Materials Science Forum Vols. 591-593

Abstract: Calcium carbonate (CaCO3) is a natural occurring mineral or it can be produced by chemical means. This last product is often referred to as Precipitated Calcium Carbonate (PCC). PCC has various advantanges since it exhibits higher brightness and opacity. There can be considerable variation in the properties of the PCC depending upon the process conditions used to prepare it. In this work, the effect of different factors on the particle size of PCC has been investigated. The experiments were performed by bubbling CO2 in slaked lime suspensions. The pH of the lime slurry was 12 or higher at the beginning, but it decreased, down to an equilibrium pH of 8± 1, when the reaction was considered complete.

Abstract: Soils play a decisive role in the manufacture of building materials, especially, tiles and bricks. Production techniques cause negative impacts on the environment and add additional time to the production process. Three samples obtained from Paraíba State were characterized by thermal characterization, granulometric and physical-mechanical techniques. Thermal analysis was undertaken in two different atmospheres, at a flow rate of 110 ml/min and a heating rate of 10oC/min. Evaluation of the soils through their thermal characteristics for use in soil-cement bricks production is elucidated.

Abstract: There is a substantial increase on carbon-carbon composites use for engineering applications, considering its high temperature properties and low specific mass. However the machining costs are relatively high, and new cutting tools, mainly ceramics, must be developed to overcome such difficulty, aiming cost reductions. In this work, silicon nitride based ceramics has been prepared , by pressureless sintering of silicon nitride powders and appropriate amounts of Al2O3,Ce2O3, Y2O3 and AlN. Cutting tools were prepared from the sintered materials, with geometry according to ISO1832. Selected cutting tools were also diamond coated by a hot filament-assisted Chemical Vapor Deposition (HFCVD) diamond coating process. Carbon Fiber Reinforced Carbon (CFRP) composites machining was performed, to evaluate the diamond coating influence on machining performance. After the tests, the uncoated tools presented severe flank wear and shorter life than the diamond coated ceramic tools. This flank wear is caused by the abrasive carbon powder generated during the facing operation. On CVD diamond coated α-SiAlON ceramic tools, no flank wear was observed, and the cutting edge remained unmodified, even for severe test conditions, such as high cutting length and speed. Carbon particles, originated from the machined composite, do not promotes diamond film rupture, but instead, acts as lubricant film and reduces composite surface initial roughness.

Abstract: Silicon nitride based composite has been prepared, using a direct mixture process as alternative route. Silicon carbide whiskers were mixed directly to silicon nitride based powders containing yttrium and neodymium oxide as sintering aids. Uniaxial hot pressing was used to prepare sintered samples. Crystalline phases were identified using x-ray diffractometry. Intergranular phases were analyzed using Scanning Electron Microscopy. Some mechanical properties (microhardness and fracture toughness) were also evaluated. The obtained high values of fracture toughness were correlated to the activation of toughening mechanisms, such as crack bridging and crack deflection.

Abstract: Aiming identification of the components most affected by corrosion under saline environment conditions, we have carried out X-ray diffraction measurements in ceramic and bond materials, all in the powder form. The ceramic is analyzed before and after thermal annealing at 1000°C, showing the same DRX peaks, although better defined after annealing. Identification suggests the presence of Al6Si2O13 (Mullite) and SiO2 (Quartz). Analysis of the junction (bond) material shows similar peaks, but a metallic preponderance is observed. Thermal annealing of the junction is done at much lower temperature, because it melts in the range 135°C-170°C, when a whitish smoke begins to show up along with strong sulfur odor.

Abstract: Due to their high hardness and wear resistance, Si3N4 based ceramics are one of the most suitable cutting tool materials for machining cast iron, nickel alloys and hardened steels. However, their high degree of brittleness usually leads to inconsistent results and sudden catastrophic failures. This necessitates a process optimization when machining superalloys with Si3N4 based ceramic cutting tools. The tools are expected to withstand the heat and pressure developed when machining at higher cutting conditions because of their high hardness and melting point. This paper evaluates the performance of α-SiAlON tool in turning Ti–6Al–4V alloy at high cutting conditions, up to 250 m min−1, without coolant. Tool wear, failure modes and temperature were monitored to access the performance of the cutting tool. Test results showed that the performance of α-SiAlON tool, in terms of tool life, at the cutting conditions investigated is relatively poor due probably to rapid notching and excessive chipping of the cutting edge. These facts are associated with adhesion and diffusion wear rate that tends to weaken the bond strength of the cutting tool.

Abstract: Silicon nitride samples were formed by pressureless sintering process, using neodymium oxide as sintering aid. The short term compressive creep behavior was evaluated over a stress range of 50-300 MPa and temperature range 1200-1350 0C. Post sintering heat treatments in nitrogen with a stepwise decremental variation of temperature were performed in some samples and microstructural analysis by X-ray diffractometry and Transmission Electron Microscopy showed that the secondary crystalline phases which form from the remnant glass is dependent upon heat treatment. For the non heat treated samples, glassy regions were revealed, by centered dark field images, using diffuse scattered electrons, to be located at three and four point grain junctions. No direct evidence of microstructural changes involving dislocation generation or motion was detected in the stress and temperature range studied. Stress exponents near unity, related to grain boundary accommodation processes were obtained for low temperatures and for heat treated samples. The behaviors for the heat treated samples were correlated in terms of depletion of metallic rareearth ions and impurities from grain boundaries and triple junctions, with subsequent crystallization of the primary glass. The non heat treated samples showed higher creep rates at higher stresses and temperatures. The deformation processes in these cases were correlated to stress concentrations at grain boundary and triple point junctions, caused by grains rotation and sliding, accommodated by cavitation.

Abstract: The α-SiAlON ceramic cutting tool insert is developed. Silicon nitride and additives powders are pressed and sintered in the form of cutting tool inserts at temperature of 1900 oC. The physics and mechanical properties of the inserts like green density, weight loss, relative density, hardness and fracture toughness are evaluated. Machining studies are conducted on grey cast iron workpiece to evaluate the performance of α-SiAlON ceramic cutting tool. In the paper the cutting tool used in higher speed showed an improvement in the tribological interaction between the cutting tools and the grey cast iron workpiece resulted in a significant reduction of flank wear and roughness, because of better accommodation and the presence of the graphite in gray cast iron. The above results are discussed in terms of their affect at machining parameters on gray cast iron.

Abstract: There has been a great interest for improving the machining of cast iron materials in the automotive and other industries. Comparative studies for tool used to machine grey cast iron (CI) and compacted graphite iron (CGI) on dry machining were also performed in order to find out why in this case the tool lifetime is not significantly higher. However the machining these materials while considering turning with the traditional high-speed steel and carbide cutting tools present any disadvantages. One of these disadvantages is that all the traditional machining processes involve the cooling fluid to remove the heat generated on workpiece due to friction during cutting. This paper present a new generation of ceramic cutting tool exhibiting improved properties and important advances in machining CI and CGI. The tool performance was analyzed in function of flank wear, temperature and roughness, while can be observed that main effects were found for tool wear, were abrasion to CI and inter-diffusion of constituting elements between tool and CGI, causing crater. However the difference in tool lifetime can be explained by the formation of a MnS layer on the tool surface in the case of grey CI. This layer is missing in the case of CGI.

Abstract: SiCf/SiC composite has a high potential for applications such as thermal protection systems due to its high oxidation resistance, high heat emissivity and low through-the-thickness (transverse) thermal conductivity, mainly if hollow fibers are used as reinforcement phase of the composite. PAN polymer is a precursor widely used and studied for developing high performance carbon fibers. Most commercial carbon fibers are basically derived from PAN polymer. In this work, the influence of the fiber specific mass on the morphology of SiC fiber obtained by conversion reactions from PAN oxidized fiber precursor was investigated. These oxidized fibers were obtained by controlling the time and temperature during the process of stabilization/oxidation.