alpha-SiAlON-SiC composites obtained by gas-pressure sintering and hot-pressing

The objective of this work was the obtaining in situ of alpha-SiAlON-SiC composite, using an alternative rare-earth oxide mixture, RE2O3, as sintering additive, by two different sintering processes. As sintering additive, 20 vol.% of AlN-RE2O3 in a molar ratio of 90: 10 was mixed to the alpha-Si3N4 powder. In the Si3N4-AlN-RE2O3 powder mixture, 0, 10, 15 and 20wt.% of SiC were added. The powder batches were milled, dried and compacted by cold isostatic pressing. Two different sintering processes were used: gas-pressure sintering at 1950 degrees C for 1 h under 1.5 MPa of N-2 atmosphere, or uniaxial hot-pressing at 1750 degrees C, for 30 min under pressure of 20 MPa. The sintered samples were characterized by X-ray diffraction, scanning electron microscopy and mechanical properties. XRD patterns indicate only alpha-SiAlON (alpha') and beta-SiC as crystalline phases. It was observed that the SiC addition did not influence the alpha-SiAlON formation, although the growth of elongated alpha'-grains is substantially decreased. The hot-pressed composites presented better mechanical properties, exhibiting fracture toughness of 5 MPa m(1/2) and hardness around 21.5 GPa. (c) 2007 Elsevier B.V. All rights reserved.

Thermoregulatory responses of goats in hot environments

Notwithstanding the solar radiation is recognized as a detrimental factor to the thermal balance and responses of animals on the range in tropical conditions, studies on the amount of thermal radiation absorbed by goats therein associated with data on their production and heat exchange are still lacking. Metabolic heat production and the heat exchange of goats in the sun and in the shade were measured simultaneously, aiming to observe its thermal equilibrium. The results showed that black goats absorb twice as much as the white goats under intense solar radiation (higher than 800 W m(-2)). This observation leads to a higher surface temperature of black goats, but it must not be seen as a disadvantage, because they increase their sensible heat flow in the coat-air interface, especially the convection heat flow at high wind speeds. In the shade, no difference between the coat colours was observed and both presented a lower absorption of heat and a lower sensible heat flow gain. When solar radiation levels increases from 300 to 1000 W m(-2), we observed an increase of the heat losses through latent flow in both respiratory and cutaneous surface. Cutaneous evaporation was responsible for almost 90 % of the latent heat losses, independently of the coat colour. Goats decrease the metabolic heat production under solar radiation levels up to 800 W m(-2), and increase in levels higher than this, because there is an increase of the respiratory rate and of the respiratory flow, but the fractions of consumed oxygen and produced carbon dioxide are maintained stable. The respiratory rate of black goats was higher than the white ones, under 300 W m(-2) (55 and 45 resp min(-1)) and 1000 W m(-2) (120 and 95 resp min(-1), respectively). It was concluded that shade or any protection against solar radiation levels above 800 Wm(-2) is critical to guarantee goat's thermal equilibrium. Strategies concerning the grazing period in accordance with the time of the day alone are not appropriate, because the levels of radiation depend on the latitude of the location.

Tribocorrosion behaviour of hot pressed CoCrMo−Al2O3 composites for biomedical applications

Alumina/alumina wear couple can lower the wear rates and thus metallic ion releasing on load bearing metallic implant materials. However, the low fracture toughness of ceramics is still a major concern. Therefore, the present study aims to process and to triboelectrochemically characterise the 5 and 10 vol.-%Al2O3 reinforced CoCrMo matrix composites. Corrosion and tribocorrosion behaviour of the composites were investigated in 8 g L−1 NaCl solution at body temperature. Corroded and worn surfaces were investigated by a field emission gun scanning electron microscope equipped with energy dispersive X-ray spectroscopy. After tribocorrosion experiments, wear rates were calculated using a profilometer. Results suggest that Al2O3 particle addition decreased the tendency of CoCrMo alloy to corrosion under both static and tribocorrosion conditions. However, no significant influence on the corrosion and wear rates was observed in composites mainly due to increased porosity and insufficient matrix/reinforcement bonding.

Experimental and numerical study of heat transfer in hot machined workpiece using infrared radiation

One of the greatest problems found in machining is related to the cutting tool wear. A way for increasing the tool life points out to the development of materials more resistant to wear, such as PCBN inserts. However, the unit cost of these tools is considerable high, around 10 to 20 times compared to coated carbide insert, besides its better performance occurs in high speeds requiring modern machine tools. Another way, less studied is the workpiece heating in order to diminish the shear stress material and thus reduce the machining forces allowing an increase of tool life. For understanding the heat transfer influences by conduction in this machining process, a mathematical model was developed to allow a simplified numerical simulation, using the finite element method, in order to determine the temperature profiles inside the workpiece.