Spatio-temporal behaviour of atomic-scale tribo-ceramic films in adaptive surface engineered nano-materials

Atomic-scale, tribo-ceramic films associated with dissipative structures formation are discovered under extreme frictional conditions which trigger self-organization. For the first time, we present an actual image of meta-stable protective tribo-ceramics within thicknesses of a few atomic layers. A mullite and sapphire structure predominates in these phases. They act as thermal barriers with an amazing energy soaking/dissipating capacity. Less protective tribo-films cannot sustain in these severe conditions and rapidly wear out. Therefore, a functional hierarchy is established. The created tribo-films act in synergy, striving to better adapt themselves to external stimuli. Under a highly complex structure and non-equilibrium state, the upcoming generation of adaptive surface engineered nano-multilayer materials behaves like intelligent systems - capable of generating, with unprecedented efficiency, the necessary tribo-films to endure an increasingly severe environment.

XPS data on the tribo-films formation. Figure 1 shows general photoelectron spectra from the worn area on the rake surface of the tool with mono-and multilayer TiAlCrSiYN-based coatings after length of cut 15 m (running-in stage of wear). By comparing these spectra, it is clear that the phase composition of the wear products differ significantly for the mono-and multilayered coatings. The amount of titanium oxide is significantly lower on the worn surface of the multilayer coating. In contrast, the amount of silicon and aluminum content in tribo-ceramics is much higher.
There are two lines of silicon and aluminum at different kinetic energies in these spectra. This gives us a unique opportunity to determine the thickness of the tribo-oxide film. Tribo-oxides, which form on the surface of worn tools, have complex phase and chemical composition. The dynamically regenerating films of simple and complex meta-stable tribo-oxides and spots of un-oxidized initial nitride phase are appearing on the worn surface. Under these conditions, the precise measurement of film thickness using the Electron Spectroscopy for Chemical Analysis (ESCA) method is difficult [1].
However, it is possible to make an estimate by measuring the divergence from the standard ratio of intensities of the photoelectron lines for corresponding mean free passes. A line of silicon and aluminum 2s and 2p appears on the photoelectron spectrum of the wear surface of the multilayer coating after cutting length of 15 m (Figure 1, a). Standard line intensity ratio of silicon (ISi2s/ISi2p) = 0.988. The mean free pass for these lines is 30.75 and 31.70 A, respectively. Standard line intensity ratio of aluminum (IAl2s/IAl2p) = 1.188, experimental -1.15. The mean free pass for these lines is 27.33 and 28.05 A, respectively. This means the thickness of the oxide film is somewhat less than 27.33-28.05 A.
The experimentally obtained value of the ratio of silicon (ISi2s/ISi2p) = 0.383, which is almost three times lower than the standard photo-ionization cross sections corresponding to the thickness of the oxide film of the order of 18.8 A. We can attribute this thickness to the mullite tribo-ceramic films because XPS   Figure 2, c exhibits the difference in the tribo-chemical behavior of the multilayer and mono-layer coatings. For the multilayer coating, the content of mullite tribo-phase on the surface is increased while the amount of sapphire-like oxide and initial nitride is significantly lower compared to the monolayer. It can be seen by analyzing the chemical shift in O1s line (Figures 3, a-b) that the products of wear in the multilayer coating contain only mullite, sapphire and chromium tribo-oxides.
In contrast titanium oxide is detected on the worn surface of the monolayer coating. Figure 4 shows the photoelectron spectra Si2s and Y3d 3 / 2 from the worn surface of the multilayer (a, b) and monolayer (c) coatings. Formation of Y oxides is detected. Small amount of stoichiometric Y 2 O 3 oxide and non-equilibrium Y 1-x O x oxide are detected on the worn surface. With wear of the multilayer coating the amount of stoichiometric Y 2 O 3 oxide grows vs. the non-equilibrium oxide. Reverse pattern is observed for the monolayer coating. The contents of these oxides are less than a fracture of at %. However we have to note Y 2 O 3 has many beneficial properties, especially in conjunction with alumina, including: high chemical and thermodynamic stability; low thermal conductivity [3][4][5] as well as high temperature lubrication properties [6].
Fine structure analysis of photoelectron Si2s line shown in Figure 4 indicates the mullite tribofilms formation on the worn surface. Phase analysis of the tribo-films formed during the running-in stage (after length of cut of 15 m) is summarized in Figure 3 in the main text.