Although particle detachment is a common phenomenon associated with most tribological processes, it seldom occurs that each piece of elemental debris can be considered as the result of a single event. Such an association has been revealed by the systematic study of a specific system, where a pin of graphite is made to rub against thoroughly polished steel. While the discontinuous nature of the transfer film allows a quantitative assessment of the volume of transfer he to be made by 3D optical-profilometry, the linear dependence of the rate of particle detachment dhe/dn (n=number of rubbing cycles) with the logarithm of sliding speed v strongly suggests the existence of a particular type of stick–slip, where each stick may lead to the detachment of a debris particle. The variations in size of these debris with environment as revealed by AFM, further suggest that the global rate of particle detachment is of the form: dhe/dn=Nxi, where N is the number of stick–slip events per rubbing cycle, x the proportion of stick events leading to a cohesive rupture, and i the mean volume of an elemental particle. While this relation is apparently supported by most experimental results, its actual validation can only be made by experiments at the level of single (nanoscale) asperities, carried out under well-controlled experimental conditions. 相似文献
The morphology of polyethylene (PE) crystallized at the melt/atomically flat substrate interface was studied using atomic force microscopy (AFM). Our attention is concentrated on isothermal crystallization of PE on HOPG and MoS2 substrates at high temperatures up to 135 °C. By quenching after different times of crystallization, it was possible to “freeze” the lamellar morphology at various stages of its development at a given supercooling. After detachment of the PE sample from the substrate, individual lamellae (even 150 nm thick) and stacks of the edge-on lamellae after different stages of growth were observed. The similarity of the individual lamellae with those grown from the hexagonal phase under high pressure (characteristic tapered edges), allows to conclude that at the interface, even under normal pressure, the crystallization proceeds according to the mechanism of lamellar thickening growth. 相似文献
Calibration of the spring constant of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules.
We have developed a compact and easy-to-use reference standard for this calibration. The new artifact consists of an array of 12 dual spiral-cantilever springs, each supporting a mirrored polycrystalline silicon disc of 160 μm in diameter. These devices were fabricated by a three-layer polysilicon surface micromachining method, including a reflective layer of gold on chromium. We call such an array a Microfabricated Array of Reference Springs (MARS). These devices have a number of advantages. Cantilever calibration using this device is straightforward and rapid. The devices have very small inertia, and are therefore resistant to shock and vibration. This means they need no careful treatment except reasonably clean laboratory conditions.
The array spans the range of spring constant from around 0.16 to 11 N/m important in AFM, allowing almost all contact-mode AFM cantilevers to be calibrated easily and rapidly. Each device incorporates its own discrete gold mirror to improve reflectivity. The incorporation of a gold mirror both simplifies calibration of the devices themselves (via Doppler velocimetry) and allows interferometric calibration of the AFM z-axis using the apparent periodicity in the force–distance curve before contact. Therefore, from a single force–distance curve, taking about one second to acquire, one can calibrate the cantilever spring constant and, optionally, the z-axis scale. These are all the data one needs to make accurate and reliable force measurements. 相似文献
In cells plasma membrane, sphingomyelin (SM) plays a key role in the formation of a category of lipid microdomains enriched in cholesterol (Chl) often referred to as rafts. Atomic force microscopy (AFM) was used to analyze the mesoscopic topography of enriched SM microdomains in supported bilayers made of SM/dioleoylphosphatidylcholine (SM/DOPC) and SM/palmitoyl-oleoyl-phosphatidylcholine (SM/POPC) equimolar mixtures, in buffer, at room temperature. Gel–fluid phase separation occurs in both SM/DOPC and SM/POPC bilayers. The gel phase SM-enriched microdomains adopt a variety of size, shape and mesoscopic structure, from homogeneous flat domains of a few hundreds of nanometer in diameter to domains of several micrometers made of closely packed globular structures. Gel–gel phase separation in SM domains is also observed which gives rise to different structures for the diunsaturated and the mixed-saturated PC species. These differences could also extend to the interactions with Chl. This suggests that studies on rafts formation commonly performed using SM/DOPC mixture as a model should also include the physiologically more relevant POPC species. 相似文献
CF4 and CO2 plasma treatments have been used to modify the barrier properties of a polyamide 12 (PA12) towards permeant molecules, which
present opposing characteristics: water and toluene. The surface modifications were observed by atomic force microscopy, X-ray
photoelectron spectroscopy and surface Gibbs function measurements. Both treatments lead to different surfaces; one is rather
hydrophobic (with CF4) whereas the other is more hydrophilic (with CO2. The effect of this modification on permeametric properties has been investigated by liquid water and liquid toluene permeation
measurements. Our results show opposite effects of the two treatments. CF4 plasma treatment leads to a reduction of water and toluene permeability. With CO2 plasma treatment, in terms of permeation, two different behaviours were observed, an increase and a decrease of permeancy
for water and toluene respectively. These results are in full agreement with those obtained for the surface characterization,
and confirms change in the polymeric surface affinity for the permeant leading to a variation of the materials permeancy. 相似文献
Of the several techniques available for the surface modification, plasma processing has proved to be very appropriate. The low temperature plasma is a soft radiation source and it affects the material only over a few hundred Å deep, the bulk properties remaining unaffected. Plasma surface treatment also offers the advantage of greater chemical flexibility. PET films are widely used for packaging and electrical insulation. The studies of adhesion and printability properties are important. In the present study PET films are treated in air plasma for different time of treatment. The improvement in adhesion is studied by measuring T-peel and Lap shear strength. In addition, printability of plasma treated PET films is studied by cross test method. It has been found that printability increases considerably for plasma treatment of short duration. Therefore it is interesting to study the surface composition and morphology by contact angle measurement, ESCA and AFM. Surface energy and surface roughness can be directly correlated to the improvement in above-mentioned surface related properties. It has been found that the surface oxidation occurs containing polar functional groups such as C-O, COO. A correlation of all such observations from different techniques gives a comprehensive picture of the structure and surface composition of plasma treated PET films. 相似文献
Our work was focused on deposition of volatile hydrocarbons of carbonaceous precursor on corundum wafer, taking advantage of a metallic catalyst incorporated in precursor. Coal tar-pitch, namely a fraction soluble in toluene, served as precursor material for deposition of filamentous material. The toluene-soluble fraction of tar-pitch originally contained metallic particles of iron and nickel. During heat treatment up to 1000°C, metallic particles accompanied the volatile hydrocarbons conducive to forming a filamentous deposit. The deposit obtained demonstrates a semicrystalline material that has an irregular filamentous structure with an average filament diameter of 30 µm. The presence of catalysts after the deposition process was proved in the deposit but catalysts were not found in the residuum. 相似文献