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 surface s at scales ranging from atomic and molecular to microscales. These te chniques have been used to study surface roughness, adhesion, friction , scratching/wear, indentation, detection of material transfer and bou ndary lubrication and for nanofabrication/nanomachining purposes. Micr o/nanotribological studies of materials of scientific and engineering interest have been conducted. Commonly measured roughness parameters a re found to be scale dependent, requiring the need of scale-independen t fractal parameters to characterize surface roughness. Measurement of atomic-scale friction of a freshly cleaved highly orientated pyrolyti c graphite exhibited the same periodicity as that of corresponding top ography. However, the peaks in friction and those in corresponding top ography were displaced relative to each other. Variations in atomic-sc ale 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 th is variation. Directionality in the friction is observed on both micro - and macroscales which results from the surface preparation and aniso tropy in surface roughness. Microscale friction is generally found to be smaller than macroscale friction as there is less ploughing contrib ution in microscale measurements. Microscale friction is load dependen t and friction values increase with an increase in the normal load app roaching the macrofriction at contact stresses higher than the hardnes s of the softer material. The wear rate for single-crystal silicon is negligible below 20 mu N and is much higher and remains approximately constant at higher loads. Elastic deformation at low loads is responsi ble for negligible wear. The mechanism of material removal on a micros cale is studied. At the loads used in the study, material is removed b y the ploughing mode in a brittle manner without much plastic deformat ion. 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 mea surement of nanoindentation hardness and Young's modulus of elasticity , with the depth of indentation as low as 1 nm. Hardness of ceramics o n the nanoscale is found to be higher than that on the microscale. Cer amics exhibit significant plasticity and creep on the nanoscale. Scrat ching and indentation on nanoscales are powerful ways to screen for ad hesion and resistance to deformation of ultra-thin films. Detection of material transfer on the nanoscale is possible with AFM. Boundary lub rication studies and measurement of lubricant-film thickness with a la teral resolution on a nanoscale have been conducted using AFM. Self-as sembled 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 co mpared to that at macroscales. Therefore, micro/nanotribological studi es may help define the regimes for ultra-low friction and near-zero we ar.