Volume 106, Issue 11

We have evaluated the orientation dependence of plasma etch rates for single crystal silicon using SF 6 /Ar and CF 4 /Ar/O 2 chemical gases in an inductively coupled plasma dry etcher. Etch rates for different crystal orientations having various surface conditions were measured. The results indicate that the <111> crystal lattice orientation is etched at a lower rate than the <100> orientation. The dependence of the silicon etch rate on the crystal orientation is derived from differences in the local neighborhood of the exposed surface atoms, such as number of neighbors, dangling bond, and surface bond. Finishing the silicon parts with mirror-finish polishing results in surfaces with a very thin subsurface damage layer that exhibits a lower plasma etch rate. This compares to the much thicker damage layer caused by abrasive machining of the surface during grinding steps, and to the portion of this that remains after polishing.

Mechanical properties of metallic materials are often optimized by a specific heat treatment to adjust a required grain size. Thereby solute atoms, impurities or precipitates play an important role due to their retarding forces on the grain boundary movement. However, they not only stabilize small grain sizes during a heat treatment, it is also suggested that they introduce abnormal grain growth whereby for a small amount of grains the grain size increases tenfold and more. On the one hand abnormal grain growth impedes the adjustment of a required grain size; on the other hand it can be used to introduce bimodal grain structures which are known to combine opposing mechanical properties such as a high toughness and a high ductility. Therefore, the mechanisms of abnormal grain growth are investigated by varying the content of additives during the deposition process. Particle pinning is suggested as the mechanism responsible for abnormal grain growth at least in the initial stage, while a second phase may introduce extraordinary cubic grains growing abnormally in the late stage.

The present study focuses on the effect of alloying elements, namely, strontium, titanium, zirconium, scandium and silver, individually or in combination, on the performance of a newly developed Al-2 %Cu-based alloy. A total of thirteen alloy compositions were used in the study. Castings were prepared employing the low pressure die casting technique. The microstructures of selected samples were examined using optical microscopy and electron probe microanalysis. Hardness measurements were also carried out on these alloys using a Brinell hardness tester. The results showed that adding Ti in the amount of 0.15 wt.% in the form of Al-5 %Ti-1 %B master alloy is sufficient to refine the grains in the cast structure in the presence of 200 ppm Sr (0.02 wt.%). Addition of Zr and Sc did not contribute further to the grain refining effect. The main role of addition of these two elements appeared in the formation of complex compounds with Al and Ti. The results obtained from the low pressure die casting samples were compared with those produced using traditional gravity die casting. No heat treatment was applied.

The effects of Sn and Ca on the microstructure and mechanical properties of Mg–Zn–Y alloys have been investigated. The results showed that Sn had an obvious effect on the microstructure, but its effect was smaller than that of Ca. The grain size first decreased as the Sn/Ca content increased and then slightly increased when the Sn/Ca content exceeded a critical value. These microstructural changes are related to the hindering effect on grain growth and constitutional supercooling resulting from the segregated Sn/Ca atoms at the advancing solid/liquid fronts. The hardness of the alloys continuously increased as the Sn/Ca content increased because of the improved solid solution strengthening and the increased concentration of Sn- or Ca-rich intermetallic compounds. The tensile properties of the alloys increased as the Sn/Ca content increased below the critical value and then decreased when the Sn/Ca content exceeded the critical value. The increased tensile properties were attributed to the improved grain bonding strength in addition to the grain refinement strengthening. The subsequent decrease in these properties was mainly ascribed to the impaired grain bonding strength and grain coarsening. Correspondingly, the fracture regime changed from intergranular to transgranular modes, and then back to the intergranular mode.

Hollow sphere reinforced aluminum matrix composites – so-called syntactic foams – were produced by means of low pressure infiltration. Commercial AlSi12 aluminum alloy was applied as the matrix material. The overall volume fraction of the reinforcing iron (GM) and ceramic (GC) hollow spheres was maintained at ∼64 vol.%, but their ratio was varied in 20 vol.% steps between 100 %GM-0 %GC and 0 %GM-100 %GC reinforcement composition. The wear properties of the produced specimens were investigated using the pin-on-disk method in dry and lubricated conditions. In the case of dry sliding conditions, the coefficient of friction increased with the ceramic reinforcement content. The pure iron reinforcement showed the lowest coefficient of friction and also the lowest wear rate. From the point of view of the worn surface, the full GC reinforcement was found to be the most promising with relatively smooth surfaces and low amount of wear debris. Based on the discussed results, aluminum matrix syntactic foams would be promising materials for moderately loaded sliding parts.

In the present study, functionally graded aluminium/zirconia metal matrix composite was fabricated using a liquid metallurgy route followed by centrifugal casting process and a hollow cylindrical component of dimensions 150 × 150 × 16 mm was obtained. Microstructural characteristics and mechanical properties along the radial direction were analysed to check the graded properties throughout the thickness of the casting. The tribological behaviour of the functionally graded composite was studied using a three body abrasion tester by varying the applied loads in the range of 33 – 80 N at two different rotating speeds of 100 and 200 rpm. The microstructural examination results showed that the outer surface of the composite has a particle rich zone due to centrifugal force as well as the density difference between alloy and reinforcement. Hardness was found to be higher at the outer surface and tensile strength was higher at the upper section of the composite due to the higher segregation of reinforcement particles. The abrasive wear test results revealed that the wear rate increases as applied load increases and decreases as rotating speed increases. The wear resistance was found to be maximum at the outer periphery compared to other surfaces under all conditions. Scanning electron microscopy analysis of worn surfaces revealed fine and shallow grooves on the external zone ensuring better wear resistance.

In this paper, two titanium–titanium boride composites aiming at 20 % and 40 % (by volume) of titanium boride reinforcement were processed through spark plasma sintering. The mechanical properties such as fracture toughness, indentation creep, contact stiffness, indentation hardness and Young's modulus of the processed composites were evaluated by means of nano-indentation. The Young's modulus, indentation hardness and contact stiffness increase with the increase in volume fraction of titanium boride, while the fracture toughness and indentation creep decrease. The titanium composite with 38.5 % (by volume) titanium boride showed improved mechanical properties compared to the composite with 24 % (by volume) titanium boride reinforcement. The morphological influence of TiB reinforcement on the mechanical properties was also discussed.

This paper reports on the effect of phase formation on the dielectric behaviour of nanostructured calcium copper titanate (CaCu 3 Ti 4 O 12 ) thin film and powder prepared by using the sol–gel method. X-ray diffraction peaks were indexed as a cubic phase belonging to the Im3 space group for both film and powder. The monophase with a highly crystalline nature was obtained at lower temperature for thin film compared with powder. Micrographs of CaCu 3 Ti 4 O 12 thin film and nanopowder confirmed the presence of nano-dimensional particles. The functional groups and optical properties of both film and powder were characterized by using the Fourier transform infrared spectrum and the photoluminescence spectrum. The CaCu 3 Ti 4 O 12 thin film showed higher dielectric permittivity than powder, which strongly suggests that nanostructured thin film is a more efficient material than powder for microelectronic devices.

Potato starch-based carbon particles were prepared by low-temperature carbonization in methyl silicone oil at 220 °C for 15 h and subsequent carbonization at 600 °C for 1 hour under nitrogen. The as-prepared carbon particles were molded, and then heat-treated. The results confirmed that carbon particles could keep the original shape of the potato starch and exhibit high monodispersity with smooth surfaces. The size distribution of carbon particles ranged from 10 to 100 μm and the blocks molded from the carbon particles exhibited relatively superior mechanical strength with a flexural strength of 85.7 MPa. Additionally, it was found that the low-temperature carbonization in oil is crucial in the preparation of this kind of carbon particle.

Sc-doped AlN films with (100) orientation have high surface acoustic wave velocity and excellent optical properties. Sc-doped AlN thin films based on flexible substrate were prepared by RF reactive magnetron sputtering at pressure levels ranging from 0.3 Pa to 1.5 Pa. The sputtering rate, crystal quality, and electric properties of the Sc-doped AlN films were investigated. The results show that pressure greatly influences the preparation of Sc-doped AlN thin films. Under low pressure, (002) oriented Sc-doped AlN film was prepared; with increasing pressure, the fraction of the (100) plane increased. Furthermore, the electrical properties were closely related to the crystal quality, achieving a maximum resistivity of 3.5 × 10 12 Ω · cm, a minimum leakage current of 0.65 × 10 −8 A, and a maximum piezoelectric response of 5.2 pC · N −1 .

Pure VB 2 powders with a particle size less than 300 nm were synthesized by a novel borothermal reduction of NH 4 VO 3 in Ar atmosphere at 900 – 1 000 °C with or without the presence of NaCl/KCl. Appropriate amounts of NaCl/KCl addition not only accelerated the reaction rate but also inhibited the grain growth of VB 2 particles.