical properties than those of a traditional metal; however, their poor machinability lead to worse surface quality and serious cutting tool wear. In this study, wire electrical discharge machining (WEDM) is adopted in machining Al₂O₃p/6061Al composite. In the experiments, machining parameters of pulse-on time were changed to explore their effects on machining performance, including the cutting speed, the width of slit and surface roughness. Moreover, the wire electrode is easily broken during the machining Al₂O₃p/6061Al composite, so this work comprehensively investigates into the locations of the broken wire and the reason of wire breaking.The experimental results indicate that the cutting speed (material removal rate), the surface roughness and the width of the slit of cutting test material significantly depend on volume fraction of reinforcement (Al₂O₃ particles). Furthermore, bands on the machined surface for cutting 20 vol.% Al₂O₃p/6061Al composite are easily formed, basically due to some embedded reinforcing Al₂O₃ particles on the surface of 6061 aluminum matrix, interrupt the machining process. Test results reveal that in machining Al₂O₃p/6061Al composites a very low wire tension, a high flushing rate and a high wire speed are required to prevent wire breakage; an appropriate servo voltage, a short pulse-on time, and a short pulse-off time, which are normally associated with a high cutting speed, have little effect on the surface roughness.     

Elevated Temperature Solid Particle Erosion Performance of Plasma-Sprayed Co-based Composite Coatings with Additions of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>

In this paper, investigation into solid particle erosion behavior of atmospheric plasma-sprayed composite coating of CoCrAlY reinforced with Al₂O₃ and CeO₂ oxides on Superni 76 at elevated temperature of 600 °C is presented. Alumina particles are used as erodent at two impact angles of 30° and 90°. The microstructure, porosity, hardness, toughness and adhesion properties of the as-sprayed coatings are studied. The effects of temperature and phase transformation in the coatings during erosion process are analyzed using XRD and EDS techniques. Optical profilometer is used for accurate elucidation of erosion volume loss. CoCrAlY/CeO₂ coating showed better erosion resistance with a volume loss of about 50% of what was observed in case of CoCrAlY/Al₂O₃/YSZ coating. Lower erosion loss is observed at 90° as compared to 30° impact angle. The erosion mechanism evaluated using SEM micrograph revealed that the coatings experienced ductile fracture exhibiting severe deformation with unusual oxide cracks. Reinforced metal oxides provide shielding effect for erodent impact, enabling better erosion resistance. The oxidation of the coating due to high-temperature exposure reforms erosion process into oxidation-modified erosion process.     

Fabrication and Properties of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> Composite Coatings

Al₂O₃ /xZrO₂ (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al₂O₃ and nano-sized monoclinic-ZrO₂ powders. Microstructural investigation showed that the coatings comprised well-separated Al₂O₃ and ZrO₂ lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al₂O₃ and tetragonal-ZrO₂ with trace of original α-Al₂O₃ and monoclinic-ZrO₂ phases. The effect of ZrO₂ addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO₂ improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.     

Dependency of fracture toughness of plasma sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on lamellar structure

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of critical strain energy release rate G _Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al₂O₃ coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G _Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al₂O₃ coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G _Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G _Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding.     

The Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powder Morphology on the Properties of Cu-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites Designed for Functionally Graded Materials (FGM)

In order to meet the requirements of an increased efficiency applying to modern devices and in more general terms science and technology, it is necessary to develop new materials. Combining various types of materials (such as metals and ceramics) and developing composite materials seem to be suitable solutions. One of the most interesting materials includes Cu-Al₂O₃ composite and gradient materials (FGMs). Due to their potential properties, copper-alumina composites could be used in aerospace industry as rocket thrusters and components in aircraft engines. The main challenge posed by copper matrix composites reinforced by aluminum oxide particles is obtaining the uniform structure with no residual porosity (existing within the area of the ceramic phase). In the present paper, Cu-Al₂O₃ composites (also in a gradient form) with 1, 3, and 5 vol.% of aluminum oxide were fabricated by the hot pressing and spark plasma sintering methods. Two forms of aluminum oxide (αAl₂O₃ powder and electrocorundum) were used as a reinforcement. Microstructural investigations revealed that near fully dense materials with low porosity and a clear interface between the metal matrix and ceramics were obtained in the case of the SPS method. In this paper, the properties (mechanical, thermal, and tribological) of composite materials were also collected and compared. Technological tests were preceded by finite element method analyses of thermal stresses generated in the gradient structure, and additionally, the role of porosity in the formation process of composite properties was modeled. Based on the said modeling, technological conditions for obtaining FGMs were proposed.