Microtribological Properties of Two-Phase Al2O3 Ceramic Studied by AFM and FFM in Air of Different Relative Humidity

Commercially available, monolithic alumina ceramic was modified using CO₂-laser irradiation by surface remelting and adding HfO₂ powder. Scanning electron microscopy revealed that the microstructure of the modified ceramic consisted of a fine lamellar eutectic Al₂O₃–HfO₂ phase embedded in the Al₂O₃ matrix. Differences in the microtribological properties of the matrix and the eutectic phase could be measured by friction force microscopy (FFM) during unlubricated sliding contact with a silicon tip at room temperature as a function of relative humidity of the surrounding air and normal load. The dependence of the friction coefficient and the pull-off force on humidity was explained by the formation of lubricating tribochemical surface layers and described by theoretical models.     

Investigation of adhesive and frictional behavior of GeSbTe films with AFM/FFM

Frictional force microscope (FFM) was used to investigate the nanoscale frictional behavior of GeSbTe films deposited by magnetron sputtering. The effects of relative humidity, scanning velocity and surface roughness on friction were taken into account. Besides, the frictional behavior of GeSbTe films with different compositions was analyzed. Experimental results show that the coefficient of friction of GeSbTe films is almost independent of scanning velocity, while the frictional force decreases with increasing velocity. Both the relationship of friction vs. normal load and that of friction vs. RMS keep relatively linear, and the coefficient of friction increases with the increase in RMS. The influence of humidity on adhesion between the tip and the GeSb₂Te₄ film is more significant than that between the tip and the Ge₂Sb₂Te₅ film.     

Evidence of commensurate contact and rolling motion: AFM manipulation studies of carbon nanotubes on HOPG

We report on experiments in which multiwall carbon nanotubes (CNTs) are manipulated with AFM on a graphite (HOPG) substrate. We find certain discrete orientations in which the lateral force of manipulation dramatically increases as we rotate the CNT in the plane of the HOPG surface with the AFM tip. The three-fold symmetry of these discrete orientations indicates commensurate contact of the hexagonal graphene surfaces of the HOPG and CNT. As the CNT moves into commensurate contact, we observe the motion change from sliding/rotating in-plane to stick–roll motion.     

The effect of surface roughness on the vibration behavior of AFM piezoelectric MC in the vicinity of sample surface in air environment based on MCS theory

Recent developments in the field of piezoelectric materials have led to the increasing use of piezoelectric materials in a variety of Atomic Force Microscopy (AFM). Utilizing piezoelectric layer as a sensor and actuator not only reduces the size of microscope but also enhances the quality of surface topography in Micro and Nano scales. In the current study, the effect of surface roughnesson the vibration behavior of AFM piezoelectric micro cantilever (MC) has been investigated in Micro and Nano scales according to the types of the surface roughness. Furthermore, the micro cantilever modelling has been schemed based on the Modified Couple Stress (MCS) theoryin order to model the vibration amplitude of AFM piezoelectric MC that precisely indicates the measured surface roughness. Besides, according to the various modelling of surface roughness, the effect of roughness radius on the minimum and maximum amplitude of Piezoelectric MC has been studied based on the geometry of roughness in air environment. In this environment, the effect of environmental forces including van der Waals, Capillary and contact forces on the vibration amplitude of MC forms the basis of surface topography which has, also, been studied in this article. Moreover, the present study intends to investigate the effect of surface roughness on the vibrating amplitude of MC in both the Tapping and Non-Contact Modes.     

Imaging the substructure of antibodies with tapping-mode AFM in air: the importance of a water layer on mica

Monoclonal antibodies (immunoglobulin G; IgG) against the N-terminal domain of the A subunit of DNA gyrase have been imaged using tapping-mode atomic force microscopy under ambient conditions on hydrophilic mica surfaces. The familiar tri-nodal submolecular resolution of IgG (i.e. 50-kDa resolution) has been achieved when operating the microscope with the tip predominantly in the attractive force regime. Under common laboratory conditions of about 40% relative humidity, the resolution of this substructure was not achieved owing to motion of the antibodies on the surface and/or image distortion from tip–sample instabilities. Reproducible imaging of the tri-nodal antibody substructure was achieved by desiccating the samples for extended periods of time (1 week or more) before imaging. This effect is attributed to the presence of a humidity-dependent thin water layer (a few molecules or nanometres thick), which has been observed previously using the surface force apparatus and scanning polarization force microscopy. Desiccation of the mica surfaces allowed enough water to be removed from the mica surface to prevent this effect. Degradation in the image quality over the imaging period of an hour or two was observed, owing to re-adsorption of water under the ambient laboratory conditions.     

Molecular dynamics simulation of the contact process in AFM surface observations

In the present work, several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between the specimen surface and probe tip in surface observations by an atomic force microscope (SFM) or friction force microscope (FFM). In the simulation, a three‐dimensional model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of the cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well‐defined Cu{100} surface. From the simulation results it has been verified that the surface images and the two‐dimensional atomic‐scale stick‐slip phenomenon, just as is the case for real AFM/FFM surface observations, can be detected from the spring force acting on the cantilever. From the evaluation of the behaviour of specimen surface atoms, the importance of the specimen stiffness in deciding the cantilever properties can also be understood. The influence of the probe tip shape on the force images is also evaluated. From the results it can be verified that the behaviour of the specimen surface atoms as well as the solid surface images in AFM/FFM surface observations can be understood using the molecular dynamics simulation of the model presented.     

Friction property study of the surface of ZDDP and MoDTC antiwear additive films using AFM/LFM and force curve methods

The friction properties and material differences of the surface of ZDDP and MoDTC antiwear additive films, which give clear evidence of different friction coefficients in a pin-on-disc test, have been studied using atomic force microscopy (AFM)/lateral force microscopy (LFM) and force curve methods. The AFM/LFM observations show that the friction force on the surface of MoDTC additive films over the sliding area of a steel disc is lower and the friction force of ZDDP additive films is higher than that of afilmless area. Lateral force scope-trace evaluations reveal that the ratio of the friction forces on the surface of the ZDDP film, the filmless area, and the MoDTC film under the same normal force is approximately 1.5:1.0:0.7. Force curve measurements indicate that the surface materials of the ZDDP film, thefilmless area, and the MoDTC film differ according to their attractive forces, that is 29 nN for the ZDDP film, 22 nN for the filmless area, and 12 nN for the MoDTC film. These results correspond to the friction behaviour in the pin-on-disc test and also agree with the idea of the formation of solid MoS₂ lubricant from MoDTC additives on the surface of the antiwear film.     

FTIR spectroscopy and nanotribological comparative studies: influence of the adsorbed water layers on the tribological behaviour

Under ambient conditions, a water film is always present on a silica substrate and generates additional capillary forces between the nanotip and the studied surface. In the present paper, we report AFM measurements of pull-off and friction forces as a function of the temperature and a comparative FTIR spectroscopy study. The AFM results show a net decrease of the forces as the temperature increases, while the IR spectroscopy indicates that the liquid film is removed at high temperature. Consequently, we deduce that a liquid neck is created between the tip and the surface and that the forces measured are mostly capillary forces. The present work shows that temperature studies with AFM can be a useful way to probe the influence of the capillary force in turn to characterize surface properties.     

Investigation of intrinsic crystalline Si(111) and amorphous silicon surfaces in air with STM

Crystalline (111) and amorphous silicon surfaces have been studied by scanning tunnelling microscopy. An orientated, reproducible, corrugated structure has been observed on Si(111) surfaces. The voltage dependence of the corrugation amplitude may be attributable to surface states. The surfaces of amorphous silicon thin films show some reproducible structure in the range of a few tens of ångströms, observable only when the applied voltage between the tip and sample is between ?1·3 and +0·4V.     

Influence of test parameters on friction and wear results obtained in oscillating sliding tests with 100cr6 steel against sic‐based materials

In order to characterise materials for tribological applications, model tests with simple contact geometry are in widespread use. Friction and wear behaviour can be determined with high accuracy from this type of test. Tests with oscillating sliding motion have the advantage that only small‐sized specimens are required. However, the fact that the results of model tests are affected considerably by the choice of test parameters is often overlooked. In order to check the influence of test parameters on friction and wear results, tests were performed in which the relevant parameters (stroke, frequency, load, relative humidity of the surrounding air) were varied. Comparative studies with different SiC‐based materials (SiC, SiSiC, and SiC‐TiC) against steel (100Cr6) under unlubricated conditions at room temperature show that in all cases the relative humidity is an important parameter, influencing the friction and wear results substantially. Additionally, several parameters can modify friction and wear behaviour more or less significantly. The effects of test parameters on friction and wear are discussed here on the basis of the wear mechanism. Some of the consequences of this for planning test series and for the practical use of test results are noted.     

液氮介质中Si3N4/9Cr18配副摩擦磨损性能研究

超低温介质中工作的轴承由于不能采用润滑油或脂润滑,其球环材料配副摩擦磨损特性将对轴承的工作性能产生很大影响。采用球盘点接触形式,在5 N载荷和0.4 m/s相对滑动速度下试验研究了超低温全钢轴承和混合式轴承中9Cr18/9Cr18和Si3N4/9Cr18两种球环配副在常温干摩擦和液氮环境中的摩擦磨损性能。结果表明9Cr18/9Cr18配副在低温下由于粘着趋势减弱而使摩擦因数和磨损均低于常温下。Si3N4/9Cr18配副在低温介质中由于试件表面吸附、氧化和摩擦化学反应受到抑制使摩擦因数高于常温下,同样由于无摩擦化学磨损发生,而只是以脆性断裂为主,其磨损低于常温下。Si3N4/9Cr18在常温下摩擦因数低于9Cr18/9Cr18配副,而在低温下则高于9Cr18/9Cr18配副。2种环境中陶瓷/钢配副的磨损均小于相应条件下钢/钢配副的磨损。     

On the influence of surface finish of SSiC on the results of tribological laboratory tests in the case of water lubricated oscillating sliding

The development of materials for tribological applications requires characterisation of their friction and wear behaviour. This characterisation is often based on tribological model testing, working with simple shaped specimens, running under well-defined conditions. Discrepancies in test results may arise from the fact that non-standard tests with different sets of operational parameters are used. An essential starting point for the development of tribological standards is knowledge of the main parameters that influence the tribological properties. One aspect that is not often adequately taken into account is the surface finish of the test specimens. Results are presented of friction and wear tests with self-mated SSiC couples (ball on disc), running in water in an oscillating sliding mode. The surfaces of the disc were ground, lapped, or polished, respectively, running against a polished ball. For certain test conditions, the influence of the surface finish on wear is found to be negligible, while for other conditions, the wear rate can differ by one order of magnitude or even more. The consequences for the standardisation of wear tests of ceramic materials are discussed.     

Effects of oil hydraulic pressure on surface crack growth in rolling/sliding contact

Rolling/sliding contact fatigue tests were conducted using a pair of transparent discs containing a hole or a surface crack produced artificially on the disc surface. The crack initiation from the hole or the growth of the surface crack were compared with predictions by numerical analyses based on fracture mechanics. Experiments and analyses showed that Way's hypothesis on pitting should be accepted as a plausible mechanism. The experimental results verified several important theoretical predictions on crack growth under rolling and/or contact conditions.     

Tribocarbonisation of a synthetic engine oil in lubricated piston ring/cylinder bore sliding contact and its relation to friction and wear

Tribocarbonisation of a fully formulated synthetic engine oil, an API SJ/SAE 5W‐30 containing an organic molybdenum friction modifier, was investigated in an Optimal SRV® tribotester, with a Mo‐coated piston ring and a cast iron cylinder bore tribopair in lubricated sliding contact and under stepwise heating conditions. The friction characteristics were determined by the friction coefficient curve which showed that two local minimum values occurred as the temperature increased stepwise. The local minimum friction coefficient at the lower temperature of 290°C was the result of the formation of MoS₂ and MoO₃, tribochemically generated by MoDTC and ZDTP. For the other local minimum friction coefficient at the higher temperature of 400°C, FT‐IR and Raman spectroscopic examinations of the worn tracks on the cylinder bore samples indicated that tribopyrolysis of the oil components and simultaneous polycondensation into carbonaceous species had occurred. Detailed Raman analyses showed that the carbonaceous species included a disordered phase and an ordered phase characterised, respectively, by the D‐line (1370 cm⁻¹) and G‐line (1580 cm⁻¹). The peak positions and sizes of the graphite crystallites involved varied according to temperature, and were related to the specific points on the friction coefficient versus temperature curve. Tribochemistry could enhance pyrolysis of the oil and facilitate the production of the carbonaceous species and growth of the graphite crystallites.     

The composition of lubricant-derived surface layers formed in a lubricated cam/tappet contact II. Effects of adding overbased detergent and dispersant to a simple ZDTP solution

The surface films formed by a set of oils of related additive chemistry, but with differing detergent and dispersant contents, have been chemically characterized using a combination of surface analysis techniques. The films were formed in a cam/tappet friction apparatus with a direct acting bucket tappet geometry. In the absence of overbased detergent, the amorphous films were composed essentially of inorganic zinc phosphates formed by the ZDTP anti-wear additive, and evidence of higher molecular weight phosphates (e.g. metaphosphates) was found. Adding overbased detergent and dispersant resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favour of ortho- and pyro-phosphates.