Surface softening in metal-ceramic sliding contacts: an experimental and numerical investigation.

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten-carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation.

Surviving the surf: the tribomechanical properties of the periostracum of Mytilus sp.

We investigated the friction and wear behavior as well as the mechanical properties of the periostracum of Mytilus sp. Tribological properties were determined with a reciprocal sliding microtribometer, while mechanical characterization was performed using a nanoindenter. Measurements were performed in dry and wet conditions. On the dry periostracum we found a low friction coefficient of 0.078±0.007 on the young parts and a higher one of 0.63±0.02 on the old parts of the shell. Under wet, saline, conditions we only observed one average coefficient of friction of 0.37±0.01. Microscopic ex situ analysis indicated that dry periostracum wore rather rapidly by plowing and fatigue, while it exhibited a high wear resistance when immersed in salt water. The Young's modulus and hardness of the periostracum were also investigated in both dry and wet conditions. Under dry conditions the Young's modulus of the periostracum was 8±3GPa, while under wet conditions it was 0.21±0.05GPa. The hardness of dry periostracum samples was 353±127MPa, whereas the hardness of wet samples was 5±2MPa. It was found that, in the wet state, viscous behavior plays a significant role in the mechanical response of the periostracum. Our results strongly indicate that the periostracum can provide an important contribution to the overall wear resistance of Mytilus sp. shell.

Physical and biological properties of barium cross-linked alginate membranes.

We describe the manufacture of highly stable and elastic alginate membranes with good cell adhesivity and adjustable permeability. Clinical grade, ultra-high viscosity alginate is gelled by diffusion of Ba2+ followed by use of the "crystal gun" [Zimmermann H. et al., Fabrication of homogeneously cross-linked, functional alginate microcapsules validated by NMR-, CLSM- and AFM-imaging. Biomaterials 2003;24:2083-96]. Burst pressure of well-hydrated membranes is between 34 and 325kPa depending on manufacture and storage details. Water flows induced by sorbitol and raffinose (probably diffusional) are lower than those caused by PEG 6000, which may be related to a Hagen-Poiseuille flow. Hydraulic conductivity, L(p), from PEG-induced flows ranges between 2.4x10(-12) and 6.5x10(-12) m Pa(-1)s(-1). Hydraulic conductivity measured with hydrostatic pressure up to 6 kPa is 2-3 orders of magnitude higher and decreases with increasing pressure to about 3x10(-10) m Pa(-1)s(-1) at 4kPa. Mechanical introduction of 200 microm-diameter pores increases hydraulic conductivity dramatically without loss of mechanical stability or flexibility. NMR imaging with Cu2+ as contrast agent shows a layered structure in membranes cross-linked for 2h. Phase contrast and atomic force microscopy in liquid environment reveals surface protrusions and cavities correlating with steps of the production process. Murine L929 cells adhere strongly to the rough surface of crystal-bombarded membranes. NaCl-mediated membrane swelling can be prevented by partial replacement of salt with sorbitol allowing cell culture on the membranes.