② Surface modification of nanoparticles.

The so-called surface modification, also known as modification, is based on the characterization of the surface of nanodiamonds.

Research has shown that the surface of nanodiamonds contains a large number of functional groups and dangling bonds, which are the reasons for their high chemical activity, strong adsorption performance, high surface energy, and easy aggregation. As the name suggests, functional groups are atoms or atomic groups that determine the chemical properties of organic compounds. Hanging bond is a chemical bond. Generally, the crystal lattice terminates suddenly at the surface, and each atom in the outermost layer of the surface will have an unpaired electron, that is, an unsaturated bond. This bond is called a hanging bond. However, almost all scholars are unable to provide specific types of so-called functional groups, and are mostly vague, such as diamonds with oxygen-containing functional groups 0H (hydroxyl), C=0 (carbonyl), C00H (carboxyl), etc. Nevertheless, we tentatively assume that there are active centers on the surface of nanoparticles that can undergo chemical reactions.

In fact, the solid nanomaterials we can come into contact with, such as diamond powder, are already aggregates of nano diamond. There are two ways to depolymerize these aggregates and evenly disperse them in non-polar liquid media (such as lubricating base oil): mechanical methods and chemical methods.

If only mechanical methods are used to depolymerize nano diamond aggregates, the stability of the suspension system is poor, and the particles are easily re aggregated. If only chemical methods are used, it is impossible to unravel the nano diamond hard aggregates. Mechanochemical methods can be used to utilize the synergistic effect of mechanical force, surfactants, and hyperdispersants to efficiently pulverize nano diamond aggregates at high energy. At the same time, the surface of nano diamond, especially the newly generated surface during the pulverization process, can be modified to change its functional group composition and adjust its hydrophilic and hydrophobic properties, thereby achieving stable dispersion of nano diamond in the medium.

Here, I would like to draw your attention to two terms: surfactants and hyperdispersants. You must have heard of 'titanium coupling agent', which is a widely used titanium ester substance in the resin and plastic industries. In fact, it is a surface active substance, also known as a 'molecular bridge', that connects inorganic substances (diamond) with organic substances (lubricant additives).

Of course, in addition to titanium coupling agents, commonly used surface modifiers include organic acids, organic amines, organic thiophosphoric acids, and polyisobutene bissuccinimide, which can be connected to functional groups or dangling bonds on the surface of diamond. The following figure shows the preparation of nano nickel ions and the use of oleic acid as a modifier to prepare modified nano nickel.

The higher the content of nanodiamonds, the better the wear resistance of the oil.

However, as mentioned above, the dispersion stability of nanoparticles is still being explored on a laboratory scale, and many so-called characterization, modification, and performance tests that are taken for granted have ended without results. It can be seen that the uniform and stable dispersion of nanoparticles is still the bottleneck for their application in lubricating oil, let alone large-scale applications.