Surface Review and Letters

Ti-48%Al-2%Cr-2%Nb-(0, 0.5, 1) at.%W alloys were synthesized via the powder metallurgical route, and cyclically oxidized at 800, 900, or 1000°C in air for up to 100 h in order to find the effects of W on their oxidation characteristics. At 800°C, they oxidized relatively slowly, and the scales were thin and adherent. At 900°C, the scales began to spall locally. At 1000°C, they spalled repetitively during oxidation. Cr, Nb, and W improved the cyclic oxidation resistance of TiAl alloys. The oxides formed were TiO₂ and Al₂O₃, which contained the alloying elements of Cr, Nb, and W. Nitrides formed were TiN and Ti₂AlN.

The morphological control of crystalline cerium oxide particles was realized by homogeneous precipitation process followed by calcination in air at 400°C. The effects of room temperature pre-aging time on the morphologies of final products were investigated. Monodispersed rod-like cerium carbonate precursor was produced at 70°C for 2 h using the solution without pre-aging treatment. In contrast, monodispersed spherical precursor and plate-like precursor were obtained under the same conditions after pre-aging the solution at 25°C for 72 and 144 h, respectively. Ceria particles with similar morphologies and particle size to those of carbonate precursor could be obtained after calcination in air at 400°C. The monodispersed spherical, rod-like, and plate-like cerium oxide particles were successfully synthesized.

Titanium alloys are superior of biocompatibility, mechanical properties and chemical stability. The biocompatibility of Ti alloy is related to the surface effect between human tissue and implant. Therefore, the purpose of this study is to investigate the bioactivity of Ti alloy by alkali and acid chemical surface treatment; and the biocompatibility of Ti alloy was evaluated by in vitro test. Higher bone-bonding ability and bioactivity of the substrate were obtained by the formation of apatite layers on the Ti alloy in simulated body fluid. The microstructures of apatite layer were investigated by scanning electron microscope (SEM) and the formed phases were analyzed with X-ray diffraction (XRD).

Needle-shaped PbTiO₃ (PT) template particles were successfully synthesized using two different types of approaches, molten salt synthesis (MSS) and thermal synthesis method. In eutectic NaCl–KCl molten salt system, homogeneous and nonagglomerated needle-shaped PT particles were produced from PbO and needle-like TiO₂ compound heated at 800–850°C for 2 h. The effect of heating temperature on the morphology of formed powders was studied in order to identify the optimum processing parameters. The template formation mechanism is discussed. Using thermal synthesis method, comparable needle-shaped PT crystalline particles were also obtained in one step. Plate-shaped PT powders were derived via flux synthesis. The effect of additives such as MnCl₂ and LiF on the morphology of derived PT was studied. Adding a certain amount of MnCl₂ led to the formation of well-defined plate-shaped PT particles. The structure and morphology of synthesized powders were characterized by means of X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM).

Chitosan–gold nanocomposites were fabricated via a seed-mediated goldshell growth over chitosan–gold nanocomplex and examined as a potential biomedical agent for drug delivery. Chitosan–gold nanocomplex was formed by the electrostatic interaction between of cationic chitosan nanoparticles (ca. 50 nm) and anionic gold colloids (ca. 1–3 nm) and the subsequent reduction of gold salts was conducted to form the gold clusters over the chitosan nanoparticles in the presence of reducing ascorbic acid. The resultant nanocomposites (or nanoshells) exhibited the absorption peak around 580 nm that was clearly red-shifted by ca. 60 nm as compared to gold nanoparticles. Surface morphology and plasmonic properties of chitosan–gold nanocomposites were characterized by transmission electron microscopy, field emission scanning electron microscopy (including EDX), ultraviolet–visible spectroscopy, and zeta-potential meter.

ZnO with 2D flower-like and 1D rod shape were obtained from simple and rapid hydrolysis of Zn nanopowder. The Zn nanopowders were incorporated into distilled water with acetic acid and then the solution was stirred at 60°C for 8 h. The nanoflower-like and rod shape were formed without any surfactant. It seems that the acetic acid played a role of controlling PH and etching the oxide layer on the surface of metal nanopowders to enhance rapid reaction with distilled water. X-ray diffraction patterns for all samples exhibited that the resultant precipitates were completely transformed to ZnO powder. It is clearly observed that the morphological changes of ZnO with reaction time in aqueous solution follows chestnut bur → flower → tetrahedron → rod sequences during the hydrolysis reaction.

MgB₂/carbon fibers have been synthesized by the combination of RF-sputtering of B and thermal evaporation of Mg, followed by co-evaporation. First, boron layer was deposited by RF-sputtering on the carbon fiber with average diameter of 7.1 μm. Later this coated layer of B was reacted with Mg vapor to transform into MgB₂. Since the MgB₂ reaction proceed with Mg diffusion into the boron layer, Mg vapor pressure and the diffusion time had to be controlled precisely to secure the complete reaction. Also the deposition rate of each element was controlled separately to obtain stoichiometric MgB₂, since Mg was evaporated by thermal heating and B by sputtering system. The sintered B target was magnetron sputtered at the RF-power of ~200 W, which corresponded to the deposition rate of ~3.6 Å/s. With the deposition rate of B fixed, the vapor pressure of Mg was controlled by varying the temperature of tungsten boat with heating element control unit between 100 and 900°C. The MgB₂ layers with the thickness of 200–950 nm could be obtained and occasionally MgO appeared as a second phase. Superconducting transition temperatures were measured around ~38 K depending on the deposition condition.

In order to optimize the condition for the semi-solid Al–Zn–Mg aluminium alloy fabricated by cooling plate method, the Taguchi design was used. The cooling plate method effectively separating the grains formed from the mold wall can be used to form a semi-solid material by flowing molten metal over an inclined Cu plate and casting in a mold for the near-net shape component. In Taguchi's design method, the higher signal vs noise (S/N) ratio the better. Therefore, the manufacturing conditions were arranged as a table of orthogonal arrays (L₉(3⁴)), and the influence of two factors, pouring temperature and cooling plate angle, was examined. From the observed microstructures, the grain size and aspect ratio were measured by image analyzer. The results indicated that the pouring temperature exerts the main effect on the spherical microstructures since the S/N ratio, which is the sensibility of the surrounding environment, was the highest. The optimum condition for the Al–Zn–Mg alloy was a cooling plate angle of 40° and a pouring temperature of 680°C. The grain size and aspect ratio were 70 μm and 1.3, respectively.

In this paper, alumina film was deposited using supersonic micronozzle in nano-particle deposition System (NPDS). Powder deposition at room temperature is important in the field of film deposition since high processing temperature can be a serious limitation for the deposition on flexible substrate. Previously, many studies have been reported on particle deposition, such as aerosol deposition method (ADM) or cold spray method. However, these deposition methods cannot be applied to various types of powders. Recently, NPDS using aluminum nozzle was designed to resolve these problems but it cannot deposit precise patterns less than 1 mm. In this study, alumina particles were deposited using Silicon-based micronozzle in NPDS. Three-dimensional silicon micronozzle was fabricated using semiconductor processing method, specifically deep reactive ion etching (DRIE) method. The silicon micronozzle fabricated by Bosch process is advantageous over the conventionally used nozzle, since the hardness of silicon is higher than that of aluminum and the lifetime can be increased. In this study, alumina nano-particles were accelerated to supersonic level at the neck of micronozzle and deposited on the substrate in a low vacuum condition. The film characteristics were evaluated using field-emission scanning electronic microscope (FE-SEM) and alpha step to measure its thickness of the deposited layer. The deposition result showed that alumina powders were successfully deposited using the fabricated micronozzle by means of NPDS.

Titanium carbide (TiC) has a low density, relatively high thermal and electrical conductivity. TiC is also very stable, with a melting temperature of 3100°C and does not undergo phase transformations. These properties have attraction to apply cutting tool extensively. Recently, in order to improve the properties of cermet tool materials, most researches have been carried out on the development of new binder. Ni₃Al intermetallic is an interesting material because it has a great advantage of application for some structural materials, especially at the high temperatures. Ni₃Al has low density, high strength and good corrosion resistance properties, which recommend them for the application of high temperature structural material. In this study, we had investigated the effects of mechanical alloying time and annealing temperature on the Ni₃Al–Al₂O₃/TiC phase formation. Raw powders to fabricate Ni₃Al–Al₂O₃/TiC composite powders were Ni, Al, TiH₂ and activated carbon. These powders were ball-milled in planetary milling jar in argon atmosphere. Ball-milled powders were annealed up to 1200°C in Ar + 4%H₂ atmosphere. The mean particle size of 300 nm was obtained after 10 h ball-milling. Ni₃Al–Al₂O₃/TiC composite powders were formed by annealing for 1 h over 1100°C.

SiC-mixed Sn–58Bi composite solder bumps were successfully fabricated via an electroplating process. For the composite solder bump fabrication, ultrasonically dispersed SiC nanoparticles were added to the plating solutions. DSC analysis indicated that the melting temperature of SiC-mixed Sn–58Bi solders was the same as that of the non-mixed Sn–58Bi. Shear strengths of Sn–58Bi+SiC solder bumps were 6% higher than that of non-mixed solder bumps. The thicknesses of intermetallic compound were almost the same for both Sn–58Bi and Sn–58Bi+SiC samples. The Sn–58Bi+SiC composite solder bumps had finer lamellar structures than non-mixed Sn–58Bi. From the fracture surface analysis, fracture occurred at solder bump matrix, not at joint interface. Therefore, the addition of the SiC nanoparticles in the Sn–58Bi solders decreased the grain sizes, which increased the shear strengths.

A micron-sized WC-CrCo powder was coated onto an IN718 substrate using high-velocity oxy-fuel (HVOF) thermal spraying. To further improve the surface properties, the HVOF coating was heat-treated by a CO₂ laser. The surface properties of both the coating and the laser-heated coating were then compared. The HVOF optimal coating process (OCP) for a coating with the highest surface hardness was determined with the Taguchi program. The friction and wear behaviors of the coating, an electrolytic hard chrome (EHC)-plated coating and IN718, were comparatively investigated via a reciprocating sliding wear test at both 25 and 450°C. The friction coefficient (FC) for all three samples decreased when the sliding surface temperature increased from 25 to 450°C. The FC of the coating decreased with increasing surface temperature: 0.33 ± 0.02 at 25°C to 0.26 ± 0.02 at 450°C; the coating had the lowest FC among the three samples. At both temperatures, the coating wear depth (WD) was smaller than those of the EHC sample and IN718. At room temperature, WC-CrCo and the EHC coatings had good wear resistance and had only a shallow WD. IN718, however, had poor wear resistance with 50 μm deep grooves created from fretting corrosion that arose during the 1500 reciprocating slides (2.5 Hz, 10 min sliding wear test). At 450°C, the coating WDs were much shallower than those for the EHC coating and IN718: 0.5-μm deep grooves compared to 60–70-μm deep grooves. These results proved that the coating provided a protective coating for IN718 and other metal components. With the OCP coating fabricated from the powders on the IN718 surface, the surface hardness increased 316% from 399 Hv to 1260 Hv. Furthermore, by laser heating the coating surface for 0.6 s, the hardness increased 44% from 1260 ±30 Hv to 1820 ±100 Hv, porosity decreased more than five times from 2.2 ± 0.3% to 0.4 ± 0.1%, and the coating thickness decreased 17% from 300 to 250 μm. These results showed that both the WC-CrCo powder coating and the laser-heating improved the surface properties of IN718.

A HfB₂-SiC ceramics were fabricated via a reactive hot pressing using Hf, B₄C, and Si as precursors. The reaction temperature for the reactive hot pressing between 1800 and 1900°C was determined by reaction of the precursor at different temperatures from 900 to 1800°C. The effective size reduction of precursors was investigated by vibration milling, which exhibited a critical role to achieve high densification and uniform microstructure. Also, it has affected the oxidation behavior of HfB₂–SiC in air. Vibration milled sample showed uniform surface of SiO₂ layer, but sample which was fabricated by as-received powder exhibited non-uniform oxidation behavior. Examination of the mechanical properties showed that particle size reduction via vibration also led to improved flexural strength, hardness and fracture toughness.

Niobium powder was fabricated by metallothermic reduction process using K₂NbF₇ as the raw material, KCl and KF as the diluents and Na as the reducing agent. The apparatus for the experiment was designed and built specifically for the present study. Varying properties of niobium powder depending on reaction temperature and excess of reducing agent were analyzed. The niobium particle size increased significantly as the reduction temperature increased from 993 to 1093 K. The particle size was fairly uniform at a given reaction temperature, varying from 0.2 μ m to 50 nm depending on the reaction temperature. The yield of niobium powder increased from 58 to 83% with an increase in reaction temperature. The average particle size of niobium powder was improved from 70 nm to 0.2 μ m with the increase in the amount of Na excess. In addition, the yield rate of Nb powder was 82% in the 5% excess sodium.

In the present study, the characteristics of the Ti powders fabricated by Hydride–Dehydride (HDH) were analyzed in terms of particle shape, size and size distribution. Ti powders were subjected to roll compaction and their microstructure and green densities were evaluated in terms of particle size, powder morphology, roll gap and rolling speed. Effects of blending elements having different powder sizes on densification properties were analyzed. The strip thickness was proportional to the roll gap up to 0.9 mm and the density of titanium strip was decreased with the increase in roll gap. As the roll speed increased, the strip density and thickness were decreased by using -200 mesh Ti powder. However, the effect of rolling speed for -400 mesh Ti powder was not greater than that of -200 mesh powder. The highest density by 93% was achieved by using -400 mesh Ti powder at 0.1 mm roll gap, however edge cracks and alligator cracks were occurred.

Micro-spur gears were fabricated by powder injection–compression molding and micro-two-step gears were produced through stacking and sintering of the green parts. Shrinkage of micro-gears by sintering was larger in teeth and with increase of compression pressure, shrinkage was decreased due to increase of density in green parts. Surface roughness of sintered body was reduced to a few hundreds of nanometers by ultrasonic micro-polishing. The joining of green bodies by stacking and sintering was achieved using the pressurized sintering and insert of feedstock powders between both green bodies.

The Fe–Al–X(Cu, Ni, Ti) composites reinforced with graphite particles were fabricated employing the powder metallurgy process for slide bearing applications. In all samples, graphite particles of 6 wt.% were added. Elemental powders were mixed to specific compositions with a powder lubricant, and then the mixed powders were compacted at 500 MPa. The green compacts were heated to 450°C to remove the lubricant and sintered at 1200°C for 1 h. The sintering was conducted in vacuum. The sintering of binary Fe–Al–graphite system showed low density and growth in dimension. Complex addition of Cu, Ni and Ti improved the sinterability of green compacts due to occurrence of partial liquid phases. Hardness of sintered samples strongly depended on dimensional change during sintering. The samples of high hardness showed better wear resistance properties.

A single pulse of 0.57–1.1 kJ/0.3 g-spherical Ti-powder in size range of 30–70 μm, using a constant 450 μF capacitor, was applied to produce fully porous Ti- and W-compacts by electro-discharge sintering. Microstructure examination and hardness measurement of the porous Ti-compacts revealed that more stable necks formed with increasing electrical input energy up to 1.1 kJ. On the contrary, W-compacts using rectangular W-powders in size range of 5–15 μm are composed of solid region with micropore and porous layer with weak particle bonding indicating heterogeneous structure. These results suggest that the powder morphology plays a crucial role to control the porous microstructure.

Hard materials, in particular tungsten carbide (WC) hard material has been used in many industries as cutting tool and abrasive materials. For improving toughness, binders are added into carbide materials. The effect of Co as binder on the mechanical properties in the nanosized WC was discussed. The hardness and fracture toughness were also investigated using 30 kgf load Vickers indenter. The nanosized WC-8, 10 and 12 vol.% Co composites were successfully fabricated without grain growth by high frequency induction heated sintering (HFIHS), which is the rapid sintering method, due to short sintering time 1140°C. The nanosized WC–Co composites fabricated by HFIHS have better mechanical properties than WC–Co composite fabricated by commercial sintering.

In this study, the effect of the reinforcement on the wear behavior of hypereutectic Al–Si composites was investigated by performing a ball-on-disk test. The specimens were manufactured by hot press after gas atomizing. Al–20Si–5TiC composite exhibited superior wear resistance than other composites used in this study.

ZnO nanosheet and nanorod arrays were selectively synthesized on zinc substrate through the reaction of zinc foil with dilute alkaline solution at 120–180°C for 12–24 h. The microscopic morphology and crystal phase of these products were characterized by scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). The growth mechanism and morphology evolution of the nanostructured ZnO arrays were investigated. Due to their uniform morphologies and large surface areas, the as-prepared ZnO nanosheet and nanorod arrays may be used as photovoltaic films and gas sensors for future applications.

Taking wollastonite as raw material, aluminium sulfate and water glass as coating modifier, a new type of wollastonite coated with nano-aluminium silicate powder-materials was obtained by chemical depositing method. By modificating this powder-materials with silane and filling it in PA6, the PA6 composite materials were obtained. The modificated powder-materials were characterized by SEM, BET, XRD, EDX, and FT-IR. The mechanism of this powder-materials modificated by silane was discussed. It was shown that the surface of wollastonite was coated with nano-aluminium silicate evenly, the average grain size of aluminium silicate was 54 nm, filling 30% of this powder-materials modified by silane in PA6, the mechanical properties and the heat distortion temperature of the PA6 composites filled by it were notably enhanced.