Abstract
Experimental investigations of nano-scale spatio-temporal effects that occur on the friction surface under extreme tribological stimuli, in combination with thermodynamic modeling of the self-organization process, are presented in this paper. The study was performed on adaptive PVD (physical vapor deposited) coatings represented by the TiAlCrSiYN/TiAlCrN nano-multilayer PVD coating. A detailed analysis of the worn surface was conducted using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) methods. It was demonstrated that the coating studied exhibits a very fast adaptive response to the extreme external stimuli through the formation of an increased amount of protective surface tribo-films at the very beginning of the running-in stage of wear. Analysis performed on the friction surface indicates that all of the tribo-film formation processes occur in the nanoscopic scale. The tribo-films form as thermal barrier tribo-ceramics with a complex composition and very low thermal conductivity under high operating temperatures, thus demonstrating reduced friction which results in low cutting forces and wear values. This process presents an opportunity for the surface layer to attain a strong non-equilibrium state. This leads to the stabilization of the exchanging interactions between the tool and environment at a low wear level. This effect is the consequence of the synergistic behavior of complex matter represented by the dynamically formed nano-scale tribo-film layer.
Highlights
The running-in period is the actual stage of wear when self-organization begins and progresses towards the post-running-in when wear rate stabilizes [1]
These tribological conditions are in a strong non-equilibrium state [8] and, present an ideal case study of the self-organization process during friction [1], with surface-engineered nano-material experiments [9,10,11]
This paper demonstrates an example of adaptive coating [23] represented by a nano-multilayer (TiAlCrSiYN/TiAlCrN PVD) coating, which is capable of sustaining severe operating conditions [24]
Summary
The running-in period is the actual stage of wear when self-organization begins and progresses towards the post-running-in (stable stage) when wear rate stabilizes [1]. High-performance machining represents a unique case of an extreme external environment [3,4,5], due to a combination of high temperatures, around 1000–1300 ◦ C, and pressures of 2–5 GPa [6,7] during high-speed cutting As such, these tribological conditions are in a strong non-equilibrium state [8] and, present an ideal case study of the self-organization process during friction [1], with surface-engineered nano-material experiments [9,10,11]. The entire tribo-film layer has a complex chemical composition during cutting and as was shown previously, an amorphous/crystalline nano-scale structure [18,19] In this way, the tribo-film layer, to some degree, represents what is known as complex matter, which is a matter that has informed, self-organized, evolutive properties [20,21]. The key part of this investigation is concerned with the complex interaction between a number of non-linear processes on the friction surface at the nano-scale, under outlined extreme tribological conditions
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