Friction, Scratching/Wear, Indentation and Lubrication on Micro- to Nanoscales

Abstract

A review with 40 refs. At. force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribol. studies of engineering surfaces at scales ranging from at. and mol. 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/nanotribol. studies of materials of scientific and engineering interests, 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. Measurements of at.-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topog. However, the peaks in friction and those in corresponding topog. were displaced relative to each other. Variations in at.-scale friction and the obsd. displacement has been explained by the variations in interat. 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 obsd. on both micro- and macro scales which results from the surface prepn. and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less plowing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approx. const. for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamondlike carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nano scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness and Young's modulus of elasticity measurements with depth of indentation as low as 1 nm. Hardness of ceramics on nano scales is found to be higher than that on micro scale. Ceramics exhibit significant plasticity and creep on nanoscale. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin films. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thickness with a lateral resoln. on a nanoscale have been conducted using AFM. Self-assembled monolayers and chem.-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribol. studies may help define the regimes for ultra-low friction and near zero wear. [on SciFinder(R)]