Nanoparticles, which range from metals and metal oxides to carbon nanomaterials, have started to play an important role as lubricant additives because of their exceptional thermal stability at elevated temperatures.
The size of nanoparticles enables them to adhere to and smooth out irregularities in the contact surfaces. Furthermore, their high surface-to-volume ratio facilitates interaction with their environment.
Nanoparticle additives or nano-additives have a particle size of less than 100 nanometers (nm) in at least one dimension. In comparison, a human hair is approximately 80,000- 100,000 nm wide.
Nano-additives have demonstrated their ability to reduce friction and wear of lubricants even at concentrations of less than 1 wt%. This has been made possible by several methods:
- The ball-bearing effect, where nanoparticles effectively act as ball bearings between two surfaces, thus reducing friction and wear;
- Protective film effect, where the nanoparticles form an anti-wear film;
- Mending effect, where nanoparticles deposit onto a surface and compensate for the loss of mass;
- Polishing effect, where nanoparticles act as an abrasive, reducing surface roughness.
With the growth of nanotechnology, it is not surprising that nanomaterials constitute one of the most revolutionary technologies in the field of tribology. Due to the myriad of materials, extensive research has been conducted to find the nano-additives with the optimum performance.
One factor that has been extensively considered is the high surface energy of nanoparticles that promotes aggregation, especially when the temperature or pressure rises. Chemical modification or functionalisation of the nanoparticle surface is the key to the uniform dispersion of nanoparticles in the base lubricant and the synthesis of high-performance lubricants.The surface coating with the functional groups prevents material transfer and cold-welding of contact surfaces. The hybrid structure means these nanoparticles are soft outside and rigid inside, enabling good lubricant functionality with excellent load-bearing capability.
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