Open Access Review Article Issue
A review of current understanding in tribochemical reactions involving lubricant additives
Friction 2023, 11 (4): 489-512
Published: 18 November 2022

Lubricants have played important roles in friction and wear reduction and increasing efficiency of mechanical systems. To optimize tribological performance, chemical reactions between a lubricant and a substrate must be designed strategically. Tribochemical reactions are chemical reactions enabled or accelerated by mechanical stimuli. Tribochemically activated lubricant additives play important roles in these reactions. In this review, current understanding in mechanisms of chemical reactions under shear has been discussed. Additives such as oil-soluble organics, ionic liquids (ILs), and nanoparticles (NPs) were analyzed in relation to the tribochemical reaction routes with elements in metallic substrates. The results indicated that phosphorus, sulfur, fluorine, and nitrogen are key elements for tribochemical reactions. The resulting tribofilms from zinc dithiophosphates (ZDDP) and molybdenum dithiocarbamate (MoDTC) have been widely reported, yet that from ILs and NPs need to investigate further. This review serves as a reference for researchers to design and optimize new lubricants.

Open Access Research Article Issue
Drag-reduction of 3D printed shark-skin-like surfaces
Friction 2019, 7 (6): 603-612
Published: 06 November 2019

The marvels of the slippery and clean sharkskin have inspired the development of many clinical and engineering products, although the mechanisms of interfacial interaction between the sharkskin and water have yet to be fully understood. In the present research, a methodology was developed to evaluate morphological parameters and to enable studying the effects of scale orientation on the fluidic behavior of water. The scale orientation of a shark skin was defined as the angle between the ridges and fluid flow direction. Textured surfaces with a series orientation of scales were designed and fabricated using 3D printing of acrylonitrile butadiene styrene (ABS). The fluid drag performance was evaluated using a rheometer. Results showed that the shark-skin-like surface with 90 degree orientation of scales exhibited the lowest viscosity drag. Its maximum viscosity reduction was 9%. A viscosity map was constructed based on the principals of fluid dynamic. It revealed that the drag reduction effect of a shark-skin-like surface was attributed to the low velocity gradient. This was further proven using diamond nitrogen-vacancy sensing where florescent diamond particles were distributed evenly when the velocity gradient was at the lowest. This understanding could be used as guidance for future surface design.

Open Access Research Article Issue
Morphology and electric potential-induced mechanical behavior of metallic porous nanostructures
Friction 2020, 8 (3): 604-612
Published: 16 August 2019

Understanding mechanical behaviors influenced by electric potential and tribological contacts is important for verifying the robustness and reliability of applications based on metallic porous nanostructures in electrical stimulations. In this work, nickel-based metallic porous nanostructures were studied to characterize their mechanical properties and morphologically dependent contact areas during application of an electric potential using a nanoindenter. We observed that the indentation moduli of nickel-based metallic porous nanostructures were altered by pore size and application of electric potential. In addition, the structural aspects of the surface morphology of nickel-based porous nanostructures had a critical effect on the determination of contact area. We suggest that the relation between electric potential and the mechanical behaviors of metallic porous nanostructures can be crucial for building mechanically robust functional devices, which are influenced by electric potential. The morphological shape characteristics of metallic porous nanostructures can be alternative decisive factors for manipulation of tribological performance through regulation of contact area.

Open Access Research Article Issue
Morphology-influenced wetting model of nanopore structures
Friction 2016, 4 (3): 249-256
Published: 09 September 2016

Understanding the wetting behavior of nanostructures is important for surface design. The present study examined the intrinsic wettability of nanopore structures, and proposed a theoretical wetting model. Using this model, it was found that the wetting behavior of nanopore structures depends on the morphology of a surface. To accurately predict the wetting behavior of nanopore structures, correction factors were introduced. As a result, the proposed wetting model can be used to predict the wettability of nanopore structures for various engineering purposes.

Open Access Research Article Issue
Y2O3 nanosheets as slurry abrasives for chemical-mechanical planarization of copper
Friction 2013, 1 (4): 327-332
Published: 20 July 2013

Continued reduction in feature dimension in integrated circuits demands high degree of flatness after chemical mechanical polishing. Here we report using new yttrium oxide (Y2O3) nanosheets as slurry abrasives for chemical-mechanical planarization (CMP) of copper. Results showed that the global planarization was improved by 30% using a slurry containing Y2O3 nanosheets in comparison with a standard industrial slurry. During CMP, the two-dimensional square shaped Y2O3 nanosheet is believed to induce the low friction, the better rheological performance, and the laminar flow leading to the decrease in the within-wafer-non-uniformity, surface roughness, as well as dishing. The application of the two-dimensional nanosheets as abrasive in CMP would increase the manufacturing yield of integrated circuits.

Open Access Research Article Issue
Spatial evolution of friction of a textured wafer surface
Friction 2013, 1 (1): 92-97
Published: 26 March 2013

Mechanical failure of integrated circuits and micro-electro-mechanical systems (MEMS) demands new understanding of friction in small devices. In present research, we demonstrated an in situ approach to measure sliding friction of a patterned surface composing multi-materials and structures. The effects of materials and surface morphology on friction and electrical contact resistance were investigated. The material transfer at the interface of dissimilar materials was found to play dominating roles in friction. The current work provides important insights from the fundamentals of friction that benefit the design of new micro-devices.

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