Coarse graining and localized plasticity between sliding nanocrystalline metals.

Tribological shearing of polycrystalline metals typically leads to grain refinement at the sliding interface. This study, however, shows that nanocrystalline metals exhibit qualitatively different behavior. Using large-scale atomistic simulations, we demonstrate that during sliding, contact interface nanocrystalline grains self-organize through extensive grain coarsening and lattice rotation until the optimal plastic slip orientation is established. Subsequently, plastic deformation is frequently confined to localized nanoshear bands aligned with the shearing direction and emanating from voids and other defects in the vicinity of the sliding interface.

Expression and purification of recombinant M-Pol I from Saccharomyces cerevisiae with alpha-1,6 mannosylpolymerase activity.

Mannan outer chain N-glycan structures are yeast/fungal-specific typically found on secreted and cell wall glycoproteins. Mannan outer chains consist of an alpha-1,6 polymannose backbone attached to a Man(8-10)(GlcNAc)(2) core. The backbone contains branches of alpha-1,2 mannose residues, terminated with alpha-1,3 mannose and decorated with alpha-1,2 mannose phosphate. Mannan biosynthesis starts in the Golgi with the initial polymerization of the alpha-1,6 linked mannose backbone by the M-Pol I complex. Constructs encoding soluble portions of the M-Pol I subunits, Mnn9p and Van1p from Saccharomyces cerevisiae, were expressed in Pichia pastoris. Both subunits had to be expressed in the same strain to obtain the recombinant proteins. Recombinant M-Pol I was made only by the KM71 strain transformed with two vectors: one encoding Mnn9p and the other encoding Van1p. Soluble secreted M-Pol I was purified by sequential chromatography on DEAE-Trisacryl, GDP-Hexanolamine-Sepharose and Superdex 200. Characterization of the purified complex indicates that recombinant M-Pol 1 is a Mnn9p-Van1p heterodimer. Purified M-Pol I was active with alpha-1,6 mannobiose as acceptor and GDP-mannose as donor. HPLC identified five products confirmed to be 3-7 mannose residues long. Digestion with linkage-specific alpha-mannosidases revealed that the linkage formed is exclusively alpha-1,6. No alpha-1,2 mannosyltransferase activity, reported previously for M-Pol I immunoprecipitates from cell extracts was detected. These results provide further information on the role of M-Pol I in mannan biosynthesis.

Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C.

Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol's three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule's complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

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.

Friction and wear mechanisms of tungsten-carbon systems: a comparison of dry and lubricated conditions.

The unfolding of a sheared mechanically mixed third-body (TB) in tungsten/tungsten carbide sliding systems is studied using a combination of experiments and simulations. Experimentally, the topographical evolution and the friction response, for both dry and lubricated sliding, are investigated using an online tribometer. Ex situ X-ray photoelectron spectroscopy, transmission electron microscopy, and cross-sectional focused ion beam analysis of the structural and chemical changes near the surfaces show that dry sliding of tungsten against tungsten carbide results in plastic deformation of the tungsten surface, leading to grain refinement, and the formation of a mechanically mixed layer on the WC counterface. Sliding with hexadecane as a lubricant results in a less pronounced third-body formation due to much lower dissipated frictional power. Molecular dynamics simulations of the sliding couples predict chemical changes near the surface in agreement with the interfacial processes observed experimentally. Finally, online topography measurements demonstrate an excellent correlation between the evolution of the roughness and the frictional resistance during sliding.

Structure of Kre2p/Mnt1p: a yeast alpha1,2-mannosyltransferase involved in mannoprotein biosynthesis.

Kre2p/Mnt1p is a Golgi alpha1,2-mannosyltransferase involved in the biosynthesis of Saccharomyces cerevisiae cell wall glycoproteins. The protein belongs to glycosyltransferase family 15, a member of which has been implicated in virulence of Candida albicans. We present the 2.0 A crystal structures of the catalytic domain of Kre2p/Mnt1p and its binary and ternary complexes with GDP/Mn(2+) and GDP/Mn(2+)/acceptor methyl-alpha-mannoside. The protein has a mixed alpha/beta fold similar to the glycosyltransferase-A (GT-A) fold. Although the GDP-mannose donor was used in the crystallization experiments and the GDP moiety is bound tightly to the active site, the mannose is not visible in the electron density. The manganese is coordinated by a modified DXD motif (EPD), with only the first glutamate involved in a direct interaction. The position of the donor mannose was modeled using the binary and ternary complexes of other GT-A enzymes. The C1" of the modeled donor mannose is within hydrogen-bonding distance of both the hydroxyl of Tyr(220) and the O2 of the acceptor mannose. The O2 of the acceptor mannose is also within hydrogen bond distance of the hydroxyl of Tyr(220). The structures, modeling, site-directed mutagenesis, and kinetic analysis suggest two possible catalytic mechanisms. Either a double-displacement mechanism with the hydroxyl of Tyr(220) as the potential nucleophile or alternatively, an S(N)i-like mechanism with Tyr(220) positioning the substrates for catalysis. The importance of Tyr(220) in both mechanisms is highlighted by a 3000-fold reduction in k(cat) in the Y220F mutant.