Relationship between thermal and sliding wear behavior of Al6061/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metal matrix composites

Al6061 alloy and Al6061/Al₂O₃ metal matrix composites (MMCs) were fabricated by stir casting. The MMCs were prepared by addition of 5, 10 and 15 wt% Al₂O₃ particulates and the size of particulates was taken as 16 μm. The effect of Al₂O₃ particulate content, thermal properties and stir casting parameters on the dry sliding wear resistance of MMCs were investigated under 50–350 N loads. The dry sliding wear tests were performed to investigate the wear behavior of MMCs against a steel counterface (DIN 5401) in a block-on-ring apparatus. The wear tests were carried out in an incremental manner, i.e., 300 m per increment and 3,000 m in total. It was observed that, the increase in Al₂O₃ vol% decreased both thermal conductivity and friction coefficient and hence increased the transition load and transition temperature for mild to severe wear during dry sliding wear test.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on sintering behavior and microwave dielectric properties of ZnTa<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics

ZnTa₂O₆ ceramics with various amount of Al₂O₃ additive were synthesized by a conventional mixed-oxide route. The grain growth of ZnTa₂O₆ ceramics was accelerated with Al₂O₃ additive. However, excessive addition (>1.0 wt%) of Al₂O₃ leaded to abnormal grain growth. With Al₂O₃ addition, the Al₂O₃ additive did not solubilized into ZnTa₂O₆ structure but resulted in forming the second phase. The Al₂O₃ addition resulted in the lower sintering temperature of ZnTa₂O₆ ceramics and improved microwave dielectric properties. The dielectric constant (ε_r) of the samples did not change much and ranged from 32.41 to 34.33 with different amount of Al₂O₃ addition. The optimized quality factor (Q × f) was higher than 70,000 GHz as a result of the denser ceramics. The temperature coefficient of resonant frequency (τ _f ) of the doped ZnTa₂O₆ ceramics could be optimized to near-zero.     

Tribological performance of nano-Al2O3 reinforced polyamide 6 composites

Nano-Al₂O₃ and micron Al₂O₃ reinforced polyamide 6 composites (PA6/Al₂O₃) were prepared via in situ polymerization. The effects of nano-particle content and wear condition on the tribological properties of the composites were measured using a MM-200 block on ring wear tester. The worn surface was investigated with a scanning electronic microscope and a metallurgical microscope and the wear mechanism was proposed. It was found that nano-Al₂O₃ improved the wear resistance of monomer casting polyamide 6 with the optimal content of nano-Al₂O₃ being approximately 3 wt.%. The wear rate of PA6/Al₂O₃ nano-composite was lower than that of the composite with micron Al₂O₃ and increased slowly while its friction coefficient decreased gradually with load. However, the friction coefficient of the composite was somewhat larger than that of monomer casting polyamide 6 and lower than that of the composite with micron Al₂O₃.     

Analysis of morphology and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> layers

The paper presents the characterization of obtaining Al₂O₃ oxide layers on AlMg₂ aluminum alloy as a result of hard anodizing by the electrolytic method in a three-component electrolyte. The Al₂O₃ layers obtained on the AlMg₂ alloy in the three-component SBS electrolyte were subjected to detailed microstructural investigations (by means of a scanning electron microscope). By using X-ray diffraction, the phase compositions of obtained oxide layers were examined. It was found that the Al₂O₃ oxide layers obtained via hard anodizing in a three-component electrolyte are amorphous. The chemical composition of the Al₂O₃ layers is presented and compared with the results of stechiometric calculations for the Al₂O₃ layer. Surface morphologies of the obtained oxide layers are characterized and discussed in nano- and microscopic scales. The surface morphologies of the layers obtained have a significant influence on their properties, including their susceptibility to further modification (e.g., to incorporation of graphite), their wear resistance, and the capacity for sorption of lubricants.     

Wear and mechanical properties of epoxy/SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> composites

Preparation of epoxy/SiO₂-TiO₂ composites is investigated in this paper. The products are characterized by FT-IR spectroscopy. Results of FT-IR spectroscopy and atom force microscope (AFM) demonstrated that epoxy chains have been covalently bonded to the surface of the SiO₂-TiO₂ particles. The particles sized of SiO₂-TiO₂ are about 20–50 nm, which characterized by AFM. The properties of composites such as impact strength, flexural strength, tensile strength and ring-on-block wear are also investigated. Dry sliding wear tests showed that the SiO₂-TiO₂ particles could improve the wear resistance of the epoxy matrix even though the content of the SiO₂-TiO₂ particles was at a relatively low level (1.95–2.65 wt%). This makes it possible to develop novel type of epoxy-based materials with improved wear resistance for various applications. The worn surface was observed by scanning electron microscopy (SEM), and mechanisms for the improvement are discussed in this paper     

Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO<Subscript>2</Subscript>, MgO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript>

Biologically derived hydroxyapatite from calcinated (at 850 °C) bovine bones (BHA) was doped with 5 wt% and 10 wt% of SiO₂, MgO, Al₂O₃ and ZrO₂ (stabilized with 8% Y₂O₃). The aim was to improve the sintering ability and the mechanical properties (compression strength and hardness) of the resultant BHA-composites. Cylindrical samples were sintered at several temperatures between 1,000 and 1,300 °C for 4 h in air. The experimental results showed that sintering generally occurs at 1,200 °C. The BHA–MgO composites showed the best sintering performance. In the BHA–SiO₂ composites, extended formation of glassy phase occurred at 1,300 °C, resulting in structural degradation of the resultant samples. No sound reinforcement was achieved in the case of doping with Al₂O₃ and zirconia probably due to the big gap between the optimum sintering temperatures of BHA and these two oxides.     

Tribological properties of MoS<Subscript>2</Subscript> and carbon fiber reinforced polyimide composites

The tribological properties of carbon fiber reinforced polyimide (PI) composites with different MoS₂ containing sliding against GCr15 steel were comparatively evaluated on an M-2000 model ring-on-block test rig. The wear mechanisms were also comparatively discussed, based on scanning electron microscopic examination of the worn surface of the PI composites and the transfer film formed on the counterpart. It was found that small incorporation of MoS₂ was harmful to the improvement of friction and wear behaviors of carbon fiber reinforced PI composites. However, it was found that the increasing filler of MoS₂ significantly improved the wear resistance and decreased the friction coefficient of carbon fiber reinforced PI composites. It was also found that the tribological properties of MoS₂ and short carbon fiber reinforced PI composites were closely related with the sliding condition such as sliding rate and applied load.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Microstructure investigations and mechanical properties of an Al‐Al2O3 MMC produced by semi‐solid solidification

The influence of the semi‐solid solidification production parameters (shear rate and agitation time) and the concentration of reinforcing particles on the microstructure formation and mechanical properties of a 520 aluminum alloy reinforced with Al₂O₃ particles was investigated. Depending on the content of reinforcing particles and the stirring conditions different rosette structures were formed. The type of wear mechanism (delamination or adhesion) depends on the size of the rosettes and the distribution of Al₂O₃ reinforcements. Best mechanical properties were obtained for metal matrix composites reinforced with 12 wt% of Al₂O₃ stirred at a shear rate of 2100 s^–1 for 1800 s. These samples showed tensile strength and yield stress similar to the commercial A520 alloy. The hardness and wear resistance were improved by the addition of Al₂O₃ particles, meanwhile the elongation to fracture was reduced.     

Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

Microstructure evolution and mechanical properties of Al<Subscript>2</Subscript>O<Subscript>3sf</Subscript>/AZ91D magnesium matrix composites fabricated by squeeze casting

The squeeze casting process was used to fabricate Al₂O_3sf/AZ91D magnesium matrix composites before thixoforging. The microstructural evolution process in Al₂O_3sf/AZ91D was investigated during partial remelting. Tensile mechanical properties of thixoforged automotive component were determined and compared with those of squeeze casting formed composites. The results show that the microstructural evolution during partial remelting exhibited four stages: the formation of liquid, structural fragmentation, the spheroidization of solid particles, and final coarsening. As the holding time increases, the size of solid particles decreases initially and then increases. However, the size of solid particles decreases monotonously as the temperature increases. Increasing holding time or temperature promotes the degree of spheroidization. It is also shown that the cylindrical feedstock of the Al₂O_3sf/AZ91D composites can be thixoforged in one step into intricate shapes in the semi-solid state. The tensile tests indicate that the yield strength and ultimate tensile strength for Al₂O_3sf/AZ91D thixoforged from starting material fabricated by squeeze casting and partial remelting are better than those of Al₂O_3sf/AZ91D fabricated by squeeze casting. This research confirms that thixoforging is a practical method for the near net shape forming of magnesium matrix composites.     

Low-temperature sintering and microwave dielectric properties of Mg<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript>-LMZBS composites

The Mg₃B₂O₆ ceramics with lithium magnesium zinc borosilicate (LMZBS) glass were prepared at a lower sintering temperature. The effects of the glass addition on the densification, phase development, microstructure and microwave dielectric properties of the Mg₃B₂O₆ ceramics were investigated. The addition of LMZBS glass improved the densification and lowered the sintering temperature of Mg₃B₂O₆ ceramics from 1,300 to 950 °C. X-ray diffraction patterns showed that Mg₃B₂O₆ transformed into Mg₂B₂O₅ and a new phase, Li₂ZnSiO₄, crystallized from the glass phase. Because of the high dielectric performance of these phases, Mg₃B₂O₆ mixed with 55 wt% LMZBS sintered at 950 °C for 3 h had ε_r = 6.8, Q × f = 50,000 GHz, and τ_f = ?64 ppm/°C at 7.28 GHz. The chemical compatibility of ceramic-glass composites with Ag was also investigated for LTCC.     

Modification of the structural,textural, and mechanical properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al composite on the addition of an Al-SBA-15-type mesoporous phase

The formation of the pore structure of metal–ceramic materials based on Al₂O₃/Al composites has been studied in detail by modifying them via the incorporation of SBA-15-type mesoporous materials. The composition, pore structure, pore size, and morphology of the composite particles have been studied using nitrogen adsorption/desorption isotherms, scanning electron microscopy, and X-ray diffraction. We have produced Al-SBA-15/Al₂O₃/Al monoliths that combine properties of both adsorbents and permeable materials, but the incorporation of Al-SBA-15 into an Al₂O₃/Al matrix reduces the permeability and mechanical strength of the composites.     

Microstructure and property characterisation of 3-3 Al(Mg)/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interpenetrating composites produced by a pressureless infiltration technique

3-3 Interpenetrating composites, consisting of 3-dimensionally interpenetrating matrices of two different phases, are interesting materials with potentially superior properties when compared with traditional metal matrix composites. In the present research, gel-cast Al₂O₃ foams with open porosity in the form of spherical cells connected by circular windows were pressurelessly infiltrated using an Al-8 wt% Mg alloy. Electron backscatter diffraction (EBSD) analysis revealed that the alloy had a large grain size with single grains generally inhabiting multiple cells. The flexural strength of the composites, tested using 3-point bending, was ~350 MPa, rather high when compared to other Al-alloy-based Al₂O₃ composites. The strength increased with both decreasing foam density and cell size. The reasons for the high strength are good metal–ceramic interfacial bonding, crack bridging by plastic deformation of the metal phase and crack deflection.     

Effects of atomic oxygen exposure on the tribological performance of ZrO2-reinforced polyimide nanocomposites for low earth orbit space applications

The effects of atomic oxygen exposure on pure polyimide and nano-ZrO₂ reinforced polyimide composites were investigated in a ground-based simulation facility. The experimental results indicated that the surface structure of both pure polyimide and ZrO₂/polyimide composites were destroyed by atomic oxygen attack, but the addition of nano-ZrO₂ particles in polyimide could obviously decrease the mass loss, which showed that ZrO₂ could enhance the atomic oxygen resistance. The results of ZrO₂/polyimide composites before and after atomic oxygen exposure showed that atomic oxygen irradiation aggravated the friction and wear of the ZrO₂/polyimide composites. The wear mechanism was mainly abrasive particles wear arising from the ZrO₂-rich layer on the surface of composites. The ZrO₂/polyimide composites with 1 wt% nano-ZrO₂ owns the lowest varying rate of the friction coefficient and wear rate before and after atomic oxygen exposure, which showed stable friction and wear properties and was expected to become a kind of potential tribological materials for practical spacecraft designation.     

Einfluß des ZrO2-Zusatzes auf mechanische Eigenschaften und den ungeschmierten Gleitverschleiß von TiB2-ZrO2-Mischkeramiken

Effect of ZrO₂ addition on mechanical and tribological properties of TiB₂-ZrO₂-composites TiB₂-ZrO₂-composites were produced by hot-pressing in argon. The mechanical and tribological properties of the composites were investigated in relation to the amount and the type of the ZrO₂ added. Addition of ZrO₂ reduced the sintering temperature of the TiB₂-ZrO₂-composites and led to growth inhibition of the TiB₂ phase. ZrO₂ also increased the bending strength and fracture toughness of the composites. Tribological tests were carried out using laboratory tribometers under unlubricated oscillating sliding contact against Al₂O₃-, Si₃N₄- and SiC-counterbodies. Abrasive wear of the ceramics against 80 mesh SiC grits was studied by using an abrasive wheel test. During sliding contact the composites showed high wear resistance but caused extensive wear of the counterbodies and high friction coefficients compaired to self-mated monolithic Al₂O₃- and ZrO₂-ceramics. Volumetric wear loss of the TiB₂-ZrO₂-composites due to abasive SiC grits was by a factor up to 30 lower than that of a commercial monolithic Al₂O₃. Wear resistance of the composites increased with decreasing amount of ZrO₂ and was higher for the composites with addition of partially stabilized tetragonal ZrO₂ than for those with addition of monoclinic ZrO₂.     

Microstructure and mechanical properties of 3Y-TZP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites fabricated by liquid phase sintering

3Y-TZP/Al₂O₃ composites were pressureless sintered with the addition of TiO₂-MnO₂ and CaO-Al₂O₃-SiO₂ glass. The densification, microstructure and mechanical properties of the composites were investigated. It was found that the composites could be densified at a temperature as low as 1400^C by liquid phase sintering. The majority of the grain sizes for both Al₂O₃ and ZrO₂ were below 1 m because of the lower sintering temperature. A bending strength of 934 ± 28 MPa and fracture toughness of 7.82 ± 0.19 MPam^1/2 were obtained for 3Y-TZP/Al₂O₃ (20 vol%) composite. Transformation toughening is considered the responsible toughening mechanism.     

Accelerated hydrogen desorption from MgH<Subscript>2</Subscript> by high-energy ball-milling with Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The effect of the addition of Al₂O₃ (50 wt%) on the dehydrogenation of MgH₂ was investigated. Composites of the oxide and the hydride were prepared in two ways: by milling the components separately or by co-milling them together in a gear-driven planetary ball mill for 10 min. The co-milled composite (MgH₂–Al₂O₃) released approximately 90% of the maximum hydrogen storage capacity within 30 min under a pressure of 0.003 MPa at 250 °C. In contrast, the composite of the separately milled components did not release hydrogen even after 2 h under the same conditions. BET measurement with nitrogen gas showed a negligible difference in the specific surface areas between the co-milled and separately milled composites. However, the saturation amount of hydrogen gas for the co-milled composite was 30% larger than that of the mixture of separately milled hydride and oxide. The activation energy for hydrogen desorption from the co-milled composite, calculated on the basis of the surface-controlled model was 80 kJ mol⁻¹, a value that is 50 kJ mol⁻¹ lower than that of mixture of the separately milled MgH₂ and Al₂O₃.     

Synthesis,crystal structure and luminescence in Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript>

Bluish green emitting phosphor, Ca₃Al₂O₆:Ce³⁺, is prepared by low-temperature combustion method. X-ray diffraction, photoluminescence, scanning electron microscopy techniques are used to characterize the synthesized phosphor. The most efficient bluish green (483 nm) emission is observed under the excitation by near UV light. The emission characteristics are credited to 5d → 4f type transitions in Ce³⁺. The luminescence properties of Eu²⁺ are predicted for the first time from those of Ce³⁺. Also, photoluminescence of Eu³⁺ is studied in the same host. The emission spectrum of Ca₃Al₂O₆:Eu³⁺ shows the peak at 592 (orange) and 614 nm (red) wavelengths. Ca₃Al₂O₆:Ce³⁺phosphor can be a potential blue phosphor for field emission display, solid-state lighting and LED.     

Fabrication and mechanical properties of WC particulate reinforced Cu<Subscript>47</Subscript>Ti<Subscript>33</Subscript>Zr<Subscript>11</Subscript>Ni<Subscript>6</Subscript>Sn<Subscript>2</Subscript>Si<Subscript>1</Subscript> bulk metallic glass matrix composites

The mechanical behavior of the WC particulate (WC_p) reinforced Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ bulk metallic glass (BMG) matrix composites has been examined. The mechanical properties are improved with increasing WC_p content up to 20 wt%. The ultimate compression strength and plastic strain of the composite containing 20 wt% WC_p are 2.4 GPa and 2.4%, while those of the monolithic BMG are 1.6 GPa and ∼0%, respectively. The multiple shear band formation and crack deflections through WC particles have been identified as the main mechanism for the improved toughness.