Thermal oxydation during extrusion of polyolefins

International audienceThe thermal oxidation during extrusion of polyethylene was studied under inert gas (nitrogen and CO2) and compared with extrusion under air atmosphere. A strong reduction of degradation rate was found by optical observation and by infrared analyses. A mechanism for the change of color is proposed

An energy description of wear mechanisms and its applications to oscillating sliding contacts

To quantify wear rates, the Archard approach is classically applied. It relates the wear volume to the product of the sliding distance and the normal load. A wear coefficient is then extrapolated and is supposed to establish the wear resistance of the studied material. This synthesis shows that this approach does not work when the friction coefficient is not constant. It appears to be much more relevant to consider the interfacial shear work as a significant wear parameter. This approach is applied to study the wear response of different steels and then extended to different hard TiN, TiC coatings under reciprocating sliding conditions. By identifying wear energy coefficients the wear quantification can be rationalized and the wear resistance of the studied tribosystems can be classified. This also appears to be a convenient approach to interpret the different wear mechanisms. Metallic materials involving plastic strain are analyzed from FEM computations. The energy balance confirms that a minor part of the dissipated energy is consumed by plasticity, whereas the major part participates in the heat and debris flow through the interface. When a load energy approach is introduced an accumulated density of the dissipated energy variable is considered to quantify the TTS (Tribologically Transformed Structure) formation. A wear ”scenario” of metallic structures is then discussed. This energy wear approach is applied to analyze hard coating wear mechanisms focusing on abrasion and oxidation phenomena. The local wear energy analysis is transposed, thus allowing the lifetime of hard coatings to be quantified

Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5Hz have been carried out under 50-150N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50µm to 200µm at a constant value of relative humidity of 50% at 296K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical 1 dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated

Rapport de la campagne SURTROPAC 15 à bord du N.O. NOROIT du 18 juillet au 15 août 1991 de 20°S à 10°N le long du méridien 165°E

Dans le cadre du programme international TOGA, la campagne SURTROPAC 15 a eu lieu du 18 juillet au 15 août 1991, le long du méridien 165°E de 20°S à 10°N. Cette campagne a été réalisée par le groupe ORSTOM-SURTROPAC de Nouméa, Nouvelle-Calédonie, avec la participation du groupe ORSTOM-PROPPAC (Nouméa) et du PMEL-NOAA (Seattle). Elle s'est déroulée à bord du navire océanographique LE NOROIT de la flotte océanographique nationale. Au cours de la campagne SURTROPAC 15, les travaux suivants ont été effectués : 64 stations à la sonde CTDO2 (0-1000 m) avec prélèvements à la rosette, 64 tirs XBT, des mesures en continu du courant absolu (0-400 m) à l'aide d'un profileur de courant à effet Doppler acoustique, des mesures de la température et de la salinité de surface, toutes les 2 minutes, des observations météorologiques toutes les 3 heures, ainsi que des relevages et poses de mouillages. Ce rapport décrit le déroulement de la campagne ainsi que le matériel et les méthodes utilisés. Il présente également les figures correspondant aux premiers résultats. (Résumé d'auteur

Dinitrogen-Fixing Cyanobacteria in Microbial Mats of Two Shallow Coral Reef Ecosystems

Dinitrogen-fixing organisms in cyanobacterial mats were studied in two shallow coral reef ecosystems: La Reunion Island, southwestern Indian Ocean, Sesoko (Okinawa) Island, and northwestern Pacific Ocean. Rapidly expanding benthic miniblooms, frequently dominated by a single cyanobacterial taxon, were identified by microscopy and molecular tools. In addition, nitrogenase activity by these blooms was measured in situ. Dinitrogen fixation and its contribution to mat primary production were calculated using 15N2 and 13C methods. Dinitrogen-fixing cyanobacteria from mats in La Reunion and Sesoko showed few differences in taxonomic composition. Anabaena sp. among heterocystous and Hydrocoleum majus and Symploca hydnoides among nonheterocystous cyanobacteria occurred in microbial mats of both sites. Oscillatoria bonnemaisonii and Leptolyngbya spp. occurred only in La Reunion, whereas Hydrocoleum coccineum dominated in Sesoko. Other mats dominated by Hydrocoleum lyngbyaceum, Phormidium laysanense, and Trichocoleus tenerrimus occurred at lower frequencies. The 24-h nitrogenase activity, as measured by acetylene reduction, varied between 11 and 324 nmoles C2H2 reduced µg−1 Chl a. The highest values were achieved by heterocystous Anabaena sp. performed mostly during the day. Highest values for nonheterocystous cyanobacteria were achieved by H. coccineum mostly during the night. Daily nitrogen fixation varied from nine (Leptolyngbya) to 238 nmoles N2µg−1 Chl day−1 (H. coccineum). Primary production rates ranged from 1,321 (S. hydnoides) to 9,933 nmoles C µg−1 Chl day−1 (H. coccineum). Dinitrogen fixation satisfied between 5% and 21% of the nitrogen required for primary production.Ocean Development Sub-Committee of France–Japan S&T Cooperation; Mitsubishi cooperation; The Ministry of Education, Science, Sport, and Culture of Japan; Hanse Wissenschaftskolleg; Delmenhorst and Alexander-von-Humboldt Foundation; Bad Godesberg; Germany supported collaborative research on cyanobacterial diversit

A biomarker based on gene expression indicates plant water status in controlled and natural environments

Plant or soil water status are required in many scientific fields to understand plant responses to drought. Because the transcriptomic response to abiotic conditions, such as water deficit, reflects plant water status, genomic tools could be used to develop a new type of molecular biomarker. Using the sunflower (Helianthus annuus L.) as a model species to study the transcriptomic response to water deficit both in greenhouse and field conditions, we specifically identified three genes that showed an expression pattern highly correlated to plant water status as estimated by the pre-dawn leaf water potential, fraction of transpirable soil water, soil water content or fraction of total soil water in controlled conditions. We developed a generalized linear model to estimate these classical water status indicators from the expression levels of the three selected genes under controlled conditions. This estimation was independent of the four tested genotypes and the stage (pre- or post-flowering) of the plant. We further validated this gene expression biomarker under field conditions for four genotypes in three different trials, over a large range of water status, and we were able to correct their expression values for a large diurnal sampling period.Comment: Plant, Cell & Environment, 201

Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance char