The contribution of plastic sink-in to the static friction of single asperity microscopic contacts

We report microscale friction experiments for diamond/metal and diamond/silica contacts under gigapascal contact pressures. Using a new nanoprobe technique that has a sufficient dynamic range of force and stiffness, we demonstrate the processes involved in the transition from purely interface sliding at the nanoscale to the situation where at least one of the sliding bodies undergoes some plastic deformation. For sliding of micrometre-sized tips on metallic substrates, additional local plastic yielding of the substrate resulting from tangential tractions causes the tip to sink into the surface, increasing the contact area in the direction of loading and resulting in a static friction coefficient higher than the kinetic during ploughing. This sink-in is largely absent in fused silica, and no friction drop is observed, along with lower friction in general. The transition from sink-in within the static friction regime to ploughing in the sliding friction regime is mediated by failure of the contact interface, indicated by a sharp increase in energy dissipation. At lower contact pressures, the elastic interfacial sliding behaviour characteristic of scanning probe or surface force apparatus experiments is recovered, bridging the gap between the exotic realm of nanotribology and plasticity-dominated macroscale friction.

Role of asperities on the transition from seismic to aseismic slip using an experimental fault slip system

<p>Faults are common geological structures distributed at various depths within the Earth with different behaviors: from seismic to aseismic. The frictional stability of faults is linked to the properties of asperities that make the contact between fault surfaces. Investigating the interaction between asperities and its link with the frictional stability of faults aims at a better understanding of the intrinsic relationships between the observations of earthquake swarms and the slow local aseismic transient. Here we propose an experimental approach, which allows a customized interface sliding slowly under a well-controlled normal load, to study this problem. This interface consists of asperities modeled by poly-methyl-methacrylate (PMMA) balls in a softer, polymer base representing the parts of the fault that are easily deformed, facing a transparent flat PMMA plate. We employ a high-resolution camera for in-situ optical monitoring of the local deformation of the interface while loaded. We also attach acoustic sensors to capture the dynamics events attesting to local dynamic ruptures. We connect our observations with a mechanical model derived from a high-precision topography of the customized interface. We investigate the effects of various internal parameters of natural fault systems, including the density of asperities, their rigidity or the contrast of rigidity compared to the base, on the evolution of the frictional stability under variable normal load and of the behavior of the population of asperities at the transition between seismic and aseismic slip. Our results, bring new observations on the mechanics of swarm and fault transient.</p>

Damping Behavior of Layered Aluminium and Aluminide Coatings on AISI 316 Austenitic Steel

Several coating configurations on AISI 316 steel were obtained by a hot dipping process followed by isothermal interdiffusion. Six different kind of multilayered specimens were produced and characterized. These coatings, typically employed as bond coat in thermal barrier coating (TBC), can also be effective as vibration reduction elements at intermediate and high temperatures. This preliminary work was focused on the microstructural design and processing effects of the coatings. The damping of the produced specimens was measured up to 450 °C and compared with that of the steel substrate. The most performing coatings contain an Al-Si layer and exhibit a steep damping increase above 200 °C, reasonably due to dislocation movements by plastic straining of soft alloy layer and to the interface sliding between layers with different elastic moduli.

Damping Behavior of Layered Aluminium and Aluminide Coatings on AISI 316 Austenitic Steel

Several coatings configurations of combined aluminizing and diffusion layered aluminide on 316 steel were produced and characterized. These coatings, typically employed as thermal barrier coating (TBC), can also be effective as vibration reduction elements at intermediate and high temperatures. This preliminary work has been focused on the microstructural design and processing effects of the coatings. The damping of the produced specimens was measured up to 450°C and compared with that of the steel substrate. The most performing coatings contain an Al-Si layer and exhibit a steep damping increase above 200 °C, reasonably due to dislocation movements by plastic straining of soft alloy layer and to the interface sliding between layers with different elastic moduli.

Enhancing Fracture Toughness and Stress Energy Release Rate of Vinyl Ester Matrix Using Dual Reinforcement of CNT and GNP

ABSTRACTIn this paper, we report a method of increasing fracture toughness (KIC) and strain energy release rate (GIC) of vinyl ester matrix by adopting a dual reinforcement strategy. Reinforcements were carbon nanotubes (CNT) and graphene nanoplatelets (GNP). Both categories of nanoparticles were functionalized with COOH. The idea was to enhance crack bridging and interface sliding with CNT inclusions, given their high aspect ratio. In addition, promote crack-tip blunting and cross-linking density with GNP inclusions, due to their platelet structures. Both KIC and GIC were measured using ASTM D5045-14. An exhaustive experimental study revealed an optimum loading of both nanoparticles to be 0.25 wt% CNT and 0.5 wt% GNP, based on the highest combination of KIC and GIC values. We observed that stress intensity factor, KIC, of neat vinyl ester increased by 43% from 1.14 to 1.62 MPa*(m½). Meanwhile, the improvement in GIC was even greater with an increase of 65%, i.e., from 370 to 610 J/(m2). Differential scanning calorimetry (DSC) studies showed a discernible shift in glass transition temperature (Tg) from 123 to 128°C. The slight temperature increase was similar in thermogravimetric analysis (TGA). We observed the maximum thermal decomposition temperature (Tp) increase from 410 to 414°C, as was evident in the derivative TGA (DTG) curves.

Microstructure and Damping Capacity of AlNp/Mg-Al Composites with Different Particle Contents

The AlN particles reinforced magnesium-aluminum matrix composites were fabricated by powder metallurgy and the damping mechanism was discussed. The results showed that the best damping capacity of composite reached with the addition of 6wt% AlN reinforcement, while the AlN particles were uniformly dispersed in the matrix. The damping capacity of composites decreases with the increasing of the reinforcement content and the experimental frequency. The internal friction peak related to dislocation appearance in the temperature ranges of 100-150°C. In addition, another internal friction peak of composites between 200 and 250°C arose, which was related to interface sliding.