Critical Step Height Research Articles

Transient plasto-elastohydrodynamic lubrication concerning surface features with application to split roller bearings

The use of roller bearings as crankshaft main bearings has shown potential in reducing the fuel consumption of internal combustion engines. An effective way to mount the roller bearing onto the crankshaft is to split the outer ring. However, this may lead to a severe out-of-roundness in the split region when the bearing is mounted, further implying increased noise, vibrations and contact stresses. In this work, a novel approach to study the plasto-elastohydrodynamic contact using commercial finite element software is developed. The modelling approach is based on the contact moving in space, allowing for the stress history based on the lubricant pressure to be studied in a straight-forward manner. The model is first utilised to study the influence of asperities on the lubricating conditions, indicating that stresses may exceed the yield strength of the material due to the transient effects taking place when the surface feature is over-rolled. Thereafter, the model is used to analyse the step in a mounted crankshaft roller bearing with the purpose of specifying a critical step height, which implies zero plasticity and thereby a reduced risk of accumulated damage in the vicinity of the step.

Computational Investigation of Step Excrescence Sensitivity in a Swept-Wing Boundary Layer

The interaction between an existing stationary crossflow instability and a two-dimensional step excrescence placed into a swept boundary layer on a lifting surface has been investigated computationally. Backward-facing steps were found to not amplify the stationary modes. The existence of an additional local traveling instability was found in the recirculation region following the step. Further time-resolved calculations were recommended to confirm this mode as the cause of breakdown. Forward-facing steps were found to affect the transition location only once a critical step height was exceeded. This height was associated with sudden amplification of stationary crossflow modes and transition moving forward to the step. A physics-based correlation based on linear stability theory and associated with the height of the core of the stationary crossflow vortex was proposed for that critical height, which agreed well with results for the geometry and Reynolds numbers tested experimentally. Critical forward-facing step heights were in the majority of cases higher than those associated with backward-facing steps for the same conditions. Moreover, the physics associated with two-dimensional excrescences in swept-wing flow was fundamentally different from that associated with other types of roughness, as well as all roughnesses in two-dimensional flowfields. Upon further validation, the proposed correlation for forward-facing steps would provide a reasonable manufacturing criteria available to airframe designers that would prove less restrictive and more accurate than existing empirical criteria.

Hydraulic and sediment transport properties of autogenic avulsion cycles on submarine fans with supercritical distributaries

AbstractSubmarine fans, like other distributive systems, are built by repeated avulsion cycles. However, relative to deltas and alluvial fans, much less is known about avulsions in subaqueous settings. In this study, we ran a set of subaqueous fan experiments to investigate the mechanics associated with autogenic avulsion cycles of self‐formed channels and lobe deposits on steep slopes. The experiments used saline density currents with crushed plastic to emulate sustained turbidity currents and bed load transport. We collected detailed hydraulic and bathymetric measurements and made use of a 1‐D laterally expanding density current model to better understand different aspects of the avulsion cycle. Our results reveal three major components of the avulsion cycles: (1) distributary channel incision, extension, and stagnation; (2) mouth bar aggradation and hydraulic jump initiation; and (3) hydraulic jump sedimentation and upstream retreat. Interestingly, in all but one experiment, the avulsion cycles led to fans that remained perched above the basin slope break. Experimental data and hydraulic theory were used to unravel actual mechanics associated with cycles. We found that channels stopped extending into the basin due to a decay in sediment transport capacity relative to sediment supply and that the reduction in capacity was primarily an outcome of expansion‐driven velocity reduction; dilution played a secondary role. Once channel extension ceased, mouth bar deposits aggraded to a thickness approximately equal to the critical step height needed to create a choked flow condition. The choke then initiated a hydraulic jump on the upstream side of the bar. Once formed, the jump detained a majority of the incoming sediment and forced the channel‐to‐lobe transition upstream, filling the channel with steep backset bedding and capping the entire channel with a mounded lobate deposit. These intrinsic processes repeated through multiple avulsion cycles to build the fan.

In-Plane Orientation Control of 2,7-Diphenyl[1]benzothieno[3,2-b][1]benzothiophene Monolayer on Bismuth-Terminated Si(111) Vicinal Surfaces with Wettability Optimization

We report the in-plane orientation control of a high-mobility organic semiconductor: 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). As previously reported for the monolayer pentacene, it was revealed that bunched steps on the vicinal Si(111) with a bismuth termination break the surface 3-fold symmetry and reduce the multiple orientation of the DPh-BTBT grains into a quasi-single orientation. Interestingly, the critical step height necessary for the orientation control was higher than that of pentacene. We examined several mechanisms of orientation control and concluded that the facet nanostructure fabricated by step bunching works as an anisotropic template for the nucleation. We also show the wettability optimization of the bismuth-terminated silicon surface and show that the growth mode of DPh-BTBT is dependent on the surface nanostructure of Bi–Si.

How Does Crystalline Substrate Plasticity Modify Thin Film Buckling?

We report experimental atomic force microscopy observations and analytical modeling of buckling structures of thin films deposited on single crystal substrates. The formation of straight-sided blisters just above the step structures resulting from the dislocations emergence has been observed and explained in the framework of the Föppl-von Karman theory of thin plates. A critical step height above which the buckling may occur has been determined and the asymmetry of the resulting blisters has been explained. Finally, the new buckling criterion has been compared with the classical one in the plane case and allows us to explain the blisters localization on step structures.

Pinning of a Contact Line on Nanometric Steps during the Dewetting of a Terraced Substrate

We study the dewetting of polystyrene films on an alumina surface. We show that the morphology of dewetting holes is drastically modified by the nanometric steps on the surface. Nevertheless, below a critical step height of the order of the polymer chain dimension, the contact line is not anymore sensitive to the defects. This method thus gives an estimation of the limit of validity of macroscopic descriptions of wetting when going down to molecular dimensions.

Mechanism and an empirical model of the fixed abrasivepolishing process on a web-format tool

Several chemical–mechanical planarization characterization test wafers were polished to understand the polishing mechanism of the fixed abrasive process. Oxide thickness removal in the “active” (up) and the “recessed” (down) regions of the wafer was monitored for different times of polish. It was found that there was no significant removal in the recessed areas until the step height was reduced to about 100 Å, and the polish rate in the active area decreased rapidly once this critical step height had been attained. At this critical step height, the polish rate of the down areas started to increase and approached that of the up area, with both eventually reaching the negligibly low removal rate of the blanket wafer. The drop in the polish rate of the up area, after planarity had been attained, was fitted to an exponential model.

Resistance to spreading of liquids by sharp edged microsteps

Sharp edges significantly inhibit the spreading of liquids on solid surfaces. For sharp edges with a step height of macroscopic dimensions, previous studies showed that the inhibition can be explained by the Gibbs inequality condition, which does not consider the height of the step. It is important to know for which step sizes the Gibbs inequality breaks down and to determine if there exists a critical step height δ c beyond which liquid spreading is no longer inhibited. In this study the spreading was observed of liquids on mica surfaces which, when freshly cleaved, contain well-defined sharp steps; the step dimensions were determined by scanning electron microscopy. Steps 60 nm in height were found to inhibit spreading appreciably. From extrapolation from experiments performed with various step sizes, it can be deduced that δ c is of the order of 10 −g m or smaller. Steps with a ragged geometry and with rounded edges did not inhibit liquid spreading appreciably.