Graphene Failure under MPa: Nanowear of Step Edges Initiated by Interfacial Mechanochemical Reactions

The low wear resistance of macroscale graphene coatings does not match the ultrahigh mechanical strength and chemical inertness of the graphene layer itself; however, the wear mechanism responsible for this issue at low mechanical stress is still unclear. Here, we demonstrate that the susceptibility of the graphene monolayer to wear at its atomic step edges is governed by the mechanochemistry of frictional interfaces. The mechanochemical reactions activated by chemically active SiO2 microspheres result in atomic attrition rather than mechanical damage such as surface fracture and folding by chemically inert diamond tools. Correspondingly, the threshold contact stress for graphene edge wear decreases more than 30 times to the MPa level, and mechanochemical wear can be described well with the mechanically assisted Arrhenius-type kinetic model, i.e., exponential dependence of the removal rate on the contact stress. These findings provide a strategy for improving the antiwear of graphene-based materials by reducing the mechanochemical interactions at tribological interfaces

Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to Adist, Aforce, and Abond, respectively) are not equivalent at all, while we interestingly clarify that only Adist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only Abond is proportional to the friction force indicating that Abond is the dominant one for determining materials’ frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction

Superlubricity Enabled by Pressure-Induced Friction Collapse

From daily intuitions to sophisticated atomic-scale experiments, friction is usually found to increase with normal load. Using first-principle calculations, here we show that the sliding friction of a graphene/graphene system can decrease with increasing normal load and collapse to nearly zero at a critical point. The unusual collapse of friction is attributed to an abnormal transition of the sliding potential energy surface from corrugated, to substantially flattened, and eventually to counter-corrugated states. The energy dissipation during the mutual sliding is thus suppressed sufficiently under the critical pressure. The friction collapse behavior is reproducible for other sliding systems, such as Xe/Cu, Pd/graphite, and MoS₂/MoS₂, suggesting its universality. The proposed mechanism for diminishing energy corrugation under critical normal load, added to the traditional structural lubricity, enriches our fundamental understanding about superlubricity and isostructural phase transitions and offers a novel means of achieving nearly frictionless sliding interfaces

Mitochondrial Protein PGAM5 Regulates Mitophagic Protection against Cell Necroptosis

<div><p>Necroptosis as a molecular program, rather than simply incidental cell death, was established by elucidating the roles of receptor interacting protein (RIP) kinases 1 and 3, along with their downstream partner, mixed lineage kinase-like domain protein (MLKL). Previous studies suggested that phosphoglycerate mutase family member 5 (PGAM5), a mitochondrial protein that associates with RIP1/RIP3/MLKL complex, promotes necroptosis. We have generated mice deficient in the pgam5 gene and surprisingly found PGAM5-deficiency exacerbated rather than reduced necroptosis in response to multiple <i>in vitro and in vivo</i> necroptotic stimuli, including ischemic reperfusion injury (I/R) in the heart and brain. Electron microscopy, biochemical, and confocal analysis revealed that PGAM5 is indispensable for the process of PINK1 dependent mitophagy which antagonizes necroptosis. The loss of PGAM5/PINK1 mediated mitophagy causes the accumulation of abnormal mitochondria, leading to the overproduction of reactive oxygen species (ROS) that worsen necroptosis. Our results revise the former proposal that PGAM5 acts downstream of RIP1/RIP3 to mediate necroptosis. Instead, PGAM5 protects cells from necroptosis by independently promoting mitophagy. PGAM5 promotion of mitophagy may represent a therapeutic target for stroke, myocardial infarction and other diseases caused by oxidative damage and necroptosis.</p></div

PGAM5 fails to stabilize PINK1 in HT-29 cells.

<p>(A) HT-29 cells transduced with shRNA against PGAM5 were treated with TCZ to induce necroptosis. Cell viability was evaluated by the MTT assay. (B) Mitochondria in HeLa cells and HT-29 cells were stained with anti-Tomm20 and mitochondrial morphology was evaluated by confocal. (C) PGAM5 was knocked down by lentiviral shRNA in HeLa, HT-29 and SY5Y cell lines. Then PINK1 mRNA in the control (NS) and Knock-down cells (PGsh) were quantified by RT-PCR. (D) HT-29 and HeLa cells were treated with CCCP for 3 hours and PINK1 was detected by western blot. * indicated full length PINK1 and Δ indicated cleaved PINK1.(E)HT-29 control (NS) and PGAM5 knock-down cells (PGsh) were treated with CCCP for 3 hours followed by western blot for detecting indicated proteins.</p

PGAM5 stabilizes PINK1 to protect cell from necroptosis.

<p>(A) Mitochondrial extracts of WT and Pgam5 KO MEFs treated with DMSO (D), CCCP(C), or TCZ (T) for 3 hours were analyzed by immunoblot as above. (B) PINK1 immunoblot of mitochondrial fractions from WT and KO hearts subjected to I/R (+) or perfusion only (-). (C) Cell viability of Pink1 WT/KO MEFs was quantified by MTT assay. Black bars represent WT controls and white bars the respective KO. Cells were treated with TCZ, TCZ plus Nec1, or TCZ plus BHA for 12 hours. (D) Cell viability of WT and Pink1 KO MEF cells was transduced with scrambled (NC) or Pgam5-specific shRNA (PG) lentiviruses, treated with TCZ, and then tested for viability by the MTT assay at indicated treatments. <i>p</i> <0.01 by Student <i>t</i>-test.</p

PGAM5 deficiency exacerbates necroptosis.

<p>(A) Phase contrast photomicrographs of WT or KO MEFs treated with DMSO (a, b) or TCZ (c, d) for 24 hr. Cell viability was evaluated by the MTT assay, and propidium iodide staining (inset c, d). (B) Cell viability results for WT (black) and KO (white) MEFs, which were treated with TCZ with or without 50 μM Nec1 or 100 μM BHA; (C) WT and Pgam5 KO MEFs were treated with DMSO or TCZ for 6 hr and then stained with the mitochondria membrane potential dependent dye DAPI (blue), anti-HMGB1 (green) and Mitotracker (red); (D) WT and Pgam5 KO MEFs were treated with TCZ for 3 hrs, and followed by purification of mitochondria. Western blots with the indicated antibodies are shown. Tubulin and VDAC are loading controls. (E) Representative TEM micrographs of necrotic WT and Pgam5 KO MEFs treated with TCZ or DMSO vehicle for 12 hrs. (F) Jurkat I2.1 (Caspase 8 deficient) cells transfected with nonspecific (NS) or PGAM5 (siPG) siRNAs were treated with 0.5 ug/ml human TNF-α for 24 hrs with or without necrostatin (Nec1), then cell death was quantified by taking the percentage outside of the life gate (indicated by the box using dot plots with PI staining on the y-axis and forward scatter on the x-axis (left panels). Quantification is shown in a bar graph (right upper panel) and the quality of the knockdown shown by Western blot (right lower panel).</p