Low Contact Stiffness Research Articles

The effect of interfacial wear debris on the friction-induced stick-slip vibration

To address the influence of wear debris on friction-induced stick-slip vibration (FISSV), a method involving grooving the friction disc was proposed. FISSV simulation experiments were conducted using a test setup. A finite element model (FEM) was established for dynamic simulations. The results show that wear debris significantly influences FISSV. Debris involved in friction will result in significant abrasive wear. Extensive debris entering the interface causes the worn surface to have numerous small contact plateaus with lower contact stiffness, leading to pronounced FISSV. Conversely, the presence of larger contact plateaus on the worn surface increases contact stiffness, making the friction system less prone to generating FISSV. Filling groove with graphite effectively suppresses FISSV. Controlling debris flow is crucial for suppressing FISSV.

A Dual-Modal Hybrid Gripper with Wide Tunable Contact Stiffness Range and High Compliance for Adaptive and Wide-Range Grasping Objects with Diverse Fragilities.

The difficulties of traditional rigid/soft grippers in meeting the increasing performance expectations (e.g., high grasping adaptability and wide graspable objects range) of a single robotic gripper have given birth to numerous soft-rigid coupling grippers with promising performance. However, it is still hard for these hybrid grippers to adaptively grasp various objects with diverse fragilities intact, such as incense ash and orange, due to their limited contact stiffness adjustable range and compliance. To solve these challenging issues, herein, we propose a dual-modal hybrid gripper, whose fingers contain a detachable elastomer-coated flexible sheet that is restrained by a moving frame as a teardrop shape. The gripper's two modes switched by controlling the moving frame position can selectively highlight the low contact stiffness and excellent compliance of the teardrop-shaped flexible sheets and the high contact stiffness of the moving frames. Moreover, the contact stiffness of the teardrop-shaped sheets can be wide-range adjusted by online controlling the moving frame position and offline replacing the sheets with different thicknesses. The compliance of the teardrop-shaped sheets also proves to be excellent compared with an Ecoflex 10 fingertip with the same profile. Such a gripper with wide-range tunable contact stiffness and high compliance demonstrates excellent grasping adaptability (e.g., it can safely grasp several fragile strawberries with a maximum size difference of 18 mm, a strawberry with a left/right offset of 3 cm, and a strawberry in two different lying poses) and wide-range graspable objects (from 0.1 g super fragile cigarette ashes to 5.1 kg dumbbell).

The Potential of a Nanostructured Titanium Oxide Layer with Self-Assembled Monolayers for Biomedical Applications: Surface Properties and Biomechanical Behaviors

This study investigated the surface properties and biomechanical behaviors of a nanostructured titanium oxide (TiO) layer with different self-assembled monolayers (SAMs) of phosphonate on the surface of microscope slides. The surface properties of SAMs were analyzed using scanning electron microscopy, X-ray photoemission spectroscopy, and contact angle goniometry. Biomechanical behaviors were evaluated using nanoindentation with a diamond Berkovich indenter. Analytical results indicated that the homogenous nanostructured TiO surface was formed on the substrate surface after the plasma oxidation treatment. As the TiO surface was immersed with 11-phosphonoundecanoic acid solution (PUA-SAM/TiO), the formation of a uniform SAM can be observed on the sample surface. Moreover, the binding energy of O 1s demonstrated the presence of the bisphosphonate monolayer on the SAMs-coated samples. It was also found that the PUA-SAM/TiO sample not only possessed a higher wettability performance, but also exhibited low surface contact stiffness. A SAM surface with a high wettability and low contact stiffness could potentially promote biocompatibility and prevent the formation of a stress shielding effect. Therefore, the self-assembled technology is a promising approach that can be applied to the surface modification of biomedical implants for facilitating bone healing and osseointegration.

SENSITIVITY ANALYSIS OF A CALIPER FORMED ATOMIC FORCE MICROSCOPE CANTILEVER BASED ON A MODIFIED COUPLE STRESS THEORY

ABS TRACT: A relationship based on the modified couple stress theory is developed to investigate the flexural sensitivity of an atomic force microscope (AFM) with assembled cantilever probe (ACP). This ACP comprises a horizontal cantilever, two vertical extensions and two tips located at the free ends of the extensions, which forming a caliper. An approximate solution to the flexural vibration problem is obtained using the Rayleigh–Ritz method. The results show that the sensitivities of AFM ACP obtained by the modified couple stress theory are smaller than those evaluated by the classical beam theory at the lower contact stiffness. The results also indicate that the flexural sensitivities of the proposed ACP are strong size dependant when the thickness of the cantilever is close to the material length scale, especially at lower contact stiffness. Furthermore, the greatest flexural modal sensitivity occurs at a small contact stiffness of the system, in which the ratio of the cantilever thickness to the material length scale and the distance between the vertical extensions are also small. In this situation, the distance between the vertical extensions and the clamped end of the cantilever and also the vertical extensions lengths are large. The results reveal that the sensitivity of the right sidewall tip is higher than that of the left one.

Effect of the Abrasive Size on the Friction Effectiveness and Instability of Brake Friction Materials: A Case Study with Zircon

The friction-induced vibration triggered at the sliding interface between the gray iron disk and brake friction material was studied by changing the size of the zircon particles in the friction material. The friction tests were performed using a reduced brake dynamometer and the friction characteristics of the friction materials containing zircon particles with sizes of 3, 50, and 100 μm were analyzed. Our results show that the properties of the sliding surface were strongly affected by the entrenchment of the abrasive particles in the friction layers during sliding. The friction effectiveness was inversely proportional to the size of the abrasive, while friction instability was pronounced when smaller zircon particles were used. The smaller zircon particles produced larger plateaus on the sliding surface with low contact stiffness. However, the contact plateaus with the low contact stiffness showed higher amplitudes of the friction oscillations, suggesting a surface with low stiffness also can produce high propensity of friction instability during sliding. Based on the friction stability diagram and surface properties, such as contact stiffness and surface roughness, it was suggested that the static coefficient of friction, which was changed as a function of dwell time, was crucial to understand the cause of friction-induced force oscillations and propensity of friction instability of brake friction materials.

Increasing the Image Contrast of Atomic Force Microscope by Using Improved Rectangular Micro Cantilever

The resonant frequency of flexural vibrations for an atomic force microscope (AFM) cantilever has been investigated using the Euler-Bernoulli beam theory. The results show that for flexural vibration the frequency is sensitive to the contact position, the first frequency is sensitive only to the lower contact stiffness, but high order modes are sensitive in a larger range of contact stiffness. By increasing the height H, for a limited range of contact stiffness the sensitivity to the contact stiffness increases. This sensitivity controls the image contrast, or image quality. Furthermore, by increasing the angle between the cantilever and sample surface, the frequency decreases.

Effects of Damping on the Vibration Frequency of Atomic Force Microscope Cantilevers Using the Timoshenko Beam Model

The modal frequencies of flexural vibration for an atomic force microscope (AFM) cantilever immersed in fluids have been derived based on Timoshenko beam theory, including the effects of rotary inertia and shear deformation, and a closed-form expression for the resonant frequencies of vibration modes has been obtained. The effects of quality factor and contact stiffness on the modal frequency are analyzed. The results show that the damping effect on the vibration frequency of AFM cantilever is obvious, especially for high-order modes and low contact stiffness. The vibration frequency of AFM cantilever shows a marked decrease with low quality factors. In addition, the effects of rotary inertia and shear deformation on the ratio of vibration frequency of a Timoshenko beam to that of an Euler beam are significant, especially for high-order modes and low quality factors. It indicates that the Timoshenko beam theory is able to evaluate the frequencies of flexural vibration of the higher modes for the AFM cantilever immersed in liquids.

Depth-sensing indentation of low-density brittle nanoporous solids

Many applications of low-density nanoporous dielectrics are limited by their poor mechanical properties. Although nanoindentation is often used to evaluate the mechanical behavior of such materials, the physical meaning of the inelastic parameters measured by various indentation methods is not clear. Here, we study low-density nanoporous silica monoliths (aerogels) by nanoindentation using the most common indenter geometries (spherical, pyramidal and flat punches) and discuss the parameters measured. Results suggest that the deformation of the nanoporous silica monoliths studied in this work is controlled by elastic bending and fracture of nanoligaments with no signs of plasticity. The contact pressure (Meyer hardness) increases with increasing strain and does not represent the foam “crushing pressure”. The critical load for Hertzian fracture obeys the Auerbach law, and the formation of radial cracks and brittleness are strongly suppressed by the presence of nanopores. We also discuss the choice of indenter geometry and provide recommendations that can be used to overcome some typical challenges of indentation studies of low-density nanoporous solids, including very low contact stiffness for indenters with small contact areas, large surface roughness inherent to this class of materials, spatial non-uniformity (skin layers on monoliths), viscoelasticity and elastic nonlinearity.

Deformation behavior of PZN–6%PT single crystal during nanoindentation

The deformation behavior of [001]T- and [011]T-cut single crystal solid solution of Pb(Zn1/3Nb2/3)O3–6% PbTiO3 (PZN–6%PT) in both unpoled and poled states has been investigated by nanoindentation. Nanoindentation experiments reveal that material pile-up and local damage around the indentation impressions are observed at ultra-low loads. These pile-ups and local damage cause a pop-in event (i.e. a sudden increase in displacement at an approximately constant load) in the nanoindentation load–displacement curve (P–h curve). Detailed studies of the relationships between indentation load (P), displacement (h) and harmonic contact stiffness (S) suggest that there is a surface layer, possibly due to crystal fabrication processes, which possesses different mechanical properties from the interior. The thickness of this surface layer is estimated to be approximately 300 nm. Furthermore, it is found that [011]T-cut crystal is stiffer than [001]T-cut crystal. On the other hand, both [001]T- and [011]T-cut crystals in unpoled state possess lower contact stiffness than poled crystals. This finding suggests that poling improved the mechanical property of the crystal. In summary, poled [001]T-cut crystals have an elastic modulus of (107 ± 6) GPa and a hardness of (5.1 ± 0.4) GPa. In contrast, the modulus for [011]T-cut crystals is not constant but increases with indentation depth.

Cells anchored upon a thin organic film with different nano-mechanical properties

By applying dynamic contact module and particular measurement of phase angles, harmonic contact stiffness ( S) along with the measured displacement ( D) of different self-assembled monolayers (SAMs) adsorbed on Au can be distinguished. The relatively ordered and hydrophobic ODT and DDT molecules adsorbed on Au form high contact stiffness, which are presumably unfavorable substrates for a cell to adhere upon. Short-chain MUA molecules adsorbed on Au provides a hydrophilic characteristic with a relatively low contact stiffness, which may significantly promote cell adhesion. It is, therefore, estimated that the behavior of a cell adhered on SAMs/Au is correlated not only with their outermost chemical species but also with a proper d S/d D matrix acting as a cushion.