Atomic Force Microscopy/friction Force Microscopy Research Articles

Mechanical Property Evaluation of ZnO Thin Films Using Nanoindentation and Scanning Probe Microscope

The characteristics of morphology, friction and nanotribological properties of ZnO thin films were achieved by means of x-ray diffraction, scanning probe microscopy (SPM), and nanoindentation. The ZnO thin films were deposited by a radio frequency magnetron sputtering system. Surface geometry and friction analysis were derived from atomic force microscopy/friction force microscopy (AFM/FFM). The hardness and Young’s modulus of the ZnO thin films were investigated by nanoindentation measurements with a Berkovich indenter. The films exhibited an increase in the hardness with decreasing load i.e. the indentation size effect (ISE) was found. In addition, the nanoscratched mechanical property of the films was discussed.

Fabrication and atomic force microscopy/friction force microscopy (AFM/FFM) studies of polyacrylamide–carbon nanotubes (PAM–CNTs) copolymer thin films

A novel polyacrylamide–carbon nanotubes (PAM–CNTs) copolymer has been prepared by ultraviolet radiation initiated polymerization. The PAM–CNTs copolymer was characterized by the instruments of Fourier transform infrared spectroscopy, UV–vis absorbance spectra, fluorescence spectra and transmission electron microscope. The morphology and microtribological properties of PAM–CNTs thin films on mica were investigated by atomic force microscopy/friction force microscopy (AFM/FFM). The friction of the films was stable with the change of applied load and the friction coefficient decreased significantly as the CNTs addition. The results show that the rigid rod-like CNTs in polymer would enhance load-bearing and anti-wear properties of the thin films.

Nanofrictional Properties of Dendron Langmuir–Blodgett Films

Abstract The nanofrictional properties of Langmuir–Blodgett (LB) films of three dendrons (G2-COOH, G3-COOH, and G4-COOH) are characterized using atomic force microscopy/friction force microscopy (AFM/FFM). The result shows that the frictional responses of LB films of three dendrons decrease with increasing generation. The origin of the frictional differences measured from these LB films is also discussed.

Atomic force microscopy/friction force microscopy study on phase separation of mixed LB films containing amphiphiles with bulky probe units

Abstract Polyion complexed mixed LB films of amphiphiles with long FC chains (PFECA [CF 3 (CF 2 ) 8 (CH 2 ) 2 O(CH 2 ) 2 COOH] or perfluorodecanoic acid (PFDA) [CF 3 (CF 2 ) 8 COOH]) and those with long hydrocarbon (HC) chains, 3-, 6-, 9- or 12-(9-anthroyloxy) stearic acids (3-, 6-, 9- or 12-ANSTAs) deposited on Si substrates were characterized by atomic force microscopy/friction force microscopy (AFM/FFM). AFM/FFM images of 1:1 mixed LB films of PFDA and 6-, 9-, or 12-ANSTA showed phase separated structures, while those of PFDA and 3-ANSTA did not. AFM/FFM images of 1:1 mixed LB films of PFECA and 3-, 6-, or 9-ANSTA did not show phase separated structure. These results show that the amphiphile molecules with long HC chain which have a bulky probe unit at the position in the methylene chain close to head group (e.g. 3-ANSTA) can mix well with the amphiphile molecules with long fluorocarbon (FC) chains (PFDA or PFECA). Similar results were also observed in the mixed LB films of PFDA (or PFECA) and doxylstearic acids. According to the π – A isotherms for these amphiphiles with bulky probe unit, the molecular sectional area in the monolayers for these amphiphile molecule increased as the position of the probe unit approached to the head group. Since the cohesion energy for molecules decrease with the increase in the inter-molecular distance, the molecular mixing between the amphiphile with FC chain and that with HC chain which have a bulky probe unit at the position closer to head group can be attributed to the decrease of cohesion energy due to the increase in the inter-molecular distance. In the AFM/FFM image for 1:1 mixed LB films of PFDA and 12-ANSTA, the following interesting phenomenon were observed. After a scanning with large load over the part of an island composed of 12-ANSTA, which is higher than the sea area before these scanning, the height of the whole area of the island became lower than the sea area. From this result, it was confirmed that this mixed LB film has a ‘side by side’ structure.

Tribology on the macroscale to nanoscale of microelectromechanical system materials: A review

Silicon-based microelectromechanical system (MEMS) devices are made from single-crystal silicon, polycrystalline silicon (polysilicon) films obtained by low-pressure chemical vapour deposition and certain ceramic films. For high-temperature applications, SiC films are being developed to replace polysilicon films. Tribology in the MEMS devices requiring relative motion is of importance. Atomic force microscopy/friction force microscopy (AFM/FFM) and nanoindentation techniques have been used for tribological studies on a microscale to nanoscale on materials of interest. These techniques have been used to study surface roughness, friction, scratching and wear, indentation and boundary lubrication of bulk and treated silicon, polysilicon films and SiC films, Macroscale friction and wear tests have also been conducted using the ball-on-flat tribometer. Measurements of microscale and macroscale frictional forces show that friction values on both scales of all the silicon samples are about the same for different silicon materials and higher than that of SiC. The microscale values are lower than the macroscale values as there is less ploughing contribution in the microscale measurements. Surface roughness has an effect on friction. In microscale and macroscale tests, C+-implanted, oxidized and plasma-enhanced chemically vapour-deposited oxide-coated single-crystal silicon samples exhibit much larger scratching and wear resistance than untreated samples. Polysilicon films and undoped single-crystal silicon show similar friction and wear characteristics. The doping of polysilicon film does not significantly affect its tribological properties. Microscratching, microwear and nanoindentation, and macroscale friction and wear studies indicate that SiC films are superior to the other materials currently used in MEMS devices. Higher hardness and fracture toughness of the SiC film is believed to be responsible for its superior mechanical integrity and lower friction. Chemically grafted self-assembled monolayers and chemically bonded liquid lubricants show promising performance for boundary lubrication in MEMS devices.

STUDY ON MICRO FRICTION AND ADHESION FORCE OF OTS SELF-ASSEMBLED FILM(Ⅰ)EXPERIMENT AND ANALYSIS OF FRICTION

The friction and adhesion characteristics of the self-assembled film of octadecyltrime-thoxysilane (OTE SAM) on the surface of SiO2 under the relative humidity of 5%—99% were studied using atomic force microscopy/friction force microscopy(AFM/FFM). Experimental results indicate that OTE SAM can not only improve the tribological properties of SiO2 surface,but also show well anti-wear properties under the pressure of the load of 200nN, or Hertz pressure of 0.8GPa. Due to the strong hydrophilicity, the friction force between the FFM tipe and SiO2 surface increases with the relative humidity when it is lower than 50%HR and then decreases sharply. However, due to the well hydrophobe property of OTE SAM, the friction force between FFM tipe and the surface of OTE SAM is much lower than that of the FFM/SiO2 surface and slightly decrease with relative humidity. Therefore, OTE SAM will be a good boundary lubricant of high speed recording system.

Nanoscale tribophysics and tribomechanics

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. This invited keynote paper presents a state of the art of nanoscale tribophysics and tribomechanics.

Phase separation of mixed LB films containing spin probe molecules

Polyion complexed mixed Langmuir–Blodgett (LB) films of amphiphiles with long fluorocarbon chains (PFECA [CF 3(CF 2) 8(CH 2) 2O(CH 2) 2COOH] or perfluorodecanoic acid (PFDA) [CF 3(CF 2) 8COOH]) and those with long hydrocarbon chains (stearic acid (STA) or doxylstearic acids (5-, 10- or 16-doxyslstearic acid) deposited on Si, glass substrates or polyethylene terephthalate (PETP) sheets were characterized by atomic force microscopy/friction force microscopy (AFM/FFM) and electron spin resonance (ESR) spectroscopy. AFM/FFM images of 1:1 mixed LB films of PFECA (or PFDA) and 16-DXSTA showed clear phase-separated structure, while those of PFECA (or PFDA) and 5-DXSTA did not. All the values of the frictional force for the LB films, which did not show clear phase-separated structure, were intermediate between those for PFECA and STA. ESR spectra of mixed LB films of PFECA and 16-DXSTA were almost the same as that of a polyion complexed LB film of pure 16-DXSTA. On the contrary, the ESR spectra of mixed LB films of PFECA, STA and 5-DXSTA were very different from those of mixed LB films of 5-DXSTA and STA. From these results it was concluded that in the polyion complexed 1:1 mixed LB films of 5-DXSTA and PFECA, 5-DXSTA and PFDA, and 10-DXSTA and PFDA, the amphiphilic molecules with fluorocarbon chains and those with hydrocarbon chains are mixed with each other on the molecular level. This finding shows that phase separation of mixed LB films containing amphiphiles with fluorocarbon chains and those with hydrocarbon chains can be controlled by the selection of the structure of amphiphiles.

Micro/nanoscale friction and wear mechanisms of thin films using atomic force and friction force microscopy

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. At most solid–solid interfaces of technological relevance, contact occurs at numerous asperities; a sharp AFM/FFM tip sliding on a surface simulates just one such contact. However, asperities come in all shapes and sizes. To study the effect of tip radius, experiments are conducted using an AFM tip with radii ranging from about 50 nm to 14.5 μm. The effect of the radius on adhesive forces and coefficient of friction at different relative humidities is measured. It is found that adhesive forces at low humidities do not change with tip radius whereas these increase with tip radius at high humidities. Coefficient of friction increases with the tip radius at all humidities. Samples coated with perfluoropolyether lubricant, which are hydrophobic in nature, are less sensitive to the environment and the tip radius. It appears that hydrophobicity of liquid films can be studied using large radii tips. Another objective of this study is to understand material removal mechanisms on microscale. Silicon surfaces are micromachined using a sharp diamond tip in an AFM. It is found that wear rate as well as the coefficient of friction is negligible up to a certain load and these increase rapidly above this load. The critical load corresponds to the local hardness of the silicon implying that the wear rate is negligible and the coefficient of friction is very low if the deformation is primarily elastic. The range of scanning velocity used in this study has little effect on the wear rate. SEM studies show that at light loads used in AFM, material is removed by ploughing and fine particulate debris is observed. At higher loads, cutting type including ribbon-like debris is observed. SEM and TEM studies of the wear region suggest that the material is removed in a brittle manner or chipping without major dislocation activity and crack formation. Evolution of wear of thin films also has been studied. Wear is found to be initiated at nano-scratches. These fundamental studies have provided insight to molecular origins of friction and wear mechanisms.

Micro/nanotribology using atomic force microscopy/friction force microscopy: State of the art

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of materials of scientific and engineering interest have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurement of atomic-scale friction of a freshly cleaved highly orientated pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement have been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope, suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macroscales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than macroscale friction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load, approaching the macrofriction at contact stresses higher than the hardness of the softer material. The wear rate for single-crystal silicon is negligible below 20 μN and is much higher and remains approximately constant at higher loads. Elastic deformation at low loads is responsible for negligible wear. The mechanism of material removal on a microscale is studied. At the loads used in the study, material is removed by the ploughing mode in a brittle manner without much plastic deformation. Most of the wear debris is loose. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nanoscratches. AFM has been modified to obtain load-displacement curves and for measurement of nanoindentation hardness and Young's modulus of elasticity, with the depth of indentation as low as 1 nm. Hardness of ceramics on the nanoscale is found to be higher than that on the microscale. Ceramics exhibit significant plasticity and creep on the nanoscale. Scratching and indentation on nanoscales are powerful ways to screen for adhesion and resistance to deformation of ultra-thin films. Detection of material transfer on the nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thickness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolayers and chemically bonded lubricant films with a mobile fraction are superior in wear resistance. Friction and wear on micro- and nanoscales at low loads have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help define the regimes for ultra-low friction and near-zero wear.

Microtribological studies of doped single-crystal silicon and polysilicon films for MEMS devices

Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) have been used to study surface roughness, friction, scratching/wear and indentation hardness, and Young's modulus of elasticity of p-type (lightly boron-doped) single-crystal silicon (referred to as ‘undoped’), p +-type (boron-doped) single-crystal silicon, polysilicon, and n +-type (phosphorus-doped) LPCVD polysilicon films. Undoped silicon samples are very smooth (r.m.s. = 0.08 nm). Surface-roughness values for p +-type silicon and lapped polysilicon bulk samples are comparable. As-deposited polysilicon films are rougher than undoped and doped single-crystal silicon, unlapped and lapped polysilicon bulk samples. Microscale friction measurements show that the friction values for undoped silicon, p +-type silicon and lapped polysilicon bulk samples are comparable. Polysilicon films exhibit the highest coefficient of friction as compared to other samples, due to the roughness effect. Macroscale friction values for undoped silicon and polysilicon bulk are comparable with those of p +-type silicon sample having the highest value and polysilicon films having the lowest values. The microscale friction values for all samples are lower than the macroscale friction values as there is less ploughing contribution in microscale friction measurements. Local variation in microscale friction is found generally to correspond to the local surface slope. Directionality in the friction is generally observed on the microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/wear studies indicate that scratch and wear resistance of p +-type silicon is poor as compared to other samples used in this study. Nanoindentation studies indicate that the p +-type silicon sample exhibits lowest hardness and Young's modulus as compared to the other samples, which is responsible for high macrofriction and poor microscratch and microwear behavior of the p +-type silicon sample. Microtribological behavior and nanohardness and Young's modulus values for polysilicon bulk and n +-type polysilicon film are comparable.

Microtribological Characterization of Self-Assembled and Langmuir-Blodgett Monolayers by Atomic and Friction Force Microscopy

Abstract : Organized and dense monolayers were produced by chemical grafting of molecules and by Langmuir Blodgett (L-B) deposition. Friction, wear and indentation of these films on a microscale have been studied using atomic force microscopy friction force microscopy (AFM/FFM). For comparison, macroscale friction and wear measurements have also been made. Mechanisms of friction, wear and lubrication of chemical grafted and L-B films are discussed. These studies show that coefficient of friction, wear resistance, and nanoindentation properties of C18 double grafted films are superior to L-B films. It is concluded that chemical grafted films may be suitable for lubrication of microcomponents.