Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Isolation and characterisation of a low molecular weight inhibitor (of chymotrypsin and human granulocytic elastase and cathepsin G) from leeches

Two protein proteinase inhibitors were isolated and purified from the leech Hirudo medicinalis by means of gel filtration and ion-exchange chromatography. They inhibit chymotrypsin, subtilisin and the granulocytic neutral proteases elastase and cathepsin G. They proved to be homogeneous in polyacrylamide and dodecylsulfate gel electrophoresis and by end group analysis; only threonine was found as N-terminal amino acid residue using the dansylation technique. These inhibitors, which we call eglins, are stable in neutral and weakly acid (pH 3) solutions and resist non-specific proteolysis. From the amino acid compositions, a molecular weight of 6 600 - 6 800 is calculated for both inhibitory proteins, which is in good agreement with a value of about 6000 estimated by dodecylsulfate electrophoresis. The eglins contain an unusually large amount of hydrophobic amino acid residues but no methionine, isoleucine or--a rarity--cysteine residues or disulfide bridges. To our knowledge, the eglins are the first examples of proteinase inhibitors of the protein type which are not stabilized by disulfide bridges.     

-Apatite rheumatism, unusual aspects-

OBJECTIVES: The goal of this study was to evaluate the effects of cryopreservation of teeth on dentin bond strength as a function of remaining dentin thickness. METHODS: Flat occlusal surfaces of human dentin were prepared in 54 freshly extracted teeth and 54 thawed, cryopreserved teeth. In each group, 18 bonds were performed in superficial dentin, 18 in mid-coronal, and 18 in deep dentin. A resin composite cylinder, 3 mm in diameter and in height, was bonded orthogonally to the surface. After storage in distilled water at room temperature for 1 wk, the bonded cylinders underwent shear testing at a crosshead speed of 0.5 mm min-1. The mean remaining dentin thickness was calculated after longitudinally sectioning the debonded samples through the center of the bonded area. Non-parametric statistical analyses were used to correlate the shear bond strength with the remaining dentin thickness among the storage modes and within the different dentin regions. RESULTS: The lowest shear bond strength values were found in the deep dentin of both fresh and cryopreserved dentin, while the values in deep and mid-coronal dentin were not significantly different in fresh and cryopreserved dentin. In the superficial and mid-coronal dentin of cryopreserved samples, the shear bond strength values were identical. There was a significant difference between the shear bond strength values in the superficial dentin of fresh teeth compared to the values for cryopreserved teeth. SIGNIFICANCE: According to the experimental conditions, tooth cryopreservation shows some promise as a substitute for freshly extracted teeth, provided that the experiments are performed in midcoronal and deep dentin.     

Prognosis of the local tool wear in gear finish hobbing

Gear finish hobbing is a method for soft finishing of external cylindrical power transmission gears. The application of this process is required when the process chain of gear production is to be set up completely free of coolant. Since this finishing technology is relatively new, there is some practical experience, but no fundamental knowledge regarding economical process design. One challenge in process design is that unbalanced tool wear typically leads to geometrical deviations of the machined gear profile. The aim of the investigations described in this paper is to develop a wear model which is capable to predict the local tool wear of gear finish hobbing tools. This model is based on analogy cutting trials and geometrical analyses of the chip geometries in gear finish hobbing. The results of the model are validated by fly-cutting trials with different local loads on the tool. The validation shows that an optimization of the local tool wear development can be achieved by means of the prediction model. Therefore, an optimization of the technological process parameters can be carried out based on this model to reach an efficient process design.     

Velocity effects in metal forming and machining processes

Velocity is probably the most important parameter in manufacturing, influencing performance, cost, productivity, energy and resources efficiency as well as safety and environmental issues. This paper presents basic phenomena as well as other important effects which are linked to velocity as a process parameter. In addition, applications, for example superplastic forming or high speed cutting, which have been founded on uncommon process velocities are discussed in the context of technological developments which have taken place over the past several years.     

Fundamental Wear Mechanisms when Machining Austempered Ductile Iron (ADI)

Austempered Ductile Iron (ADI) is characterised by improved mechanical properties but low machinability compared to conventional ductile iron materials and steels of similar strengths. The mechanical properties of ADI are achieved by a very fine austenitic-ferritic microstructure. However this unusual microstructure significantly affects mechanical and thermal machining properties. A keen understanding for the interactions of microstructure, chip formation, machining properties, cutting material and wear mechanisms is essential for the optimisation of the cutting process. This paper describes material and machining investigations as well as cutting simulations to reveal the wear mechanisms being responsible for the low machinability of ADI.     

The flight activity of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) and Teretrius nigrescens Lewis (Coleoptera: Histeridae) in Kenya

The invasive storage pest, the larger grain borer (LGB) Prostephanus truncatus (Horn), was introduced into the maize-deficit, semi-arid areas of the eastern region in Kenya in the early 1980s. In spite of containment efforts and the introduction of the predator Teretrius nigrescens Lewis from Mexico, LGB has spread to the main maize production zone in western Kenya during the past five years. The present work presents results from a 28-month monitoring effort of LGB and its predator, using pheromone traps at five locations along an east-west transect across Kenya. LGB occurred in all regions with highest trap catches in the high potential maize production zones in Western Kenya. T. nigrescens had not spread to western Kenya and trap catches were very low and mostly zero in Eastern Kenya, even in the area where it was released during the 1990s, suggesting that it became locally extinct after initial establishment. LGB flight activity was closely related to relative humidity, temperature and vapour pressure deficit. A model based on climatic factors accurately predicted seasonal trends of LGB flight behaviour in Kakamega and Mombasa but not in Kitale and Thika. It was concluded that models that rely on the direct effect of climate cannot predict LGB flight accurately enough to allow assessment of the impact of T. nigrescens on a regional basis. It is suggested that other factors such as the availability of stored grain and thus the indirect effect of climate via enhanced or reduced crop production play a major role in the flight activity of LGB and T. nigrescens.     

Tool life increase of coated carbide tools by micro blasting

In today’s production engineering nearly every cutting tool is coated. Since using the technology of coating different process chains for reconditioning have been developed. The last innovation is micro blasted coatings, which is the main topic of this paper. Concerning this topic influences on tool life have been investigated. Investigations on the tool life especially of hobbing tools were made thereby and coherences between coating bond, roughness and other attributes were discovered. The investigations were made with an aluminum oxide and a zirconium oxide as blasting abrasives and a variation of the blasting parameters pressure and period. The final result is an optimization of tool wear behavior by micro blasting of the coating.