Nanoscale lubrication
Friction and wear are major causes of mechanical failures and dissipative energy losses in industries such as in the automotive, aerospace and manufacturing industries. This energy loss could be saved by the proper use of lubricants. Friction depends on the two surfaces in contact, the operating conditions, the sliding velocity, and so on and the fact is that friction is a very complicated phenomenon, little understood fundamentally, and dependent on processes taking place at the nanoscale.To minimize frictional energy losses both solid and liquid lubricants have been developed which will reduce equipment maintenance and prolong component lifetime. Application of various types of lubricants alone does not solve the problem. These issues are studied with great focus by emerging technologies at the nanoscale.
Lubrication
Lubrication is the application of a substance capable of reducing friction, heat, and wear when introduced as a film between solid surfaces which is necessary to reduce damage to the moving surfaces and to enable reliable operation. Fundamentally, the phenomena of friction, wear, and lubrication involve molecular mechanisms occurring on a nanometer scale, and hence a good understanding of lubricant behavior on this scale is critical to developing new technologies for reduction of loss due to friction. Conventional lubrication schemes rely on the formation of a solid or liquid interface between mating parts where a lubricant slides against itself. But application of liquid lubricants causes the viscosity of the fluid to impede motion of micro - and nanoscale parts and surface tension can cause these parts to warp and adhere.
Nanoscale lubrication
Tribological principles applicable to micro - and nanoscale devices (i.e., nanotribology) focus primarily on surface interactions at the original interface to controlling friction at the nanoscale. To solve this problem researchers are developing techniques to perform fundamental measurements of atomic - and nanoscale forces to determine the contributions to these forces from individual material properties, including electrical conductivity, thermal conductivity, structure and composition. This can be done by using instruments like Raman spectroscopy with high-speed, high-resolution atomic force microscopy for the in situ measurements of the thermal properties of materials.
AFM has allowed to study the interaction between approaching surfaces at length and force scales that were previously unattainable and understand the science of adhesion, lubrication, friction and wear can now be understood at the sub-nanometer scale.
Georgia tech researchers have come to the following conclusions:
Viscosity and other concepts that we commonly use are taken from bulk behavior, and one of the questions we must answer is whether it is appropriate to adopt the same concepts on the molecular levels.
Increased friction caused by nanoconfinement-induced layering poses a significant concern for future devices. But the researchers have proposed several countering techniques:
• Chemically altering the long-chain molecules to include branched structures that inhibit the formation of layers. The researchers have shown that a nanoconfined liquid made of branched alkane molecules has a lower viscocity then a confined liquid of the same molecular weight but made of straight chain molecules. This behavior is opposite to that found in much larger environments.
• Roughening the surfaces of the confining plates to disrupt the molecular ordering. Instead of forming ordered layers, the molecules closest to the rough surfaces adhere to them, leaving free-flowing molecules in between.
• Varying the distance between the two confining surfaces in an oscillatory manner, just enough to keep the lubricant molecules in a "frustrated" state of disorder. Varying the distance by one Angstrom in a 20-Angstrom gap should be enough to prevent the layering. The frequency of the applied oscillations depends on the characteristic molecular relaxation times and the viscosity of the lubricant, which in turn are governed by the nature and structure of the fluid molecules.
Engine lubrication
Modern engine lubricating oil is a complex, highly engineered mixture, up to 20 percent of which may be special additives to enhance properties such as viscosity and stability and to reduce sludge formation and engine wear. But, unfortunately phosphorus is a chemical poison for automobile catalytic converters, reducing their effectiveness and life span, so industry chemists have been searching for ways to replace or reduce its use.
Researchers of National Institute of Standards and Technology have established that a titanium compound added to engine oil creates a wear-resistant nanoscale layer bound to the surface of vulnerable engine parts, making it a credible substitute for older compounds that do not coexist well with antipollution equipment.
US researchers have shown that hybrid lubricants developed by them simultaneously exhibit lower interfacial friction coefficients, enhanced wear and mechanical properties, and superior thermal stability in comparison with nanocomposites created at low nanoparticle loadings.