Materials Science Forum Vols. 510-511

Abstract: In this work, silicon coatings were deposited using VPS and APS technologies. Their structure and composition were characterized using XPS, SEM, and XRD. The porosity and surface-roughness of coatings were evaluated. The results showed that the VPS silicon coating possessed lower porosity compared with the APS silicon coating. The APS silicon coating surface was almost all covered by many small particles whose size is about 100 nm. The Si 2p XPS spectra indicated that silicon oxide existed in both coatings. The oxidation may have occurred during the plasma spray process. However, the VPS process could inhibit silicon from being oxidated.

Abstract: The candidate gases for the replacement of SF6 in magnesium melt protection were reviewed from the viewpoint of environmental effects, melt protection properties, and chemical properties. The surface films formed under fluorine bearing gases were analyzed by using XPS. The XPS results revealed that the surface films consist of MgO and MgF2. The fluorine content of the surface film increases with increasing gas concentration and exposure time. The Pilling- Bedworth ratio (PBR) of MgO was 0.73, and the porous magnesium oxide could not cover the whole melt surface. PBR of MgF2 was 1.29, and the surface film which contains MgF2 could cover a lager area of the melt surface. This higher PBR of composite surface film prevented further oxidation and evaporation of magnesium.

Abstract: In this paper, using sodium phosphate and calcium nitrate as reagents, nano-hydroxyapatite (n-HA) was synthesised under 8 conditions. The morphology and crystallinity of these prepared HA were investigated by Transmission Electron Microscope (TEM) and X-ray diffraction pattern (XRD). The Ca/P molar ratio of n-HA was tested by chemical method. In order to find a better way to obtain purer nano-hydroxyapatite, the phases transition and purification of these 8 samples before and after sintering at 1000°C were compared. The results indicated that all these samples showed similar, poorly crystallized apatite structures before sintering. The adding order between calcium and phosphate would affect the crystallinity, structure and Ca/P molar ratio of hydoxyapatite significantly. XRD analysis showed that adding sodium phosphate into calcium nitrate could gain more HA phase after sintering at 1000°C. The pH of reaction system had obvious effects on the structure of hydroxyapatite. Besides the pH of reactants also affected the structure of hydoxyapatite.

Abstract: Microstructure observation and kinetic analysis were conducted on fine Ni-20Cr powder (spherical shape and 5 m in average particle diameter) to understand the sintering mechanism of fine metallic powder during pulsed electric current sintering (PECS). Insulation of the sample during PECS was carried out to investigate the influences of pulsed electric current passing through the sample. Temperature at the sample/die interface was measured as sample temperature. Pulsed electric current did not influence densification. The microstructure observation revealed that the necks between particles had very small curvature radius, which means the neck formation by compressive deformation of particles owing to creep. As results of the kinetic analysis of the densification, the creep rate of fine Ni-20Cr powder was two digits larger than the extrapolated values of the steady-state creep on PECS of coarse Ni-20Cr powder and creep tests reported previously. The larger creep rate of fine Ni-20Cr powder during PECS may be influenced by not only smaller grain size of powder particles but also contribution of the initial creep, which was faster than the steady-state creep.

Abstract: Density, purity, grain, and grain size distribution are the key factors that influence the transparency of an optical ceramics. To achieve high transmittance, efforts should be made to eliminate or minimize scattering or absorption of light. The aim of this paper is to verify the effect of two-step sintering technique for translucent alumina. The density and transmittance of the samples were enhanced by pre-heat treating at 800°C for 50 hours before the high temperature sintering. Abnormal grain growth was successfully suppressed by a two-step sintering technique and a small amount of MgO as additives. It is a very effective technique for designing homogeneous microstructure and high transparency in high purity alumina.

Abstract: Radio frequency interference characteristics of plate-like Fe-Si-Al alloy powders-polymer composites were investigated for effective noise suppression. Broadband reflection loss characteristics were exhibited in the composites below a critical compaction pressure of 1.56 x 108 N/m2, while narrowband characteristics were shown in the composites above the critical pressure. Although the composites at 2.34 x 108 N/m2 had the highest complex permeability and complex permittivity, the loss properties did not show the broadband characteristics. The peculiar broadband characteristics were obtained in the composites with wideband dependency of the imaginary part of permittivity and permeability. These findings led us to conclude that the broadband electromagnetic interference characteristics of the composites are caused by the coupling effects of magnetic and dielectric resonance in the radio frequency range.

Abstract: The rod-shaped bulk composites consisting of Al-10Ni-6Ce and Al-4Fe-0.6Mo-1.1V- 0.3Zr alloy (mixing ratio; 0.7:0.3, 0.5:0.5 and 0.3:0.7) and corresponding monolithic alloys were produced to a full density via powder forging process. The process involved pre-compaction of rapidly solidified alloy powders and subsequent isothermal forging at 673K. The forged Al-10Ni- 6Ce alloy exhibited nano-scaled crystalline particles, such as fcc-Al, Al3Ni, Al4Ce and Al11Ce3 phase, coexisting with an amorphous phase. In the case of the forged Al-4Fe-0.6Mo-1.1V-0.3Zr alloy, an equiaxed grain structure was observed to exist with uniformly distributed nano-scaled Al- Fe based intermetallics. The monolithic Al-10Ni-6Ce alloy had a considerably high maximum compressive strength (MCS) of 1.35 GPa without showing any compressive plastic strain (CPS). In contrast, the monolithic Al-4Fe-0.6Mo-1.1V-0.3Zr alloy possessed noticeably high CPS of 25% with the MCS of 0.71GPa. The composites acquired the CPS varying from 1 to 5.8 % and the MCS from 1.26 to 0.74 GPa, with increment of the volume fraction of Al-4Fe-0.6Mo-1.1V-0.3Zr alloy from 0.3 to 0.7.

Abstract: In order to solve problems modular type camshafts have, that is, reducing weight, increasing oil filling time and designing multi-valve system, research was initiated about feasibility for DfE (Desing for Environmrnt) about ultralight camshaft . Through LCA (Life Cycle Assessment) and properties and process evaluation, an attempt has been made to develop cost-effectvie process for ultiralight aluminum camshaft without the problems modular type camshaft have. The results clearly show 50% weight reduction achieved and better environmental impact of the ultralight aluminum camshaft compared with that of modular camshafts.

Abstract: This work studied the microstructure and tribological behavior of Al2O3/TiO2/ZrO2 nanocomposites to be used in hip joint replacements. To increase the fracture toughness of alumina and enhance the tribological properties, nanometer sized particles of TiO2 and ZrO2 were added. The specimens contained 10 mol% of TiO2, different percentages of ZrO2 (0, 2.5, 5, 7.5, 15 and 20 mol %), and alumina as a remainder. Disks were hot pressed at 1500 °C and 25 MPa, in an Argon atmosphere for 1 hour. Given the fact that the sample containing 7.5 mol% of ZrO2 showed the lowest wear volume, it was hot pressed again at 1500, 1400, 1300 and 1200 °C to assess the best temperature condition for sintering. For a composition of 7.5 mol% of ZrO2 the lowest coefficient of friction (0.1-0.3) and the lowest wear volume (0.0046 mm3) were achieved. A direct relation between mechanical and tribological properties was not found. However, these nanocomposites may be considered as a candidate for a new generation of hip joint replacement material.