Nanoscale friction and wear maps

Nanoscale Abrasive Wear of Polyethylene: A Novel Approach To Probe Nanoplastic Release at the Single Asperity Level

There is a growing concern that nanoplastic pollution may pose planetary threats to human and ecosystem health. However, a quantitative and mechanistic understanding of nanoplastic release via nanoscale mechanical degradation of bulk plastics and its interplay with photoweathering remains elusive. We developed a lateral force microscope (LFM)-based nanoscratch method to investigate mechanisms of nanoscale abrasive wear of low-density polyethylene (LDPE) surfaces by a single sand particle (simulated by a 300 nm tip) under environmentally relevant load, sliding motion, and sand size. For virgin LDPE, we found plowing as the dominant wear mechanism (i.e., deformed material pushed around the perimeter of scratch). After UVA-weathering, the wear mechanism of LDPE distinctively shifted to cutting wear (i.e., deformed material detached and pushed to the end of scratch). The shift in the mechanism was quantitatively described by a new parameter, which can be incorporated into calculating the NP release rate. We determined a 10-fold higher wear rate due to UV weathering. We also observed an unexpected resistance to initiate wear for UV-aged LDPE, likely due to nanohardness increase induced by UV. For the first time, we report 0.4-4 × 10-3 μm3/μm sliding distance/μN applied load as an initial approximate nanoplastic release rate for LDPE. Our novel findings reveal nanoplastic release mechanisms in the environment, enabling physics-based prediction of the global environmental inventory of nanoplastics.

Rheological and tribological characterization of novel modified graphene/oil-based nanofluids using force microscopy

In this study, it is aimed to improve the lubrication and anti-wear characteristics of nanofluids produced by the distribution of silane-modified graphene nanosheets into the base oil without any surfactant or dispersant. Nanofluids are among the hottest research topics currently studied in the literature due to their interesting thermal and rheological properties. Graphene nanosheet with unique physicochemical properties is a good alternative as a nanofluid component and a lubricant. In this study, the behavior of nanofluidic films on the material was investigated by using scanning probe techniques, phase-contrast microscopy, and friction force microscopy techniques. Due to stick-slip behavior and rheological properties that are dominant in the studied ranges, problems were encountered in performing tribological analyzes with friction force microscopy. On the other hand, these results have been beneficial in determining tribological factors in nanoscale. The presented nanofluids showed non-Newtonian behavior at high concentrations and shear rates and shown an improved tribological performance up to 43% in friction coefficient, 91% in wear, and 46% in thermal conductivity compared to the base oil.

Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays

Normal and friction forces between immobilized two-dimensional (2D) homogeneous non-close-packed colloidal arrays made of different particles are compared in aqueous media. Soft pH-responsive (microgels) and nonresponsive hard (silica) particles of different sizes were used to create the 2D arrays. The results show that the friction of soft responsive structured layers can be successfully modulated by varying the pH, with a friction coefficient varying by nearly 3 orders of magnitude (10^-2 to 1). This important change in lubricating properties is mainly correlated with the particle swelling behavior, i.e., the friction coefficient decreasing exponentially with an increase in the swelling ratio regardless of the size, surface coverage, and degree of ionization of the particles. In addition, the robustly attached microgel particles were able to sustain high pressure (up to 200 atm) without significant surface damage. The 2D arrays of nonresponsive hard particles also gave rise to a very low friction coefficient (μ ≈ 10^-3) under similar experimental conditions and could sustain a larger pressure without damage (≤600 atm). The low friction dissipation observed between the hard arrays resulted from a rolling mechanism. Even though rolling requires nonimmobilized particles on the substrates, the results show the importance of attaching a certain proportion of particles on the surfaces to reduce friction.