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

Effect of dopant size and dopant concentration on the crystallization pressure of phase change materials: The role of local order and non-local interactions

Ge0.15Sb0.85 is a phase change material that undergoes a transition from a semi-conducting glass to a crystalline metal when being densified. In this work, we investigate some parameters controlling the crystallization pressure Pc in related compounds by conducting high-pressure experiments on initially amorphous, germanium- and silicon-doped antimony. We find that the amorphous phase is stabilized to higher pressures when the dopant size is decreased. This result can be easily rationalized under the assumption that in the glass, Ge and Si atoms occupy tetrahedral or related small coordination shells. When pressure increases, the larger Ge atoms move sooner into the larger octahedral shells, which are characteristic for the crystal, than the smaller Si atoms do. We also find that Pc increases quickly with Ge and Si concentrations. This observation implies that the pressure-induced change of coordination cannot be a local, elementary event, but that the four-coordination of the group-14 atoms is stabilized by the presence of other four-coordinated atoms

Mechanical properties of zinc and calcium phosphates

Recent studies on a variety of metal phosphates (MP) have revealed that MPs tend to be soft at ambient pressure if the coordination on the metal cation is low and the degree of hydration or hydrogenation is high, while they are stiff otherwise. In addition, the softer MPs were found to stiffen dramatically more quickly with increasing pressure than the stiffer MPs. Here we review these findings and support their relevance with new results on the mechanical properties of tribofilms aged in air of relative humidity, which were produced from commercial, zinc phosphate-containing lubricant packages via heating and rubbing. We find that the films can soften quite substantially after having been exposed to humidity, as to be expected from the studies of bulk MPs. Moreover, when the hydrated films are exposed to high loads, the force-distance withdrawal curve becomes identical to that of unaged, non-hydrated films. A straightforward explanation of this observation is that large pressure reverses the hydration of the tribofilms. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010