Improving the oxidation resistance of refractory metals via aluminum diffusion coatings and halogen effect

Recently, refractory metals and their alloys have received increasing attention for the purpose of substituting Ni-based single crystal superalloys. Compared to commonly used materials, refractory metals have significantly higher melting points while their mechanical properties seem adequate at high temperatures. The main challenge for their application is, that refractory metals show low oxidation resistances at elevated temperatures. Aluminum diffusion layers are promising coatings to suppress harmful oxidation by forming a protective oxide layer. This study deals with the application of such aluminum reservoir layers and with the investigation of their protective properties. Four different unalloyed refractory metals, molybdenum, tantalum, tungsten and niobium, were used as substrate materials. The coatings were manufactured via a pack cementation process carried out for 8 h at 1000°C. Homogeneous intermetallic layers with thicknesses up to 49 µm and high aluminum contents were characterized using optical microscope, EPMA, and XRD analysis. The oxidation resistance of the samples was investigated using thermogravimetric analysis. The experiments were carried out at 1300°C for up to 100 h in synthetic air. Via mass change curves the oxidation kinetics were analyzed as well as the formed oxide layers using, again, optical microscopy, EPMA, and XRD analysis. It was found that an additional application of a halogen treatment can significantly reduce the oxidative attack of the substrate and support the formation of a continuous protective Al2O3 layer. Furthermore, the effect of varying the amount of halogen on oxide layer formation is shown. The Al2O3 growth mechanism and aluminum depletion of the underlying reservoir layer in the different refractory metals were investigated by comparing uncoated, coated, and additional halogen-treated samples

Membrane-Active Peptides and the Clustering of Anionic Lipids

AbstractThere is some overlap in the biological activities of cell-penetrating peptides (CPPs) and antimicrobial peptides (AMPs). We compared nine AMPs, seven CPPs, and a fusion peptide with regard to their ability to cluster anionic lipids in a mixture mimicking the cytoplasmic membrane of Gram-negative bacteria, as measured by differential scanning calorimetry. We also studied their bacteriostatic effect on several bacterial strains, and examined their conformational changes upon membrane binding using circular dichroism. A remarkable correlation was found between the net positive charge of the peptides and their capacity to induce anionic lipid clustering, which was independent of their secondary structure. Among the peptides studied, six AMPs and four CPPs were found to have strong anionic lipid clustering activity. These peptides also had bacteriostatic activity against several strains (particularly Gram-negative Escherichia coli) that are sensitive to lipid clustering agents. AMPs and CPPs that did not cluster anionic lipids were not toxic to E. coli. As shown previously for several types of AMPs, anionic lipid clustering likely contributes to the mechanism of antibacterial action of highly cationic CPPs. The same mechanism could explain the escape of CPPs from intracellular endosomes that are enriched with anionic lipids