Interfacial layer is a layer formed on the surface of solid by adjacent liquid. Liquid in the close vicinity of solid surface is more ordered and displayed modified thermophysical properties.
You have to analyse the structure of layers formed by nanolubricants at the molecular level.. where you have to study both fluid and particles interaction dealing with particular nanoparticle and base lubticant.
The nanosize addtitives to lubricants may have different effect on the worn surface. They may decompose in friction and then create an anti-wear film. Or they simply oxidize and create oxide films. Using solid powder additives also creates a problem of deagglomerating the particles in the fluid lubricant. The particles may precipitate on the worn surfaces and then aggglomerate again in the surface pattern valleys. in order to reduce friction, the particles shoud precipitate on the real contact areas. Therefore real antiwear films are films are patchy.
Given all that such methods as XPES and Auger are in often use to detect the films. These methods allow detecting either metals of their oxides within micron size films.